Matrix-assisted Laser Desorption/Ionization of Low Molecular Weight Compounds by Fourier Transform Laser Microprobe Mass Spectrometry with External Source

Abstract

Fourier transform laser microprobe mass spectrometry is used to perform matrix-assisted laser desorptionl ionization (MALDI) experiments on low molecular weight compounds in the range up to 2 kDa The optimal molar ratio of analyte to matrix is determined. The use of fructose as co-matrix and the effect of laser irradiance on the mass spectra are studied. The typical power density in MALDI is compared to the laser only mode. Specific experiments concerning the cationization of the analyte are described. The observations are interpreted in terms of previous concepts about DI in laser mass spectrometry of solids.Laser microprobe mass spectrometry (LMMS) employs a pulsed high-power UV laser to permit one-step desorption and ionization (DI) of a solid with a spot of 1-5 pm. The ions are mass analysed, initially by time-of-flight (TOF) mass spectrometry, where the mass resolution was deemed to be insufficient,' although a significantly improved TOF LMMS was described recently.' Alternatively, focused laser ionization is coupled to Fourier transform (FT) MS using dual-cell instruments3* or instruments with an external source and a single cell?LMMS generates structural ions from organic compounds and performs detailed speciation of inorganic substances. Sample preparation is minimal, while stability of the specimen in the vacuum is the only requisite. The qualitative but complete characterization of organic and inorganic surface components is appreciated in science and i n d~s t r y .~.~ Besides local analysis, the use of especially FT LMMS for off-line high-performance liquid chromatography (HPLC) applications is interesting, exploiting the combination of LMMS with high resolution and minimal analyte consumption. The residue from a single HPLC peak on an analytical column provides sufficient material for detailed MS investigation of thermolabile compounds. In contrast to local analysis, sample preparation may involve dissolution or the use of additives. Taking advantage of the benefits that result from the application of matrix-assisted laser desorption/ionization (MALDI) becomes obvious.'-'' Although MALDI literature primarily deals with high molecular weight (MW) compounds and TOF MS, the use of this method to improve DI is also beneficial to the analysis of small molecules in the molecular weight range pharmaceutical industry, MALDI FT LMMS was applied to small polyfunctional molecules and association complexes of molecular weight under 3 ma. The purpose of this paper is to describe the results from an introductory study. First of all, optimization of sample preparation and instrumental methodology was required to reach micro-analytical reproducibility standards. In comparison with TOF MS and defocused laser irradiation, the 5 pm spot in FT LMMS makes local sample homogeneity critical, while the specific acceptance of the system, with respect to the width of the ion energy distribution, the initial ion emission angles and the moment of ion formation with respect to the laser pulse, is largely different.' Specifically...

Systematization of the Mass Spectra for Speciation of Inorganic Salts with Static Secondary Ion Mass Spectrometry

“…This idea is recently supported by simulations that take into account molecular dynamics and reaction enthalpies along the primary ion trajectory. , Additionally, the DI model stresses the role of the analyte-dependent local pressure in the selvedge and the time domain of ion formation and mass analysis. It has been proven to provide an adequate basis to rationalize organic and inorganic mass spectra in LMMS, (matrix-assisted) laser desorption MS, and S-SIMS. − …”

Fourier transform mass spectrometry—advancing years (1992-mid. 1996)

“…An external source Fourier transform laser microprobe mass spectrometer (FT-LMMS) has been developed by Van Kollensperger et al, 1996), and has been used for site-specific laser desorption (LD) analysis of organic and inorganic compounds.…”

A New Electrostatic Transfer Line for Improved Transmission in Fourier Transform Laser Microprobe Mass Spectrometry with External Ion Source

“…Experimental evidence shows that the initial ion population in focused laser irradiation of solids comprises contributions from the direct emission of ions from the sample surface and from ion formation in the selvedge. 6,[39][40][41][42] The first contribution is expected to yield a forward-peaked emission, whereas the latter is an essentially random angle process. [43][44][45] The relative contribution of each type of process depends on the analyte.…”