Anal. Chem. 1986, 58, 2811-2813
region for the component. However, when overlap was severe, as in the PdEtio, the minimum overlap criterion obtained the best estimate. In this paper, we have shown that when only one of the two vectors describing a pure component is known, the other vector lies in a subset of possible solutions of the corresponding eigenvector space. Further, it is shown that the pseudoinverse of the given vector with the matrix is one possible solution. However, if the sample matrix has a zero base line, then better estimates of the vector can be made by minimization of the fraction of negative elements in the residual matrix or the overlap between the derived vector and the residual matrix. The general procedures were successfully demonstrated for a synthetic two-component mixture. The algorithms were then tested on a real three-component mixture of porphyrins. When only one of the components is known, the results show that both the minimum negativity and the minimum overlap criteria provide more reasonable distributions and better estimates for the concentration than the pseudoinverse method for any of the three components. Our results are consistent with the work of Vandeginste et al. (9, l o ) , in which it was found that the concept of minimal overlap could act as a powerful constraint in the deconvolution of two-dimensional multicomponent data sets. We should point out as well that the model used by Harris and colleagues (7, 8) also strongly enforces this constraint. The reader is cautioned that the minimum overlap constraint is most likely to be valid when the spectrum is known but the retention behavior is unknown. Fortunately, this is the case most likely to occur in practice.
ACKNOWLEDGMENT The authors wish to thank E. R. Davidson and D. B. Skoropinski for many helpful discussions.
2811
LITERATURE CITED (1) Warner, Isiah M.; Davidson, Ernest R.; Christian, Gary D. Anal. Chem. 1977, 49, 2155. (2) Lawson, Charles L.; Hanson, Robert J. Solving Least Squares Problems; Prentice-Hail: Engiewood Cliffs, NJ, 1974. (3) . . Burns, David H.; Callis, James B.; Christian, Gary D. Anal. Chem. 1988, 58, 1415. (4) Lawton, C. J.; Sylvester, E. R. Technometrics 1971, 13, 617. (5) Warner, Isiah M.; Christian, Gary D.; Davidson, Ernest R.; Caiiis, James B. Anal. Chem. 1977. 4 9 . 564. (6) Sharaf,Muhammad, A.; Kowaiski; Bruce R. Anal. Chem. 1982, 5 4 , 1291. (7) Knorr, F. J.; Thorsheim, H. R.; Harris, Joel M. Anal. Chem. 1981, 53, 821. (8) Frans, S. D.; McConneii, M. L.; Harris, Joel M. Anal. Chem. 1985, 57, 1552. (9) Vandeginste, Bernard; Essers, Raymond; Bosman, Theo; Reijnen, Joost; Kateman, Gerrit Anal. Chem. 1985, 57, 971-985. (10) Vandeginste, Bernard; Derks, Wiibert; Kateman, Gerrit Anal. Chim. Acta 1985, 173, 253-264. (11) Gemperiine, Paul J. J . Chem. I n f . Comput. Sci. 1984, 2 4 , 206. (12) Ho, Chu-Ngi; Christian, Gary D.; Davidson, Ernest R. Anal. Chem. 1980, 52, 1071. (13) Glanelli, Mary-Lu; Burns, David H.; Caiiis, James B.; Christian, Gary D.; Andersen, Niels, H. Anal. Chem. 1983, 55, 1858. (14) McCue, Matthew; Maiinowski, Edmund R . J . Chromatogr. Sci. 1983, 21, 229. (15) Appellof, Carl J.; Davidson, Ernest R. Anal. Chem. 1981, 53, 2053. (16) Ho, Chu-Ngl; Christian, Gary D.; Davidson, Ernest R. Anal. Chem. 1981, 53, 92. (17) Lorber, Avraham Anal. Chim. Acta 1984, 164, 293. (16) Sanchez, Eugenio; Kowalski, Bruce R. Anal. Chem. 1986, 58, 499. (19) Said, A. S. AIChE J . 1959, 5 , 69. (20) Deming, Stanley N.; Morgan, Stephen L. Anal. Chem. 1973, 4 5 , 276A. (21) Leggett, David J. Anal. Chem. 1977, 4 9 , 2098. (22) Dantzig, G. Linear Programming and Extensions ; Princeton University Press: Princeton, NJ, 1963.
RECEIVED for review March 13, 1986. Accepted June 9, 1986. This work was supported by NSF Grant IS1 8415075 and NIH Grant R01 GM 22311-06.
Determination of Selenium and Tellurium in the Gas Phase Using Specific Columns and Atomic Absorption Spectrometry Sirinart Muangnoicharoen, K. Y. Chiou, and 0. K. Manuel* Chemistry Department, University of Missouri, Rolla, Missouri 65401
Total seienlum and tellurium In the gas phase were analyzed afler adsorption on gold-coated beads and charcoal. The thermally eluting gas was trapped on columns fliled with quartz beads that were cooled in an ice bath. The beads were boiled in dilute HCI, and the resultlng solution was analyzed for Se and Te by graphlte furnace atomic absorption spectrometry. Our results demonstrate that gold-coated beads efflclently trap gaseous Se and Te at a low gas flow rate, but at hlgher flow rates charcoal traps are more expedient. With charcoal traps, lt was found that local alr samples contain Se in the range of 0.92-3.05 ng m-a and Te in the range of 0.10-0.34 ng m-3. Detection llmlts down to about 0.1 ng m-' allow the ready detection of Se and Te in rural alr wlth a preclsion of about &6% at the nanogram level of Te and about f4% at the nanogram level of Se.
The chalcogen elements-S, Se, and Te-have similar chemical properties and are of environmental interest. It has been suggested that the gaseous forms of these elements may
be involved in their atmospheric cycle (1). There have been many papers published of the gaseous form of S and a few reports on Se in the gas phase, but we have found no reports on Te in the gas phase. Total Se vapor in ambient air was reported to span a wide range, 0.006-5 ng m-3 ( 2 ) . Another study (3) identified biological alkylation as the main source for volatile organic Se in air. Alkylated selenium species have been studied by several workers (4-8). The total alkylated Se in the area of a sewage digestion tank was 0.2-5.4 ng m-3 ( 4 ) . Several workers successfully separated different species of alkylated Se compounds from air (4-8). Since the total airborne Se and T e levels in air are very low, an efficient sampling technique that will preconcentrate the anale is very necessary. The common air sampling procedure (7,8) is based on cryogenic trapping in which an efficient cooling system is needed. This method has limited utility because of ice formation from moisture in air and blockage of the trap. Activated carbon and some other common gas chromatographic stationary phases were found successful a t trapping volatile Se compounds; however, the thermal desorption procedure is incomplete (9).
0003-2700/66/0356-2611$01.50/00 1986 American Chemical Society
2812
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
( a ) SAMPLE COLLECTION SYSTEM
PREFILTER
Table I. Collection Efficiency of Se at Various Flow Rates (n = 5)
ADSORPTION TUBES ROTAMETER
collector gold-coated beads
VACUUM
WMP
(b) DESORPTION SYSTEM
charcoal
flow rate, Se added, Se found, L min-l clg clg 1.0
1.00
2.0
1.00
5.0
1.00
1.0 2.0
1.00
5.0
1.00
1.00
70
retained
1.09 f 0.10 109.3 f 10.2 0.96 f 0.06 95.7 f 5.9 0.88 f 0.06 87.9 f 6.0 1.06 f 0.06 106.6 f 5.6 0.97 f 0.05 97.5 f 5.3 1.00 f 0.03 100.5 f 3.2
ROTAMETER
Table 11. Collection Efficiency of Te at Various Flow Rates (n = 5)
TRANSFORMER CRYOGENIC TRAP AT O'C
TUBING PUMP
Figure 1. Schematic drawing of the apparatus used for (a) trapping
and (b) desorption of selenium and tellurium.
In nature, Se and Te are commonly found associated with deposits of gold and silver. Gold-coated beads have been successfully used as a selective collector for several volatile mercury compounds in air (10, 11). This paper presents a study of the feasibility of using gold-coated beads as a collector for volatile Se and T e compounds in ambient air and compares this technique with a granulated charcoal collector. Several authors (12-14) separate Se from other elements prior to detection by using extraction techniques. Atomic absorption spectrometry is one of the most extensive methods for the analysis of trace levels of Se and T e (15-18). For the work described here, we used ion exchange to separate T e from Se and other elements, and graphite furnace atomic absorption spectrometry for the detection of these elements. T h e total amounts of Se and Te in t h e gaseous phase of the local atmosphere are given. EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer 530 atomic absorption spec-
trometer with a HGA-2200 graphite furnace, together with a deuterium arc background corrector was used. Se and T e Perkin-Elmer hollow cathode lamps were used. Instrumental conditions include a nitrogen carrier gas at a flow rate of 30 mL min-', a drying temperature of 100 OC, a charring temperature of 500 "C, and an atomizing temperature of 2700 "C (19). The peak heights of the Se line at 196.0 nm and Te line a t 214.3 nm were used to determine the amounts of these elements present in samples and standards. As shown in Figure la, air samples were pulled through adsorption tubes with a pump, and the flow rate was controlled by an adjustable flowmeter. As shown in Figure lb, a tubing pump is used to maintain a low flow of air during thermal desorption. The electric tube furnace used to desorb the adsorbed gas was made of quartz (13 mm o.d., 11 mm id., 110 mm length) wired with 7 f t of nichrome wire (10 Q m-'). The heater was connected to a 10-A, 120-V variable transformer to allow continuous operating temperatures up to 1000 "C. The furnace was wrapped in asbestos paper and covered with alumina fiber to maintain constant temperature throughout the furnace. The temperature difference in the furnace is less than 10%. Sample Collection. The granulated charcoal (14-60 mesh) was supplied by Sigma Chemical Co. No chemical pretreatment was applied. The preparation of gold coating on quartz beads (20-80 mesh) was done by using 99.995% gold powder (Johnson Matthey, Inc.), dissolved in aqua regia and coated on self-made acid-washed quartz beads. The coating procedure was done by the methods described earlier (10, 11). The coating was about 10% by weight. The collector tubes were fabricated of quartz (9 mm o.d., 7 mm i.d., 130 mm length) with a small identation at 20 mm from each end of the tube. The packing materials were kept in the tube by acid-washed, quartz wool plugs. Prior to use, the adsorption tube containing gold-coated beads was heated to 1000 "C for 4-5 min with nitrogen carrier gas of 100 mL min-'.
collector gold-coated beads
flow rate, Te added, Te found, L min-' bug clg 1.0
2.0 5.0
charcoal
1.0 2.0 5.0
*
%
retained
0.59 0.53 0.62
0.58 0.05 0.46 f 0.04 0.42 f 0.05
98.7 f 8.5 87.1 f 8.4 66.9 f 8.0
0.57 0.55 0.61
0.57 f 0.02 99.8 f 2.9 0.54 f 0.01 98.0 f 2.5 0.62 f 0.01 101.6 f 2.03
The apparatus used to collect air samples is shown in Figure la. A vacuum pump was used to draw air samples through a cellulose membrane filter (0.2 pm pore size and 25 mm diameter), and then into two to three continuous collector tubes. The flow rate was controlled by means of an adjustable flowmeter. The connections between each part were made by very short pieces of Tygon tubing. Adsorption Test. Standard solutions of Se and Te were prepared by dissolving SeOz, 99.99% purity (Gallad Schlensinger Chemical Mfg. Corp.), HzSe04,96% purity (BDH Chemicals Ltd.), TeO,, >99% purity (Fisher Laboratory Chemicals), and HzTe0,.2H20, 99.5% (BDH Chemicals Ltd.) in concentrated HCl, and then diluting to standard volume. An aliquot of each solution was heated for 30 min in a quartz tube with an electric tube furnace to a maximum temperature of 1000 OC with a carrier gas of air drawn into a collector tube at various flow rates. Following the collector tube were two gas traps filled with 3 N HCl to dissolve any unadsorbed gases. Desorption Procedure. The apparatus used for desorption is shown in Figure lb. The collector tube was heated with the electric tube furnace to the desired temperature (1000 "C for gold-coated beads and completely burnt for charcoal) with an air flow rate of 100 mL min-'. The thermally desorbed gases were collected in two, ice-cooled U tubes filled with quartz beads, followed by two gas traps filled with 3 N HCl. The quartz-filled U tubes were washed with distilled water and then with 3 N HCl. The solution was evaporated to near dryness and taken up in 0.05N HC1, and the Se and Te were then separated from the other interference by ion-exchange chromatography (19). The resulting solutions were then placed in sample cups and automatically transferred to the graphite furnace in 20-pL pipets. Blank levels of both gold and charcoal adsorption tubes were measured. The results are the same as for doubly distilled water, near the limit of detection for T e and Se. RESULTS A N D DISCUSSION Performance of Collector Traps. T h e efficiencies of
collector traps for Se and T e are summarized in Tables I and 11. It was observed that both gold and charcoal quantitatively retain inorganic vapor of Se and T e at a flow rate of 1L min-'. However, at higher flow rates (>5 L min-') less than 90% of the amounts of Se and T e are adsorbed on gold-coated beads. When charcoal was used as a collector both Se and Te are efficiently trapped even at the higher flow rates. It is also noted that the trapping efficiency of the gold column is greater for Se than for Te; about 66% of T e was adsorbed at a flow rate of 5 L min-' while the Se efficiency was about 87%. From a n ambient air sampling system with a n flow rate of 10 L
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table 111. Chemical Yields for Thermal Desorption of Se from Collector Traps at 1000 OC, Air Flow Rate 100 mL m i d (n = 3)
Table V. Amount of Total Se and Te in Vapor Phase in Rolla, MO, between January 1986 and March 1986
vol of air, m3
total amt of Se, ng m-3
total amt of
date 1/21-1126 2/05-2114 2/26-3101 3103-3/01 3/07-3112
45.0 56.1 43.3 54.4 48.2
2.96 3.05 1.54 0.92 2.87
0.34 0.32 0.15
2.27 f 0.97
0.23 f 0.11
amt
collector
element
added, pg amt found, pg 7% recovery
gold-coated beads
Se4+ Ses+
1.00 1.00
1.05 f 0.12 0.99 f 0.05
105.4 f 12.2 99.0 f 5.4
charcoal
Se4+ See+
1.00 1.00
1.06 f 0.06 1.02 f 0.05
106.6 f 5.6 102.0 f 5.3
av
Table IV. Chemical Yields for Thermal Desorption of Te from Collector Traps at 1000 "C, Air Flow Rate 100 mL m i d (n = 3) amt
collector
element
added, pg amt found, pg 7% recovery
gold-coated beads
Te4+ Tea+
0.54 0.63
0.52 0.03 0.63 f 0.04
96.2 f 5.3 100.3 f 6.0
charcoal
Te4+ Tee+
0.48 0.60
0.48 f 0.01 0.61 f 0.02
100.9 f 2.6 102.4 f 2.6
*
min-l, Se and T e were found only on the f i t charcoal column. This demonstrates that essentially all (>98%) of the Se and T e were trapped in charcoal at this flow rate. No differences were observed in the Se and T e contents of fresh columns and those of the second and third columns. The latter therefore served as blanks. Thermally Desorption Results. The gaseous forms of Se(IV), Se(VI), Te(IV), and Te(V1) were trapped on columns at a flow rate of 1L min-l and desorbed by heating up to loo0 OC in an air flow. The results shown in Tables I11 and IV assume 100% efficiency for trapping. For gold-coated beads; the system was heated under constant air flow. The use of N2carrier gas at the same flow rate was also studied for Se(IV), but this resulted in its incomplete removal. We attribute this to the ease with which Se(1V) is reduced to the free metal. The optimum condition for the removal of these trapped ions was found to be 1000 OC for 30 min at an air flow rate of 100 mL min-'. A blank for each run was also analyzed by using another gold column that had not been exposed to the selenium or tellurium compounds. Desorption of selenium and tellurium compounds from the charcoal column required combustion in the electric tube furnace. This was achieved under the same conditions employed for desorption from gold, i.e., 1000 "C at an air flow rate of 100 mL min-'. The time required for complete combustion varied from 1 to 2 h. The resulting gas was found to be completely trapped in ice-cooled quartz beads. Analyses of Se and T e were performed in the usual manner. A blank was also analyzed with each sample. The complete burning of charcoal quantitatively removed the Se and Te. Since a fresh charcoal collector must be used each time, there is no memory effect from the history of the collector. Ambient Air Analysis. Charcoal was found to be more practical than gold-coated beads for routine air sampling because Se and T e are a t ultratrace levels in local air. For this reason, about 40 m3 of air must be collected for reliable analyses of Se and Te. The use of charcoal a t a flow rate of 10 L min-' is much faster than using gold-coated beads a t flow rate less than 2 L min-l. Local air samples were collected outside the third floor of the chemistry building, University of Missouri-Rolla, Rolla, MO, in the period of January to March 1986. The collection site is about lo00 m southeast of Interstate Highway 1-44 and
2813
Te, ng m-3
0.10
0.25
100 m east-southeast of the University coal-fired power plant. The height of the smoke stack relative to the sampling site is about 35 m. Prevailing winds are from the west-southwest. The air was drawn through a 0.2 Fm pore filter and then into a train of three charcoal traps. Our results are tabulated in Table V. The first charcoal column was found to be an efficient collector for total Se and T e in air. The second and third columns were found to contain Se and T e a t the blank level. These served as blanks in the experiment. From local air sample analysis, Se was found in the range of 0.92-3.05 ng m-3 and T e was found in the range of 0.10-0.34 ng m-3.
CONCLUSIONS In this study of methods for determining Se and T e in air, it is shown that gold-coated beads and charcoal have efficiencies near 100% for trapping these two elements at low flow rates ( 1L min-l). However, at higher flow rates, the efficiency of the gold-coated beads decreases but that of charcoal remains near 100% for flow rates up to 5 L min-'. The detection limits of the method employe here are about 0.1 ng m-3 for both Se and Te, with a precision of about &6% at the nanogram level of T e and about f 4 % at the nanogram level of Se. N
ACKNOWLEDGMENT Special thanks are due to E. B. Bolter for his kindness in letting us use the atomic absorption spectrometer, to H. Ross for his advice in air sampling, and to P. Johnson for typing. Registry No. Se, 7782-49-2; Te, 13494-80-9; Au, 7440-57-5.
LITERATURE CITED (1) Duce, R. A.; Hoffmann, G. L.; Zoller, W. H. Science 1975, 187, 59-61. (2) Mosher, B. W. J . Geophys. Res. 1983, 88, 6761-6768. (3) Radzuik, B.; Loon, J. V. Sci. TotalEnviron. 1976, 6 , 251-257. (4) Reamer, D. C.; Zoiler, W. H. Science 1980, 208, 500-502. (5) Chau. Y. K.; Wong, P. T. S . Science 1976, 192, 1130-1131. (6) Jiang, S.;Robberecht, H.; Adams, F. Amos. Environ. 1983, 17, 111-1 14. (7) Wong, P. T. S.;Goulden, P. D. Anal. Cbem. 1975, 4 7 , 2279-2281. (8)Jlang, S.; De Jonghen: Adams, F. Anal. Cbim. Acta 1982, 136, 163-1 90. (9) Evans, C. S.;Asher, C. J.; Johnson, C. M. Aust. J . 8iol. Sci. 1968, 21, 13-20. (10) Braman, R. S.;Johnson, D. L. Environ. Sci. Techno/. 1974, 8 , 996-1003. (1 1) Dumarey, R.; Dams, R.; Hoste, J. Anal. Chem. 1985, 5 7 , 2638-2643. (12) Dilli, S.;Sutikno, I. J . Chromatogr. 1984, 300, 256-301. (13) Chung, C. H.; Iwamoto, E.; Yamamoto, M.; Yamamoto, Y. Spectrochim. Acta. Part 6 1980, 398, 459-466. (14) Nakayama, M. Talanta 1984, 3 1 , 269-274. (15) Verlinden, M.: Deelstra, H.: Adriaenssens, E. Talanta 1981, 28, 637-646. (16) Andreae, M. 0. Anal. Chem. 1984, 56, 2064-2066. (17) Maher, W. A. Anal. Lett. 1984, 17(A10), 979-991. (18) Yu, M. Q. Talanta 1983, 30, 265-270. (19) Chiou, K. Y.; Manuel, 0. K. Anal. Chem. 1984, 56, 2721-2723.
RECEIVED for review May 28, 1986. Accepted July 21, 1986.
