Reductive electrochemical analysis method with continuous deoxygenation
An improved method for the continuous removal of dissolved oxygen from a flowing liquid containing sample bands, the invention being effective to remove the interference of the dissolved oxygen for purposes of reductive electrochemical detection of analyte of interest. As an example of utility, using a silicone rubber membrane in a shell and tube design, typically >99 percent of the dissolved oxygen can be removed by permeation from aqueous solutions within a residence time of six seconds. Applications of the technique to the post-column removal of dissolved oxygen from ion exchange and reverse phase chromatographic mobile phases prior to reductive electrochemical detection are particularly described and claimed.
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Liquid chromatography with electrochemical detection (LCEC) has become a widely accepted technique for trace analysis as evidenced by numerous technical publications in the area since 1970. Applications of oxidative LCEC have been primarily in the determination of trace organic species such as phenols, cathecolamines, pharmaceuticals, and antioxidants. Unlike oxidative LCEC, relatively few applications of reductive LCEC have been reported even though a large number of compounds are amenable to reductive detection.
Reductive LCEC suffers from several difficult problems which limit the method. Reduction of hydrogen ion can contribute to background current and limit the useful cathodic range. Simultaneous reduction of trace metals in the mobile phase can also contribute to high background current or alter detector performance. However, the most apparent limitation of reductive LCEC is the need to remove dissolved oxygen from the mobile phase prior to detection. Dissolved oxygen, at normal saturation levels (and in much lesser amounts), is both a direct and indirect interference for reductive detection. For example, in acid medium and at a mercury cathode (vs. SCE), oxygen is reduced in two steps:
O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O.sub.2 E.sub.1/2 =-0.5 V
H.sub.2 O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.2H.sub.2 O E.sub.1/2 =-1.0 V
and its presence results in intrinsically high background current and thus limited detection sensitivity. Also hydrogen peroxide, an intermediate in the oxygen reduction process, can detrimentally react with components in the mobile phase.
Various technical approaches have been developed to minimize the limitations of reductive LCEC posed by the presence of oxygen saturation of the mobile phase. These have included rigorous purging of eluent and sample solution with an inert gas; a method which is effective but cumbersome and kinetically slow. Other approaches which have been tried have involved, e.g., pre- or post-column addition of a chemical reductant (limited by reaction kinetics and solubility in chromatographic solvents); electrochemical reactors based on porous silver oxide to preferentially reduce oxygen prior to detection (not compatible with detection of easily reducible analytes); and signal conditioning techniques such as dual electrode detection and reverse pulse amperometry (which disadvantageously require special electronics to be effective). None of these techniques, however, has proved to be entirely satisfactory.
THE INVENTIONThe invention is an improved liquid chromatography method using reductive electrochemical detection wherein the detection interferences due to inherent concentrations of dissolved oxygen in the mobile phase are minimized. This is accomplished by use of an oxygen-permeable membrane in a post-column configuration with a liquid chromatography column to continuously remove dissolved oxygen from the mobile phase (column effluent).
As an example of utility, the method can be practiced to lower dissolved oxygen to levels as low as about 0.01 ppm, or to an amount wherein, for all practical purposes, little or no detrimental effects will be observed. Further, benefits and advantages of the invention include a rapid stabilization time relative to inert gas purging techniques; broad applicability since no dilution or chemical change of the mobile phase is involed (such as occurs when adding chemical reductants or using oxygen-selective reduction approaches); and additionally the invention does not require samples to be deoxygenated prior to injection.
TERMS"Chromatographic Column" means any suitable chromatographic stationary phase, usually on supports such as particles, but also other supports, e.g., capillaries and the like, useful in performing liquid chromatographic separations of electro-reducible anolytes.
"Eluent" or "Mobile Phase" means an electro-inactive liquid solution suitable for use with the chromatographic column for performing liquid chromatographic separations and electrochemical detection of given electro-reducible analytes.
"Reverse Phase Chromatography" means broadly liquid chromatography methods using a chromatography column comprising a generally nonpolar stationary phase and using eluent or mobile phase which is an aqueous solution, and which may include polar organic phases miscible with the water phase; and which single phase or polyphase eluent is compatible with reductive electrochemical detection according to this invention.
"Ion Exchange Chromatography" means broadly liquid chromatography methods using a chromatography column comprising an ion exchanger stationary phase and eluent or mobile phase which is an aqueous electrolyte solution compatible with reductive electrochemical detection according to this invention.
"Electrochemical Cell" or "Electrochemical Detector" means an analytical device having a working electrode and a reference electrode across which may be applied a potential to selectively reduce for analysis purposes analytes in the cathodic range of from about 0 to -2.5 V. Such detectors useable in the invention are of diverse types using working electrodes, e.g., of mercury, carbon and carbon-black based electrodes, gold, gold/mercury amalgams, platinum, various metallic oxides, etc., and are described, e.g., by Snyder et al., Introduction to Modern Liquid Chromatography, 2nd Ed., pgs 153-161 (incorporated by reference).
"Electro-reducible Analyte" means a sample specie which may be determined by electrochemical reduction at the working electrode of an electrochemical detector.
"Thin Channel" means the unpacked bore or bores of a single tube or multiple tubes arranged in parallel; the packed bores of single or multiple tubes containing packing means, e.g., as described in U.S. patent application Ser. No. 300,143, filed Sept. 8, 1981 (incorporated fully herein by reference); and shall also include, e.g., flat membranes pressed against grooves to form a thin channel (as described, e.g., in U.S. Pat. No. 3,751,879). Any thin channel in which laminar band broadening is no worse than a tube, the bore of which is unpacked and is 1 mm I.D., and the length of which is 3 meters, is intended to be covered within the broad meaning of the term "thin channel".
"Interfering Amount of Oxygen" means a concentration of dissolved oxygen in liquid of greater than 1 ppm.
"Oxygen-Permeable Membrane" means a material which will permeate dissolved oxygen (not necessarily selectively) while retaining a detectably useful amount of sample analyte of interest.
GENERAL DESCRIPTION OF THE INVENTIONThe invention as it relates to an improved liquid chromatographic method using reductive electrochemical detection comprises:
(a) chromatographically displacing a sample of interest from a liquid chromatographic column to separate said sample into a sample band comprising an electro-reducible analyte of interest which emerges in the liquid effluent of the column, the liquid effluent containing an interfering amount of dissolved oxygen;
(b) thereafter deoxygenating the effluent by flowing the effluent through a thin channel comprised of an oxygen-permeable membrane, the effluent communicating through the membrane with an oxygen-depleted zone in which the oxygen partial pressure is less than about 20 mm Hg, the effluent being in said channel and communicating with said zone for a period of time sufficient to reduce the dissolved oxygen in the effluent to les than 1 ppm; and
(c) thereafter and without allowing a substantial return of dissolved oxygen into the effluent, analyzing the effluent for the analyte of interest using reductive electrochemical detection.
The invention has further utility than strictly reductive LCEC applications, these being in respect to flow analysis apart from chromatographic separations. The invention as it broadly relates to the latter comprises an improved flow analysis method using electrochemical detection which comprises:
(a) adding to a flowing stream of an electro-inactive carrier liquid, a known volume of liquid sample containing an electroactive analyte which is reducible at a potential at which oxygen responds, the flow stream with sample added containing an interfering amount of dissolved oxygen;
(b) thereafter deoxygenating the carrier liquid containing the sample by flowing the liquid through a thin channel comprised of an oxygen-permeable membrane, the liquid communicating through the membrane with an oxygen depleted zone in which the oxygen partial pressure is less than 20 mm Hg, the liquid being in said channel and communicating with said zone for a time sufficient to reduce the dissolved oxygen in the liquid to less than 1 ppm; and
(c) thereafter and without allowing a substantial return of dissolved oxygen into the liquid, analyzing the liquid for an analyte of interest using a potential for reductive electrochemical detection thereof at which oxygen responds.
The invention in its most preferred form uses a single tube membrane, the bore of which comprises a thin channel through which the effluent of the column is fed for deoxygenation. The tube is contained in a chamber or shell which defines an oxygen-depleted zone maintained by, e.g., evacuation, inert gas flushing, or by filling with a liquid which chemically reacts with oxygen which is permeated from the liquid through the wall of the tube membrane. Broadly, the invention is intended to cover any suitable method for attaining an oxygen-depleted zone in which the oxygen partial pressure is less than about 20 mm Hg; and effectively deoxygenates the effluent (or liquid) to a level less than 1 ppm (weight percent basis).
Suitable membranes employed are oxygen-permeable, but not necessarily oxygen-selective. For example, nonpolar membranes are generally ideally suited for use in the invention to resist permeation of polar solvents such as are typically used in reverse phase and ion exchange liquid chromatography methods. Generally, thin-walled silicone rubber membranes have been found to be most suitable for use in practicing the invention but other thin-walled oxygen-permeable membranes are contemplated to have utility, e.g., Teflon.RTM., ethyl cellulose, poly-4-methyl-1-pentane (TPX), nitrite silicone rubber, and polydimethylcyclohexane. These materials all have published permeability coefficients ranging from 30.times.10.sup.10 to 350.times.10.sup.10 (see J. Brandrup, Polymer Handbook, 2nd Edition). Polydimethylsiloxane or slicone rubber has a published permeability coefficient of about 600.times.10.sup.10 and is generally impermeable to the liquid solvents involved and is commercially available in the thin-walled structures desired, and hence constitutes a satisfactory and highly preferred membrane material for use in the invention.
Detrimental band broadening in liquid chromatography according to the invention is minimized by using thin channels, e.g., tube membranes of preferably less than about 0.5 mm I.D. Also, the teaching of Stevens et al., Ser. No. 300,143; and Stevens et al., Ser. No. 112,579, filed Jan. 16, 1980, (the latter also incorporated by reference) are relied upon and incorporated for the purpose of showing various ways of forming thin channels, which reduce laminar form band broadening. However, in this invention, in contrast to certain embodiments of Stevens et al., most preferred membranes are thin walled unpacked tubes. These "unpacked" tube membranes are preferred since the thin walled tube membranes found most suitable for use in the invention are often easily susceptible to damage caused by moderate or high back pressures which can be detrimentally contributed to, e.g., by the "particle" packings employed by Stevens et al., Ser. No. 300,143.
THE DRAWINGYet further objects and advantages of the invention will, in part, be pointed out, and, in part, apparent from the following detailed description taken together with the accompanying Drawing wherein:
FIG. 1 is an elevational view of a preferred embodiment of apparatus designed for use in practicing the LCEC analysis method of the present invention;
FIG. 2 is an enlarged cross-sectional view of the apparatus of FIG. 1, illustrating the detail of the deoxygenating device;
FIGS. 3-5 are reproductions of a graph and certain response curves developed using the method of the invention which are associated with certain of the working Examples, below.
DETAILED DESCRIPTION OF THE INVENTIONReferring to FIG. 1, there is shown a schematic view of a LCEC liquid chromatographic apparatus which comprises a chromatographic or analytical column 10. The chromatographic column comprises a housed chromatographic separating means typically in the form of a particulate packing or gel through which sample is eluted to separate the sample into its component analyte species. Diverse types of separating means may be used to construct a suitable chromatographic column useful for the purposes of the invention, e.g., as described extensively, e.g., by Snyder et al. Typically used are reverse phase and ion exchange columns described, e.g., by Snyder et al.
Preferred means to add eluent or mobile phase to chromatograhic column 10 comprises an eluent reservoir 12 containing a suitable electro-inactive eluent solution 14, the latter of which is withdrawn from the reservoir by a chromatographic pump 16 equipped with an optional pulse damping coil (not shown) and pressure gauge 18.
Preferred means for adding sample comprises, e.g., a syringe-loadable sample injection valve 20. Sample added to the system at valve 20 is swept through the apparatus by the pumped eluent solution to chromatographic column 10. The sample elutes in the effluent of column 10, with component analyte species thereof appearing chromatographically displaced as sample bands in a background of the eluent or mobile phase.
A deoxygenating device 22 includes a single tube membrane or hollow fiber 24 (see also FIG. 2), into the bore of which the column 10 effluent is fed. The opposite outer surface of the membrane tube is contained within an oxygen-depleted zone maintained within a shell or chamber 26, by the preferred mechanism of a vacuum pump 28. The effluent emerges from the bore of the tube membrane with the dissolved oxygen level at less than 1 ppm, and is ultimately fed through oxygen-impervious tubing 30, suitably of stainless steel, to an electrochemical detector 32.
The detector illustrated in the drawing of FIG. 1 is a representation of the Model 310 polarographic detector (from EG&C Princeton Applied Research Corporation) which uses a static mercury drop working electrode 34 (SMDE) upon which the liquid effluent is directed to reduce the electro-reducible species analyte of interest in the column effluent. This chemical reduction over a working range of between about 0 to -2.5 V (for mercury vs. the Ag/AgCl reference electrode 36) produces electrical signals, which after state of the art amplification and signal processing are displayed on a suitable recorder or readout device, thus providing a quantitation of the sample bands of interest.
Referring exclusively to FIG. 2, a preferred deoxygenating device 22 is constructed, e.g., of a cylinder 38 which is seated in recessed surfaces of end caps 40, 42, respectively; and a hermetic seal is formed using preferably double O-rings 44, 46. The above assemblage is fastened in compression by screws 48, thereby forming a hermetically tight chamber about the enclosed tube membrane 24. The membrane is desirably coiled, and its opposite ends are extended through openings of chromatographic fittings 50, 52, threadably received in end cap 42.
The fittings 50, 52 provide the inlet and outlet for connecting the tube membrane with the chromatographic column and detector, respectively. The extreme ends of the tube membrane are secured in fittings 50, 52 using, e.g., RTV rubber (suitably Dow Corning RTV 732) which is used to form flanges 54, 56 formed in situ with an intermediate metal washer 58. The opposite end cap 40 contains a pneumatic fitting 60 adapted for connection to tubing leading to the vacuum pump 28, and a plug 62 for relieving the vacuum on the inside of shell 26 after completion of the analysis. The device 22 is preferably constructed of a Plexiglass or Lucite plastic cylinder, and plastic end caps such as of polypropylene, and preferably the fittings are standard plastic chromatographic fittings.
The operation of the invention is now described. In operating the apparatus, vacuum pump 28 is activated to form an oxygen-depleted zone about membrane 24. Typically about 20-30 minutes are required to remove the last residual traces of O.sub.2 from shell 26. Upon reaching this equilibrium or steady-state condition, sample is injected and the effluent consisting of the liquid eluent and displaced sample bands is immediately fed to the inlet bore of the tube membrane. During the residence of the effluent in the membrane, the dissolved oxygen in the effluent is lowered to a concentration of less than 1 ppm. For example, assuming the shell is evacuated to 1 mm Hg and all the pressure remaining is assumed to be oxygen (a nearly correct assumption in typical cases), the dissolved oxygen of the effluent is reduced to about 0.05 ppm, a result closely obeying Henry's Law. The effluent so deoxygenated is then immediately fed to the detector where reductive LCEC is effectively practiced.
While a preferred embodiment of device 22, and its mode of operation, have been described with respect to a liquid chromatographic apparatus, it should be apparent that the chromatographic column may be eliminated, e.g., such as where there is no sample interferences present. Thus, simply a flow analysis system is contemplated wherein sample or analyte of known volume would be injected into a suitably electro-inactive carrier liquid, fed through the device 22, and then onto detector 32 for measuring the injected anolyte. This variation of the invention is also illustrated further in certain of the Examples, below.
EXAMPLE 1This example illustrates the removal of dissolved oxygen from a sample solution of 0.1M HClO.sub.4 at ambient temperature and pressure conditions. The sample solution is pumped at 1.5 ml/min using a pulse-dampened Altex Model 110A pump, through a silicone rubber tube membrane (3 meters.times.0.25 mm I.D..times.0.89 mm O.D.) within a shell evacuated to 0.9 mm Hg. The sample solution is then immediately advanced from the outlet of the deoxygenating device to an electrochemical detector comprising an EG&G Princeton Applied Research Model 310 Static Mercury Drop Electrode (SMDE). The detector is used as supplied by the manufacturer except modified with a stainless steel inlet port to prevent oxygen from re-entering the sample solution prior to the detector. Vacuum is supplied to the deoxygenating device by a Sarvac Model 8804 vacuum pump, and the amount of oxygen removed from the sample solution is calculated as: ##EQU1## where i.sub.s =current at E=-0.5 V vs Ag/AgCl of an air-saturated solution
i.sub.v =current with deoxygenating device evacuated and at equilibrium.
The results of this example using the described experimental apparatus and sample solution is shown in FIG. 3. Initially, using an applied voltage E=-0.50 V, a large cathodic current is measured due to the reduction of dissolved oxygen at the mercury electrode. This current rapidly decreased following evacuation of the shell of the deoxygenating device, and a stable background current is achieved after about 25-30 minutes. Using the data generated, it is calculated in this example that about 98 percent of the dissolved oxygen has been removed, and the residence time in the membrane tube is 6.1 seconds.
EXAMPLE 2The transport of dissolved oxygen across the membrane wall is directly proportional to the oxygen pressure difference .DELTA.P across the membrane. Several different means of maximizing .DELTA.p (i.e., minimizing the external oxygen pressure) can be effectively used in order to remove dissolved oxygen from liquids within the scope of the invention. These different approaches include: purging the shell of the deoxygenating device, e.g., with an inert gas (.about.2000 cc/min); evacuating the shell; and flushing the shell with a suitable liquid, e.g., countercurrent flushing the shell with a suitable oxygen reducing liquid. The successful performance of these several approaches is illustrated in Table I below.
TABLE I
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Comparison of Deoxygenation Methods
% O.sub.2
Residence
Deoxygenating System
Removed Time
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Membrane Device 99 6.1
Shell Vacuum/1 mm Hg
Membrane Device 97 6.1
Nitrogen Purged Shell
Membrane Device 98 6.1
Helium Purged Shell
Membrane Device 96 6.1
0.2 M NaOH/0.2 M NaHSO.sub.3
Shell Solution
Membrane Device 90 6.1
0.2 M NaOH/1.0 M NaHSO.sub.3
Shell Solution
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EXAMPLE 3
Assuming a sufficient length of tube membrane is used, the amount of dissolved oxygen removed should be determined by the oxygen partial pressure inside the shell of the deoxygenating device. This effect is shown in FIG. 4. Here, two silicone rubber tube membranes of different internal diameter and wall thickness, but of same length, were tested at varying flow rates. In both cases, with sufficient residence time the minimum concentration of dissolved oxygen is in close agreement with that predicted from the thermodynamic limit of the system, Henry's Law:
C=kp
where
C=concentration of dissolved oxygen (ppm)
p=partial pressure of oxygen (cm Hg)
k=constant=5.4.times.10.sup.-1 at 25.degree. C.
With shorter residence time, the concentration of dissolved oxygen is apparently fixed by the kinetics of mass transport to the inner membrane wall.
EXAMPLE 4In this example, the effect of minimizing external pressure on dissolved oxygen concentration (i.e., current i.sub.v) is determined and compared with that calculated using Henry's Law. The experimental data is developed using the apparatus and conditions of Example 1 and using as the sample solution 0.1 HClO.sub.4. The observed data and comparative caculated data are given in Table II.
TABLE II
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Shell
Test Pressure Observed Dissolved Oxygen (ppm)
No. (mm Hg) i.sub.v (.mu. amps)
Experimental
Calculated
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1 755 14.0 -- 8.7
2 222 4.50 2.8 2.5
3 181 3.55 2.2 2.1
4 139 2.70 1.7 1.6
5 98 1.89 1.2 1.1
6 56 0.95 0.5 0.6
7 44 0.74 0.5 0.5
8 31 0.45 0.3 0.4
9 15 0.24 0.2 0.2
10 4 0.15 0.09 0.04
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From the observed data of Table II, it is shown that nearly linearly as the pressure in the shell of the deoxygenating device decreases, the concentration of dissolved oxygen (current i.sub.v) also decreases, and in fact, is very close in conformance to the decrease predicted from theoretical calculations. Also it may be observed that Test Nos. 1-5 are not in conformance with the invention since the dissolved oxygen level is not reduced to less than 1 ppm; whereas Test Nos 6-10 are illustrative of the method of the invention.
EXAMPLE 5To be chromatographically useful, a deoxygenating device according to the invention must be compatible with mobile phases normally encountered in liquid chromatography. The effectiveness of the deoxygenating device was successfully tested using supporting electrolytes representative of mobile phases commonly encountered in ion exchange and reverse phase chromatography since reductive LCEC is amenable primarily to these separation modes. Specifically, the deoxygenating device of this invention was used satisfactorily with the following typical aqueous ion exchange mobile phases: 0.1M HClO.sub.4 ; 0.1M NaOH; and 0.1M KCl. In addition, the deoxygenating device of the invention, and the practice of the method was successfully tested with respect to the following reverse phase mobile phases: 0.02M H.sub.3 PO.sub.4 in a solution of 50% methonol/50% water (v/v); 0.1M NaH.sub.2 PO.sub.4 /0.002M H.sub.3 PO.sub.4 in a solution of 50% acetonitrile/50% water (v/v), pH=4.4.
EXAMPLE 6This example shows the analytical utility of the invention with respect to the reverse phase separation and reductive electrochemical detection of nitro-aromatic compounds (see FIG. 5). Both background response and baseline noise are sufficiently lowered so as to allow detection of the listed compounds in amounts less than 20 nonograms.
Claims
1. An improved liquid chromatographic method using reductive electrochemical detection which comprises:
- (a) chromatographically displacing a sample of interest from a liquid chromatograhic column to separate said sample into a sample band comprising an electro-reducible analyte of interest which emerges in the liquid effluent of the colummn, the liquid effluent containing an interfering amount of dissolved oxygen;
- (b) thereafter deoxygenating the effluent by flowing the effluent through a channel comprised on an oxygen-permeable membrane, the effluent communicating through the membrane with an oxygen-depleted zone in which the oxygen partial pressure is less than about 20 mm Hg, the effluent being in said channel and communicating with said zone for a period of time long enough to lower the dissolved oxygen in the effluent to less than 1 ppm; and
- (c) thereafter and without allowing a substantial return of dissolved oxygen into the effluent, analyzing the effluent for the analyte of interest using reductive electrochemical detection.
2. The method of claim 1 wherein the chromatography column is an ion exchange column.
3. The method of claim 1 wherein the chromatography column is a reverse phase column.
4. The method of claim 1 wherein the oxygen-depleted zone is produced by evacuation.
5. The method of claim 4 using a membrane comprising silicone rubber.
6. The method of claim 5 using a tubular membrane.
7. The method of claim 6 using an oxygen-depleted zone in which the oxygen partial pressure is less than 10 mm Hg, and wherein the effluent is in communication with said zone for a time long enough to lower the dissolved oxygen in the effluent to about 0.5 ppm or less.
8. An improved flow analysis method using reductive electrochemical detection which comprises:
- (a) adding to a flowing stream of electro-inactive carrier liquid, a known volume of liquid sample containing an electro-active analyte species which is reducible at a potential at which oxygen responds, the flow stream with sample added containing an interfering amount of dissolved oxygen;
- (b) thereafter deoxygenating the carrier liquid containing the sample by flowing the liquid through a channel comprised of an oxygen permeable membrane, the liquid communicating through the membrane with an oxygen-depleted zone in which the oxygen partial pressure is less than 20 mm Hg, the liquid being in said channel and communicating with said zone for a time sufficient to lower the dissolved oxygen in the liquid to less than 1 ppm; and
- (c) thereafter and without allowing a substantial return of dissolved oxygen into the liquid, analyzing the liquid for an analyte of interest using reductive electrochemical detection.
9. The method of of claim 8 wherein the oxygen-depleted zone is produced by evacuation.
10. The method of claim 9 using a membrane comprising silicone rubber.
11. The method of claim 10 using a tubular membrane.
12. The method of claim 11 using an oxygen-depleted zone in which the oxygen partial pressure is less than 10 mm Hg, and wherein the carrier liquid containing the sample is in communication with said zone for a time long enough to lower the dissolved oxygen in the effluent to about 0.5 ppm or less.
| 3463615 | August 1969 | Sochor |
| 3751879 | August 1973 | Allington |
| 4265634 | May 5, 1981 | Pohl |
| 4268279 | May 19, 1981 | Shindo et al. |
| 4388411 | June 14, 1983 | Lovelock |
- Kissinger et al.; Reductive Mode Thin-Layer Amperometric Detector for L C: J. of L C 4(10), 1777-1795, (1981). Manufactures Bulletin, ERC 3000 On--Line Degasser. Stevens et al.; Hollow Fiber Ion-Exchange Suppressor for Ion Chromatography; Anal. Chem. 1981, 53, pp. 1488-1492.
Type: Grant
Filed: Dec 19, 1984
Date of Patent: Feb 4, 1986
Assignee: The Dow Chemical Company (Midland, MI)
Inventor: Robert E. Reim (Midland, MI)
Primary Examiner: Barry S. Richman
Assistant Examiner: Michael S. Gzybowski
Attorney: Burke M. Halldorson
Application Number: 6/683,451
International Classification: G01N 100; B01D 1300;
