Cluster ion structures formed by positive and negative chemical ionization of lactic and other hydroxyacids

Both positive ion and negative ion chemical ionization mass spectra of hydroxycarboxylic acids (hydroxyethanoic acid, 3-hydroxypropanic acid, 2-hydroxypropanoic acid and 1-phenyl-1-hydroxyethanoic acid) show intense oligomeric ions when the samples are evaporated into a chemical ionization source. The observation of oligomeric anionic and cationic species is unusual, and the parallel behavior observed between the positive and negative ion mass spectra is striking. These results are explicable in terms of evaporation of oligomers and their dehydration products from the hot probe, although gas phase clustering reactions of singly charged ions are not excluded. Hydrogen bonding and dehydration provide bonding within each cluster. The structures of the ions have been confirmed by recording the collision induced dissociations of individual cluster ions via their mass analyzed ion kinetic energy spectra. Temperature dependence of the chemical ionization mass spectra provides a method for distinguishing hydrogen bonding from covalent bonding and gives further structural information on the cluster ions.     

Electrospray ionization mass spectra of amino acid phosphoramidates of adenosine

Amino acid phosphoramidates of adenosine were synthesized and determined by positive and negative ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). The fragmentation pathways were investigated. In the positive ion mass spectra abundant characteristic fragment ions appeared, and many complementary ions were found. In the negative ion mass spectra only a few fragment ions were observed, and most of them contained phosphoryl groups. The results show that ESI-MS is a useful tool for structural determination of amino acid phosphoramidates of nucleosides.     

Formation of lithiated adducts of glycerophosphocholine lipids facilitates their identification by electrospray ionization tandem mass spectrometry

Electrospray ionization (ESI) tandem mass spectrometry (MS) has simplified analysis of phospholipid mixtures, and, in negative ion mode, permits structural identification of picomole amounts of phospholipid species. Collisionally activated dissociation (CAD) of phospholipid anions yields negative ion tandem mass spectra that contain fragment ions representing the fatty acid substituents as carboxylate anions. Glycerophosphocholine (GPC) lipids contain a quaternary nitrogen moiety and more readily form cationic adducts than anionic species, and positive ion tandem mass spectra of protonated GPC species contain no abundant ions that identify fatty acid substituents. We report here that lithiated adducts of GPC species are readily formed by adding lithium hydroxide to the solution in which phospholipid mixtures are infused into the ESI source. CAD of [MLi⁺] ions of GPC species yields tandem mass spectra that contain prominent ions representing losses of the fatty acid substituents. These ions and their relative abundances can be used to assign the identities and positions of the fatty acid substituents of GPC species. Tandem mass spectrometric scans monitoring neutral losses of the head-group or of fatty acid substituents from lithiated adducts can be used to identify GPC species in tissue phospholipid mixtures. Similar scans monitoring parents of specific product ions can also be used to identify the fatty acid substituents of GPC species, and this facilitates identification of distinct isobaric contributors to ions observed in the ESI/MS total ion current.     

Negative fast atom bombardment ionization of aromatic sulfonic acids: unusual sample-matrix interaction

The most intense ion(s) in negative ion fast atom bombardment (FAB) mass spectra of 2- and 4-benzaldehyde sulfonic acid (BSA) in glycerol or 3-nitrobenzyl alcohol matrix corresponds to a covalent association of the analyte with one or two matrix molecules accompanied by the elimination of a molecule of water. The molecular ion [M - H](-), however, is of low abundance. The identity of the resulting ions [M + nA - H(2)O - H](-) (where M is the analyte and A is the matrix) was confirmed by exact mass measurement using the peak matching technique. These covalent matrix-analyte complexes were not observed when the sulfonic acid functionality in BSA was substituted with COOH, NO(2), and OH or when the sulfonic acid was in salt form. These observations indicate that the free sulfonic acid group in BSA is responsible for the covalent adduct formation. To our knowledge, analyte-matrix covalent association in negative ion FAB spectra of BSA has not been reported previously.     

Laser desorption fourier transform mass spectra of ascorbic and isoascorbic acids

Positive and negative ion laser desorption Fourier transform mass spectra of ascorbic acid, isoascorbic acid, and their sodium and potassium salts have been determined. A number of differences between these spectra and previously reported laser microprobe mass analyzer (LAMMA) spectra are found. In contrast with earlier results, m/z 175 anions are present in all negative ion spectra, as are cluster ions. Thus, it should not be concluded that the radical salts are electroneutral, as claimed in the earlier study.     

Laser desorption mass spectrometry of squaric acid and its salts

The laser desorption mass spectrometry of the oxocarbon squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) and its salts of the form A₂C₄O₄ (A = cation) is described. Both positive and negative ion spectra were obtained. The positive ion spectrum of the acid is characterized by an ion corresponding to loss of CO from [M + H]⁺. The negative ion spectrum shows an intense [M ? H]^? peak in addition to a dimer species. The alkali salt spectra contain [M + A]⁺ in the positive mode and [M ? A]^? and an intense [C₄HO₄]^? in the negative mode. The smaller alkali salts also have an [M + H]⁺ adduct ion. Unlike the alkali squarates, the ammonium salt shows ions corresponding to losses of neutrals from the molecular adduct in the positive ion spectrum and a dimer species in the negative ion spectrum. Molecular weight information was obtained in all cases. A (bis) dicyanomethylene derivative of potassium squarate was also studied. Some field desorption mass spectrometry results are presented for comparison.     

Electrospray mass spectrometry in the structural characterization of cephalosporins

Six cephalosporins of pharmacological interest, cephalexin, cephuroxime, cephazolin, cephoperazone sodium salt, cephatrizin free acid and cephonicid disodium salt, were analysed by electrospray mass spectrometry. [M - Na]- anions were produced in high yield in the case of cephalexin, cephuroxime, cephazolin and cephoperazone, leading to signals at least two orders of magnitude more intense than those related to [M + Na]+ cations observed in the positive ion mode. In cephatrizin, [M - H]- represented the most abundant species, whereas in cephonicid the [M - 2Na + H]- anions were easily produced. No fragment ions were detectable in the electrospray spectra of any of the compounds, and MSn turned out to be essential to draw the fragmentation patterns. Most of these patterns were related to the substituent of the 7-aminocephalosporin nucleus, suggesting that the nucleus itself is highly stable.     

PDMS-chemistry of angiotensin II and insulin in glucose glass thin films

A plasma desorption mass spectrometry study was made on the properties of glucose and glucose/glucuronic acid thin films as matrices for amino acids, small and large peptides and insulin. Amino acids and small peptides are distributed throughout the film as it is formed from aqueous solutions and the mass spectra are similar to what is observed for nitrocellulose matrices. AngiotensinII (angII), insulin, and reduced insulin containing the separated A- and B-chains concentrate at the surface of the film due to the hydrophobic interaction. Extensive positive and negative fragmentation patterns are observed for angII using the glucose glass film. The fragment ions appear to be formed from layers just below the surface of the film. The co-matrix of glucuronic acid/glucose produces a higher molecular ion yield. The spectrum of insulin in glucuronic acid/glucose consists mainly of positive ions with a fragmentation pattern from the B-chain. The spectrum of reduced insulin using a nitrocellulose matrix gives B-chain ions but glucose/glucuronic acid gives A-chain ions in both the positive and negative ion spectra. The fragmentation patterns of the A-chain and B-chain ions are sensitive to the nature of the matrix. An extensive negative ion A-chain fragmentation pattern was observed with glutamate ions serving as the charge centers. The reasons for the behavior of the A- and B-chain fragmentation patterns in these matrices is not clear.     

Use of fast atom bombardment mass spectrometry for the characterization of nitroaromatic isomers

Positive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]⁺ and M⁺ are observed in the positive ion FAB spectra; ions such as [M ? H] ^? and M^?˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.     

甾体磷酰胺类缀合物在电喷雾质谱中的裂解特征

Novel steroidal phosphoramidate conjugates of 3'-azido-2',3'-dideoxythymidine(AZT)and amino acid esterswere synthesized and determined by positive and negative ion electrospray ionization mass spectrometry.The MSfragmentation behaviors of the steroidal phosphoramidate conjugates have been investigated in conjunction withtandem mass spectrometry of ESI-MS/MS.There were three characteristic fragment ions in the positive ion ESImass spectra,which were the Na adduct ions with loss of steroidal moiety,amino acid ester moiety from pseudomolecular ion(M Na)~ ,and the phosphoamino acid methyl ester Na adduct ion by α-cleavage of the phosphora-midate respectively.The main fragment ions in negative ion ESI mass spectra were the ion(M-HN_3)~-,the ion(M-AZT-H)~-,and the ion(M-steroidal moiety-H)~- besides the pseudo molecular ion(M-H)~-.Thefragmentation patterns did not depend on the attached amino acid ester moiety.     

蒽醌及其衍生物的THz时域光谱研究

利用太赫兹(terahertz,THz)时域光谱技术室温下进行了蒽醌及其衍生物2_甲基蒽醌、蒽醌_2_磺酸钠、蒽醌_2,6_二磺酸钠、蒽醌_2,7_二磺酸钠在10～55cm-1(0.3～1.65THz)频谱范围内的光谱测量。结果表明,蒽醌及其衍生物在此波段有不同的吸收特征,它们的吸收可能是由于晶格振动引起的。THz时域光谱不仅能够鉴别分子结构存在微小差别的化合物而且还能鉴别同分异构体。     

Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1--investigations of alkali metal chloride and sodium salt cluster ions

Salt cluster ions of alkali metal chlorides ACl (A = Li(+), Na(+), K(+), Rb(+) and Cs(+)) and sodium salts NaB (B = I(-), HCOO(-), CH(3)COO(-), NO(2)(-), and NO(3)(-)), formed by electrospray ionization, were studied systematically by mass spectrometry. The influences on the total positive ion and negative ion currents of variation of solvent, solution concentration, desolvation temperature, solution flow-rate, capillary voltage and cone voltage were investigated. Only cone voltage was found to influence dramatically the distribution of salt cluster ions in the mass spectra observed. Under conditions of normal cone voltage of approximately 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of approximately 10 V, the distribution of cluster ions detected is characterized by a relatively low average mass/charge ratio due to the presence of multiply charged cluster ions; in addition, there is a marked reduction in cluster ions having a magic number of molecules. Product ion mass spectra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and closest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixed clusters of NaCl with HCl and NaOH, respectively. The effects of ionic radius on 20 distributions of cluster ions for 10 salts were investigated; however, the fine structure of these effects is not readily discerned.     

Direct correlation of the crystal structure of proteins with the maximum positive and negative charge states of gaseous protein ions produced by electrospray ionization

Electrospray mass spectrometric studies in native folded forms of several proteins in aqueous solution have been performed in the positive and negative ion modes. The mass spectra of the proteins show peaks corresponding to multiple charge states of the gaseous protein ions. The results have been analyzed using the known crystal structures of these proteins. Crystal structure analysis shows that among the surface exposed residues some are involved in hydrogen-bonding or salt-bridge interactions while some are free. The maximum positive charge state of the gaseous protein ions was directly related to the number of free surface exposed basic groups whereas the maximum negative charge state was related to the number of free surface exposed acidic groups of the proteins. The surface exposed basic groups, which are involved in hydrogen bonding, have lower propensity to contribute to the positive charge of the protein. Similarly, the surface exposed acidic groups involved in salt bridges have lower propensity to contribute to the negative charge of the protein. Analysis of the crystal structure to determine the maximum charge state of protein in the electrospray mass spectrum was also used to interpret the reported mass spectra of several proteins. The results show that both the positive and the negative ion mass spectra of the proteins could be interpreted by simple consideration of the crystal structure of the folded proteins. Moreover, unfolding of the protein was shown to increase the positive charge-state because of the availability of larger number of free basic groups at the surface of the unfolded protein.     

The effects of electron and chemical Ionization Modes on the MS Profiling of Whole Bacteria

Free fatty acid profiling of whole bacteria [Francisella tularensis, Brucella melitensis, Yersinia pestis, Bacillus anthracis (vegetative and sporulated), and Bacillus cereus] was carried out with direct probe mass spectrometry under 70-eV electron ionization (EI) and isobutane chemical ionization in both the positive (CI+) and negative modes (CI-). Electron ionization produced spectra that contained molecular ions and fragment ions from various free fatty acids. Spectra acquired with isobutane chemical ionization in the positive mode yielded molecular ions of free fatty acids as well as ions from other bacterial compounds not observed under EI conditions. Spectra obtained with negative chemical ionization did not contain as much taxonomic information as EI or CI+; however, some taxonomically significant compounds such as dipicolinic acid and poly(3-hydroxybutyrate) did produce negative ions. All ionization modes yielded spectra that could separate the bacteria by Gram-type when observed with principle components analysis (PCA). Chemical ionization in the positive ion mode produced the greatest amount of differentiation between the four genera of bacteria when the spectra where examined by PCA.     

GO/PVA nanocomposites with significantly enhanced mechanical properties through metalion coordination

In this study, a simple and efficient way is demonstrated to create strong interfacial interaction between graphene oxide(GO) filler and poly(vinyl alcohol)(PVA) matrix through metal ion coordination. The coordination bonding provides efficient load transfer during the tensile process, and enhances the mechanical properties of the nanocomposites significantly. After being coordinated with Cu(Ⅱ) ions, GO/PVA composites show much higher Young's moduli and yield stresses than pure PVA and noncoordinated GO/PVA. UV–vis and FTIR spectra are performed to confirm the successful coordination between GO and PVA. Ethylene diamine tetraacetic acid disodium salt(EDTA-2 Na) is used to confirm the important role of coordination in enhancing the composites. This research provides a new approach to manufacture polymer-matrix nanocomposites with significantly improved mechanical performances.     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

Production and fragmentation of negative ions from neutral N-linked carbohydrates ionized by matrix-assisted laser desorption/ionization

Although negative ion fragmentation mass spectra of neutral N-linked carbohydrates (those attached to Asn in glycoproteins) provide much more structural information than spectra recorded in positive ion mode, neutral carbohydrates are reluctant to form negative ions by matrix-assisted laser desorption/ionization (MALDI) unless ionized from specific matrices such as nor-harmane or adducted with anions such as chloride. This paper reports the results of experiments to optimize negative ion formation from adducts of N-linked glycans with respect to ion abundance and fragment ion production. The best results were obtained with 2,4,6-trihydroxyacetophenone (THAP) as the matrix with added ammonium nitrate as the salt providing the anion. This approach is demonstrated to be applicable for a wide range of N-linked glycan structures. Phosphate adducts, analogous to those that are usually encountered in electrospray spectra from N-glycans released by protein N-glycosidase F, were produced by addition of ammonium phosphate to the matrix but in relatively low yield allowing competitive ionization of endogenous anionic compounds leading to complex spectra. Fragmentation of the nitrate adducts, which were formed in higher yield, generally paralleled that seen by collision-induced dissociation following ionization by electrospray, with the first stage of the dissociation being the elimination of the nitrate with a proton from one of the hydroxyl groups of the sugar. The spectra of the resulting [M-H](-) species displayed very specific fragment ions, mainly cross-ring and C-type glycosidic cleavage products, that revealed more structural (linkage and branching) information of the compounds than the mainly glycosidic cleavage products that dominated the positive ion spectra.     

Negative ion mass spectroscopy of several cyclic sulfones

Conclusions The formation of negative ions by several cyclic sulfone molecules was studied. The introduction of electron-withdrawing substituents or unsaturated bonds markedly stabilizes the negative molecular ion. A difference is observed in the negative ion mass spectra of isomers, while the positive ion mass spectra obtained upon electron impact are identical.Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2477–2482, November, 1985.     

Mass-spectrometric studies of new 6-nitroquipazines—serotonin transporter inhibitors

Six synthesized 6-nitroquipazine derivatives were examined by electron ionization (EI) and electrospray ionization (ESI) mass spectrometry in positive and negative ion mode. The compounds exhibit high affinity for the serotonin transporter (SERT) and belong to a new class of SERT inhibitors. The EI mass spectra registered in negative ion mode showed prominent molecular ions for all the compounds studied. All EI mass spectra and all ESI mass spectra showed similar fragmentation pathways of molecular ions, but the pathways differed between EI and ESI. The differences were explained with the aid of theoretical evaluation of the stability of the respective radical ions (EI MS) and protonated ions (ESI MS).     

Positive and negative ion mass spectrometric studies of dioximes of α-diketones and their nickel complexes

The positive and negative ion mass spectra of glyoxime, methylglyoxime, dimethylglyoxime, diphenyl glyoxime and of their nickel(II) complexes are reported. Both the positive and negative ion mass spectra of the dioximes show loss of OH^˙ and H₂O from the molecular ion to give fragment ions which probably have cyclic furazan type structures. The positive ion spectra of the complexes fragment mainly by loss of ligand radicals whereas the negative ion spectra show mainly loss of OH^˙ and H₂O.