Signal enhancement in matrix-assisted laser desorption/ionization by doping with Cu(II) chloride

Analysis of deprotonated acids with silicon nanoparticle-assisted laser desorption/ ionization mass spectrometry

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/µl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.

Dissipation of charge on MALDI-TOF polymeric chips using an electron-acceptor: analysis of proteins

Disposable polymeric devices used for direct sample pretreatment/signal enhancement and MALDI-MS analyses of biomolecules suffer from the accumulation of electric charge and related positive mass shifts due to photoelectric effects on non-conductive surfaces after irradiation with lasers. The effects of surface charging on the mass shift of protein spectra during MALDI-TOF/MS measurements on copolymeric sample array chips were studied. Methyl viologen is used to monitor the development of charge and is demonstrated to be an effective electron scavenger. The use of such reducible species leads to increased accuracy, signal homogeneity, and resolution for mass spectral measurements of proteins in mixtures with sinapinic and 2,5-dihydroxybenzoic (DHB) acids. This approach can be used on a wide range of nonconductive support materials.