Ion Electrospray Ionization Mass Spectrometry Research Articles

Characterization of oligomers from methylglyoxal under dark conditions: a pathway to produce secondary organic aerosol through cloud processing during nighttime

Abstract. Aqueous-phase oligomer formation from methylglyoxal, a major atmospheric photooxidation product, has been investigated in a simulated cloud matrix under dark conditions. The aim of this study was to explore an additional pathway producing secondary organic aerosol (SOA) through cloud processes without participation of photochemistry during nighttime. Indeed, atmospheric models still underestimate SOA formation, as field measurements have revealed more SOA than predicted. Soluble oligomers (n = 1–8) formed in the course of acid-catalyzed aldol condensation and acid-catalyzed hydration followed by acetal formation have been detected and characterized by positive and negative ion electrospray ionization mass spectrometry. Aldol condensation proved to be a favorable mechanism under simulated cloud conditions, while hydration/acetal formation was found to strongly depend on the pH of the system and only occurred at a pH<3.5. No evidence was found for formation of organosulfates. The aldol oligomer series starts with a β-hydroxy ketone via aldol condensation, where oligomers are formed by multiple additions of C3H4O2 units (72 Da) to the parent β-hydroxy ketone. Ion trap mass spectrometry experiments were performed to structurally characterize the major oligomer species. A mechanistic pathway for the growth of oligomers under cloud conditions and in the absence of UV-light and OH radicals, which could substantially enhance in-cloud SOA yields, is proposed here for the first time.

Hydrophilic interaction liquid chromatography/positive ion electrospray ionization mass spectrometry method for the quantification of perindopril and its main metabolite in human plasma

A validated method based on liquid chromatography/positive ion electrospray-mass spectrometry (LC-ESI/MS) is described for the quantification of perindopril and its active metabolite, perindoprilat, in human plasma. The assay was based on 500-microL plasma samples, following solid-phase extraction using Oasis HLB cartridges. All analytes and the internal standard (trandolapril) were separated by hydrophilic interaction liquid chromatography using a SeQuant Zic-HILIC analytical column (150.0 x 2.1 mm i.d., particle size 3.5 microm, 200 A) with isocratic elution. The mobile phase consisted of 10% 5.0 mM ammonium acetate water solution in a binary mixture of acetonitrile/methanol (60:40, v/v) and pumped at a flow rate of 0.10 mL min(-1). Quantitation of the analytes was performed with selected ion monitoring (SIM) in positive ionization mode using electrospray ionization interface. The assay was found to be linear in the concentration range of 5.0-500.0 ng mL(-1) for perindopril and perindoprilat. Intermediate precision were found less than 3.5% over the tested concentration ranges. A run time of less than 6.0 min for each sample made it possible to analyze a large number of human plasma samples per day. The method is the first reported application of HILIC in the analysis of angiotensin-converting enzyme inhibitors and can be used to quantify perindopril and perindoprilat in human plasma covering a variety of pharmacokinetic or bioequivalence studies.

Electrospray Ionization Tandem Mass Spectrometry of Phosphonate Peptides

Phosphonate peptides were synthesized and investigated by positive ion electrospray ionization mass spectrometry in conjunction with tandem mass spectrometry. It was found that, for the sodiated adduct, there were two rearrangement reactions. In path A, there was C-C covalent bond cleavage with consecutive migration of a hydrogen atom to lose the amino alcohol molecule. In path B, two consecutive covalent cleavages of the C-N and P-O bonds occurred.

Assigning glucose or galactose as the primary glycosidic sugar in 3‐O‐mono‐, di‐ and triglycosides of kaempferol using negative ion electrospray and serial mass spectrometry

Kaempferol 3-O-beta-glucopyranoside, kaempferol 3-O-beta-galactopyranoside and higher glycosides of these two flavonoids with alpha-rhamnose at C-2 and/or C-6 of the primary sugar were studied by negative ion electrospray ionisation and serial mass spectrometry in a three-dimensional (3D) ion trap mass spectrometer. Kaempferol 3-O-beta-glucopyranoside and kaempferol 3-O-alpha-rhamnopyranosyl(1-->6)-beta-glucopyranoside could be distinguished from their respective galactose analogues by differences in the ratio of the radical aglycone ion [Y(0) - H](*-) to the rearrangement aglycone ion Y(0) (-) following MS/MS of the deprotonated molecules. Kaempferol 3-O-rhamnopyranosyl(1-->2)-beta-glucopyranoside and kaempferol 3-O-alpha-rhamnopyranosyl(1-->2)[alpha-rhamnopyranosyl(1-->6)]-beta-glucopyranoside could be distinguished from their respective galactose analogues by differences in the product ion spectra of the [(M - H) - rhamnose](-) ion following serial mass spectrometry. In the triglycoside, it was deduced that this ion resulted from the loss of the rhamnose substituted at 2-OH of the primary sugar by observing that MS/MS of deprotonated kaempferol 3-O-beta-glucopyranosyl(1-->2)[alpha-rhamnopyranosyl(1-->6)]-beta-glucopyranoside showed the loss of glucose and not rhamnose. Thus the class of sugar (hexose, deoxyhexose, pentose) at C-2 and C-6 of the primary sugar can be determined. These observations aid the assignment of kaempferol 3-O-glycosides, having glucose or galactose as the primary glycosidic sugar, in LC/MS analyses of plant extracts, and this can be done with reference to only a few standards.

Mass spectrometry in the characterization of ambers. II. Free succinic acid in fossil resins of different origin

Baltic amber contains high levels of succinic acid, most of which is part of the polymer framework, and only recently has the presence of microcrystals of free succinic acid been documented by scanning electron microscopy and X-ray diffraction analysis. The determination of succinic acid can be used for an easy identification of Baltic amber among other fossil resins and to distinguish it from imitations, widely diffused on the market, produced with natural or modified recent or sub-fossil resins, such as copals. We report here a simple method, based on negative ion electrospray ionization mass spectrometry, for evaluating the free succinic acid content in water/methanol extracts of ambers from different origin, using a sample of about 20 mg. The limit of quantification is better than 1 ppm and, when applied to a set of amber samples of different origin, it was able to distinguish those of Baltic origin (showing free succinic acid levels in the range 50-400 ppm) from the others (for which succinic acid was undetectable, i.e. at levels lower than 1 ppm).

Binding studies of nNOS‐active amphibian peptides and Ca2+ calmodulin, using negative ion electrospray ionisation mass spectrometry

Amphibian peptides which inhibit the formation of nitric oxide by neuronal nitric oxide synthase (nNOS) do so by binding to the protein cofactor, Ca2+calmodulin (Ca2+CaM). Complex formation between active peptides and Ca2+CaM has been demonstrated by negative ion electrospray ionisation mass spectrometry using an aqueous ammonium acetate buffer system. In all cases studied, the assemblies are formed with a 1:1:4 calmodulin/peptide/Ca2+ stoichiometry. In contrast, the complex involving the 20-residue binding domain of the plasma Ca2+ pump C20W (LRRGQILWFRGLNRIQTQIK-OH) with CaM has been shown by previous two-dimensional nuclear magnetic resonance (2D NMR) studies to involve complexation of the C-terminal end of CaM. Under identical conditions to those used for the amphibian peptide study, the ESI complex between C20W and CaM shows specific 1:1:2 stoichiometry. Since complex formation with the studied amphibian peptides requires Ca2+CaM to contain its full complement of four Ca2+ ions, this indicates that the amphibian peptides require both ends of the CaM to effect complex formation. Charge-state analysis and an H/D exchange experiment (with caerin 1.8) suggest that complexation involves Ca2+CaM undergoing a conformational change to a more compact structure.

Quantification of Citalopram or Escitalopram and Their Demethylated Metabolites in Neonatal Hair Samples by Liquid Chromatography-Tandem Mass Spectrometry

Citalopram and escitalopram are highly selective serotonin reuptake inhibitors widely used in the treatment of depression. They exhibit adverse drug reactions and side effects, however, and the development of specific methods for their determination is of great interest in clinical and forensic toxicology. A liquid chromatography-tandem mass spectrometry method has been developed and validated for the assay of citalopram, escitalopram, and their demethylated metabolites in 10-mg hair samples. The analytes were extracted by incubation in methanol and liquid/liquid extraction with diethyl ether/dichloromethane. Gradient elution on a narrow bore C18 column was realized using clomipramine-d3 as an internal standard. Positive ion electrospray ionization and tandem mass spectrometry determination by collision-induced dissociation were performed in an ion trap mass spectrometer. The method exhibited a linear range of 25 to 2000 pg/mg, a quantification limit of 25 pg/mg for all analytes, relative standard deviations in the range of 12.10 to 9.80 (intraassay), and 13.80 to 11.78 (interassay), and accuracies (as percent recovery of the spiked standards) in the range of 90% to 110%; it was applied to the determination of citalopram and escitalopram and their metabolites in hair samples of two newborns to document their in utero exposure to the drugs. The method proved suitable for neonatal hair analysis of citalopram or escitalopram and was applied to two real cases of gestational exposure.

Measurement of Total Acid Number (TAN) and TAN Boiling Point Distribution in Petroleum Products by Electrospray Ionization Mass Spectrometry

We report a new method for rapid measurement of total acid number (TAN) and TAN boiling point (BP) distribution for petroleum crude and products. The technology is based on negative ion electrospray ionization mass spectrometry (ESI-MS) for selective ionization of petroleum acid and quantification of acid structures and molecular weight distributions. A chip-based nanoelectrospray system enables microscale (<200 mg) and higher throughput (20 samples/h) measurement. Naphthenic acid structures were assigned based on nominal masses of a set of predefined acid structures. Stearic acid is used as an internal standard to calibrate ESI-MS response factors for quantification purposes. With the use of structure-property correlations, boiling point distributions of TAN values can be calculated from the composition. The rapid measurement of TAN BP distributions by ESI is demonstrated for a series of high-TAN crudes and distillation cuts. TAN values determined by the technique agree well with those by the titration method. The distributed properties compare favorably with those measured by distillation and measurement of TAN of corresponding cuts.

Monitoring of N-nitrosodiethanolamine in cosmetic products by ion-pair complex liquid chromatography and identification with negative ion electrospray ionization mass spectrometry

A high-performance liquid chromatographic (HPLC) method was developed for the determination of N-nitrosodiethanolamine (NDELA), which is a non-volatile N-nitrosoamine. In this method, sodium 1-octanesulfonate has been used as an ion complexation agent for NDELA. The resulting complex was analyzed by reversed-phase high-performance liquid chromatography (RP-HPLC) using a Eurospher-100 C18 column (250 × 4.6 mm, 5 μm), water/acetonitrile (95/5, v/v) and UV detection at 234 nm. The detector response was linear in the range 0.03–10.00 μg mL −1 and the limit of detection was 0.01 μg mL −1. The complexation of the NDELA and sodium 1-octanesulfonate was confirmed by negative ion electrospray ionization mass spectrometry at m/ z 327 [M–Na] − and also MS/MS measurements of the resulting fragments. For real sample analyses, NDELA was measured in cosmetic products after clean up by solid-phase extraction (SPE) with C 18 cartridge for water-soluble samples and liquid–liquid extraction (LLE) by methylene chloride for less water-soluble samples. The average values for NDELA recovery by SPE and LLE were 86.9 and 51.8% and relative standard deviations (RSDs%) were 0.9 and 5.6%, respectively. The presented method is easy to use, robust and can be implemented on any reversed-phase HPLC system with UV detection.

Study on [(4‐substituted benzoylamino)phenylmethyl]phosphonic acid diisopropyl esters under electrospray ionization tandem mass spectrometric conditions

Phosphonate esters were synthesized and investigated by positive ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). It was found that for the sodiated adduct there were two novel rearrangement reactions, in which the phosphoryl oxygen atom migrated to the carbonyl carbon and a cleavage occurred at the amide bond, or a benzylamine fragment was lost and the phosphorus was reduced. However, when the methyl group substituted for the isopropyl group, these migrations were inhibited. A possible mechanism was proposed, and high-resolution mass spectrometry was applied to identify the formula of these novel ions. These results showed that ESI-MS is a useful tool for the structural determination of phosphonate esters.

Anacardic Acids and Ferric Ion Chelation

6-Pentadeca(e)nylsalicylic acids isolated from the cashew Anacardium occidentale L. (Anacardiaceae), commonly known as anacardic acids, inhibited the linoleic acid peroxidation catalyzed by soybean lipoxygenase-1 (EC 1.13.11.12, type 1) competitively without prooxidant effects. Their parent compound, salicylic acid, did not have this inhibitory activity up to 800 pm, indicating that the pentadeca(e)nyl group is an essential element to elicit the activity. The inhibition is attributed to its ability to chelate iron in the enzyme. Thus, anacardic acids chelate iron in the active site of the enzyme and then the hydrophobic tail portion slowly begins to interact with the hydrophobic domain close to the active site. Formation of the anacardic acids-ferric ion complex was detected in the ratio of 2:1 as the base peak in the negative ion electrospray ionization mass spectrometry. Hence, anacardic acids inhibit both Eox and Ered forms.

Synthesis and Electrospray Ionization Mass Spectra of N‐(1,3, 2‐Dioxaphosphorinan‐2‐ylmethyl)thiophosphoramidates

AbstractN‐(1,3,2‐Dioxaphosphorinan‐2‐ylmethyl)thiophosphoramidates were synthesized and determined by NMR spectra and positive ion electrospray ionization mass spectrometry (ESI‐MS) in conjunction with tandem mass spectrometry (MS/MS). The fragmentation pathways were investigated. The results show that these characteristic ions in ESI mass spectra are useful in the structural determination of N‐(1,3,2‐dioxaphosphorinan‐2‐ylmethyl)thiophosphoramidates.

Highly Specific Quantification of Ergotamine in Urine, Blood, and Hair Samples by Liquid Chromatography-Tandem Mass Spectrometry

Ergotamine has been used for therapeutic purposes since the 1950s, usually to treat vascular headache. It is highly toxic and in large, repeated doses can produce all the symptoms of ergot poisoning. A selective and sensitive method, based on liquid chromatography-tandem mass spectrometry (LC-MS2), has been developed for quantifying ergotamine in biological fluids with use of a quick and easy sample preparation. Ergotamine and the internal standard, trideuterated lysergic acid diethylamide, were extracted from human urine, blood, and hair by means of liquid-liquid extraction at alkaline pH. Gradient elution on a cyanopropyl column was used for chromatographic separation. Positive ion electrospray ionization and tandem mass spectrometry determination by collision-induced dissociation were performed in an ion trap mass spectrometer. The method was validated and successfully applied to a case of iatrogenic ergotism resulting from the intake of ergotamine tartrate for treating headache. For the first time, ergotamine was identified and quantified in hair. The ergotamine concentrations measured were 320 pg/mL in blood, 100 pg/mL in urine, 24 pg/mg in proximal hair, and 15 pg/mg in distal hair.

Selectivity of an indolyl berberine derivative for tetrameric G‐quadruplex DNA

Negative ion electrospray ionization mass spectrometry (ESI-MS) was used to compare the binding affinities and stoichiometries of the alkaloid berberine, a 13-substituted indolyl berberine derivative, SS14, and the chemotherapeutic agent, daunomycin, for 16-mer double-stranded (ds) DNA (D1 and D2) and for an 8-mer tetrameric quadruplex, Q1 (d(TTGGGGGT)(4)). Under the experimental conditions presented here, ESI mass spectra of Q1 showed that the major ions were from Q1 with three ammonium ions bound in the structure. Ions from Q1 with four ammonium ions were of lower abundance. In agreement with other work, there were multiple binding sites on the dsDNA and the quadruplex for daunomycin and berberine. The binding of SS14 to both dsDNA and Q1 was less extensive. Although the binding affinity of SS14 for Q1 was modest, this compound showed a clear preference for Q1 DNA over D1 or D2 DNA. Berberine and daunomycin bound with greater affinity to both types of DNA secondary structure, with the former showing a slight preference for Q1 over D1 while the latter showed a slight preference for D1 over Q1. While at least five berberine molecules bound to Q1, this quadruplex could accommodate only two SS14 molecules. These investigations show that SS14 is a promising lead compound for drugs that may selectively bind quadruplex over duplex DNA.

Simultaneous determination and pharmacokinetic study of oxymatrine and matrine in beagle dog plasma after oral administration of Kushen formula granule, oxymatrine and matrine by LC‐MS/MS

A rapid, specific and sensitive LC-MS/MS method was developed for the determination of oxymatrine (OMT) and matrine (MT) in beagle dog plasma. The method was applied to study the pharmacokinetics of OMT and MT after oral administration of OMT, MT and Kushen formula granule (KFG) containing equivalent amounts of OMT and MT in a three-period crossover design. The analysis was carried out on an Acquity UPLC BEH C(18) column by linear gradient elution with 0.01% acetic acid-water-methanol as mobile phase. Detection was by positive ion electrospray ionization (ESI) mass spectrometry with multiple-reaction monitoring (MRM). Linear calibration curves were both obtained over the concentration range 15-2000 ng/mL, with a limit of quantification of 15 ng/mL. The matrix effect was minimized. The intra- and inter-day precisions (RSDs) were less than 12.4 and 14.7%, respectively, and the accuracy (RE) was from -2.1 to 2.7%. The validated method was used to determine the concentration-time profiles of OMT and MT. The results indicated that the absorption of OMT and MT after oral administration of KFG was significantly greater than that after oral administration of pure components.

Highly Sensitive Determination of Diverse Ceramides in Human Hair Using Reversed‐Phase High‐Performance Liquid Chromatography–Electrospray Ionization Mass Spectrometry

Since ceramides (CERs) play roles in signal transduction and cell regulation, CERs of human hair might be responsible for apoptosis during keratinization, in addition to their structural barrier and water-holding functions. Although, we previously developed a method for comprehensive profiling of the CERs in hair, that method was too insensitive to quantitatively characterize the CERs in a small amount of hair samples. The aim of this study was to develop a novel method for the highly sensitive determination of the diverse CERs. The method developed is negative ion electrospray ionization mass spectrometry (ESI-MS) coupled to reversed-phase high-performance liquid chromatography (RP-HPLC) using methanol containing 10 mM ammonium acetate as a mobile phase. By this method, 48 peaks derived from 73 kinds of CERs were simultaneously determined in selected ion monitoring measurement using one calibration line of the standard N-palmitoyl dihydrosphigosine, based on extremely small differences in the molar responses among different species of CERs, followed by the calculation of the actual levels using corrections for (13)C and (2)H effects. This method had extremely high sensitivity as indicated in the limit of quantification being in the femtomolar range. Other quantitative validation data, such as reproducibility, linearity and recoveries, were all sufficient. The quantitative levels of CERs determined by RP-HPLC-ESI-MS were comparable with those determined by thin-layer chromatography. This method was successfully applied to the characterization of levels of CERs in only 1-mm pieces derived from a single hair fiber and revealed the presence of interindividual and intraindividual variations of the CER composition. This RP-HPLC-ESI-MS method can be a powerful tool for future research on physicochemical and physiological roles of CERs in hair.

Determination of five nitrobenzoic acids in groundwater by solid-phase extraction and liquid chromatography–mass spectrometry

A method involving solid-phase extraction (SPE) and reversed-phase liquid chromatography-mass spectrometry (LC-MS) has been developed for determination, in groundwater, of nitrobenzoic acids associated with 2,4,6-trinitrotoluene production. Pre-concentration on a co-polymer-based SPE cartridge enabled quantitative extraction of the analytes from water. Investigation of negative ion electrospray and atmospheric-pressure chemical ionization mass spectrometry indicated the sensitivity of APCI was more than twice that of ESI. An 15N-labeled internal standard was used to achieve more accurate quantitation and mass assignment. Recovery was better than 80% when 10 mL water was extracted with the SPE cartridge. Combination of SPE with LC-MS analysis resulted in method detection limits of less than 5 microg L-1. The method has been used for analysis of groundwater samples collected from a site of a former ammunition plant. Contamination with nitrobenzoic acids was determined at microg L-1 levels.

Reduction of nitriles to amines in positive ion electrospray ionization mass spectrometry

Some compounds readily form [M+46]+ adduct ions during positive ion electrospray ionization mass spectrometry ((+)ESI-MS) analysis. These [M+46]+ ions were characterized as [M+CH3CH2NH2+H]+ by accurate mass determination. Ethylamine involved in the adduct was proposed to be the reduction product of acetonitrile and this was confirmed using deuterated acetonitrile. Other nitrile-containing compounds tested, including isobutyronitrile and benzonitrile, also formed the adduct ions of the respective amine forms under (+)ESI-MS conditions. Hydrogen/deuterium exchange experiments demonstrated that the reductive hydrogen originated from water. Reduction of nitriles (R-CN) to their respective amines (R-CH2NH2) under (+)ESI-MS conditions expands the ability to identify nitrile-containing chemical unknowns.

Characteristic Fragmentation Behavior of Steroidal Phosphor‐ amidate Conjugates in Electrospray Ionization Tandem Mass Spectrometry

AbstractNovel steroidal phosphoramidate conjugates of 3′‐azido‐2′,3′‐dideoxythymidine (AZT) and amino acid esters were synthesized and determined by positive and negative ion electrospray ionization mass spectrometry. The MS fragmentation behaviors of the steroidal phosphoramidate conjugates have been investigated in conjunction with tandem mass spectrometry of ESI‐MS/MS. There were three characteristic fragment ions in the positive ion ESI mass spectra, which were the Na adduct ions with loss of steroidal moiety, amino acid ester moiety from pseudo molecular ion (M+Na)+, and the phosphoamino acid methyl ester Na adduct ion by α‐cleavage of the phosphoramidate respectively. The main fragment ions in negative ion ESI mass spectra were the ion (M−HN3)−, the ion (M−AZT−H)−, and the ion (M−steroidal moiety−H)− besides the pseudo molecular ion (M−H)−. The fragmentation patterns did not depend on the attached amino acid ester moiety.

Payne rearrangement during analysis of epoxyalcohols of linoleic and α-linolenic acids by normal phase liquid chromatography with tandem mass spectrometry

Hydroperoxides of polyunsaturated fatty acids can be transformed to epoxyalcohols and keto fatty acids by metal enzymes, hematin, and various catalysts. In the current study, we used hematin to transform 9-hydroperoxy-10 E,12 Z-octadecadienoic acid and 13-hydroperoxy-9 Z,11 E-octadecadienoic acid to epoxyalcohols (with trans epoxide configuration) and to keto fatty acids. The products were separated by normal phase high-performance liquid chromatography (NP-HPLC) and analyzed using postcolumn addition of isopropanol/water and online negative ion electrospray ionization mass spectrometry (MS). The tandem MS (MS/MS) spectra were studied using analogs prepared from [9,10,12,13- 2H 4]linoleic acid (18:2 n−6) and from α-linolenic acid (18:3 n−3). We also studied the MS/MS spectra of epoxyalcohols formed from 11-hydroperoxy- and 8-hydroperoxy-9 Z,12 Z-octadecadienoic acids. Results were confirmed by MS/MS analysis of a series of authentic standards. MS/MS ions of 9-keto-10 E,12 Z-octadecadienoic acid and 13-keto-9 Z,11 E-octadecadienoic acid could be explained by keto–enol tautomerism. MS/MS spectra of regioisomeric allylic epoxyalcohols differed in relative intensities of characteristic ions. The MS/MS spectra of the epoxyalcohols with 1-hydroxy-2,3-epoxy-4 Z-pentene or 3-hydroxy-1,2-epoxy-4 Z-pentene elements were virtually identical and showed two characteristic ions that differed by 30 in m/ z values (CH(OH)). The results suggested that epoxide migration (Payne rearrangement) occurred during collision-induced dissociation. We conclude that regioisomeric allylic epoxyalcohols can be identified by their MS/MS spectra, whereas regioisomeric epoxyalcohols can be identified by MS/MS in combination with their retention times on NP-HPLC.