Matrix-enhanced secondary ion mass spectrometry: the influence of MALDI matrices on molecular ion yields of thin organic films

In this work the effect in secondary ion mass spectrometry (SIMS) of several frequently used matrix-assisted laser desorption/ionisation (MALDI) matrices on the secondary ion intensities of low molecular weight (m/z 400-800) organic dyes and a pharmaceutical is tested. Matrix (10(-1) M) and analyte (10(-2) M) solutions were made in methanol. Mixtures with several concentration ratios were prepared from these solutions and spincoated on Si substrates prior to time-of-flight (TOF)SIMS analysis. In some cases the presence of the MALDI matrices caused a considerable increase in the positive secondary (protonated) molecular ion signals. Enhancements of a factor of 20 and more were recorded. Generally, of the matrices used, 2,5-dihydroxybenzoic acid and 2,4,6-trihydroxyacetophenone brought about the highest intensity increases. It was also shown that matrix-enhanced (ME-)SIMS is capable of lowering the detection limits for molecule ions. However, the enhancement effect is strongly influenced by the analyte/matrix combination and its concentration ratio. As a result, finding an optimal analyte/matrix mixture can be a very time-consuming process. Mostly, the presence of the matrices causes changes in the relative ion intensities in the TOFS-SIMS spectra. Compared to the spectra recorded from samples without matrices, only a few additional peaks, such as signals that originate directly from the applied matrix or adduct ions, are observed in the mass spectra. Sometimes molecule ions and some characteristic fragments at high m/z values, that cannot be recorded without matrix, do appear in the spectrum when a matrix is present. In the negative mode no enhancement effect is observed on applying the studied MALDI matrices. The results obtained from samples treated with MALDI matrices are also compared to SIMS results for the same samples after Ag and Au metallisation (MetA-SIMS). For three of the four tested compounds Au MetA-SIMS resulted in higher ion yields than ME-SIMS. For both techniques possible mechanisms that can account for the enhancement effect are proposed. Copyright (c) 2005 John Wiley & Sons, Ltd.