Anal. Chem. 1990, 62, 1895-1899
Table 11. Comparison of Sensitivities and Detection Limits for Lithium
sensitivity (1% abs), pg
system0 MMLD-GFAA HCL-GFAA HCL-GFAA
detection limit, pg
(ng/mL)*
(ng/rnL)*
ref
4 (0.4) 10 (1)
6 (0.6) 5 (0.5) 1 (0.1)
this system 12 13
4 (0.4)
O Key GFAA, graphite furnace atomic absorption; HCL, hollow cathode lamp; MMLD, multiple mode laser diode. *Based on 10wL samples and 2a of blank.
1.20
,
I I
/
1895
source. Since the background and reference lines are from the same source and are close to the analyte line, accurate correction and measurement are possible. With a solution equal to or greater than 0.2 pg/mL Li, the entire resonance line was absorbed, indicating the laser line width is narrower than the width of the absorption profile. I t has been shown that peak ratio measurement gave superior signal precision with the MMLD-flame AA system (IO);this method of measurement was used in this system. Table I1 shows the detection limit and the sensitivity obtained with this system, for lithium, and the limit and sensitivity are compared with those of conventional HCL-GFAA systems. Similar values are seen (Table 11). Figure 3 shows the calibration curve. The linear dynamic range is approximately 2 orders of magnitude. If several laser diodes are used, simultaneous multielement AA measurement is feasible; this implementation is much simpler using coherent laser radiation than using incoherent hollow cathode lamp radiation. The limitation of the present system is that laser diodes are currently only available with wavelengths suitable for a few elements. Shorter wavelength laser diodes, however, will soon be available. LITERATURE C I T E D
/
/
Walsh, A. Spectrochim. Acta 1955, 7, 108. Sullivan, J. V. Prog. Anal. At. Spectrosc. 1981, 4 , 311. Jones, B. T.; Smith, B. W.; Winefordner, J. D. Anal. Chem. 1989, 67, 1670.
00
20.0
40 0
60
0
80 0
L'vov, B. V. Spectrochim. Acta 1961, 77, 761. Slavin, W.; Manning, D. C. Prog. Anal. At. Spectrosc. 1982, 5 , 243. Kahn, H. L. At. Absorpt. Newsl. 1968, 7, 40. Smith, S. 8.;Hieftje, G. M. Appl. Spectrosc. 1983, 37, 419. De Loos-Vollebregt, M. T. C.; de Galan, L. Prog. Anal. At. Spectrosc.
100 0
LI c o n c e n t r a t i o n (ng/rnl)
1985, 8 , 47.
Hergenrcder, R.; Niemax, K. Spectrochim. Acta 1988, 438, 1443. Ng, K. C.; Ali, A. H.; Barber, T. E.; Winefordner, J. D. Unpubllshed work. Ng, K. C.; Ali, A. H.; Barber, T. E.; Winefordner, J. D. Appl. Spectrosc., in press. Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice-Hall, Inc.: Englewocd Cliffs, NJ, 1988; p 300. Analytical Values in Flame AA , Furnace AA and ICP; Thermcd Jarrell Ash, 1987.
Flgure 3. Lithium (10 pL sample volume) analytical calibration curve
for the diode laser graphite furnace atomic absorption system. the laser radiation and subtracted to measure the net absorption. R E S U L T S AND DISCUSSION Figure 2 shows the spectral lines of the MMLD radiation, with one of the lines at the resonance wavelength of lithium (670.780nm). The lines were positioned by tuning the laser diode temperature and/or the current. The intensity of each line was wavelength dependent; the most intense line (mode) could not be tuned to the desired wavelength position, nor could the lines (modes) be tuned to obtain equivalent intensity. However, the spacing (-0.3 nm) between lines did not appear to change with tuning. The lines adjacent to the lithium line can be realized for simultaneous background correction and for peak ratio measurement. The real-time simultaneous background correction is beneficial since the GF background and/or atomic absorptions may change with time, and the simultaneous peak ratio measurement can improve signal precision by compensating for the fluctuation in the radiation
* Author to whom correspondence should be sent. On leave from Department of Chemistry, California State University at Fresno, Fresno, CA 93740-0070.
'
K i n C. Ng' Abdalla H.Ali Tye E. B a r b e r J a m e s D. Winefordner* Department of Chemistry University of Florida Gainesville. Florida 32611
RECEIVED for review March 5,1990. Accepted May 18,1990. This research was supported by NIH-5-R01-GM38434-03.
Surface Acoustic Wave Detector for Screening Molecular Recognition by Gas Chromatography Sir: The generation of interfacial structures capable of molecular recognition at the surfaces of chemical and biological sensors is a crucial element in the fabrication of these devices. For biosensors, the approach to achieving the desired selectivity has been almost exclusively through the use of particular species such as antibodies, enzymes, and molecular receptors extracted from biological milieu (1). For chemical sensors designed to detect organic molecular species there has to date been a heavy reliance placed on both conventional adsorption 0003-2700/90/0362-1895$02.50/0
at solid interfaces and partitioning of the analyte into stationary-phase-like films (2). We are interested in the study of selective molecular recognition at interfaces through specific functional group interactions between gas-phase analyte molecules and chemically modified surfaces. The aim of this approach is to achieve the rational design of chemical sensing devices through a better understanding of the molecular recognitive process combined with computational methods for structure comparison, host-guest docking, and molecular 0 1990 American Chemical Society
1886
ANALYTICAL CHEMISTRY, VOC. 62, NO, 17, SEPTEMBER 1. 1990
dynamics studies ( 3 ) . In a previous report we described the use of a surface acoustic wave (SAW) device in combination with molecular modeling to study the selective interactions between a number of nitroaromatic species and surface-immobilized alkylamino functional groups (4). It was found that the exoerimentallv observed selectivitv of bindina- could be explained in terms of the strength and number of hydrogen bonds formed hetween the analyte and the surface 'receptor" site. As an extension of this work, we report here the design of a gas chromatograph (GC) with SAW detector. This provides the potential for the rapid screening of chemically modified surfaces for selective binding against a large number of gas-phase analytes. We describe the instrumentation and preliminary results for the adsorption of isopropyl alcohol to a bare (unmodified) quartz surface. The generation of acoustic waves in piezoelectric materials has been discussed by various authors (5-7). The theory and application of SAW devices have also been extensively reviewed (6-11) and will not be detailed here except as relevant to the instrument design and discussion. Most of the SAW devices used as chemical sensors employ a delay line configuration in which a Rayleigh wave is generated by one interdigital transducer (IDT) and received by a second. A feedback amplifier is connected between the two IDTs to form an oscillator circuit. The fundamental resonant frequency (fo) of the oscillator is determined by the acoustic propagation velocity (v0) and the acoustic wavelength (A)
fo = uo/x
(1)
In order to provide temperature and pressure compensation, it is usual to employ a dual-delay line configuration in which one oscillator is used as the sample and one the reference channel. The difference in frequency between the two is then measured
Af =
IU0)enmple
- (f0)reremel
(2)
In order for the electrical circuit to work, the phase condition for the oscillator must be satisfied. This is expressed as (wt/u) *E = 2nr (3)
+
where L is the distance between IDT centers for one delay line, w is the frequency (rad 5-9, h i s the amplifier phase shift, and n is an integer.
INSTRUMENTATION Several requirements were identified for the design of the SAW detector. These included the ability to control the temperature of the device and to distinguish between specific and nonspecific adsorption effects, the minimization of band-broadening within the detector, and the ability to compare the response with a second detector, e.g. a flame ionization detector (FID), to confirm elution for those compounds which might give no or only a partial response to the SAW detector. These requirements led to the following design criteria: (1) the gas volume in contact with the device should be a minimum: (2) it should be possible to control the temperature of the device to within at least 1 "C to a maximum of about 200 "C; (3) the sample and reference channels must he kept separate; ( 4 ) the housing must incorporate all necessary electrical connections: and (5)the electrical connections and IDTs must not be exposed to the gas flow. Criteria 3 and 5 became necessary even though they present significant technical problems because of the dual-delay line configuration and the nature of the experiments envisaged. In current SAW sensors, an adsorbent film is coated onto one side of the device and the analyte stream allowed to pass over the entire surface. This means that the observed shift in Af on analyte adsorption actually represents differential adsorption of analyte into the film with respect to nonspecific adsorption to the uncoated
Flgure 1. Exploded view of the SAW detector housing. Cables and electrical connections are omiued for chrity. See text for details.
reference channel. Adsorption to the IDTs may also occur. For our purposes this represents an undesirable and potentially misleading complication, especially since saturation of surface binding sites will occur a t lower analyte concentrations than would be the case for adsorbent film coatings. Further, the protocols for chemical modification of the SAW crystal surface will not work on the gold electrodes of the IDTs. Our design differs from that of Wohltjen and Dessy (22,13)in that they used single delay line devices, the entire surface area of the crystal being exposed to the gas stream. (These authors also used LiNb03 dual delay line devices, but both delay lines were connected in parallel.) The detector housing is shown in exploded view in Figure 1 and has overall dimensions of 2 in X 4 in X 1.5 in. A gasket was used to keep the sample and reference gas flows separate and confined to an area of 5 x 5 mm (approximately 40 pL volume) between the IDTs. This represents a compromise between device sensitivity and exposing the IDTs to the sample stream. The lid, base, and main body were all staides4 steel. A shallow channel was machined in the main body to receive the SAW crystal, the top of the crystal being flush with the connector blocks. The inlet and outlet tubing was matched to the column internal diameter and connecting tubes in the GC (0.3 mm id.) and connected with zero dead volume '/le in. Upchurch fittings. The gaskets were cut from either silicone or Viton rubber (Ontario Rubber, Rexdale, ON) using a steel rule die. Temperature was controlled by a Model CN9111 controller (Omega Engineering, Inc., Stamford, CT) connected to a Chromalox CIR1012 heating cartridge (100 W) and a copper-constantin thermocouple mounted directly helow the crystal. The temperature was read to *O.l "C. The power delivered by the heating cartridge was limited by two 100-R resistors connected in series to provide better temperature stability. The SAW crystals (Microsensor Systems, Inc., Springfield, VA) were ST-cut, X-propagating quartz dual-delay line devices operating a t 52 MHz. The necessary electrical circuitry was provided by a CEM-52 unit from the same source and allowed the sample, reference, and difference frequencies to be monitored. This was modified by removing the sample cell from the circuit board and installing 5 0 4 BNC connections. The cables and electrical connections were mounted in Teflon blocks and held in place using glass-filled Teflon clamps. Electrical connections were made with RG58AU 5 0 4 hightemperature cable (Anixter-Turmac, Cooksville, ON). Impedance matching was accomplished with 0.47-pH rf chokes
ANALYTICAL CHEMISTRY. VOL. 62. NO, 17. SEPTEMBER 1. 1990 REFERENCE I
FrequencylHz)
Flgure 2. Gas flow system for steady-state measurements: R I . R2. R3. rotameter flow guages; V1. six-port switching valve.
mounted inside the connector blocks. At high frequencies, the length of connecting cable becomes important since, unless it is very much less than or very much greater than the wavelength of the electrical signal, it makes a significant contribution to aE.In order for eq 3 to hold, the cable length must be equal to a whole number of wavelengths. At 52 MHz (correcting for the propagation velocity of the particular cable used-69% of the speed of light), the electromagnetic wavelength is 3.98 m. Therefore 1.99-m lengths of cable were used to connect each end of each delay line to the rest of the electrical circuit. EXPERIMENTAL S E C T I O N Apparatus and Chemicals. For all measurements the SAW detector was placed in the detector oven of a Varian Model 2700 gas chromatograph. This had been modified for capillary GC, as described previously (4). The sample material was isopropyl alcohol (IPA) (>99.5% purity, BDH) which was used as received. Frequency measurements were made with a Hewlett-Packard HP5328A counter connected to an Apple Macintosh I1 computer via a National Instruments GPIB interface bus. The necessary software was written and compiled by using Microsoft QuickBASIC. This included SavitzkyGolay filteringof the data where appropriate and is designated in the figures below by XaYZ where X is the number of points included in the filter "window" and Y, 2 the order of the polynomial and differentialused,respectively. S t e a d y s t a t e Measurements. For steady-state adsorption measurements. the SAW detector was connected to the gas flow system shown in Figure 2. All connections in the gas flow system were made with in. 0.d. Teflon tubing and Swagelok connectors. Argon was used as the carrier gas. Moisture and hydrocarbon traps were placed in-line before the inlet regulator. Experiments were performed at ambient and elevated (65 "C) temperatures and at ambient temperature with diluent flow. The temperature of the SAW detector was continuously monitored in all cases. All flow rates were measured with a soap bubble flowmeter. A t ambient temperature the sample and reference gas flow rates were 20.8 and 26.3 mL mi&, respectively, while at elevated temperature they were 16.9 and 11.0 mL min-'. For a diluent flow of 6.3 mL min-' the sample and reference flows were 11.0 and 25.3 mL min-I, respectively. It was found that the temperature recorded by the thermocouple under ambient conditions (no heating applied) stabilized at 29 'C (8 "C above room temperature). This was attributed to the heating effect of the acoustic energy dissipated within the crystal. GC Measurements. For GC measurements, the sample channel of the SAW detector was connected to the column outlet after the make-up gas connection. The reference channel was connected to a 50 cm length of stainless steel capillary tubing mounted in the second column position. The FID was not connected. Replicate injections were made of 1pL of IPA without split injection using nitrogen as the carrier gas. The column flow rate was 1 mL min-' and the make-up flow rate 35 mL min-'. Injector and column temperatures were 125 and 120 O C , respectively. The detector was operated without heating and stabilized at 30 "C. No attempt was made to optimize either the
1897
Time lrnln)
FreauencvlHz)
10
14
1s
22
Smoothlng factor Is: 2 % 3 0
26
2
T i m e lrnln)
Frequency I H z )
24100
I
15
37
19
41
41
I
45
factor Is: Sa 2 0 Tlme l m l n ) Adswption profiles for IPA (a)at r m temperature. (b) Wim diiuent flow, and (c) at elevated temperature. See text for details. Srnoothlnq
Figure 3.
detection or chromatographic conditions. RESULTS AND DISCUSSION Steady-State Measurements. Results for the adsorption of isopropyl alcohol (IPA) onto the bare quartz surface of the SAW crystal plotted as Af (Hz) against time (min) are shown in Figure 3. For the undiluted IPA sample stream, the ohserved shift in Af was typically 12 kHz at 29 "C and 550 Hz at 65 "C. For the dilute sample stream, the observed shift was 1.45 kHz. The large increase in the baseline value of Af between Figures 3a and 3b can be attributed to the difficulty of ohtaining a reproducible and even clamping action of the gasket across the crystal. Uneven clamping will result in different stresses being applied to each side of the crystal, changing the observed values of f o for the sample and reference channels and so changing A f . In some cases, the shift in A f was such that it suggested either the loss of the fundamental oscillation
1898
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
frequencies (Af equivalent to the difference between the second harmonic frequencies of each channel) or “modehopping” of one or both channels (non-Rayleigh wave acoustic propagation). The difference in flow rate between the two experiments was found to have a negligible effect on 4relative to the size of the shifts observed for IPA adsorption. Figure 3c shows that the shift in Af on exposure to IPA vapor at elevated temperature is in the opposite direction to those observed at ambient temperature. This was probably caused by a switch in the magnitude of the sample and reference channel frequencies. The crystal was initially connected so that the lower frequency side was the sample channel, giving an increase in Af on adsorption of analyte (eq 2). The lid of the detector housing was not clamped until after the temperature had stabilized to allow for thermal expansion. If the clamping was uneven so that (fO)sample was larger than (fO)reference, then Af would decrease. The signal was also noisier than that observed for the equivalent measurement at ambient temperature. The variation of temperature with frequency for X-propagating ST-cut quartz SAW devices can be described by a quadratic function (14) with negative coefficients, Le., frequency decreases with increasing temperature. Therefore at higher temperatures, the values of fo for each channel are more sensitive to small fluctuations in temperature. In principal, the dual delay line configuration should compensate for such fluctuations since both channels will experience the same effects so that the value of Af should remain constant. In practice this is never fully achieved even at room temperature where f,, is essentially independent of temperature (15). One reason for this is that unless the crystal is thin, temperature changes may not equilibrate across the crystal very quickly. Another reason is that applied stress changes the temperature coefficients of frequency (16) so that a differential stress present across the SAW devices used in this study would have a significant effect on the temperature stability of Af. There are a number of methods for improving the temperature stability of SAW oscillators (16),some of which are currently being tested for suitability to the detector configuration described here, including the use of an external oscillator, parallel acoustic paths across the SAW crystal, and improved control of the detector temperature. The large “spikes“ observed in the adsorption/desorption profiles (eg. Figure 3b, at 13 and 23 min) were in part caused by the pressure fluctuations occurring when valve V1 was switched. Similar spikes were also observed however when the valve was not switched (Figure 3b, 20 and 28 min). These spikes were caused by movement in the vicinity of the detector and were found to be due to insufficient electrostatic screening of the housing, even when it was placed inside the GC detector oven. This problem was overcome by using a second metal enclosure within the GC detector oven. Because of the design of the SAW crystal and the high frequency of operation, there is always some degree of coupling between the two channels. This coupling results in the two channels “pulling in” to one another until, when Af is small ( < 2 kHz), the two channels simply lock together ( A i becomes zero). Coupling only becomes a serious problem when the expermental setup is such that changes in coupling (due to relative movements of the connecting cables, for example) can occur during the course of a measurement. It is therefore important to fix all the components of the system firmly in place. GC Measurements. A typical IPA peak is shown in Figure 4. As can be seen, the peak was observed even though the signal was somewhat noisy and subject to drift. Thermal drift of the rf electronics was found to be a major factor in this case, as well as the noise sources noted above. It was also found that the baseline stabilized slowly following large changes in
Frequency (Hz) 6840
6805
6770
6735
6700
1
I 51
51 8
52 6
53 4
Smoothing f a c t o r is. 25a30
54 2
55
Time ( m i n )
Figure 4. Typical peak profile for IPA. See text for details.
temperature or flow rate (gas pressure) or clamping of the detector lid, making it important to allow sufficient time for thorough thermal and mechanical equilibration prior to any measurements. Baseline noise levels were estimated from the peak-to-peak deviation from a straight line of the original data. Short-term noise ( 5 min) was 36 Hz, giving signal-to-noise (S/N) ratios of 9.2 and 3.3, respectively. This represents the limit of detection (LOD) if the longer term noise component is taken into account. A much better LOD could be obtained by reducing the noise and increasing the SAW crystal sensitivity. An increase in sensitivity could be achieved either by increasing the area of the SAW crystal exposed to the gas stream or by using a higher frequency device (17,18). The former would be difficult to achieve without exposing the IDTs to the gas flow and increasing the band-broadening contributed by the detector. The latter would result in a smaller area making chemical modification of the surface more difficult due to problems of scale, although a moderate increase in device frequency might yield a satisfactory increase in sensitivity without greatly decreasing the available area. A 1-pL sample volume represents significant overloading of the column, so it is essential to decrease this. This would require a reduced S / N ratio and increased sensitivity, as discussed above. Alternatively, a larger diameter column could be used. A capillary column would give the highest resolving power, while a packed column would allow greater sample loading. We have recently switched to a megabore capillary column, which allows a higher sample loading than the capillary column described above while retaining high column efficiency.
SUMMARY The SAW detector configuration described in this paper can be used to measure both steady-state and transient adsorption processes. Although the experimental system is not yet fully optimized, preliminary measurements on the adsorption of simple alcohols to bare and chemically modified quartz crystals (not reported here) suggest that studies of the rates of adsorption and desorption together with the size and direction of the observed relative frequency shifts can be used to probe the nature of different surface sites of the quartz in a manner analogous to recent studies using other techniques (19,ZO). The ability to control the temperature of the SAW device is particularly important in this respect. Further work is currently underway to improve the overall performance of the detector system, particularly with respect to improving the signal-to-noise ratio. This will allow steady-state and
Anal. Chem. 1990, 62, 1899-1902
transient adsorption studies to be performed on both bare and chemically modified quartz surfaces. The use of a GC configuration coupled with the resolving power of capillary columns provides the potential for the rapid screening of chemically modified surfaces for molecular recognition. Further work in this direction is currently in progress. Registry No. IPA, 67-63-0; silica, 60676-86-0.
LITERATURE CITED Blosensors: Fundemenfals and Appllcafions; Turner, A. P. F., Karube, I., Wilson, G. S., Eds.; Oxford University Press: Oxford, U. K., 1987. ChemiCel Sensor Technology; Seiyama, T., Ed.; Elsevier: New York, 1989; Vol. 1 and 2. Thompson, M.; Frank, M. D.; Heckl, W. M.; Marassi, F. M.; Vlgmond, S. J. I n Chemical Sensor Techno&y; Seiyama T., Ed.; Elsevier: New Yark. 2. rr DO _ 237-254. . -..., 1888: . _ _ _ Val. ,. -, _ .. Hecki, W. M.; Marassi, F. M.; Kallury, K. M. R.; Stone, D. C.; Thompson. M. Anal. Chem. 1990. 62. 32-37. Auld, B. A. Acoustic Fieus and Waves in Sol&; Wiley-Interscience: New York, 1973; Vol. 1 and 2. Dana, S. Surface Acoustic Wave Devices ; Prentlce-Hall: Enalewood Cliffs, NJ, 1986. Ristic, V. M. Rinclples of Acoustic Devlces; Why-Interscience: New Yark. . -. .., 1883. .- - -. D'Amico, A.; Verona, E. Sens. Actuators 1989. 77, 155-166. Bailantine, D. S.; Wohltjen, H. Anal. Chem. 1989, 67, 704A-715A. Fox, C. G.; Alder, J. F. Anelyst 1989, 774, 997-1004. Nieuwenhulzen, M. S.; Venema, A. Sensors Materials 1989, 5 , 261-300.
__
1899
(12) Wohltjen, H.; Dessy, R. Anai. Chem. 1979, 57, 1458-1464. (13) Wohltjen, H.; Dessy, R. Anal. C h m . 1979. 57, 1465-1470. (14) Hauden, D.; Michel, M.; Bardeche, G.; Gagnepain, J.J. Appl. Phys. Lett. 1977, 37,315-317. (15) Aider, J. F.; Fox, C. G.; Przybylko, A. R. M.; Rezgui, N.-D. D.; Snook, R. D. Analyst 1989, 714, 1183-1185. (16) Lewis, M, IEEE s,,,,,p, (Roc,) ,979, 612-622, (17) Wohltjen, H. Sens. Acfuators 1984, 5 , 307-325. (18) Wohltjen, H.; Snow, A.; Ballantine, D. I€€€ Unrason. Symp. (Roc .) 1985. 66-72. (19) Airokk C.; Santos, L. S., Jr. Thermochlm. Acta 1988, 704, 111-119. (20) Bernstein, T.; Michel, D.; Pfeifer, H.; Fink, P. J. Collokl Interface Sci. 1981, 84, 310-317.
Michael Thompson* David C. Stone Department of Chemistry University of Toronto 80 St. George Street Toronto, Ontario M5S 1Al Canada RECEIVEDfor review January 25,1990. Accepted May 17,1990. Support from the Institute of Chemical Science and Technology, Canada, and the Natural Sciences and Engineering Research Council of Canada for this work is gratefully acknowledged.
TECHNICAL NOTES Evaluation of Aluminum Canisters for the Collection and Storage of Air Toxics Alex R. Gholson, R. K. M. Jayanty,* and Julia F. Storm' Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709 Whole air collection techniques for determining volatile organic compounds (VOCs) in air have been widely used to study the effects of VOCs on atmospheric photochemistry and to monitor for toxic compounds in indoor environments, in the vicinity of point and area sources, and in ambient air. The passivated stainless steel canister has become the container of choice for whole air sampling because of the low background, ruggedness, and storage stabilities for most organic compounds (1-3). Limitations in the use of these containers include the surface reactivity with many oxygen-, nitrogen-, and sulfur-containing compounds, which results in significant wall losses, and the relatively high cost of the containers. As part of an audit material development program for hazardous waste incineration, the stability of 25 toxic organic compounds in high-pressure aluminum cylinders has been documented for up to 5 years a t a concentration as low as 5 ppbv. Of the organic compounds in the aluminum cylinder, five contained either nitrogen or oxygen (acetone, 1,4-dioxane, 2-butanone, acetonitrile, and acrylonitrile) and were found to be stable. Two oxygen-containing compounds, ethylene oxide and propylene oxide, were found to be unstable in the aluminum cylinder (4). A recent study showed that aluminum gas sampling loops provided better results for sampling oxygenated organics at the parts per billion by volume (ppbv) concentration than stainless steel loops (5). The stability of a compound in a high-pressure cylinder and in a flowing stream of gas in a sampling loop are substantially different than in a static, low-pressure (100-200 kPa) air sample. Current address: State of North Carolina, Division of Environmental Management, P.O. Box 27687, Raleigh, NC 27611. 0003-2700/90/0362-1899$02.50/0
Nevertheless, the findings suggest that aluminum may be a good material for sampling organic compounds at the part per billion level. This work presents the results of stability studies for 23 organic compounds in aluminum canisters and passivated aluminum canisters. Stability in the aluminum canister is a function of (1) surface reactions between the analytes and the canister walls, (2) reactions between analytes, and (3) reactions between analytes and other compounds present in the sample. To evaluate the aluminum canisters, the effects of the second and third mechanisms were minimized by using standard gas mixtures containing relatively nonreactive analytes prepared in either nitrogen or air. Because of the effect seen with water on passivated stainless steel canisters, water was added to some of the canisters. Water is believed to compete for active sites on the walls of the canisters, helping to passivate the surface. The amounts of water added was not great enough to result in condensation loss of the polar organics.
EXPERIMENTAL SECTION Canisters. Four spherical, 6-L aluminum canisters and one spherical, 6-L stainless steel canister provided by Andersen Samplers, Inc., were used for this evaluation. Two of the aluminum canisters and the stainless steel canister were passivated with the Summa process (Molectrics Corp.). A stainless steel bellows valve was fitted at the opening in the top of each canister. All canisters were cleaned with a series of evacuations and Nz pressurization at 150 "C followed by a pressurization with humidified N2 The canisters were then evacuated, refilled with N,, and blank checked by analyzing approximately 300 mL of N2from the canister. Sample Preparation. Stability studies were performed by using two gaseous mixtures of organic compounds. Samples were 0 1990 American Chemical Society
