Influence of matrix effects by inorganic ions in single-cell direct mass spectrometry

[Abstract] Direct nanoelectrospray ionization mass spectrometry (nanoESI-MS) is extremely useful for single-cell analysis to detect endogenous metabolites as well as exogenous compounds that have been taken up. However, during ionization, coexisting constituents, especially sodium chloride, can cause severe matrix effects, resulting in decreased detection sensitivity and fluctuating peak intensities for the compounds of interest. Herein, conventional ESI and nanoESI were examined to determine the extent of ionization suppression by salts using amino acids as typical cellular constituents in addition to caffeine and acetaminophen as exogenous compounds. The corresponding isotope-labeled standard compounds were also tested to compensate for fluctuations. All the tested compounds showed significantly decreased ion intensities with both conventional ESI and nanoESI when ionized with high-concentration salt solutions. However, when the nanospray tip with the smallest inner diameter was used for nanoESI, the peak intensities were 2-3 times higher than those obtained using conventional ESI and their peak intensities recovered to 60% compared to the salt-free samples. These findings clearly demonstrate that nanoESI has advantages over conventional ESI in terms of reducing ionization suppression effects for small molecules in biological samples such as cells containing inorganic salts, particularly sodium chloride, at high concentrations. Furthermore, the use of stable isotope-labeled compounds corresponding to each analyte was revealed to allow adequate correction of the ion peak intensity fluctuations for each analyte during direct nanoESI-MS analysis of single HepG2 cell samples, even when coexisting salts cause severe ion suppression.