I
CORRESPONDENCE
Microanalysis of Solids by Atomic Absorption and Emission Spectroscopy Using an R. F. Furnace SIR: Attempts have been made to extend the application of atomic absorption spectroscopy to the analysis of solid samples by replacing conventional flames with resistance furnaces (1-4). Although their detection limits are impressive, no practical analytical method has been offered. Part of the problem is the awkwardness of the systems for processing samples. Recent studies in this laboratory using an R. F. induction furnace as a source reveal the possibilities of a simple and rapid method for the direct analysis of microsamples of solids or evaporated solutions. An induction heated graphite crucible serves as a thermal means of vaporization and atomization, while the excitation of the atomized sample is achieved mainly due to the helium plasma formed by the R.F. field. As with flame sources, both atomic emission and absorption can be used, but this technique provides more control for chosing the preferred one. Experimental Facilities. The furnace, a graphite crucible (2.2 cm o.d., 1.6 cm i.d., 11 cm length), is surrounded by an insulating layer of carbon black, and placed in a quartz thimble. This assembly is raised into position through the center of the induction coils. The system as a whole is sealed and protected by a helium environment (see Figure 1). In this study a Leco induction furnace (Leco No. 537-100) was used. It consumes 4.5 kW, operating at 3 Mc and is capable of reaching temperatures as high as 2500 “C as measured by optical pyrometry. For absorption measurements the light beam from the hollow cathode lamp passes through the plasma plume located over the mouth of the graphite crucible and is absorbed by the sample vapor. The beam then travels to a Jarrel-Ash 0.5 meter Ebert mounting scanning monochromator with an 1180 ruling/mm grating blazed to 3000 A. Entrance slit height and width are kept at 19 millimeter and 10 microns respectively. A 1P-28 photomultiplier is attached to the exit slit of the monochromator. The output signal from the photomultiplier is amplified by a Keithley 417 Picoameter or an Ithaco 355 lock-in amplifier when chopping of the beam is necessary, and then sent to an L & N Speedomax G recorder. The time constant of the recording electronics is 300 milliseconds, For atomic absorption sources commercial Westinghouse hollow cathode lamps operated with a d.c. power supply were used. External Optics. For atomic absorption, a 30-mm diameter 100-mm focal length quartz lens focuses the chopped (177 cps) hollow cathode beam at the sample vapor plume. A second quartz lens (65-mm diameter, 150-mm focal length) focuses the partially absorbed beam on the entrance slit of the monochromator. In atomic emission, only the plasma-vapor plume is focused. (1) A. S. King, Astrophys. J., 56, 318 (1922). (2) H. Massrnan, Spectrochim. Acta, 23B,215 (1968). (3) Ray Woodriff, Ronald W. Stone, and Andrew M. Held, Appl. Spec., 22, 408 (1968). (4) B. V. L’vov, Spectrochim. Acta, 24B,53 (1969).
Quartz window
Helium Inlet
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Sornple introduction chornber
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Helium inlet
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Figure 1. R.F. source chamber.
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Sample introduction chamber
Figure 2. Sample introduction chamber Sample Introduction. To avoid opening the reaction tube to the atmosphere, samples are introduced through a Teflon (DuPont) sample chamber (Figure 2) which is flushed continuously with helium. The sample chamber is brought into position above the furnace mouth and is rotated, causing the sample to fall into the crucible. Two different substrates on which the samples are deposited are used to introduce them into the furnace. These include small graphite cups and tantalum foil disks. In the case of the solution analysis, the sample is deposited on the substrate by means of a microsyringe and then dried. For solids the sample is weighed on the tantalum disk and then immobilized with 1-2 microliters of a dilute collodion solution. Analysis Procedure. Degassing of the system is carried out at 950-mA plate current for 30 minutes. The furnace is ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
0
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Figure 3. Emission signals for elements detected in freeze dried liver
Table I. Detection Limits by Atomic Emission and Absorption
Element Ag A1
Au Bi
Ca Cd Cr Fe Ga Hg I
In Mg
P Pb S
Si Te Zn
Analytical line (A) 3280.683 3944.032 2675.95 3067.716 4226.728 3261.057 2288.018 4254.346 3719.935 4032.982 2536.519 3650.146 2061.63 4101.773 2852.129 2553.28 3639.580 4433.4" 2575. 6b 2516.123 2142.75 2383.25 3075.901 2138.56
Excitation energy (eV)
3.78 3.14 4.63 4.04 2.90 3.80 5.4 2.91 3.3 3.1 4.88 8.86 5.9 3.0 4.3 7.1 4.4 4.9 5.8 5.8 4.03 5.8
Emission detection limit (gm)
Absorption sensitivity
10-10 10-9 10-9 5 x 10-10 10-9 10-11
10-10
(gm)
10-8 10-11
10-9 10-8 10-9 10-11 10-9 5 x 10-8 10-9 10-10 10-8 10-9 10-8 10-8 5 x 10-8 10-9 10-9 10-11
10-9
10-10
10-11
Molecular band head of Sz. Molecular band head of CS.
then set at the appropriate plate current value, depending on the temperature and plasma energy required for the particular analysis. RESULTS AND DISCUSSION
Detection limits for those elements that have been studied are given in Table I. For this preliminary study the detection 810
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7 , JUNE 1970
limit was taken as that amount of element which produces an emission signal twice that of the background signal. For absorption measurements the sensitivity is that amount of material which results in 1% absorption. Although there are more precise methods of establishing detection limits, we chose the present method since the results are preliminary and further understanding and control of a number of experimental parameters should result in extension of these limits. For immediate application to analysis of samples many of the elements normally encountered would be well above the limits found. The R.F. furnace approach indicates broad scope with particular applicability to the determination of nonmetals and other elements that are difficult to determine by flame emission. The excitation energies for the chosen analytical lines do not exceed 8 eV with best results occurring for those transitions involving 3-5 eV. The efficiency of the plasma as an excitation source, when all the factors that determine its features are kept constant, depends solely on the amplitude of the R.F. field strength. Thus proper variations of plate-current values can be used to control the ratio of excited to ground state atoms to such an extent as to produce either optimal atomic absorption or atomic emission conditions for a given analytical line. It should be possible to extend the list of applicable elements when further control of the system parameters has been investigated. Those shown to play an important role are (1) helium flow rate, (2) design of crucible and its position in the furnace, (3) the substrates on which the samples are deposited, and (4) optimization and stabilization of the R.F. field. The R.F. field influences both the temperature of the furnace and the plasma energy. Using evaporated solution samples up to 10 microliters containing the individual analytes, the precision of the measurements with the present system is l-4z depending upon the element. When powdered samples were directly analyzed, the precision was 4-10z. Although most measurements made here were based on peak height, some were also made by integrating the signal over the total observation time with comparable results.
An important feature of the technique is the speed and simplicity of running samples. Using microsamples it is possible to introduce a new sample every 30 seconds. Emission signals for the elements: Pb, Cd, K, Na, Rb, Te, Al, Cs, Sr, P, Zn, Mn, Cu, Ag, Cs, and Mg obtained from 0.2-1 mg freeze dried liver samples (Figure 3) are typical of the sharp lines of 1-10 sec duration normally obtained. Preliminary studies already have shown positive quantitative results. Although the results are of a preliminary nature, they indicate the approach is capable of producing a simple
and rapid method of broad scope for elemental analysis of a wide variety of solid materials. G . H. MORRISON YAIRTALMI Department of Chemistry Cornell University Ithaca, N.Y. 14850 RECEIVED for review February 9, 1970. Accepted March 31, 1970. This work was supported by the Advanced Research Projects Agency through the Cornell Materials Science Center.
AIDS FOR ANALYTICAL CHEMISTS Modification of a 30 MHz Plasma Torch for Gas Analysis and Comparison with a 2450 MHz Plasma C . David West Department of Chemistry, Occidental CoIIege, Los Angeles, CaI$ 90041
PAPERSby Cooke, Lisk, and others (1-4) have demonstrated the sensitivity and selectivity with which gas chromatography effluents, in particular, may be determined with a microwave plasma emission detector. In contrast to the substantial work done with microwave plasma detectors, little work has been done on the use of 30 MHz (non-inductive) plasmas for gas analysis. The purpose of this paper is to report on a sideby-side comparison of the two plasma detectors for this application.
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EXPERIMENTAL Apparatus. 30 MHz PLASMA.The r.f. plasma used is electronically identical to that used previously (5). The oscillator power supply is capable of 500 watts (200 mA at 2500 V) but for the work described here an input power of only 75 to 100 watts was used to drive the oscillator. Figure 1 shows the design of the torch tip and chamber adopted for gas analysis with the 30 MHz plasma. To understand the orientation of this torch tip with respect to the oscillator tank coil, refer to Figure 1 of Reference 5, where an expanded view of coil and tip is shown. The conventional conical tip shown in this previous paper is removed and replaced by the molybdenum unit labelled C in Figure 1 of this paper. A simplified top view as well as a front view are shown in the figure. The different crosshatched areas indicate separately removable or adjustable sections of the tip. A 1-mm i.d. by 3-mm 0.d. quartz chimney ( A ) is fused to a 13-mm 0.d. Vycor base, which then fits gas-tight over the high-temperature O-ring (E). The central tip may be adjusted up and down by the inner threads shown so as to be positioned just inside the 1-mm capillary when ( A ) is in position. The plasma is thus formed entirely inside this capillary. For attachment to the chromatographic column effluent, a standard l/16-in.stainless tube was connected to the outlet (1) C. A. Bache and D. J. Lisk, ANAL.CHEM.,37,1477 (1965). (2) Ibid., 38, 783 (1966). (3) Ibid., p 1757. (4) . , A. J. McCormack, G. G. C. Tong, - and W. D. Cooke, ibid., 37, 1470 (1965). (5) C. D. West and D. N. Hume, ibid., 36, 412 (1964).
C Figure 1. Torch tip and quartz chimney A . Quartz chimney B. High temperature O-ring C. Assembled unit. All parts molybdenum. Separately cross-hatched areas indicate the three separately removable sections
of the column, passed into the tank coil of the oscillator, and terminated at a plug containing a 1-mm hole through its center. The tip was attached by means of the threads shown to the end of the tank coil, where it nearly bottoms out on the aforementioned plug. This arrangement allows but little hold-up volume as the gas flows from the stainless tube through the six equally spaced holes of ( C ) and into the capillary (A). This assembly gave detection limits significantly lower than any of several others tested and was used for the results described here. Chimneys of a larger diameter bore had less sensitivity and stability, whereas smaller ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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