Lead deposition, spectra

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

In soft ionization methods the excess energy deposited onto the ionized molecule is very small and stable even-electron ions are formed. This leads to easy determination of the molecular weight of the analyte, but as fragmentation is absent or it occurs to a very low extent, structural information is missing in the mass spectrum. However, one can obtain structural information by causing ion fragmentation out of the source by means of tandem mass spectrometry experiments (see below). 

It is evident that these "incorrect" procedures in the example lead to a shift of the band towards higher wave numbers, because the band characteristic of the deposit is located on a sloped and not on a flat section of the catalyst spectrum. Furthermore, the band shape is slightly changed, and the absolute intensity values are much smaller. 

MALDI suffers from some disadvantages such as low shot-to-shot reproducibility and strong dependence on the sample preparation method. Each laser shot ablates a few layers of the deposit at the spot where the laser irradiates. This can produce variation in the shot-by-shot spectrum. Also, the impact position on the surface of the deposit can lead to spectral variations. Improvement of the deposit homogeneity gives a better reproducibility of the signal. This is very important if precise quantitative results must be obtained. A given... 

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