Subsquent experiments showed that the charring temperature as well as the molybdenum concentration had a significant bearing on the per cent recovery of added selenium obtained. A sample (Lab No. G3860) for which recovery had been low was analyzed a t various charring temperatures using two different concentrations of molybdenum. In Figure 4, t)e per cent recovery of added selenium obtained under these varying conditions is plotted us. the charring temperature. Note that, in a11 cases, a significant improvement in per cent recovery is obtained when a concentration of 100 mg/l. Mo is used as compared to the recovery obtained when 50 mg/l. Mo is used. Also, the per cent recovery obtained is definitely a function of charring temperature, with the recovery increasing as the charring temperature increases until a temperature of 1000 “C is reached. Above this temperature, the recovery drops off slightly. The main reason why per cent recovery is a function of charring temperature is probably because the higher charring temperature causes volatilization of interfering anions which results in increased recovery values. However, this would not explain why the recovery drops off a t temperatures above 1000 “C. Another possible explanation may be that the increased molybdenum concentration, coupled with an increased charring temperature, results in the formation of a higher molecular weight heteropolymolybdate anion which is more successful a t isolating the selenium from interfering substances. Precision Studies. The precision of the atomic absorption measurement at the 0.010 mg/l. Se level was determined. The standard deviation of ten replicate analyses of 10 wg/l. Se standard in 5% HC1 was found to be f0.75 gg/l.
Se, giving a relative standard deviation of 7.5%. This precision test did not include the digestion procedure. The precision of the entire procedure, including the digestion step, would probably be poorer.
CONCLUSIONS A rapid and highly specific method for determining selenium in water and industrial effluents has been developed. Unfortunately, the digestion method required by the Environmental Protection Agency for “total” metals analysis results in loss of selenium on some samples. Improvements in the digestion procedure will be necessary before the full value of the method can be utilized in monitoring environmental waters for selenium content.
LITERATURE CITED (1) Bureau of National Affairs, Washington, D.C., Environ. Rep., 4, 666 (1973). (2) American Public Health Association, “Standard Methods for the Exam!nation of Water and Wastewater,” 13th ed.,1971, p 295 (3) F. J. Marcie. Environ. Sci. Techno/., 1, 164(1967). (4) J. H. Wiersma and F. G. Lee, Environ. Sci. Techno/.,5, 1203 (1971). (5) J. M. Rankin, Environ. Sci. Techno/.,7 , 823 (1973). (6) C. S Rann and A. N. Hambly, Anal. Chin?,Acta, 32,346 (1965). (7) H. K. Y. Lau.and P. F. Lott, Talanfa, 18, 303 (1971). (8) D. C. Manning, At. Absorption Newslett., 10, 123 (1971). (9) J. S. Caldwell, R. J. Lishka, and E. F. McFarren, J. Amer. Wafer Works Ass., 85, 731 (1973). (10) R. B. Baird, S. Pourian, and S. M. Gabrielian. Anal. Chem., 44, 1887 (19721. (11) k. 6. Baird, S. Pourian and S. M. Gabrielian, Preprints, 166th National Meeting of the American Chemical Society, Aug. 1973, 13, Paper 15. (12) Environmental Protection Agency, Cincinnati, Ohio, “Methods for Chernical Analysis of Water and Wastes,” 1971, p 88.
RECEIVEDfor review July 31, 1974. Accepted November 18, 1974.
Determination of Lithium in Microliter Samples of Blood Serum Using Flame Atomic Emission Spectrometry with a Tantalum Filament Vaporizer J.
K. Grime and T. J. Vickers
Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
A method is described for the determination of lithium in microliter samples. A conventional flame atomic emission system has been adapted such that the atomization and vaporization processes are Isolated; the former occurrlng in either an air/hydrogen or air/acetylene flame, the latter from a heated tantalum filament. The disadvantages of using electrothermal atomization alone have thus been minimized. The technique has been employed in the determination of lithium in three matrices, viz,, water, artificial serum, and reconstituted human serum. The limits of detection (signalhoise = 2) in aqueous samples, for the air/H2 and air/ C2H2 flames are 0.009 nanogram (0.0018 ppm/5pI) and 0.003 nanogram (0.0006 ppm/5@) respectively. Lithium is determined at normal levels in human sera (0.01 ppm), using only a 5-microliter sample. It is recommended that serum samples be determined by either calibration with artificial serum standards or by the method of standard additions. 432
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It was determined in 1949 that lithium salts could be used effectively in the treatment of manic-depressive psychosis ( I ) . The normal level of lithium in blood serum is approximately 0.01 gg ml-I (2);during treatment, however, the level is usually maintained at 7 wg ml-l (3). Toxic symptoms have been reported for concentrations as low as 11.2 wg ml-l (4),and, accordingly, the serum lithium level must be carefully monitored in order that this level is not exceeded. The need for an accurate and reproducible method of analysis requiring a minimal amount of sample is therefore apparent. Conventional flame atomic absorption and atomic emission spectrometry have been successfully used to determine abnormal levels of lithium in blood serum (3-6). It has been suggested that there is no significant difference in sensitivity between the two techniques (4, 6, 7 ) , although atomic emission is generally accepted as the more convenient. The normal level of lithium in sera is, however, below the
1975
Table I. Instrumental Specifications a n d Operating Parameters Monochromator: Heath (McPherson) EU 700 with 0.1-mm slit width s e t for 670.8 nm. Photomultiplier : EM1 50-mm S-20 phototube (9558QB) with Harrison 6515A dc power supply and dc amplifier/integrator . Recorders : 1) Tektronix Type 564 storage oscilloscope with camera attachment (Type C12) Type 2B67 Time base 1 s e c division-', Type 3A3 dual-trace differential amplifier 0.51.O volt division". 2) Hewlett-Packard 680 s t r i p chart recorder. 10-volt output. Chart speed, 2 in. min-'. Vaporization chamber: Pyrex glass. Dimensions : Height 35 mm. D i m . 65 mm. Filament: 0.127-mm tantalum foil. Capacity 1-15 p1 (see Figure 3 for dimensions). Operating currents: desolvation 3- 5 A approx. Vaporization: 40 A appr ox. Filament power supply: 50 amperes max. at 4.0 V (approx) ac power supply (see Figure 1). Burner: Alkemade type burner. Flame conditions: Au,",: Air 8.6 1. min-', H, 2.4 1. min", Ar 1.3 1. min-' Air/C,H,: Air 8.6 1. min-', C,H, 0.6 1. min-' Ar 1.3 1. min-'. The burner was adapted to permit the argon and sample vapor to enter the central chamber through the base of the burner.
Ll T4
Filament
Figure 1. Circuit diagram for filament power supply T I , T2 = variable transformer, 10 A, General Electric 9T92Y8; T3 = filament transformer, Allied 6K134VER; T4 = 10: 1 step-down transformer composed
of 3 Allied 6K119VER control transformers connected in parallel; A-0-50A AC ammeter, Singer Co.. Metrics Division Model MIEW-7A; for drying and ashing switch 2, SW2, is closed and switch 1, S W 1 , is set in position 2; for vaporization, switch 2 is opened and switch 1 is placed in position 1
C
Figure 2. Vaporization chamber assembly
sample vapor outlet. E = brass electrodes, C = rubber seal, D = aluminum collar, E = sample injection port, and F = argon inlet A = argon and
limit of detection quoted in all these methods. Furthermore, to prevent blockage of the burner orifices, deproteinization and/or dilution of the sample (usually 1 : l O ) is recommended. It should also be noted that methods including classical nebulization techniques require a minimum of one milliliter of sample. In a n effort to produce a microsampling method for the determination of' lithium in serum, a n attempt was made to extend previous work using a modified Delves Cup procedure (8) in the atomic emission mode. The results, however, proved to be unsatisfactory, because of t h e inadequate vaporization temperatures produced in the nickel cup. Recently a carbon rod atomizer has been utilized t o permit the determination of microliter capillary samples containing lithium ( 9 ) . Other electrothermal devices that could undoubtedly be considered included graphite or tantalum filaments and platinum or tungsten wire loops. Such devices have proved both sensitive and reproducible. in both absorption and emission modes. There are, however, inherent disadvantages in the use of electrothermal atomizers and these have been summarized in a recent report (10). The report describes the incorporation of a tantalum filament vaporizer with a n inductively coupled plasma. T h e sepsration of the vaporization and atomization processes in this way minimizes the limitations imposed when electrothermal atomization is used in isolation, uiz.,critical opti-
Flgure 3. Tantalum filament
mization reproducibility, recombination and nucleation, incomplete vaporization and dissociation. The present paper describes a similar system utilizing a tantalum filament vaporizer associated with a conventional flame system. The technique has been employed in the determination of microliter samples of lithium in three different matrices: water. artificial serum. and reconstitut>ed human serum. EXPERIMENTAL Apparatus. Instrument specifications and nperatint: parameters are listed in Table I. No attempt was made t o focus the emitted radiation using lenses or mirrors; however. the flame was placed as close as possible t o the entrance slit of the monochromator (approximately 6 cm). The glass chamber and tantalum filament are shown i n Figures 2 and 3 . The glass chamber encloses the electrodes and filament and also provides an inert atmosphere when flushed with argon.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
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P
Flgure 4. Change of peak shape with concentration A = 0.1 ppm Li. B = 1.0 ppm Li, and C = 10.0 ppm Li. Sensitivity decreases by factor of 10 in going from A to B and from B to C
Figure 5. Effect of amplifier time constant and filament current on
(0005)
peak shape
(0051
IO 5)
I5 0)
(5001
(500)
LITHIUM CONCEhTRATION (ppm,S+I SAMPLE ) ABSOLUTE AMOLNT OF SAMPLE (NANOGRAMS) (NUMBER IN PAqENTHESES I
A = 1 ppm Li, 0.5-sec time constant (35 A): B = 1 ppm Li, 0.05-sec time constant (35 A); C = 1 ppm Li, 0.05-sec time constant (29 A)
Figure 6. Analytical curves for Li The tantalum filament (Figure 3) was fabricated locally. A die was constructed to allow filaments of uniform characteristics to be stamped from the 0.127-mm thickness tantalum foil (Alpha Inorganic No. 00331). A small indentation was made in the center of the filame,nt to accommodate the sample !
