Nonmetal Analysis of Micro Quantities of Solids by Emission Spectrum E. L. GUNN Humble O i l a n d Refining Co., Baytown, Tex.
The high-voltage discharge of a commercially available spectrographic source unit has been utilized to produce the emission spectra of certain nonmetals, halogens and sulfur, in minute solid samples. An analytical technique has been employed which has been found to be especially suitable for the rapid analysis of micro quantities of materials occasionally encountered in a petroleum laboratory-i.e., ash matter, deposits, surface films, paints, erosion products, and precipitatesfor which ordinary chemical methods are inadequate or inapplicable because of the limited amount of sample available. Calibrations for sulfur and chlorine in inorganic solids have been established to cover the working ranges of 1 to 18% sulfur and 1 to 45% chlorine. The relative accuracy, measured by the average deviation from theory in these ranges, is about 18%. Certain variables which may affect the excitation characteristics of selected spectral lines have been considered.
T
methods used have demonstrated that, in sample preparation as in excitation, a n inordinate effort is not required to obtain their emission spectra. Mansfield, Fuhrmeister, and F r y ( 5 ) fabricated a substance to be analyzed into a silver pellet which then was subjected to a high energy electric discharge. Since this method of sample preparation appeared to be an easy and convenient way of introducing the sample into the spark it was selected for investigation with the commercially available spark soul ce unit regularly used in this laboratory. EQUIPMENT
The source of excitation used for nonmetals excitation is the Model 4700 high precision source unit supplied by Applied Research Laboratories (ilRL). T h e spectrograph is a n ARL 1.5-meter instrument with a grating of 24,000 lines per inch and affording a dispersion of about 7 A. per mm. in the region of spectral measurement used. Photometry was made with the ARL comparator-densitometer. STANDARDS AND SAMPLE PREPARATION
HE emission spectrum provides one of the most direct and
unequivocal methods known for the detection and identification of the elements. It has been very generally applied in the analysis of about 70 of the metal- or metalloid-type element* in various substances encountered in research or industrial laboratories. It has been somewhat less commonly applied to the detection and determination of either the nonmetal or inert elements. This, of course, constitutes one limitation to the general utility of the emission spectrograph as an analytical instrument. For example, not infrequently a minute quantity of a substance is submitted to the petroleum refining laboratory in which nonmetal composition is of paramount importance. Thus, a significant incentive exists for overcoming those limitations which account for the instrument not being as commonly used for nonnietals as it has been for metals in micro amounts of solids. The less frequent applications of emission spectrometric methods to nonmetals (or to inert elements) resides in special considerations of the spectrum characteristics involved or thr techniques by which the emimion spectrum desired may be produced in analysis. The characteristic spectrd lines of most of the metals require excitation potentials of the order of 4 to 8 electron volts, whereas for desired lines of the nonmetal or inert elements the potentials may tie 2 to 4 times as high as f o r the metals. lMoreooer, excitation of the high potential spectral lines becomes an increasingly greater problem in instances where easily excited elements coexist with those elements difficult to excite; the effective energy potential of the discharge zone thus may be controlled by the low excitation element?, so that the higher energy levels required for nonmetals are seldom or never attained. Some of the more intense spectral lines of the nonmetals occur in the far or vacuum ultraviolet region; these lines are, of course, unavailable with the conventional optical systems ordinarily used in spectrographic analysis because either quartz lenses or air in the optical path absorbs the radiation. I n case the material analyzed is liquid or gaseous a t normal conditions of temperature and pressure, still further complications may be encountered in attaining satisfactory excitation of the sample in its existing state. Hence, excitation of nonmetals as they occur in solid compounds is, in some respects, the easiest method of obtaining their spectra. Several techniques have been described (1, 4-6, 8) in which use is made of a highly condensed high voltage spark discharge for exciting the high energy level lines of nonmetals. The
Calibration Standards. Reagent compounds of known compo$tion are used for spectrographic calibration-i.e., blends containing sulfur and chlorine are prepared from high-purity reagent chemicals t o contain varying but known amounts of these nonmetals. It is emphasized that the compounds used be adequately powdered so as to pass a 200-mesh screen.
Figure 1.
Curve for Determining Chlorine in Inorganic Solids
Internal Standard. The internal standard powder for blending with the sample is a mixture which consists of 1 part potassium bromide (Baker’s c.P.) and 3 parts of precipitated silver powder (Amend Drug and Chemical Co. ). These powdered components must pass a 200-mesh screen. Since silver powder is very malleable, the mixture is not ground. The components are admixed by continuous stirring for 5 minutes with a small glass rod. Sample Powdering. T h e dry sample, standard or unknovm, is thoroughly ground Tyith a micro mortar and pestle, weighed into a small glass vial with a precision analytical balance, then thoroughly blended with an equal weight of internal standard. Pellet Preparation. Satisfactory pelleting can be performed either by a pelleting machine or by a manually operated set. For manual preparation, a Stokes 3/1B-inchdie set with upper and lower punch is used. A 3/8-inchshim is used between the shoulder 1815
ANALYTICAL CHEMISTRY
1816 of the short punch and the die t o allow for adequate filling depth. -4pproximately 6 mg. of the sample-internal standard portion is added t o the die with a micro spatula. Silver powder is tamped into the die by firm hand pressure on the upper punch until the die is almost filled. Another sample-internal standard portion finally is added and the upper punch given three or four $harp taps with a hammer to press the silver into a firni pellet of about l/c-inch length. Thus, after carefully removing the pellet from the die, two spectra may be obtained by alternate sparking of opposite ends of the pellet. If the sample amount permits, 4 spectra should be obtained to improve the precision of the measurements. The counter pellet is made in the same manner as the sample pellet except that no sample-internal standard blend is introduced. Scrupulous care must be exercised to avoid contamination in preparing and handling the pellets. Two 2-inch pieces of stainless steel tubing of 3/16-inch inside diameter and split lengthwise with i/r-inch hacksaw slitq may be used for holding the pellets firmly in sparking.
chlorine contents were calculated froni the weights of the coniponents used in preparing them. Compounds that were used as the basis oi calibrations foi the analyses of Table I were in the form of blends containing known ratios of Baker's C.P. potassium sulfate and pota~siuiii chloride. The analysis value of each single spectrum rworded on the substance analyzed is shown; normally, four spectra are measured to improve precision in a determination on a substance. The results presented show the absolute deviation of each analysis value froin theory with the difference froni theory expressed as relative per cent. The average deviation from theory for chlorine and sulfur is about 18%. Slightly better results were obtained on the potassium sulfate-potassium chloride blend than were obtained on other inorganic substances analyzed; this
SPECTROGRAPHIC TECHNIQUE
The high voltage case of the -1700 unit is used for excitation and the most highly condensed sparklike discharge available is used. The air inductance coils are removed from the circuit so that a minimum of residual inductance remains. A nominal r a d i o - f r e q u e n c y current of 8 amperes flows during a sparking time of 30 seconds. The gap distance between the ellets is fixed a t 0.5 mm. witR a metal spacer and the gap position is at the optical axis of the instrument. A 40-micron slit setting is used and total passage of light is allowed with no shutter or filter being placed in the path to attenuate the incident beam. One or more control spectra of standards may be recorded on the same plate or film with an unknown in case references for line identification are desired. The exposed emulsion is processed and measured in a conventional manner. The Spectrum Analysis KO.1 emulsion is calibrated for the spectral region of measurement by the homologous iron line method ( 3 ) . The recording, m e a s u i P nient, and interpretation foi, standards or for unknowns are carried out in the same manner. TKOcalibration working curves for chlorine and sulfur are shown in Figures 1 and 2, rcspectively.
Table I.
Spectrographic Analysis of Solid Substances for Sulfur and Chlorine Content Chlorine, 7 Sulfur, % Diff. ~Diff. rinaly-
Rel.
Substance
Theory
31s
Diff.
Ferrous ammonium sulfate
16.3
14 4
-1 9
13.0
-2.4
.5.0 4 8 4.4
+1.1 +O 7
AV.
Blend, FelOa, PhS, KCl
5.2
6.2 7.3 5.4 5 7
+1.0
