LESA MS Imaging of Heat-Preserved and Frozen Tissue: Benefits of Multistep Static FAIMS

We have previously
demonstrated liquid extraction surface analysis
(LESA) high field asymmetric waveform ion mobility spectrometry (FAIMS)
mass spectrometry imaging of proteins in thin tissue sections of brain
and liver. Here, we present an improved approach that makes use of
multiple static FAIMS parameters at each sampled location and allows
a significant improvement in the number of proteins, lipids, and drugs
that can be imaged simultaneously. The approach is applied to the
mass spectrometry imaging of control and cassette-dosed rat kidneys.
Mass spectrometry imaging of kidneys typically requires washing to
remove excess hemoglobin; however, that is not necessary with this
approach. Multistep static FAIMS mass spectrometry resulted in a 6-
to 16-fold increase in the number of proteins detected in the absence
of FAIMS, in addition to smaller increases over single step static
FAIMS (chosen for optimum transmission of total protein ions). The
benefits of multistep static FAIMS mass spectrometry for protein detection
are also shown for sections of testes. The numbers of proteins detected
following multistep FAIMS increased between 2- and 3-fold over single
step FAIMS and between 2- and 14-fold over LESA alone. Finally, to
date, LESA mass spectrometry of proteins in tissue has been undertaken
solely on fresh frozen samples. In this work, we demonstrate that
heat-preserved tissues are also suitable for these analyses. Heat
preservation of tissue improved the number of proteins detected by
LESA MS for both kidney and testes tissue (by between 2- and 4-fold).
For both tissue types, the majority of the proteins additionally detected
in the heat-treated samples were subsequently detected in the frozen
samples when FAIMS was incorporated. Improvements in the numbers of
proteins detected were observed for LESA FAIMS MS for the kidney tissue;
for testes tissue, fewer total proteins were detected following heat
preservation; however, approximately one-third were unique to the
heat-preserved samples.