1514
ANALYTICAL CHEMISTRY
I n general, the phenomenon of self-ahsorption seems to be decreased using the chamber. This is probably due to the direction of gas flow which would tend to sweep the arc vqmpors aut of the optical path. DISCUSSION
Special atmosphere excitation in generalized semiquantitative analysis has distinct advantapes. The ratio of line intensities to background intensities is increased in a manner which lowers the limit of detection of an element. It is also more practical with this procedure to decrease the sample size, the sample employed being one half the standard Harvey m-eight. As the Harvey method of calculation depends upon the concept of minimum detectability of a line rather than true signal-noise ratio, the increase in signal-noise ratio does not directly correlate to the increased sensitivities. Dupliente or triplicate determina-
tions of the measured constants gave an a v e r a s devintiou from the mean of 5.3% for 32 lines. The method is n d l within the limits of error attributed to the original semiquantitative procedure. It has been proposed ( I ) that the method vould be especially good for the determination of elements of the rare earth group. LITERATURE CITED
(1) Harvey, C. E., correspondence. (2) Harvey, C. E., "Semi-Quantitative Spectrographic Analysis." Analysis," Applied Reseeroh Lsbomtoriea, 1947. (3) Omstein. L. S., and Brinkman, H., Physica. 1,797 (1933-34). (193334). (4) 0wen.L. E., J . Optical SOC.Am.,41.139 (1951). ( 5 ) Smith, D. M.,snd Wiggins. G. M., Analyst, 74,95 (1949). (6) Valles, B. L..Reimer, C. B., and Loofbourow. J. R.. J . Ontical SOC.An..40.751 (1950).
R E O ~ ~ VFebruaiy ED 2, 1951 1951.
Determination of Calcium, Aluminum, and Iron in Magnesite Spectrographic Rotating Electrode Solution Method R. E. MOSHER', E. J. BIRD,
AND
A. 1. ROYLE, Wayne Uniuersity, Detroit I , Mieh.
RECENT years the emission spectrograph has found inINcreasing use for the quantitative determinatiou of the metallic constitutenta of solutions. Numerous methods for the spectrographic analysis of solutions are described by Brode ( d ) . The customary hut time-consuming procedure is to evaporate t o dryness a drop of the solution t o be analy~edin s. craterieed carbon electrode, which i s subsequently subjected t o arc or spark exoitation (7). Solutions have been analyaed by direct introduction through a capillary into the arc or spark (6),and by the employment of a parous graphite cup through which the solution slowly diffuses (4). A more recent method is one in which a rotating carbon disk electrode is partially immersed in the solution to he analyzed. As the disk rotates, the sample is picked up on the surface of the electrode and brought under the influenceof high-voltage spark excitation. The details of this system for the analysis of biological materials are described by Boyle elal. ( I ) .
oped in D-19 developer for 2 minutes a t 70" F. A single film is sufficient for nine exposures. As triplicate exposures are made on sample and standard, a single film will accommodate two samples and one standard. If the material being analyzed is fairly uniform, it is not necessary to employ the splitAeld filter m-hich permits the measurement of n greater range of concentration. In this situation, 8. narrower single-field filter may be used, making possible sixteen exposures per film, which reduces the time of analysis somewhat and c o n s e i ~ film. e~
APPARATUS AND SPECTROGRAPHIC CONDITIONS
This communication describes a rapid and accurate method for the routine spectrographic solution determination of calcium, iron, and aluminum in magnesium-bearing ores by the rotating electrode technique. The apparatus in use is an Applied Researoh Laboratories 1.5meter grating spectroeraph equipped with a rotatina disk elec-
i i diameter and 0.125 i
