Matrix effects, internal energies and MS/MS spectra of molecular ions sputtered from surfaces

Desorption ionization (DI) involves the transfer of material from a condensed phase to a collision-free environment (ref. 1,2). Tandem mass spectrometry (ref. 3), used with desorption ionization, improves the signal-to-noise ratio for spectra of individual analytes present in complex matrices, provides evidence that fragmentation in DI is typically due to gas phase dissociations of energized but intact molecular ions after they have left the surface, and allows the compositions of desorbed ions to be characterized. A complementary approach to improving analytical performance and obtaining further information on the species and processes of desorption ionization is to be found in the examination of the sample in the presence of matrix materials. Some matrices act as reagents which yield an appropriate ionized form of the analyte (ref. 4), either during or prior to energization of the sample, while others serve to isolate analyte molecules and reduce intermolecular analyte reactions (ref. 5). Particularly complex matrices are those encountered when examining samples directly from chromatographic materials or in their natural state, for example, crude extracts of plant materials. Examples of analyses in these situations are given.

Ammonium chloride acts as a valuable matrix material which, even at sample dilutions of 10³, can cause an increase in both absolute secondary ion yields and in spectral persistence (ref. 6,7,8). This matrix has beneficial effects in SIMS, FAB and LD mass spectra and has the advantage of being totally transparent except under high flux conditions. It is shown to decrease ion internal energies, presumably by providing a sputtered ion with a shield of solvating molecules which are readily lost as NH₃ and HCl, thereby carrying away excess energy. Cluster ions [(NH₄)_n+1Cl_n]⁺ are observed in FAB and shown by MS/MS to undergo ready desolvation. These cluster ions are remarkable for the absence or low intensity of clusters where the total number of anions and cations is a prime number and for the high intensity of clusters which may be made up of regular arrays of atoms, e.g., 3×3×3 or 3×3×5.

A qualitative model of desorption ionization, advanced some years ago (ref. 9), accommodates the observations reported here using MS/MS and matrix effects. The chief features of this model are (i) isomerization (loss of identity) of the input energy, (ii) desorption of preformed ions or intact molecules, (iii) ion/molecule reactions such as cationization occurring in the selvedge region, (iv) dissociation of energetic (metastable) ions well-removed from the surface. In most cases just a few types of ionic species are sputtered from the surface and their unimolecular chemistry determines the chief features of the desorption ionization mass spectrum.