摘要:

AbstractThe principles of time‐of‐flight mass spectrometry (TOFMS) are described with a view to understanding the strengths and weaknesses of this method of mass analysis in the context of current applications of mass spectrometry and the more familiar scanning instruments. Fundamental and instrumental factors affecting resolving power, sensitivity and speed are examined. Methods of gating ion populations and the special requirements in the detection and digitisation of the signals in the TOFMS experiment are discus

ISSN:1076-5174    

DOI:10.1002/jms.1190301102

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractMethyl and ethylendo‐ andexo‐bicyclo [2.2.2] oct‐5‐ene‐2‐carboxylates exhibit different mass spectral behaviour under i‐Bu‐chemical ionization (CI) conditions and upon collision‐induced dissociation (CID) of their MH+IONS. Theendo‐esters exhibitm/z135 [MH  MeOH]+and [MH  EtOH]+ions, which are absent in the i‐Bu‐CI mass spectra of theexo‐isomers. This stereospecific behavior is explained in terms of the proton bridging requirement in the alcohol elimination from MH+ions of esters. Such proton bridging between the alkoxycarbonyl group and the double bond is possible in theendo‐esters, but not in theexo‐isomers. Hydrogen‐deuterium exchange processes, observed in the CID spectra of the MD+ion of theendo‐esters, support the proposed mechanistic pathway. Hydrogen bridging with the double bond also plays a role in the alcohol elimination processes of stereoisomeric bicyclo

ISSN:1076-5174    

DOI:10.1002/jms.1190301103

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractEnergy resolved mass spectrometry (ERMS) was used to characterize the dissociation processes of four diethyl alkyl phosphonates where the alkyl R group attached to the central phosphorus atom ranged from methyl to n‐butyl. An atmospheric pressure ionization triple quadrupole mass spectrometer was used in the analyses. Dissociation of the MH+and further dissociation of the resulting progeny ions was induced between the atmospheric—vacuum sampling orifice and the first ion focusing RF‐only quadrupole, Q0, instead of the central Q2 quadrupole. The major dissociation pathways of the four phosphonates could be characterized by two sequential McLafferty rearrangements yielding an alkyl phosphonic acid. This acid further dissociated losing H2O followed by ROH to produce PO+, or the same process occurred but in the reverse order, i.e. first losing ROH followed by H2O to yield PO+. The latter process was the energetically favored process. The alkyl group attached to the phosphorus atom of the protonated alkyl phosphonic acid was observed to have a significant influence on the stability of the protonated alkyl phosphonic acid. As the R group increased in size with a concomitant increase in electron‐donating power, the alkyl phosphonic acid was less prone to dissociation than other alkyl phosphonic acids which possessed smaller R groups. A significant difference was also noted in the alkyl phosphonic acid dissociation processes; methyl phosphonic acid dissociated predominantly via H2O loss unlike the other acids which favored the loss of ROH o

ISSN:1076-5174    

DOI:10.1002/jms.1190301104

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractLiquid chromatogrpahy‐electrospray tandem mass spectrometry allows identification of dansylated basic amino acids and polyamines with a high degree of specificity, including unambiguous acid and 2 hydroxycadaverine, andN1‐ andN8‐acetylspermidine. In favorable cases, characteristic fragmentation mechanisms permit identification even in the absence of authentic reference compounds. The method readily identified acetyldiaminopropane, citrul‐line, and diacetylspermine as metabolic products ofErwinia am

ISSN:1076-5174    

DOI:10.1002/jms.1190301105

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe relative stabilities, isomerizations and dissociations of ionized dimethyl sulfoxide (DMSO), its three C2H6OS+·isomers and of all their 14 conceivable CH3OS+fragments (1–14), have been investigated byab initiocalculations at the MP2/6–31G(d,p)//6–31G(d,p) + ZPE and G2 levels of theory, and by multiple‐stage two‐ and three‐dimensional mass spectrometry performed in a pentaquadrupole instrument. Theab initiorelative energies of the isomers, their connecting transition states, and their dissociation thresholds were used to elaborate potential energy surface diagrams that precisely corroborate and unify several previously divergent experimental observations on these systems. The most kinetically favorable isomerization of (CH3)2SO+·(I) to itsaci‐form CH2S(OH)CH 3+·(II) displays a transition state considerably lower in energy than the threshold for its direct dissociation by CH3⋅ loss. Therefore, low‐energy, long‐lived metastable ions I are predicted to isomerize to II, and to dissociate in turn to CH2S+OH (2) upon CH3⋅ loss. Ions I excited a few electronvolts above the threshold are, on the other hand, predicted to dissociate directly to CH3S+O (1). Isomerization of I to the most stable C2H6OS+·ion, that is CH3SOCH 3+·(III), displays the most energetic transition state and should be negligible, whereas III is predicted to dissociate by CH3⋅ loss preferentially to 1 rather than 5 (CH3OS+). Nine CH3OS+isomers, that is 1, 2, (HCS…OH2)+(3),+CH2S(O)H (4), 5, HC(SH)OH+(9), CH2OSH+(10), CH2+OSH (11), and (HCO…SH2)+(14), were found as true minima on the RHF/6–31G(d,p) potential‐energy surface, and some of their isomerization barriers and dissociation thresholds were estimated. Tandem and multiple‐stage (MS3) mass spectrometric experiments show that non‐dissociating DMSO+·ions produce, upon collision‐induced dissociation (CID), a mixture of approximately 40% of 1 and 60% of 2, whereas the CID chemistry of 1 and 2 is affected considerably by the collision energies employed. Both the experimental and theoretical results on 1 and 2 allow a detailed interpretation of their complex dissociation chemistry, which clarifies the nature of most of their indirect fragments. Such fragments are proposed to be formed via the common isomerization/dissociation sequences 1 → 2 → 9 ⇌ 3 → HCS+(m/z45) + H2O, and 1 → 2 → [7, HOCH2S+] → S + CH2OH+(m/z31). These processes are favored at lower collision energies, whereas direct dissociation of 1 to

ISSN:1076-5174    

DOI:10.1002/jms.1190301106

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA simple and rapid method for counting the number of internal disulfide bridges in a protein by incubation with 2‐mercaptoethanol and electrospray mass spectrometry analysis of the products was developed. 2‐Mercaptoethanol yields intermediate mixed disulfides during reduction of a protein. This results in a molecular weight increase of the protein by 78 Da per disulfide bond, which can easily be determined by electrospray mass spectrometry (ESMS). The number of mercaptoethanol adducts observed by ESMS reveals the number of disulfide bridges in the peptide or protein. Since the protein–mercaptoethanol–disulfide bonds are themselves further reduced by excess mercaptoethanol, the course of the reaction has to be followed in order to detect the maximum number of intermediates. Owing to the volatility of mercaptoethanol, samples can be taken out of the reaction solution for MS analysis without prior purification. Successful experiments were carried out using proteins with one, two, four or six SS‐bonds, covering a mass range from about 1 to

ISSN:1076-5174    

DOI:10.1002/jms.1190301107

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA series of 3,3‐dimethyl‐1‐(isoxazol‐3‐yl)triazenes, potential anticancer agents, were studied by electron impact (EI) ionization and fast atom bombardment mass spectrometry. Their behaviour was compared with that of dacarbazine, 5‐(3,3‐dimethyl‐1‐triazenyl)‐(1H)‐imidazole‐4‐carboxamide, which is employed in the treatment of several neoplastic conditions. An interesting EI‐generated decomposition pathway was observed, consisting in the primary formation of [NH2CH3]+cations, involved in the metabolic pathway of triazenes, as the alkylating agent responsible for the anticancer properties of the drug. A higher thermodynamic stability of the examined compounds than decarbazine was observed, which reasonably reflects th

ISSN:1076-5174    

DOI:10.1002/jms.1190301108

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractSulfonylurea herbicides have been studied using negative‐ion desorption chemical ionization (DCI) mass spectrometry (MS) and DCI‐MS/MS techniques. Both [M  H]−and M−·ions were observed in the DCI mass spectra. The collisionally activated dissociation (CAD) spectra were characteristic of the structure of the herbicides examined. The fragmentation pattern was established based on the tandem mass spectra of thein situdeuterated sulfonylureas and on the comparison of the CAD spectra of sulfonylurea fragment ions with the CAD spectra of the ions derived from saccharin (o‐benzoic sulfimide) and methyl 2‐(aminosul

ISSN:1076-5174    

DOI:10.1002/jms.1190301109

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe production and detection of doubly charged organic cations were studied using a previously unreported approach—electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. Owing to its ultra‐high mass resolution, accurate and stable mass calibration, multiple stages of collisionally activated dissociation (CAD) allowing tandem mass spectrometry and extremely long ion storage times (>1 min), this technique has proven to be a powerful method for the detection and study of doubly charged organic compounds. CAD studies were used for the identification of ion–molecule reactions and for the investigation of the propensity towards gas‐phase SN1 and SN2 reaction pathways. To date exclusively SN1‐type reactions in the gas phase have been detected. Three doubly charged pyridinium salts were chosen as model compounds for t

ISSN:1076-5174    

DOI:10.1002/jms.1190301110

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA method to measure the enrichment of deuterium in the hydrogens bound to C‐6 of glucose has been refined and taken to the micro‐scale. Glucose (≤0.2 mg) in small quantities of plasma samples is oxidized with sodium periodate. The formaldehyde formed contains the two hydrogens bound to C‐6 of glucose. The formaldehyde is condensed with ammonia to form hexamethylenetetramine (HMT), which can be directly determined by gas chromatography/mass spectrometry. All of the 12 hydrogens of HMT are from the formaldehyde and therefore deuterium enrichment as low as 0.1% on hydrogens bound to C‐6 of glucose can be measured with a coefficient of variation of less than 3%. The method can be used to quantify gluconeogenesisin vivoin humans by administering [2H2]O at an enrichment in total body water of only 0.5%, resulting in reproducibly measurable labeling of the hydrogens bound to C‐6 of glucose. The method is presented because of its potential value for the measurement of gluconeogenesis in humansin vivoat safe and acceptable dose

ISSN:1076-5174    

DOI:10.1002/jms.1190301111

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY