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

Abstract

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.

Kinetics of Surface-Induced Dissociation of N(CH3)4+ and N(CD3)4+ Using Silicon Nanoparticle Assisted Laser Desorption/Ionization and Laser Desorption/Ionization

“…As mentioned earlier, SPALDI requires less laser intensity than LDI [34,35], which can lead to lower internal energy deposition during the ionization process. If an ion has high internal energy before SID, only a small amount of additional energy is required to overcome the energy barrier for dissociation.…”

“…Although our modified MALDI TOF instrument was successfully combined with SID to explore fast dissociation processes, the study of small molecules was complicated by MALDI matrix interference. Recently, the Wysocki group has reported ionization via silicon nanoparticle assisted laser desorption/ionization (SPALDI) [33][34][35]. SPALDI has been successful in generating molecular ions from analytes ranging in size from small organics such as N(CH 3 ) 4 ϩ to peptides without matrix assistance and with much lower laser fluence than MALDI or laser desorption/ionization (LDI).…”

Less common applications of monoliths: V. Monolithic scaffolds modified with nanostructures for chromatographic separations and tissue engineering

“…However, nanoparticles may serve not only as affinity probes for selective extraction but also as matrices for direct analysis by surface‐assisted laser desorption/ionization (SALDI) MS []. Chemically modified silicon nanoparticles were used in silicon nanoparticle‐assisted laser desorption/ionization (SPALDI) MS analysis of sulfonic and fatty acids [].…”

Determination of Paclitaxel Distribution in Solid Tumors by Nano-Particle Assisted Laser Desorption Ionization Mass Spectrometry Imaging

“…The use of inorganic fine particles [15] is based on their physical properties, like high photo-adsorption, low heat capacity and large surface area. This ensures rapid heating [16], highly localized and uniform energy deposition [17], resulting in efficient sample desorption and ionization. Moreover, nanoparticle-assisted laser desorption ionization (nPALDI), when used for MSI, provides better spatial resolution than conventional matrixes.…”