Anal. Chem. 1987, 59,2313-2316
model” (18). This model is not intended to describe the actual morphology of the bonded phase, but rather it is used to explain trends in retention that arise from differences in molecular shape of the solute. With this representation, solute molecules fit into “slots” within the bonded phase during retention. Long, narrow PAH are retained longer than square isomers, following the trend of length to breadth ratio of the solutes ( L I B ) (19). The retention of planar and nonplanar PAH is similarly described; planar PAH are retained longer than otherwise similarly shaped nonplanar PAH (e.g. coronene vs. phenanthrophenanthrene) because planar PAH fit into more “slots” and interact more stronglywith the bonded phase than nonplanar PAH. By extention of this model to include phase thickness, thin phases could be viewed as having shallow slots, whereas thick phases (polymeric and long chain monomeric phases) would have deep slots. Because greater solute-bonded phase interaction would be possible with thick phases, greater retention and shape selectivity should result. For decreasing phase thickness (slot depth), solute-bonded phase interaction is reduced as less of the solute fits within the slot.
LITERATURE CITED Karger, B. L.: Gant, R. J.; Hartkopf. A.; Weiner, P. H. J. Chromatogr. 1978, 128, 65-78. Horvath, C.; Melander, W.; Moinar, I . J. Chromafogr. 1978, 125, 129-1 56. Horvath, C.; Melander, W. J. Chromafogr. Sci. 1977, 15, 393-404. Lochmuller, C. H.; Wilder, D. R. J. Chromatogr. Sci. 1979, 17, 574-579. Berendsen, G. E.; de Galan, L. J. Liq. Chromatogr. 1978, 1 , 56 1-586. Berendsen, G. E.; Pikaart, K. A.; de Galan, L. J. Liq. Chromatogr. 1980, 3, 1437-1464. Hennion, M. C.; Picard, C.; Caude, M. J. Chromatogr. 1978, 166, 21-35.
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(8) Cheng, W.; McCown. M. J. Chromatogr. 1985, 318, 173-185. (9) Tanaka, N.; Sakagami, K.; Araki, M. J. Chromafogr. 1980, 199, 327-337. (10) Spacek, P.; Kubin, M.; Vozka, S.; Porsch, B. J. Liq. Chromatogr. 1980, 3 , 1465-1480. (11) Sander, L. C.; Wise, S. A. J. Chromatogr. 1984, 316, 163-181. (12) Martire, D. E.; Boehm, R. E. J. Phys. Chem. 1983, 8 7 , 1045-1062. (13) Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504-510. (14) Sander, L. C.; Wise, S.A. Polynuclear Aromatic Hydrocarbons: Nghth International Symposium on Mechanisms, Methods and Metabolism ; Cooke, M. W., Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1983; pp 1133-1 144. (15) Sander, L. C.; Glinka, C. J.; Wise, S. A. Chemicaily Modified Surfaces: Silanes, Surfaces and Interfaces; Leyden, D. E., Ed.; Gordon and Breach Science Publishers: New York, 1986; pp 431-445. (16) Verzele, M.; Mussche, P. J. Chromatogr. 1983, 254, 117-122. (17) Sander, L. C.; Wise, S.A. Advances in Chromatography; Marcel Dekker: New York, 1986; Vol. 25, pp 139-218. (18) Wise, S. A.; Sander, L. C., HRC CC, J. High Resoiut. Chromatogr. Chromatogr Commun. 1985, 8 , 248-255. (19) Wise, S.A.; Bonnett, W. J.; Guenther, F. R.; May, W. E. J. Chromatogr. Sci. 1981, 19, 457-465. (20) Berendsen, G. E.; de Galan, L. J. Chromatogr. 1980, 196, 21-37. (21) Tchapla, A.; Colin, H.; Guiochon, G., Anal. Chem. 1984, 5 6 , 62 1-625. (22) Sander, L. C.; Giinka, C. J.; Wise, S. A,, unpublished research. (23) Shah, P.; Rogers, L. B.; Fetzer, J. C. J. Chromafogr. 1987, 388, 41 1-419. (24) Giipin, R. K.; Squires, J. A. J. Chromafogr. Sci. 1981, 19, 195-199. (25) Claudy, P.; Letoffe, J. M.; Gaget, C.; Morel, D.; Serpinet, J. J. Chromafogr. 1985, 329, 331-349. (26) Morel, D.; Serpinet, J.; Letoffle, J. M.; Claudy, P. Chromafographia 1988, 2 2 , 103-108.
.
RECEIVED for review March 12,1987. Accepted June 8,1987. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
CORRESPONDENCE Sampler-Sensor for Preconcentration and Quantitation of Atmospheric Hydrogen Sulfide Sir: Increasing concern over atmospheric environmental problems has necessitated the design and development of new analytical techniques in order to introduce alternatives to the presently employed methods. The use of solid sorbents for collection of pollutants in air has gained general acceptance, and criteria for this type of system have often been presented (I). In general practice, solid sorbents are subjected to a desorption process in order to prepare an analyte solution prior to an analytical detection procedure of choice. Therefore, in general, the solid sorbents employed are designed to provide physical and chemical characteristics for efficient takeup of analyte and yet allow effective desorption procedures. This results in a high overall recovery value necessary for required accuracy. Collection of atmospheric H2S on a Cd(I1)-exchanged zeolite as a solid sorbent (2)and application of several analytical techniques as diverse as X-ray fluorescence spectrometry, combustion analysis by nondispersive infrared (IR) measurement, diffuse reflectance Fourier transform infrared (FTIR) spectrometry, visible spectrometry, and photoacoustic spectrometry on both intact solid sorbent and its leached solution after conversion to methylene blue have been studied (3). The present paper reports the initial data on a novel
sampler-sensor that uses a filter paper pretreated system as a solid sorbent sampler having an air channel in the shape of a planar spiral which gives a direct visual readout of low levels of H2S.
EXPERIMENTAL SECTION Sampling Cells. Photographs of two cells used in this study with similar design but different dimensions are shown in Figure 1,where darker regions correspond to air channels. An aluminum mold was used to prepare the cells to provide gas sampling channel when they are applied on the surface of flat solid sorbent. A RTV (room temperature vulcanizing) silicone rubber compound, RTV630, General Electric Co., was employed to prepare the cells. RTV630 polymerizing mixture was applied between the mold and a Plexiglas disk, which had a thickness of 0.70 cm and a diameter of 9.0 cm. The product is polymerized RTV630 as shaped by the mold and attached to the Plexiglas disk. An entrance hole is in the center of the disk and has a diameter of 3.0 mm; an exit hole is on the periphery where the gas channel ends and has a diameter of 1.5 mm. The surface is resilient and when pressed onto the flat surface of the solid sorbent, a spiral gas sampling tunnel is formed starting in the center. A C-clamp was used to press the cell together with sufficient power to prevent any leaks and yet to allow the required flow rate. Cell 1 has an air channel width
0003-2700/87/0359-2313$01.50/0 0 1987 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY. VOL. 59. NO. 18, SEPTEMBER 15.1987
50i.Il 30
Cell 1
00021 M cacti)
100
A
5
10
15
YO 20 "2' 25
3. CaWalbn plotobtahed by ushlgcd 1 and 0.0021 MWII) as the p&malmmsokmon. mW rate was 30 munln; sampllngthm was 1.0 h. Concenbath was varied between 0.097 and 8.14 ppm H,S. Best Im, equalen and conelation coelf~cient( R )are y = 2.32.92 and 0.9900, respectbeiy.
+
FIgw 1. Top view photographs of samplhg cells: (A) call 1. (B) d
2.
in this solution for 3 min, then removed and air-dried. The concentrations of Cd(I1) solutions were 0.0021 M for cell 1and 0.44 M for cell 2. In another set of experiments with cell 2,O.lS M PbCI,, Fisher Certified Reagent, was used as the pretreating medium. Standard H,S Stream.. For low concentration range experiments with cell 1, a gas sampler, Dynacalibrator Model 230-50-2, and permeation devices in both tubular and wafer form were used as supplied by Vici Mehnica Prepurified N, by A h , Inc,was used as carrier gas. In experiments dealing with higher Ha quantities with cell 2,990 ppm H,S in N,was employed as supplied by Linde Specialty Gases, Union Carbide. Sampling Systems and Procedures. For low-concentration experiments,the samplingtrain started with a standard gas stream source, Dynacalibrator 230-50-2.Flow was controlled prior to and following cell 1 to control any leaka. The gas stream was introduced to cell 1through its center, while required humidity was provided by bubbling the carrier gas through water prior to entering the cell. Following the cell exit was a Du Pont Model P-4W constant-flowair sampler that was used to introduce the gas stream into the cell at a desired flow rate by suction. Between the air sampler and the cell, humidity traps of dry ice with isopropyl alcohol and Drierite beds were employed in order to mure that the gas stream entering the Du Pont sampler is dry,avoiding any adverse effects on the internal components of the sampling pump. Samplings were carried out at a constant flow rate of 30 mL/min for 1.0 h, and H,S concentration was varied. For experimentswith higher sample amounts of 990 ppm H,S in N,was used as the standard gas source. Flow was measured and controlled prior to sampling in cell 2. In this case, solid sorbent papers were used immediately following the pretreatment with Cd(I1) solution. without drying. Outlet pressure of the gas source was 1.0 psig or 1.01 atm, and flow rate was 49 mL/min. Samplings were carried out at constant flow rate and H,S concentration and sampling duration was varied. In the experimentswith cell 1, after the samplingstage the tilter paper containing the analyte was exposed to UV radiation from a lamp supplied by Ultra-Violet Products, Inc., Model UVS-12. The path containinganalyte was bright pink, which could be wily differentiated from light purple background. Visual inspection was employed under UV radiation to measure the length of sampling path with analyte by using a topographic map measurer. Measurements were carried out similarly where cell 2 was employed, except that no W light source was needed and a bright yellow spiral was readily observed on white background.
Ha,
L
,inpur g of H2S 2. AInSOM 3.0 sabent mln. obtained by using d l 2 whkh collected 188
of 0.14 cm with a 0.33-em pit& this cell was employed for low amomla of Ha.Cell 2 w88 employed m samplinghigher amounts ofH&thiscell has an air channel width of 0.40cm witha O.€&cm pitch. Both cella had an air channel depth of 0.08 em. Preparation of SoIld Sorbent. Filter papers (11.0 cm),B&A Grade "0" by Allied Chemical were used as support material. An aqueous solution of CdClz~2'/,H,0, Baker analyzed reagent, was prepared as pretreating medium. Filter papers were immersed
RESULTS AND DISCUSSION
Linear correlation was found between the observable length of solid sorbent containing analyte and amount of HzS introduced Calibration plots are given in Figures 3 and 4. With cell 1,concentration was varied with constant flow rate for 1.0 h; concentration and flow rate were constant but sampling
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
Cm
'Ot
501 I 30
I
Y 0
./
*/
/
0.44 M C d ( 1 1 )
/
/ I
500
I
1000
2000
Figure 4. Calibration plot obtained by using cell 2 and 0.44 M Cd(I1) as the pretreatment solution. Flow rate was 49 mL/min; concentration was 990 ppm. Sampling tlme was varied between 1 and 24 min. Best line equation and correlation coefficient ( R ) are y = 0 . 0 3 5 ~-I-1.04 and 0.9961, respectively.
duration was varied with cell 2. In both cases the system had a linear analytical response with respect to the total amount of H2S introduced. The amount of analyte collected has been reported to be independent of inlet concentration and flow rate in a study for takeup of amines on silica gel (4). Presence of similar behavior in our case, however, would require further substantiation by more data. Nevertheless, present results are indicative of such independency, which should prove very useful in providing flexibility for the style of calibration. Cell 1 has higher sensitivity than cell 2 as it would be expected from its dimensions; both range and slope values clearly show this behavior in Figures 3 and 4. In addition to the physical differences between the sampling channels, one should also note that 0.0021 M Cd(I1) solution was used for cell 1, whereas this figure was 0.44 M for cell 2. Thus, these combined properties made cell 1more sensitive. Homogeneity of Cd(I1) on filter papers was checked by analyzing leach solutions with inductively coupled plasma atomic emission spectroscopy (ICP-AES), and a reproducibility of 2.5% or lower standard deviation was found for different portions of a filter paper. Factors affecting sensitivity in these sampling cells are currently under investigation. Dimensions of sampling channel and concentration of Cd(I1) in pretreating solution seem to be two important parameters for sensitivity and dynamic range. Linear analytical behavior for both cells was observed through the entire length of channels up to breakthrough. Therefore, the size of the cell and thus the total path length of the spiral can be varied for the desired upper limit of dynamic range. On the other hand, a cell with a narrower sampling channel will have higher sensitivity where physical limitations such as flow rate should be considered. In a different set of experiments, 0.18 M PbC12 was used as the pretreatment solution for cell 2 where all other parameters were kept the same as before. A brown-black stain was formed and its length was linearly correlated to amount of H2S introduced. The calibration line between 60 and 400 pg H2S had a slope of 0.14 cm/pg H2S, an intercept of 1.47 cm and a correlation coefficient of 0.9961. A typical result is shown in Figure 2, for the higher contrast with filter paper constituted an advantage for photography compared to the yellow stain of the Cd(I1)-H,S system. When Pb(N03)2instead of PbC1, was used, no results were obtained, possibly because of surface oxidation of H2Sby NO3- ions. For all the experiments, viewed under either visible or ultraviolet light, and with Cd(I1) or Pb(II), colored regions persisted for at least 6 months after sampling.
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Humidity was required in all sets of experiments. Previously, Cd(I1) exchanged zeolite was used as a solid sorbent bed for H2S in this laboratory, where presence of humidity caused efficient collection of analyte at the very front part of the bed; but in dry medium, this behavior was not observed although recovery was still quantitative ( 2 ) . The sampling system used in the present study is significantly different than the aforementioned one. Nevertheless, an analogy can be drawn from the behaviors in the two systems and a model for takeup may be suggested for the latter case, in the presence of water with a fairly even distribution of analyte on solid sorbent and a sharp front, which makes measurements of the length of exposed path feasible. For calibration purposes, three filter papers were used and three measurements were made on each after the sampling. Precision for length measurements on each paper was better than 3.0% mean deviation for concentrations higher than 1.0 ppm H2S (2.8 pg of H2S),and values as high as 9.0% mean deviation were obtained for lower concentrationssuch as 0.097 ppm H2S (0.25 pg of H2S). The analysts using the topographic map measurer were rather inexperienced in the manipulation of this device. Improved experience and/or employment of a more repeatable measuring technique should result in higher precision. Precision among filter papers of the same set of three ranged between 1.0% and 7.0% mean deviation. As a better understanding of the factors affecting the process is achieved, better precision figures may be expected. A thorough interference data would be required for the present sampling technique. Such a study was carried out previously in our laboratory where Cd(I1)-exchangedzeolite was used (Z), resulting in no significant interference and/or adverse effects from a number of potential interferant5 with the exception of dimethyl suKde and dimethyl disulfide, which caused recoveries higher than 10070, and exposing of solid sorbent to ozone to yield low recoveries. Some form of CdS is expected to be responsible for the retention of H2S and formation of color in both present and previous ( 2 ) studies, where the styles of introducing the analyte to solid sorbent as well as the support material for active sites, namely zeolite and filter paper, are different. In studies with Cd(I1)-exchanged zeolite, several spectrometric techniques were employed either for leach solution from solid sorbent or in vitro style as in the case of photoacoustic spectrometry (3). The gain in specificity by employing spectrometry is a certain advantage, which the present method is lacking as the human eye has only limited performance in the visible region. However, there are other positive consequences of not using an involved spectrometric determination procedure as wiIl be described below. Any analytical procedure involving the use of solid sorbents for gas pollutants commonly has the following steps: (1) sample collection and preconcentration, (2) desorption of analyte and preparation of a solution, (3) determination of analyte by a suitable method. The total time required for the whole procedure is a sum of these three steps. Particularly, step 2 is a common source of contaminations or losses and human errors. While step 3 can provide competent precision and accuracy figures with the present state of almost perfect analytical instrumentation, errors and irreproducibility in the first two steps would generally degrade the figures of analytical merits. The method suggested in this report provides the elimination of step 2 with obvious advantages. Step 3, on the other hand, takes only a few minutes and has the potential of even higher speeds with a transparent overlay scale, which could be applied directly on colored spiral path. Therefore, the time required for all process or sample throughput is basically a function of sample collection time. In cases where a TWA (time weighted average) value is needed, sample
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Anal. Chem. 1987, 59,2316-2320
collection speed would be commonly limited to 8 h, which will be followed by a fast measurement step, and total analysis time would be limited virtually by step 1. Since no complicated equipment and instruments would be kept occupied, sampling at different sites or by different persons should be achieved in an inexpensive manner. Another advantage is the elimination of problems regarding breakthrough. In most solid sorbents, breakthrough behavior requires an effective control ( I ) , where in the cell proposed in this study, breakthrough occurs when all the path available is colored. Therefore, the analyst can easily observe breakthrough. Upon immediate observation of breakthrough, one may use shorter sampling times or flow rates or a less sensitive sampling cell with larger gas channels and/or higher Cd(I1) concentration on solid sorbent. Since dynamic range seems to be controlled by aforementionedvariables, several cells with various sensitivities can be used at one site for improved dynamic range to avoid breakthrough, which would result in meaningless data. Further studies on this novel sampling device will help in better characterization of the process as well as in achieving better analytical performance. Registry No. CdCl,, 10108-64-2; H2S, 7783-06-4.
LITERATURE CITED (1) Melcher, Richard G.; Langner, Ralph R.; Kagel. Ronald 0. Am. I n d . Hyg. ASSOC. J . 1978, 3 9 , 349-361. ( 2 ) Vasireddy, Sivasankararao; Street, Kenneth W., Jr.; Mark, H. E., Jr. Anal. Chem. 1901, 53, 868-873. (3) Street, Kenneth W., Jr.; Mark, H. E., Jr.; Vasireddy, Sivasankararao; LaRue-Filio, Rebecca A.; Anderson, C. William; Fuller, Michael P.; Simon, Stephen J. Appl. Spectrosc. 1985, 3 9 , 68-72. (4) Wood, G. 0.; Anderson, R. G. Am. I n d , Hyg. Assoc. J . 1975, 3 6 ,
538.
’On leave of absence from the Department of Chemistry, Middle East Technical University, Ankara, Turkey.
Rebecca LaRue 0. Yavuz Ataman’ Daniel P. Hautman Gregory Gerhardt Hans Zimmer H a r r y B. Mark Jr.* Department of Chemistry and the Edison Sensor Technology Center University of Cincinnati Cincinnati, Ohio 45221
RECEIVED for review April 2, 1987. Accepted June 1, 1987. This research was supported in part by the Edison Sensor Technology Center.
AIDS FOR ANALYTICAL CHEMISTS Scintillation-Type Ion Detection for Inductively Coupled Plasma Mass Spectrometry Le-Qun Huang, Shiuh-Jen Jiang, and R. S. Houk* Ames Laboratory-US. Department of Energy and Department of Chemistry, Iowa State University, Ames, Iowa 50011 Inductively coupled plasma mass spectrometry (ICP-MS) has become an important new technique for elemental and isotopic analysis (1-3). In all the ICP-MS instruments constructed to date, the ion signals are monitored by an electron multiplier, which is generally of the Channeltron variety ( 4 ) . Although good analytical performance can certainly be obtained with Channeltron electron multipliers, they do have several undesirable characteristics as detectors for ICP-MS. A Channeltron has a limited lifetime of approximately 1year under normal analytical use. The general use of pulse counting techniques, in which a high bias voltage is applied to the multiplier so that its gain is saturated, probably accelerates the rate of gain loss. The response of a Channeltron is linear up to count rates of approximately 1 x lo6 counts s-l; above this value, calibration curves tend to droop. Deviation from linear response a t high count rates particularly limits the concentration range over which either very large or very small isotope ratios may be determined (5) and can be a source of error when isotope dilution is employed for quantitation (6). Conceivably, the linear range could be improved by employing a segmented Channeltron that permits both analog current measurements and pulse counting a t the same time. There is some evidence that the detector gain can be degraded temporarily by scanning the mass analyzer across an intense peak, e.g., >lo7 counts s-l. If a peak for a trace analyte is monitored soon after the intense peak, this “fatigue”can affect the accuracy with which the trace constituent may be determined or limit the rate at which the spectrum can be scanned (7). Although computer-controlled peak hopping routines can be devised to avoid exposing the detector to the
intense ion beam, this often requires prior knowledge of the major element composition of the sample and can be inconvenient. The scintillation device depicted by items J-L in Figure 1 is an alternate ion detector for mass spectrometry. Its operation is based upon the following sequence of events. Ions from the mass analyzer (G) strike the negative target (J)and generate secondary electrons. These electrons are accelerated from the target (ca. -5 kV) to a thin grounded metal film coated on the surface of a scintillator (K). The film thickness is chosen so that the secondary electrons can penetrate the film and deposit energy in the scintillator. Energy deposition promotes some of the scintillator molecules to excited electronic states. The excited molecules then emit photons at visible wavelengths that are sensed by the photomultiplier (L). This technique is often referred to as Daly detection after its inventor (8-10). A scintillation-type detector offers some potential advantages in the problem areas identified above for Channeltron electron multipliers. A t the very least, the gain of the scintillation detector should not deteriorate with time. In the present work, the analytical figures of merit of a scintillation-type detector are compared with those of a Channeltron for ICP-MS. EXPERIMENTAL SECTION The sampling interface and MS part of the apparatus are depicted in Figure 1. Instrumental components and operating conditions are identified in Table I. ICP-MS Apparatus. The basic features of the ultrasonic nebulizer, ICP, sampling interface, ion optics, mass filter, and vacuum system have been described previously; pertinent al-
0003-2700/87/0359-2316$01.50/0 0 1987 American Chemlcal Society
