Anal. Chem. 1992, 6 4 , 151-155
amine can be achieved at an electrochemically pretreated TTF-TCNQ electrode. At -75 mV, neither ascorbate, sulfite, nor dopamine are oxidized a t a bare platinum electrode (Figure la). At the TTF-TCNQ electrode however, ascorbate is oxidized at -75 mV whereas neither sulfite nor dopamine are oxidized (Figure lb). This is clearly observed under potentiostated conditions, where ascorbate produces a large signal whereas no anodic current is elicited by dopamine or sulfite (not shown). Using a bare TTF-TCNQ electrode, routine analysis of ascorbatecontaining samples can be performed within a few seconds without sample pretreatment. The electrode yields a welldefined response over a wide range of ascorbate concentrations, matrix compositions, pH, buffer concentrations, potentials, and sample oxygen concentrations. Therefore, the operation conditions can be optimized for specific applications. This flexibility, together with the option of easy automation, makes the "F-TCNQ electrode a useful tool for the analysis of ascorbate-containing samplea and complements the recently published application of mediator-modified graphite electrodes for the determination of ascorbate (28). ACKNOWLEDGMENT We thank NSERC (Canada) and the McGill Graduate Faculty for support of this research. Registry No. TTF,31366-253; TCNQ, 1518-16-7;L-ascorbic acid, 50-81-7; L-ascorbate, 299-36-5. REFERENCES OIHver, M. I n The Vh%mh~.~. 2nd ed.;Sebrell, W. H.. Harris,R. S., Eds.; Academic Press: New York. 1967; Vol. 1, pp 359-367. Nagy, G.; Rice, M. E.; Adams, R. N. Life Sci. 1082. 37,2611-2616. Schenk, J. 0.; Miller, E.; Adams. R. N. Anal. Chem. 1082, 54, 1452-1454.
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(4) &in, M. I n Ascorbic acM: Chemistry, metabolism, and uses; Seib, P. A., Toibert, B. M., Eds.; American Chemical Society Advances in Chemistry Series No. 200; American Chemical Soclety: Washington, DC, 1982; pp 369-379. (5) Leinweber, F. J.; Monty, K. J. Methods Enzymol. 1087. 743, 15-17. (6) Smith, V. J. Anal. Chem. 1987,5 9 , 2256-2259. (7) Laker, M. F.; Hofmann. A. F.; Meeuse, J. D. Clln. Chem. 1080,2 6 , 627-630. (6) Obzansky, D. M.; Richardson, K. E. Clin. Chem. 1083, 2 9 , 1815-1819. (9) Crawford, 0. A.; Mahony, J. F.; Gyoery, A. 2. Clin. Chim. Acta 1085, 747, 51-57. (IO) Brdlcka, R.; Zuman, P. Collect. Czech. Chem. Commun. 1050, 75, 766-779. (11) Roe. J. H. I n The Vitamins, 2nd ed.; Gyorgy. P., Pearson, W. N., Eds.; Academic Press: New York, 1967; Vol. 7, pp 27-51. (12) Pongracz, G. 2.Anal. Chem. 1071,253, 271-274. (13) Matsumoto, K.; Kamikado, H.; Matsubara, H.; Osajlma, Y. Anal. Chem. l08& 6 0 , 147-151. (14) Matsumoto, K.; Yamada, K.; Osajima, J. Anal. Chem. 1081, 53. 1974-1979. (15) Hkchman. M. L. I n Chemical Analysis; Eking, P. J., Wlnefordner, J. D., Eds.; J. Wlley & Sons: New York 1976; Vol. 49. (16) McKenna, K.; Boyette, S. E.; Brajter-Toth, A. Anal. Chim. Acta 1088, 206, 75-84. (17) Freund. M. S.; Brajter-Toth, A. Anal. Chem. 1080,6 7 , 1048-1052. (18) . . Freund. M. S.; Braiter-Toth, A.; Ward, M. D. J . Nectroanal. Chem. 1000. 289, 127-141. (19) Hill, B. S.; Scolari, C. A.; Wilson, G. S. Phil. Trans. R . Soc. London A 1BBO. 333. .- - - - , 63-89. - ... (20) Ferrarls, J.; Cowan, D. 0.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1073. 95. 948-949. (21) Zhao, S.;Lennox, R. B. Anal. Chem. 1991,6 3 , 1174-1176. (22) Gunasingham. H.; Tan, C.-H. Anal. Chim. Acta 1000, 229, 83-91. (23) Hennlger, G. Allmenta 1081, 2 0 , 12-14. (24) . . The Merck Index, 10th ed.; Windholz, M., Ed.; Merck & Co. Inc.: Rahway. NJ. 1983. (25) Zhao, S.; Korell, U.; Cuccla, L.; Lennox, R. 8. Submltted for publication In J. Phys . Chem . (26) Matsumoto, K.; Hamada, 0.; Ukeda, H.; Osajima, Y. Anel. Chem. 1088,58. 2732-2734. (27) . . Wbhtman. R. M.: May, L. J.; Michael, A. C. Anal. Chem. 1088,6 0 , 769A-779A. (28) Kulys, J.; Drungiline, A. Nectroanalysis 1001, 3, 209-214.
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RECEIVED for review June 19,1991. Accepted October 8,1991.
Mercury-Coated Carbon-Foam Composite Electrodes for Stripping Analysis of Trace Metals Joseph Wang,* Albert Brennsteiner, and Lucio Angnes Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Alan Sylwester a n d Robert R. LaGasse Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 Nils Bitsch Radiometer Analytical AIS, DK-2880 Bagsvaerd, Denmark The advantages and characterlstlcs of mercury-coated carbon-foam composne electrodes for stripplng analysis of trace metals are described. The enhanced perimeter-to-area ratlos characterizing these composite surfaces offer hlgh preconcentratlon efflciencles from quiescent solutlons. Additional advantages accrue from the lower oxygen-reductlon and mercuryoxldatlon background-current components. Scannlngtunnellng and scannlng electron mlcroscoples offer valuable lnslghts into the unlque microstructure of the mercury flhn and substrate. Exploratory experknents have shown the dependence of the stripplng response upon numerous experlmental variables. Convenlent quantitatlon of lead In drlnklng water Is accomplished with qulescent solutlon and short depodtlon period. Slnce neither stlrrlng nor deoxygenation is required, composltsbased stripplng electrodes should be valuable for field and remote operations. 0003-2700/92/0364-0151$03.00/0
INTRODUCTION Because of its inherent sensitivity, stripping analysis has been widely used for measuring trace metals in numerous matrices ( 1 ) . A proper choice of the working electrode is crucial for the success of the stripping operation. Mercury-film electrodes, particularly those based on a rotating glassy-carbon disk, have been traditionally used for achieving high sensitivity and reproducibility (2). The investigation of new electrode materials, as substrates for the mercury film, has continued to receive a great deal of attention. In particular, the introduction of ultramicroelectrodes exhibits great potential for stripping analysis. Several studies indicate that the stripping response of microelectrodes compares favorably with that observed at conventional electrodes (3-9). In particular, such electrodes offer several attractive features for stripping analysis, including enhanced diffusional flux (leading to higher 0 1992 American Chemical Society
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signal-to-noise ratios or use of quiescent solutions), work in low ionic strength solutions or assays of microliter samples. Such advantages have been illustrated using mercury-coated carbon-fiber (5), carbon-ring (6),or hemispherical (7) ultramicroelectrodes. In this paper we describe the characteristics and advantages of performing stripping measurements a t mercury-coated carbon-foam composite electrodes. Composite electrodes, with surfaces consisting of uniform (array) or random (ensemble) dispersion of conductor region within a continuous insulating matrix have gained considerable attention in recent years (10). Such electrodes couple the advantages of microelectrodeswith significantlyhigher currents due to larger surface areas. The analytical advantages of composite electrodes become more apparent as the size of the conductor region decreases and the perimeter-to-area ratio (PIA)increases. Recent work in these laboratories (11)and that of Weber (12) has illustrated the attractive features of new composite electrodes based on microcellular carbon-foam/epoxy surfaces. These composite materials are prepared by carbonization of poly(acrylonitri1e) foams, followed by filling the void volume of the resulting porous carbon with the insulator (13). By controlling the preparation/ carbonization conditions, it is possible to tailor the carbon foam for specific applications. In particular, the morphology and cell size can be manipulated to judiciously reduce the active site dimensions and tailor the P I A to meet specific analytical needs. Improved performance, relative to other composites presently in use (12),can thus be achieved. In addition to their unique composite character and associated nonlinear diffusional behavior, the carbon-foam/epoxy materials possess many other attractive properties for use as substrates for mercury films for stripping measurements of trace metals. These include high hydrogen overvoltage (even versus glassy carbon (II)), good electrical conductivity, and mechanical stability. The last advantage permits polishing to a highly smoothed two-dimensional surface. Unlike single microelectrodes employed in early stripping work (3-9), the mercury-coated composite surface couples the enhanced deposition efficincy with a large “collective” stripping current (from the individual sites). Such characteristics and advantages of mercury-coated carbon-foam composite electrodes are elucidated in the following sections, together with high-resolution microscopic examinations. EXPERIMENTAL SECTION Apparatus. Stripping voltammetry was performed with a BAS lOOA electrochemical analyzer (BioanalyticalSystems (BAS)), in connection with a Houston Instruments digital plotter (KIPLOT DMP-series). The 10-mL cell (Model VC-2, BAS) was joined to the working electrode, reference electrode (Ag/AgC1(3 M NaCl), Model RE-1, BAS), and platinum-wireauxiliary electrode through a hole in its Teflon cover. Potentiometric stripping analysis (PSA) was performed with the TraceLab system (Radiometer, Inc.). Data obtained for the carbon-foam surface were compared with those of 3-mm-diameter glassy-carbon disks (MF-2012 (BAS) or Radiometer Inc.). Surface images were obtained with a Phillips 501B-SEM and a Nanoscope II STM system (Digital Instruments). Reagents. Double-distilled water was used to prepare all solutions. The lead, cadmium,copper, and mercury stock solutions (lo00 mg/L) were obtained from Aldrich and diluted as required for making standard additions. A 0.05 M acetate buffer solution (pH 4.5) served as the blank electrolyte. Drinking-water samples, collected at the NMSU laboratory, were not treated prior to analysis. Procedure. The preparation of the carbon-foam composite electrode was described earlier (11). Most of the work was performed with 3-mm-diameter composite disks based on a low density, “flake morphology”foam (11). The surface was handpolished with 3-, 1-, and 0.05-pm alumina slurries (3 min each) to yield a shiny (mirrorlike) appearance. A thorough rinsing with double-distilledwater was used to remove the excess alumina.
Figure 1. Scanning electron micrograph of a mercury-coated carbon-foam composite electrode.
The mercury film was preplated prior to each experiment. For this purpose, the acetate buffer solution, was spiked with 5 mg/L mercury and purged with nitrogen for 5 min. A potential of -0.9 V was then applied at the working electrode while the solution was stirred. After 5 min, the potential was switched to 0.0 V and held there for 2 min. Background and sample measurements were subsequently carried out by applying the deposition potential (usually -1.0 V) for a selected time, using quiescent or stirred solutions. The subsequent stripping step was performed (with a quiescent solution), usually with the square-wavestripping mode. PSA was carried out similarly in the presence of 80 mg/L mercury ion as the oxidant. The mercury film was removed at the end of the day by wiping the electrode face with a soft wetted tissue paper.
RESULTS AND DISCUSSION Surface Characterization. Surface techniques, such as scanning-tunnelingmicroscopy (STM) and scanning electron microscopy (SEM) were employed first to study the microscopic structure of the mercury film. Figure 1 shows a scanning electron micrograph of a typical surface region of the mercury-coated foam composite electrode. As expected, the mercury is deposited along the conductive domains of the composite. The film, however, is discontinuous, consisting of numerous individual spherical microdroplets (of 0.4-0.8-pm diameter). Apparently, the carbon region of the composite surface consists of sites of varying activity for the mercury plating. Analogous SEM observationsof isolated droplets were reported for the commonly used mercury-coated glassy-carbon electrodes (14). Such discontinuity of the mercury film further enhances the nonlinear diffusional character, hence offering improved preconcentration efficiency (as desired for stripping measurements). Such character is further facilitated by the large separation between the discontinuous mercury “lines”. Overall, only a very small fraction of the geometric area (3%) is covered by the mercury film. In contrast, the fraction of carbon area of the composite was estimated earlier to be
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
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nm loo00
Flgure 2. STM image of a carbon-foam composite surface. Conditions: tunneling current, 2 nA; tip bias voltage, 0.05 V.
’*’
POTENTIAL (V) Flgure 4. Potentiometric stripping analysis of a sample containing 100 ppb lead and cadmium and 200 ppb copper, at glassy-carbon (A) and foam composite (B) mercury-coated electrodes. Conditlons: 1-mln deposition at -1 .O V from a stked solution; PSA with 80 mglL mercuic ions (as oxidant) and 3000-Hz measurement frequency.
I /
0.0
POTENTIAL [VI
-1.0
Figure 3. Squarewave stripping voltammograms for 20 ppb cadmium, iead, and copper, at mercury-coated glassy-carbon (A) and carbonfoam composite (B) electrodes. Condnions: 3-min deposition at -1.0 V from a quiescent solution. Square-wave amplitude, 30 mV; step, 4 mV; frequency, 30 Hz. Analogous voltammograms following deposition from a stirred solution are shown as dotted lines.
4% (11). In addition to information about the mercury film, such mercury-deposition/SEM experiments shed new light into the carbon-phase network (compared to previous, unsuccessful, attempts to visualize the foam composite material (12)). No droplets were observed for the uncoated electrode. Additional insights into the nature of the composite substrate can be achieved via a high-resolution STM characterization. We have recently documented that STM can map the spatial variations of the conductive and insulating regions of composite electrodes, such as carbon-paste surfaces (15). An analogous STM characterization of the carbon-foam composite electrode is shown in Figure 2. Since STM is restricted to the electrically conducting carbon region, the tunneling disappears abruptly as the tip passes over the insulating epoxy region. The resulting image thus offers a unique visualization of the distribution, shape, and dimension of the carbon-phase network. Such a network consists of connected “lines” of 1-pm average width. Stripping Measurements. The enhanced plating current densities (and hence preconcentration efficiency) displayed by the mercury-coated carbon-foam composite electrode eliminate the need for forced-convectionduring the deposition step. Figure 3 illustrates typical stripping voltammograms for a quiescent solution containing 20 ppb (pg/L) cadmium, lead, and copper, obtained at the mercury-coated glassy-carbon (A) and carbon-foam composite (B) electrodes. The Composite
surface yields well-defined and sharp peaks, good resolution between neighboring signals, low background current, and a wide potential window. The copper peak (Ep= -0.15V) is not affected by a rising mercury-oxidation background current. The improved signal-to-backgroundcharacteristics observed at the composite electrode permit convenient quantitation at the ppb level. Also shown in Figure 3 (dotted lines) are analogous voltammograms obtained following deposition from a stirred solution. Because of the nonlinear diffusional character, the composite surface results in significantly higher ratios of current peaks obtained in quiescent and stirred solutions (i,,q/ip,s). For example, ie,q/ip,svalues of 0.40 and 0.11 can be estimated for the cadmium response at the coated composite and glassy-carbon electrodes, respectively. Unlike the very low current outputs characterizing earlier stripping work at individual ultramicroelectrodes (3-9), the present composite surface couples the use of quiescent solutions with relatively large (-500 A)“collective”stripping currents. This advantage of “arraylike”composite electrodes (over individual ultramicroelectrodes) are particularly apparent when trace analysis is concerned. No detectable signals were observed in analogous measurements with single microdisk glassycarbon electrodes (10-pm diameter). Only larger (in one dimension) individual microelectrodes, e.g. cylinder or band (of refs 3-9) can yield measurable stripping signals with conventional instrumentation. Various stripping modes were compared for their response for trace metals at the mercury-coatedcarbon-form electrode. Among the voltammetric schemes, favorable signal-to-background characteristics were obtained with the square-wave and differential pulse operations. Linear-scan stripping voltammetry, in contrast, yielded inferior performance due to a large charging background current contribution. The foam-based electrodes are also very suitable for a potentiometric stripping operation. Figure 4 compares the potentiometric stripping response at mercury-coated glassy-carbon (A) and carbon-foam (B) electrodes. Favorable signal-to-background characteristics are observed at both electrodes. Notice, again,the improved detection of copper at the mercury-coated foam electrode. Additional improvements in the sensitivity (over the coated glassy-carbon surface) were observed in potentiometric stripping measurements following deposition from quiescent solutions (not shown). High reproducibility and stability are also indicated from the elimination of forced convection. A prolonged series of 20 repetitive measurements
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5
Lu
ar ar
3
4 50
V
I
1
0.0
POTENTIAL (V]
I
I
-1.0
Flgure 5. Square-wave stripping voltammograms of 10 ppb cadmium, lead, and copper in a nondeaerated solution at glassy-carbon (A) and foam composite (B) mercury-coated electrodes. The corresponding background response is shown as dotted lines. Conditions are as in Figure 3, using a stirred solution during the deposition.
Flgure 6. Effect of square-wave pulse amplitude on the stripping peak current (A) and width (B). Response to 20 ppb cadmium following 3 min of preconcentration at -1.0 V from a stirred (400 rDm) solution. Other conditions are as in Figure 3.
5&*
L
of 100 ppb lead yielded a mean potentiometric stripping signal of 2136 counts, with a range of 1990-2298 counts, and a relative standard deviation of 3.8% (1-min deposition; not shown). Subsequent work was carried out using square-wave stripping voltammetry. It has been shown earlier (16) that stripping analysis can be performed in the presence of dissolved oxygen if squarewave voltammetry is used during the stripping step. Such capability is attributed to the electrolytic depletion of oxygen at the surface prior to the fast square-wave scanning operation. Figure 5 illustrates square-wave stripping voltammograms at mercury-coated glassy-carbon (A) and foam composite (B) electrodes, for a nondeaerated solution containing 10 ppb lead, cadmium, and copper. The corresponding blank voltammograms are shown as dotted lines. Both electrodes exhibit a broad background current peak (at ca. -0.6 V), indicating the presence of some oxygen a t the surface. The larger oxygen peak at the coated glassy-carbon surface severely affects the quantitation of cadmium and lead. In contrast, such measurements at the composite-based surface are feasible, due to the significantly smaller oxygen contribution. Apparently, the microarray character of this electrode facilitates the depletion of oxygen from the surface (compares to that at conventional electrodes). Figure 6 examines the effect of the square-wave pulse amplitude upon the stripping response of the coated foam electrode. The peak current increases rapidly with the increase in the amplitude between 5 and 25 mV and then it starts to level off. The increased amplitude resulted also in peak broadening from a blI2(half-peak width) of 53 to 67 mV. The exact reason for the discontinuity in bl!l (observed between 30 and 50 mV) is not clear. The peak current also increased linearly with the square-wave frequency (over 2-100 Hz), with a slight peak broadening from a b1/2 of 60 to 72 mV over this range (not shown). Most subsequent work employed a square-wave amplitude of 30 mV, coupled to a frequency of 30 Hz. The effect of the deposition time on the square-wave stripping peak current was studied using an unstirred 100 ppb lead solution. The preconcentration duration was varied between 0 and 10 min. As expected, a linear relationship was obtained, with a slope of 137 nA/min and a correlation coefficient ( R ) of 0.999. Calibration data, acquired with quiescent solutions of cadmium (10-80 ppb), yielded defined and sharp stripping peaks; the resulting calibration plot was highly linear (R = 0.998), with a slope of 37.3 nA.L/pg. A
L LLI
E E 3
v
POTENTIAL (V) Flgure 7. Square-wave stripping voltammograms for a drinking-water sample (a), as well as for subsequent concentration increments of 1 ppb lead (b-e). Preconcentration was at -1.0 V for 3 min from a quiescent solution. Sample: 8 mL of drinking water -t 2 mL of acetate buffer (pH 4.5). Other conditions are as in Figure 3.
detection limit of 0.05 ppb was estimated from analogous measurements of 1 ppb cadmium, following 10 min of deposition (not shown). The properties of the mercury-coated foam composite electrode (particularly the large “collective”currents obtained with quiescent, nondeaerated samples) are particularly attractive for designing small portable instruments for on-site monitoring of trace metals. For example, the mercury-coated composite electrode holds a great promise for on-site assays of drinking water for the trace lead level. Figure 7 shows typical stripping voltammograms,illustrating such assay. With short (3 min) deposition times and unstirred solutions, such an electrode yields a well-defined lead peak for the sample (a) and subsequent standard additions of 1ppb (b-e). A lead level of 1.35 ppb was thus calculated for this sample. In conclusion, microcellular carbon-foam/epoxy composite surfaces have been shown to be very suitable substrates for mercury films for stripping measurements of trace metals. The reduced active site dimension (compared to other porous electrodes, e.g. reticulated vitreous carbon, used to prepare two-dimensional composite surfaces) enhances the microarray character and hence the stripping performance. Since neither stirring nor deoxygenation is required, composite-based stripping electrodes should be valuable for field and remote operations. The ability to control the properties of carbonized poly(acrylonitri1e) foams over a wide range (13)should lead to tailor-made composite materials suitable for various sensing applications. New high-density foams, with smaller conducting sites and cell sizes (13c), can further enhance the
Anal. Chem. 1992, 6 4 , 155-158
analytical performance. Special attention should be given to the construction of these electrodes (in comparison to readily available glassy-carbon substrates). These (and similar) composite materials are thus expected to play a growing role in stripping analysis. Registry No. C, 7440-44-0; Hg,7439-97-6; Cd, 7440-43-9;Cu, 7440-50-8; Pb, 7439-92-1; HzO, 7732-18-5.
REFERENCES (1) Wang, J. Stripping Analysis : prtnciples , Inshumentation and Applica tbns: VCH: Deerfiild Beach. FL. 1985. (2) Florence, T. M. J. Electroanai. Chem. InterfacialElectrmhem. 1970, 2 7 , 273-281. (3) Harman, A. R.; Baranski. A. S. Anal. Chim. Acta 1990,239, 35-46. (4) Bond, A. M.; Luscombe, D. L.; Tan, S. N.; Waiter, F. L. Nectroanalysis 1990, 2,195-202. (5) Wang, J.; Tuzhi, P.; Zadeii, J. Anal. Chem. 1987, 5 9 , 2119-2122. (6) Wong. D. K. Y.; Ewing, A. G. Anal. Chem. 1990, 62. 2697-2702. (7) Schuize. G.; Frenzei. W. Anal. Chlm. Acta 1984, 759, 95-103. (8) Huiliang, J.; Hua, C.; Jagner, D.; Renman, L. Anal. Chlm. Acta 1987, 193, 61. (9) Sottery, J. P.; Anderson, C. W. Anal. Chem. 1987, 5 9 , 140-144. (10) Tallman, D. E.; Petersen, S. L. Electroana&sis 1990,2 , 499-510.
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(11) Wang, J.; Brennsteiner, A.; Sylwester, A. P. Anal. Chem. 1990,6 2 , 1102- 1104. (12) Davis, B. K.; Weber, S. G.; Sylwester, A. P. Anal. Chem. lQS0,62, 1000- 1003. (13) (a) Sylwester, A. P.; Aubert, J. H.; Rand, P. B.; Arnold. C., Jr.; Clough, R. L. Pol)”. Mater. Scl. Eng. 1987,5 7 , 113-118. (b) U.S. Patent 4,832,881, 1989. (c) Aubert, J. H.; Syiwester, A. P. Chemtech 1991. April, 234-239. (14) Stullkova, M. J. Electroanal. Chem. Interfacial Electrochem. 1973. 48, 33-45. (15) Wang, J.; Martinez, T.; Yank, D. R.; McCormick, L. J. Electroanel. Chem. Interfacial Ebctrmhem. 1990,286, 265-272. (16) Wojciechowski, M.; Balcerzak, J. Anal. Chem 1990,6 2 , 1325-1331.
RECEIVED for review July 9,1991. Accepted October 8,1991. This work was supported by grants from Sandia National Laboratories (under DOE Contract No. DE-AC0476DP00789) and the US.Environmental Protection Agency (Grant No. CR-817936-010). Mention of commercial products does not constitute endorsement by the US.EPA or the US. DOE. L.A. acknowledges a fellowship from Fundaciio de Amparo 6 Pesquisa do Estado de S5o Paul0 (FAPESP), Brazil.
Calibration of a Windowless Photoacoustic Cell for Detection of Trace Gases Gyorgy Z. Angeli* and Anikd M. Sdlyom
Analtron Applied Research Company, POB 63, H-1311 Budapest, Hungary Andrhs Miklds
Institute of Isotopes, Hungarian Academy of Sciences, POB 77, H-1525 Budapest, Hungary Dane D. Bicanic
Laser Photoacoustic Laboratory, Department of Agricultural Engineering and Physics, Agricultural University, Duivendal 1, 6701 AP Wageningen, The Netherlands
The Instrumental flgure of performance, F , of a windowless, gas-phase photoacoustic cell, defined as the ratio of the normalized photoacoustic signal to the absorbance, is shown to be dependent on the sample, in contrast to prevlous assumptions. However, for a particular gas, F is Independent of concentration in the ppm range. A method of calibration of such a cell Is outllned In the absorbance range from 2 X IO-’ to 5 X The estimated detection limits at a slgnal to noise ratio of unity were 7.4 ppm, 0.43 ppb, and 1.3 ppb for carbon dioxide, ammonia, and ethylene, respectlveiy.
INTRODUCTION Photoacoustic (PA) spectroscopy is a sensitive method for the detection of gases at low concentrationand as such appears to be a very attractive means of studying atmospheric pollution. The use of the photoacoustic (or, as it is often termed, the optoacoustic) effect, originally discovered by Bell ( I ) , in combination with a strong laser infrared source was initially applied to detect weak absorption in gases (2). Technological developments in the field of lasers and high-sensitivity pressure detectors contributed to the substantial progress of photoacoustic spectroscopy ( 3 , 4 ) . In recent years numerous
* Current address: Department of Chemistry, University of Arizona, Tucson, A2 85721.
studies have demonstrated the feasibility of infrared-laser photoacoustic spectroscopy in environmental applications and in agriculture (5-11). In our recent experiments a step tunable C02 laser radiation source-since its emission spectrum (9.4-10.6 pm) overlaps ( 1 1 ) with the absorption fingerprint of various pollutants-was used in conjunction with a windowless (open) resonant cell with a high acoustic quality factor designed for trace analysis of ambient air (12). In the recently reported calibration procedure of the above-mentioned windowless cell, the PA system was considered as an analytical detector and the calibration itself was performed by conventional methods. In order to facilitate the comparison of various analytical PA systems, we suggest the introduction of a different quantity characterizing the PA instrument. This new parameter is consistent with the traditional terminology of analytical absorption spectroscopy, enabling direct comparison of PA spectroscopy with other spectroscopic methods. Terminology. The intensity I, absorbed by the sample can easily be calculated from Bouguer-Lambert-Beer’s law as
I, = Io[l - 10-A] = It[loA- 11 where Io and It are the incident and transmitted powers, respectively (in watts), while A is absorbance. If A
