Comparison of mass spectrometric techniques for generating molecular weight information on a class of ethoxylated oligomers

The results of fast atom bombardment (FAB), time-of-flight secondary ion mass spectrometry (ToF-SIMS), matrix-assisted laser desorption/ionization (MALD/I), electrospray ionization (ESI), and field desorption (FD) analyses of ethoxylated oligomers of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (Surfynol^® 104) were compared.Each of these desorption mass spectrometry (MS) techniques can produce spectra of unfragmented cationized oligomers. From the observed ion series we calculate average molecular weight information. We have compared the results of mass spectrometric analyses of a series of ethoxylated Surfynol surfactants. Our data indicate that FAB, ToF-SIMS, MALDI/I, and ESI produce similar results for the lower molecular weight species, but that as the average molecular weight increases FAB and SIMS produce slightly lower results than MALD/I and FD. This could be due to increased fragmentation. ESI produced a result similar to FAB and SIMS for the highest average molecular weight material. Further experiments compare the mass spectral results with gas chromatographic quantitative data. Although gas chromatography is not expected to accurately analyze the higher mass oligomers, we observe significant differences in intensities of the short-chain oligomers (especially the 0- and 1-mers) when compared to the desorption mass spectrometer results. These differences may reflect poor cationization efficiency for very short oligomer chains in the mass spectrometric analyses.     

Comparison of bovine and porcine beta-lactoglobulin: a mass spectrometric analysis

Nano-electrospray-ionization mass spectrometry (nano-ESI-MS) is applied to comparison of bovine and porcine beta-lactoglobulin (BLG and PLG). The conformational and oligomeric properties of the two proteins under different solvent and experimental conditions are analyzed. The pH-dependence of dimerization is described for the pH range 2-11. The results indicate maximal dimer accumulation at pH 6 for BLG and pH 4 for PLG, as well as a lower stability of the PLG dimer at pH 4 compared to BLG at pH 6. Conformational stability appears to be higher for BLG at acidic pH, but higher for PLG at basic pH. The higher stability of BLG at low pH is revealed by means of either chemical or thermal denaturation. Equilibrium folding intermediates of both proteins are detected. Finally, conditions are found that promote dissociation of the BLG dimer at pH 6 into folded monomers.     

Thermogravimetry for measuring the molecular mass distribution of perfluorinated oligomers

A method for determinoffing the molecular mass distribution (MMD) in mixtures of perfluorinated oligomers based on analysis of the differential thermogravimetric (DTG) curves (temperature dependences of the rate of mass loss) is proposed. It is indicated that this method allows us to measure the MMD of oligomers whose boiling temperature is less than the temperature of their thermal decomposition, and the total portion of longer oligomers. This method was applied to the determination of MMD of products of the radiation-chemical telomerization of tetrafluoroethylene in solution.     

Uniform oligomers as reference materials for molecular weight determination of polymers

 Uniform oligomers are special oligomers having no molecular weight distribution. They have been used as reference materials for polymer characterization, especially for accurate molecular weight determination in lower molecular weight ranges. These oligomers are used for different purposes. First, they are used in the calibration for chromatographic or spectroscopic data. Second, they are applied to identifying peaks appearing in chromatographies such as supercritical fluid chromatography (SFC) of oligomeric mixtures. Finally, they could give accurate instrumental or physicochemical constants used in polymer characterizations. The uniform oligomers have been prepared by using preparative chromatographies as separation methods. In some cases, step-wise syntheses have been applied together with the chromatographic methods. Recently, we prepared uniform oligostyrenes and oligo(oxyethylene)s whose molecular weights are greater than 1000 by using preparative size exclusion chromatography. More recently, using preparative SFC, some Japanese researchers have prepared high molecular weight and uniform oligomers such as oligo(methylmethacrylate)s. In this article, we briefly review recent progress mainly made by Japanese researchers in the field of preparation, analysis and applications of these pure oligomers. Received: 20 September 1996 Accepted: 3 December 1996     

Comparison of mass spectrometric methods for studying thermally labile compounds: Rifapentine

Results of 16 different mass spectrometric ionization and sample-introduction methods are compared for the case of a thermally very labile antibiotic, rifapentine. These suggest that extensive thermal decomposition occurs during evaporation when the sample can come into contact with hot metal parts, usually the source housing. The intensity of the molecular ion and the extent of fragmentation depend on various parameters, such as the ionization process, positive or negative-ion detection and the type of sample introduction. The most informative methods for rifapentine seem to be ‘in-beam’ electron impact, negative ionization with particle beam and direct liquid introduction interfaces and positive- and negative-ion fast atom bombardment.     

Two-dimensional correlation spectroscopy techniques applied to ion trap tandem mass spectrometric analysis: nitroaromatics

The technique of two-dimensional (2D) correlation spectroscopy is combined with ion trap tandem mass spectrometric (MS/MS) analysis to provide insight into the energetics and behavior of gas-phase ions. The 2D correlation method is demonstrated by the analysis of the fragment ions produced by the fragmentation of molecular radical cations of nitrobenzene and m-nitrotoluene at a series of applied potentials that effect collision-induced dissociation (CID). The asynchronous correlation intensities show that competitive loss of NO to form [M-NO]+ proceeds at a lower CID energy than loss of NO2 to form [M-NO2]+. The formation of the ion corresponding to [M-NO-CO]+ is shown by the method to occur at a lower CID energy than the formation of [M-NO2]+, but at a higher CID energy than [M-NO]+ for both nitrobenzene and m-nitrotoluene.     

Investigation of endoglucanase selectivity on carboxymethyl cellulose by mass spectrometric techniques

The benefits of applying cellulose selective enzymes as analytical tools for chemical structure characterization of cellulose derivatives have been frequently addressed over the years. In a recent study the high selectivity of cellulase Cel45A from Trichoderma reesei (Tr Cel45A) was utilized for relating the chemical structure to the flow properties of carboxymethyl cellulose (CMC). However, in order to take full advantage of the enzymatic hydrolysis the enzyme selectivity on the cellulose substrate must be further investigated. Therefore, the selectivity of Tr Cel45A on CMC was studied by chemical sample preparation of the enzyme products followed by mass spectrometric chemical structure characterization. The results strongly suggest that, in accordance with recent studies, also this highly selective endoglucanase is able to catalyze hydrolysis of glucosidic bonds adjacent to mono-substituted anhydroglucose units (AGUs). Furthermore, the results also indicate that substituents on the nearby AGUs will affect the hydrolysis.     

Comparison of in-tube sorptive extraction techniques for non-polar volatile organic compounds by gas chromatography with mass spectrometric detection

A commercial in-tube sorptive extraction device, known as solid-phase dynamic extraction (SPDE), has been evaluated for the extraction of non-polar volatile aromatic analytes from aqueous solutions in both headspace and liquid injection modes. An automated sampler is used with a gas-tight 2.5 ml syringe equipped with a special needle that is coated on the inside with a non-polar extraction phase. After absorption onto the phase, the analytes were thermally desorbed directly into a GC-MS system. The technique was evaluated for the determination of furan, benzene and toluene. The sensitivity for toluene was greatly improved on using SPDE compared to static headspace. A slight increase in sensitivity was observed for benzene but none for determination of furan. Estimated limits of detection ranged from 0.2 to 2 microg/l.     

A novel method to determine the molecular weight of the oligomers of biodegradable polymer

The biodegradable polymers used in controlled release applications are primarily insoluble polymers which undergo chemical or enzymatic hydrolysis to form soluble monomeric or oligomeric units. To determine the endurance of the polymer after implantation or injection into the body it is important to determine the rate of elimination of the intermediates formed upon the hydrolysis of the polymer. It is essential to identify these intermediates before its elimination rate can be determined. Identification of these intermediates is a problem because they are difficult to isolate, and are thermally and hydrolytically unstable. A new technique is proposed, taking advantage of the neighboring group effect on the hydrolysis of ionic and non-ionic oligomers, to determine the molecular weight of unknown species that are formed upon hydrolysis of the polymer. The technique involves the determination of the specific acid and base catalytic rate constants. To demonstrate the technique, a time sequence synthesis was carried out to synthesize several oligomers that would be forming upon the hydrolysis of poly(butylene tartrate). Hydrolysis studies were conducted with these oligomers and the samples were analyzed by liquid chromtography. The observed rate constant for the hydrolysis under acidic and alkaline pH medium were calculated from the terminal slopes of the first order plot. The observed rate constants were further utilized to calculate the specific acid and base catalytic rate constants. These specific rate constants, along with the micro-hydrogen and -hydroxyl ion catalytic rate constants, were used to determine the molecular weight of the intermediate species. The molecular weight obtained from the kinetic parameters was in excellent agreement with the results obtained from fast atom bombardment mass spectrometry. The same type of analysis can be extended to any multifunctional group compound which has repeat units and can undergo a specific reaction which can be accurately measured.     

Cyclization of molecular ions: The mass spectrometric fragmentation of allene-carboxylic esters

A series of six 2,4,4-substituted allenecarboxylic esters R₁R₂C?C?C(R₃)COOAlk has been found to exhibit O-Alkyl cleavage as the most prominent feature of both their 70 and 12 eV mass spectra. Loss of linearity of the allene system upon ionization, followed by conversion of the molecular ion to a 5-membered cyclic isomer, is thought to induce this unusual mode of bond rupture. Oxygen-migration from the ester function to C-4 as is indicated by the subsequent decomposition of the [M — Alk] ions to [R₁? C?O]+ and [R₂? C?O]⁺ fragments can be taken as direct evidence in favour of this rationale.     

Molecular speciation of microparticles: application of pattern recognition techniques to laser microprobe mass spectrometric data

Attempts were made to enhance the ability of laser microprobe mass spectrometry (LAMMS) to identify molecular species in individual microparticles by applying pattern recognition methods. Principal component analysis (PCA) and canonical discriminant analysis were applied to LAMMS data for nickel-containing environmental particles. Detailed comparison of the two statistical methods demonstrated the utility of PCA. The successful application was highly dependent on the use of appropriate spectral normalization and feature extraction techniques prior to PCA. Although the test system involved only a small number of standard compounds, the LAMMS data were complicated by the effects of intra-particle heterogeneity common to environmental samples and by instrumental limitations. Pattern recognition techniques provided more accurate quantitative assignments of molecular species than were available by qualitative inspection of characteristic cluster ions or by simple spectral subtraction to compare particle data with a library of standard compounds. Results were substantiated by comparison with bulk analysis studies using wet chemical techniques.     

UPLC/MSE; a new approach for generating molecular fragment information for biomarker structure elucidation

The original article to which this Addendum refers was published in Rapid Commun. Mass Spectrom. 2006; 20 : 1989–1994     

Comparison of gas chromatographic and gas chromatographic/mass spectrometric techniques for the analysis of TNT and related nitroaromatic compounds

The use of capillary column gas chromatography and gas chromatography/mass spectrometry for the analysis of a series of standard solutions (0.1 to 10 μg/ml) of 2,4,6-trinitrotoluene (TNT) and eight other nitroaromatic components was evaluated. The techniques included gas chromatography with electron capture detection (GC/ECD), full scan and selected ion monitoring gas chromatography/mass spectrometry with electron impact ionization (EI/FS and EI/SIM), full scan and selected ion monitoring gas chromatography/mass spectrometry with positive ion chemical ionization using methane reagent gas (PICI/FS and PICI/SIM), and full scan and selected ion monitoring gas chromatography/mass spectrometry with negative ion chemical ionization using methane reagent gas (NICI/FS and NICI/SIM). The performance of the techniques was comapared by determining the linear response range, precision, and detection limits of the analyses.     

Comparison of metal and metal oxide media for phosphopeptide enrichment prior to mass spectrometric analyses

Several affinity resins consisting of ionic metals or metal oxides were investigated for their phosphopeptide enrichment capabilities with subsequent mass spectrometric analyses. Commercially-available enrichment metal oxide affinity chromatography (MOAC) resins using manufacturer’s and/or published protocols were compared and evaluated for the most efficient and selective method that could be implemented as a standard enrichment procedure. From these comparative analyses, using a tryptic digest of casein proteins, it was determined that in our hands, two of the resins out-performed the others based on a variety of criteria, including the number of phosphorylation sites identified during MS analyses, the lower numbers of nonspecifically bound peptides observed, and the limits of detection. Applicability of these enrichment resins to a complex biological mixture was investigated. For this work, a mixture of avian histones was digested, subjected to titanium dioxide phosphopeptide enrichment, and analyzed by mass spectrometry. Eight phosphorylated tryptic peptides were observed following enrichment and subsequent LC/MS/MS analyses. Of note, seven of the eight phosphopeptides were not observed without titanium dioxide enrichment. From these analyses, four sites of phosphorylation were unequivocally determined, two of which have not been reported previously. Four additional phosphopeptides were observed; however, the site of phosphorylation could not be distinguished but was localized to one of two possible amino acids. These methods should aid in the investigation of proteins post-translationally modified with phosphate, especially those present at low concentrations as was demonstrated by successful enrichment at the femtomole level.     

Characterization of low molecular weight hydrocarbon oligomers by laser desorption/ionization time-of-flight mass spectrometry using a solvent-free sample preparation method.

A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards.     

Approaches for the analysis of low molecular weight compounds with laser desorption/ionization techniques and mass spectrometry

This review summarizes various approaches for the analysis of low molecular weight (LMW) compounds by different laser desorption/ionization mass spectrometry techniques (LDI-MS). It is common to use an agent to assist the ionization, and small molecules are normally difficult to analyze by, e.g., matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) using the common matrices available today, because the latter are generally small organic compounds themselves. This often results in severe suppression of analyte peaks, or interference of the matrix and analyte signals in the low mass region. However, intrinsic properties of several LDI techniques such as high sensitivity, low sample consumption, high tolerance towards salts and solid particles, and rapid analysis have stimulated scientists to develop methods to circumvent matrix-related issues in the analysis of LMW molecules. Recent developments within this field as well as historical considerations and future prospects are presented in this review.     

Matrix-assisted and polymer-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of low molecular weight polystyrenes and polyethylene glycols

Recently, matrices based on oligomers of dioxin and thiophene (polymer-assisted laser desorption/ionization (PALDI)) have been described for mass spectrometric (MS) analysis of low molecular weight compounds (Woldegiorgis A, von Kieseritzky F, Dahlstedt E, Hellberg J, Brinck T, Roeraade J. Rapid Commun. Mass Spectrom. 2004; 18: 841-852). In this paper, we report the use of PALDI matrices for low molecular weight polymers. An evaluation with polystyrene and polyethylene glycol showed that no charge transfer ionization occurs. Ionization is mediated through metal ion adduction. Comparison of matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) data for two very low molecular weight polymers with data obtained from size-exclusion chromatography (SEC) revealed a systematic difference regarding mean molecular weight and dispersity. Further, the mass spectra obtained with PALDI matrices had a higher signal-to-noise ratio than the spectra obtained with conventional matrices. For polymers with higher molecular weights (>1500 Da), the conventional matrices gave better performance. For evaluation of the MALDI spectra, three non-linear mathematical models were evaluated to model the cumulative distributions of the different oligomers and their maximal values of Mw, Mn and PDI. Models based on sigmoidal or Boltzmann equations proved to be most suitable. Objective modeling tools are necessary to compare different sample and instrumental conditions during method optimization of MALDI analysis of polymers, since the bias between MALDI and SEC data can be misleading.