Table I. Phosphorescence Characteristics of Molecules Studied Concentration Linear range of near Slope of correlation Limit of Limit of linearity linear portion coefficient detection, pg/rnlc detection pg/mld 104 0.964 0.9995 2 . 4 x 10-3 2.0 x 10-3 104 1.111 0.9771 1 . 3 x 10-3 1 . 0 x 10-4
Numbere of grams Compound at limit of detection m-Tryptophan" 4 . 7 x 10-11 Sulfapyradineb 2.5 x 10-11 3-Indole pyruvic acid* 104 0.893 0.9954 2.6 x 10-3 ... 5 . 1 x 10-11 a Solvent 5 % WjW sodium iodide:water, adjusted to pH 13 with sodium hydroxide (6). Solvent 5 % WjW sodium iodide:water. c Limits of detection in this study. Best previously reported limit of detection. e Volume of sample retained in capillary tube (calculated as 19.7 pl) for the present study. The sample used in previous studies was about 500 pl.
(Suprasil) quartz capillary tube, which gave a lower background signal; and a thin film polarizer which has a better transmittance in the ultraviolet than the Glan-Thompson previously used. A single polarizer in the excitation radiation beam, was used for two reasons: first, the absorption and luminescence spectra of the quartz capillary tube are polarized (1) and can be minimized by proper orientation of the polarizer; second, the absorption and luminescence of molecules in snowed frozen matrices are essentially depolarized by virtue of diffuse reflectance within the snowed matrix (4,and should be independent of polarizer orientation (the polarizer parameter which most affects the luminescence signal is the ultraviolet transmittance). The range of linearity and limits of detection reported in Table I, compare favorably with those previously obtained in clear rigid glasses. Because of the small volume of sample solution retained in the capillary tube, the total number of grams of solute necessary for a determination is in the picogram region. This should be of particular importance in biological and clinical applications where sample size is often limited. The average slopes of the analytical curves (logarithm of phosphorescence signal us. logarithm of analyte concentration) reported in this study and those reported in a previous study (1) are 0.94 which approaches the ideal slope of unity for phosphorescence signals in snowed matrices (4). Both positive and negative deviations from a slope of unity have been observed; however, much more data are necessary before a conclusion can be drawn as to whether an individual molecule perturbs the crystalline matrix sufficiently enough to alter the
slope or whether the deviations can be attributed to instrumental parameters influencing the precision and accuracy of signal measurements. Present theory assumes the scattering coefficient s, of the entire sample is not a function of solute molecule or its concentration ( 4 ) . A series of ten replicate determinations of 1 pg/ml of 2thiouracil in 10% VjV methanol-water were measured to determine the relative precision of using the rotating sample cell arrangement; the relative standard deviation obtained was 1.47%. This is an improvement of nearly an order of magnitude over the stationary quartz capillary tube system previously used (I); the physical dimensions of the previous capillary tube prevented use of the spinner assembly. The major noise contribution in our system arises from wobble of the capillary tube, caused by a poor fit of the turbine sleeve in the spinner assembly when the tube is spinning. If the capillary tube were held more rigidly in the turbine sleeve, the precision (1.47% in our case) would be reduced even more. The improved precision and limits of detection as well as the greatly facilitated sample handling procedure for phosphorimetric analysis in predominately aqueous solvents should offer additional advantages for use of phosphorimetry for routine analyses in biological and clinical applications.
RECEIVED for review November 12, 1971. Accepted December 21, 1971. Research was carried out as part of a study on the phosphorimetric analysis of drugs in blood and urine, supported by a U.S. Public Health Service Grant (GM-1137309).
Sandwich KBr Disk for Scanning Volatile Pesticides Paul A. Giang Entomology Research Division, Agricultural Research Service, US.Department of Agriculture, Beltsoille, Md. 20705
VOLATILE PESTICIDES invariably evaporate so rapidly that there is not time enough to scan the whole infrared spectrum from 4000 to 200 cm-l with an infrared spectrophotometer. In the laboratory at Beltsville, we have recently found that an excellent spectrum for such pesticides can be obtained by placing a sample of the pesticide in a freshly prepared KBr disk in the barrel of a Wilks Mini-Press (I) and covering this disk with (1) Wilks Scientific Corporation, Norwalk, Conn., Data Sheet No. 16, November 15, 1968. 1340
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
mother KBr disk. This technique allowed us to scan some important volatile chemical pesticides. PROCEDURE
Place a bolt in the barrel of a Wilks Mini-Press (Catalog Number MP-5) and advance five full turns. Deposit 300 mg of KBr on the surface of the bolt inside the barrel, and tap the unit gently to spread the salt uniformly. Insert the second bolt, turn it until it is finger tight, and place the whole press in a bench vise. With a torque wrench, gradually exert 25-
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Figure 1. Infrared spectra of reference samples of volatile pesticides WAVELENGTH (MICRONS)
& - - ' : " 3500 "'
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Figure 2. Infrared spectrum of 8 pg of a sample of residue compared with a reference spectrum of 0.5 mg of the same compound
to 30-lb pressure on the top bolt. In general, a disk made in this manner is clear and transparent. Loosen and remove the top bolt. Scratch a small shallow depression (about 2 to 3 mm diameter) in the center of the disk, but leave the other parts of the disk intact. Remove any loose pieces of KBr into an agate micromortar. With a micropipet, place an appropriate amount of the sample of pesticide to be analyzed in the mortar, and mix the KBr well with the sample. Then transfer the sample matrix to the shallow depression in the disk. Cover the whole disk in the barrel with another 200 mg of KBr, and press it again with bolts and the torque wrench until the pressure is 25 to 30 lb. Remove the bolts, and insert the barrel directly in the sample beam of the spectrophotometer. Prepare a similar KBr disk (without sample) with another Wilks Mini-Press barrel and bolts, and use it in the reference beam. For scanning the spectrum, adjust the gain control, suspension, etc., and set the instrument at 1X or at any other expansion desired. Some spectra obtained by this procedure are shown in Figure 1.
DISCUSSION
If the sample of pesticide is a solution, the solvent must be evaporated completely after the solution is mixed with the KBr in the agate mortar and before it is transferred into the depression in the disk. Therefore, a low-boiling solvent such as spectranalyzed methylene chloride or pentane is preferred in making the sample solution. In subsequent experiments, we have found that this disk sandwich procedure for volatile chemicals can also ,be used with minute amounts of certain samples of pesticidal residues (Figure 2 ) . If the sample is placed entirely in the path of the beam of infrared energy, a spectrum of maximum intensity can easily be obtained. RECEIVED for review October 26, 1971. Accepted December 21, 1971. Mention of a pesticide or a proprietary product in this paper is for identification only and does nct constitute a recommendation or an endorsement of this product by the U.S. Department of Agriculture.
Commercial Tungsten Filament Atomizer for Analytical Atomic Spectrometry Mark Williams and E. H. Piepmeier' Department of Chemistry, Oregon State Uniuersity, Corvallis, Ore. 97331
NON-FLAME ELECTRICALLY HEATED means of atomizing samples for analytical atomic absorption and atomic fluorescence have recently received renewed attention because of their ability to determine picogram amounts of elements in microgram samples. Winefordner and Elser ( I ) have briefly reviewed some of the graphite furnaces, filaments, and wire lamps used in atomic fluorescence. Tantalum foils have been studied (2, 3) and applied ( 4 ) to blood samples. The carbon rod atomizer has been used for clinical samples ( 5 , 6 ) and graphite furnaces for biological samples (7), snow (7), and air samples (8). The importance of correcting for light scattering, particularly when biological samples are used, has been discussed (9). Platinum and tungsten wire loops have been studied (10) and an automated system using a platinum wire loop has been built (11). In this investigation, a rigid spiral-wound tungsten filament from a commercially available light bulb (General Electric 1
Author to whom requests for reprints should be sent.
(1) J. D. Winefordner and R. C. Elser, ANAL.CHEM., 43,(4), 24A (1971). (2) H. M. Donega and T. E. Burgess, ibid., 42, 1521 (1970). (3) J. Y.Hwang, S. D. Smith, and A. L. Malenfant, 10th National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., Oct. 1971, paper 137. (4) J. Y . Hwang, ibid., paper 71. (5) M. D. Ames, P. A. Bennett, K. G. Brodie, P. W. Y.Lung, and J. P. MatouSek, ANAL.CHEM.,43,211 (1971). ( 6 ) D. P. Sandoz, K. G. Brodie and J. P. MatouSek, 10th National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., Oct. 1971, paper 136. (7) R. Woodriff, B. Culver and D. Shrader, ibid., paper 139. ( 8 ) R. Woodriff and J. F. Lech, ibid., paper 145. (9) S. R. Koirtyohann and Helen Severs, ibid., paper 134. (10) M. P. Bratzel, Jr., R. M. Dagnall, and J. D. Winefordner, Appl. Spectrosc., 24, 518 (1970). (1 1) S. R. Goode and S. R. Crouch, The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1971, paper 80. 1342
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
QUARTZ
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Figure 1. Exploded view of argon sheath and mounting for tungsten filament
No. 1763) was used to atomize 3-111 samples. A method of compensating for background tungsten emission is presented. The nominally 6-V, 4-A filament was driven from an easily programmed precision power supply (MP-1026, McKeePedersen Instruments, Danville, Calif.). The atomic vapor was observed by atomic absorption. Matrix effects for complex samples might be expected to be more severe for a wire loop system than for a furnace or rod because spectral observations are usually made in the cooler region above the wire loop rather than in the heart of a furnace or hollow rod. Our need was for a system that could measure nanogram amounts of a single element dissolved in a small amount of solution. The precision construction and ready availability of the tungsten filaments together with the simplicity of our sample matrix led to the study of the wire loop system rather
