SIMS state mode

It is the intent of this Symposium to make some progress in the understanding of the similarities and differences of SIMS and FAB when these methods are used to produce molecular ions of biomolecules. Certainly some progress has been made in understanding some of the similarities in terms of the mode of primary energy transfer. Liquid matrix vs. solid matrix is a much more difficult question but the experimental facts at this time support the notion that the liquid matrix, under high primary flux excitation, has features that are not present in a solid state SIMS experiment. Where does 252-Cf-PDMS fit in this picture Is it really SIMS and nothing more Perhaps this is a question more philosophical than substantive at this point. The 252-Cf-PDMS method is a maverick in... 

In SIMS a primary ion beam (1-20 keV) causes ejection (sputtering) of neutrals and ions from a solid specimen the secondary ions are mass-analyzed. In static mode, sampling is restricted to the first 1-3 atom layers at the surface by keeping the sputtering rate low (0.1 nm/hour). The ejected ions and ion clusters give information on the nature and may give clues to the chemical state of species present. 

As a point of interest, Karpfen et al. have also computed the vibrational frequencies of the cyclic geometry, which represents not a minimum on the PES, but rather a transition state, and hence the imaginary frequency listed in Table 3.11 for the bending motion that would transform this cyclic geometry into one of the minima. It might be noted that the asymmetric and symmetric internal HF stretching modes in the cyclic dimer are of very sim-... 

Complex mixtures of PAH were found in almost all samples. Table III summarizes the results of the analysis of samples from New England (Samples 1-11), the continental United States (Samples 18-30), the north and south Pacific Ocean (Samples 31-41) and Africa and South America (Samples 42-50). Concentrations are given for PAH with seven molecular weights representing over 25 alkylated and nonalkylated compounds. The quantities were determined using the SIM mode of the mass spectrometer with external standards, as described previously. 

Figure 3.11 summarizes such key experimental points. As a first point, we have to choose the appropriate ionization method for the detection of small metabolites, we have alternative choices other than MALDI, such as secondary ion mass spectrometry (SIMS) [15], nanostructure-initiator mass spectrometry (NIMS) [20,21], desorption/ionization on silicon (DIOS) [22], nanoparticle-assisted laser desorptiopn/ ionization (nano-PALDI) [23], and even laser desorption/ionization (LDI) [24,25]. We consider that MALDI is stiU the most versatile method, particularly due to the soft ionization capability of intact analyte. However, other methods each have unique advantages for example, SIMS and nano-PALDI have achieved higher spatial resolution than conventional MALDI-IMS, and above aU, these mentioned alternative methods are all matrix-free methods, and thus can exclude the interruption of the matrix cluster ion. Next, if MALDI is chosen, experimenters should choose a suitable matrix compound, solvent composition, and further matrix application method for their target analyte. All these factors are critical to obtain sufficient sensitivity because they affect efficiency of analyte extraction, condition of cocrystallization, and, above all, analyte-ionization efficiency. In addition, based on the charge state of the analyte molecule, suitable MS polarity (i.e., positive/ negative ion detection mode) should be used in MS measurement. Below, we shall describe the key experimental points for MALDI-IMS applications of representative metabolites. 

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