Reduced matrix effects for anionic compounds with paired ion electrospray ionization mass spectrometry

It is well-known that matrix effects in high performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC-ESI-MS) can seriously compromise quantitative analysis and affect method reproducibility. Paired ion electrospray ionization (PIESI) mass spectrometry is an approach for analyzing ultra-low levels of anions in the positive ion mode. This approach uses a structurally optimized ion pairing reagent to post-column associate with the anionic analyte, subsequently forming positively charged complexes. These newly formed complex ions are often more surface-active as compared to either the native anion or the ion pairing reagent. No studies have examined whether or not the PIESI approach mitigates matrix effects. Consequently, a controlled study was done using five analytes in highly controlled and reproducible synthetic groundwater and urine matrices. In addition, two different mass spectrometers (linear ion trap and triple quadrupole) were used. Compared to the negative ion mode, the PIESI-MS approach was less susceptible to matrix effects when performed on two different MS platforms. Using PIESI-MS, less dilution of the sample is needed to eliminate ionization suppression which, in turn, permits lower limits of detection and quantitation.     

Gas chromatography electrospray ionization mass spectrometry analysis of trimethylsilyl derivatives

A method based on the analysis of trimethylsilyl (TMS) derivatives by capillary gas chromatography electrospray ionization mass spectrometry (GC–ESI/MS) was proposed. To improve separation, analytes were derivatized to their TMS derivative. During ESI analysis, TMS derivatives may hydrolyze back to their polar native form and are thus suitable for ESI analysis. Several types of analytes were studied to investigate the potential of the approach. Not all TMS derivatives hydrolyzed back to their native form as anticipated. Incomplete hydrolysis was observed for TMS‐organic acids and TMS‐nonchlorinated phenols. For TMS‐chlorophenols, the observation of only the [M ? H]^? ion suggested that these phenols were hydrolyzed back to their native form. For TMS‐beta agonists, the hydrolysis rate was low; therefore, the hydrolysis product was not detected. Both TMS‐chlorophenols and TMS‐beta agonists provide a sensitivity in the range of low parts per billion (0.25–5 ng/ml and 0.5–10 ng/ml respectively). Copyright © 2016 John Wiley & Sons, Ltd.     

High mass resolution breath analysis using secondary electrospray ionization mass spectrometry assisted by an ion funnel

In this study, we used secondary electrospray ionization mass spectrometry assisted by an ion funnel (IF) operating at ambient pressure to find compounds in the mass range of 100–500 m/z in online breath fingerprinting experiments. In low‐resolution experiments conducted on an ion trap instrument, we found that pyridine is present in breath of individuals long after drinking coffee. In high‐resolution experiments conducted on a Fourier transform ion cyclotron resonance, we found more than 30 compounds in the mass range of 100–500 m/z in analogous online breath experiments. More than a third of these compounds have molecular weights above 200 Daltons and have not been mentioned in previous studies. In low‐resolution experiments as well as experiments without the IF, these compounds could not be detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural characterization of acetylpyridinium-ethyl pyruvate adducts by electrospray ionization mass spectrometry

The reactions of ethyl pyruvate with acetic anhydride and pyridine were studied by electrospray ionization mass spectrometry (ESI-MS). Ethyl 2-acetoxy-2-pyridiniumpropionate (1) (m/z 238) resulting from the reaction of the acetylpyridinium cation with ethyl pyruvate, and the adduct of ethyl 2-acetoxyacrylate with a pyridinium cation (2), bound together by non-covalent interactions (m/z 238), were identified by ESI-MS for the first time. Structures 1 and 2 cannot be distinguished, probably because one may be converted into the other and vice versa.     

Oligosaccharide sequences in Quillaja saponins by electrospray ionization ion trap multiple-stage mass spectrometry

Ten different samples with 13 previously identified saponin structures from Quillaja saponaria Molina were investigated by electrospray ionization ion trap multiple-stage mass spectrometry (ESI-ITMS(n)) in positive and negative ion modes. Both positive and negative ion mode MS(1)-MS(4) spectra were analyzed, showing that structural information on the two oligosaccharide parts in the saponin can be obtained from positive ion mode spectra whereas negative ion mode spectra mainly gave information on one of the oligosaccharide parts. Analysis of MS(1)-MS(4) spectra identified useful key fragment ions important for the structural elucidation of Quillaja saponins. A flowchart involving a stepwise procedure based on key fragments from MS(1)-MS(3) spectra was constructed for the identification of structural elements in the saponin. Peak intensity ratios in MS(3) spectra were found to be correlated with structural features of the investigated saponins and are therefore of value for the identification of terminal monosaccharide residues.     

Assignment of acetyl groups to O-2 and/or O-3 of pectic oligogalacturonides using negative electrospray ionization ion trap mass spectrometry

Partially acetylated and methylated oligogalacturonides produced by enzymatic hydrolysis of sugar beet pectin were analysed by negative electrospray ionization ion trap mass spectrometry (ESI-ITMS). The (18)O labelling of the oligomer reducing end allowed the precise assignment of the fragments resulting from glycosidic bond and cross-ring cleavages. The collisional-induced dissociation of the C(i) and Z(j) fragment ions through sequential MS(n) experiments always displayed (0, 2)A-type cross-ring cleavage ions which were related to C(2)H(4)O(2) losses. These (0, 2)A ions appeared to be highly diagnostic ions allowing the precise location of the acetyl groups to the O-2 and/or O-3 of the acetylated galacturonic acid residues.     

Portable electrospray ionization mass spectrometry (ESI-MS) for analysis of contaminants in the field

Hitherto analysis of chemicals in the field using mass spectrometry (MS) has been limited to analysis of non-polar and thermally stabile organic compounds using either a direct gas leak or a membrane inlet as MS interface. Recently, Professor R. Graham Cooks’ group demonstrated that miniature mass spectrometers operating at elevated pressures (>0.13?Pa (1?·?10^?3??Torr)) can be combined with electrospray ionization (ESI) for analysis of polar as well as thermally labile organic compounds. We present a simple miniaturized ESI unit for analysis of small liquid samples using miniature mass spectrometry. The ESI unit operates without pumps and supplementary sheath gases, which makes it very simple to handle in the field. 20–30?µL of sample solution is simply dropped into a small cavity in the ESI unit, where after the spray is initiated by applying high voltage to it. The miniaturized ESI unit was tested in combination with a miniature mass spectrometer (the Mini 10 developed by Professor R. Graham Cooks, Purdue University, IN) and we found that common herbicides (Atrazine, Prometryne, Terbutryne and Triadimefone) could be detected with detection limits around 1?mg?L^?1 and with a quantitative reproducibility of +/?30%. These characteristics, although high for environmental samples, are comparable to detection limits obtained with other ESI units used with miniature mass spectrometers and represent an early step forward towards a future field instrument. A major advantage of the capillary spray cell is its direct compatibility with micro extraction techniques for sample pre-concentration.     

Determination of cyclic structure for polydithiane using electrospray ionization mass spectrometry

Disulfide polymers obtained by ring‐opening polymerization have been considered to have a possibility of a cyclic catenane structure as judged from their test properties on the loss modulus and stress–strain. Electrospray ionization mass spectrometry (ESI‐MS) was used to detemine molecular structure of polydithiane and polyoxyethylene to find whether they are present as a cyclic or as a linear structure. The results indicated that the polydithiane possesses the cyclic structure, and analysis of the isotope distribution of the spectral ions further showed that the polymer consists entirely of the cyclic structure without contamination of a linear polymer. A linear chain polymer with a benzylmercaptan end group was synthesized, and the ESI‐MS analysis revealed that the polymer was a mixture of both the cyclic and the linear polymer. The cyclic polymer is probably formed by back‐biting of the highly reactive sulfur radicals that had been formed during the polymerization reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4403–4406, 2000     

Detection of protein from detergent solutions by probe electrospray ionization mass spectrometry (PESI-MS)

Detergents are necessarily used for different extraction protocols of proteins from biological cells or tissues. After the extraction, elimination of detergent is necessary for the better performance of electrospray ionization mass spectrometry (ESI-MS). Elimination of detergents is laborious and time-consuming, and also sample loss may be unavoidable. Probe electrospray ionization (PESI) developed in our laboratory has been found to be tolerant to the presence of salts and buffers in sample solutions. In this report, it was examined whether PESI is applicable to the sample solutions that contain high-concentration of detergents. It was found that PESI is highly tolerant to the presence of sodium dodecyl sulphate, cetyl trimethylamminium bromide, Triton X100 and 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate compared with conventional ESI and nanoESI. Therefore, PESI can be a potential analytical tool for direct analysis of protein extracts and digests containing high-concentration detergents.     

基质辅助激光解吸电离质谱和电喷雾电离质谱在辣根过氧化物酶糖肽结构分析中的应用

糖链结构的质谱解析是今后糖蛋白分析中的重要研究内容,其中完整糖肽的分析,由于可以同时获得糖基化位点和对应糖链的结构信息,更具有重要意义和研究前景。本工作对质谱软电离技术在完整糖肽分析中的应用进行了研究,其中包括了基质辅助激光解吸电离(matrix-assisted laser desorption ionization, MALDI)和电喷雾电离(electrospray ionization, ESI)技术。通过平行使用两种串联质谱(tandem mass spectrometry, MS/MS)分析策略: MALDI-MS/MS和ESI-MS/MS对目标糖蛋白——辣根过氧化物酶进行分析,并讨论了其互补性。结果表明,MALDI和ESI技术各有优劣,结合串联质谱分析,可获得糖肽的糖链结构信息;两条路线互补使用,在揭示蛋白质糖基化修饰(位点和结构)的研究中十分必要。     

Desorption electrospray ionization mass spectrometry of DNA nucleobases: implications for a liquid film model

Desorption electrospray ionization (DESI) mass spectrometry has been implemented on a commercial ion‐trap mass spectrometer and used to optimize mass spectrometric conditions for DNA nucleobases: adenine, cytosine, thymine, and guanine. Experimental parameters including spray voltage, distance between mass spectrometer inlet and the sampled spot, and nebulizing gas inlet pressure were optimized. Cluster ions including some magic number clusters of nucleobases were observed for the first time using DESI mass spectrometry. The formation of the cluster species was found to vary with the nucleobases, acidification of the spray solvent, and the deposited sample amount. All the experimental results can be explained well using a liquid film model based on the two‐step droplet pick‐up mechanism. It is further suggested that solubility of the analytes in the spray solvent is an important factor to consider for their studies by using DESI. Copyright © 2009 John Wiley & Sons, Ltd.     

The mass spectral analysis of isolated hops A-type proanthocyanidins by electrospray ionization tandem mass spectrometry

Comprehensive mass spectral fragmentation patterns have been established for sequencing chromatographically isolated A-type proanthocyanidins (PAs) using electrospray ionization tandem mass spectrometry (ESI-MS(n)) in the positive ion mode similar to those used for sequencing previously reported B-type PAs. Sequence-identifying fragmentations for A-type PAs include heterocyclic ring fission (HRF), retro-Diels-Alder (RDA) fission, benzofuran-forming (BFF) fission, and quinone methide (QM) fission. There is commonality in fragmentation patterns between A-type and B-type PAs, but distinguishing features in the mass spectral patterns between the two classes include 2-Da mass differences in the pseudo molecular ions, the propensity for the A-type PAs to undergo QM fissions and yield bis-quinoid ions as opposed to mono-quinoid ions in the upper unit of the sequence, and the reluctance of A-type linkages to undergo RDA, BFF, and BFF/H(2)O fissions from the upper unit. The positions of one or more A-type (C2-->O-->C7') ether linkages have been located in sequences of PAs ranging in chain lengths of two to five monomer units using ESI-MS(n) data. Using the fragmentation information from ESI-MS(n) experiments, a total of 17 PAs were structurally sequenced by systematic real time ESI-MS(n). Among them ten A-type and six B-type hop PAs are reported here for the first time.     

Examination of structurally selective derivatization of vitamin D(3) analogues by electrospray mass spectrometry

The structural specificity of vitamin D derivatization by PTAD (4-phenyl-1,2,4-triazoline-3,5-dione) was probed using synthetic analogues and ion trap mass spectrometry. EB 1089, a vitamin D(3) analogue which contains a second site for Diels--Alder cycloaddition on its side-chain, allowed the examination of derivatization modes and comparisons of ion fragment structures. The origins of a PTAD-vitamin D(3) ion fragment, commonly used in metabolite characterization and quantitation of vitamin D(3) analogues (m/z 314), were established; ion trap mass spectrometry revealed that the PTAD comprises a portion of this diagnostic fragment, and is not lost by a retro-Diels--Alder step. Furthermore, the unique structure of the EB 1089 side-chain also permits facile determination of its side-chain metabolism. Use of PTAD derivatization and detection of metabolite-specific ion fragments identify hydroxylation at the end of the EB 1089 sidechain. It is believed that the results from these studies provide a clearer understanding of the mass spectrometry of triazolinedione derivatives, not only in the specific case of EB 1089, but also in their application to other vitamin D compounds.     

Identification and fragmentation of hydrolyzed aluminum species by electrospray ionization tandem mass spectrometry

Earlier characterization of some hydrolysis products of AlCl₃·6H₂O was confirmed by electrospray ionization tandem mass spectrometry with increasing collision energy of projectile ions. At lower collision energies, the aqua ligands were stripped off. At higher energies, two hydroxo groups formed a bridging oxo group with loss of one water molecule. Aluminum complexes could also capture aqua ligands in the collision chamber so long as the parent ion did not fragment, and the fragment ion spectra broadened toward higher m/z values. The chloro ligands were eliminated as hydrochloric acid. The aluminum cores remained highly intact. Copyright © 2004 John Wiley & Sons, Ltd.     

Chiral analysis of amphetamine-type stimulants using reversed-polarity capillary electrophoresis/positive ion electrospray ionization tandem mass spectrometry

Reversed-polarity (RP) capillary electrophoresis/positive ion electrospray ionization mass spectrometry (CE-ESI+ MS) and tandem mass spectrometry (MS/MS) were utilized for simultaneous chiral separation of nine amphetamine-type stimulants (ATS) (dl-norephedrine, dl-norpseudoephedrine, dl-ephedrine, dl-pseudoephedrine, dl-amphetamine, dl-methamphetamine, dl-methylenedioxyamphetamine, dl-methylenedioxymethamphetamine, and dl-methylenedioxyethylamphetamine). Using highly sulfated gamma-cyclodextrin (SU(XIII)-gamma-CD) as a chiral selector, the nine ATS were completely separated within 50 min. The migrated ATS-CD complex was dissociated at the ESI interface, and only ATS molecules went into the MS detector so that all 18 individual enantiomers were identified by their mass spectra. The detection limit of MS/MS was 10 times more sensitive than those for single MS. Seized d-methamphetamine hydrochloride samples dissolved at high concentration (20 mg/mL) were analyzed. Impurities originating in the precursor such as l-ephedrine and d-pseudoephedrine were detected and identified by tandem mass spectra.     

Initiation of free‐radical polymerization by peroxypivalates studied by electrospray ionization mass spectrometry

Initiation by tert‐butyl peroxypivalate (TBPP), tert‐amyl peroxypivalate (TAPP), 1,1,3,3‐tetramethylbutyl peroxypivalate (TMBPP), or 1,1,2,2‐tetramethylpropyl peroxypivalate (TMPPP) of radical polymerization of methyl methacrylate in toluene solution at 90 °C was studied via polymer end‐group analysis using electrospray ionization mass spectrometry (ESI‐MS). Conclusive peak assignments allowed for measuring the type and concentration of the fragments that actually initiate macromolecular growth after thermal decomposition of these peroxypivalates. It was found that the pivaloyloxy radical moiety undergoes instantaneous decarboxylation to yield an initiating tert‐butyl radical. The alkoxy radical moiety, on the other hand, may generate, via β‐scission reaction, different types of carbon‐centered radicals (together with a ketone) or may undergo a 1,5‐H‐shift reaction, by which reaction an oxygen‐centered radical is transformed into a carbon‐centered hydroxy radical. This hydrogen shift reaction was found in case of TMBPP. Surprisingly, no evidence for initiating alkoxy radicals could be found, not even in case of initiation by TBPP, where the intermediate tert‐butoxy radical undergoes a rapid chain‐transfer reaction with the solvent toluene. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4266–4275, 2004     

Sensitive detection of drug vapors using an ion funnel interface for secondary electrospray ionization mass spectrometry

In this study, we use an ion funnel (IF) at ambient pressure to enhance the sensitivity of secondary electrospray ionization (SESI). Atenolol, salbutamol and cocaine as test compounds are delivered to the SESI interface in the gas phase and are charged with three nano electrosprays. In our experiments, we show that the compounds can be detected at concentrations in the low pptv range, which is an increase of two orders of magnitude compared with the results without the IF. With a standard SESI interface, the compounds could not be detected at all. With the use of the SESI IF interface for the headspace analysis of bananas and limes, we can detect many more compounds and at higher intensities than with a standard SESI interface. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of non-covalent complexation between catechin derivatives and peptides by electrospray ionization mass spectrometry

The recent development of electrospray ionization mass spectrometry (ESI-MS) has allowed its use to study molecular interactions driven by non-covalent forces. ESI-MS has been used to detect non-covalent complexes between proteins and metals, ligands and peptides and interactions involving DNA, RNA, oligonucleotides and drugs. Surprisingly, the study of the interaction between polyphenolic molecules and peptides/proteins is still an area where ESI-MS has not benefited. With regard to the important influence of these interactions in the biological and food domains, ESI-MS was applied to the detection and the characterization of soluble polyphenol-peptide complexes formed in model solution. The ability to observe and monitor the weak interactions involved in such macromolecular complexation phenomena was demonstrated for monomeric and dimeric flavonoid molecules (catechin-derived compounds) largely encountered in plants and plant derived products. Intact non-covalent polyphenol-peptide complexes were observed by ESI-MS using different experimental conditions. Utilizing mild ESI interface conditions allowed the detection of 1 : 1 polyphenol-peptide complexes in all tested solutions and 2 : 1 complexes for the dimers and galloylated polyphenols (flavanols). These results show that there is a preferential interaction between polymerized and/or galloylated polyphenols and peptide compared with that between monomeric polyphenols and peptides. Thus, ESI-MS shows potential for the study of small polyphenolic molecule-peptide interactions and determination of stoichiometry.     

Determination of binding sites in carboplatin-bound cytochrome c using electrospray ionization mass spectrometry and tandem mass spectrometry

Interaction of carboplatin with cytochrome c (Cyt. c) has been investigated by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS). ESI-MS studies revealed that the ring-opened adducts of carboplatin with Cyt. c were formed in the stoichiometric ratio of 1:1 and 2:1 at pH 5.0 and 37 degrees C and in the stoichiometric ratio of 1:1 only at pH 7.0 and 37 degrees C. It was also found that Cyt. c could be cleaved by carboplatin at pH 2.5 and 50 degrees C. The cleaved fragments of Cyt. c were determined by ESI-MS and MS/MS analysis to be Glu66 approximately Met80, Ac-Gly01 approximately Met65, Glu66 approximately Glu104, Ac-Gly01 approximately Met80 and Ile81 approximately Glu104. The carboplatin prefers to anchor to Met65 first, then to Met80. To further confirm the binding site of Met, AcMet-Gly was used as the model molecule to investigate its interaction with carboplatin and its hydrolysis reaction. On the basis of species detected during the reaction monitored by ESI-MS, a possible pathway of the cleavage reaction was proposed.     

Structural characterization and identification of dibenzocyclooctadiene lignans in Fructus Schisandrae using electrospray ionization ion trap multiple-stage tandem mass spectrometry and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry

The electrospray ionization ion trap multiple-stage tandem mass spectrometry (ESI-MSⁿ) and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry (ESI-FT-ICR-MSⁿ) have been applied successfully to the direct investigation of a number of dibenzocyclooctadiene lignan constituents from the methanol extracts of the Fructus Schisandrae in the positive ion mode. The detailed structural characterization of the same skeleton and different peripheral substituents had been studied and the precise elemental compositions of ions at high mass resolution had been obtained. So the fragmentation mechanisms could be clarified. And the lignan components in Schisandra chinensis (Turcz.) Baill. fruits (SCF) and Schisandra sphenanthera Rehd. et Wils. fruits (SSF) were identified by comparing the structural information and fragmentation mechanisms. Then a pair of isobaric compounds was differentiated. Meanwhile these two similar fruits were distinguished. The research results demonstrated that ESI-MSⁿ technique is a sensitive, selective and effective tool for the direct analysis and rapid determination of constituents in complex mixtures from nature products. And these should be useful for the identification of similar compounds and differentiation of similar species from Chinese herbs.