carbon electrodes for liquid chromatography detection

Anal. Chem. 1982, 54, 1169-1174 1169

specificity procedures for establishing standard reference material (SRM) values of phenols and miscellaneous hydrocarbons in shale oil. With LCEC, simpler procedures may be followed to profile electroactive solutes. The selectivity of LCEC for trace phenols is exemplified in Figure 7. The base line shift with LCEC is iminimal since the detector is operated a t low gain. Of course, the selectivity prevents the determination of important polyaromatic hydrocarbons. The choice of methodology is dependent on the goals desired. Provided that the solutes are electroactive, simpler manipulations as described here enable the analyst to shorten analysis times and improve method reliability.

LITERATURE CITED (1) Rand, M. C., Cireenberg, A. E., Taras, M. J., Ed. "Standard Methods for

the Examlnation of Waste and Wastewater", 14th ed.; American Pub- lic Health Association: Washington, DC, 1976.

(2) Edgerton, T. I?.; Moseman, R. F.; Lores, E. M.; Wrlght, L. H. Anal. Chem. 1980, 52, 1774-1777.

(3) McKague, A. B. J. Chromatogr. 1981, 208, 287-293. (4) fed. Regist. 1979, 4 4 , (Dec. 3), 69464-69575. (5) Ogan, K.; Katz, E. Anel. Chem. 1981, 53 , 160-163. (6) Kuwata, K.; Ueborl, M.; Yamazaki, Y. Anal. Chem. 1981, 53 ,

153 1-1534.

(7) Schabron, J. F.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1979, 51, 1426-1 433.

(8) Ugiand, K.; Lundanes, E.; Grelbrokk, T. J. Chromatogr. 1981, 213,

(9) Sparacino, C. M.; Minick, D. J. Environ. Sci. Technol. 1980, 14, 860-882.

(10) Shoup, R. E. "Bibliography of Recent Reports on Liquid Chromatogra- phylEiectrochemistry"; Bioanalytlcal Systems Inc.: West Lafayette, IN, 1980.

(11) Armentrout, D. N.; McLean, J. D.; Long, M. W. Anal. Chem. 1979,

(12) Weisshaar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981,

(13) Shoup, R. E., unpublished results. (14) Schauwecker, P.; Frei, R. W.; Ernl, F. J. Chromatogr. 1977, 136, 63. (15) Bushway, R. J. J. Chromatogr. 1981, 211, 135-143. (16) Werkhoven-Goewle, C. E.; Brinkman, U. A. Th.; Frei, R. W. Anal.

Chem. 1981, 53 , 2072-2080. (17) Hackman, M. R.; Brooks, M. A. J . Chromatogr. Blomed. Applications

1981,22, 179-190. (18) Lund, U. J. Liq. Chromatogr. 1981, 4 , 1933-1945. (19) Hertz, H. S.; Brown, J. M.; Cheder, S. N.; Guenther, F. R.; Hiipert, L.

R.; May, W. E.; Parris, R. M.; Wise, S. A. Anal. Chem. 1980, 52, 1650.

63-90.

51, 1039-1045.

53, 1809-1813.

RECEIVED for review December 18,1981. Accepted March 24, 1982.

Carbon Electrodes for Liquid Chromatography Detection

J. D. McLean

Dow Chemical U.S.A., Michigan Dlvision Analytical Laboratories, 1602 Building, Midland, Michigan 48640

Graphlte electrodes wore treated wlth various lmpregnatlng agents to Improve signal-to-nolse ratios for analytical measurements, and surface morphology was examlned by scan- nlng electron microscopy. Electrodes were formulated from carbon black and various polymeric blndlng agents and found to possess superior qualltles for use as worklng electrodes for llquld chromalography cletectors. Parameters evaluated were percent carbon loading, type of blndlng agent, method of electrode fabrlcatlon, and preparatlon of electrode surface. The electrodes were applled to the detection of halogenated phenols and lnhlbltors In monomer systems.

Wax impregnation has been the method of choice to improve the performance of graphite electrodes for analytical electrochemical measurements. A detailed study of various waxes was reported by Gaylor ( I ) . Adams (2) described the preparation of the carbon-paste electrode and Kissinger (3) has applied this electrode to the detection of species eluting from liquid chromatography (LC) columns.

The electrochemical detection of various halogenated phenols was studied ai, several electrode materials after ap- propriate LC separation. Carbon paste and wax-impregnated graphite were found to be unsuitable, as a result of dissolution of the binder in acetonitrile solvents and loss of performance, respectively. Previously, a carbon black polyethylene combination was found to plossess unique and desirable properties (4) . While the polyethylene carbon black (PECB) electrode material outperformed all other forms of carbon tested, only one composition (50/50 by weight) was evaluated. This paper

reports a detailed study of a variety of carbon black polymer electrodes.

EXPERIMENTAL SECTION Apparatus. The LC equipment employed consisted of an

Altex Model l1OA pump, a Chromatronix Model R6031V6K rotary injection valve with a 250-pL injection loop, a Laboratory Data Control (LDC) 12.7 X 330 mm Cheminert glass column with movable Teflon plungers to permit adjustment of the column bed length, and a tubular anode electrochemical detector ( 4 ) employing a Ag/AgCl reference electrode. The scanning potentiostat employed was designed by J. Holland of Michigan State University. A Varian Model (2-2500 variable range recorder was employed to record all chromatograms. The various carbon electrodes were prepared as previously described ( 4 ) and machined to 3/16 in. 0.d. X 3/16 in. long tubular electrodes with 1/16 in. diameter bores. Carbon black anodes are the subject of pending patents. An electrochemical LC detector employing a similar cell design and a carbon black polyethylene working electrode has recently become available from Chromatix of Sunnyvale, CA, as the Model CMX-20. A Cambridge Mark IIA scanning electron microscope was employed for surface morphology studies. Electrodes were mounted so that the inside surface of the bore could be observed. By use of a magnification of 50X, the edges of the in. bore could be kept in the field of view.

Reagents. The LC column was slurry packed with Aminex 50W-X4, 20-30 pm particle size strong cation exchange resin available from Bio-Rad Laboratories, Richmond, CA. Fresh resin was washed in the column with UV grade acetonitrile and then with 1 N sulfuric acid prior to use. Because of the strong acid eluent, the Delrin column end fittings normally supplied with the column were replaced with ones machined from stainless steel.

The eluent employed was a 29% acetonitrile-water mixture containing 0.05 N sulfuric acid to suppress phenol ionization and to provide supporting electrolyte for the electrochemical detector.

0003-2700/82/0354-1 169$01.25/0 0 1982 American Chemical Society

1170 ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982

i

'. ! ! . <-.* *: ' ..

Flgure 1. (Top) Scanning electron photomicrograph 01 a graphite electrde surface: polished. wax impregnated. low background noise. SOX magnification. (Bottom) Same: high background noise.

UV grade acetonitrile was obtained from Burdick and Jackson, Muskegon, MI.

Phenolic standards were technical grade or better and were used without further purification.

Detector Operation. For chromatographic experiments, eluent flow was established through the cell at a rate of 1.3 mL/min and a potential sufficient to oxidize the various chlo- rophenol isomers (+1.20 V) was applied between the electrodes with the carbon electrode having the positive polarity. Within periods of time from a few minutes to 1 h (depending upon the electrode composition), the residual current decreased from >1 PA to a steady state of -15 nA at which time chromatographic measurements were initiated. Chromatograms required ap- proximately 35 min and a t least two injections were made per electrode. Between all chanees to a new electrode comDosition.

"*,

Flgure 2. (Top) Scanning electron photomicrograph of a graphne electrode surface: untreated, unpolished, lOOOX magnification. (Bonom) Same: wax impregnated, polished

was studied to see if any correlations could be found with electrode performance. Figure 1 shows the differences in smoothness of the bores of good and bad electrodes with the bad electrode having several major surface imperfections aligned in opposition to the eluent flow path. Figure 2 at lOOOX magnification shows the ability of wax to fill the voids and smooth the graphite surface.

Nujol was also evaluated as an impregnating agent for tubular graphite electrodes. Electrodes were vacuum impregnated and excellent signal-to-noise levels were obtained. Figure 3 shows a tubular graphite electrode before and after Nujol treatment. The untreated electrode which gave high hackgound noise levels has distinct ridges present on the surface even after polishing to remove machining marks.

R 51) %I PKCR electrode k n u k u, exhibit g u d p r r f o r m k wa9 inserted into the deterwr and tested 1.0 assure that the flow cell

These microridger. which are aligned against the path of flow, mav contrihuce to turbulent flow and thus increased back-

and the chromatographic system were functioning properly. Response factors could be repeated within *20% for different electrodes of any given composition.

For voltammetric results, a constant composition solution of compound in 50150 methanol/H,O, 0.2 M in LiCl was pumped directly through the detector employing a flow rate of 1.8 mL/min produced by a Model 403 Scientific Industries, Inc., peristaltic pump. While the solution was being pumped through the detector, a voltammetric scan was recorded.

RESULTS AND DISCUSSION During work with tubular wax-impregnated graphite elec-

trodes, inconsistent performance was often observed due to radically different background noise levels, even though identical preparation from the same lot of graphite was employed. Surface morphology of 'good" and 'had" electrodes

ground currents. The same electrode, after treatment with Nujol gave low background noise and has no observable ridges at the surface. Residual pools of excess Nujol can be seen in several places.

The direction of polish (rotary vs. reciprocating) was ex- amined to assess the effect on background current levels of electrodes. Graphite electrodes were machined and divided into three groups of two each. One group was not polished at all. One group was polished with 600 mesh alundum on a pipe cleaner with a reciprocating motion by hand. The final group was polished with the same material hut with a rotary motion with a lathe. One electrode from each group was impregnated with Nujol. Electrodes were individually tested for background noise employing the flow cell with +0.8 V (sufficient to oxidize the diphenylamine standard) applied to

ANALYTICAL CHEMISTRY, VOL. 54, NO. 7. JUNE 1982 1171

Table I. Graphite Electrodes"

Comparison of Impregnating Agents for

signal/ signall impregnating agent noise impregnating agent noise

none 0.2 paraffin 8.0 benzyl acrylate <0.1 RTV 732 Silicone 11 acrylic acid <0.1 Fluoroluhe S-30 12 Polyethylene Glycol <0.1 styrene- 29

200 Diacrylate divinylbenzene epoxy resin 0.3 Nujol 31 a Based on oxidation of diphenylamine in 1:l alcohol-

H,O.

r

kl ...' :'

Flgure 3. (Top) Scanning electron photomicrograph of a graphite eleclrcde surface: polished, untreated. high background noise. 50X magnification. (Bottom) Same: Nujol treated, low background noise.

. ,

, 1 I I , 1 0 5 10 15 20 0 5 10

Time. Minull, Tlmp Ml""ts8

Hand.Pd#rhd, RPliDrOCdllng MOIIP". LBthe~POI8Ihed R O W " N " P T r o o l d Mom". NYiOi Trealpd

Figure 4. Effect of types of surface polish on graphite electrode performance.

the working electrode with electrolyte flowing through the cell a t 1.8 mL/min. Each electrode which had no Nujol treatment gave off-scale current readings of >1.0 WA regardless of the type of surface polish. The direction of polish has a critical effect on the background current as shown in Figure 4. This could he due to the elimination of turbulent flow on a mic- roscale a t the electrode surface. In the rotary polished case,

turbulent flow may dominate when the polishing grooves are opposed to the direction of electrolyte flow. If such turbulence occurred, it could induce uneven mass transport which could yield high and erratic background currents. Reciprocating polish, however, would align the polishing grooves with the direction of flow and allow laminar flow which in turn would favor uniform mass transport to the electrode surface. The presence of residual polishing compound could also be a factor contributing to high background currents, since the lathe probably forces more polishing compound into the carbon pores than does the hand-polishing method. Polishing marks and residual polishing compound are not observable in scanning electron photomicrographs due to the high contrast of the carbon background.

Electrodes treated with Nujol and hand polished with a reciprocating motion gave excellent results when employed as LC detectors but with 29% acetonitrile eluent high background currents returned after several days use. Although the electrodes could he regenerated by further Nujol treatment, it was desired to have a more permanent coating. Good results have been reported with a silicone rubber based graphite electrode (5) and with a styrene-impregnated graphite electrode (6) which was polymerized prior to use. Thus, a number of impregnating agents, which would potentially have improved solvent resistance, were selected for evaluation.

In order to compare hydrodynamic performance of various treated electrodes, we pumped solutions of constant composition through the electrochemical flow cell, and a voltammetric scan was made a t a scan rate of 10 mV/s. To evaluate performance, we calculated a ratio from the limiting current a t the plateau of the wave to the residual current from the solvent-electrolyte alone. Table I shows the ratio for the oxidation of diphenylamine for a variety of impregnating agents. Impregnation with various acrylates and epoxy resin did not produce useful results. Fluoroluhe, a fluorinated mineral oil, improved electrode performance but it did not possess the desired solvent resistance and thus electrode performance degenerated in acetonitrile solutions. Good initial performance was obtained with styrenedivinylbenzene which was polymerized in situ by heating hut the electrode cracked and subsequent experiments gave erratic response.

Glassy carbon does not require any impregnating agents to reduce noise levels and it has been well characterized for use in electrochemical measurements (7). Material obtained from Tokai of Japan did not perform well in tubular configuration for trace determinations of various phenols (4 ) but the electrode's internal surface was not polished. The electrode bore was then polished with decreasing particle size metallurgical grade diamond lapping compound of 15,6. and 0.1 wn particle size, but no significant improvement was observed and evidence for surface fouling was demonstrated for tetra- and pentachlorophenol. Glassy carbon is difficult to machine and polish and therefore is not amenable for trace determinations in a tubular configuration. Good results have been reported by a number of authors for glassy carbon in a

1172 ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982

Table 11. Effect of Carbon Black Loading on Electrode Performance

(signal/noise)/ppm

coniposi tion“ phenol 2-c1 2,6-C1, 2,4,6-C13 2,3,4,6-C14 c15 25% black/75% PE 20 15 13 14 12 13 40% black/60% PE 70 40 60 50 40 40 50% black/50% PE 830 430 370 190 100 80 35% black/65% PP 80 55 55 40 40 40 40% black/60% PP 820 350 270 130 90 80 50% black/50% PP 500 250 2 70 220 160 150

a PE = polyethylene, PP = polypropylene,

Table 111. Effect of Binding Agent on Electrode Performance

(signal/noise)/ppm

2,4,6-C13 2,3,4,6-C1, c15 compositiona phenol 241 2,643,

40% black/60% PE 70 40 60 50 40 40 40% black/60% PP 360 180 160 80 45 40 50% black/50% PE 27 0 200 220 160 110 110 50% black/50% PP 500 250 270 220 160 150

a PE = polyethylene, PP = polypropylene.

Table IV. Effect of Pressing Procedure on Electrode Performance

composition a

(signal/noise)/ppm phenol 2-c1 2,641, 2,4,6-C13 2,3,4,6-C1, C15

electrode no. 1, 50% black/50% PE; 380 190 200 120 70 60

electrode no. 2, 50% black/50% PE; 41 0 200 170 100 50 50 pressed once

pressed once

pressed twice

pressed twice

a PE = polyethylene.



thin-layer planar configuration which can be polished more readily.

Since impregnation with oils, monomers, and resins failed to provide the desired combination of low background noise and solvent resistance, combinations of graphite and polymers were evaluated. Klatt reported good results with Teflon- graphite electrodes (8) in aqueous systems, but similar electrodes proved unsuitable for LC detectors (4). Mascini reported good results with polythene graphite electrodes (9) using 230 mesh graphite (maximum particle size of 63 pm). A similar electrode was prepared from 75 pm material but the resulting “electrode” had very high internal resistance. Further grinding of the graphite in a ball mill and sieving resulted in 1 wm graphite particles which when combined 50/50 by weight with polyethylene produced a usable electrode with resistance of a few hundred ohms. This electrode gave comparable signal-to-noise ratios to the Nujol-impregnated graphite electrode (4) and possessed the desired solvent resistance.

Carbon blacks have particle sizes in the 25-75 nm range (up to 40 times smaller than the finest available graphite) so electrodes were prepared from carbon black and several polymers and evaluated as LC detectors. In order to obtain signal-to-noise ratios efficiently for a number of compounds, we injected a mixture of unsubstituted phenol and mono-, di-, tri-, tetra-, and pentachlorinated phenols into a chromatographic system where the electrode of the desired carbon black/polymer composition was employed as the detector. (Signal/noise)/ppm of phenol was calculated from chromatographic peak heights and average base line widths which were measured just prior to the first eluting peak (phenol) and just after the last eluting peak (pentachlorophenol).

The effect of carbon black loading on electrode performance was evaluated for polyethylene (PE) and polypropylene (PP) binding agents. The lowest percent carbon loading studied was 20% in P E and while the electrode functioned, only minima1 response was obtained with inconsistent results. Table I1 demonstrates the effect of carbon black loading from 25 to 50% in PE and PP. In all cases, except phenol and mono- and dichlorophenol a t the 50% carbon black level in PP, increasing carbon content increases the performance significantly. The 50% PP electrode had to be polished to obtain a low enough background current to allow useful measurements to be made while none of the other electrodes required polish. This likely accounts for the variations observed in Table I1 for this electrode composition. Loadings of 50% carbon black are very near the maximum possible because above that level carbon black will not stay in the binder during electrode fabrication.

Table I11 shows the effect of the binding agent on electrode performance. While the binding agent has some influence, it is not as dramatic as the change in performance with increasing percent carbon black. Because the 50% PP electrode required polishing to obtain reasonable performance, the 50% P E electrode was also polished so a direct comparison could be made. From a signal-to-noise point of view, polypropylene appears to offer some advantage. This is outweighted, however, by the much longer equilibration time (6X) and much higher absolute background current (10-1OOX) as well as the polishing requirement.

Table IV shows the effect of the pressing procedure used in preparation of rods employed for electrode stock. The double-pressing procedure was initially tested as an aid to

ANALYTICAL CHEMISTRY. VOL. 54, NO. 7. JUNE 1982 - 1173

Table V. Effect of Polishing on Electrode Performance

composition' (signal/noise)/ppm

phenol 2-CI 2,6-c1, 2,4,6-c1, 2,3,4,6-C14 CI, 35% black/65% PP, unpolished 40 25 30 20 13 13 35% black/65% PP, polished 80 55 55 40 40 40 40% black/60% PP, unpolished 360 180 160 80 45 40 40% black/60% PP. polished 820 350 270 130 90 80 50% black/50% PE; unpolished 830 430 370 190 100 80 50% black/50% PE, polished 270 200 220 160 110 110

PP = polypropylene; PE = polyethylene.

Flpure 5. Scanning electron photomiCrOgraph of a 50150 PECB electrode surface: unpolished. untreated. 55X magnliication.

elimination of trapped bubbles in rod interiors. These bubbles were often encountered during machining and c a d rejection of a number of electrodes. The benefit of improved signal- to-noise ratio was unexpected but may be a result of better contact between particles or a thinner film of polymer-coating around particles. Table IV also shows repeatability of signal-tunoise ratios between pairs of electrodes made from the same carbon black polymer composition.

Table V demonstrates the effect of polishing on electrode performance. A clean, dry pipe cleaner was passed through the electrode bore five times without use of any polishing agent. Care was taken to avoid scratching the surface with the pipe cleaner wire when inserting the cleaner. With the '/le in. bore of the electrode, the pipe cleaner does not fit very tightly and thus, a very gentle polish has a profound effect. The use of 0.3 wm alumina as a polishing aid produced poor resulta. For PP electrodes, signal-tc-noise ratica for all compounds increased after polishing. For PE electrodes, however, the relative response for the first four eluting phenols decreased, with phenol reapow decreasing by more than a factor of 2. The fact that response of certain compounds decreases suggesta that surface adsorption of eluting compounds may be involved. A second explanation for differences between PE and PP electrodes is the difference in hardness of the polymer films. Polishing should expose more carbon surface and thus increase response if hard polymer surfaces are present. However, in the case of a soft polymer, such as PE, the polymer could be smeared by polishing and actually cover carbon sites which could decrease response. For PE electrodes, the combination of drill speed and polymer melting point allow production of optimal surface finish, as shown in Figure 5, without the need of polishing for the specific phenolic compounds tested. Anderson (10) also observed changing response characteristics for species electrolyzed a t graphite Kel-F electrodes with varying degrees of surface polish. Thus, it may be possible to produce electrodes with preferential response characteristics by varying the binding agent and the type of surface preparation but, a t present, it is not known

L l 1 1 I , 0 0.2 0.4 0.6 0.8 ,.a

AOOhed P.,C"II.I. "OK%

F b m 6. OxKaM d diphenylamine on a graphite -ode: pdished. Fluorolube heated.

I , I I 1 I , 0 0 2 0 4 0 6 0 8 1 0 1 2

i\pp,,Pd pal.",,^,. Volts

Flgun 7. Oxidaw of diphenylamine on a polyethylene carbon black electrode: unpollshed. unheated.

how to predict such performance. Further evidence for the uniqueness of the PECB electrode

was found by comparing current-voltage curves for oxidation of diphenylamine at graphite and carbon black electrodes. At a graphite surface, only a single oxidation wave for diphenylamine is observed as shown in Figure 6. Figure I indicates that the same oxidation proceeds in two or three steps a t the PECB electrode. This may be due to the fact that solvent/electrolyte breakdown occurs a t a lower potential on graphite, thus preventing observation of further oxidation steps. The current oscillations observable in Figures 6 and I are due to surges in the flow caused by the peristaltic pump.

1174 Anal. Chem. 1982, 5 4 , 1174-1178

100 ppm /Hydroquinone

--- 0 l o 20 0 l o 20 0 IO 20

Time, Minuter

Flgure 8. Determination of inhibltors in monomers with a PECB electrochemical detector with replicate injections of an alkyl-substituted styrene monomer.

An improved method was desired for determination of polymerization inhibitors in an alkylated styrene monomer. p-tert-Butylcatechol (TBC) is almost universally used as an inhibitor and antioxidant in manufacture and storage of styrene (11). An ASTM procedure is available for determination of TBC in styrene (IZ), but an extraction is involved. Mixed inhibitor systems are also employed and such phenolic compounds can be separated by LC but the monomer itself interferes with UV detection. Because use levels of inhibitors are in the 10-100 ppm range ( I I ) , monomer can be diluted 1000-fold in the 29% acetonitrile eluent system and injected directly onto the column used for phenol separations. Suf- ficient sensitivity is available with the PECB electrochemical detector to determine inhibitors with no interference from the

monomer. Figure 8 shows separation of two inhibitors, TBC and hydroquinone, in a monomer system and demonstrates the excellent reproducibility of the procedure. Thus, monomer and inhibitors do not passivate the PECB electrode.

ACKNOWLEDGMENT The help of M. W. Long in selecting and preparing polymer

electrodes, H. D. Woodcock for machining electrodes, and R. E. Reim for evaluating the electrodes is gratefully acknowl- edged. Thanks are also due to V. A. Stenger and R. M. Van Effen for helpful discussion and Harry Baker for photomi- crography.

LITERATURE CITED (1) Gaylor, V. F.; Conrad, A. L.; Landerl, J. H. Anal. Chem. 1957, 29 ,

(2) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969; pp 280-283.

(3) Kissinger, P. T.; Refshauge, C.; Drelllng, R.; Adams, R. N. Anal. Lett. 1973, 5 , 465-477.

(4) Armentrout. D. N.; McLean. J. D.: Long, M. W. Anal. Chem. 1979,

224-228.

51 , 1039-1045.

288-293 (5) Pungor, E.; Feher, 2; Nagy, G. Magyar Kemiai Fokoirat 1971, 77,

- - - - - -. (6) Clem, R. G.; Sciamanna, A. F. Anal. Chem. 1975, 47 , 276-280. (7) Panzer, R. E.; Elving, P. J. J . Nectrochem. SOC. 1972, 119,

( 8 ) Klatt, L. N.; Connell, D. R.; Adams, R. E. Anal. Chem. 1975, 4 7 ,

(9) Mascini, M.; Pallozzl, F.; Llbertl, A. Anal. Chim. Acfa 1973, 64,

(IO) Anderson, J. E.; Tallman, D. E.; Chesney. D. J. Anal. Chem. 1978, 50, 1051-1058.

(1 1) Hodges, K.; McLean, J. D. "Encyclopedia of Industrial Chemical Analysis"; Whey: New York, 1974; Vol. 18, Styrene, pp 285-313.

(12) ASTM D 2120-68 "Test for Inhibitor, 4-Tertiary-Butylcatechol in Sty- rene Monomer"; American Society for testing and Materials: Philadel- phia, PA, 1970.

864-874.

2470-2472.

126-13 1.

RECEIVED for review December 14, 1981. Accepted March 25, 1982.

Refractive Index and Absorption Detector for Liquid Chromatography Based on Fabry-Perot Interferometry

Steven D. Woodruff and Edward S. Yeung"

Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 500 11

The combination of Fabry-Perot interferometry and a single frequency laser is probably the most sensitive way to detect refractive index changes. The added finesse and the increased monochromaticity provide an order-of-magnitude improvement in detectability over commercial high-performance llquid chromatography refractive index detectors. I f a second laser Is used to interact with the anaiyte, absorption can be monitored as a change in refractive Index. For a 60-mJ laser pulse, we have achieved detectabiifty 2 orders of magnitude better than standard absorption detectors In high-performance liquid chromatography.

With the increasing interest in environmental, clinical, and other biological problems, there is a growing need for trace analytical methods that are suitable for complex organic mixtures. While gas chromatography (GC), particularly in combination with mass spectrometers, has been successfully

applied to the volatile species, high-performance liquid chromatography (HPLC) is often the choice for the nonvolatile components. Although HPLC technology has made big gains recently ( I ) , the overall separatory power is still not compe- titive with GC. This is why new concepts for HPLC detectors can become beneficial. Further, since small sample sizes are required for these highly efficient HPLC separations, the detectors must be improved with respect to their detecta- bilities. Of the three most commonly used HPLC detectors, the fluorometric detector (2, 3) has already been developed sufficiently to be suitable for most situations. For non- fluorescing samples, the absorption detector must be used, but the detection of small differences in two large signals limits conventional detectors to the low3 to range in absorbance. When the species of concern does not show convenient absorption bands, e.g., saturated organic compounds, the refractive index (RI) detector is commonly used, despite its poor sensitivity. Since the scope of application of HPLC is inversely related to the detectability of the detectors, it will be useful

0003-2700/82/0354-1174$01.25/0 0 1982 American Chemical Society

carbon electrodes for liquid chromatography detection

Documents