sulfur compounds, as in refined products the sulfur compounds of high volatility are largely removed in processing. For crude oils, the presence of HsS and high volatility sulfur compounds might be expected t o lead to high results when a nonvolatile sulfur compound is employed as standard; as described below, however, this effect is not observed. Presumably the volatile sulfur components in crude oil are lost before analysis. Dibenzyl disulfide was chosen for use as the standard compound for use alone in MIBK to prepare the sulfur calibration graph and for addition of known amounts of sulfur to those oils analyzed by the standard additions technique. Results. The results obtained for the determination of sulfur in oils by direct comparison with a calibration curve produced from dibenzyl disulfide in MIBK and also by use of the standard additions technique are shown in Table 11. Each of the values in column 4 represents a n oil sample taken through the sample dilution and recommended procedure. The absorbance for each of these diluted samples was measured six times and the mean value used to calculate the sulfur content. The three values shown for each oil were obtained in different experiments conducted on different days over a period of several weeks; the standard deviations shown in column 5 therefore also take into account the long term repeatability. The values obtained by AAS for the oil samples whose sulfur contents were determined by reference to the calibration curve for dibenzyl disulfide in MIBK show a small systematic negative error compared to the mean values obtained by X-ray fluorescence. This most probably results from a slight mismatch in the viscosity characteristics between the diluted samples and standard solutions. This could possibly be eliminated if a sulfur-free base oil of similar viscosity was available t o add t o the standard solutions to match the nebulization characteristics. Even without matching the standard and sample solution viscosities closely, however, the accuracy attained compares quite favorably with the spread of results obtained between laboratories for sulfur determination in similar oils using the X-ray fluorescence spectrometry method recommended by the American Society for Testing and Materials (5). No significant systematic error was observed in the values for sulfur content obtained for the oils treated by the standard additions technique. This suggests that the
assumption that a mismatch between sample and standard viscosity characteristics is responsible for the error observed in the conventional calibration technique is correct; when suitable sulfur-free base oils are not available, therefore, use of the standard additions technique is preferable, particularly for examination of samples of low sulfur content (
