Effects of Inlet Capillary Temperature in Atmospheric-Pressure Infrared Laser-Ablation Plasma Postionization Mass Spectrometry

Mass spectrometry imaging (MSI) can be used to survey numerous molecular species from a wide variety of surfaces, including biological tissue sections. Atmospheric-pressure (AP) infrared laser-ablation plasma postionization (IR-PPI) has recently been shown to allow matrix free analysis of small molecules from both fresh frozen and formalin fixed paraffin embedded (FFPE) tissue. Detected ion intensities in IR-PPI as well as other AP inlet modalities such as desorption electrospray ionization (DESI) show a strong dependence on the inlet capillary temperature. In this study, the relationship between detected ion intensity and inlet capillary temperature is evaluated, between room temperature and 650 °C, for analyte pipetted on various substrates, as well as fresh frozen and FFPE tissue, by IR-PPI. Temperature trends for exemplar ions of interest show a variety of dependencies with optimal temperatures observed throughout this temperature range. For example, detection of lactate [M-H]^- <i>m</i>/<i>z</i> 89.0244 is optimal at ∼100 °C, glutamine [M-H]^- <i>m</i>/<i>z</i> 145.0618 at ∼250 °C, arachidonic acid [M-H]^- <i>m</i>/<i>z</i> 303.2324 at ∼150 °C and PI(18:0/20:4) [M-H]^- <i>m/</i>z 885.5488 at ∼500 °C. Data reduction and clustering of these data by uniform manifold approximation and projection (UMAP) and k-means provides a summary of all temperature trends within the data and association of different ions with these trends are presented. Finally, the implications of different inlet capillary temperature settings in tissue MSI are demonstrated by comparing detected glucose and lactate ion intensities in response to different inlet temperatures in mouse brain. The choice and control of inlet temperature are shown to be critical variables for the interpretation of biological MSI data in AP modalities.