Study of the cyanoacrylate fuming mechanism by matrix‐assisted laser desorption/ionization mass spectrometry

Despite cyanoacrylate fuming being widely used in the forensic science field, its mechanism is not well understood. In this study, matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry is used to study latent fingerprints that have been cyanoacrylate fumed in an attempt to gain insight into the fuming mechanism. In the negative mode mass spectrometry data, four compounds related to the polymerization of cyanoacrylate are identified and their structures are determined from accurate mass and MS/MS. A mechanism is proposed for the formation of these compounds that are regarded as intermediates in the polymerization reaction. In addition, based on the fuming of standard endogenous compounds, we suggest that fatty acids and amino acids are the major catalytic nucleophiles that initiate the polymerization reactions.     

Study of synthetic aliphatic copolyamides by time-of-flight matrix assisted laser desorption/ionization mass spectrometry

Synthetic copolyamides based on aliphatic diamines (1,3-propanediamine and 1,4-butanediamine) and dichlorides of aliphatic carboxylic acids (adipic and sebacic acid dichlorides) were investigated using time-of-flight matrix assisted laser desorption/ionization mass spectrometry. Their mass spectra showed peaks for cationized (Na⁺ and K⁺) and protonated (less intense peaks) oligomers with NH₂-NH₂, NH₂-COOH, or COOH-COOH end groups. No cyclic oligomers were detected in the samples. The compositions of oligomers were determined, and the relative reactivities of homologous comonomers in polycondensation were estimated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1320–1324, July, 2007.     

A novel approach of combining thin-layer chromatography with surface-assisted laser desorption/ionization (SALDI) time-of-flight mass spectrometry

A novel means of combining thin-layer chromatography (TLC) with laser desorption/ionization mass spectrometry using a liquid matrix is proposed. Surface-assisted laser desorption/ionization (SALDI) mass spectrometry, which uses a mixture of a micrometer-sized carbon powder (graphite or activated carbon, the SALDI solid) and 15% sucrose/glycerol, dissolved in an equal volume of methanol (SALDI liquid) as a SALDI matrix, is used for laser desorption mass analysis. The ablation of carbon powder from a pencil drawing was used as an alternative to the SALDI solid. The liquid matrix resembled that used in a conventional SALDI matrix system. A line was drawn before separation with a pencil on the track of the sample developed on the TLC plate. After TLC separation, approximately 0.1 microl of SALDI liquid was directly applied to the chromatographic spots on the TLC plate. Porphyrins were used to demonstrate this combination owing to the visible colors of this type of compound. The analyte signal can be easily detected by irradiating the laser along the pencil line on the TLC plate. An additive, p-toluenesulfonic acid, is added to the SALDI liquid to enhance the signal's intensity. This additive dramatically improves the signal-to-noise ratio. A detection limit of approximately 500 pg is demonstrated for porphines, which is 50 times better than that corresponding to conventional TLC SALDI.     

Recent advances in the preparation of adsorbent layers for thin‐layer chromatography combined with matrix‐assisted laser desorption/ionization mass‐spectrometric detection

This review is focused on planar chromatography hyphenated with mass‐spectrometric detection for analysis of low‐molecular‐mass solutes. Various kinds of hyphenations are discussed with attention paid to the preparation of thin layer plates suited both for the mass‐spectrometric detection of the resolved solutes direct from thin‐layer plates and for indirect mass‐spectrometric detection of the resolved solutes, performed by scraping, extracting, purifying, and concentrating the analyte from the thin‐layer chromatography plate. Plates with monolithic layers are relatively new for thin‐layer chromatography but they can successfully be combined with mass‐spectrometric technique in a pursuit of comprehensive local sample composition information. Preparation of monolithic layers of different porosity and structure based on organic, inorganic, and composite materials is illustrated together with examples of successful separation and detection of low‐molecular‐mass solutes by means of matrix‐assisted and surface‐assisted laser desorption mass spectrometry.     

Charge derivatization by 4-sulfophenyl isothiocyanate enhances peptide sequencing by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

High-sensitivity, rapid identification of proteins in proteomic studies normally uses a combination of one- or two-dimensional electrophoresis together with mass spectrometry. The simplicity and sensitivity of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) have increased its application in recent years. The most common method of 'peptide fingerprinting' often may not provide robust identification. Normally additional sequence information by post-source decay (PSD) MALDI-TOFMS provides additional constraints for database searches to achieve highly confident results. Here we describe a derivatization procedure to facilitate the acquisition of such sequence information. Peptide digests from a skin-expressed protein were modified with 4-sulfophenyl isothiocyanate. The resulting peptides carry a fixed negative charge at the N-terminal end and the resulting PSD spectrum is dominated by C-terminal y-type ions. The sequence information in most cases can be obtained manually or with simple programming tools. Methods of optimizing the procedure and increasing the sensitivity are discussed.     

Derivatization of small biomolecules for optimized matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is a powerful tool for the measurement of low molecular mass compounds of biological interest. The limitations for this method are the volatility of many analytes, possible interference with matrix signals or bad ionization or desorption behavior of the compounds. We investigated the application of well-known and straightforward one-pot derivatization procedures to circumvent these problems. The derivatizations tested allow the measurement and the labeling of alcohols, aldehydes and ketones, carboxylic acids, alpha-ketocarboxylic acids and amines.     

Characterization of some synthetic Ru and Ir complexes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was applied to the analysis of Ru(OCOCF(3))(2)(CO)(PPh(3))(2), Ru(OCOC(3)F(7))(2)(CO)(PPh(3))(2), Ir(tBuppy)(3) and Ir(ppy)(2)(acac) complexes. A troublesome problem in the MALDI-TOFMS characterization of these metal complexes is the possible replacement of complex ligands by matrix. In this contribution, 10 matrices, ranging from acidic to basic, were investigated: alpha-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA), dithranol, 2,4,6-trihydroxyactophenone (THAP), 6-azo-2-thiothymine (ATT), norharman, 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB), 4-nitroaniline (NA) and 2-amino-5-nitrophyridine (ANP). With most of the matrices, including the neutral and basic ones, matrix substitution of ligand could clearly be detected. Based on the experimental results, possible mechanisms of matrix substitution were discussed. It was demonstrated that the ligand exchange process might also occur through the gas-phase reactions initiated by laser shots. Among the matrices tested, DCTB was found to be the best one for the complexes that are prone to ligand exchange by matrix.     

Dehydrogenation of tertiary amines in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) analysis of various compounds synthesized in our laboratory, strong [M - H]+ ion peaks were often observed for the molecules with tertiary amino groups. In this work, the MALDI TOF MS behavior of two groups of compounds that incorporate tertiary amino moieties was investigated. One group is bisurea dimethylanilines (BUDMAs) prepared for the study of molecular recognition in thermoplastic elastomers, and the other group is the poly(propylene imine) diaminobutane dendrimers. The results clearly demonstrate the appearance of the [M - H]+ ions. In order to understand the possible mechanisms for the generation of these ions, a series of model compounds, ranging from primary to tertiary amines, were investigated. Unlike the tertiary amines, no [M - H]+ ion peaks were recorded for the primary amines, and only barely detectable ones, if any, for some secondary amines. It appears that the tertiary amino groups play an important role in the formation of these ions. In addition to MALDI TOF MS analysis, these samples were also applied to electrospray ionization (ESI) MS where no [M - H]+ ions were observed. The results indicate that the generation of [M - H]+ ion is due to the unique MALDI conditions and is likely to be formed via dehydrogenation of a protonated tertiary amine resulting in an N=C double bond. The absence of [M - H]+ ion peaks for the primary and secondary amines is probably because upon their formation these ions could easily transfer one proton to the corresponding amines in the MALDI gas-phase plume, yielding neutral imines that cannot be detected by MS.     

Detection of water-soluble vitamins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using porphyrin matrices

The detection of water-soluble vitamins B(1), B(2), B(6), B(12) and C by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOFMS) was attempted by studying 17 porphyrin matrices. Comparative studies of porphyrin matrices, useful mass spectral window, matrix/analyte molar ratio (M/A), ultraviolet-visible absorption characteristics and quantitative results were made. Most porphyrin matrices provide a useful mass spectral window in the low-mass range. The optimal M/A increases with increasing molecular mass of the vitamin. Vitamin B(12) possesses the highest molecular mass and requires a higher M/A. The presence of hydroxyl or carboxyl groups in the porphyrin is an indicator of a useful MALDI matrix. Vitamins B(2) and B(6) were readily ionized upon irradiation with a 337 nm laser without the use of any porphyrin matrix. Improved linearity and sensitivity of the calibration curves were obtained with samples prepared with a constant M/A. The limits of detection and quantitation are at the picomole level. The results indicate that MALDI-TOFMS with porphyrin matrices is a rapid and viable technique for the detection of low molecular mass water-soluble vitamins.     

Automated normal phase nano high performance liquid chromatography/matrix assisted laser desorption/ionization mass spectrometry for analysis of neutral and acidic glycosphingolipids

The coupling of nano high-performance liquid chromatography (nanoHPLC) with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) via an automatic spotting roboter was developed and adapted for the first time for the analysis of complex mixtures of glycosphingolipids (GSLs). The 2,5-dihydroxybenzoic acid and 6-azo-2-thiothymine matrix systems were adjusted to concurrently meet the requirements for reproducible and homogeneous crystal formation with the liquid chromatography (LC) eluent under the variable LC solvent composition over the course gradient and high ionization efficiency of the GSL species, without the need for recrystallization. Precise adjustment of the automatic spotting parameters in terms of matrix flow rate, on-tip collection time of the matrix/LC eluent solution and the matrix spotting mode, i.e., continuous and discontinuous, was accomplished to collect individually nanoHPLC-separated species within distinct spots and consequently recover by MALDI MS screening all major and minor GSL species in the mixtures. The nanoHPLC/MALDI MS coupling protocol was developed and applied to a mixture of neutral GSLs purified from human erythrocytes and a monosialoganglioside mixture expressed by the murine MDAY-D2 cell line. Additionally, on-line nanoHPLC/MALDI doping with lithium cations of individually separated neutral GSLs was introduced to enhance data interpretation of the GSL MS pattern, while preserving the same level of information and ultimately to enhance structural assignment of components of interest. The method is demonstrated to be highly sensitive, reaching the low femtomole level of detection of individual GSL species and is highlighted as a versatile analytical tool for glycolipidomic studies. Figure Automatic LC/MALDI MS profiling of glycosphingolipids Mostafa Zarei and Stephan Kirsch contributed equally to this work.     

基质辅助激光解吸电离飞行时间质谱研究2: 四硫 富瓦烯化合物

采用基质辅助激光解吸电离飞行时间质谱(MALDI-TOF-MS),对四硫富瓦烯化合物进行质谱表征。在所用的实验条件下,样品很容易解吸电离生成单电荷分子离子,得到单同位素分辨的质谱图。26种实际样品的质谱分析结果表明;MALDI-TOF-MS可以比其它质谱方法更有效、更方便地用于此类化合物的质谱分析,解决了此类化合物不易进行质谱鉴定的难题。     

Identification of formylglycine in sulfatases by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

C(alpha)-Formylglycine, the catalytic amino acid residue in the active site of sulfatases, is generated by post-translational modification of a cysteine or serine residue. We describe a highly sensitive procedure for the detection of C(alpha)-formylglycine-containing peptides in tryptic digests of sulfatase proteins. The protocol is based on the formation of hydrazone derivatives of C(alpha)-formylglycine-containing peptides when using dinitrophenylhydrazine as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The hydrazone derivatives desorb and ionize with high efficiency and can be detected in the sub-femtomole range. The presence of C(alpha)-formylglycine is indicated by a mass increment of 180.13 u, corresponding to the hydrazone moiety, and also by a unique C-terminal fragment ion, characteristic of sulfatases, that becomes prominent in MALDI post-source decay mass spectra of the hydrazone derivatives.     

Phosphopeptide detection and sequencing by matrix-assisted laser desorption/ionization quadrupole time-of-flight tandem mass spectrometry

A prototype matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDI-TOF) tandem mass spectrometer was used to sequence a series of phosphotyrosine-, phosphothreonine- and phosphoserine-containing peptides. The high mass resolution and mass accuracy of the instrument allowed the localization of one, three or four phosphorylated amino acid residues in phosphopeptides up to 3.1 kDa. Tandem mass spectra of two different phosphotyrosine peptides permitted amino acid sequence determination and localization of one and three phosphorylation sites, respectively. The phosphotyrosine immonium ion at m/z 216.04 was observed in these MALDI low-energy CID tandem mass spectra. Elimination of phosphate groups was evident from the triphosphorylated peptide but not from the monophosphorylated species. The main fragmentation pathway for the synthetic phosphothreonine-containing peptide and for phosphoserine-containing peptides derived from beta-casein and ovalbumin was the beta-elimination of phosphoric acid with concomitant conversion of phosphoserine to dehydroalanine and phosphothreonine to 2-aminodehydrobutyric acid. Peptide fragment ions of the b- and y-type allowed, in all cases, the localization of phosphorylation sites. Ion signals corresponding to (b-17), (b-18) and (y-17) fragment ions were also observed. The abundant neutral loss of phosphoric acid (-98 Da) is useful for femtomole level detection of phosphoserine-peptides in crude peptide mixtures generated by gel in situ digestion of phosphoproteins.     

Application of off-line size-exclusion chromatographic fractionation-matrix assisted laser desorption ionization time of flight mass spectrometry for proanthocyanidin characterization

In this work, we wanted to devise a reliable method to characterize polymerized forms of tannins, their structural information and mass distribution. Size-exclusion chromatography (SEC) is a chromatographic technique used to determine molecular mass (weight) distributions of polymers. One important step in the data treatment is the modeling of the calibration curves. Polystyrenes (PS) are standards usually used because no commercial procyanidin (PC) standards are available. An off-line coupling of SEC and MALDI was carried out to measure differences between polystyrenes and procyanidins. Thus, a new calibration curve was established; from 1000 to 8000 Da, there is a good correlation between the MALDI and PS calibration curves, in this field the PS calibration is correct and enables true mass determination. For masses above 8000 Da, PS calibration overestimates the real molecular weight of PC, overestimation of 53%. And for masses below 1000 Da, PS calibration underestimates their real molecular weight (10-15%). This means that to truly characterize PC, calibration based on PC standards is required.     

Inline pneumatically assisted atmospheric pressure matrix‐assisted laser desorption/ionization ion trap mass spectrometry

Atmospheric pressure (AP) matrix‐assisted laser desorption/ionization (MALDI) is known to suffer from poor ion transfer efficiencies as compared to conventional vacuum MALDI (vMALDI). To mitigate these issues, a new AP‐MALDI ion source utilizing a coaxial gas flow was developed. Nitrogen, helium, and sulfur hexafluoride were tested for their abilities as ion carriers for a standard peptide and small drug molecules. Nitrogen showed the best ion transport efficiency, with sensitivity gains of up to 1900% and 20% for a peptide standard when the target plate voltage was either continuous or pulsed, respectively. The addition of carrier gas not only entrained the ions efficiently but also deflected background species and declustered analyte–matrix adducts, resulting in higher absolute analyte signal intensities and greater signal‐to‐noise (S/N) ratios. With the increased sensitivity of pneumatically assisted (PA) AP‐MALDI, the limits of detection of angiotensin I were 20 or 3 fmols for continuous or pulsed target plate voltage, respectively. For analyzing low‐mass analytes, it was found that very low gas flow rates (0.3–0.6 l min^?1) were preferable owing to increased fragmentation at higher gas flows. The analyte lability, type of gas, and nature of the extraction field between the target plate and mass spectrometer inlet were observed to be the most important factors affecting the performance of the in‐line PA‐AP‐MALDI ion source. Copyright © 2010 John Wiley & Sons, Ltd.     

Stable isotope labeling for matrix-assisted laser desorption/ionization mass spectrometry and post-source decay analysis of ribonucleic acids

Matrix-assisted laser desorption/ionization mass spectrometry is a powerful analytical tool for the structural characterization of oligonucleotides and nucleic acids. Here we report the application of stable isotope labeling for the simplified characterization of ribonucleic acids (RNAs). An (18)O label is incorporated at the 3'-phosphate of oligoribonucleotides during the enzymatic processing of intact RNAs. As implemented, a buffer solution containing a 50 : 50 mixture of H(2)O and (18)O-labeled H(2)O is used during endonuclease digestion. Upon digestion, characteristic doublets representative of the isotopic distribution of oxygen are noted for those products that contain 3'-phosphate groups. This approach is used to distinguish readily endonuclease digestion products from incomplete digestion products and non-specific cleavage products. In addition, RNase digestion products containing the characteristic isotopic doublet can be selected for further characterization by post-source decay (PSD) analysis. PSD products carrying the 3'-phosphate group will appear as a doublet, thereby simplifying fragment ion assignment.     

Desorption electrospray ionization ‐ orbitrap mass spectrometry of synthetic polymers and copolymers

Desorption ElectroSpray Ionization (DESI) ‐ Orbitrap Mass Spectrometry (MS) was evaluated as a new tool for the characterization of various industrial synthetic polymers (poly(ethylene glycol), poly(propylene glycol), poly(methylmethacrylate), poly(dimethylsiloxane)) and copolymers, with masses ranging from 500 g.mol⁻¹ up to more than 20 000 g.mol⁻¹. Satisfying results in terms of signal stability and sensitivity were obtained from hydrophobic surfaces (HTC Prosolia) with a mixture water/methanol (10/90) as spray solvent in the presence of sodium salt. Taking into account the formation of multiplied charged species by DESI‐MS, a strategy based on the use of a deconvolution software followed by the automatic assignment of the ions was described allowing the rapid determination of M_n, M_w and PDI values. DESI‐Orbitrap MS results were compared to those obtained from matrix‐assisted laser desorption/ionization‐ time‐of‐flight MS and gel permeation chromatography. An application of DESI‐Orbitrap MS for the detection and identification of polymers directly from cosmetics was described. Copyright © 2012 John Wiley & Sons, Ltd.     

Effective detection of peptides containing cysteine sulfonic acid using matrix-assisted laser desorption/ionization and laser desorption/ionization on porous silicon mass spectrometry

Cysteine sulfonic acid-containing peptides, being typical acidic peptides, exhibit low response in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. In this study, matrix conditions and the effect of diammonium hydrogencitrate (DAHC) as additive were investigated for ionization of cysteine sulfonic acid-containing peptides in MALDI. A matrix-free ionization method, desorption/ionization on porous silicon (DIOS), was also utilized to evaluate the effect of DAHC. When equimolar three-component mixtures of peptides carrying free cysteine, cysteine sulfonic acid, and carbamidomethyl cysteine were measured by MALDI using a common matrix, alpha-cyano-4-hydroxycinnamic acid (CHCA), no signal corresponding to cysteine sulfonic acid-containing peptide could be observed in the mass spectrum. However, by addition of DAHC to CHCA, the peaks of cysteine sulfonic acid-containing peptides were successfully observed, as well as when using 2,4,6-trihydroxyacetophenone (THAP) and 2,6-dihydroxyacetophenone with DAHC. In the DIOS mass spectra of these analytes, the use of DAHC also enhanced the peak intensity of the cysteine sulfonic acid-containing peptides. On the basis of studies with these model peptides, tryptic digests of oxidized peroxiredoxin 6 were examined as a complex peptide mixture by MALDI and DIOS. In MALDI, the peaks of cysteine sulfonic acid-containing peptides were observed when using THAP/DAHC as the matrix, but this was not so with CHCA. In DIOS, the signal from cysteine sulfonic acid-containing peptides was suppressed; however, the use of DAHC significantly enhanced the signal intensity with an increase in the number of observed peptides and increased signal-to-noise ratio in the DIOS spectra. The results show that DAHC in the matrix or on the DIOS chip decreases discrimination and suppression effects in addition to suppressing alkali-adduct ions, which leads to a beneficial effect on protonation of peptides containing cysteine sulfonic acid.     

A novel approach for analysis of oligonucleotide-cisplatin interactions by continuous elution gel electrophoresis coupled to isotope dilution inductively coupled plasma mass spectrometry and matrix-assisted laser desorption/ionization mass spectrometry

In this work we present a novel approach for in vitro studies of cisplatin interactions with 8-mer oligonucleotides. The approach is based on the recently developed coupling of continuous elution gel electrophoresis (GE) to an inductively coupled plasma-sector field mass spectrometer (ICP-SFMS) with the aim of monitoring the interaction process between this cytostatic drug and the nucleotides. In contrast to existing methods, the electrophoretic separation conditions used here allow both the determination of the reaction kinetics in more detail as well as the observation of dominant intermediates. Two different nucleotides sequences have been investigated for comparison purposes, one containing two adjacent guanines (5'-TCCGGTCC-3') and one with a combination of thymine and guanine (5'-TCCTGTCC-3'), respectively. In order to gain further structural information, MALDI-TOF MS measurements have been performed after fraction collection. This allows for identification of the intermediates and the final products and confirms the stepwise coordination of cisplatin via monoadduct to bisadduct formation. Furthermore, the ICP-MS results were quantitatively evaluated in order to calculate the kinetics of the entire process.