Utility of matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry for the analysis of low molecular weight compounds

Abstract

A range of low molecular weight compounds (<800 Da) have been examined by matrix-assisted laser desorption time-of-flight mass spectrometry to demonstrate the general analytical utility of this technique. The compound classes investigated included: carbohydrates, quaternary ammonium salts, sterols, nucleosides, purine and pyrimidine bases, amino acids, catecholamines, opioids, antibiotics, prostaglandins and a range of macrocyclic metal complexes of porphyrins and phthalocyanines. Many of the compounds tested are of biochemical, geochemical, industrial and/or therapeutic interest. Most organic analytes gave intense protonated molecules, but some were characterized by sodium adduct ions (i.e., [M + Na]'). The metal-containing compounds formed radical-cation molecular ions. In all instances the ions detected could be assigned, and excellent agreements between calculated and experimental mass values were obtained.Matrix-assisted laser desorption/ionization (MALDI) combined with time-of-flight (TOF) mass spectrometry has found general acclaim as an approach to syntheticand bio-polymer analysis. MALDI allows the direct analysis of mixtures, and hence, analyte impurities can usually be identified and characterized. Sub-picomole amounts of sample are adequate for multiple analyses, results are available in minutes, and most of the sample can be recovered directly from the target. The increasing availability of inexpensive commercial instruments has promoted widespread use of MALDI-TOFMS, particularly for biopolymers, but studies of compounds of low molecular weight (i.e., ca. 800 Da or less) have been re~tricted.'-~ Here we present experimental results for the MALDI analysis of a range of low molecular weight compounds including carbohydrates, quaternary ammonium salts, sterols, nucleosides, purine and pyrimidine bases, amino acids, catecholamines, opioids, antibiotics, prostaglandins and a range of macrocyclic metal complexes of porphyrins and phthalocyanines. Many of the compounds tested are of biochemical, geochemical, industrial and/or therapeutic significance. EXPERIMENTAL InstrumentationAnalyses were performed on a Finnigan Lasermat MALDI linear time-of-flight mass spectrometer (Finnigan MAT, Heme1 Hempstead, UK). Much of the work described in this paper was performed on a standard Lasermat, but later data were obtained on the instrument following upgrade to Lasermat 2000 specifications. The system uses a nitrogen laser (337 nm; 3 ns pulse duration), and the laser fluence is adjusted by a variable-beam attenuator. Samples were applied to stainless-steel slides with a circular sample application area of 2 mm diameter. The Lasermat software allows the user to irradiate one of four possible target regions * Author for correspondence within this area. Under standard operating conditions, and for the mass range 100-1000Da, the resolving power ( m / A m , full width at half maximum height) of the Lasermat 2000 is in the range ca. 100-200 (or 50-100 for the Lasermat). Materials and methodsWater was purified by a Millipor...

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

“…However, most ionization including cationization is believed to occur in the gas phase [39]. Although MALDI ionization mechanisms are not fully understood, there exists strong experimental evidence that secondary ionization processes occur in the laser plume and are responsible for metal-ligand exchanges [21,40,41]. Sodiated analytes are believed to come from sodium cation transfer reactions that occur in the gas phase [42].…”

Elemental sulfur as a versatile low-mass-range calibration standard for laser desorption ionization mass spectrometry

“…Historically, LDI-MS, and its more extensively used variant, MALDI-MS, are used for mass determination of large molecules [1]. However, many useful applications of LDI-MS in the low mass region (Ͻm/z 500) have also emerged, and the technique has become widely accepted [2,3]. For example, organic acids [4,5], salts of oxyanions [5], amine-based chelating compounds [5], amino acids [6,7], amino acid derivatives [8], aromatic amines [9], oligosaccharides [10], and pharmaceuticals [11] have been investigated by LDI-MS.…”

MALDI-TOF MS analysis of urinary nucleosides

“…Lidgard et al used DHB and CHCA matrix for determination of nucleosides and other small molecules by MALDI-TOF°MS° [18].°In°our°experiments,°DHB°was discovered to be the most suitable matrix for analyzing nucleosides by MALDI-TOF MS with high sensitivity. However, we observed reduction of several nucleosides, especially adenosine and its metabolites, as shown°in°Figure°2.°Similar°effects°were°observed°in previous°examinations°by°Koomen°and°Russell° [19].°To avoid this, CHCA would be the preferable matrix, but with less sensitivity.…”