Atomic Absorption Spectroscopy with Inductio n-Co upled PIasmas SIR: Two recent publications have reported the use of induction-coupled plasmas as excitation sources for the optical emission spectroscopic analysis of solutions ( 1 , 4 ) . These plasmas possess several distinctive features which suggest that they may be superior to combustion flames as absorption cells for atomic absorption spectroscopy. First, the much higher temperature and longer residence time of particles in the plasma should lead to a greater degree of conversion of the aerosol to free atoms. Second, greater control can be exercised over the chemical environment in plasmas than in flames. Thus, the lifetime of free atoms in the plasma may potentially be extended by providing an optimal chemical environment. Third, the very high temperature core region, through which the sample must pass, should minimize the depressant effects of “chemical” interferences-Le.! formation of stable refractory compounds by the element being studied-which are common in flames. Greenfield, Jones, and Berry ( 1 ) have already demonstrated that the depressant effect on calcium emission by phosphate or aluminum ions is virtually eliminated in the tail flame of their plasma. Fourth, the background radiation emitted by the tail flame of an argon plasma fed by an aqueous aerosol is markedly less than that of a combustion flame, especially those which burn hydrocarbons. Even when an a.c. photometric system is employed, the photomultiplier noise arising from the irradiation of the detector with uninterrupted background radiation is appreciable (proportional to the square root of the light intensity). Thus, this noise component should be markedly less for a plasma than for hydrocarbon flames. Fifth, the much greater energy available in a plasma gives it the potential capability of vaporizing refractory solid samples directly. Although we have not yet confirmed all of these expectations, the obervations presented in this communication demonstrate that inductioncoupled plasmas are versatile and useful absorption cells for atomic absorption spectroscopy.
F‘HOTOMWLTIWER
-‘SPECTROMETER
Figure 1 .
Optical system for atomic absorption
mounting scanning spectrometer with 1180 rulings/mm. grating blazed for 5000 A. Reciprocal linear dispersion of 16A./mm. in first order. Primary Source. Commercial hollow cathode lamps operated with d.c. power supply. External Optics. Primary radiation chopped at 13 C.P.S. and passed three times through discharge as parallel beam as shown in Figure 1. Portion of the plasma traversed by the primary radiation extended from 8.4 cm. t o 10.5 cm. above the core and was 1.0 cm. wide. Hollow cathode image was focused a t the slit. Slit Width. Fifty microns, with slit height of 6 or 10 mm. dependent on size of hollow cathode. Detector. EM1 6255B photomultiplier coupled t o the 13 C.P.S. amplifier of Perkin-Elmer Model 13 infrared spectrophotometer. Photocurrent recorded on Leeds and Northrup Speedomax recorder. Time constant of the recording electronics was approximately 2 seconds. RESULTS AND DISCUSSION
Typical detection limits and sensitivities observed for elements in aqueous solution are given in Table I. The detection limit is defined here as the concentration in solution required to produce an absorption signal equal to twice the standard deviation in the background fluctuation. The sensitivity
Table 1.
Detection Limits and Sensitivities of Elements
Detection Limit
EleLine ment (-4.) (pg./ml.) A1 3092.8 1 3944.0 1 3961.5 0.6 Ca 3933.7a 0.7 4226.7 0.2 Mg 2852.1 0.06 Nb 3535.3 100 4058.9 40 4079.7 40 4100.9 30 Re 3460.5 30 Ti 3635.5 15 7 3642.7 3653.5 9 4667 6 5 4681.9 9 5174.8 5 W 4008.8 3 4659.9 3 V 3184.0 2 Y 3327.95 20 3620.9 10 3710.30 20 4077.4 20 4102.4 10 4643.7 10 0 Ion lines.
Semitivitv (pg./&
for 1% absorption) 1 0.9 1 1 0.6 0.1 50 60 70 60 130 20
20 20 15 15 20 10 9 3 40 40 30 40 40 40
EXPERIMENTAL FACILITIES
The induction-coupled plasma, the aerosol generator, and their mode of operation are essentially the same as except for the described previously (4, following modifications : Plasma. Coolant tube: 19 cm. total length, extending 8.2 cm. beyond coil. Spectrometer. Jarrell-Ash, Boston, Mass., Model 82000, 0.5 meter Ebert
WAVELENGTH (A.)
Figure 2.
Emission spectrum of plasma tailflame VOL 38, NO. 2, FEBRUARY 1966
337
j
6
P
o
5
10
IS
20
CONCENTRATION RATIO: P 0 4 / C o
Figure 3. Interference effect of phosphate on calcium absorption The data of David were taken from David, D. J., Analyrf 85, 495 (1 960).
is that concentration required to produce 1% absorption. Several significant conclusions can be drawn from the data presented in Table I. Although only 0.12 ml. of aerosol per minute passed through the plasma, the observed detection limits and sensitivities for the strong monoxide-forming elements (Al, Nb, Ti, W, Y) are comparable t o the best reported flame absorption values (2, 3 ) . Since aerosol flow rates for flames are commonly an order of magnitude greater, the comparable sensitivities observed indicate that either the degree of free-atom formation in the plasma is considerably greater or monoxide formation is greatly reduced.
-
'
The data in Table I also show that useful sensitivities are observed for absorption lines in the 4600 to 5200 A. spectral region. This wavelength interval is often avoided in flame spectroscopy because the strong CZband emission increases the d.c. noise component of the photomultiplier detector. The zone of the plasma examined in this study has virtually no band or continuum emission even when an aqueous aerosol is added. The only significant background emission, aside from the argon lines, in the zone of the plasma examined in this study is the 3064 A. OH system (see Figure 2). It is worth noting that the hollow cathode emission lines employed in this investigation produced photocurrents from 8 to 200 times greater than the band head a t 3063 A. Our expectations that chemical interferences would be markedly reduced in the plasma were confirmed, as shown in Figures 3 and 4, by the behavior of calcium in the presence of phosphate or aluminum ions. In contrast to the sharp depressive effect of these ions observed in flame absorption, the calcium absorption in the plasma shows a slight but surprising increase. Although this apparent enhancement invites speculation, it seems advisable to defer discussion until more definitive information on the various processes occurring in the plasma is available.
i
$40 I "01 :
L /
"
'INDUCTON COUPLED
-0
PLASMA
AIR-PIXTYLENE FLAME (DAVID1
a
AIR- ACETYLENE F L A M E (ISU)
0
s
IO
IS
20
CONCENTRATION RATIO: AllCo
Figure 4. Interference effect of aluminum on calcium absorption The data of David were taken from David, D. J., Analyst 85, 495 ( 1 960). LITERATURE CITED
(1) Greenfield, S., Jones, I. Ll., Berry, C. T., Analyst 89, 713 (1964).
(2) Manning, D. C., Atomic Absorption Newsletter 4, 267 (1965). (Perkin-
Elmer. Corp., Norwalk, Conn.).'
(3) Slavin, Walter, Atomic Absorption Newsletter, No. 24, 15 (1964), (Perkin-
Elmer Corp., Norwalk, Conn.).
(4) Wendt, R. H., Fassel, V. A., ANAL.
CHEM.37, 920 (1965). H. WENDT RICHARD VELMER A. FASSEL Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa RECEIVED for review September 27, 1965. Accepted December 13, 1965.
Gas Chromatographic Separation of Hydrazine, Monomethylhydrazine, and Water SIR: There has been no literature available describing a satisfactory method for the gas chromatographic separation of a mixture of hydrazine, monomethylhydraaine (MMH), and water, although a number of procedures for the separation of hydrazines, water, and various hydrazine mixtures have been published (I,%'). The method described here shows an excellent separation of water, MMH, and hydrazine on a column using 10% Dowfax 9N9 on Teflon 6 ( 3 ) . EXPERIMENTAL
Chemicals, The hydrazine and M M H are products of the Olin Chemical Corp., Lake Charles, La. The Teflon 6 (registered trademark for tetrafluoroethylene (TFE) fluorocarbon resin) is a product of E. I. d u Pont de Nemours & Co., Wilmington, Del. The Dowfax 9N9, a product
338
ANALYTICAL CHEMISTRY
of the Dow Chemical Co., Midland, Mich., has a chemical composition of
Apparatus. All analyses were performed with a Perkin-Elmer Model 154-C vapor fractometer using a dual chamber thermistor thermal conductivity cell. Chromatograms were recorded on a Leeds and Northrup 5-mv. recorder, a t a chart speed of l/z inch per minute. An Instron automatic integrator, two counter model, was used in integrating the peaks. A sample size of approximately 5 111. was used in each case. The column packing was prepared by air-drying a slowly stirred slurry of Dowfax 9N9 in methanol in amounts to produce a 10% Dowfax 9N9 coating, and subsequently was dried in a vacuum oven a t 100' C. The material was then chilled, screened through a 30mesh sieve, and carefully packed in a
l/d-inch o.d., 6-fooblong stainless steel column. The column was stabilized overnight a t 150' C. at a 20 ml. per minute helium flow rate. During analysis, a temperature of llOo C. and a helium flow rate of 40 ml. per minute were maintained. The column was conditioned prior to each day's use by one or more injections of approximately 10 pl. of hydrazine. Although separation required only about 7 minutes, an additional 10 minutes were allowed before the next analysis was performed. Sample Preparation. Samples of hydrazine and M M H were analyzed and an by an acidimetric method (4, 0.2% hydrazine analysis of 97.0 and 90.9 =t0.2y0MMH was obtained. Test mixtures were prepared by adding different amounts of water to hydrazine and to MMH, and by mixing hydrazine and MMH in various percentages. The per cent composition by weight of some of the mixtures is given in Table I.
*
