Direct injection high efficiency nebulizer-inductively coupled plasma mass spectrometry for analysis of petroleum samples

Direct injection high efficiency nebulizer (DIHEN)-inductively coupled plasma mass spectrometry (ICPMS) is investigated for analysis of petroleum samples dissolved in volatile organic solvents. To minimize solvent loading, the solution uptake rate is reduced to 10 μl/min, far less than the level (85 μl/min) commonly used for aqueous sample introduction with the DIHEN, and oxygen is added to the nebulizer gas flow and outer flow of the ICP. Factorial design is applied to investigate the effect of nebulizer tip position within the torch and the nebulizer and intermediate gas flow rates on the precision and the net signal intensity of the elements tested for multielemental analysis. Cluster analysis and principal component analysis are performed to distinguish the behavior of different isotopes, oxide species and doubly charged ions. The best operating conditions at a solution uptake rate of 10 μl/min are: RF power=1500 W, nebulizer gas flow rate=0.10–0.12 l/min, intermediate gas flow rate=1.5 l/min and DIHEN tip position=3–4 mm below the top of the torch intermediate tube. Acceptable recoveries (100±10%) and good precision (less than 3% relative standard deviation) are obtained for trace elemental analysis in organic matrices (a certified gas oil sample and a custom-made certified reference material) using flow injection analysis. Because of high blank levels, detection limits are 1–3 orders of magnitude higher for organic sample introduction than those acquired for aqueous solutions.