Anal. Chem. 1989, 6 1 , 1989-1993
1989
TECHNICAL NOTES I n Situ Laser Activation of Glassy Carbon Electrochemical Detectors for Liquid Chromatography: Demonstration of Improved Reversibility and Detection Limits K e n t S t e r n i t z k e a n d R i c h a r d L. McCreery*
Department of Chemistry, T h e Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 Craig S . B r u n t l e t t a n d Peter T. Kissinger
Bioanalytical Systems, 2701 Kent Avenue, West Lafayette, Indiana 47906 INTRODUCTION The development of liquid chromatography/electrochemistry (LCEC) provided a very sensitive technique for a variety of redox compounds, many of which are biologically important. The marriage of the separation ability of LC and the selectivity and low detection limits of amperometric flow detectors has made LCEC the method of choice for many neurotransmitters, vitamins, pharmaceuticals, and a variety of other systems (1, 2). With the advent of capillary zone electrophoresis, electrochemical detectors have extended detections limits to femtomole levels, with analytical volumes in the subnanoliter range (3). In addition to the high sensitivity resulting from Faraday's law, LCEC has an additional advantage of variable applied potential. Selectivity beyond that already provided by the chromatographic process may be realized for analytes having different redox potentials. Several technological approaches for exploiting these advantages have been discussed (4-7).
The LCEC experiment is dependent on heterogeneous electron-transfer kinetics at the electrode/solution interface and can be adversely affected by changes in the electrode surface. Kinetic effects in LCEC have two common manifestations in detector sensitivity and stability. First, slow kinetics may result in poor sensitivity for certain analytes. An example of relevance to this report is the oxidation of glutathione (GSH) at carbon electrodes. Slow heterogeneous kinetics require a high oxidation potential at the detector, leading to high background current and poor detection limits. Stated more generally, there is a variety of important redox systems that could be detected with LCEC if their chargetransfer kinetics were more favorable. Second, the sensitivity of an LCEC detector may degrade with time due to electrode passivation, usually by irreversible deposition of solution species or electrolysis products on the electrode surface. The resulting film may inhibit electron transfer and decrease detector response. Selectivity is dependent upon kinetics as well. The ability to resolve analytes a t the detector on the basis of redox potential is an important aspect of LCEC (5). Whether the detector sensitivity and selectivity are low because of inherently slow charge-transfer kinetics or because of passivation, a means to improve the electron-transfer rate constant ( K O ) would be valuable. Due to several practical advantages such as wide potential range, low background current and desirable mechanical properties, glassy carbon (GC) has been used widely for LCEC. The electron-transfer kinetics of a GC electrode are known to vary greatly with pretreatment history, and several methods have been demonstrated for improving electrochemical kinetics on glassy carbon. These include rigorous polishing *Author to whom correspondence should be addressed.
procedures (8-1 0), flame, inert atmosphere, vacuum, and low-pressure heat treatments (11-13), radio frequency plasmas (14, 15), electrochemical pretreatments (16-24), and laser irradiation (25-28). While any of these may be done prior to cell assembly, the activated surface is sensitive to pollution, and deactivation is likely during cell mounting and startup. Only the last two methods can be carried out in situ during LCEC analysis, thus reducing contamination. Electrochemical pretreatments (ECP) have been developed for LCEC, to result in a repeatable, in situ activation technique (23,24). It was demonstrated that passivated GC electrodes could be restored repeatedly by using ECP directly in the LCEC cell. A very different approach to activation is exposure of the GC surface to intense laser pulses, also in situ. Not only did laser activation restore passivated GC surfaces, but it also increased k o greatly for several redox systems with very slow kinetics (26, 27). This work was undertaken to address several questions. First, can the advantages demonstrated for laser activation in conventional electrochemical experiments be realized in an LCEC detector? Second, can a passivated LCEC electrode, specifically GC, be renewed in situ with laser activation? Third, can a wider range of analytes be detected with laserassisted LCEC? EXPERIMENTAL SECTION Activation was conducted with a Quantel580-10NdYAG laser, using the same optical arrangement and alignment procedure described previously (26,27). Unless noted otherwise, three pulses of the Nd:YAG fundamental (1064 nm, 7-10 ns) were employed for activation, and the center of the laser beam covered the entire GC working electrode surface. Three laser pulses were used to average out the significant spatial and pulse-to-pulse variation in laser power density, which was approximately k20%. Power densities were determined by dividing average power through a known aperture by the pulse duration, repetition rate, and aperture area. The electrode was a single 3.2-mm glassy carbon disk in a Kel-F block. A modified Model TL-5A cell (BioanalyticalSystems, West Lafayette, IN) held the working electrode block and Ag/AgCl reference electrode (3 M in NaC1). All potentials reported are relative to this reference electrode. A window was mounted in the cell block across from the working electrode to allow entry of laser light, as shown in Figure 1. All laser activation was conducted in the LCEC cell, with solvent present and flowing. Prior to cell assembly the GC was polished successively with LO-, 0.3-, and 0.05-pm alumina and ultrasonicated. The detector unit was a BAS LC-4B potentiostat operated at constant potential, and output from the detector was recorded by a Labmaster analog-to-digitalconverter (ScientificSolutions, Solon, OH) and IBM PC compatible computer. The working electrode was switched off during laser irradiation to avoid overloading the detector with the large electromagnetic interference and current spikes accompanying laser activation.
0003-2700/89/0361-1989$01.50/00 1989 American Chemical Society
I990
ANALYTICAL CHEMISTRY. VOL. 61, NO. 17, SEPTEMBER 1, 1989
R
hv
a I --w
From Column I
Flgure 1. Design of the thin4ayer cell used for experiments. See text lor details. An unmodified Model BAS PM-3OA pump was used with a 10-pm silica C-18 bonded phase column (Alltecb Associates), 25 X 0.46 cm.All separations were ismatic, and injectionswere made with a 20-pL sample loop. A Teflon tube delivered the column eluent to the cell block mounted in front of the laser on an optical table. Mobile phases were made with NANOpure II water (Syhron Barnstead) and filtered with 0.45-pm membranes (Gelman Scientific). Degassing was performed hy bubbling argon through the mobile phase for 20 min prior to use. All chemicals and solvents were of reagent grade and used as received.
RESULTS AND DISCUSSION The optical and electrochemical aspects of the apparatus depicted in Figure 1are very similar to those of previous work on laser activation, with the exception of the thin-layer hydrodynamic cell and high current sensitivity. The most pronounced effect of laser activation on the LCEC response is an increase in the heterogeneous electron-transer rate constant, k', particularly for GC electrodes. The effect of improved kinetics on the LCEC chromatogram is an increase in observed peak current for systems exhibiting slow kinetics on unactivated electrodes. The result of increased ha of important to LCEC is detection of redox systems a t applied potentials closer to the thermodynamic E O . As will be demonstrated, increased ko provides advantages in selectivity and detection limits. An example of laser activation for LCEC is shown in Figure 2 for the case of a chromatogram of a mixture of three oxidizable species. At an applied potential of +0.05 or +0.6 V, glutathione (GSH), resorcinol, and 5-hydroxytryptamine (5-HT) exhibit small chromatographic peaks on a conventional GC electrode, due to slow electron transfer. Laser activation significantly improves the peak currents for these systems by improving the electron-transfer rate. At the same time, background has not increased in proportion to signal and settles quickly after laser activation. For unactivated GC, GSH and resorcinol are barely observable above background but exhibit well-defined peaks at either +0.5 or +0.6 V after in situ laser activation. GSH is a tripeptide with a thermodynamic redox potential of -0.57 V vs Ag/AgCl (at pH 7.4; E"' is ca. -0.15 V vs Ag/ AgCl a t pH 0) and should be amenable to oxidative LCEC detection. However, very slow electron-transfer kinetics on GC inhibit oxidation to the disulfide within the available potential range. The potential dependence of the LCEC response for GSH is shown in the hydrodynamic voltammogram (HDV) of Figure 3. The points in Figure 3 are the peak heights for equal amounts of GSH injected at a range of Ea++ values. Figure 3, curve B shows that GSH does not oxidize on polished GC, until Eapp is greater than +1.0 V vs Ag/AgCl, with an El,* above +1.2 V. In order to obtain a useful LCEC response on unactivated GC, the high EBpp required will lead
0
6
12
TIME
18 (MINUTES)
nAmps
4
-
A
0 0
24
6
12
TIME
18
24
(MINUTES)
Flgure 2. LCEC chromatograms for the separation of glutaUdone (GSH). reswcinol, and 5HT after in@c+lm of 0.60. 60.0, and 0.60 nmol. respectively. Mobile phase: 0.050 M monochloroacetic acid (MCAA)-methanol (251). pH 2.9. flow rate 1.5 mL min-'. (A) E, +0.50 V: (E) +OB0 V. Lower traces are lor p o l M OC: upper traces were mined 5 min after three 30 MW laser pulsas. Upper trace in Figure 28 was displaced upward by 50 nA for c W i . Peak identitiss: 1, solvent front; 2. GSH: 3, resorcino1: 4, 5-HT.
200 .. 160 ..
A
120 ..
*..A 0
"
0.4
....-x -
__r,..
0.6 E vs Ag/AgC1
1.2 (VOLTS)
Flgure 3. Hydrodynamic voltamnqrams for the oxidation of glutathione on GC injections 01 0.40 nmol in 20 pL. Mobile phase: 0.040 M MCAA-methanol (1001). pH 2.9, llow rate 1.0 mL min-'. A is for three 30 MW cm+ pulses applied initially, with one renewal pulse at each potential. B is lor wnventionally polished GC.
to high baclrground and low signal-bbackground ratio. Laser activation leads to a more useful hydrdynamic voltammogram
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
PHENOL
A
i
LASER
1991
t
0.0
+ z
I200 nA
W
a a 3
U
0
20 30 40 50 TIME (MINUTES) Flgure 4. Renewal of an electrode fouled with phenol. Conditions: ascorbic acid injections of 0.20 nmol, phenol 160 nmol; mobile phase, 0.050 M MCAA-methanol (20:1), pH 2.95, flow rate 1.5 mL min-'; laser, three 35 MW cm-' pulses at 38 min; Eapp +0.900 V. 10
with an Ellz of +0.76 V vs AgIAgC1, and thus more easily quantified LC peaks. Although the charge-transfer kinetics are still slow and the Ellz is still much more positive than the thermodynamic E"', the electron-transfer rate has been increased to a point where LCEC analysis is tremendously improved. The resulting improvement in the detection limit for GSH is discussed below. The benefits of laser activation for LCEC applications presented thus far are dependent on improved electrontransfer kinetics. As reported previously, an additional aspect of laser activation is removal of surface films, which may foul solid electrodes. For the case of passivating films produced from phenol oxidation, laser treatment in a conventional cell restored an inactive electrode apparently indefinitely (25). Phenol oxidation results in a passivating film similar to irreversibly adsorbed products that may form from a variety of common analytes and nonelectroactive biomolecules. This film deters the mass transport of some analytes to the electrode surface, or the electron transfer itself. The resulting decreased response to a common analyte and the removal of this passivating film are both demonstrated in Figure 4. Initially, ascorbic acid (AA) exhibits an LCEC response on a laser-activated GC electrode with a peak height of 210 nA. After six injections of large amounts of phenol, the ascorbic acid peak current is reduced to 120 nA. Three laser pulses restored the peak current to 202 nA. This increase in current is due exclusively to film removal since at 0.90 V ascorbic acid is already at its diffusion limit before electrode activation. As mentioned in the Introduction, the LCEC detector can, in principle, resolve analytes on the basis of their redox potentials, through the use of different applied potentials. For example, AA and dihydroxyphenylacetic acid (DOPAC) have thermodynamic potentials that differ by 0.18 V, with DOPAC more positive. Based on thermodynamic E", an LCEC detector operating at 0.3 V vs AgJAgCl should detect only AA, while one a t +0.5 or greater should detect both compounds. Unfortunately, the electron-transfer kinetics for both systems are so slow that their voltammetric peaks are shifted positive by several hundred millivolts, to the point where they severely overlap. As shown by the series of chromatograms in Figure 5A, it is difficult to find an Eapp where only one component is detected. If Eappis low enough so DOPAC is not detected, (e.g. +0.3 V), the AA response is greatly diminished. As demonstrated for conventional voltammetry (26),laser activation can greatly improve resolution by increasing k o to the point where redox systems appear near their thermodynamic potentials. On the laser-activated surface (Figure 5B), AA
20
0
TIME
20
0
(MINUTES)
Flgure 5. Chromatograms of mixtures of AA (0.30 nmol) and DOPAC (1.80 nmol) at various applied potentials. Mobile phase: 0.050 M MCAA-methanol (17:1), pH 2.9, flow rate 1.5 mL min-'. (A) polished GC;(B) following three 30 MW cm-' laser pulses. Single laser pulses occurred after each chromatogram in series B. ,E values indicated are positive relative to Ag/AgCI.
Table I. GSH Detection Limits detectn lim, pmolb
noise, pA
sensitivity: pA/pmolb
(SIN = 3)
18.3 784 16.5 39.5
41.5 366 96.1 304
1.32 6.43 0.52 0.39
p-p
E w +1.0 V, no laser +1.5 V,no laser +0.6
V, laser activated
+LO V, laser activated
ODetermined from five or more injections covering at least an order of magnitude of GSH injection amount. *Picomolesof injected GSH. and DOPAC exhibit Ellz values that are well separated (by of +0.2-0.3 V on the activated surface, ca. 0.25 V). At an Eapp AA exhibits a diffusion-limited response, while the DOPAC peak is negligible. Clearly the activated GC surface permits resolution on the basis of redox potential. As demonstrated in Figure 3 for GSH, a consequence of increased k" is detection of redox systems at values closer to their thermodynamic potentials. In the case of GSH, LC detection is feasible a t 0.6-0.9 V rather than 1.0-1.3 V vs Ag/AgCl. Since background current from both GC surface oxidation and interferents generally increases with more positive potentials, a higher signal-to-background ratio would be expected at +0.6 than at +1.0 V. The effects of potentials and laser activation on GSH detection limits are listed in Table I. Noise is defined as the peak-to-peak variation in detector response with no peaks eluting; signal is the LC peak height above the average base line. For an unactivated surface, required to detect GSH led to higher background the high Eapp and noise, and a relative poor detection limit. With Eapp= 1.0 V, laser activation increased the noise, but increased the signal by a greater factor, resulting in improved signal-to-noise ratio and a lower detection limit. At +0.6 V, where an unactivated electrode showed negligible response, the laseractivated surface exhibited a significantly lower detection limit than that of the conventional GC electrode operating at higher applied potential. It should be noted that this comparison was carried out with synthetic GSH samples. For actual analytical samples, interferents that may oxidize at high potentials may not appear at +0.6 V. Thus laser activation may reduce contributions to background from oxidizable inter-
1992
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989 E
l I
I
d
i
Flgwe 6. Chromatograms showing decay of actiiatlon for GSH on Gc: injections, 0.40 nmol; mobile phase as in Figure 3. At t = 0, the electrode was conventionally polished GC. At t = 40 min, 10 30 MW cm-' laser pulses were applied. Single renewal pulses were applied at t = 6.7, 13.5, and 21.2 h. faw:+0.800 V. Peaks are labeled as follows: A, last GSH peak before laser; B, laser pulse; C, first GSH injection after laser; D, second injection after laser.
ferents, as well as improve the signal-to-noise ratio. The principal motivation for the activation of GC toward GSH oxidation is to provide an alternative to mercury film electrodes for GSH in biological materials (29). The duration and reproducibility of laser activation on LCEC chromatograms were examined in more detail for the case of glutathione on GC. Loss of surface activity would be exhibited in the HDV (similar to Figure 3) as a shift to more positive potential. Deactivation is accelerated by the passage of mobile phase over the electrode surface, presumably because of enhanced mass transport of adsorbates. We observed that loss of surface activity is extremely slow under static conditions in a thin-layer cell. Several factors were observed to affect activation lifetime, including laser power, purity of mobile phases, filtering, and flow rate. Figure 6 shows a series of chromatograms for GSH at four different times after activation. At 0.8 V, the applied potential is below the mass transport limit with or without laser activation and laser activation yields a significant increase in GSH peak height. Although the peak height decays slowly after activation, it may be restored with additional laser pulses, as shown in Figure 6 for a period covering 23 h. Although the GSH peak height decreased by 58% over a 6-h period after activation, it was completely restored by a single laser pulse. Peak heights are reproducible following subsequent activations within the margin of error expected from injection to injection. The laser also induces an increase in background current, which decays to base line. Apparently the background current is due to activation to surface faradaic reactions which are pronounced a t 0.80 V. The magnitude of the laser-induced background increase depends on potential and detector sensitivity, with larger backgrounds always appearing at higher potential. For low potential or relatively high analyte concentrations, laser-induced background is negligible, but for very low analyte concentrations, the passivation period following laser treatment was significantly shorter than that following polishing, in some cases a few seconds instead of many minutes. From the practical standpoint, the principal advantage of laser activation is the ability to activate analyte redox processes to a greater extent than background processes. The longevity of laser activation depends on the redox system being detected, for reasons related to the electrontransfer rate constant. GSH is a case where E l l z on the activated surface is still far from the thermodynamic E"'. Laser
activation increases k" by several orders of magnitude, but the applied oxidation potential for LCEC detection of GSH is still very positive of E"'. As activity gradually decreases with time, k" decreases and the peak response is reduced. GSH represents a difficult case where LC response is sensitive to k" and therefore to electrode surface stability. A different situation arises with ascorbic acid, for which the activated El,* is close to the thermodynamic E". For the large k" values exhibited by AA on activated GC, the LC response is not charge-transfer-limited and variations in k" no longer affect LCEC response. After laser activation, the surface must deactivate substantially before charge-transfer kinetics again become a factor and the LCEC response suffers. The practical ramification of this effect is variable sensitivity of different redox systems to the time elapsed after electrode activation. For GSH detected at 1.0 V, the LCEC response decreased by 21% during a 130-min period following laser activation. For AA detected at +0.250 V, the LCEC peak height decreased by less than 5% over a 70-min period, and no decrease was observed for 75 min if Eappwas +0.450 V. Thus k" appears to decrease for many systems folowing laser activation, as would be expected from surface adsorption of impurities. However, the LCEC responses of some systems (e.g. GSH) are more sensitive to k" changes than others, and the charge-transfer-limited systems exhibit the greatest variation with time after activation. Even with these difficult systems, however, electrode activity may be restored by an additional laser activation pulse. In situ laser activation of LCEC detectors has objectives similar to those of the electrochemical activation methods for GC (16-24) and metal electrodes (30). While electrochemical pretreatments have the advantage of simplicity and low cost, there are fundamental differences in the effects on surface chemistry. Potential pulses applied to Au and Pt electrodes (30) are very effective for a variety of electrocatalytic reactions, but are not applicable to GC because they rely on reversible oxide film formation. The oxidation of GC is a more complex and irreversible process, with the surface of the oxidized GC currently being uncharacterized a t the molecular scale (31, 32). Of particular importance to the LCEC application is the potentially high background current observed on oxidized GC, which apparently results from surface redox groups (28). In contrast, laser activation reduces oxides and enhances k" by exposing graphitic edge plane (28, 31). Although the comparative efficacy of laser vs electrochemical activation will be application dependent, it is important to recognize that they are fundamentally different processes.
CONCLUSIONS All of the effects of laser activation of GC electrodes used for LCEC result from the enhanced electron-transfer rate following laser treatment. Laser activation is fast and repeatable, and can be carried out in situ without cell disassembly. Enhanced k" permits detection for several example analytes to be conducted at less extreme potentials and may reduce interferences. Although laser activation may increase background current on GC, the increased sensitivity to analyte often more than compensates, and both selectivity and detection limits may be better. For analytes exhibiting high k" on activated GC (e.g. AA), laser activation yields a long-lived improvement in response, while the response for kinetically slow analytes (e.g. GSH) slowly decreases with time. In either case, response can be restored immediately with additional laser pulses. The degree of these improvements will depend on the nature of the analyte, other substances in the sample, and the properties of the mobile phase (pH, solvent, etc.). While not examined here, other electrodes of value for LCEC, such as metals and composite electrodes, may also exhibit improved performance upon laser activation, particularly with
Anal. Chem. 1989, 61, 1993-1996
respect to the removal of passivating films. On a final note regarding practicality, the Nd:YAG laser employed for this work is a versatile research laser with high cost compared to that of the LCEC detector. However, initial experiments with a small Nz laser have demonstrated activation with a much lower cost laser (ca. $3500).
ACKNOWLEDGMENT We acknowledge Jan Pursely of BAS for valuable advice on chromatographic conditions and crucial equipment repairs. LITERATURE CITED Shoup, R. E. I n High Performance Liquid Chromatography;Academic Press: 1986; Vol. 4, p 91. Stulik, K.; Pacakova, V. CRC Crk. Rev. Anal. Chem. 1984, 74, 297. Knecht, L. A.; Guthrie, E. J.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479. Lunte, C. E.; Kissinger, P. T.; Shoup, R. E. Anal. Chem. 1985, 5 7 , 1541. White, J. G.; St. Claire, R. L.; Jorgenson, J. W. Anal. Chem. 1988, 58, 293. Lavrich, C.; Kissinger, P. T. Chromafogr. Sci. 1985, 32, 191. Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1417A. Hu, I . F.; Karweik, D. H.; Kuwana, T. J . Electroanal. Chem. Inferfacia1 Electrochem. 1985, 786, 59. Kumau, G. N.; Wiliis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545. Thorton, D. C.; Corby, K. T.; Spendei, V. A,; Jordan, J.; Robbot, A,; Rustrom, D. J.; Gross, M.; Ritzler, G. Anal. Chem. 1985, 5 7 , 150. Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632.
1993
Fagan, D. T.; Hu, I. F.; Kuwana. T. Anal. Chem. 198$, 5 7 , 2759. Wightman, R. M.; Deakin, M. I?.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J . Electrochem. SOC. 1984, 737, 1578. Miller, C. W.; Karweik, D. H.; Kuwana, T. Anal. Chem. 1981, 53, 2319. Evans, J.; Kuwana, T. Anal. Chem. 1979, 57, 358. Engstrom, R. C. Anal. Chem. 1982, 5 4 , 2310. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136. Wang, J.; Hutchins. L. D. Anal. Chlm. Acta 1985, 5 0 , 1056. Moiroux, J.; Eiving, P. J. Anal. Chem. 1978, 5 0 , 1056. Falat, L.; Cheng, H. Y. J . Elecfmanal. Chem. I n t e r f a c i a l E l e c t m . 1983, 757, 393. Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386. Wang, J. Anal. Chem. 1981, 53, 2280. Wang, J.; Tuzhl. P. Anal. Chem. 1986. 58, 1787. Wang, J.; Lin. M. S. Anal. Chem. 1988, 6 0 , 499. Hershenhart, E.; McCreery, R. L.; Knight, R. D. Anal. Chem. 1984, 56, 2256. Poon, M. J.; McCreery, R. L. Anal. Chem. 1988. 58, 2745. Poon, M. J.; McCreery, R. L. Anal. Chem. 1987, 5 9 , 1615. Poon, M. J.; Engstrom, R. C. McCreery, R. L. Anal. Chem. 1988, 6 0 , 1725. Allison, L. A.; Shoup. R. E. Anal. Chem. 1983, 55, 8. Neuburger, G. C.; Johnson, D. C. Anal. Chem. 1987, 5 9 , 203. Bowling, R. J.; Packard, R. T.; McCreery, R. L. Langmuk 1989, 5 , 663. Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 6 0 , 1459.
RECEIVED for review December 30, 1988. Accepted May 1, 1989. This work was supported by the Air Force Office of Scientific Research and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
Diffusion Apparatus for Trace Level Vapor Generation of Tetramethyllead P. R. Fielden* and G . M. Greenway' Department of Instrumentation and Analytical Science, UMIST, P.O. Box 88, Manchester M60 1 8 0 , U.K. The generation of vapor standards and measurement of the diffusion coefficients of vapors is an important aspect of the development and calibration of methods for trace vapor analysis in atmospheres. Dynamic vapor generation systems can be based on a wide range of production methods such as gas stream mixing (l),injection methods, evaporation and chemical reactions ( 2 ) ,permeation devices (3), diffusion apparatus ( 4 ) , electrolytic methods (5),and gas-phase titrations (6). The toxicity of the compound must be considered when deciding which method should be employed. Usually for toxic compounds the vapor standards required will be of very low concentrations. In this work vapor samples of tetramethyllead (TML) were prepared near the Occupational Exposure Limit of 0.15 mg m-3 (7). Permeation devices and diffusion apparatus are the most appropriate methods for the production of low vapor concentrations; however permeation devices are difficult to develop and construct for toxic compounds because of their basic design (8). Diffusion apparatus provides a simple method for preparing mixtures of vapor-containing atmospheres and determining diffusion data ( 4 ) . The apparatus works by maintaining the liquid phase of a vapor in a reservoir which is kept a t constant temperature. The liquid is then allowed to evaporate and the vapor diffuses through a capillary tube into a flowing gas stream. If the rate of diffusion of the vapor and the flow rate of the diluent gas are known, the vapor concentration in the resultant mixture can be calculated. Present address: Department of Chemistry,University of Hull, Cottingham Road, Hull HU6 7 R X , U.K. 0003-2700/89/0361-1993$01.50/0
More recent designs allow for rapid changes in the concentration of the vapor standards by altering the diffusion path length (9,lO). These methods require syringes or taps to alter the volume of liquid present. This is not suitable for toxic compounds because there is a danger of the pure liquid leaking, and the need for significant volumes of liquid renders such an approach potentially hazardous. Another important consideration when handling toxic compounds is the method of measuring the diffusion rate. The gavimetric method is not always practicable. Alternatively the diffusion rate can be found by monitoring the position of the liquid meniscus in an open precision capillary tube as a function of time (11). The gradient (X)of a graph of the square of the variation in diffusional path length (1 2, vs time is given by
where p is the density of the liquid a t temperature T, P is pressure in diffusion cell at the open end of the capillary (Pa), p is the partial pressure of the diffusing vapor at temperature T (Pa), M is relative molecular mass of the vapor, R is the gas constant (8.314 J K-l mol-'), D is the diffusion coefficient (m2s-l), and T i s the temperature (K). At a fixed temperature and pressure the diffusion rate can be calculated from
S = XAp/21
(2)
where (S) is rate of diffusion of vapor out of the capillary tube (kg s-l), and 1 is diffusion path length (m). A is the cross sectional area of the diffusion tube (m2). 0 1989 American Chemical Society
