Microwave
induced plasma atomic emission spectroscopy (MP-AES)
with nitrogen gas was employed for the determination of trace elements
in petroleum fractions with a boiling point above 1000 °F, using
direct dilution in an organic solvent. Nitrogen produces robust plasma
for the analysis of fractions having an American Petroleum Institute
(API) gravity range from −2° up to 9.7° and elemental
composition as follows: S (1.0–5.5 wt %) and N (200–14 000
mg kg–1). A satisfactory limit of quantification
and spike recoveries at low and high concentration levels were determined
for Na, K, Ca, V, Fe, Ni, and Mo present in different residua samples.
The recoveries obtained from the analysis of three QC test materials
were within ±10% of the actual and or certified values. It was
found that V, Fe, and Ni are the most predominant elements present
in these samples and the effect of the source of sustained plasma
gas as well as sample preparation method was focused on the quantification
of these three elements. Using a direct dilution method, V calculated
recoveries using NIST 8505 CRM (104% and 109%) are found independently
of the plasma sustained gas source (argon vs nitrogen). Comparing
sample preparation methods (wet ashing vs direct dilution), recoveries
of 99% and 109% were determined. Furthermore, for Ni, regardless of
the fact that the NIST 8505 CRM reference value is not available,
the calculations using our measured value of 51 mg kg–1, indicate that that nickel quantification is not sensitive to either
sample preparation or detection methods, giving recovery of 100% in
all cases. For Fe, the quantification is susceptible to a sample preparation
method; microparticles, which are possible to be present in such fractions,
cannot be homogeneously dispersed into the organic media, leading
to bias to low measurements in most cases. It seems that better results
are obtained using wet acid digestion. Finally, it was shown that,
when using nitrogen-based plasma (MP-AES) versus argon-based plasma
(ICP-OES), excellent agreement in all cases for vanadium and nickel
was determined, indicating that MP-AES represents an alternative for
analysis of these complex samples.
