Anal. Chem. 1981, 53, 544-546
544
hexane extractions made on each sample, most of the rubber (averaging 77%) is removed in the first extraction. Small sample sizes provided higher first extraction percentages (Table 11). Additional improvements in technique have increased that t o an average of 85% with the first extraction of 5-g samples. Also small samples reduce cost, reduce time of analysis, and provide improved extraction percentages.
(8) Meryman, H. T. "Cryobiology"; Academic Press: London, 1966; Chapter I . (9) Bidwell, R. G. S. "Plant Physiology", 2nd ed.; Macmillan: New York, 1979; Chapter 28. (10) Anderson, J. O., University of Arizona, Department of Plant Sciences, Tucson, AZ, 1980, personal communication. (11) Garrot, Donald J., Jr., unpublished data, University of Arizona, Department of Plant Sciences, Tucson, AZ, 1980.
Donald J. Garrot, Jr.* D u a n e L. Johnson D. D. R u b i s
LITERATURE C I T E D (1) Hammond, B. L.; Polhamus, L. G. "Research on Guayule (Parthenium argentaturn): 1942-1959", U . S . Dep. Agric. Tech. BUN. 1985, No. 1327, 109-131. (2) Hanson, G. P. "Breeding Improvement of Rubber Yield in Guayule: 3rd Progress Report"; National Science Foundation: Washington, DC, 1978; pp 88-94. (3) Gresa, E. C., B. F. Goodrich Co., Brechsville, OH, 1980, Dersonal corn mudcation. (4) Mehta, I. J.; Dhillon, S. P.; Hanson, G. P. Am. J . Bot. 1979, 66, 796-804 (5) Baue,-T. "Guayule Natural Rubber Development Project: 1st Year Report"; California Department of Food Agriculture, Division of Plant Ind., Sacramento, CA, 1979; p 24. (6) "Guayule: An Alternative Source of Natural Rubber"; National Academy of Science; Washington, DC, 1977; Chapter 6. (7) Glymph, E. M.; Nivert, J. J., presented at the International Rubber Study Group: 25th Assembly; Offices of the Secretarht, London, 1978.
Department of Plant Sciences University of Arizona Tucson, Arizona 85721 Gerald M. Dill Department of Plant Pathology and Crop Physiology Louisiana State University Baton Rouge, Louisiana 70803
RECEIVED for review August 25, 1980. Accepted December 12, 1980. Financial support from the National Science Foundation, PFR-7911723, is gratefully acknowledged.
AIDS FOR ANALYTICAL CHEMISTS Concentration of Dilute Pesticide Solutions Gerd Puchwein Landwirtschaftlich-chemische, Bundesversuchsanstatf Linz, Wleningerstrasse 8, A-4025 Linz, Austria
In pesticide residue analysis the final determination of content very often uses methods such as gas chromatography, high-pressure liquid chromatography, or thin-layer chromatography requiring only a few microliters of a cleaned extract. On the other hand most cleanup procedures end up with a final volume of about 1-10 mL. The potential advantages of a substantial reduction of scale of cleanup procedures are obvious provided the sensitivity of the method is preserved (e.g., cut in costs for highly purified organic solvents, reduced hazard due to manipulating inflammable and/or toxic solvents). A prerequisite for a reduction of scale of cleanup procedures is a method for reliably concentrating very dilute solutions to a small and defined volume. The concentration step can be a major source of error due to uncontrollable losses of pesticides ( I ) . This especially holds true if a reduction of volume below 1 mL is desired. A few devices for concentrating such solutions to low volume have already been described (2-6). Usually special glassware is needed, which either renden these methods dependent upon a good glass blower or calls for the acquisition of special equipment, if commercially available (microconcentrator, Supelco Inc.). Our idea was to make use of commercially available parts as far as possible and to work with inexpensive disposable glass tubes as receiving vessels for the concentrated solutions. EXPERIMENTAL S E C T I O N Apparatus. The heart of the device (Figure 1) consists of a concentration vessel, B, with a bulb of 6 mL content and a Luer
tip end connected to a Hamilton hypodermic needle, N, with CTFE Hub (KF-722). The needle extends into the upper opening of a disposable 200-rL one-way pipet, R (Clay Adams, Micropet 200 pL), which is placed inside a T-shaped tube, T. Two ends are sealed gastight by screw caps, SC (Schott, GL 14), and a septum, S (Teflon rubber laminated disks, 12 mm, Pierce), at the upper end and a Teflon-lined silicon ring at the lower end. The glass tube, G, acts as a sleeve which firmly holds in place the receiving tube, R. It can easily be inserted into and withdrawn from tube T, after loosening the screw cap. The pipets with sealed lower ends serve as the receiving vessels for the concentrated solutions. The side arm of tube T is connected via a Swagelok reducing union (B-400-6-2)(not shown in the figure) and 1-mm Teflon tubing to a three-way ball valve (Whitey B-41XS2V). The two other outlets of the valve are connected to a nitrogen line and to a 10-mL disposable plastic syringe, by l/g in. copper tubing and 1-mm Teflon tubing, respectively. Nitrogen flow can be adjusted by a standard laboratory pressure regulator and a needle valve. An aluminum bar, A, 250 mm long, with square cross section (40 x 40 mm) and suitable bores holds five vessels, B. The aluminum bar is electrically heated at two of its long sides by a heating jacket, H, made from heating wire (length = 2.5 m, R = 50 Q ) sandwiched between mica sheets for electrical insulation. For thermal insulation the two heated long sides are covered with two asbestos (5 mm) and two PVC sheets (5 mm) while for all other sides Teflon foil (1.5 mm) is used. The heating jacket is connected to an ordinary laboratory power regulator (220 V); the aluminum bar is grounded. The temperature of bar A is controlled by a contact thermometer and a relay. The whole =tup comisthg of the heated bar, A, tubes, T, valves, and syringes is mounted on a panel, P, and placed in a hood. Only the concentration vessels, B, and the tubes, T, had to be made to order by a local
0003-2700/81/0353-0544$01.00/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
545
Table I. Recoveries of Pesticides from 4-mL Solutions Subjected to Concentration solution typea a b
solvent petroleum ether hexane petroleum ether hexane
no. of experiments
recovery, 9% x
S
12
96.2
3.1
19 4
93.2 94.1
3.3 2.2
4
93.9
2.4
Pesticide concentrations of solutions a and b are given in the text.
I
receiving tubes, complete mixing is ensured within seconds. The tubes are closed with plugs made from 2-mm Teflon tubing to prevent solvent losses during storage. The procedure was performed with pesticide solutions containing: (a) hexachlorobenzene (0.25 ng/mL); (b) hexachlorobenzene and a-BHC (0.25 ng/mL), lindane (0.35 ng/mL), heptachlor and aldrin (0.5 ng/mL), 6-BHC, a-chlordane, ychlordane, 6-heptachlor epoxide, dieldrin, and pp’-DDE (1ng/mL), endrin and pp’-TDE (2 ng/mL); op’-DDT and pp’-DDT (2.5 ng/mL) using either petroleum ether (boiling range 40-60 “C) or hexane as solvent with temperature settings of 55 and 73 “C, respectively. Time requirement for concentrating 4 mL was about 20 min in either case. Recovery rates were determined by gas-liquid chromatography with an electron-capture detector following standard techniques. U
Figure 1. Concentrating device, side vlew (detail): B, concentration vessel; A, aluminum bar; H, heating jacket; M, 100-pL mark; N, hypodermic needle; S , septum; SC, screw caps; R, receiving vessel; T, T-shaped tube; G, glass tube serving as support for R; P, panel.
glass blower. As no calibrations are needed both are inexpensive. All other parts apart from standard equipment employed can be produced in any average workshop and were home built. Procedure. All glassware coming into contact with the pesticide solutions and the needle have to be thoroughly cleansed before use. The pipets are rinsed with petroleum ether in a wide chromatographic column. After the pipets are dried in an electric oven, one end is sealed with a Bunsen burner. After an additional solvent wash and drying they are stored in a desiccator ready for use. For cleaning of the concentration vessel between uses a solvent wash through B is usually sufficient. Naturally, it can also simply be removed from the device and disconnected from the needle for cleaning purposes. After the appropriate temperature depending on the type of solvent used has been reached, a nitrogen gas flow regulated to about 15 mL per vessel is turned on. The nitrogen flow is fed into tube T and forced through the needle into the concentration vessel, B. Up to 5 mL of the solution to be concentrated is added to the bulb and kept gently bubbling by the gas stream which prevents the solution from running into the receiving tube. The solvent is thus removed under partial reflux until the volume remaining in the narrow lower part of vessel, B, is below the 100-pL mark, M. Switching the three-way ball valve from the nitrogen line to the vent releases the pressure inside tube, T, and allows the concentrated solution to run into the receiving vessel, R. This can be assisted by connecting tube, T, to the syringe, and exerting slight suction. Finally, the inside of vessel, B, is rinsed with small volumes of solvent delivered from a Hamilton 25-pL syringe, the receiving vessel is withdrawn after collecting the washes and filled up to mark with solvent. Thorough mixing of the final volume is essential to ensure a homogenous solution in the narrow tubes. This can be achieved by prolonged vortex mixing or by repeatedly pumping the solution with a 10-pL syringe provided the syringe has a needle long enough to reach the bottom of the receiving tubes. A superior alternative method is to use a glass-coated 1 cm piece of thin iron wire as a mixing bar. After the final volume is adjusted, the small mixing bar can be put into the receiving tubes. When the mixing bar is agitated from outside by rapidly moving a magnet along the
RESULTS AND DISCUSSION The experiments are summarized in Table I. The recoveries of the different pesticides of solution b did not show any significant bias. The standard deviation of the individual recoveries from the mean recovery of all 15 pesticides of solution b was only 2% (mean of five determinations) including the error of the gas chromatographic determination itself. A relative standard deviation of less than 0.5% is given by the manufacturer for the volume of the disposable pipets. Our own weighings of tubes filled with distilled water show that the sealing of one end reduces the volume to 198 f 2 p L (i f s). As the volume of the vapor phase compded to the liquid is small in these tubes, losses due to evaporation of solvent by repeated opening of the tubes are negligible. This is a definitive advantage over flasks where the concentrated solutions make up only a small fraction of their total volume. As each receiving vessel is only used once, repeated cleaning of the narrow tubes is avoided and the danger of cross contamination eliminated. Hence these cheap tubes are considered a real alternative to fairly expensive special flasks. The gas bubbling through the heated solution prevents bumping and sputtering and effectuates thorough mixing of the solution under partial reflux. This is very important as our own experiences support the view ( 5 ) that ineffective washing of the exposed glass surface of the concentration vessel is a main source of uncontrollable losses. An addition of a high boiling solvent such as a mineral oil (3) or heptane (2) to “trap” the residues was not found to be necessary. As the solution collects in the lower nonheated part of B during the last stage of concentrating, the rate of evaporation is slowed down, which minimizes the danger of inadvertent evaporating t o dryness. The device allows concentration under very mild conditions. A temperature of only about 5 “C above the boiling point of the solvent is sufficient for the heating block. The nitrogen flow creates an inert atmosphere. Hence it is thought that the device might be applicable whenever dilute solutions containing labile or volatile compounds have to be reduced to a small volume. Apart from the solutions of organochlorine pesticides, organophosphorus insecticides in acetone were successfully concentrated with this device.
546
Anal. Chem. 1981, 53, 546-549
ACKNOWLEDGMENT
(3) Beroza, Morton; Bowan, Malcolm C.; Bierl, Barbara A. Anal. Chem. 1972, 44, 2411-2413. (4) Solomon, John Anal. Chem. 1979, 51, 1861-1863. (5) Junk, A.; Richard, J. J.; Grieser, M. D.; Withk, D.; WRhk. J. L.; Arguello, M. D.; Vick, R.; Svec, H. J.; FrRz, J. S.; CaMer, G. V. J . Chromatogr. 1974, 99, 745-762. ( 6 ) Dunges, W. "Praechromatographische Mlkromethoden"; A. HClthlg Verhg: Heidelberg, Basel, New York, 1979; Chapter 3.
The expert technical assistance of G. Schmidinger and L. Kappinger is acknowledged.
LITERATURE CITED (1) "Pesticide Analytical Manual"; U.S. Department of HeaRh, Education and Welfare: Rockville, MD, 1972; Vol. 1, Chapter 1. (2) Zimmerli, B. M M . Gebiete LebensmMelunters. Hyg. 1973, 64, 528-532.
for review September
49
lg80. Accepted December
2, 1980.
Preparation of Multielement Solutions for X-ray Fluorescence Analysis with a Liquid-Aerosol Generator Robert B. Kellogg, Nancy F. Roache, and Barry Dellinger" Environmental Sciences, Northrop Services, Inc., P.O. Box 123 13, Research Triangle Park, North Carolina 27709
The number of techniques for analyzing solutions by X-ray fluorescence are far too numerous to describe individually ( I , 2 ) . Techniques such as preconcentration and precipitation are time consuming or not applicable if many elements are present in the sample. The most direct method of analysis is to use liquid-specimen holders (I). In this method the solvent as well as the Mylar film used to contain the liquid in the holders greatly absorbs the radiation from low atomic number elements. Such a method is best suited for relatively high concentrations of high atomic number elements. The capillary matrix method ( 2 )utilizes an array of capillaries which is dipped into the solution to be analyzed drawing an accurately known volume. One then touches an absorbent filter with the capillary array wetting the filter followed by freeze-drying. This leaves a pattern of dry spots containing the sample. One is unquestionably limited to analysis of very high atomic number elements because of the strong absorption by the filter material. After attempting the above techniques, we were convinced that rapid analysis of solutions against our t h i n - f i i calibration standards was most feasible through total evaporation of the solvent and subsequent analysis of the particulate matter. Such a method was readily achievable only through a liquid aerosol generation technique. This article describes in detail the results obtained in this laboratory by using this method.
EXPERIMENTAL SECTION Apparatus The liquid-aerosol generator, shown in Figure 1, consists of a Collison nebulizer (BGI, Inc., Waltham, MA), a nebulizer vessel, a dilution air coupling with a stainless she1 sleeve, a mixing chamber, and a filter holder (Millipore Corporation, Bedford, MA). All parts are made of Pyrex (Corning)except the nebulizer and stopper assembly, which are machined from Teflon (DuPont Co.), and the dilution air coupling sleeve and fdter holder, which are stainless steel. The construction materials reduce sample contamination and facilitate cleaning. The nebulizer vessel was constructed so that the bottom tip of the nebulizer clears the vessel by approximately 0.5 mm. This construction, which deviates from previous descriptions (3), is necessary because of the small volumes (5-10 mL) to be prepared by use of this apparatus. Also, a six-jet nebulizer is used to achieve higher aerosol generation rates with less preparation time. The dilution air coupling is fitted with a sleeve for introducing compressed air through the air inlet. The air flows between the sleeve and the coupling and then through a series of holes around the coupling's circumference, enters the appratus, and dries the aerosol. The dried particles then pass into the 10 cm by 46 cm cylindrical mixing chamber before being deposited on the filter. Operating conditions for the aerosol generator are given in Table I. Ease of cleaning reduces the possibility of cross-contamination during analysis and, thus, was an important design criterion during 0003-2700/81/0353-0546$01 .OO/O
Table I. Operating Conditions of Sample Aerosol Generator parameter
measurement
nebulizer air flow dilution air flow sample volume sampling time
7.6 L/min at 25 psi 18-22 L/min 5-15 mL 1-30 min
the development of the generator. All parts of the generator, except the nebulizer and dilution air coupling, can be adequately cleaned with deionized water. The dilution air coupling is cleaned with a mild soap solution and a brush. The nebulizer is rinsed and submerged in deionized water, operated for a few seconds and then rinsed in methanol, and dried by a burst of air. Characterization Deposit Uniformity. Aerosol droplets generated for use in X-ray fluorescence analysis must be uniformly deposited across the measurement area because of nonuniform photon flux density of the excitation source. A series of 11 6.35mm diameter plugs were removed from the 35-mm diameter deposit from a sample containing S, K, and Cd to assess the uniformity of the deposit for samples from the aerosol generator. The plug pattern is shown in Figure 2. Each plug was fixed in the center of a sample holder with two-sided transparent tape. The count rate for S, K, and Cd was measured for each plug and normalized to the average for each element, respectively. These normalized count rates were then averaged (with 20 error bars) for each of the five distances from the axis of rotation. (The samples are rotated during analysis.) The results are shown in Figure 2. Evaluation of these data indicates that no one position on the filter received abnormally high or low aerosol deposits, indicating that the deposit is sufficiently uniform for analytical purposes. Generation of Calibration Standards. The liquid aerosol generator was used to prepare calibration standards for sulfur. Standard solutions were prepared from 0.1% KzSO4 and were collected on O.&pm pore size Nuclepore filters (Nuclepore Corporation, Pleasanton, CA) that had been exposed for 24 h to a constant temperature and relative humidity of 21 "C and 45%. After collection of the particulate, the samples were stored under the same conditions for 24 h, weighed to determine the areal concentration, and analyzed by X-ray fluorescence. A linear least-squares fit of the absorption corrected intensity data on Seven standards gave a sensitivityof 254.3 f 1.0counts cm2pg-* s-' which is in good agreement with 260.7 i~ 1.3 counts cm2pg-' s-' obtained with thin film standards from Micro-Matter Co., Seattle, WA. Similar good agreement was found for Na2S04and (NH&SO4. Analysis of Aerosol Samples. The applicability of the aerosol generator for the analysis of a broad range of acid-soluble elements was assessed. A solution was prepared by mixing 1mL of a lo00 ppm standard of each of the following cations: Al,Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Sn, Sb, Cs, and W (F & J Scientific, Monore, CT). Because of the nonavailability of sulfur standards from F & J Scientific and contamination of our 0 1981 American Chemical Society
