Anal. Chem. 1988, 58,1197-1202
1197
Evaluation of Metallic Foils for Preconcentration of Sulfur-Containing Gases with Subsequent Flash Desorption/Flame Photometric Detection R. A. Kagel and S . 0. Farwell*
Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Ag, NI, Pd, Pt, Rh, and W folls were examined for their collective efflclencles toward seven sulfur-containing gases, Le., H,S, CH,SH, CH,SCH,, CH,SSCH,, CS,, COS, and SO,. Lowand sub-part-per-bllllon (v/v) concentrations of these lndlvldual sulfur gases In air were drawn through a fluorocarbon resln cell contalnlng a mounted 30mm X 7-mm X 0.025-mm metal foil. The preconcentrated specles were then thermally desorbed by a controlled pulse of current through the foll. The desorbed sample plug was swept In precleaned zero air from the fluorocarbon resln cell to a flame photometric detector. Sampling flow rate, amblent temperature, sample humidity, and common oxidants were examined for their effects on the collectlon efflclencles of these sulfur compounds on platlnum and palladlum folls. Analytical characterlstlcs of this metal foll collectlon/flash desorptlon/flame photometrlc detector (MFC/FD/FPD) technique Include a sulfur gas detectability of less than 50 pptr (parts per trllllon) (v/v), a response repeatability of at least 95 %, and field portable collection cells and lnstrumentatlon. The resuHs are dlscussed both In terms of potential analytlcal appllcatlons of MFC/FD/FPD and In terms of thelr relationship to characterized models of gas adsorption on solid surfaces.
The determination of airborne sulfur-containing gases at low- and sub-part-per-billion (v/v) concentrations is complicated by three major factors: (a) current lack of a sulfur-selective detector that possesses both adequate direct detectability and field portability, (b) potential interferences due both to the large number of other compounds present in such air samples and to the nonspecific responses of present sulfur detectors, and (c) the affinities of certain sulfur-containing gases toward adsorption, absorption, and chemical reaction. Therefore, quantitative measurements of atmospheric sulfur-containing gases a t these ultratrace levels require an efficient sample preconcentration step prior to the actual detection phase. These sample enrichment techniques have included various types of chemically impregnated filters (1-5), solid adsorbents (6-8), metal-coated glass beads (9, IO),and cryogenic traps (11-14). In addition to these preceding techniques, other systems have been devised for the collection and subsequent quantitative determination of total gaseous sulfur a t low- and sub-part-per-billion (v/v) concentrations (15,16). One such approach developed in our laboratory involved the oxidative conversion of reduced sulfur gases to sulfur dioxide, which was subsequently collected on glass fiber filters (16). The extracts from these filters were then analyzed for sulfur via flash vaporization/flame photometry (FV/FPD) using small platinum boats. Occasionally, a serious positive blank response interference was noted while using this method. Investigation into the source of this blank contamination finally revealed that the high background problems were coincident with the use of carbon disulfide in a nearby laboratory. Thus, in this
instance, carbon disulfide vapors were apparently collected on the surface of the platinum boat during the short time the flash vaporization cell was opened to the laboratory air for application of the liquid filter extract. Further experiments showed that sub-part-per-billion (v/v) concentrations of seven different sulfur gases (i.e., H2S, COS, CH3SH, CS2, CH3SCH3, CH3SSCH3,and SO2) all gave significant flash desorption/ flame photometric detector (FD/FPD) responses when such standards were passed over the Pt foil at 200 mL/min for only 5 min. In addition, these repeatable FD/FPD signals seemed proportional to the sulfur gas concentration and the exposure time. Such preliminary observations prompted the analytical project discussed in this paper. Previous investigations of sulfur gas sorption to metallic surfaces have resulted primarily from efforts to explain sulfur’s poisoning effect on certain catalysts (17), especially platinum (18-20). However, Pierce (21) has described an apparatus that uses the sulfur poisoning effect of hydrogen permeable metals to measure total gaseous sulfur compounds. The chemisorbed sulfur atoms reduced the hydrogen transport rate through a palladium foil and this hydrogen transport rate was monitored either via a katharometer or by a change in the resistivity of the metal itself (22). This paper describes our use of a metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) system to evaluate the sorptive affinities and capacities of nine different metal foils toward the seven sulfur species listed above. The experimental collection efficiencies and compound selectivities are both reported and examined with respect to their theoretical implications. In addition, the potential applications of this MFC/FD/FPD method are discussed.
EXPERIMENTAL SECTION Flash Desorption/Flame Photometric Detection Apparatus. The solid-state electronic control device for the rapid thermal desorption of the sorbates on the metal foils was identical with the unit previously described by Farwell et al. (16). Figure 1 shows a drawing of a typical cell used for sample preconcentration and flash desorption. The top and bottom halves of the cells were cut from a 2.5 cm thick Teflon sheet to form two 4.3-cm X 4.3-cm X 2.5-cm blocks. A 1.3 cm x 3.2 cm X 1 cm deep portion of the bottom half WBS machined out to accommodate the mounted metal foil. Two holes were drilled into the bottom half so that the threaded 2-mm X 5-cm stainless steel posts could be screwed into the cell cavity containing the metal foil. Inlet and outlet 1.5 mm i.d. holes, centered directly over and under the central area where the foil was mounted, were drilled in the top and bottom portions of the cell, respectively. A right angle hole was tapped in the bottom cell exit so as to position the outlet connection on the side of the bottom half rather than straight down between the metal posts. Male l/g in. unions made of Teflon were used for the sample inlet and outlet connections to l/s in. Teflon tubing. The two cell halves were held together with four 4-mm X 7-cm bolts and corresponding nuts. Inside each Teflon cell, four small nuts and washers were used to hold the metal foil in place on two threaded stainless steel posts. The ends of the metal strips were each compressed between two washers, one above and one below the strip with the rounded washer sides toward the metal strip.
0003-2700/86/0358-1197$01.50/00 1986 American Chemical Society
1198
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
n
1
COMPRESSED A'R
I
E%,
MULTIBED
DILUTION
VENT
CARTRIDGES
VENT
DESORPTION
INTEGRATOR
DEVICE PUMP
Figure 2. Block diagram of the MFC/FD/FPD system. Symbols: 0 represents Teflon valves; A represents flow controllers; T represents the permeation devices in the constant temperature bath; solid arrow heads and dotted arrow heads represent the gas flow paths during sample collection and desorption, respectively.
Figure 1. Drawings of the assembled (A) and disassembled (B) MFC/FD cell.
Table I. Metal Foils Investigated minimum thickness, metal purity, % mm Pt
99.95
0.025
Pt Pd Au Ag Rh W
99.9 99.9 99.9 99.9 99.9 99.9
0.025 0.025 0.01 0.025 0.025 0.025
Mo
99.97 99.999 99.9
0.025 0.025 0.025
Sn Ni
supplier Ernest F. Fullam, Inc. Alfa Alfa Alfa Alfa Alfa Ernest F. Fullam, Inc. Alfa Alfa Alfa
optimum desorption voltage, V 12.0 12.0 13.0
10.0 10.0 14.0 10.5 10.0
10.0 14.0
A circular V-shaped sealing gasket was machined into the bottom side of the top Teflon block. The cell was connected to the flash desorption control device via banana plugs. The flame photometric detector was a Meloy Labs Model SA-285 continuous sulfur analyzer. The only modification was a 1m X 0.32 cm i.d. Teflon tube that bypassed the regular inlet solenoid valve and connected directly to the FPD. A HewlettPackard Model 3390A operated in the peak height mode was used to record the linearized signal from the Meloy SA-285. Figure 2 shows a schematic of the overall MFC/FD/FPD instrumentation and corresponding gas calibration system. Metal Foils and Calibration Standards. Table I lists the metal foils used in this study and their purities, thicknesses, commercial sources, and experimental flash desorption voltages. The metal foils were cut to identical strip sizes of 0.7 cm X 3 cm, and 3 mm diameter holes were punched near both ends of these strips to allow them to be connected to the metal posts within the Teflon cell bodies. Upon installation in a cell, each metal foil was repeatedly flash desorbed at a discharge voltage high enough to heat the foil to a bright cherry red glow. After 20 such desorptions, the top half of the cell was attached and the cell was connected to the FPD. The foil cleansing process was then re-
peated until further thermal desorptions yielded no detectable FPD response. Optimum discharge voltages for the thermal desorption of each different metal were selected by exposing a specific metal foil to different sulfur-containing gases and noting the voltage where the second successive desorption after exposure produced an FPD response that was less than 10% of the first desorption's response. This approach gave optimal FD/FPD signal-to-noise ratios for typical Pt, Pd, Rh, Ni, and W foil lifetimes of several thousand thermal desorptions. Because of combined effects due to resistivity, melting point, and foil thickness, the Au and Sn foils in Table I could not be resistively desorbed without breaking. At a minimum desorption voltage of 10.0 V, the Mo and Ag foils could only be used from 10 to approximately 50 times before breaking. Digital pyrometer measurements during flash desorption of three different metal foils gave desorption temperatures of -900 "C for Au, -1300 "C for Pd, and -1500 "C for Pt when the respective discharge voltages corresponded to those listed in Table I. The following sulfur compounds with their corresponding minimum purities were obtained from Matheson Gas Products: HzS, 99.9%; COS, 97.5%; CH,SH, 99.5%; and SOz, 99.9%. The CSz was J. T. Baker Analyzed Reagent Grade with a minimum purity of 99%. The CH3SCH3and CH3SSCH3were Eastman Reagent Grade with respective minimum purities of 98% and 97%. These seven sulfur compounds were used to prepare Teflon-wafer type permeation devices according to recognized procedures (23, 24). The permeation devices were maintained at 30 & 0.01 "C in a constant temperature water bath and were calibrated gravimetrically with a Sauter Model 404 analytical balance. Purified sulfur-free air was prepared by passing compressed laboratory air through a multibed solid adsorbent filter cartridge (13). The standard sulfur gas concentrations were prepared by use of the dilution procedures previously described (13,16,25).Standard gas concentrations in the 1to 20 ppb (v/v) range could be prepared with a precision of approximately &lo% from the gravimetrically calibrated permeation devices whose loss rates ranged from 20 ng of S/min to 200 ng of S/min. For certain sulfur compounds of higher boiling point, e.g., CH3SSCH3,permeation devices having loss rates below 10 ng of S/min were gravimetrically calibrated over a 6-month period. Such low loss rates allowed standard concentrations down to 50 pptr (v/v) to be prepared with a relative precision of approximately &25%. The inability to determine the instantaneous loss rate of any given permeation device using gravimetric calibration was responsible for a minimum inaccuracy of about &lo%,which was the limiting factor in determining the potential compound to compound differences in MFC/FD/FPD responses. The liquid sulfate standards used to calibrate the FD/FPD were prepared from known dilutions of H2S04(J.T. Baker) with distilled-deionized water. The sulfur concentration of the stock sulfate solution was independently verified by sulfur determinations on an inductively coupled plasma atomic emission
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
spectrometer (Applied Research Laboratories Model 35000C) whose sulfur response at 182.04 nm was calibrated with aqueous standards prepared from KzS04. MFC/FD/FPD Procedures. The liquid standard calibration curves were obtained by applying separate 1.0-pL aliquots of the aqueous HzS04standards (1-20 ng of S/wL) via a Hamilton syringe to the precleaned metallic strip mounted in the Teflon cell. Each standard application was allowed to dry for 5 min under a sulfur-free air flow of 200 mL/min and then flash vaporized into the FPD. These aqueous H2S04standards were also used to routinely check the sensitivity of the FPD. Collection of gaseous samples was performed by drawing a known flow rate of sample, usually 1.0 L/min, through a MFC cell that had been previously flash desorbed to a background FPD response level. The sampling lines were then disconnected, the inlet tubing to the FPD was connected to the top Teflon cell fitting, and an excess flow of sulfur-free air was connected to the bottom cell fitting through a tee fitting vented to atmosphere. Once the cell was connected on-line to the FPD, the pump within the Meloy SA-285 pulled the sulfur-free air from the cell into the FPD at a rate of 200 mL/min. Next, the flash desorption device was charged to the desired voltage and subsequently discharged through the circuit containing the metal strip. The pulse of thermally desorbed species was then pulled into the FPD for sulfur-selective quantification. The individual sulfur gas collection efficiencies for the metal foils were obtained in two different ways. In one approach, FD/FPD responses from the aqueous sulfate standards were treated as the 100% collection efficiency value. Then, the collection efficiency for each sulfur gas was computed by comparing the known quantity of sulfur drawn into the MFC cell (as determined from the gaseous standard prepared via the gravimetrically calibrated permeation device) and the quantity of sulfur desorbed from the metal foil (as determined from the FD/FPD calibration curve prepared via aqueous sulfate standards). This procedure yields a combined collection/thermal desorption efficiency value that may differ from the absolute collection efficiency if the recovery from the desorption process is incomplete for either the gaseous or liquid standards. Even when the desorptions are complete both for gaseous and liquid standards, differences between the absolute and experimental collection efficiency values could still occur due to possible selectivity differences in the FPDs response to samples applied to the metal foil as gases or liquids. In the second approach, two MFC cells that were precisely constructed to produce statistically identical FD/FPD response curves were used in series. Gaseous calibration standards of each sulfur compound were drawn through the two cells connected in tandem, first with and then without a metal foil in the first cell, Le., cell no. 1. The reduction in FD/FPD response from cell no. 2 after a metal foil was installed in cell no. 1 was considered to be due to sulfur gas adsorption onto the metal foil in cell no. 1. The ratio of these two responses can be used to calculate the respective collection efficiency since the FD/FPD response curves were linear and their 95% confidence limits for the intercept, as calculated by a & tsB,included the origin. This latter procedure also does not depend on liquid standards. Rather than removing and reinstalling the metal foil in cell no. 1 for each efficiency determination, the simpler alternative of comparing the response from cell no. 2 to the response from cell no. 1gave equivalent efficiency values when the following relationship was used: % collection efficiency =
cell no. 2 response cell no. 1 response
)
x 100
RESULTS AND DISCUSSION Liquid Standard Curve. Table I1 shows typical FD/FPD response data to nanogram quantities of sulfur applied as aqueous H2S04standards to a Pt foil. On the basis of triplicate determinations of these ten standards from 0.15 ng of S to 10.0 ng of S, the resultant liquid standard curve has an unweighted regression equation of y = 232.8~- 14.77 with s b = 4.45, sa = 19.65, and r = 0.9983. Equivalent linear FD/FPD responses were obtained for standards prepared from Na2S04 and (NH,),S04; however, H2S04standards were employed in
1199
Table 11. Representative MFC/FD/FPD Response Data for Liquid and Gaseous Standards standard
amt of S, ng
mean response data, n = 3
H,S04(aq)
0.15 0.36 0.72 1.44 1.98 2.88 4.50 7.88 9.12 10.00 0.8 1.6 3.2 4.4 6.4 8.8 10.0 17.5 20.0 1.4 2.8 4.2 8.0 9.0 12.0 18.0 27.0
33 f 3 85 f 8 155 f 6 298 f 11 451 f 18 615 f 32 1052 f 44 1818 f 99 2156 f 99 2275 f 46 93 f 9 156 f 6 305 f 6 435 f 5 638 f 32 882 f 8 1024 f 6 1807 f 66 2191 f 27 86 f 5 171 f 5 255 f 6 486 f 7 552 f 6 704 f 15 1093 f 13 1661 f 43
H2S(dn
cos(dn
a The respective ng of S values correspond to the amount of sulfur that entered the MFC cell as H2Sor COS.
this work. To achieve maximum response repeatability, it, was imperative to place the solution aliquot of the sulfate standard on the same central portion of the foil and to employ a constant flash desorption voltage. Determination of MFC Collection Efficiencies for Gaseous Standards. Typical MFC/FD/FPD response data for H2Sand COS standards on B Pt foil are also listed in Table 11. The H2S standard curve has an unweighted linear regression equation of y = 1 0 7 . 8 ~- 34.10 with s b = 2.19, s, = 19.4, and r = 0.9983. The conventional linear regression equation for the COS standard curve is y = 61.28~- 5.60 with s b = 0.823, sa = 9.30, and r = 0.9993. As noted above, one method for the determination of the MFC/FD/FPD collection efficiencies of the different sulfur gases is to normalize the slope of the liquid linear standard curve to 100. Then, the normalized slopes of the gaseous linear calibration curves are equivalent to the collection efficiencies for the gases. For example, if the slope of 232.8 for the preceding liquid standard curve is normalized to 100, then the H2Sand COS calibration curves have corresponding normalized slopes of 46 and 26, respectively, and hence yield collection efficiencies of 46% for H2Sand 26% for COS. This method for the determination of gaseous collection efficiencies assumes that a liquid sulfate standard will produce the same FD/FPD response as an equal amount of sulfur deposited on the metallic foil by chemisorption from the gaseous state. The collection efficiencies for the seven sulfur gases on Pt and Pd as determined by the tandem MFC cell method, which does not rely upon liquid standards, were statistically identical with the collection efficiency values obtained from the first method that compares the regressed slopes of gas and liquid calibration curves. Thus, within the measurement imprecision of
