Direct analysis of trace elements in crude oils by high-repetition-rate femtosecond laser ablation coupled to ICPMS detection

IR-femtosecond pulses were used at high repetition rates (up to 10 kHz) to ablate viscous crude oils for the determination of trace elements by ICPMS. A special internal glass cap was fitted into the ablation cell to minimise oil splashes and remove big particles that would be otherwise spread into the cell. Laser ablation in static and dynamic conditions (i.e. the laser beam being moved rapidly at the surface of the sample) was studied together with some fundamental parameters like repetition rate and fluence. Signal sensitivity and stability were found to be strongly affected by repetition rate and fluence, though not in linear manner, and in some circumstances by the laser beam velocity. Sample transport efficiency was found to decrease with increasing repetition rate, probably due to stronger particle agglomeration when increasing the density of primary particles. ICPMS plasma atomisation/ionisation efficiency was also found to be affected to some extent at the highest repetition rates. Moderate repetition rate (1 kHz), high fluence (24 J cm⁻²) and fast scanning velocity (100 mm s⁻¹) were preferred taking into account signal intensity and stability. Sample transport elemental fractionation was also evidenced, particularly as regards to carbon due to volatilisation of volatile organic species. Matrix effect occurring when comparing the ablation of transparent (base oil) and opaque (crude oil) samples could not be completely suppressed by the use of IR femtosecond pulses, requiring a matrix matching or a standard addition calibration approach. This approach provided good accuracy and very low detection limits in the crude oil, in the range of ng g⁻¹.