Ionization matrix effects in plasma-based ambient mass spectrometry sources

Ambient desorption/ionization mass spectrometry (ADI-MS) is an emerging field that aims to eliminate sample pretreatment and separation steps by directly desorbing and ionizing analytes from a sample surface for analysis by mass spectrometry. Although numerous applications of ADI-MS have been presented, little has been done to characterize problems caused by matrix effects. In the present study, ionization-related matrix effects were investigated for three plasma-based ADI-MS sources: the flowing atmospheric-pressure afterglow (FAPA), direct analysis in real time (DART), and the low-temperature plasma (LTP) probe. Small amounts of vapor-phase matrices were mixed with a continuous stream of gaseous analyte and introduced into each ionization source. A decrease in analyte signal upon introduction of a matrix signaled an ion-suppression event. When the matrix species had a proton affinity the same as or greater than that of the analyte, all three sources suffered analyte signal suppression, even at moderate matrix-to-analyte concentration ratios. In every case, the FAPA was the least susceptible to the ion suppression process. In contrast, when the proton affinity of the matrix species was lower than that of the analyte, no matrix effect was observed with DART, although an effect persisted for both FAPA and LTP. Indeed, matrix-to-analyte mole ratios of 10 were sufficient to entirely suppress analyte ion signals in the LTP. These findings demonstrate that matrix effects in ADI-MS are important for qualitative as well as quantitative analyses.