Dual parallel electrospray ionization and atmospheric pressure chemical ionization mass spectrometry (MS), MS/MS and MS/MS/MS for the analysis of triacylglycerols and triacylglycerol oxidation products

Two mass spectrometers, in parallel, were employed simultaneously for analysis of triacylglycerols in canola oil, for analysis of triolein oxidation products, and for analysis of triacylglycerol positional isomers separated using reversed-phase high-performance liquid chromatography. A triple quadrupole mass spectrometer was interfaced via an atmospheric pressure chemical ionization (APCI) interface to two reversed-phase liquid chromatographic columns in series. An ion trap mass spectrometer was coupled to the same two columns using an electrospray ionization (ESI) interface, with ammonium formate added as electrolyte. Electrospray ionization mass spectrometry (ESI-MS) under these conditions produced abundant ammonium adduct ions from triacylglycerols, which were then fragmented to produce MS/MS spectra and then fragmented further to produce MS/MS/MS spectra. ESI-MS/MS of the ammoniated adduct ions gave product ion mass spectra which were similar to mass spectra obtained by APCI-MS. ESI-MS/MS produced diacylglycerol fragment ions, and additional fragmentation (MS/MS/MS) produced [RCO](+) (acylium) ions, [RCOO+58](+) ions, and other related ions which allowed assignment of individual acyl chain identities. APCI-MS of triacylglycerol oxidation products produced spectra like those reported previously using APCI-MS. APCI-MS/MS produced ions related to individual fatty acid chains. ESI-MS of triacylglycerol oxidation products produced abundant ammonium adduct ions, even for those molecules which previously produced little or no intact molecular ions under APCI-MS conditions. Fragmentation (MS/MS) of the [M+NH(4)](+) ions produced results similar to those obtained by APCI-MS. Further fragmentation (MS/MS/MS) of the diacylglycerol fragments of oxidation products provided information on the oxidized individual fatty acyl chains. ESI-MS and APCI-MS were found to be complementary techniques, which together contributed to a better understanding of the identities of the products formed by oxidation of triacylglycerols.     

Applications of HPLC-MS in compound Ilex pubescens extract study

In this paper, high performance liquid chromatography (HPLC) along with mass spectrometry (MS) and HPLC along with a diode array detector (DAD) was used to study the compound Ilex pubescens extract. Two ionization techniques: electro spray ionization (ESI) and atmospheric pressure chemical ionization (APCI) were used in this work. The liquid chromatograms obtained by DAD, total ion chromatograms (TIC) from positive-and negative-ion ESI-MS and the positive-and negative-ion APCI-MS were compared. The liquid chromatograms obtained by TIC from ESI-MS provided more information on chromatographic peaks than those obtained by DAD or TIC from APCI-MS. It is suggested that the fingerprints of the compound Ilex pubescens extract should be provided by the liquid chromatograms obtained by DAD together with TIC from the negative-ion ESI-MS. The molecular weights of the nine main components in an HPLC-DAD chromatogram were determined by the corresponding positive-and negative-ion ESI and the positive-and negative-ion APCI mass spectra information. In the liquid chromatogram obtained by TIC from the negative-ion ESI-MS, the molecular weights of 23 main components were determined based on the corresponding positive-and negative-ion ESI mass spectra information.     

Optimization of HPLC-APCI-MS conditions for the qualitative and quantitative determination of total solanesol in tobacco leaves

HPLC coupled online with atmospheric pressure chemical ionization MS (APCI-MS) technique was evaluated for the qualitative and quantitative determination of solanesol in extracts of tobacco leaves. The solanesol and other compounds in the extract were separated on an Alltima C(8) (4.6 mm x 250 mm) column with methanol and water (98:2 v/v) as mobile phase, with flow rate of 0.8 mL/min and UV detection wavelength of 211 nm. In the APCI(+) mode, abundant stable [M-H(2)O + H](+) ion (m/z at 613.5) was observed, with low abundance of other fragmentation ions. A comparison of APCI-MS and ESI-MS techniques showed that APCI mode is more sensitive than ESI mode, and thus better suited for solanesol analysis. When comparing UV 211 nm and APCI-MS in SIM for solanesol quantification, the former offered better precision and reproducibility, but the latter was more than 200 times sensitive in detection. The developed method has been successfully applied to the analysis and comparison of solanesol concentration in different tobacco leaf samples.     

Liquid chromatography with dual parallel mass spectrometry and (31)P nuclear magnetic resonance spectroscopy for analysis of sphingomyelin and dihydrosphingomyelin. I. Bovine brain and chicken egg yolk

Liquid chromatography coupled to atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) mass spectrometry (MS), in parallel, was used for detection of bovine brain and chicken egg sphingolipids (SLs). APCI-MS mass spectra exhibited mostly ceramide-like fragment ions, [Cer-H(2)O+H](+) and [Cer-2H(2)O+H](+), whereas ESI-MS produced mostly intact protonated molecules, [M+H](+). APCI-MS/MS and MS(3) were used to differentiate between isobaric SLs. APCI-MS/MS mass spectra exhibited long-chain base related fragments, [LCB](+) and [LCB-H(2)O](+), that allowed the sphinganine backbone to be differentiated from the sphingenine backbone. Fragments formed from the fatty amide chain, [FA(long)](+) and [FA(short)](+), allowed an overall fatty acid composition to be determined. The presence of both dihydrosphingomyelin (DSM) and sphingomyelin (SM) sphingolipid classes was confirmed using (31)P NMR spectroscopy.     

Mass spectrometric and tandem mass spectrometric behavior of nitrocatechol glucuronides: A comparison of atmospheric pressure chemical ionization and electrospray ionization

The mass spectrometric (MS) and tandem mass spectrometric (MS/MS) behavior of six nitrocatechol-type glucuronides using atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) was systematically studied, and the effect of operation parameters on the fragmentations are presented. The positive ion APCI- and ESI-MS spectra showed an intense protonated molecule and the respective negative ion spectra a deprotonated molecule with minimal fragmentation. The main fragment ions in the MS/MS spectra of the protonated and deprotonated molecules were [M + H - Glu]+ and [M - H - Glu]-, respectively, formed by the loss of the glucuronide moiety. The measured limits of detection indicated that ESI is a significantly more efficient ionization method than APCI in the negative and positive ion modes for the compounds studied. MS/MS was found to be less sensitive, but more reliable and simple than MS due to the absence of chemical noise.     

Atmospheric pressure ionization mass spectrometry of purine and pyrimidine markers of inherited metabolic disorders

Purines and pyrimidines are of interest owing to their significance in processes in living organisms. Mass spectrometry is a promising analytical tool utilized in their analysis. Two atmospheric pressure ionization (API) methods (electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)) in both negative and positive modes applied to selected purine and pyrimidine metabolites (markers of inherited metabolic disorders) were studied. APCI is less sensitive to alkali metal cations present in a sample and offers higher response than ESI for studied compounds. Both of the techniques afford quasi-molecular ions, but fragmentation also occurs to a certain extent. However, the application of collision-induced dissociation of quasi-molecular ions is essential to confirm a certain metabolite in a sample. Fragmentation of both positive and negative ions was evaluated using multi-stage mass spectrometric experiments. Typical neutral losses correspond to molecules NH(3), H(2)O, HCN, CO, H(2)NCN, HNCO and CO(2). The ion [NCO](-) arises in the negative mode. The cleavage of the glycosidic C-N bond is characteristic for relevant metabolites. Other neutral losses (CH(2)O, C(2)H(4)O(2) and C(3)H(6)O(3)) originate from fragmentation of the glycosidic part of the molecules. In addition to fragmentation, the formation of adducts of some ions with applied solvents (H(2)O, CH(3)OH) was observed. The composition of the solution infused into the ion source affects the appearance of the mass spectra. Tandem mass spectra allow one to distinguish compounds with the same molecular mass (uridine-pseudouridine and adenosine-2'-deoxyguanosine). Flow injection analysis APCI-MS/MS was tested on model samples of human urines corresponding to adenosine deaminase deficiency and xanthine oxidase deficiency. In both cases, the results showed potential diagnostic usefulness.     

多溴联苯醚代谢物的色谱保留和质谱特征

研究了多溴联苯醚(PBDEs)代谢物中常见的18种羟基PBDEs(HO-BDEs)、15种甲氧基PBDEs(MeO-BDEs)的气相色谱、反相液相色谱分离和质谱碎片特征。MeO-BDEs在反相液相色谱(RPLC)的保留行为受疏水作用支配,而HO-BDEs则还受到除疏水作用以外的特殊作用影响。LC/MS中大气压化学电离(APCI)源适用于HO-BDEs检测,而APCI源以及电喷雾离子源(ESI)对MeO-BDEs都没有响应。质谱碎片具有显著取代位效应:在LC/APCI-MS中,邻位取代HO-BDEs的碎片离子基峰均为脱一个溴的结构[M-H]--HBr,而对位取代的基峰则为准分子离子[M-H]-;在电子电离(EI)源质谱仪下,邻位取代MeO-BDEs有脱溴甲烷碎片离子(M. -BrCH3),对位取代有特征的脱甲基自由基结构(M. -.CH3),间位取代则无上述两种碎片离子。     

Supercritical fluid extraction and quantification of aflatoxins in Zizyphi Fructus by liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry

An integrated method combining supercritical fluid extraction (SFE) with liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry (LC/APCI-MS/MS) was developed and successfully applied to quantify aflatoxins (AFs) in Zizyphi Fructus (fruits of Zizyphus jujube), a traditional Chinese medicine. To minimize the potential interferences caused by the complex matrix in Zizyphi Fructus, a SFE pretreatment was performed. In addition, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) spectra were also compared. The results showed that the calibration curves of AFB(1), AFB(2), AFG(1), and AFG(2) were all linear over the range of concentration from 1 to 50 ng/g, the squared correlation coefficients (r(2)) were over 0.995, and the detection limits of the method were between 0.17 and 0.32 ng/g. It showed high recovery and good precision in quantitating AFs in Zizyphi Fructus without further clean-up. Further, fragmentation pathways of protonated AFs in APCI-MS/MS were clearly proposed which could predict the existence of AFB or AFG series. To test the empirical validity of the proposed methodology in this paper, eight random samples of Zizyphi Fructus collected from supermarkets and traditional Chinese medicine stores in different geographical areas of Taiwan were analyzed. The results indicated that low levels of AFs were detected in only one of them.     

Fragmentation pathways of some benzothiophene radical cations formed by atmospheric pressure chemical ionisation

Polycyclic aromatic sulfur‐containing compounds (PASHs) are commonly found in fossil fuels and are of considerable importance in environmental studies. This work presents detailed studies on the fragmentation patterns of radical cations formed from four representative PASHs, benzo[b]thiophene, dibenzothiophene, 4‐methyldibenzothiophene and 4,6‐dimethyldibenzothiophene, using tandem atmospheric pressure chemical ionization mass spectrometry (APCI‐MS/MS). Understanding these fragmentation patterns can be a useful aid in the analysis of PASHs employing APCI or electron ionization (EI‐MS/MS), either alone or in conjunction with liquid or gas chromatography. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of metabolites of a novel histamine H(2)-receptor antagonist, lafutidine, in human liver microsomes by liquid chromatography coupled with ion trap mass spectrometry

The metabolism of lafutidine in human liver microsomes was studied using liquid chromatography/ion trap mass spectrometry with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources. A total of 14 metabolites were identified including hydroxylated lafutidine and sulfonyl lafutidine as the major metabolites. The chemical properties and the MS(n) behaviors of lafutidine and all of its identified metabolites were studied in detail. Lafutidine had a fragmentation pattern as a result of homolytic bond cleavage in the MS/MS spectrum. This cleavage can form an odd-electron ion with the loss of furan-2-ylmethyl radical (-81 Da with a proton shift), which then sequentially loses neutral groups in the MS(3) spectrum. This fragmentation sequence was also observed from the metabolites with the unchanged sulfinyl moiety. When the sulfinyl moiety was oxidized to the sulfonyl moiety, this fragmentation sequence did not exist, which could be used to identify S-oxidation metabolites of lafutidine. In general, N-oxides could produce distinct [M+H-O](+) ions under LC/APCI-MS due to the thermal activation in the desolvation region of the API source, which could be used to identify N-oxidation metabolites of lafutidine. In order to avoid the possibility of false positives, the MS/MS spectrum of the [M+H-O](+) ion was compared with that of the non-N-oxidation metabolites or parent drug in the APCI source. If they were consistent, the structure could be finally confirmed. The exact masses for lafutidine and lafutidine N-oxide fragment ions were determined using an LTQ/Orbitrap mass spectrometer.     

Drug impurity profiling by capillary electrophoresis/mass spectrometry using various ionization techniques

Capillary electrophoresis/mass spectrometry (CE/MS) is predominantly carried out using electrospray ionization (ESI). Recently, atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) have become available for CE/MS. With the VUV lamp turned off, the APPI source may also be used for CE/MS by thermospray ionization (TSI). In the present study the suitability of ESI, APCI, APPI and TSI for drug impurity profiling by CE/MS in the positive ion mode is evaluated. The drugs carbachol, lidocaine and proguanil and their potential impurities were used as test compounds, representing different molecular polarities. A background electrolyte of 100 mM acetic acid (pH 4.5) provided baseline separation of nearly all impurities from the respective drugs. APPI yielded both even‐ and odd‐electron ions, whereas the other ionization techniques produced even‐electron ions only. In‐source fragmentation was more pronounced with APCI and APPI than with ESI and TSI, which was most obvious for proguanil and its impurities. In general, ESI and TSI appeared the most efficient ionization techniques for impurities that are charged in solution achieving detection limits of 100 ng/mL (full‐scan mode). APPI and APCI showed a lower efficiency, but allowed ionization of low and high polarity analytes, although quaternary ammonium compounds (e.g. carbachol) could not be detected. Largely neutral compounds, such as the lidocaine impurity 2,6‐dimethylaniline, could not be detected by TSI, and yielded similar detection limits (500 ng/mL) for ESI, APPI and APCI. In many cases, impurity detection at the 0.1% (w/w) level was possible when 1 mg/mL of parent drug was injected with at least one of the CE/MS systems. Overall, the tested CE/MS systems provide complementary information as illustrated by the detection and identification of an unknown impurity in carbachol. Copyright © 2009 John Wiley & Sons, Ltd.     

Atmospheric pressure photoionization for coupling liquid-chromatography to mass spectrometry: A review

This review presents the state-of-the-art techniques that couple liquid chromatography (LC) and mass spectrometry (MS) via atmospheric pressure photoionization (APPI). The different ionization mechanisms are discussed as well as the influence of the mobile phase composition, the nature of the dopant, etc. A comparison with other ionization sources, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), is reported, and the combination of APPI with these sources is also discussed. Several applications, covering the time period of 2005-2008, for the analysis of drugs, lipids, natural compounds, pesticides, synthetic organics, petroleum derivatives, and other substances are presented.     

液相色谱-大气压化学电离质谱法分析人参中的人参皂甙

研究了用反相高效液相色谱-大气压化学电离质谱(HPLC/APCI-MS)分析人参皂甙的方法。液相色谱采用乙腈-水流动相进行梯度洗脱,质谱采用正负离子同时扫描并结合二级质谱进行定性,用选择反应离子模式(SRM)测定检测限。实验发现虽然人参皂甙是热不稳定物质,但在大气压化学电离质谱的高温汽化过程中仍能检测到很强的负离子分子离子峰,而且随着汽化温度的升高,人参皂甙的负离子分子离子峰的强度增加。该方法对人参皂甙Rb1和Rg1的检测限分别为1.2×10-13 g和3.0×10-14 g,并检测出白参中包括丙二酰人参皂甙在内的29种人参皂甙。该法灵敏度高,重复性好,结果准确,能有效地对药材提取物中的多种人参皂甙进行检测和结构分析。     

Comparison of atmospheric pressure chemical ionization and electrospray ionization mass spectrometry for the detection of lignans from sesame seeds

In sesame seeds, high concentrations of lignans are present. When these lignans are fermented in the human colon, a range of structurally different lignans is formed. A good liquid chromatography/mass spectrometry (LC/MS) protocol for the analysis of lignans in complex mixtures is lacking. In order to develop such a protocol, electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization (APCI)-MS, both in the positive and negative ionization mode, were compared. An extract from defatted sesame meal was analyzed by APCI-MS and ESI-MS, before and after deglucosylation. APCI-MS was found to be a more generic method than ESI-MS because lignans, especially sesamolin, sesamin and pinoresinol, were better detected by APCI-MS than by ESI-MS. Positive and negative ionization modes had to be combined in order to detect all lignans in a bacterial culture grown on aglyconic, acid-treated lignans from sesame oil and defatted sesame meal. Lignans with methylenedioxy-bridged furanofuran structures mostly lack phenolic hydroxyl groups and were, therefore, optimally detected in positive ionization mode. Dibenzylbutadiene lignans, which were formed during fermentation, carry hydroxyl groups and were better detected in negative ionization mode.     

Normal-phase high-performance liquid chromatographic separations using ethoxynonafluorobutane as hexane alternative. II. Liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry applications with methanol gradients

We have reported recently that high-speed normal-phase (NP) HPLC separations of a broad range of organic compounds can be performed on cyano columns using gradients of methanol in hexane-like solvent-ethoxynonafluorobutane (ENFB), available commercially. In this communication, we demonstrate that atmospheric pressure chemical ionization (APCI) in combination with mass spectrometry (MS) can be effectively used for detection in such separations. The efficiency of APCI under conditions studied has also been compared to the efficiency of traditional electrospray ionization (ESI) in combination with MS for reversed-phase (RP) HPLC of the same compounds. The compounds included in this study were steroids, benzodiazepines, and other central nervous system-active substances, nonsteroidal anti-inflammatory drugs, tricyclic antidepressants, and beta-adrenergic blocking agents. Non-polar compounds were found to respond stronger when APCI-MS technique was used, whereas APCI and ESI ionization efficiencies were comparable when polar substances were studied. The combination of normal-phase HPLC separation conditions with mass spectral detection may expand the range of LC-MS applications traditionally associated with reversed-phase HPLC and ESI-MS detection.     

Targeted lipidomics using electron capture atmospheric pressure chemical ionization mass spectrometry

There is an increasing need to be able to conduct quantitative lipidomics analyses as a complement to proteomics studies. The highest specificity for proteomics analysis can be obtained using methodology based on electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) coupled with liquid chromatography/tandem mass spectrometry (LC/MS/MS). For lipidomics analysis it is often necessary to be able to separate enantiomers and regioisomers. This can be very challenging when using methodology based on conventional reversed-phase chromatography. Normal-phase chromatography using chiral columns can provide dramatic improvements in the resolution of enantiomers and regioisomers. However, conventional ESI- and APCI-MS/MS has limited sensitivity, which makes it difficult to conduct studies in cell culture systems where only trace amounts of non-esterified bioactive lipids are present. The use of electron capture APCI-MS/MS overcomes this problem. Enantiomers and regioisomers of diverse bioactive lipids can be quantified using stable isotope dilution methodology coupled with normal-phase chiral chromatography and electron capture APCI-MS/MS. This methodology has allowed a lipidomics profile from rat epithelial cells maintained in culture to be delineated and allowed the effect of a non-selective lipoxygenase inhibitor to be assessed.     

Effects of sample properties and mass spectroscopic parameters on electrospray ionization mass spectra of size-fractions from a soil humic acid

Size-fractions from a soil humic acid were separated by preparative size-exclusion chromatography (SEC), desalted, and concentrated by ultrafiltration and vacuum centrifugation without being subjected to any freeze-drying process. After having assessed the lack of formation of any multiple-charged ions by high-resolution Fourier transform ion cyclotron resonance electrospray ionization (ESI) mass spectrometry (MS), the size-fractions were used by direct infusion to compare the molecular ion distribution by both atmospheric pressure chemical ionization (APCI)- and ESI-MS in negative mode. The weight- (Mw) and number-averaged (Mn) molecular weight obtained by ESI-MS were invariably larger than by APCI-MS for all size-fractions, thereby indicating that ESI is more efficient than APCI to evaluate the molecular mass distribution of humic samples. No substantial difference was observed when concentration and pH of unfreeze-dried humic size-fractions were varied. The negative mode was applied to assess the effect of cone voltage from −20 to −60 V on ESI of the humic size-fractions further separated through an on-line SEC column. The resulting mass spectra and Mw and Mn values suggested that the variation of cone voltage in ESI-MS affects the ionization potential of associated humic molecules more in solution rather than their fragmentation. These findings agree with previous observations which indicated a limitation of ESI in providing consistent mass detection for a complex mixture of heterogeneous humic molecules, especially when they are aggregated by a freeze-drying process.     

Evaluation of a dual electrospray ionization/atmospheric pressure chemical ionization source at low flow rates (∼50 µL/min) for the analysis of both highly and weakly polar compounds

The atmospheric pressure ionization (API) source for a commercial mass spectrometer was modified to operate as a dual source in both the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) techniques by simultaneously utilizing the electrospray probe and the corona discharge needle. A switching box was designed to operate in either manual or programmable modes to permit rapid switching between ionization techniques without changing sources, probes, or breaking vacuum. The source can be operated using the following ionization techniques: ESI only, APCI only, ESI/APCI simultaneously, and ESI/APCI alternatingly. The optimum operating conditions for these ionization techniques were similar to the manufacturer’s original specifications except that the APCI flow rate was lower (～50 µL/min versus 1000 µL/min) and externally heated nebulizing gas was found to be desirable. A four-component mixture, introduced by flow injection, was used to demonstrate the versatility of the dual ESI/APCI source.     

The Acylpolyamines from the Venom of the Spider Agelenopsis aperta

The lyophilized venom of the spider Agelenopsis aperta (Aranea: Agelenidae) has been re‐analyzed by on‐line coupled high‐performance liquid chromatography and atmospheric‐pressure chemical‐ionization mass spectrometry (HPLC‐UV(DAD)‐APCI‐MS and MS/MS). Thus, 33 acylpolyamines with 11 different molecular masses, present in various concentrations, were detected, and their structures were disclosed. Different types of possible fragmentation as well as principles of structure elucidation are discussed. The method can be used for rapid separation and analysis of spider‐venom constituents.     

Strategies for the liquid chromatographic-mass spectrometric analysis of non-polar compounds

Electrospray ionization and atmospheric pressure chemical ionization (APCI) have evolved recently as very useful tools for the liquid chromatographic–mass spectrometric (LC–MS) analysis of polar substances. Non-polar compounds, however, are difficult to analyze with these atmospheric pressure ionization techniques due to their soft ionization mechanism. Recently, new approaches have been introduced which are likely to overcome this obstacle, at least partly. On-line electrochemical conversion of the analytes to more polar reaction products, atmospheric pressure photoionization, atmospheric pressure electron capture negative ion-MS and coordination ionspray-MS are four techniques which are presented in detail, compared and discussed critically with respect to their current status and future perspectives. Particular focus is directed from a chemical viewpoint on the substance groups which are accessible by each of the new approaches.