Anal. Chem. 1901, 63, 1759-1763
(4) Stein, Y. B.; Narang, R. S. Anal. Chem. 1982,54, 991. (5) Amin, T. A.; Llchtenberg, J. J. Anal. Chem. 1985, 57, 648. (8) HNU Systems, Inc., Instruction Manual of Photolonizatbn Detector. (7) C a s k , M. I.; Caskla. K. C. J . C h r o m a m .1980. 200,35-45. (8) Freedman. A. N. J . Chrometogr. 1880, 190, 263-273. Cox, R. D.; Earp, R. F. Anal. Chem. 1982, 54, 2265-2270. (9) (10) Drlscoll, J. N.; Hewht, 0. F. Instrumentation for "On Site" Survey and Identification of Hazardous Waste. I n d . Hyg. News 1982, May. (11) Freedman, A. N. J . Chrometogr. 1982. 236, 11-15.
(12) BsrkOwltz' Spectroscopy; J' photoebsorpHon' Academic Press: photolonlzeHon' New York, 1979. and (13) Elceman, G. A. Personal Communication.
1759
(14) Maclay, J.; Stetter, J. R.; Christensen, S. Sensws Achretors 1989,20, 277-85. (15) Stetter. J. R.; Findlay, M. W.; Maclay, G. J.; Zhang, J.; Valhinger. S.; Gopel, W. Sensws Actuators 1990,43-47. (18) VaMnger, S.; Stetter, J. R.; Gopel, W. Roceedingsof Evosensors IV, Karlsuhe. FRO, Oct 1-3, 1990.
RECEIVED for review January 8,1991. Accepted April 30,1991. This work was supported by NASA, Kennedy Space Center, SBIR No. NAS10-11561.
Solid-Surface Room-Temperature Phosphorescence Optosensing in Continuous Flow Systems: An Approach for Ultratrace Metal Ion Determination Rosario Pereiro Garcia, Yi Ming Liu, Marta Elena DIaz Garcia, and Alfredo Sanz-Medel*
Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Ouiedo, 33006 Oviedo, Spain
Solid-surface room-temperature phosphorescence (SSRTP) optosenrlng in an aqueous flow is reported here for the first the, and lt has been applied to aiunlnun control In samples of dkrlcal Interest. This technique makes use of fbw lnJection analyds (FIA) and is based on the transient Immobilization (on a strongly bask anlon-exchanger resin packed in a flow cell) of the complex formed by the phosphorogenlcreagent 8-hydroxy-7-lodo-5qulnoiinesuifonic acid (ferron) wlth aiuminum In a continuous flow carder at pH 5.5. The analytical performance characteristics of the proposed method for semiautomated analysis and control of very low levels of Ai were as follows: 2 pg/L detection limit, f3.2% preclslon analyzing 40 pg/L of the metal and most of the common Ions In biological samples did not Interfere. Only Fe( I I I ) caused serious interference, but it can be masked by 1,lOphenanthroilne. The recommended method has been successfully tested for Ai determinations at pg/L levels in sampies of particular Importance today (dlaiysls fiulds and concentrates). Basic experiments on the characteristicsof the RTP observed for different medla used to secure the necessary rigidity (micelles, veslcies, filter paper, strong anlonlc redns, etc.) have demonstrated that Dowex 1x2-100 resln seems to provide the best sheltering to the excited triplet state. Posslble lnteractlon mechanisms between phosphor and supports are suggested.
INTRODUCTION In spite of the excellent analytical features inherent to molecular phoephorimetric analysis (e.g., very high sensitivity and general good selectivity), its use had been hampered due to the necessity to resort to cumbersome cryogenic temperature techniques. The possibility of stabilizing the "triplet state" at room temperature through the immobilization of the phosphor on solid supports (I,2) or in liquid solutions using 'ordered media" (3)has opened new avenues in phos*Author to whom correspondence should be addressed. 0003-2700/91/0363-1759$02.50/0
phorescence studies and analytical phosphorimetry. Roomtemperature phosphorescence(RTP) has been applied to the determination of trace amounts of many organic compounds of interest in biochemistry and environmental chemistry (1, 4). Few papers, however, have been published for metal ions phosphorimetric determinations including the use of ordered media (5-8) or of solid supports (9JO). On the other hand, one of the most promising trends in flow injection analysis (FIA)is based on the packing of solid phases in flow cells to simultaneously preconcentrate and detect the analyte 01-13. The solid phase can consist of an untreated resin to retain in a transient way either an analyte (13)or the product of a chemical reaction (14)or, alternatively, a resin modified with a permanent immobilized reagent (15,26) or a catalyst (17). This new integrated technique provides the advantages of FIA systems (e.g., it is easy to renew reagents in the solid support, so irreversible chemistry can be applied for sensing purposes, sample pretreatment can be integrated into the system, etc.) and allows high sensitivity of detection because the analyte, which otherwise would dilute into the carrier, is then concentrated in the detector itself. Optosensing at solid surfaces has been demonstrated by using measurements of reflectance (16),absorbance (13,14), fluorescence (15),and chemiluminescence (17). It is clear that solid-surface RTP (SSRTP) detection is not well suited for flow techniques. Moreover, RTP on typical solid matrices (paper, silica, alumina, sodium acetate, poly(acry1ic acid), asbestos, etc.) is strongly quenched by moisture and O2 (1, 4). Very recently, the use of flow injection for continuous sample introduction in SSRTP (18) has been proposed; however, this methodology is not continuous in the flow because it involves sample nebulization on a paper strip followed by a drying step. We report here, for the first time, the combined use of FIA and SSRTP optosensing detection in the aqueous environment provided by the carrier used as a continuous flow stream and its application to determine very low aluminum concentrations (the toxicity of aluminum in chronic renal failure patients is well documented (19) and explains the present demand for new sensitive analytical techniques for this element). The method for A1 analysis is based on the transient immobili0 1991 American Chemlcai Society
1760
ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991 LUMINESCENCE DETECTOR
BUFFER
UJ WASTE BUFFER
PUMP
fln B
T
\ \
FERRON
,'-'\\
I
I
EXPERIMENTAL SECTION Chemicals. Analytical reagent-gradechemicals were employed for the preparation of all the solutions. A 1000 pg/mL (3.708 X M) aluminum stock solution was prepared by dissolvingpure foil (Merck)in (1+ 1)hydrochloric acid. A 0.2 M acetic acid/sodium acetate buffer solution (pH 5.5) was prepared containing 1 M NaCl and 6 X loA3M Na2S03. A 7.5 X M ferron solution containing 6 X M Na2S03,was M ferron solution by appropriate dilution prepared from a with the pH 5.5 buffer. Ultrapure water (Milli-Q 3 RO/Milli-Q2 systems, Milli-Q Corp.) was used throughout. The preparation and handling of solutions and containers to minimize contamination was carried out as described elsewhere (20). Freshly prepared sodium sulfite was present in all solutions M) for appropriate chemical de(at a concentration of 5 X oxygenation. The strongly basic anion-exchanger resins Dowex 1x2-200, Dowex 1x2-100,Dowex 1x4-100,and Dowex 1x8-100 (Sigma),the weak basic exchanger resin Amberlite IRA-68 (Sigma),and the nonionic resin Amberlite XAD 2 (Sigma) were packed in columns and cleaned by passing 2 M HCl until no atomic absorptionsignals were obtained in the effluents. OptosensingManifold. Figure 1illustrates the optosensing apparatus used. Phosphorescenceemission measurements were made on a Perkin-Elmer LS 5 fluorescence spectrometer, which employs a xenon-pulsed (10 s half-width,50 Hz) excitation source and is equipped with a Perkin-Elmer3600 data station. The delay time used typically 0.04 ms, and the gate time was 2 ms; instrument excitation and emission slits were set at 10 nm throughout this study. The triplet lifetimes were obtained by using the "Obey-Decay" application program. Steady-state phosphorescent signals were recorded with a Perkin-Elmer 560 recorder. A conventional Hellma flow cell (Model 176.52) of 25pL volume was used. At the bottom of the flow cell, a small piece of Nylon netting was placed to prevent particle displacementby the carrier. The resin was loaded with the aid of a syringe and the other end of the flow cell was keep free. The cell was then connected to the flow system and 10 min was allowed for the particles to settle down. In order to secure that the complex first retained by the solid material was in the path light, the resin level was maintained 1mm lower than that of the cell window. The resin packed in
'\
\
\ \
'\
50
Flgure 1. Optosensing manifold. A, B, C: injection valves.
zation (on a strongly basic anion-exchanger resin packed in a flow cell) of the product formed along the flow system by the phosphorogenic reagent 8-hydroxy-7-iodo-5-quinolinesulfonic acid (ferron) and aluminum. The coupling of conventional FIA with SSRTP proposed here is a new example of optosensing at active surfaces which allows the use of RTP measurements for detection in an aqueous flow. Thus, it opens new paths for fundamental and methodological studies on the SSRTP phenomenon and its application to analytical sensing of metals and, possibly, of organic compounds.
I /
650
A (nml (a)
(b)
Figure 2. Photoluminescence spectra at different delay times. (a) Resin Dowex 1x2-100 plus AI-ferron complex at pH 5.5. (b) Resin Dowex 1x2-100 at pH 5.5. (--) 0.03 ms, (-) 0.04 ms, (.) 0.05 ms.
this way could be used for longer than 3 months. A four-channel Gilson Minipuls-2 peristaltic pump was used to generate the flowing streams. Omnifit 1106 rotary valves (A, B, and C). Omnifit 2401 mixing T piece, and PTFE tubing (0.8 mm i.d.) and fittings were used between all components in the FIA system. All connections had to be sealed with high-vacuum silicone grease (BDH) to avoid O2 penetration. The pH measurementswere made by using a WTW pH meter. Model 139 (calibrated againts Radiometer buffers). The temperature in the flow-cell compartment was maintained at 22 2 "C by a circulating water bath. General Procedure. Sample or standards (0.5 mL) were injected via valve A and ferron solution (0.6 mL) through the valve B into the flow system. Both solutions,mixed in a T piece, passed through the 3.5-m reaction coil to ensure Al-ferron complex formation (see Figure 1). Finally, the complex formed went through the flow cell, D, where it was retained on the Dowex 1x2-100resin. The high phosphorescenceof the complex on the solid support was measured at spectral maxima, A,, = 400 nm and A,, = 600 nm. Once the Al phosphorescent measurement was taken, 2 mL of 6 M HC1 was injected via valve C (to strip the Al-ferron complex retained on the solid phase), before proceeding with the next sample injection. For injection in this optosensing flow system, standards and samples were prepared as follows: An appropriate aliquot of the Al(II1) standard solution was transferred into a 10-mL standard flask and 0.12 mL of 0.5 M Na2S03solution was added before the solution was brought up to the mark with 0.2 M buffer solution. For dialysis fluid analysis, the pH of 1 mL of sample was adjusted by adding 0.25 mL of 2 M HAc/NaAc (pH 5.5); then 0.03 mL of lo00 ppm 1,lO-phenanthrolinesolution was added to prevent interference from the possible presence of iron in the samples. Finally, 0.03 mL of 0.5 M Na2S03was added to get chemical deoxygenation. For dialysis concentrates, 0.5 mL of sample was treated with 0.5 mL of 2 M buffer solution, 0.04 mL of 1000 ppm 1,lOphenanthroline solution, and 0.04 mL of 0.5 M Na2S03,and then it was diluted to 2 mL with ultrapure water. Reagent blanks were prepared and measured following the same procedure.
RESULTS AND DISCUSSION SSRTP Complex Spectral Properties. The complex formed by ferron and aluminum immobilized on a strong anion-exchanger resin showed SSRTP spectra with a maximum at 600 nm. The spectra of the Al(II1)-ferron complex, immobilized on Dowex 1x2-100, at different delay times are given in Figure 2a: at short delay times (i.e., 0.01-0.03 ms), residual fluorescence from the complex and the solid support itself could be observed, but they disappeared when using
ANALYTICAL CHEMISTRY, VOL. 63,NO. 17, SEPTEMBER 1, 1991
1761
....
a
1
0
1
resin type
Flgure 3. Solid-support selection: study of the effect of mesh size and cross-linkage. [Ai] = 80 ng/mL. Experimental conditions detailed in the Experimental Section. a = Dowex 1x2-200, b = Dowex 1x2100, c = Dowex 1x4-100, d = Dowex 1x8-100.
2
3
[Nac/] M
Flgure 5. Effect of NaCl incorporated into the carrier on the phosphorescence of Ai-ferron immobilized on Dowex 1x2-100.
Table I. Effect of Diverse Ions on the Determination of 20 ng of Al(II1) Using FIA-SSRTP Optosensing -
ions
LO -
Na(U K(I) Ca(I1) MdII) Zn(I1) Cu(I1) Fe(II1)
5 w
-
. 0
S 30-
5e
20
1
I
I
I
1
I
I
I
I
I 1
(I
content, pg
re1 intensity
5000
101.5
500 500
98.7 98.4 98.6
250 0.25
0.2 0.025 0.025'
Zr(1V) I-
c1-
0.05 2500 7500
NOB-
F
lo00 0.5
1,lO-phenanthroline
25
94.2
102.2 60.0 97.3 95.9 96.5
101.5 101.2 98.4 98.7
25 pg 1,lO-phenanthroline was added.
in Figure 4: for concentrationsof ferron above lo4 MI a sharp decrease in the SSRTP was observed, probably due to the inner filter effect on the solid surface (as free ferron was retained on such a surface). Thus,a final ferron concentration of 5 X M was selected for subsequent work. In order to keep as small as possible the amount of dye retained, relatively high concentrations of various salts (to wash out free ferron from the resin) were incorporated into the carrier. Thus, the effect of sodium acetate and the effect of sodium chloride were tested: with similar results, NaCl was selected for subsequent studies, as it was the main component of samples to be analyzed and, as shown in Figure 5, its presence enhanced the phosphorescentsignal collected. A 1M concentration of NaCl was finally selected (the higher volume and charge of the A1 complex seems to prevent its washing out by the chloride anion, which releases the free reagent). Elution of the Al-ferron complex from the solid support was studied by using HCl, H2S04,and NaOH, at different concentrations, as stripping agents. HC1 and H2S04gave similar results. HCl was used in order to keep resin shrinking problems at a minimum (the same anion composition in the carrier and in the eluent); 2 mL of 6 M HC1 completely cleared up the exchanger. The analytical signal decreased slightly as the carrier flow rate increased. Thus, a flow of 0.75 mL/min was selected finally for subsequent work as a compromise between sensitivity and sampling speed. Different coil lengths from 1.5 to 7 m were studied at the selected flow rate. Complete reaction was observed for a coil length of 3.5 m or longer. Features of the Technique and Analytical Data. A thorough study of foreign ions on the photochemical characteristics of the Al-ferron complex in organized solution was previously carried out (22). The selectivity study for this SSRTP flow system was focused on those components present
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 17, SEPTEMBER 1, 1991
Table 11. Comparative Analytical Performance Characteristics of Different RTP Systems for Ultratrace Aluminum Determinations'
calibration up to, PPm
DL ( 3 ~ ~ ppb 1, reproducibility, % triplet lifetime, ms selectivity
vesicular DDAB
micellar CTAB
(7)
(6)
micellar CTAB flow system (8)
RTP optosensor
0.8
0.5
4
0.1
19 4.1
8
50 1.3
2 3.2 0.187
0.096
4.5 0.182
good
good
excellent excellent
" CTAB: Cetyltrimethylammonium bromide. DDAB: Didodecyldimethylammoniumbromide. Table 111. Results of the Determination of A1 (ng/mL) in Dialysis Fluids and Dialysis Concentrates by FIA-SSRTP Optosensing
dialysis fluid
SSRTP
ETAAAS
DF I DF I1
115.0 f 0.3 41.6 f 1.2
108.5 f 0.7 36.2 f 1.6
concentrate
SSRTP
A1 spiked
DC
210.0 & 7
200
in the samples to be analyzed (e.g., Na+, K+, Ca2+,Mg2+,C1-, CH,COO-). The results for the determination of 20 ng of aluminum following the recommended procedure are summarized in Table I. The effects of sodium chloride and sodium acetate were explained above; K, Mg, and Ca, at the concentration levels present in dialysis concentrates, did not interfere in the determination of aluminum. We also checked the effects of other cations such as Cu(II), Zn(II), and Fe(II1) that could be present in these samples as impurities. As shown in Table I, the main interference was produced by Fe(II1). It was found, however, that the quenching influence from Fe(III) could be eliminated by its masking with 1,lO-phenanthroline (8).
The effect of Zr(IV), which forms a phosphorescent complex with ferron in a micellar batch procedure (61,was also checked. No interference was observed probably due to a diverse kinetic behavior (8) in the flow system used here. Iodide and nitrate did not interfere but anions forming strong complexes with Al (e.g., fluoride) inhibited the reaction. Analytical performance characteristics of the proposed method were evaluated. Standard calibration graphs were prepared from the results of triplicate 0.5-mL injections of aluminum standard solutions and proved to be linear up to 100 ng/mL Al. The detection limit, using the 3aB criterion (uB being the standard deviation of the blank), was found to be 2 ng/mL, and the observed relative standard deviation, at 40 ng/mL A1 level, was f3.2%. As Table I1 demonstrates, the analytical performance of the described SSRTP optosensor compares favorably with other RTP methodologies developed for aluminum in solution (6-8). Analytical Applications. Following the procedure detailed in the Experimental Section, the proposed SSRTP-FIA method was applied to the determination of aluminum in dialysis concentrates and dialysis fluids. The results obtained for the dialysis fluids were compared with those obtained by electrothermal atomization atomic absorption spectrometry (ETAAAS) for the same samples, and equivalent results were obtained (Table 111). The determination of low A1 levels in dialysis concentrates
Table IV. SSRTP Characteristics of the Aluminum-Ferron Complex"
SSRTP obsd argon O2 (air)
Dowex 1x2-100 dry 1640 (87) wet 1300 (70) dry
wet
1560 (85) 256 (20)
7,
ms
0.324 0.206 0.327
Paper argon O2 (air)
dry
108 (28)
wet
34 (14) 107 (27) 27 (IO)
dry
wet
0.309 0.172 0.322 0.151
"The solid support was immersed in a solution containing [All = M. Values in parentheses corre-
1X M, [ferron] = 5 X spond to reagent blanks.
by ETAAAS is rather unreliable, and alternative techniques are urgently needed (23). Thus,a synthetic "reference" sample was prepared from a dialysis concentrate (providing zero value Al signals by ETAAAS) that was spiked with pure aluminum. As Table I11 shows, aluminum recovery in such reference concentrate is satisfactory within *5%. Mechanism Discussion. The Al-ferron immobilization process onto Dowex 1x2-100 should be mainly produced by a strong electrostatic interaction between the sulfonic groups of the complex and the ammonium moieties on the resin. In order to critically compare the photophysical behavior of the adsorbed complex in our aqueous flow system used for optosensing and that in conventional solid support RTP measurements, the A1 complex was adsorbed on filter paper and, alternatively, on Dowex 1x2-100resin and then fixed on a plastic holder. Thus, the solid is placed in the sample holder of the spectrofluorometer to study its SSRTP spectra in different environments. The SSRTP results o h ~ e in d these experiments are given in Table I V as can be seen, at room temperature, 0,quenching was facilitated by the presence of moisture both on filter paper and on the resin; the RTP intensity for a wet complex is only 1/5-1/6 of that seen for a dry complex. On the other hand, the filter paper support seemed to be much less effective than the resin Dowex 1x2-100in protecting the triplet state from moisture effect, as shown by the results under Ar atmosphere (oxygen free). In any case, the combined effect of water and air (OJ follows the general trends described for RTP on solid supports (I, 4), but with Dowex 1x2-100,the RTP is analytically useful provided that O2is removed (as is the case in the analytical procedure proposed). This fact could be explained taking into account that hydrogen bonding between the phosphor and the hydroxyl groups on filter paper could be easily disrupted by water molecules. In turn, these results tend to indicate that the aluminum complex in the resin is held more tightly, mainly through electrostatic interactions (although hydrophobic ones could act concurrently). In other words, immobilization of the phosphor on the Dowex 1x2-100 support seems to partially protect the triplet from nonradiative collisional deactivation by quenchers (e.g., 0,) even in an aqueous flow. In fact, even with no chemical deoxygenation at all in the analytical flowing system (see Figure l),a weak phosphorescence signal was observed for Al. It is very illustrative to compare this behavior with that observed for the same Al complex "immobilized" in "ordered media" of micelles or vesicles where the presence of 0, completely quenched RTP ( 6 4 , indicating that solid Dowex 1x2-100matrices shelter the phosphor from possible quenchers (e.g., 0,) much more efficiently than those "organized" as-
Anal. Chem. 1991, 63, 1763-1766
semblies. It is worthwhile to note also, see Table IV, that provided that water is absent, the RTP triplet lifetime remains substantially constant whatever the solid support or the atmosphere may be. This could be attributed to shrinking of the matrix network during the drying step with the corresponding rigidity increase. It is well-known that the presence of a heavy atom is necessary to increase the triplet-state population (4). As an iodine substituent in the ferron molecule, acting as intramolecular heavy atom, is present in all the cases described here, intersystem crossing should not change substantially. The triplet decay profiles of this Al complex were measured in those order media mentioned above and in the FIA-SSRTP system (Table 11). We observed that the triplet lifetimes at room temperature were 0.186 ms on the resin, 0.156 ms for CTAB micelles, and 0.096 ms for DDAB vesicles. So,although RTP lifetimes in liquids could vary from system to system (24), these results support the idea that a higher rigidity of the complex into the resin along with higher protection of the phosphor by the resin was probably the main factor to prevent collisional deactivation of the excited triplet state and, hence, to secure analytically useful SSRTP signal in the aqueous flow.
ACKNOWLEDGMENT We are grateful to P. Mengndez Fraga for the ETAAAS analyses. Registry No. Al, 7429-90-5; Dowex 1x2, 9085-42-1. LITERATURE CITED (1) Hurtublse, R. J. Anal. Chem. 1989, 61, 889A-894A. (2) Hutubke, R. J. h4dscuhr Llt&WSSpecboscopy: Melhods end Appllcetlons-Part I I ; Schuiman, S. J., Ed.; Wlley: New York, 1988; Chanter 1.. (3) CUM Love, L. J.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1133A-1148A. -..-I---.
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sis; Wiley: New York, .1984. (5) Sanz-Medel. A.; Martinez Garcia, P. L; Diaz Qarcia, M. E. Anal. Chem. 1987, 59, 774-778. (6) Ferndndez de ia Campa, M. R.; Diaz Gar&, M. E.; Sanz-Medei, A., Anal. Chim. Acta 1988. 212. 235-243. (7) Fernsndez de la Campa, M. R.; Llu, Y. M.; Dhz Garcia, M. E.; SanzMedei, A. Anal. Chlm. Acta 1990, 238, 297-305. (8) Liu, Y. M.; Fernindez de la Campa, M. R.; Dbz Gar& M. E.; SanzMedel, A. Anal. chkn.Acta 1990. 234, 233-238. (9) Nlshikawa, Y.; Hirakl, K.; Morlshige, K.; Murata, Y. Bunseki Kegeku 1989, 32, 729-735. (10) Endo, K.; Igarashi. S.; Yotsuyanagi, T. Chem. Lett. 1986, 1711-1714. (11) Ruzlcka, J.; Christian, G. D. Anal. Chlm. Act8 1990. 234, 31-40. (12) Valdrcel, M.; Luque de Castro, M. D. Anal. R o c . 1989, 26. 313-315. (13) Yoshimura, K. Anal. Chem. 1987, 59, 2922-2924. (14) Lizaro, F.; Luque de Castro, M. D.; Vaicarcel. M. Anal. Chim. ActB 1989, 219, 231-238. (15) Perelro, R.; Dlaz Garcia. M. E.; Sanz-Medei, A. Analyst 1990. 115. 575-579. (18) Woods, B. A.; Ruzlcka, J.; Christian, G. D. Anal. Chem. 1987, 59. 2767-2773. (17) Hool, K.; Nleman, T. A. Anal. Chem. 1988. 60, 834-837. (18) Camplgila, A. D.; Berthod, A.; Winefordner, J. D. Anal. Chim. Acta 1990, 231, 289-293. (19) Massey, R. C., Taylor, N. D., Eds. Aluminum in Foodand the Environment; Royal Society of Chemistry: Herts, U.K.. 1989. (20) Cannata, J. B.; Suarez. S. C.; Cuesta, V.; Rodriguez, R. M.; Allende, M. T.; Herrera, J.; Perez, J. Roc. Eur. Dial. Transpl. Assoc. Ew. Renal. Asspc. 1984, 21, 354. (21) Dlaz Garcia, M. E.; Sanz-Medel, A. A n d . Chem. 1988, 58, 1436- 1440. (22) Dlaz Gar& M. E.; Fedndez de la Campa, M. R.; Hinze, W. L.; Sanz-Medel. A. Mikmchim. Acfe 1988, 11 1 , 249-282. (23) Perelro Garcla, R.; Lopez Garcia, A.; Diaz Garcia, M. E.; Sanz-Medel, A. J . Anal. At. Spectrom. 1990, 5 , 15-19. (24) Kim, H.;Crouch, S. R.; Zabik, M. J.; Seiim, S. A. Anal. Chem. 1990, 62, 2365-2369.
RECEIVED for review January 24,1991. Accepted May 17,1991. This research was supported by the ‘Fundacidn para el Foment0 en Asturias de la Investigacidn Cientifica Aplicada y la Tecnologia” (FICYT) and the “Comisidn Interministerial de Ciencia y Tecnologia” (CICYT) (PM88-0813-CO2-COl).
Oscillating-Plasma Glow Discharge as a Detector for Gas Chromatography Mikio Kuzuya’ and Edward H. Piepmeier*
Department of Chemistry, Gilbert Hall 153, Oregon State University, Corvallis, Oregon 97331 -4003
The change In frequency of a 427-kHz osclllatlon In the current of a glow discharge cell at 1.8 Torr ls a linear and sendtlve measure of organk analyte concentration In the argon plasma support gas. When the plasma cell is used as a gas chromatography detector, an InJectlonof 3 X lo-‘’ mol of propand produces a peak dgnal maximum corresponding to twice the standard deviation In the background slgnal when 0.1-8 countlng tknes are used. Only 20 hnol Is present In the apparenUy acilve r q b n of the detector at the detecth Iknlt. The gas sample Is Introduced Into the plasma cell In the dlrectlon of the cathode vla a 0.34mm-dlameter hole In the center of the anode. The gas Jet that results Is surrounded near its natural outer boundary by a 0.34-cml.d. glass cylinder. Gentle sputterlng, alded by the jet strlklng the curved cathode, keeps the cathode clean.
* Author to whom correspondence should b e addressed.
Present address: De artment of Electronic Engineerin , College of Engineering, Chubu bniversity, 1200 Matsumoto-cho,kasugai-
shi, Aichi 487, Japan.
0003-2700/91/0363-1783$02.50/0
INTRODUCTION Self-sustaining oscillations in low-pressureelectrical plasmas have been known for a long time to be sensitive to plasma conditions ( I ) . Although these oscillations have been studied extensively for decades (e.g., see references in refs 2 and 3), a linear relationship between oscillation frequency and impurity concentration in the plasma support gas has not been demonstrated until now. The f i t use of a low-pressure glow discharge as a detector for gas chromatographywas reported by Harley and Pretorius (4)in 1956. They observed the voltage change across an argon discharge and detected on the order of mol of hydrocarbons. Pitkethyl(5) observed the voltage change across a glow discharge in nitrogen and showed a nearly linear response over 3 decades of concentration for hydrocarbons. Although other workers usine different dasma cells had observed long-term drift due & the deposition of carbon on the cathode (6),he observed that no carbonaceous deposits had accumulated in several weeks Of use* He proposed that the discharge itself tends to clean the surface and assist in removal 0 1991 American Chemical Society
