Post-source decay in the analysis of polystyrene by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Various secondary series are observed in matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra of polystyrene. The number and positions of the series depend on the choice of matrix and added cation. For a given treatment, series observed in linear mode are not necessarily observed in reflectron mode, and vice versa. Post-source decay analysis was used to determine that the secondary series arise at least in part from formation and decay of adducts of polystyrene with matrix species. There is some treatment-to-treatment variation, but adduct formation and decay were observed for all tested treatments. The multiplicity of secondary series makes it unclear whether post-source decay occurs for the main series (polystyrene + cation)⁺ ions under the conditions normally used for polystyrene analysis. Such ions do undergo post-source decay at laser fluences greater than normally used. Although only polystyrene was investigated in this work, other polymers may also produce adduct and decay series in MALDI analysis. Their presence can mask the presence of minor components in a sample, but at least as observed here, do not have a strong influence on molecular mass determinations.     

Characterization of fungal spores by laser desorption/ionization time-of-flight mass spectrometry

A considerable volume of research has now been completed on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to the analysis of bacteria; however, to date no definitive studies have been made using this technique on fungi. Preliminary studies on the application of the MALDI-MS methodology, previously developed for the analysis of bacteria, to the analysis of intact fungal spores are described here. MALDI-MS and electrospray mass spectrometry enable the high molecular weight analysis of proteins, glycoproteins, oligosaccharides and oligonucleotides. Using MALDI-MS with bacteria has demonstrated the ability to produce 'fingerprints' of the intact cells with the ions observed being associated with the proteinaceous components of the cell wall. This paper reports the adaptation of this technique to the direct analysis of fungal cells. The high percentage of carbohydrate in the fungal cell wall indicates that the ions observed in the mass spectrometric experiments may be of carbohydrate origin. Penicillium spp., Scytalidium dimidiatum and Trichophyton rubrum have been studied in this preliminary investigation and all show individually distinctive spectra which would appear to provide a profile of the cellular material with discrete peaks being observed over the mass range 2 to 13 kDa. The spectra obtained are reproducible within the method used but, as shown in our previous studies on bacteria, washing may selectively release components from the fungal cell wall.     

Indigo Carmine degradation by hypochlorite in aqueous medium monitored by electrospray ionization mass spectrometry

The degradation of the dye Indigo Carmine by hypochlorite in aqueous solution was monitored by electrospray ionization mass spectrometry in the negative ion mode (ESI(—)‐MS). Hypochlorite was highly efficient in removing the color of aqueous solutions of the dye. ESI(—)‐MS monitoring showed that concomitant with the Indigo Carmine consumption two transient species appeared (detected as doubly charged anions) probably formed via a net insertion of two hydroxyl groups at the exocyclic C?C bond followed by the incorporation of two (mainly) or one oxygen atoms at the indolic rings of the dye. Structures of these products were proposed based on the ESI(—)‐MS/MS data and high accuracy mass measurements. These two transient intermediates quickly decomposed, both in the condensed and in the gas phase, to yield mono‐charged anions. Based on these results, a route for the Indigo Carmine degradation by hypochlorite in aqueous solution has been proposed. Copyright © 2007 John Wiley & Sons, Ltd.     

Analysis of plant phosphatidylcholines by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The use of matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) for the quantitative determination of phospholipid (PL) molecular species has been problematic, due primarily to the formation of multiple signals (corresponding to the molecular ion and other adducts) for some classes of PL. For example, analysis of phosphatidylcholine (PC) yielded signals that corresponded to protonated and sodiated molecules in the MALDI spectrum. The resulting spectral overlap among various molecular species (e.g. [PC(16:0/18:2) + Na] and [PC(18:2/18:3)]) made it impossible to ascertain their relative amounts using this technique. Other spectral ambiguities existed among different structural isomers, such as PC(18:1/18:1) and PC(18:0/18:2). We determined that molecular species could be resolved by MALDI-TOFMS by first removing the polar head (e.g. phosphocholine) from the phospholipid to effect production of only the sodiated molecules of the corresponding diacylglycerols (DAGs). Analysis of the resulting spectrum allowed unequivocal determination of the molecular species profile of PC from potato tuber and soybean. Estimation of fatty acid composition based on the molecular species determined by MALDI-TOFMS analysis agreed with that from GC-FID analysis. Post-source decay (PSD) was used to resolve standard isomers of PC (e.g. 18:1/18:1 vs. 18:0/18:2). Our results indicated that PSD is a useful approach for resolving structural isomers of PL molecular species.     

Analysis of solid-supported oligonucleotides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

This study presents matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) as a powerful tool to analyze and characterize oligonucleotides covalently linked to a solid support during their synthesis. The analysis of the fragment ions generated either in negative or positive mode allows direct and easy access to the nucleotide sequence and identification of the internucleosidic linkage. The mechanisms of the fragmentation of the solid-supported oligonucleotides induced by MALDI-TOFMS are discussed. Copyright 2000 John Wiley & Sons, Ltd.     

Characterization of humic substances by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) and laser desorption/ionization (LDI-)TOFMS have been used to characterize Suwannee River humic substances, obtained from the International Humic Substances Society (IHSS), and Armadale soil fulvic acid (ASFA). An array of MALDI matrices were tested for use with humic substances, including alpha-cyano-4-hydroxycinammic acid (CHCA), 2-(4-hydroxyphenylazo)benzoic acid (HABA), 2,5-dihydroxybenzoic acid (DHBA), sinapinic acid, dithranol and norharmane. DHBA yielded the best results, exhibiting superior ionization efficiency, low noise, broad applicability to the analytes of interest, and most importantly producing an abundance of high mass ions, the highest observed being m/z 1848. A number of sample preparation modes were investigated; the overlayer method improved sample/matrix homogeneity and hence shot-to-shot reproducibility. The choice of the matrix, mass ratio of analyte to matrix, and the sample preparation protocol, were found to be the most critical factors governing the quality of the mass spectra. Matrix suppression was greatly enhanced by ensuring good mixing of matrix and analyte in the solid phase, proper optimization of the matrix/analyte ratio, and optimizing delayed extraction to ensure complete matrix-analyte reaction in the plume before ions are moved to the flight tube. A number of common features, in particular specific ions which could not be attributed to the matrices or to contaminants, were present in the spectra of all the humic substances, regardless of origin or operational definition. Additionally, a prominent repeating pattern of peaks separated by 55, 114 and 169 Da was clearly observed in both LDI and MALDI, suggesting that the humic compounds studied here may have quasi-polymeric or oligomeric features.     

Analysis of hydrocarbon dendrimers by laser desorption time-of-flight and fourier transform mass spectrometry

The first mass spectrometric analysis of a new class of hydrocarbon dendrimers that result from a convergent synthetic approach is reported. Molecular weights of a series of phenylacetylene dendrimers (715 to 14776 u MW) are characterized by ultraviolet matrix-assisted laser desorption (MALDltime-of-flight (TOF) mass spectrometry, direct and silver chemical ionization infrared laser desorption Fourier transform mass spectrometry @I’MSl, and ultraviolet matrix-assisted laser desorption silver chemical ionization Fourier transform mass spectromeby. New matrices and techniques were developed to facilitate analysis of the dendrimers. Mass measurement accuracies between 10 and 25 ppm are obtained for molecular ion species of the five dendrimers analyzed. Laser desorption time-of-flight and FI’MS techniques are shown to be complementary, with FTMS providing high mass resolution (27,000–67,000 resolving power) and accuracy for lower mass dendrimers (10–14 ppm) and MALD TOF yielding the highest resolution (1100 resolving power) and accuracy (25 ppm) for the largest dendrimer. These results are consistent with proposed empirical formulas.     

Characterization of Aspergillus spores by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The intact fungal spores of several strains of four Aspergillus species, Aspergillus flavus, A. oryzae, A. parasiticus, and A. sojae, were directly analyzed by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Very simple MALDI mass spectra are obtained by directly mixing spores with a matrix such as alpha-cyano-4-hydroxycinnamic acid or sinapinic acid. The mass spectra are obtained from the ablation of cell walls of spores owing to the acidity of the matrix solution. The MALDI results show that aflatoxigenic strains and non-aflatoxigenic strains have different mass peak profiles. Furthermore, the MALDI results of non-aflatoxigenic A. flavus and A. parasiticus spores resemble those of the closely related A. oryzae and A. sojae spores, respectively.     

Detection of benzopyrene-deoxyguanosine adducts by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A method for the detection of BPDE-d guanosine adducts using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is described and illustrated. The results indicate that MALDI is capable of detecting two other DNA benzopyrene adducts, which are trace products formed during the synthesis of BPDE-d guanosine. This MALDI-TOFMS method offers the potential for the detection of DNA adducts in human tissue using very limited sample purification and preparation.     

基质辅助激光解析电离-飞行时间质谱分析寡核苷酸G-四链体

我们发展了一种利用基质辅助激光解析电离飞行时间质谱技术(MALDI-TOF MS)分析对金属离子具有较高亲和力的寡核苷酸G-四链体的方法.考察了不同基质:3-羟基吡啶甲酸(3-HPA)与柠檬酸氢二铵(DHC)混合基质、3,4-二胺基苯基苯甲酮(DABP)及DABP/DHC混合基质,应用于G-四链体分析的效果.实验结果表...     

Characterization of phosphopeptides from protein digests using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and nanoelectrospray quadrupole time-of-flight mass spectrometry

A two-step mass spectrometric method for characterization of phosphopeptides from peptide mixtures is presented. In the first step, phosphopeptide candidates were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) based on their higher relative intensities in negative ion MALDI spectra than in positive ion MALDI spectra. The detection limit for this step was found to be 18 femtomoles or lower in the case of unfractionated in-solution digests of a model phosphoprotein, beta-casein. In the second step, nanoelectrospray tandem mass (nES-MS/MS) spectra of doubly or triply charged precursor ions of these candidate phosphopeptides were obtained using a quadrupole time-of-flight (Q-TOF) mass spectrometer. This step provided information about the phosphorylated residues, and ruled out nonphosphorylated candidates, for these peptides. After [(32)P] labeling and reverse-phase high-performance liquid chromatography (RP-HPLC) to simplify the mixtures and to monitor the efficiency of phosphopeptide identification, we used this method to identify multiple autophosphorylation sites on the PKR-like endoplasmic reticulum kinase (PERK), a recently discovered mammalian stress-response protein.     

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry investigations of polystyrene and poly(methyl methacrylate) produced by monoacylphosphine oxide photoinitiation

The chain-end-group composition was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analyses of low-molecular-weight polystyrene (PS) and poly(methyl methacrylate) (PMMA) produced by free-radical polymerization with a monoacylphosphine oxide, (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TPO), as a photoinitiator. Gel permeation chromatography (GPC) fractionation of the PS and PMMA samples with initial polydispersities of 1.81 and 2.81, respectively, yielded improved MALDI-TOF MS spectra. Spectral analyses of the PS fractions showed distributions attributable to PS having two diphenylphosphinyl ends and PS having one diphenylphosphinyl end and/or one 2,4,6-trimethylbenzoyl end, indicating that a combination of PS radicals with the highly reactive diphenylphosphine oxide group at one end of the chains was the predominant mode of termination. MALDI-TOF MS results for PMMA fractions provided evidence for termination primarily by disproportionation, but structure determination was confounded by the presence of isobaric peaks. Discernible peaks were obtained by MALDI-TOF MS analyses of GPC fractions of TPO-initiated poly(methyl-d₃ methacrylate-d₅), in which the major product was PMMA with a diphenylphosphinyl end group and an abstracted deuterium end group, whereas the minor combination product had two diphenylphosphinyl chain ends. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2161–2171, 2007     

Surface analysis using ultrashort laser pulses and time-of-flight mass spectrometry

An all-optical mass spectrometric system is presented which combines the advantages of picosecond-laser desorption and post-ionization with the high performance of a time-of-flight mass-spectrometer. System studies show directly that the ionization process is saturated at focal power densities of about 10¹²W/cm². The saturated ionization is used to quantificate the amount of desorbed particles. Typical desorption rates far below one single monolayer per laser pulse could been achieved.     

Intact protein analysis by matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry

Direct tandem mass spectrometric (MS/MS) analysis of small, singly charged protein ions by tandem time-of-flight mass spectrometry (TOFMS) is demonstrated for proteins up to a molecular mass of 12 kDa. The MALDI-generated singly charged precursor ions predominantly yield product ions resulting from metastable fragmentation at aspartyl and prolyl residues. Additional series of C-terminal sequence ions provide in some cases sufficient information for protein identification. The amount of sample required to obtain good quality spectra is in the high femtomolar to low picomolar range. Within this range, MALDI-MS/MS using TOF/TOF trade mark ion optics now provides the opportunity for direct protein identification and partial characterization without prior enzymatic hydrolysis.     

Differentiating structural isomers of sialylated glycans by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry

Using model acidic glycans, we demonstrate the benefits of permethylation for matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) tandem mass spectrometry. With both the linear and branched structures, extensive cross-ring fragmentation product ions were generated, yielding valuable information on sugar linkages. Elimination of the negative charges commonly associated with sialylated structures through permethylation allowed their structural analysis in the positive ion mode. Extensive A- and X-type ions were observed for the linear structures, and slightly weaker signals for the branched sialylated structures. The diagnostic cross-ring fragments, permitting a distinction between alpha2-3 and alpha2-6 linkages of the sialic acid residues, were seen in abundance. Importantly, the cross-ring fragmentation with the branched structures provides adequate information to assign sialic acid residues, with a specific linkage, to a particular antenna.     

Direct laser desorption/ionization time-of-flight mass spectrometry of conjugated polymers

Two conjugated polymers (CPs), poly(9,9-dioctylfluorene) (PF) and poly(3-octylthiophene) (PT) were analyzed by direct laser desorption/ionization time-of-flight mass spectrometry (LDI-ToF MS). Because of their strong absorption near the wavelength of the laser (337 nm), easy and transient energy transfer properties and sufficient thermal stability, CPs can be desorbed and ionized directly without a matrix. For comparison, these two polymers were also analyzed using matrix-assisted laser desorption/ionization (MALDI)-ToF MS in the positive reflectron mode. The results revealed that they are very similar in terms of quality and resolution. All results demonstrate that LDI-ToF MS is an alternative method for the mass characterization of some conjugated systems, thereby simplifying the process of sample preparation and result analysis.     

Discrimination of Penicillium isolates by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry fingerprinting

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to generate highly reproducible mass spectral 'fingerprints' for twelve Penicillium species. Prior to MALDI-TOF MS analysis, eight replicate cultures of each Penicillium species were subjected to three one-minute bead-beating cycles in an acetonitrile/trifluoroacetic acid solvent. The mass spectra contained abundant peaks in the range of m/z 5000-20 000, and allowed unambiguous discrimination between species. In addition, a biomarker common to all Penicillium mass spectra was observed at m/z 13 900. Discriminant analysis using the MALDI-TOF MS data yielded classification error rates of 0% (i.e. 100% correct identification), indicating that MALDI-TOF MS data may be a useful diagnostic tool for the objective identification of Penicillium species of environmental and clinical importance.     

Characterization of bacterial lipooligosaccharides by delayed extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption ionization (MALDI) with a time-of-flight analyzer has been used to analyze bacterial lipooligosaccharides (LOS). Crude LOS preparations from pathogenic strains of Haemophilus influenzae and Haemophilus ducreyi and a commercial preparation of lipopolysaccharide from Salmonella typhimurium were treated with hydrazine to remove O-linked fatty acids on the lipid A moiety. The resulting O-deacylated LOS forms were water soluble and more amenable to cocrystallization with standard MALDI matrices such as 2,5-dihydroxybenzoic acid and 1-hydroxyisoquinoline. Under continuous extraction conditions, O-deacylated LOS yielded broad peaks with abundant salt adducts as well as forming prompt fragments through β-elimination of phosphoric acid, that is, [M-H₃PO₄-H]. However, when a time delay was used between ionization and extraction (“delayed extraction”) a significant improvement was seen in both mass resolution and the stability of the molecular ions against β-elimination of phosphoric acid, especially in the negative-ion mode. Both an external two-point calibration and an internal single-point calibration were used to assign masses, the latter of which provided the highest degree of accuracy (better than 0.01% in most cases). At higher laser powers, the LOS molecules cleave readily between the oligosaccharide and lipid A moieties yielding a number of prompt fragments. Postsource decay (PSD) analysis of selected molecular ions provided a set of fragments similar to those seen in the linear spectra, although they were more limited in number because they were derived from a single LOS-glycoform. Both the prompt and PSD fragments provided important structural information, especially in assigning the phosphate and phosphoethanolamine substitution pattern of the lipid A and oligosaccharide portions of LOS. Last, with the addition of ethylenediaminetetraacetic acid followed by pulsed sonication, the relatively insoluble (and impure) LOS preparations yielded MALDI spectra similar to the O-deacylated LOS, although these intact LOS preparations required higher laser powers to ionize and were generally more affected by competing impurities.