492 Subsurface Geologic Methods 



spectrograph has become widely adopted, and, with the advent of specially 

 ruled gratings capable of throwing much of the incident energy into a 

 specified order, it is competing successfully with the prism instrument. 

 Where the design of grating spectrograph is such that dispersion is approx- 

 imately linear, identification of spectrum lines is facilitated by using 

 linear interpolation rather than the cumbersome Hartman formula re- 

 quired for prism spectrograms. In the United States grating spectro- 

 graphs are available from the Applied Research Laboratories, the Baird 

 Associates, and the Jarrell-Ash Company. 



Standards 



The writers used Slavin's "total-energy method" ^^ as being best 

 suited to the problem under investigation. This method utilizes an external 

 standard, which in this case consisted of a mixture of equal quantities of 

 United States Bureau of Standards standard samples No. la (argillaceous 

 limestone) and No. 88 (dolomite) of known composition. This mixture 

 gave consistent results and performed in the arc similarly to the materials 

 under investigation. Initially synthetic standards were used, consisting 

 of carefully purified calcium carbonate containing known quantities of 

 added barium sulphate, strontium sulphate, silica, and ferric, aluminum, 

 magnesium, and titanium oxides. These, however, gave erratic results, 

 primarily owing to a proved inability to obtain thorough mixing of the 

 constituents and also to the fluffy nature of the precipitated calcium car- 

 bonate, which led to improper volatilization in the arc. 



The employment of the internal-standard method is theoretically 

 sounder than that of any method using external standards, but the addi- 

 tional loss of time required in weighing out the standard and the very 

 difficult problem of obtaining thorough mixing of each unknown with the 

 standard make the method seem puite unattractive. Kvalheim ^^ has de- 

 scribed a useful internal-standard method for determining sodium, potas- 

 sium, aluminum, calcium, magnesium, iron, and manganese in minerals, 

 rocks, and slags, employing strontium carbonate as the standard material. 

 When silicon is to be determined, a mixture of beryllium carbonate and 

 sodium chloride is introduced as the internal standard. 



A most interesting method of spectrochemical analysis based on 

 measurement of line widths has been described recently by Coheur.^^ This 

 procedure would seem to have the advantage of rapidity and furthermore 

 is applicable to elemental concentrations higher than those usually meas- 

 ured in such work. 



In using Slavin's or any other external-standard method it is neces- 

 sary to photograph at least six spectra of the standard on the same film 



^^ Slavin, M., Quantitative Analysis Based on Spectral Energy: Ind. and Eng. Chemistry, vol. 10, 

 p. 410, 1938. 



^^ Kvalheim, A., Spectrochemical Determination of the Major Constituents of Minerals and Rocks: 

 Optic. Soc. America Jour., vol. 37, p. 585, 1947. 



^' Coheur, P., A Method of Quantitative Spectrochemical Analysis Based on Line Widths: Optic Soc 

 America Jour., vol. 36, p. 498, 1946. 



