Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry quantitation via in cell combination

Herein we describe a novel method for quantitation using a Fourier transform mass spectrometer (FTMS) equipped with a MALDI ion source. The unique instrumental configuration of FTMS and its ion trapping and storing capabilities enable ion packets originating from two physically distinct samples to be combined in the ion cyclotron resonance (ICR) cell prior to detection. These features are exploited to combine analyte ions from two differentially labeled samples spotted separately and then combined in the ICR cell to generate a single mass spectrum containing isotopically paired peaks for quantitative comparison of relative ion abundances. The utility of this new quantitation via in cell combination (QUICC) approach is explored using peptide standards, a bovine serum albumin tryptic digest, and a crude neuronal tissue extract. We show that spectra acquired using the QUICC scheme are comparable to those obtained from premixing the isotopically labeled samples in solution. In addition, we show direct tissue in situ isotopic formaldehyde labeling of a crustacean neuroendocrine organ, thus demonstrating the potential application of the QUICC methodology for direct tissue quantitative analysis.