Methods for generating protein molecular ions in ToF-SIMS

One of the greatest challenges in mass spectrometry lies in the generation and detection of molecular ions that can be used to directly identify the protein from the molecular weight of the molecular ion. Typically, proteins are large (MW > 1000), nonvolatile, and/or thermally labile, but the vaporization process produced by many mass spectrometry techniques including time-of-flight secondary ion mass spectrometry (ToF-SIMS) is inherently limited to generating ions from smaller compounds or fragments of the parent molecule, making the identification of proteins complex. The application of specific molecules to aid in the generation of high molecular weight ions in ToF-SIMS has been recognized for some time. In this study we have developed a matrix-SAM substrate preparation technique based on the self-assembly of a matrix-like molecule, mercaptonicotinic acid (MNA), on gold. We then compare this substrate with two existing ToF-SIMS sample preparation techniques, cationized alkane thiol and matrix-enhanced SIMS (MESIMS). The results of this study illustrate that while there is a range of methods that can be used to improve the molecular ion yield of proteins in ToF-SIMS, their efficacy and reproducibility vary considerably and crucially are linked to the sample preparation and/or protein application methods used. Critically, the MNA modified substrate was able to simultaneously induce molecular ions for each protein present in a multicomponent solution, suggesting that this sample preparation technique may have future application in proteomics and DNA analysis.