| Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been used extensively for the characterization of large biomolecules, synthetic polymers and small molecules. Typically, low molecular weight acidic matrices have been used in MALDI-MS for such analyses. Here, the use of low molecular weight nonpolar matrices for MALDI-MS is investigated. In particular, the analytical and physical properties of this class of matrices are studied.; The effect of instrumental factors on nonpolar matrix behavior was investigated. Two different MALDI target substrates, stainless steel and poly(methylmethacrylate) (PMMA), were used. The radical molecular ion abundance of nonpolar analyte ions analyzed in the presence of various nonpolar matrices, such as anthracene and 9-cyanoanthracene, were determined as a function of the MALDI target substrate. It was found that PMMA MALDI target substrates yielded higher radical molecular ion abundances for nonpolar analyte ions than stainless steel MALDI target substrates. This difference is attributed to the absence of photoelectrons which reduce charged species in the plume.; To determine whether the desorption process for nonpolar matrices is different than the desorption process for polar, acidic matrices, matrix initial ion velocities were measured in MALDI-MS. The relative initial ion velocities of the nonpolar matrices, anthracene, 9-cyanoanthracene, pyrene, and acenapthene and were investigated and compared to the relative initial ion velocities for 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA), and all-trans retinoic acid (RTA), polar matrices. It was found that DHB has a greater relative initial ion velocity than the nonpolar matrices. It does not appear that the desorption process is different between nonpolar and polar matrices based upon these results. In addition, when using any matrix, polar or nonpolar, to analyze nonpolar polymers, the addition of a metal salt increased the relative initial ion velocity of the matrix, suggesting a different desorption mechanism in these cases.; To illustrate an application of nonpolar matrices for analytical MALDI-MS, an atmospheric resid crude oil fraction was characterized. The MALDI mass spectral data for this sample was difficult to interpret when a polar matrix, 3-indole acrylic acid, was used, due to fragmentation and matrix clustering in the analyze molecular weight distribution range. The nonpolar matrices anthracene and 9-cyanoanthracene were found to be very suitable for the analysis of this sample in either linear or reflection mode time-of-flight mass spectrometry. These matrices did not lead to fragmentation or clustering, and identification and characterization of the analyze peaks were straightforward. |
