Direct determination of metals in lubricating oils by laser ablation coupled to inductively coupled plasma time-of-flight mass spectrometry

Rapid simultaneous multielemental determination of Na, Mg, Al, Ti, Cr, Fe, Ni, Co, Cu, Ag and Pb in lubricating oil samples was carried out by combining direct laser ablation (LA) of the oils with inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS). This method offers the advantage of a simplified sample preparation technique without the need for sample decomposition or sample dilution with organic solvents, although dealing with a relatively complex matrix. Because of the high data acquisition speed of the ICP-TOFMS an enormous amount of sample-information can be collected within a short period of time. The used system can acquire complete mass spectra at 20 kHz, a rate leading to data resulting from the average of 20000 individual spectra, considering a short integration interval of only 1 s. Consequently, analysis time is reduced drastically, although the multielemental capabilities are improved, compared to conventional quadrupole mass spectrometers. Two different calibration strategies were investigated and showed good agreement: external calibration based on the use of different standard oils and aqueous solution-based calibration. The analysis of a certified reference material SRM 1634b showed good accuracy for all the investigated elements. Detection limits were in the range of 0.5 ng g(-1) (Pb) to 28 ng g(-1) (Cr) and the precision was typically around 6% RSD. The good agreement obtained on fresh and used lubricating oil samples using conventional ICP-TOFMS after sample decomposition demonstrates that direct LA-ICP-TOFMS is a rapid and powerful tool for the analysis of trace and wear metals in lubricating oil.