CA1126337A - Flow-through electrochemical system - Google Patents
Flow-through electrochemical systemInfo
- Publication number
- CA1126337A CA1126337A CA324,116A CA324116A CA1126337A CA 1126337 A CA1126337 A CA 1126337A CA 324116 A CA324116 A CA 324116A CA 1126337 A CA1126337 A CA 1126337A
- Authority
- CA
- Canada
- Prior art keywords
- membrane
- species
- electrode
- diffusion
- interfering
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/28—Electrolytic cell components
- G01N27/40—Semi-permeable membranes or partitions
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S435/00—Chemistry: molecular biology and microbiology
- Y10S435/817—Enzyme or microbe electrode
Abstract
ABSTRACT OF THE DISCLOSURE
An electrochemical system using a flow-through electrode design employing a dual separation method to lower electrode poisoning due to large chemical species and interference current due to small electroactive species is disclosed. The system includes a fluid inlet to a reaction chamber and a fluid outlet from the chamber, with the chamber in contact with an electrode system. The electrode is protected from poisoning and inter-ference by a membrane system comprising a semipermeable membrane having well defined pore ranges therethrough which allow diffusion of only selected size small molecules. A unique sample stream flow control coupled with the size selective membrane combine to remove large, or macromolecule poisoning species and substantially reduce small species interference at the active surfaces of the electrode. For example, hydrogen peroxide, as an indicator for an enzyme substrate reaction, can be determined substantially free of interferences from high molecular weight species like proteins and low molecular weight species like ascorbic acid.
An electrochemical system using a flow-through electrode design employing a dual separation method to lower electrode poisoning due to large chemical species and interference current due to small electroactive species is disclosed. The system includes a fluid inlet to a reaction chamber and a fluid outlet from the chamber, with the chamber in contact with an electrode system. The electrode is protected from poisoning and inter-ference by a membrane system comprising a semipermeable membrane having well defined pore ranges therethrough which allow diffusion of only selected size small molecules. A unique sample stream flow control coupled with the size selective membrane combine to remove large, or macromolecule poisoning species and substantially reduce small species interference at the active surfaces of the electrode. For example, hydrogen peroxide, as an indicator for an enzyme substrate reaction, can be determined substantially free of interferences from high molecular weight species like proteins and low molecular weight species like ascorbic acid.
Description
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BACKCROUNU ~F T!~E INVENTION .~. .
1. Fleld Of The Invention.
Thls lnvention relates to an apparatus and method for monitoring the concentration of an electroactlve species in a flowing stream of solvent when the electroactive species of .
interest is present, noe in a continuous fashion, bue in a discrete high concentration zone or "slug". ~ore pareicularly, the invention relates to the prec~se determination of an elecero-active species of interest which is in a flowing sample stream which may include two types o f contaminating species.
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A contamlna~ing species of the first type are large molecules .
; These are usually molecules in the so-called group of macromole-cules. These specles could be exemplified by polymer fragments ln industrial fluids or blood proteins in blological fluids. These macromolecules are of such a chemistry that they may be adsorbed to the surface of an electrode and thereby "poison" the electrode by placing it in an inactive state.
Contaminants of the second type are fundamentally different from contaminants of the first type. This second type of molecule s includes small molecules, much the same size as the species of interest whose electrochemical aceivity we wish to follow. One common example ~f'interfering species is as follows. It is common in the immobllized enzyme art to perform converSiODS of substrate molecules to hydrogen peroxide (M. W. 34) and measure it polarographically by the oxldation reaction:
H202- ~ 2H + l/2 2 + 2e Unfortunately, many biological samples which are of interest also contain significant amounts of uric (M. W. 168 in the keto form) and sometimes ascorbic acid (M. W. 176). Therefore, those systems which operate without a method to exclude these low mass (compared to the macromolecules which have masses on the order of thousands to hundreds of thousands) interEering molecules from the electrode do so with fairly high interference currents being generated. In some cases it is known that the current generated by the interfering species is at least as large as the current of the sample of interest.
Since polarographic methodology is based on additive current~
the species of interest signal may be distorted by the addition ~.2fi337 of these inter~erence currents to the point where it has no analytical reliability.
~ESCRIPTION OF THE PRIOR AF~
In the past, electr~de sys-tems have been characterized by a few major types.
The general design of an electrochemical devi oe was shcwn many years ag~ by electrodes like the one designed by Leland C. Clark, Jr., and shown in U.S. Patent ~o. 2,913,386 issued November 17, 1959 and entitled "Electrochemical Devi oe Pbr Chemical Analysis". In that system an electrolyte is maintained within a tube-like body electr~de by a mem-brane whose primary func-tion is to maintain the electrolyte with the elec-trode and to allow diffusable gasses to pass thr~ugh the membrane.
These electrodes are designed for use in a static sample and have been called "dip-in" electr~des. In use, the electrode's tip is plaoe d in the solution of interest, allowed to remain in the quiet, non-flowing solution until an accurate determination is oompleted.
m is same dipping type electr~de is shown in U.S. Patent No.
3,380,905, also issued to LÆland C. Clark, Jr., on April 30, 1968 and en-titled "Electrolytic Sensor With Anodic Depolarization". This patent dis-closes a trielectrode system with a membrane structure which performs much the same function as the membrane of the previously discussed Clark electrDde.
These me~branes were essentially total liquid barriers, and were not designed and did not function to allow electr~lyte to pass the barrier.
Rather, as suggested in Clark patellt 2,913,386, the membranes were typically polyethylene which had the ability to allow gasses to diffuse there-through, but in no case liquids.
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3;~7 Clearly, these electrodes were not to ke used in flowing sample stream systems and they e~ployed liquid nonper~eable ~embranes to separate solvent of the sample from the captive or internal reference electrolyte.
While the Clark electrodes were primarily for measurement of gasses, due to the nature of their me~brane structures, they did allow interfering spe d es of the sa~e physical state as the sample of interest to interfere with the measure~ent. Fbr exa~ple, if the electrode were being used to determine solution 2 levels and there was present a sub-stantial amount of 00 or S02, these interfering species oould easily ar-rive at the electrode, just as the species of interest by permselective crossing of the membrane and generate an interfering current.
Unlike the Clark type electrode, many electrode co~binations have been designed to attempt to measure species in a flowing stream.
U.S. Patent No. 3,622,488, issued to Ramesh Chand on Novemker 23, 1971 and entitled "Apparatus EDr Measuring Sulfur Dioxide Concen-trations" shows a system to oontinuously m~nitor S02 c~ncentrations.
Again, as in the Cla~X patents, a m~brane is used to eliminate elec-trolyte loss yet allow diffusion of the S02 across the membrane to the electrode's surfaoe. While this type electrode dbes monitor the con-centration of S02 continuously it also has the drawback that interfering species will ~e allowed to reach the electrode and generate an inter-ference current.
Many solution phase flow-through electrode systems have been demoilstrated. In U.S. Patent No. 3,707,455, issued to D. B. Derr et al, on Dece~ber 26, 1972 and entitled "Measuring System" discloses a captive mab/
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enzyme reagerlt. The reagent enzyme is trapped by a membrane.
The membrane keeps the larger enzyme molecules inside a chamber and allows small molecules completely free diffusion across the membrane. Even though in a flowing stream, it is clear that small molecu]e interference in this system is still present, since a dual electrode system is used. One electrode measures species of interest plus interference and one only interference.
These systems are subject to the problems inherent in signal conditioning which affect signal reliability. Not only are electrodes like the ones discussed above subject to large molecule poisoning, but since large masses of unnecessary and interfering species arrive at the electrode and are reacted, there the electrodes are subject to more rapid degradation, concomitant failure, and drift. Also since these electrodes do measure large signals occasionally with small contributions from the species of interest, there is the problem of measuring a large volume of response with a small signal of interest and the associated signal-to-noise type problem.
SUMMARY OF T~IE INVENTION
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An object of the present invention is to provide a flow-through electrochemical system which is capable of measuring a species of interest while reducing the measurement of the interfering species greatly.
In one particular aspect the present invention provides a method of determining the solution concentration in a sample liquid stream of an electroactive species in the presence of at least one interfering electroactive species comprising placing an electrode means adjacent a flow-jl/ -5-~.~.2~i3~'37 through type reaction chamber; subsequently separating said electrode means from said reaction chamber by a membrane system comprising at least one layer of a membrane material which has ~)res there~hr.ough which allow selective diffusion across said membrane system, said m~e~brane material permitting diffusion therethrough of said at least one interfering elec-troactive spe des at a lower rate than said electroactive species; pro-viding a f,lcw path over said membrane and com~unicating with said re-action chamber,flowing a sample stream along said flow path and over said membrane system at a predetermined flow rate to allow diffusion of said electr~active species from said reaction chamber to said electrode means to the substantial exclusion of said at least one interfering electro-active species; measuring the response of said electrode means; and flowing a buffer stream along said flow path and over said membrane to allow diffusion of said electroactive species and said at least one interfering elect,roactive species into said buffer stream preparatory to the flowing of a different sample liquid stream adjacent said mem-brane for determining the solution concentration of said electro-active species therein.
In another particular aspect the present invention provides a method of electrochemically measuring the concentration of relatively low mass electroactive species of interest in a sampl.e stream in the presence of high and lcw mass interfering specles, comprises the steps of: pr~viding a flow path over a membrane protected electrode means wherein said membrane has a m~lecular mass diffusion cutoff such that said high mass m~lecules with masses higher than the mDlecular mass cutoff are excluded from the electrode;
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,then precisely controlling the flow rate of said sample stream so that the diffusion ratio of the low mass interfering species and the low mass electroactive species of interest is such that substantially all of said low mass interfering species is excluded from said electrode means; then measuring the response of said electrode means to the electrochemical reaction of said low mass species of interest at said electrode means.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram illustrating a cross-sectional view of the electrochemical cell and the flow system used therewith.
Fig. 2 illustrates the current v. potential curves for a species of interest in the presence of an interfering species.
Fig. 3 is a schematic diagram illustrating an alternative embodiment of the invention shown in Fig. 1.
Fig. 4 is a schematic flow diagram illustrating the details of the electrochemical system of the invention.
DETAILED DESCRIPTION OF THE _NVENTION
Figure 1 illustrates a preferred embodiment of the present invention. The electrochemical system of the present invention ~;, .
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includes a first housing 11 whlch lncludes a fluid lnlet 12 and a fluid outlet 13. Typlcally, the fluid inlet 12 i5 coupled to an lnlet llne 14 whlch has lntegral therewith an in~ection port 15.
Due to the composition of the membrane system used wlth this invention in the preferred embodiment, a buffer solution must continuously bathe the membrane-electrode system. This continuou buffer solution is provided from a buffer reservoir 16 which supplies a pump 17 which communicates with the fluid inlet 12.
The fluid lnlet 12 terminates ln a reaction chamber 18 which is in communication wlth the fluid outlet 13 and an electrode receiving chamber 21.
Into the electrode receiving chamber 21 ls fitted a second housing 22 which has embedded therein the electrod~s which sense _ the concentration of the electroactlve specles which pass the membrane system. The electrodes of this system comprise a refer-ence electrode 23, a counter electrode 24 and an lndicating or working electrode 25. These electrodes are embedded in the material of the second housing 22 so as to be maintained in a fixed geometric relationship. As 3een in Fig. 1, a membrane system 26 comprises in thls embodlment of the invention a single layer of a membranous material whlch allows solvent flow there-through and thereby selectlve diffusion across. The membrane 27 has sized pores 28 therein to allow the diffusion. The membrane 27 is retained closely against the face 31 of the electrode by a restraining collar 32. Since many of the species of interest are present in low concentration the electrode response and sen-sitivity are critical. Thus, a pair or temperature sensing tXermistors 33 are embedded into the first housing 11 ad~acant . .
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he reac~Lo= cha~ber 18 to trac~ the te=~erat~re o~ ~he reaotion buffer solution. As the temperature of the buffer changes the thermistor response is used to recalibrate the electronlcs l package which records and conditions the electrode readings.
¦ As seen in Fig. 1, the sensing tips of the electrodes in the electrode face 31 are in a clo3e spatial relationship with the membrane system 26.
There is always a smsll space between the electrode face 31 l and the membrane system 26. The membranes typically used with the invention must be continuolsly bathed with fluid to maintain their structural integrity. Thus, buffer flows over the face of the membrane system, tiffuses through the pores and bathes the rear face of the membrane, as well as any intervening membrane layers and the electrode face 31. This "wetting" of the membrane system also allows the proper diffusion path to e~ist between the flowing sample stream and the electrode face. The electrode materials used are typically as follows. The reference electrode 23 ls silver-silver chloride electrode, the counter electrode 24 is a platinum electrode as is the indicating electrode 25. The silver-silver chloride reference electrode ls preferred so that the bufer can function as a reference electrode filling solutlon by the incorporation of chloride ions therein. Other types of reference electrodes may be used if appropriate ad~ustments can be made to the diluent buffer supply.
The three electrode system or so-called potentiostat is preferred to a two electrode system because when there is only a referenca and working electrode in the system, current flows through the reference electrode. This can cause variation3 in the potential differenoe between the two electrodes due to the IR drop across the sample. r~lso/ when a silver-silver chloride ooating is used on a tWD electr~de system reference electrode, the silver chloride coating is eventually depleted by the reduction-oxidation reaction which occurs on the surface of the electrode. So, the use of a two electrode system, even when the two electrodes are very close together, is not favored while the three electrode potentiostat is.
When using the cell of Fig. 1 in a routine laboratory en-vironment, a peak detector and a sampling and hold circuit can be used to measure the maximum current above a base line current and the dif-ference will be proportional to the hydr~gen peroxide concentration in the specimen and is, therefore, proportional to the glucose concentration or the similar electroactive species other than hydrogen peroxide.
In the electrochemical cell shown in Fig. 1, the polarographic cell is potentiostatic. m is is the so-called "three electrode polaro-graph" as disclosed in the article, "The ~naissan oe in Polarographic and Voltammetric Analysis" by Jud B. Flato, appearing in Analytical Chemistry, Vol. 44, September 1972.
The first and second housing, which holds the electrodes, are made of a rigid, inert, electrically insulating material like glass or plastic. Polymethylmethacrylate has been used quite suc oe ssfully.
The system operates to reduce poisoning and interference of the indicating, referenoe, and counter electrode as follows. As the sample is injected into the flowing buffer stream, it is mab/
i3~ 7 S-l4388 ¦ considered to be a "slug". Slug ls defined herein as a discrete ¦ package of the sample which travels through the SySeem as a ¦ packet wi~hout substantlal dilution over the time period of the ¦ experiment. Many samples of biological origin contain large ¦ amounts of contaminating or interfering species like the non-¦ electroactive macromolecules. Examples of such macromolecules are ¦ proteins, nucleic acids or in industrial solutions, polymer and ¦ polymer fragments. A second type of interfering species that may ¦ be present in biological fluids are smaller electroactive species.
1 When analyzing for the biologically important species glucose, - I examples of such interfering species are uric acid (M.W. 168) and ascorbic acid (M.W. 176), whlch are both electroactive.
A membrane fount to be particularly suitable for this system is a cellulosic film known as SPECTRAPORTM, and available in films that have molecular mass cutoffs at 12-14,000 mass units, 6-8,000 mass units and at about 3,500 mass units. This membrane shows good long-term stability and is relatively pinhole free. The pores in the membrane perform a separation by slze roughly corresponding to the molecular mass of the species. So larger molecules, like proteins, which have masses over 14,000 are excluded by the 12-14,000 membrane, proteins over 3,500 mass units are excluded from passing to the electrode face by the 3,500 cutoff membrane, etc. This process of rough mass exclusion allows for the protection of the electrode from poisoning of the electrode by the adsorptlon of these large molecules thereon.
These membrane films are purchased from Spectrum Medical Industri s, Inc., 60916 Terminal Annex, Los Angeles~ California gO054.
The membrane also allows or selectlon of small molecules by . . ..
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3'3~ S-14388 I' a process thought to be dlffusion across the membrane system. As the sample slug passes the membrane there is a flnlte tlme for diffuslon to occur across the membrane through the slzed pores.
¦ It is known that the smaller a molecule is the faster it dlffuses ¦ ln aqueous solution Therefore, it has been discovered by the I ¦ creation of this device that if the flow rate of the sample is matched to the pore size of the membrane it i5 possible to substantially reduce the amount of one small molecule which l reaches the electrode as compared to another small molecule in ¦ the same sample. For example, if there is a very large size l difference between the species of lnterest, and the lnterfering ¦ species the flow rate could be lowered to allow more of the small species of interest to arrlve at the electrode face. If the l intèrfering species is close in size, and thus dlffusion rate, to lS ¦ the species of interest a higher flow rate may be required so that the interferlng species is not presented to the membrane system for a long enough time to be appreciably measured. Thua, lf there are a number of small interferlng specles contaminating l a sample by adiustlng the membrane and flow rate of the sample, ¦ it is possible to achieve the partial exclusion of the small interfering species, while allowing the small species of choice l to reach the electrode and react therewith.
¦ As the sample slug passes into the reaction chamber lô and l contacts the membrane system, the smallest molecule, i.e. the ¦ specles of lnterest, dlffuses towards the electrode through the membrane pores. Slmultaneously the lnterferlng specles begln to diffuse toward the electrode vla the membrane pores. The species ¦ of interest, being a smaller molecule than the interfering I
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¦molecules, arrives firse at the electrode. As thi~ diffusion toward the electrode is occurring, the sample slug is passing the membrane. As it passes the membrane the concentratiOn of the l sample grows smaller and the concentration gradient which was S ¦ driving the species of interest and the interfering species ¦ towards the electrode reverses and ~he molecules are drawn back into the sample buffer stream. By selecting both the flow rate of the sample past the membrane and the membrane characteristics, l the amount of interfering material which reaches the electrode ¦ can be very substantially reduced.
Fig. 2 shows a ploe of anodic current in nanoamperes verses anodic potential referenced to a silver-silver chloride electrode.
The graph shows the problem encountered in analyzing a sample l using a non-protected electrode and an uncontrolled flow rate.
¦ In the example of the plot curve 1 is the oxidation curve for ¦ hydrogen peroxide, a species often monitored as an indicstion of the amount of an enzyme substrate reaction. For example; glucose when reacted with the enzyme glucose oxidase is converted to l hydrogen peroxide and gluconic acid. Thus in clinical application ¦ by monitoring the amount of peroxide generated by the enzyme one may back calculate to determine the concentration of the species of interest, glucose. Curve 2 represents the oxidation curve for uric acid, a common interfering species in biological samples.
I If the systeu is being used to monltor hydrogen peroxide l (M.W. 34), for example, the uric acid and any contaminating ascorbic acid will undergo an electrochemical reaction at about the same electrode potential as does the peroxide of interest.
The main concept of this invention is to control the flow rate of ¦the sample past the membrsne system so that the smaller molecule, ~ 3'~7 S-14388 ~ ~
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H202, can dlffuse across the membrane system and be measured as the slug passes the reaction chamber but the larger, and therefore slower, interfering molecules cannot.
Note that at low potentials, around + 0.3 volts, the response due to hydrogen peroxi~e is high while the response due to uric acid is very low. ln the ideal situation the measurement would be made at this low potent~al to screen out the interference due .
to uric acid. Since the currents measured by the electrometer would be additive to total current (lT) would equal the currer~t generated by the hydrogen peroxide (iHp) and the current due to the oxidation of the interfering species uric acid (iuA), thus iT ' iHp + iUA
Unfortunately, the platinum electrode system preferred for these measurements is not active enough at these low potentials to oxidize completely the hydrogen peroxide. Therefore, the electrode must be operated at between + 0.5 to ~ 0.7 volts for best results and maximum electrode lifetime. As shown by Fig. 2, at this elevated anodic potential both the hydrogen peroxide and the uric acid are substantial contributors to the total signal.
Therefore, the use of a membrane system to exclude the uric acid or other interfering species ls necessary if accurate, relatively interference free readings are to be made. Since ehe membrane system excludes a substantial fraction of the interfering species the net current is more accurately a reflection of only the sample of interest and not sample plus interfering species. This exclusion of the interfering species from the electrode differs from presently used systems where the interference is measured by the working electrode and a second electrode and the readings then subtracted. In the case of the present lnventlon the ma~or ¦ fractlon of the lnterfering specles never reaches the electrode.
¦ As seen ln Flg. 3, the membrane system 26 may comprlse a I dual membrane havlng a flrst membrane 34 and a second membrane ¦ 35. The two me~brane system prevents the poisoning of the ¦ electrode due to any pinhole defects in the s~ngle membrane embodiment of Fig. 1. The membrane configuration of Flg. 1, ~hen used with any of the membranes described above, at a controlled l flow rate, reduces lnterference levels below the 5 mg per decilite ¦ level proposed by the Food and Drug Administration. The times ¦ typical from in~ection to sample readout are on the order of 60 seconds with a single membrane and on the order of 70-80 seconds with the dual membrane system. The system allows the effective l measurement of glucose levels found ~n human blood (70-80 mg per ; 15 l deciliter) with less than the 5 mg per deciliter interference fraction proposed by F.D.A.
Fig. 4 shows the system in a flow-through cell using a three electrode system according to the present invention.
I The system of Fig. 4 ls the same cell as shown ln Flg. 1 or ¦ Fig. 3. The cell of Fig. 4 is equipped with a reference electrode 23 in the form of a sllver wire coated with silver chloride positioned as closely as possible to the lndicating electrode 25 which is a platinum wire. The applled potential (+0.6 volts DC) l 19 applied to the input of a control amplifier 36 to which the ¦ reference electrode 23 ls also connected through voltage follower 37. The output of the control amplifier 36 is connected to the counter electrode 24 which is a platinum wire. By this design essentially no current flows through reference electrode 23 and i3~3~
sufficient 0mpensating po-tential is applied to counter electrode 24 to m~intain the potential difference between the referen oe electrode 23 ~Id the indicating electrvde 25. m e indicating elect~ode 25 is o~nnected to a small conventional current measuring device which pro-vides a current measurement which is oonverted to the sa~ple equivalent of the original specimen.
According to the present invention the aqueous buffered diluent is o~ntinuously pumped through the reaction chamber to both the electrode and membrane, as discussed above. m e sample is typically injected from a hypodermic syringe into the injection port which can be in the form of a mixing "tee" oovered with a rukber diaphragm.
The electrode response from the measurement of the electro-active species of interest is measured by a c~rrent measuring device such as a current follower. m is value is then converted to the sam~
ple equivalent of the original specimen. In the case of biological material the samples are reported as mg percent for example a glucose equivalent of an original specimen is usually reported in mg glucose/100 ml (i.e. mg percent) of specimen. These units are conventional in clinical applications.
In addition to buffer it is desirable to add salts such as potassium chloride or sodium chloride which serve to establish the referenoe potential when silver-silver chloride reference electrodes which use the buffer as a filling solution are e~ployed. A bacterial inhibitor can be incorporated in the buffered diluent to retard bac-terial interferen oe.
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~ fi3'37 Sl4388 An example of the system parameters are as follows. When the system is being used to detect hydrogen peroxide from the oxidation of glucose by glucose oxidase, an aliquot of 2.5 microliters of sample is introduced into the flowing buffer stream ~hat flows at a rate of between 0.1 to 5 ml per minute.
While, as discussed above, the flow rate must be ad~usted for each sample and membrane system, it appears that for a sample containing about 100 mg per decillter glucose, when the glucose is converted to hydrogen peroxlde and a 2.5 microliter sample is used, a flow of 0.1 to 2 ml per minute gives the optimum electrod response. It has been discovered that if the double layer membra e system of Flg. 3 1~ used a flow rate of one ml per minute gives good electrode response. The double layer system comprises two circular sections of a 12-14,000 atomic mass unit cutoff membrane of SPECTRAPOR 2TM purchased from Spectrum Medical Industries, Inc. While wlth some combinations of interfering species a membrane with smaller pore sizes may be preferred. This double layer of 12-14,000 cutoff membrane mater~al functions well over a wide range of flow rates. The time from in~ection, through sample peak to stable base line is as indicated above about 70-80 seconds.
It should be noted that while SPECTR~YORT membranes are used in the system, tests on MilliporeT membranes, type VS, VM
and PSAC have proven acceptable if flow rates are ad~usted to match the membrane. The MilliporeTMmembranes exclude in the mass range of 500-1000 mass unit~. It has been shown that of all . .
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me~branes tested, oellulosic membranes give consistently high quality results for long periods of use. Also, as membrane types are m~dified or changed totally, the loss of signal strength from the electrode may be compensated for by changing the working surface area of the elec-trDde.
To determine the usefulness of the system in determining a sample biological unkncwn the substance glucose was chosen. The glucose sample was passed through a glucose oxidase cartridge and the resultant hydr~gen peroxide was mDnitored in the presence of the F.D.A.
referen oe interferents. Table I shows the results of the interference studies. Table I ind udes three 01umns, one describing the interfering substance, the remaining two columns represent the results from tw~ dif-ferent instruments. To demonstrate that the system works well under different servi oe conditions, two instruments were similarly equipped and numerous samples of varied 0 mposition were analyzed by the separate instruments. Then, the different instruments having been subjected to different aging 0nditions, were used to analyze i & ntical sample fractions as described below. The results show ffhat while some variance is experienced between the two instruments, fflat in all, the inter-ference levels are kept below the recommended minimum.
To perfDrm the test a sample of hun~an serum was divided into two fractions. The amount of glucose was deter~ined. Fbr example, a serum sample would show about 100 mg/dl of glucose. m e halves are about five nl each. Enough solid ox solution phase inter-ferent is added to one-half of the sample to bring the blood-interferent solution to the 0ncentration indicated in parenthesis under ffhe "SUBST~N OE ADDED" column. Enough water, buffer or mab/
I ~ 2fi3~ 8-16388 .' .'. ' ,1 ¦ solven~ ls added to the second-half o f the serum to match its ¦ volume to that of the first one-half of the serum sample. The ¦ samples are run on two instruments. The dlfference between the ¦ two results for the same sample are probably due to; (1) Lndlvi-¦ dual differences b~tween platinum electrodes and (2) age and condition of the membrane system. Notice that some samples show l negative "BIAS" results. To determine the "BIAS" the samples are.
_ ¦ run, the real or "true" glucose value is determined from the l second-half sampIe. The flrst-half sample is run and the value ¦ of its glucose equivalent is subtracted fro~ the second-half reading. For example, if the glucose concentration were 100 mg/dl and the interference + glucose sample reads 101.6 mg/dl the BIAS is +1.6, as shown in "INSTRUMENT 1", item 1.
¦ Some samples show a negative bias. This is thought to be ¦ due, in one case, to latent catalase enzyme in the sample which destroys hydrogen peroxide during the course of the test and artificially lowers the "BIAS".
TABLE I_INTERFERENCE STUDY
l a) ENDOGENOUS SUBSTANCES
20 I ~
SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 Fructose (150) +1.6 0 Mannose (300) +2.8 +4.1 Galactose (300) +1.2 +0.3 Ascorbic Acid (25) +3.4 +1.8 Creatinine (25) +0.4 -0.2 l Glutathione (50) +1.8 +1.6 25 l Cltric Acid (1500) -3.8 -4.4 Hemoglobin (5000) -4.8 -2.6 NH4c~ 1.2 +0.6 Bilirubin (25) 0 0 Uric Acid (25) +1.0 +3.4 Cysteine (40) +2.0 +3.6 Lipid (600) +1.6 ~ ;3;37 S-l4388 I (b) EXOGENOUS SUBSTANCES
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_ .
SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 l L-Dopa ~10) +1.0 +2.2 1 Xylose (150) ~1.8 +0.2 Ribose (150) +1.2 +0.8 Na Salicylate (50) +1.0 +0.2 Na Diatrizoace t5Zv/v) +0.6 ~0.6 Meglumine Diatrizoate +1.6 +0.8 Tolbutamide (25) -0.8 0 l Methyl Dopa (25) ~3.0 +2.8 ¦ Streptomycin (30) -0.6 +1.0 Sulfadiazine (50) +2.2 +1.0 l De~ran 10 l (100% of plasma volume) +2.4 +1.6 l Acetyl Salicylic Acid (30) +0.2 0 ¦ (c) ANTICOAGULANTS AND PRESERVATIVES
, _ _ SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 l Na Fluoride (750) +2.4 +2.5 ~ ¦ Na aeparin (7000U/dl) -0.6 +0.4 .
Thymol (500) -4.8 -2.8 E.D.T.A. (550) +0.2 +1.2 Na Oxalate (800) +1.0 Na Citrate (2100) +2.2 +3.8 The day-to-day precision evaluated from results for aqueous l glucose standards and stable serum are seen in Table 2 (a) and l (b). In every case, the system, with results for two test lnstru-ments, showed reproducibility as the coefficient of variation ¦ (c.v.%) of less than 22. This is well below the 5% considered ¦ acceptable for most clinical uses.
l TABLE 2 PRECISION
5 l (a) AQU OUS STANDARDS (Prepared from NBS reference materialSRM No. 917) VALUE MEAN S.D. C.V.% MEAN S.D. C.Y.%
I
1 50 49.3 0.66 1.3 50.0 0.73 1.5 30 I 100 99.7 1.38 1.4 99.6 0.69 0.7 1 350 345.7 2.gO 0.8 345.4 2.7 0.8 ~ ~ 2k;;~7 S-l4388 ¦ (b) SFRUM POOLS
¦ POOL INSTBUMENT 1 INSTRUMENT 2 ¦ MEAN S.D. C.V.Z MEAN S.D. C.V.Z
¦ Low51.6 0.95 1.8 51.6 0.76 1.5 Normal 125.5 2.04 1.6 124.1 1.59 1.3 I High336.9 3.88 1.2 333.5 2.40 0.7 ¦ Therefore, clearly the unique flow-through system using a ¦ semi-permeable membrane system lowers and in some cases eliminate ¦ interference when measuring electroactive species.
¦ In accordance with the provisions of the patent statutes, ¦ the principle and mode of operation of the invention has been ¦ explained, and we have illustrated and descrlbed in the typical embodlment what is considered its best embodiment. It is under-stood that, within the scope of the appended claims, the inventio may be practiced otherwise than as specifically illustrated and l described in the typical embodiment and accompanying alternatives ¦ herein.
BACKCROUNU ~F T!~E INVENTION .~. .
1. Fleld Of The Invention.
Thls lnvention relates to an apparatus and method for monitoring the concentration of an electroactlve species in a flowing stream of solvent when the electroactive species of .
interest is present, noe in a continuous fashion, bue in a discrete high concentration zone or "slug". ~ore pareicularly, the invention relates to the prec~se determination of an elecero-active species of interest which is in a flowing sample stream which may include two types o f contaminating species.
.
~b .
~ 3~ S-14~
A contamlna~ing species of the first type are large molecules .
; These are usually molecules in the so-called group of macromole-cules. These specles could be exemplified by polymer fragments ln industrial fluids or blood proteins in blological fluids. These macromolecules are of such a chemistry that they may be adsorbed to the surface of an electrode and thereby "poison" the electrode by placing it in an inactive state.
Contaminants of the second type are fundamentally different from contaminants of the first type. This second type of molecule s includes small molecules, much the same size as the species of interest whose electrochemical aceivity we wish to follow. One common example ~f'interfering species is as follows. It is common in the immobllized enzyme art to perform converSiODS of substrate molecules to hydrogen peroxide (M. W. 34) and measure it polarographically by the oxldation reaction:
H202- ~ 2H + l/2 2 + 2e Unfortunately, many biological samples which are of interest also contain significant amounts of uric (M. W. 168 in the keto form) and sometimes ascorbic acid (M. W. 176). Therefore, those systems which operate without a method to exclude these low mass (compared to the macromolecules which have masses on the order of thousands to hundreds of thousands) interEering molecules from the electrode do so with fairly high interference currents being generated. In some cases it is known that the current generated by the interfering species is at least as large as the current of the sample of interest.
Since polarographic methodology is based on additive current~
the species of interest signal may be distorted by the addition ~.2fi337 of these inter~erence currents to the point where it has no analytical reliability.
~ESCRIPTION OF THE PRIOR AF~
In the past, electr~de sys-tems have been characterized by a few major types.
The general design of an electrochemical devi oe was shcwn many years ag~ by electrodes like the one designed by Leland C. Clark, Jr., and shown in U.S. Patent ~o. 2,913,386 issued November 17, 1959 and entitled "Electrochemical Devi oe Pbr Chemical Analysis". In that system an electrolyte is maintained within a tube-like body electr~de by a mem-brane whose primary func-tion is to maintain the electrolyte with the elec-trode and to allow diffusable gasses to pass thr~ugh the membrane.
These electrodes are designed for use in a static sample and have been called "dip-in" electr~des. In use, the electrode's tip is plaoe d in the solution of interest, allowed to remain in the quiet, non-flowing solution until an accurate determination is oompleted.
m is same dipping type electr~de is shown in U.S. Patent No.
3,380,905, also issued to LÆland C. Clark, Jr., on April 30, 1968 and en-titled "Electrolytic Sensor With Anodic Depolarization". This patent dis-closes a trielectrode system with a membrane structure which performs much the same function as the membrane of the previously discussed Clark electrDde.
These me~branes were essentially total liquid barriers, and were not designed and did not function to allow electr~lyte to pass the barrier.
Rather, as suggested in Clark patellt 2,913,386, the membranes were typically polyethylene which had the ability to allow gasses to diffuse there-through, but in no case liquids.
mab/
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3;~7 Clearly, these electrodes were not to ke used in flowing sample stream systems and they e~ployed liquid nonper~eable ~embranes to separate solvent of the sample from the captive or internal reference electrolyte.
While the Clark electrodes were primarily for measurement of gasses, due to the nature of their me~brane structures, they did allow interfering spe d es of the sa~e physical state as the sample of interest to interfere with the measure~ent. Fbr exa~ple, if the electrode were being used to determine solution 2 levels and there was present a sub-stantial amount of 00 or S02, these interfering species oould easily ar-rive at the electrode, just as the species of interest by permselective crossing of the membrane and generate an interfering current.
Unlike the Clark type electrode, many electrode co~binations have been designed to attempt to measure species in a flowing stream.
U.S. Patent No. 3,622,488, issued to Ramesh Chand on Novemker 23, 1971 and entitled "Apparatus EDr Measuring Sulfur Dioxide Concen-trations" shows a system to oontinuously m~nitor S02 c~ncentrations.
Again, as in the Cla~X patents, a m~brane is used to eliminate elec-trolyte loss yet allow diffusion of the S02 across the membrane to the electrode's surfaoe. While this type electrode dbes monitor the con-centration of S02 continuously it also has the drawback that interfering species will ~e allowed to reach the electrode and generate an inter-ference current.
Many solution phase flow-through electrode systems have been demoilstrated. In U.S. Patent No. 3,707,455, issued to D. B. Derr et al, on Dece~ber 26, 1972 and entitled "Measuring System" discloses a captive mab/
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enzyme reagerlt. The reagent enzyme is trapped by a membrane.
The membrane keeps the larger enzyme molecules inside a chamber and allows small molecules completely free diffusion across the membrane. Even though in a flowing stream, it is clear that small molecu]e interference in this system is still present, since a dual electrode system is used. One electrode measures species of interest plus interference and one only interference.
These systems are subject to the problems inherent in signal conditioning which affect signal reliability. Not only are electrodes like the ones discussed above subject to large molecule poisoning, but since large masses of unnecessary and interfering species arrive at the electrode and are reacted, there the electrodes are subject to more rapid degradation, concomitant failure, and drift. Also since these electrodes do measure large signals occasionally with small contributions from the species of interest, there is the problem of measuring a large volume of response with a small signal of interest and the associated signal-to-noise type problem.
SUMMARY OF T~IE INVENTION
.
An object of the present invention is to provide a flow-through electrochemical system which is capable of measuring a species of interest while reducing the measurement of the interfering species greatly.
In one particular aspect the present invention provides a method of determining the solution concentration in a sample liquid stream of an electroactive species in the presence of at least one interfering electroactive species comprising placing an electrode means adjacent a flow-jl/ -5-~.~.2~i3~'37 through type reaction chamber; subsequently separating said electrode means from said reaction chamber by a membrane system comprising at least one layer of a membrane material which has ~)res there~hr.ough which allow selective diffusion across said membrane system, said m~e~brane material permitting diffusion therethrough of said at least one interfering elec-troactive spe des at a lower rate than said electroactive species; pro-viding a f,lcw path over said membrane and com~unicating with said re-action chamber,flowing a sample stream along said flow path and over said membrane system at a predetermined flow rate to allow diffusion of said electr~active species from said reaction chamber to said electrode means to the substantial exclusion of said at least one interfering electro-active species; measuring the response of said electrode means; and flowing a buffer stream along said flow path and over said membrane to allow diffusion of said electroactive species and said at least one interfering elect,roactive species into said buffer stream preparatory to the flowing of a different sample liquid stream adjacent said mem-brane for determining the solution concentration of said electro-active species therein.
In another particular aspect the present invention provides a method of electrochemically measuring the concentration of relatively low mass electroactive species of interest in a sampl.e stream in the presence of high and lcw mass interfering specles, comprises the steps of: pr~viding a flow path over a membrane protected electrode means wherein said membrane has a m~lecular mass diffusion cutoff such that said high mass m~lecules with masses higher than the mDlecular mass cutoff are excluded from the electrode;
- 5a -mab/
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,then precisely controlling the flow rate of said sample stream so that the diffusion ratio of the low mass interfering species and the low mass electroactive species of interest is such that substantially all of said low mass interfering species is excluded from said electrode means; then measuring the response of said electrode means to the electrochemical reaction of said low mass species of interest at said electrode means.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram illustrating a cross-sectional view of the electrochemical cell and the flow system used therewith.
Fig. 2 illustrates the current v. potential curves for a species of interest in the presence of an interfering species.
Fig. 3 is a schematic diagram illustrating an alternative embodiment of the invention shown in Fig. 1.
Fig. 4 is a schematic flow diagram illustrating the details of the electrochemical system of the invention.
DETAILED DESCRIPTION OF THE _NVENTION
Figure 1 illustrates a preferred embodiment of the present invention. The electrochemical system of the present invention ~;, .
~ 6-i3~
'I S-1438~
includes a first housing 11 whlch lncludes a fluid lnlet 12 and a fluid outlet 13. Typlcally, the fluid inlet 12 i5 coupled to an lnlet llne 14 whlch has lntegral therewith an in~ection port 15.
Due to the composition of the membrane system used wlth this invention in the preferred embodiment, a buffer solution must continuously bathe the membrane-electrode system. This continuou buffer solution is provided from a buffer reservoir 16 which supplies a pump 17 which communicates with the fluid inlet 12.
The fluid lnlet 12 terminates ln a reaction chamber 18 which is in communication wlth the fluid outlet 13 and an electrode receiving chamber 21.
Into the electrode receiving chamber 21 ls fitted a second housing 22 which has embedded therein the electrod~s which sense _ the concentration of the electroactlve specles which pass the membrane system. The electrodes of this system comprise a refer-ence electrode 23, a counter electrode 24 and an lndicating or working electrode 25. These electrodes are embedded in the material of the second housing 22 so as to be maintained in a fixed geometric relationship. As 3een in Fig. 1, a membrane system 26 comprises in thls embodlment of the invention a single layer of a membranous material whlch allows solvent flow there-through and thereby selectlve diffusion across. The membrane 27 has sized pores 28 therein to allow the diffusion. The membrane 27 is retained closely against the face 31 of the electrode by a restraining collar 32. Since many of the species of interest are present in low concentration the electrode response and sen-sitivity are critical. Thus, a pair or temperature sensing tXermistors 33 are embedded into the first housing 11 ad~acant . .
I ~ 3~7 S-143~
he reac~Lo= cha~ber 18 to trac~ the te=~erat~re o~ ~he reaotion buffer solution. As the temperature of the buffer changes the thermistor response is used to recalibrate the electronlcs l package which records and conditions the electrode readings.
¦ As seen in Fig. 1, the sensing tips of the electrodes in the electrode face 31 are in a clo3e spatial relationship with the membrane system 26.
There is always a smsll space between the electrode face 31 l and the membrane system 26. The membranes typically used with the invention must be continuolsly bathed with fluid to maintain their structural integrity. Thus, buffer flows over the face of the membrane system, tiffuses through the pores and bathes the rear face of the membrane, as well as any intervening membrane layers and the electrode face 31. This "wetting" of the membrane system also allows the proper diffusion path to e~ist between the flowing sample stream and the electrode face. The electrode materials used are typically as follows. The reference electrode 23 ls silver-silver chloride electrode, the counter electrode 24 is a platinum electrode as is the indicating electrode 25. The silver-silver chloride reference electrode ls preferred so that the bufer can function as a reference electrode filling solutlon by the incorporation of chloride ions therein. Other types of reference electrodes may be used if appropriate ad~ustments can be made to the diluent buffer supply.
The three electrode system or so-called potentiostat is preferred to a two electrode system because when there is only a referenca and working electrode in the system, current flows through the reference electrode. This can cause variation3 in the potential differenoe between the two electrodes due to the IR drop across the sample. r~lso/ when a silver-silver chloride ooating is used on a tWD electr~de system reference electrode, the silver chloride coating is eventually depleted by the reduction-oxidation reaction which occurs on the surface of the electrode. So, the use of a two electrode system, even when the two electrodes are very close together, is not favored while the three electrode potentiostat is.
When using the cell of Fig. 1 in a routine laboratory en-vironment, a peak detector and a sampling and hold circuit can be used to measure the maximum current above a base line current and the dif-ference will be proportional to the hydr~gen peroxide concentration in the specimen and is, therefore, proportional to the glucose concentration or the similar electroactive species other than hydrogen peroxide.
In the electrochemical cell shown in Fig. 1, the polarographic cell is potentiostatic. m is is the so-called "three electrode polaro-graph" as disclosed in the article, "The ~naissan oe in Polarographic and Voltammetric Analysis" by Jud B. Flato, appearing in Analytical Chemistry, Vol. 44, September 1972.
The first and second housing, which holds the electrodes, are made of a rigid, inert, electrically insulating material like glass or plastic. Polymethylmethacrylate has been used quite suc oe ssfully.
The system operates to reduce poisoning and interference of the indicating, referenoe, and counter electrode as follows. As the sample is injected into the flowing buffer stream, it is mab/
i3~ 7 S-l4388 ¦ considered to be a "slug". Slug ls defined herein as a discrete ¦ package of the sample which travels through the SySeem as a ¦ packet wi~hout substantlal dilution over the time period of the ¦ experiment. Many samples of biological origin contain large ¦ amounts of contaminating or interfering species like the non-¦ electroactive macromolecules. Examples of such macromolecules are ¦ proteins, nucleic acids or in industrial solutions, polymer and ¦ polymer fragments. A second type of interfering species that may ¦ be present in biological fluids are smaller electroactive species.
1 When analyzing for the biologically important species glucose, - I examples of such interfering species are uric acid (M.W. 168) and ascorbic acid (M.W. 176), whlch are both electroactive.
A membrane fount to be particularly suitable for this system is a cellulosic film known as SPECTRAPORTM, and available in films that have molecular mass cutoffs at 12-14,000 mass units, 6-8,000 mass units and at about 3,500 mass units. This membrane shows good long-term stability and is relatively pinhole free. The pores in the membrane perform a separation by slze roughly corresponding to the molecular mass of the species. So larger molecules, like proteins, which have masses over 14,000 are excluded by the 12-14,000 membrane, proteins over 3,500 mass units are excluded from passing to the electrode face by the 3,500 cutoff membrane, etc. This process of rough mass exclusion allows for the protection of the electrode from poisoning of the electrode by the adsorptlon of these large molecules thereon.
These membrane films are purchased from Spectrum Medical Industri s, Inc., 60916 Terminal Annex, Los Angeles~ California gO054.
The membrane also allows or selectlon of small molecules by . . ..
. . ~.
3'3~ S-14388 I' a process thought to be dlffusion across the membrane system. As the sample slug passes the membrane there is a flnlte tlme for diffuslon to occur across the membrane through the slzed pores.
¦ It is known that the smaller a molecule is the faster it dlffuses ¦ ln aqueous solution Therefore, it has been discovered by the I ¦ creation of this device that if the flow rate of the sample is matched to the pore size of the membrane it i5 possible to substantially reduce the amount of one small molecule which l reaches the electrode as compared to another small molecule in ¦ the same sample. For example, if there is a very large size l difference between the species of lnterest, and the lnterfering ¦ species the flow rate could be lowered to allow more of the small species of interest to arrlve at the electrode face. If the l intèrfering species is close in size, and thus dlffusion rate, to lS ¦ the species of interest a higher flow rate may be required so that the interferlng species is not presented to the membrane system for a long enough time to be appreciably measured. Thua, lf there are a number of small interferlng specles contaminating l a sample by adiustlng the membrane and flow rate of the sample, ¦ it is possible to achieve the partial exclusion of the small interfering species, while allowing the small species of choice l to reach the electrode and react therewith.
¦ As the sample slug passes into the reaction chamber lô and l contacts the membrane system, the smallest molecule, i.e. the ¦ specles of lnterest, dlffuses towards the electrode through the membrane pores. Slmultaneously the lnterferlng specles begln to diffuse toward the electrode vla the membrane pores. The species ¦ of interest, being a smaller molecule than the interfering I
I ' , - 11 -~ 337 S-143~8 ~ . ' .
¦molecules, arrives firse at the electrode. As thi~ diffusion toward the electrode is occurring, the sample slug is passing the membrane. As it passes the membrane the concentratiOn of the l sample grows smaller and the concentration gradient which was S ¦ driving the species of interest and the interfering species ¦ towards the electrode reverses and ~he molecules are drawn back into the sample buffer stream. By selecting both the flow rate of the sample past the membrane and the membrane characteristics, l the amount of interfering material which reaches the electrode ¦ can be very substantially reduced.
Fig. 2 shows a ploe of anodic current in nanoamperes verses anodic potential referenced to a silver-silver chloride electrode.
The graph shows the problem encountered in analyzing a sample l using a non-protected electrode and an uncontrolled flow rate.
¦ In the example of the plot curve 1 is the oxidation curve for ¦ hydrogen peroxide, a species often monitored as an indicstion of the amount of an enzyme substrate reaction. For example; glucose when reacted with the enzyme glucose oxidase is converted to l hydrogen peroxide and gluconic acid. Thus in clinical application ¦ by monitoring the amount of peroxide generated by the enzyme one may back calculate to determine the concentration of the species of interest, glucose. Curve 2 represents the oxidation curve for uric acid, a common interfering species in biological samples.
I If the systeu is being used to monltor hydrogen peroxide l (M.W. 34), for example, the uric acid and any contaminating ascorbic acid will undergo an electrochemical reaction at about the same electrode potential as does the peroxide of interest.
The main concept of this invention is to control the flow rate of ¦the sample past the membrsne system so that the smaller molecule, ~ 3'~7 S-14388 ~ ~
I
H202, can dlffuse across the membrane system and be measured as the slug passes the reaction chamber but the larger, and therefore slower, interfering molecules cannot.
Note that at low potentials, around + 0.3 volts, the response due to hydrogen peroxi~e is high while the response due to uric acid is very low. ln the ideal situation the measurement would be made at this low potent~al to screen out the interference due .
to uric acid. Since the currents measured by the electrometer would be additive to total current (lT) would equal the currer~t generated by the hydrogen peroxide (iHp) and the current due to the oxidation of the interfering species uric acid (iuA), thus iT ' iHp + iUA
Unfortunately, the platinum electrode system preferred for these measurements is not active enough at these low potentials to oxidize completely the hydrogen peroxide. Therefore, the electrode must be operated at between + 0.5 to ~ 0.7 volts for best results and maximum electrode lifetime. As shown by Fig. 2, at this elevated anodic potential both the hydrogen peroxide and the uric acid are substantial contributors to the total signal.
Therefore, the use of a membrane system to exclude the uric acid or other interfering species ls necessary if accurate, relatively interference free readings are to be made. Since ehe membrane system excludes a substantial fraction of the interfering species the net current is more accurately a reflection of only the sample of interest and not sample plus interfering species. This exclusion of the interfering species from the electrode differs from presently used systems where the interference is measured by the working electrode and a second electrode and the readings then subtracted. In the case of the present lnventlon the ma~or ¦ fractlon of the lnterfering specles never reaches the electrode.
¦ As seen ln Flg. 3, the membrane system 26 may comprlse a I dual membrane havlng a flrst membrane 34 and a second membrane ¦ 35. The two me~brane system prevents the poisoning of the ¦ electrode due to any pinhole defects in the s~ngle membrane embodiment of Fig. 1. The membrane configuration of Flg. 1, ~hen used with any of the membranes described above, at a controlled l flow rate, reduces lnterference levels below the 5 mg per decilite ¦ level proposed by the Food and Drug Administration. The times ¦ typical from in~ection to sample readout are on the order of 60 seconds with a single membrane and on the order of 70-80 seconds with the dual membrane system. The system allows the effective l measurement of glucose levels found ~n human blood (70-80 mg per ; 15 l deciliter) with less than the 5 mg per deciliter interference fraction proposed by F.D.A.
Fig. 4 shows the system in a flow-through cell using a three electrode system according to the present invention.
I The system of Fig. 4 ls the same cell as shown ln Flg. 1 or ¦ Fig. 3. The cell of Fig. 4 is equipped with a reference electrode 23 in the form of a sllver wire coated with silver chloride positioned as closely as possible to the lndicating electrode 25 which is a platinum wire. The applled potential (+0.6 volts DC) l 19 applied to the input of a control amplifier 36 to which the ¦ reference electrode 23 ls also connected through voltage follower 37. The output of the control amplifier 36 is connected to the counter electrode 24 which is a platinum wire. By this design essentially no current flows through reference electrode 23 and i3~3~
sufficient 0mpensating po-tential is applied to counter electrode 24 to m~intain the potential difference between the referen oe electrode 23 ~Id the indicating electrvde 25. m e indicating elect~ode 25 is o~nnected to a small conventional current measuring device which pro-vides a current measurement which is oonverted to the sa~ple equivalent of the original specimen.
According to the present invention the aqueous buffered diluent is o~ntinuously pumped through the reaction chamber to both the electrode and membrane, as discussed above. m e sample is typically injected from a hypodermic syringe into the injection port which can be in the form of a mixing "tee" oovered with a rukber diaphragm.
The electrode response from the measurement of the electro-active species of interest is measured by a c~rrent measuring device such as a current follower. m is value is then converted to the sam~
ple equivalent of the original specimen. In the case of biological material the samples are reported as mg percent for example a glucose equivalent of an original specimen is usually reported in mg glucose/100 ml (i.e. mg percent) of specimen. These units are conventional in clinical applications.
In addition to buffer it is desirable to add salts such as potassium chloride or sodium chloride which serve to establish the referenoe potential when silver-silver chloride reference electrodes which use the buffer as a filling solution are e~ployed. A bacterial inhibitor can be incorporated in the buffered diluent to retard bac-terial interferen oe.
mab/
~ fi3'37 Sl4388 An example of the system parameters are as follows. When the system is being used to detect hydrogen peroxide from the oxidation of glucose by glucose oxidase, an aliquot of 2.5 microliters of sample is introduced into the flowing buffer stream ~hat flows at a rate of between 0.1 to 5 ml per minute.
While, as discussed above, the flow rate must be ad~usted for each sample and membrane system, it appears that for a sample containing about 100 mg per decillter glucose, when the glucose is converted to hydrogen peroxlde and a 2.5 microliter sample is used, a flow of 0.1 to 2 ml per minute gives the optimum electrod response. It has been discovered that if the double layer membra e system of Flg. 3 1~ used a flow rate of one ml per minute gives good electrode response. The double layer system comprises two circular sections of a 12-14,000 atomic mass unit cutoff membrane of SPECTRAPOR 2TM purchased from Spectrum Medical Industries, Inc. While wlth some combinations of interfering species a membrane with smaller pore sizes may be preferred. This double layer of 12-14,000 cutoff membrane mater~al functions well over a wide range of flow rates. The time from in~ection, through sample peak to stable base line is as indicated above about 70-80 seconds.
It should be noted that while SPECTR~YORT membranes are used in the system, tests on MilliporeT membranes, type VS, VM
and PSAC have proven acceptable if flow rates are ad~usted to match the membrane. The MilliporeTMmembranes exclude in the mass range of 500-1000 mass unit~. It has been shown that of all . .
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me~branes tested, oellulosic membranes give consistently high quality results for long periods of use. Also, as membrane types are m~dified or changed totally, the loss of signal strength from the electrode may be compensated for by changing the working surface area of the elec-trDde.
To determine the usefulness of the system in determining a sample biological unkncwn the substance glucose was chosen. The glucose sample was passed through a glucose oxidase cartridge and the resultant hydr~gen peroxide was mDnitored in the presence of the F.D.A.
referen oe interferents. Table I shows the results of the interference studies. Table I ind udes three 01umns, one describing the interfering substance, the remaining two columns represent the results from tw~ dif-ferent instruments. To demonstrate that the system works well under different servi oe conditions, two instruments were similarly equipped and numerous samples of varied 0 mposition were analyzed by the separate instruments. Then, the different instruments having been subjected to different aging 0nditions, were used to analyze i & ntical sample fractions as described below. The results show ffhat while some variance is experienced between the two instruments, fflat in all, the inter-ference levels are kept below the recommended minimum.
To perfDrm the test a sample of hun~an serum was divided into two fractions. The amount of glucose was deter~ined. Fbr example, a serum sample would show about 100 mg/dl of glucose. m e halves are about five nl each. Enough solid ox solution phase inter-ferent is added to one-half of the sample to bring the blood-interferent solution to the 0ncentration indicated in parenthesis under ffhe "SUBST~N OE ADDED" column. Enough water, buffer or mab/
I ~ 2fi3~ 8-16388 .' .'. ' ,1 ¦ solven~ ls added to the second-half o f the serum to match its ¦ volume to that of the first one-half of the serum sample. The ¦ samples are run on two instruments. The dlfference between the ¦ two results for the same sample are probably due to; (1) Lndlvi-¦ dual differences b~tween platinum electrodes and (2) age and condition of the membrane system. Notice that some samples show l negative "BIAS" results. To determine the "BIAS" the samples are.
_ ¦ run, the real or "true" glucose value is determined from the l second-half sampIe. The flrst-half sample is run and the value ¦ of its glucose equivalent is subtracted fro~ the second-half reading. For example, if the glucose concentration were 100 mg/dl and the interference + glucose sample reads 101.6 mg/dl the BIAS is +1.6, as shown in "INSTRUMENT 1", item 1.
¦ Some samples show a negative bias. This is thought to be ¦ due, in one case, to latent catalase enzyme in the sample which destroys hydrogen peroxide during the course of the test and artificially lowers the "BIAS".
TABLE I_INTERFERENCE STUDY
l a) ENDOGENOUS SUBSTANCES
20 I ~
SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 Fructose (150) +1.6 0 Mannose (300) +2.8 +4.1 Galactose (300) +1.2 +0.3 Ascorbic Acid (25) +3.4 +1.8 Creatinine (25) +0.4 -0.2 l Glutathione (50) +1.8 +1.6 25 l Cltric Acid (1500) -3.8 -4.4 Hemoglobin (5000) -4.8 -2.6 NH4c~ 1.2 +0.6 Bilirubin (25) 0 0 Uric Acid (25) +1.0 +3.4 Cysteine (40) +2.0 +3.6 Lipid (600) +1.6 ~ ;3;37 S-l4388 I (b) EXOGENOUS SUBSTANCES
I
_ .
SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 l L-Dopa ~10) +1.0 +2.2 1 Xylose (150) ~1.8 +0.2 Ribose (150) +1.2 +0.8 Na Salicylate (50) +1.0 +0.2 Na Diatrizoace t5Zv/v) +0.6 ~0.6 Meglumine Diatrizoate +1.6 +0.8 Tolbutamide (25) -0.8 0 l Methyl Dopa (25) ~3.0 +2.8 ¦ Streptomycin (30) -0.6 +1.0 Sulfadiazine (50) +2.2 +1.0 l De~ran 10 l (100% of plasma volume) +2.4 +1.6 l Acetyl Salicylic Acid (30) +0.2 0 ¦ (c) ANTICOAGULANTS AND PRESERVATIVES
, _ _ SUBSTANCE ADDED BIAS (mg/dl) (mg/dl) INSTRUMENT 1 INSTRUMENT 2 l Na Fluoride (750) +2.4 +2.5 ~ ¦ Na aeparin (7000U/dl) -0.6 +0.4 .
Thymol (500) -4.8 -2.8 E.D.T.A. (550) +0.2 +1.2 Na Oxalate (800) +1.0 Na Citrate (2100) +2.2 +3.8 The day-to-day precision evaluated from results for aqueous l glucose standards and stable serum are seen in Table 2 (a) and l (b). In every case, the system, with results for two test lnstru-ments, showed reproducibility as the coefficient of variation ¦ (c.v.%) of less than 22. This is well below the 5% considered ¦ acceptable for most clinical uses.
l TABLE 2 PRECISION
5 l (a) AQU OUS STANDARDS (Prepared from NBS reference materialSRM No. 917) VALUE MEAN S.D. C.V.% MEAN S.D. C.Y.%
I
1 50 49.3 0.66 1.3 50.0 0.73 1.5 30 I 100 99.7 1.38 1.4 99.6 0.69 0.7 1 350 345.7 2.gO 0.8 345.4 2.7 0.8 ~ ~ 2k;;~7 S-l4388 ¦ (b) SFRUM POOLS
¦ POOL INSTBUMENT 1 INSTRUMENT 2 ¦ MEAN S.D. C.V.Z MEAN S.D. C.V.Z
¦ Low51.6 0.95 1.8 51.6 0.76 1.5 Normal 125.5 2.04 1.6 124.1 1.59 1.3 I High336.9 3.88 1.2 333.5 2.40 0.7 ¦ Therefore, clearly the unique flow-through system using a ¦ semi-permeable membrane system lowers and in some cases eliminate ¦ interference when measuring electroactive species.
¦ In accordance with the provisions of the patent statutes, ¦ the principle and mode of operation of the invention has been ¦ explained, and we have illustrated and descrlbed in the typical embodlment what is considered its best embodiment. It is under-stood that, within the scope of the appended claims, the inventio may be practiced otherwise than as specifically illustrated and l described in the typical embodiment and accompanying alternatives ¦ herein.
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of determining the solution concentration in a sample liquid stream of an electroactive species in the presence of at least one interfering electroactive species comprising placing an electrode means adjacent a flow-through type reaction chamber; subsequently separating said electrode means from said reaction chamber by a membrane system comprising at least one layer of a membrane material which has pores therethrough which allow selective diffusion across said membrane system, said membrane material permitting diffusion therethrough of said at least one interfering electroactive species at a lower rate than said electroactive species; providing a flow path over said membrane and communicating with said reaction chamber;
flowing a sample stream along said flow path and over said membrane system at a predetermined flow rate to allow diffusion of said electroactive species from said reaction chamber to said electrode means to the substantial exclusion of said at least one interfering electroactive species;
measuring the response of said electrode means; and flowing a buffer stream along said flow path and over said membrane to allow diffusion of said electroactive species and said at least one interfering electroactive species into said buffer stream preparatory to the flowing of a different sample liquid stream adjacent said membrane for determining the solution concentration of said electroactive species therein.
flowing a sample stream along said flow path and over said membrane system at a predetermined flow rate to allow diffusion of said electroactive species from said reaction chamber to said electrode means to the substantial exclusion of said at least one interfering electroactive species;
measuring the response of said electrode means; and flowing a buffer stream along said flow path and over said membrane to allow diffusion of said electroactive species and said at least one interfering electroactive species into said buffer stream preparatory to the flowing of a different sample liquid stream adjacent said membrane for determining the solution concentration of said electroactive species therein.
2. The method of Claim 1 wherein said electrode means includes an indicating, reference and counter electrode.
3. The method of Claim 1 wherein said membrane system includes a single layer of membrane material of a cellulosic film having sized pores therethrough to affect a molecular mass diffusion cutoff.
4. The method of Claim 3 wherein said membrane has a molecular mass diffusion cutoff of 12-14,000 atomic mass units.
5. The method of Claim 3 wherein said membrane has a molecular mass diffusion cutoff of 6-8,000 atomic mass units.
6. The method of Claim 3 wherein said membrane system has a molecular mass diffusion cutoff of about 3,500 atomic mass units.
7. The method of Claim 1 wherein said membrane system includes a double layer of said membrane material with a molecular mass diffusion cutoff of 12-14,000 atomic mass units.
8. The method of Claim 1 wherein said membrane system includes a double layer of said membrane material with a molecular mass diffusion cutoff of 6-8000 atomic mass units and with said layers being in close contact with each other.
9. The method of Claim 1 wherein said membrane system includes a double layer of said membrane material with a molecular mass diffusion cutoff of about 3,500 atomic mass units and with said layers being in close contact with each other.
10. A method of electrochemically measuring the concentration of relatively low mass electroactive species of interest in a sample stream in the presence of high and low mass interfering species, comprises the steps of:
providing a flow path over a membrane protected electrode means wherein said membrane has a molecular mass diffusion cutoff such that said high mass molecules with masses higher than the molecular mass cutoff are excluded from the electrode;
then precisely controlling the flow rate of said sample stream so that the diffusion ratio of the low mass interfering species and the low mass electroactive species of interest is such that substantially all of said low mass interfering species is excluded from said electrode means; then measuring the response of said electrode means to the electrochemical reaction of said low mass species of interest at said electrode means.
providing a flow path over a membrane protected electrode means wherein said membrane has a molecular mass diffusion cutoff such that said high mass molecules with masses higher than the molecular mass cutoff are excluded from the electrode;
then precisely controlling the flow rate of said sample stream so that the diffusion ratio of the low mass interfering species and the low mass electroactive species of interest is such that substantially all of said low mass interfering species is excluded from said electrode means; then measuring the response of said electrode means to the electrochemical reaction of said low mass species of interest at said electrode means.
11. The method according to Claim 10 wherein said species of interest is hydrogen peroxide derived from the conversion of glucose to gluconic acid and hydrogen peroxide.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US890,093 | 1978-03-27 | ||
| US05/890,093 US4172770A (en) | 1978-03-27 | 1978-03-27 | Flow-through electrochemical system analytical method |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1126337A true CA1126337A (en) | 1982-06-22 |
Family
ID=25396246
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA324,116A Expired CA1126337A (en) | 1978-03-27 | 1979-03-26 | Flow-through electrochemical system |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4172770A (en) |
| JP (1) | JPS54154395A (en) |
| CA (1) | CA1126337A (en) |
| DE (1) | DE2911943A1 (en) |
| FR (1) | FR2421377A1 (en) |
| GB (1) | GB2017931B (en) |
| IT (1) | IT1207934B (en) |
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| CA1077566A (en) * | 1975-12-18 | 1980-05-13 | Peter H. Chang | Polarographic membrane apparatus |
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- 1979-03-23 FR FR7907397A patent/FR2421377A1/en active Granted
- 1979-03-26 IT IT7967626A patent/IT1207934B/en active
- 1979-03-26 CA CA324,116A patent/CA1126337A/en not_active Expired
- 1979-03-27 GB GB7910566A patent/GB2017931B/en not_active Expired
- 1979-03-27 JP JP3616479A patent/JPS54154395A/en active Pending
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| DE2911943A1 (en) | 1979-10-04 |
| GB2017931B (en) | 1982-06-30 |
| US4172770A (en) | 1979-10-30 |
| IT1207934B (en) | 1989-06-01 |
| JPS54154395A (en) | 1979-12-05 |
| FR2421377A1 (en) | 1979-10-26 |
| FR2421377B1 (en) | 1984-03-02 |
| IT7967626A0 (en) | 1979-03-26 |
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