At appropriate matrix:analyte mixing ratios, small to moderate sized analyte ions (lOOO-20OOOu) can fully suppress positively charged matrix ions in matrix-assisted laser desorptiodionization (MALDI) mass spectra. This is true for all matrix species, including radical cations and adducts with protons or alkali-metal ions. Full matrix suppression is also observed, regardless of the preferred d y t e ion form, be it protonated or an alkali adduct. These facts lead us to propose a mechanism for prompt, primary (not secondary gas-phase) MALDI ionization in which excited matrix molecules are the key species. At least two such excited molecules are believed to be necessary for free ion generation. This model is found to be consistent with the available data, as well as making several predictions which are confinned by new observations. The model also predicts that the matrix suppression effect will not be observable with heavy analytes because their large excluded volume precludes desorption at the neceSSary mixing ratios.Although it is not widely discussed in the literature, practitioners of matrix-assisted laser desorptionlionization (MALDI) have often noted a remarkable phenomenon: when analytes of moderate size (1000-20 OOO u) are mixed with matrix in relatively high molar ratio (1O:l to about 2OOO:l matrix-to-analyte ratio), the appearance of positive matrix ions in the mass spectrum can be completely suppressed.'.' Along with the fact that analyte fragmentation is typically weak in MALDI, this leads to nearly ideal mass spectra: strong analyte molecular ions but no other signals of any kind. Since the effect has been observed with a number of matrices'.' including nicotinic acid, a-cyanoChydroxycinnamic acid (CCA) and 2,5 dihydroxybenzoic acid (DHB), it seems to be a general phenomenon in MALDI. It would clearly be advantageous to understand the effect, so as to extend it to the widest possible range of analytes. At the same time we can also expect to learn something about the molecular-level mechanisms of the MALDI method.In their paper describing CCA as a MALDI matrix, Beavis et aL3 presented a spectrum of substance P (1 347 u) in which the matrix peaks were nearly absent. They did not specifically note this partial matrix suppression effect, nor did they study the matrix-to-analyte mixing ratio dependence of the matrix signals, but they did propose a disproportionation ionization mechanism involving nonionized matrix as an intermediate. They also noted the possibility of proton generation from the matrix as a first step, followed by efficient transfer to the analyte, but did not go on to verifj these hypotheses.Matrix-ion suppression was explicitly studied by Chan er aL,' using nicotinic acid as the matrix, and insulin (5807 u), lysozyme (-14 300 u) and myoglobin (-18 OOO u) as the analytes. They investigated matrix-to-analyte ratios of about 600 to 6OO0, and found that larger analytes required larger ratios for good suppression. They tentatively proposed a suppression model based on the competition between ma...

Polyamides are insoluble or poorly soluble in common organic solvents, which makes normal sample preparation for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry very difficult. An new analytical protocol for MALDI analysis of polyamides or other insoluble samples is described. It consists of pressing a pellet from a solid mixture of the polymer and a matrix, both in the form of finely ground powder. This sample preparation is compared with the common dried droplet sample preparation method and found to perform much better, both in terms of robustness against variation of experimental parameters and high-mass capability.

The light-induced aging of natural triterpenes used in varnishes was studied using graphite-assisted laser desorption/ionization mass spectrometry. This method was found to be superior to conventional matrix-assisted laser desorption/ionization for these materials because of higher signal levels and less chemical noise from the matrix. The dammar and mastic raw materials were found to be partially oxidized compared to their nominal composition, with up to six oxygens incorporated. The light-induced aging process leads initially to polymerization, up to trimers. Further aging leads to a decrease in average molecular weight. These observations are supplemented by FTIR and 13 C NMR spectra which show complete

This work is concerned with the improvement of a microwave liquid-crystal phase shifter using a large birefringence nematic liquid crystal. This material is a eutectic mixture of isothiocyanatotolane molecules. Microwave dielectric properties are reported and compared to the data obtained with the 5CB cyanobiphenyl material in the 26 -40 GHz frequency range using a rectangular waveguide. The phase-shifter design consists of a central cavity, where a liquid crystal is inserted, and two coplanar strip lines accesses. Its dimensions were calculated by electromagnetic simulation, using measured dielectric permittivities of the liquid crystal. The measurements were performed with a commercial Wiltron 3680 K probe test fixture. Phase-shift variations with and without bias voltage versus frequency are presented. As expected, the large-birefringence nematic liquid crystal exhibits a higher microwave dielectric anisotropy (Á" 0 ¼ 1:06 against 0.34) and the tunability of the phase shifter strongly increases (1.8 degÁcm À1 ÁGHz À1 against 0.8 degÁcm À1 ÁGHz À1 ).