813
Anal. Chem. 1983, 55, 813-814 Mylar
film
precision, it is essehtial that the amount of ash taken just fills the depression in the sample holder. Lesser amounts are free to tumble about within the holder, resulting in some loss of reproducibility. Larger amounts, on the other hand, produce some creasing and bulging of the Mylar cover, again resulting in a loss of reproducibility.
,Depression
\
Polyethylene CUP
Securing ring
CONCULSIONS The proposed sampling method offers several definite advantages over the other methods available, among them excellent precision and accuracy, as well as minimal operator involvement in sample preparation. It would seem then, that this method offers the best available alternative in cases where it is not possible to prepare a self-supporting disk or pellet from dried plant tissue. Registry No. Pb, 7439-92-1.
Monomer core
Flgure 1. Cross section of the sample holder.
Table I. Comparison of Added and Experimentally Determined Lead Levels in Spinach Leaves % added
% determined
lead
difference
0.797 0.413 0.201 0.100 0.0494 0.0101 0.00600
0.838 0.436 0.194 0.101 0.0477 0.00956 0.00602
t 5.14 t 5.57
lead
%
-3.48
LITERATURE CITED
t 1.00
Brandt, C. Stafford; Lazar, Victor A. J. Agrlc. Food. Chem. 1988,6 , 306-309. Handley, Raymond. Anal. Chem. 1980,32, 1719-1720. Ball, D. F.: Perkins, D. F. Nature (London) 1982, 794, 1163-1165. Fieldes, M.; Furkert, R. J. N. 2.J . Scl. 1971, 74, 280-291. Reuter, F. W. Anal. Chem. 1975,4 7 , 1763-1766. Whittig, L. D.; Buchanan, J. R.; Brown, A. L. J. Agric. Food Chem. l980,-8,419-421. Champion, K. P.; Whktem, R. N. Analyst(London)1987,92, 112-114. Florkowski, T.; Kuc, T.; Piorek, S. Int. J . Appl. Radlat. Isot. 1977, 28. 679-686. Evans, C. C. Analyst (London) 1970,95, 919-929. “Radiation Analyzer Manual”; Picker Instruments: Cleveland, OH, 1968; p 59. Webber, M. D. Can. J. SollScl. 1972,52, 282-284. Feinberg, Max; Ducauze, Christian Anal. Chem. 1980, 5 2 , 207-209.
-3.44 -5.35 t0.33
The precision of the sampling method was evaluated by repeated analysis of a series of standards. Ten replicate delead by terminations using a sample containing 9.89 X weight produced an average fluorescent emission intensity of 1319 counts/s with a standard deviation of 10.2 counts/s. This corresponds to a relative standard deviation of 0.776%. Similar determinations with standards containing 2.18 X and 2.50 X lead by weight yielded relative standard deviations of 0.726% and 1.18%, respectively. It is important to note that the precision of the sampling method is very dependent upon the sample size. For optimum
RECEIVED for review August 30, 1982. Accepted November 22, 1982.
Mass Spectrometer Polymer Membrane Sample Introductlon Device George J. Kallos and Nels H. Mahle” Analytical Laboratories, Dow Chemical U.S.A., Midland, Michigan 48640
In recent years attention has been given to the evaluation and use of semipermeable membranes for the determination of contaminants in air and drinking water (1-3). Hollow fiber probes, constructed of different permeable materials, were designed and evaluated as mass spectrometric sampling devices by Westover and co-workers (4). These hollow fiber devices have been applied to the routine monitoring of trace levels of toxic contaminants in aqueous solution and in air (5,6). Eustache and Histi have described a membrane device which is coupled to a mass spectrometer through a vacuum leak assembly (7). The development of a new membrane device for use with a mass spectrometer was prompted by the general unavailability of hollow fibers which constitute an obstacle to the application of these devices. The new membrane sampling device described in this paper is simple and easy to construct from readily available components.
with 100 X 100 meshes per linear in.). All metal pieces were thoroughly rinsed with water, methanol, methylene chloride, and acetofie before silver soldering and also before polymer coating. Chemicals. Experimental block copolymers of poly(amethylstyrene-dimethylsiloxane) [X4-2541 (75% PDMS), X96317 (60%PDMS), and X4-2517 (40% PDMS)] were obtained from Dow Corning Corp., Midland, MI, and have been previously described (8-10). Membrane Coating of the Probe. The polymer to be coated on the probe screen was dissolved in a solvent, preferably toluene or xylene for siloxane polymers, at concentration levels of 10-15 w t %. The precleaned metal probe was dipped into the solution for 1 min, removed and then immediately rotated 180° so that the solution drained through the opening of the 1/4 in. tube. Any solution left inside the screen portion of the probe would leave an uneven coating of the polymer after evaporation of the solvent. The probe was allowed to dry overnight. A Varian-MAT CH4B mass spectrometer was used for evaluating the membrane probe.
EXPERIMENTAL SECTION Construction of Metal Probe. The metal probe was conin. fine mesh stainless steel screen tube structed from a 1 in. X that was silver soldered to a 4 in. X 1/4 in. stainless steel tube as shown in Figure 1. A 1/8 in. x 1/4 in. diameter solid stainless steel disk was soldered (soft silver solder) on the other end of the screened tubing to serve as a plug. The screen was obtained from Newark Wire Cloth Co., Newark, NJ (0.003in. diameter steel wire
RESULTS AND DISCUSSION Only polymers which are highly permeable, selective, and possess a high degree of chemical and physical stability can be used for mass spectrometer probes. High permeability minimizes area and pressure requirements for direct sampling with a mass spectrometer. Although many polymers were coated for evaluation, the poly(dimethylsi1oxanes) and in particular the organo-siloxane block copolymers were found
0003-2700/83/0355-0813$01.50/0
0 1983 American Chemical Society
814
Anal. Chem. I@83,55, 814-816
_____
Table I. Comparison of Siloxane Membrane Probe to Hollow Fiber Robe Responses (50 ppm (v/v)) peak height, mm acrylonitrile methylene chloride m/z 53 m/z 84 (1) membranr probe prepared from 10% polymer solution ( 2 ) membrane probe prepared from 15% polymer solution (3) siloxane hollow fibrr probe a
Background pressure for water blanks was 0.4
X
22
105
14 6
71 60
ion sowce pressure,' torr 0.6 X 0.45 X 0.55 X
torr for all three cases. em2which was very close to the 1.06 cm2 for the 15-element siloxane hollow fiber probe. Organwsiloxaneblock copolymers with higher siloxane content, such as X4-2541 (75% poly(dimethylsiloxane), 25% a-methylstyrene) provided good permeability for many compounds hut lower enrichment factors. The enrichment factor of chloroform in aqueous solution with this membrane prohe was determined to be 0.6 X 10' as compared to 1.1 X 10' reported by Westover et al. (4) with the use of the siloxane hollow fiber probe. Other polymer membrane probes, including poly(viny1 alcohol), poly(cyanoacrylate),and poly(styrenea-methyLstyrene) were found to he much less permeable than the organo-siloxane block copolymer. This new membrane probe possesses the following advantages over other sampling devices: (1)It is easy to construct. (2) There is no need for gasket seals or adhesives which could lesd to vacuum leaks or memory effects. (3) A large number of polymers can be tested for applications of tailor-made separations for mass spectrometer or other instrument probes. (4) The membrane probe can he heated through the metal hy heat conduction to study Permeability of polymers a t higher temperatures. (5) It is easy to evaluate permeation properties of polymers that cannot he fabricated as hollow fihers. Registry No. Water, 1732-18-5.
LITERATURE CITED Figure 1. Diagram of membrane probe.
to he more widely applicable for a variety of organic components. T o demonstrate the usefulness of the membrane probe device as compared to the siloxane hollow fiber probe made from the same block copolymer, we prepared two membrane probes of siloxane block copolymer (X4-2517,40% poly(dimethylsiloxane), 60% a-methylsytrene) using two solutions of 10% and 15% copolymer in toluene. Fifty part-per-million (v/v) standards of acrylonitrile and the common industrial solvent, methylene chloride, were made up in water. As shown in Table I equal or better response was obtained with the membrane probe at comparable source pressures. The effective membrane probe surface area was calculated to be 1.14
(1) Klain, E.: Elcheiberger. J.: Eyer. C.; Smith. J. Water Res. 1975. 9 . 807. (2) Relszner. K. D.; West. P. W. Enwon. Scl. T e M . 1973. 7 . 526. I31 Hardv. J. K.! DBsou~tB.P. K.: Reismer. K. D.: West. P. W. Endron.
.
(8) Saam, J. C.; Ward. A.~H.:Fearon. F. W. G.'"Advances in Chemistry Series"; American Chemical Society: Washington, DC 1973: Adv. Chem. Ser. No. 129. p 239. (9) Monroe. C. M.: Ward. A. H.: Abstracts. R u b k Divisbn 01 the AmerC can Chemical Society. Toronto. Canada. MY7. 1974. No. 15. (10) Ward, A. H : Kendrick, T. C.: S a m . J. C. ."Advances in Chemistry Series". No. 142. American Chemical Society: Washington, DC. 1975. Adv. Chem. Ser. NO. 142. p 300.
RECEIVED for review July 21,1982. Accepted December 14, 1982.
Determination of Carbon Isotope Ratios in Plant Starch via Selective Enzymatic Hydrolysis Arndt Schlmmelmann' and Mlchael J. DeNlro' Department of Earth and Space Sciences. Unlversny of California. Los Aflgeles. California 90024 Previous determinations of the stable carbon isotopic compasition of biopolymers involved isolation and purification of the intact polymers from complex organic mixtures, followed by their combustion and isotopic analysis of the re'Also Program in Archaeology.
sulting carbon dioxide (e.g., ref 1-3). We present here a method based on the selective enzymatic hydrolysis of a biopolymer. The resulting monomer is collected after passage through a dialysis membrane which serves to separate it from other components present in the original biological material. 'The monomer is then combusted prior to isotopic analysis.
0 1983 American Chemical Society 0003-2101)/(1~~0955-0814$01.50~0
