Anal. Chem. 1995,67,3853-3860
Measurement of Gases by a Suppressed Conductometric Capillary - Electrophoresis Separation System -
Pumendu K. Dasgupta* and Satyajii Kar Department of Chemistty and Bicchemistty, Texas Tech Univetsiiy, Lubbock, Texas 79409-lo61
This paper describes the direct measurement of soluble ionogenic atmospheric gases by a suppressed conductometric capillary electrophoresis separation system (SuCCESS). A small circular wire loop is incorporated at the sampling end of a fused silica capillary located immediately at the tip in the same plane as the capillary. When the loop is dipped into a solution and withdrawn, a liquid film is formed on it. The film is in fluid communicationwith the capilla~~ and acts as a microreservoir. When the film end is lifted relative to the destination side, all or part of the film contents can be injected into the capillary. To perform gas sampling, a series of automated operations are conducted with a commercial CE instrument modified in a minor fashion: the film-bearingloop is lowered into a sample chamber, and air is sampled for a preset period of time at a preselected flow rate (typically 1 min at 100 cm3/min). The capillary is then lifted to introduce an aliquot 6-om the film for analysis and then dipped into the running electrolyte source vial, and electrophoresis is commenced. Under the above sampling conditions, 1 ppb SO2 can be detected. The system should be applicable for use with other detection modes and nonaqueous electrolytes. Capillary electrophoresis (CE) and the associated capillary scale technologies are rapidly and profoundly changing the way analytical separations and measurements are carried While the single most important area for these developments has undoubtedly been the separation and quantitation of large bicmolecules,4 the separation/detection of small ions has also received Separation of small ions has thus far been dominated by ion chromatography 0C).9Jo Recently, the most successful IC detection technique has also been shown to be applicable to CE, leading to suppressed conductometric capillary (1) Wu, N.; Peck, T.L.;Webb, A G.; Magin, R.;.I Sweedler, J. V. Anal. Chem. 1994,66, 3849-3857. (2) Jacobson, S. C.; Hergenroder, R ; Moore, A W., Jr.; Ramsey, J. M. Anal. Chem. 1994,66,4127-4132. (3) Schmalzing, D.; Nashabeh, W.; Yao, X-W.; Mhatre, R ; Regnier, F. E.; Afeyan, N. B.; Fuchs, M. Anal. Chem. 1995,67, 606-612. (4) Monnig, C. A; Kennedy, R T. Anal. Chem. 1994,66, 280R-314R (5) Jandik, P.; Bonn, G. K Capillay Electrophoresis of Small Molecules and Ions; VCH: New York, 1993. (6) Benz, N. J.; Fritz, J. S. J. Chromatogr. 1994,671, 437-443. (7)Salimi-Moosavi, H.; Cassidy, R M. Anal. Chem. 1995,67, 1067-1073. (8) Lucy, C. A; McDonald, T. L. Anal. Chem. 1995,67, 1074-1078. (9) Dasgupta, P. K Anal. Chem. 1992,64, 775A-783A (10) Noble, D. Anal. Chem. 1995,67, 205A-208A 0003-2700/95/0367-3853$9.00/0 0 1995 American Chemical Society
electrophoresis separation systems SUCCESS)^^-'^ that can produce low microgram per liter limits of detection &ODs) for a variety of small ions in a robust manner without special efforts toward preconcentration. One of the earliest beneficiaries of IC was the analysis of atmospheric samples, an area that has been of continuing interest to this laboratory. CEbased analyses of atmospheric filter samples have now been reported,’4J5 but in such cases, the analytical technology and the sample collection strategies are not necessarily optimally matched extraction volumes of several milliliters are obligatorily produced with an atmospheric filter sample, while microliter scale samples are adequate for providing the nanoliter scale injections made in CE. Recognizing that relative to particles, atmospheric gases can be sampled more directly and in a microscale, we previously described16a technique in which a microscale membrane-based diffusion scrubber17constitutes an integral part of the separation capillary. A small segment of a porous hydrophobic membrane capillary connected the fused silica separation capillary W C ) to a small length of an “entrance” FSC. A jacket was built around the membrane and air sampled around it, whence analyte gases of interest diffused through the pores and were trapped by the internal electrolyte. Electrophoresis was then commenced. Indirect or direct optical detection was used. Although these detection methods are not as sensitive as suppressed conductometry, respectable LODs could be obtained. The major shortcomings of the technique, however, centered around the membrane itself: the fragility of the membrane, the change in the sample transfer function over prolonged use due to soiling, and the facile evaporation of the internal liquid through the membrane pores (which necessitated a “dry flush” even during the analysis. Recently, we have introduced a liquid droplet or a film as a gas sampling interfa~e.’~J~ Such an interface is not only indefinitely renewable, but it is best deployed in a microscale, and due to the evaporative flux from the droplet/film, the approach of particles is greatly inhibited (cf. difiiophoresis due to Stefan flow).2o In the present paper, we show that a film is readily (11) Dasgupta, P. K; Bao, L. Anal. Chem. 1993,65, 1003-1011. (12)Avdalovic, N.; Pohl, C. A; Rocklin, R D.; Stillian, J. R Anal. Chem. 1993, 65, 1470-1475. (13) Dasgupta, P. K; Bao, L U S . Patent 5,358,612, Oct 25, 1994. (14) Dabek-Zlotorzynska, E.; Dlouhy, J. F. J. Chromatop. 1994,671, 389-395. (15) Dabek-Zlotorzynska, E.; Dlouhy, J. F. J. Chromutogr, 1994,685, 145-155. (16) Bao, L.;Dasgupta, P. K Anal. Chem. 1992,64, 991-996. (17) Dasgupta, P. K ACS Adu. Chem. Ser. 1993,232,41-90. (18) Liu, S.;Dasgupta, P. K Anal. Chem. 1995,67, 2110-2118. (19) Cardoso. A A; Dasgupta, P. K. Anal. Chem. 1995,67, 2562-2566. (20) Hinds, W. C. Aerosol Technology;Wiley: New York, 1982; p 161.
Analytical Chemistry, Vol. 67, No. 21, November 1, 1995 3853
Air Out c3
U
t Wire (100 pm o d . )
j
Schematic diagram of the gas sampling chamber (GSC) (modified "source vial" of Dionex CES-1): (B) gas sample inlet, (S) gas sample outlet, (T) polyethylene tube for reducing the chamber volume. Inset (not to scale) shows the expanded view of the Pt wire loop formed at the tip of the sampling end of the FSC. Figure 1.
coupled to a FSC as a sampling interface for gases. Using sulfur dioxide, an important atmospheric contaminant, as a test gas, we show that a filmcoupled SUCCESScan easily detect single digit part per billion (ppb) levels of this analyte in an 100 an3air sample. EXPERIMENTAL SECTION Equipment. The basic SuCCESS is the same as that described previously.1* A 45 cm long, 75 pm bore FSC equipped with a Na6on membrane suppressor, regenerated by 5 mM HT SO4, and a bflar wire conductance cellz1were used in conjunction with a Dionex CDM-I conductivity detector. An wire loop of 2 mm diameter was formed at the tip of the sampling end of the FSC by using 100pm 0.d. Pt wire, as depicted in Figure 1 (inset). The sample/capillary transport capabilities and the high-voltage 0power supply of a Dionex Model CESl instrument was used for complete automation of the SuCCESS based gas sampling and analysis. This instrument maintains the sampling end of the capillary and the HV electrode affixed to a common head that can make limited but programmable movements in three dimensions. A typical "normal" sequence is to move the sampling end of the running electrolytefilled FSC into a sample vial located in a programmable rotatable turret. The sampling head makes a gasket-based seal with the vial. The sample can be introduced either by (a) electromigration, (b) applying a pneumatic pressure pulse through a port in the head, or (c) grasping the vial, lifting the head along with it, and introducing the sample by gravity. The head is then returned to a "source vial" chamber where the head again makes a seal and dips into the running electrolyte; electrophoresis is then begun. The source vial contains connections that allow refilling with fresh running electrolyte or other wash liquids and pneumatic pressurization for flushing the FSC. In this work, the source vial was used as the gas sampling chamber. Minor changes were made to the source vial chamber to accommodate this, as shown in Figure 1. The bottom port (B) was enlarged and connected to a poly(tetraf3uoroethylene) (FTFE) tube through which the sample (21) Kar, S.; Dasgupta, P. K.; Liu, H.; Hwang, H. Anal. Chem. 1994,66,25372543.
3854 Analytical Chemistry, Vol. 67, No. 21, November 1, 1995
gas entered the poly(viny1edene fluoride) source vial. A polyethylene tube 0 , 9 . 5 mm i.d., was installed to reduce the effective volume of the sampling chamber (the source vial itself is 41.5 mm i.d.). Approximately 7 mm from the top, a flexible poly(viny1 chloride) tube connected tube T to a side port (S) drilled on the side of the source vial as shown. This was connected to a sampling pump or other apparatus (vide infra). For our experiments, the rotatable sample turret contained alternating vials of the liquid used for the sampling film (0.15% Hz02, 44 mM) and the running electrolyte used for the CE run (2 mM NazB407). The standard operating procedure consisted of dipping the sampling head into a Na2B407vial, pressurizing to flush the capillary with the running electrolyte,lifting the sampling head and dipping it into the film-making liquid, withdrawing it, and introducing it into the gas sampling chamber (formerly the source vial). Note that there is no signiticant hydrostatic difference between the film contents and the detector end of the capillary during sampling. Air was sampled immediately after the head sealed itself on the sampling chamber. Following the sampling period, the head was lifted to a height of 10 cm and maintained in that position for a fixed period of time to introduce an aliquot of the film contents into the capillary. The head was then returned to a fresh NazB407 vial and HV (115 kv) applied to begin the electrophoretic run. The calibrant gas generation arrangement is shown in Figure 2. House air was metered through a needle valve and flow meter (typically 70 cm3/min) through sequential columns (A-C) of activated charcoal, silica gel, and soda-lime and entered a thermal equilibration coil (EC)in a stirred (S) water bath (WB) maintained at 30 "C by a 100 W heater (H) and a mercury contact thermoregulator (TR) m o m a s Scienac) under control of relay (R). The thermally equilibrated air was admitted into the glass permeation chamber 0containing a permeation wafer device 0emitting SO2 at a gravimetricallycalibrated rate of 0.27 ng/ min. The SOTbearing air was diluted with dilution air @) (typically 50- 1500 cm3/min) metered through a needle valve and flow meter. It was split in two streams: one proceeded through a needle valve (Nl) and the other through a water-filled bubbler (WFB) and a glass wool trap (G) (to remove any entrained water droplets before being recombined again as the dilution stream. By adjusting N1, the degree of humid~cationof the dilution air stream could be controlled. Part of the diluted SO2 stream was vented to waste (W) controlled by another needle valve (N2). The rest proceeded through the gas sampling chamber (GSC). In some experiments, N2 was fully open, and the desired sampling flow rate was attained by a sampling pump (SP), equipped with its own flow control valve. In other experiments, a primary standard digital bubble meter (PS) (Gilibrator,Gilian Instrument Corp., West Caldwell, NJ) was placed at the exit of the GSC, and the sampling flow was adjusted by controlling the venting rate with N2. In some other experiments, a capacitance-type relative humidity probe (RH) was placed after the GSC to measure the RH of the sample air. All air flow rates cited in this paper are true volume flow rates at the ambient conditions of our experiments, 680 mmHg and 22 "C;these need to be multiplied by a factor of 0.828 for conversion into values at standard temperature and pressure. Unless otherwise stated, gas sampling was conducted at 100 cm3/min for 1min, and the hydrostatic sample introduction period was 20 s.
House air
c
QSC
--I
RH
PS
9
Figure 2. Schematic of SOn gas generation arrangement: (A-C) activated charcoal, silica gel, and soda-lime columns, respectively, (EC) thermal equilibration coil, (S) stir bar, (WB) water bath, (U) glass permeation chamber, (PW) SO2 permeation wafer, (TR) thermoregulator, (R) relay controller, (D)dilution air, (N1 and N2) needle valves, (WFB) water-filled bubbler, (G) glass wool trap, (W) waste, (GSC) gas sampling chamber, (FM) flow meter, (SP) sampling pump, (RH) relative humidity probe, and (PS) primary standard buble meter.
RESULTS AND DISCUSSION Test Gas and Choice of Film-FormingSolution. We chose SO2 as the test gas not only because of its importance as an atmospheric pollutant but also because the performance of the system with SO2in terms of LODs, etc., is likely to represent the lower limit. Positive polarity is used in SUCCESS, and ions electromigrate opposite to the electroosmotic flow. Weaker acid gases like HCOOH have lower mobility anions that elute fast, resulting in more easily detectable peaks relative to sulfate resulting from SOz. Other common acid gases like HONO or HCl have larger diffusion coefficient than SO2 and should thus result in more efficient collection by the film, assuming that the film composition is chosen to be an effective sink for the gas. Experiments with wet effluent diffusion denuders (WEDDs) have shown that HzOz is an efficient absorbing liquid for capturing SOz, wherein the collected gas is oxidized to Initial experiments indicated, however, that 1 mM or lower HZOZ concentrations used with WEDDs are quite insufficient in the present case; the observed signal for 10-100 ppb SO2 increases with increasing HzOz concentrations in the range 1-35 mM. In the present case, the solution contained in the film is essentially stagnant. Only the reagent present on the surface is effective for capturing the analyte. Unlike the WEDD, where the absorber flows down a surface and convective/frictional/turbulent forces can bring new reagent to the surface, here the only motive force for replenishment of the surface reagent is diffusion, a slow process in the liquid phase. Consequently, the absorber reagent concentration used should be higher. However, reagent blank increases with increasing concentration as well; this is detrimental to any type of trace analysis. We have experimented with two different HzOz stock reagents from two different manufacturers: one was 3%and the other 30%in concentration. The presence of sulfate as an impurity is particularly noticeable in the 3% HzOz stock solutions that we have experimented with: after appropriate
dilution, impurity levels are signi6cantly lower in 30% HzOz solutions. The minimum concentration of HzOz necessary to function as a fully effective sink also depends on the concentration of SO2 to be sampled and the sampling duration. Based on our experience related to ambient levels of SOz, we decided on a maximum anticipated SOZconcentration of 50 ppb and a sampling duration of 60 s. A concentration of 45 mM (-0.15%) HzOz was found to be adequate for dealing with these maximum anticipated levels. If higher amounts must be determined, the concentration of HzOz will need to be increased further. If LODs must also be maintained at previous levels, the HzOz used may need to be cleaned to remove residual sulfate (vide infra). Water by itself may serve as a suitable collection medium for some gases, but it is not ideal for collecting SOZ. Aside from lower sensitivity relative to the use of HzOz,in the absence of reactive uptake, the film becomes quickly surface saturated-strong nonlinearity is observed as a function of either sampling time or sample concentration. An alkaline medium, such as the borate solution used as the electrolyte, can also serve as an effective sink for an acidic analyte gas such as SOZ. However, in this case, it is analyzed as sulfite and detected as a monoprotic acid after suppression with consequent loss of sensitivity. Further, the sample can be partially oxidized to sulfate during electrophoresis, resulting in a broad peak that appears at a retention time intermediate for that of sulfite and sulfate, leading to acuities in quantitation. An alkaline absorbent also absorbs COZefficiently, and this results in a large carbonate peak. One other advantage with HzOz as the collecting medium, relative to the use of the running electrolyte for the purpose, is electrostacking. This can effectively occur with a low-ionic-strength,lowconductance m e dium but not with an equal or higher conductance medium23 (i an electrolyte is used for collection, some concentration is bound to occur during sampling due to evaporative losses of the solvent). ~~~
(22) Simon, P. IC;Dasgupta, P. K. Anal. Chem. 1993,65, 1134-1139.
(23)Chien, R-L.;Burgi, D. S. Anal. Chem. 1992,64,489A-496.4.
Analytical Chemistty, Vol. 67, No. 21, November 1, 1995
3855
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observations, we estimate that the radius of curvature is -4 mm. The volume of a spherical cap. V, of radius of curvature Y and height h is given by
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Electropherogramsobtained by introducing sample from the film: (a) water blank, (b) H A blank, (c-e) 20 ppbv Sa sampled with water, 45 mM H202,and 2 mM N&B& as absorber solution, respectively. Flgun 3.
Pigun 4. Photomicrograph of the liquid film formed on the loop. A thin film was made with an aqueous solution of 2.6 mM Malachite Green for easier visualization.The scale is indicated by the diameter of the wire, 100 pm.
The above issues are illustrated in Figure 3, which shows eledropherograms of (a) pure water introduced from the film, (a) 45 mM HZOZ introduced from the film, without sampling SOz, (c) 20 ppb SOZ sampled with the H20 as absorber. (d) 20 ppb SO2 sampled with the HZOZ absorber, and (e) 20 ppb SO2 sampled with 2 mM Na2B407as absorber. Loop Volume and Injection Techniques. F i i 4 shows a photomicrograph of the liquidcontaining loop. The held-up liquid has the shape of a biconvex lens, bulging in the middle to just beyond the dimensions of the capillary. Based on microscopic 3856 Analytical Chemistry, Vol. 67, No. 21, November 1, 1995
We measured the volume of the loop, by measuring the mass of water lost from a small tared water-fled vial upon the insertion and withdrawal of an initially empty wire loop, to be 880 f 70 nL (n = 12). This is in excellent agreement with the value of calculated from eqn 3, where h is 0.2 nun. When the loop is lifted with respect to the destination vial, hydrostatic introduction of the sample occu~s.Several differences with respect to conventional hydrostatic i n j d o n need to be noted. Given the same hydrostatic head, the rate of sample introduction is different in the present case due to surface tension. The rate of sample introduction was evaluated by measuring the peak area resultingfrom intducing a 0.1%Nfldimethylfonnamide solution for different periods of time and optical detection of the resulting signal. For a sample introduction period of up to 90 s, the signal was linearly related to the introduction time (uncertainty of linear slope,
