Factors Affecting Quantitative Analysis in Laser Desorption/Laser Ionization Mass Spectrometry

Microprobe laser desorption/laser ionization mass spectrometry (μ ²MS) is a sensitive and selective technique that has proven useful in the qualitative and semiquantitative detection of trace organic compounds, particularly polycyclic aromatic hydrocarbons (PAHs). Recent efforts have focused on developing μL²MS as a quantitative method, often by measuring the ratio of signal strength of an analyte to an internal standard. Here, we present evidence of factors that affect these ratios and thus create uncertainty and irreproducibility in quantification. The power and wavelength of the desorption laser, the delay time between the desorption and ionization steps, the power of the ionization laser, and the ionization laser alignment are all shown to change PAH ratios, in some cases by up to a factor of 24. Although changes in the desorption laser parameters and the delay time cause the largest effects, the ionization laser power and alignment are the most difficult parameters to control and thus provide the most practical limitations for quantitative μL²MS. Variation in ratios is seen in both synthetic poly(vinyl chloride) membranes and in "real-life" samples of Murchison meteorite powder. Ratios between similar PAHs vary less than those between PAHs that differ greatly in mass and structure. This finding indicates that multiple internal standards may be needed for quantification of samples containing diverse PAHs.