Determination of trace chelating carboxylic acids in rice by green extraction combined with liquid chromatography-mass spectrometry analysis and its application in the evaluation of old and new rice.

RATIONALE: Accurate identification of old rice samples from new ones benefits their market circulation and consumers. However, the current detection methods are still not satisfactory because of their insufficient accuracy or (and) time-consuming process. METHODS: Chelating carboxylic acids (CCAs) were selectively extracted from rice, by stirring with chelating resin and a dilute Na2CO3 solution. The green analytical chemistry guidelines for sample preparation were investigated by using the green chemistry calculator AGREE prep. The extractant was determined by liquid chromatography-mass spectrometry (LC/MS), and statistical analysis of the analytical data was carried out to evaluate the significance of the difference by ChiPlot. RESULTS: The limit of quantitation for the CCAs is in the range of 1 to 50 ng/mL, with a reasonable reproducibility. The CCAs in 23 rice samples were determined within a wide concentration range from 0.03 to 1174 µg/g. Intriguingly, the content of citric acid, malonic acid, α-ketoglutaric acid and cis-aconite acid in new rice was each found to be distinctively higher than that in old rice by several times. Even mixtures of old and new rice were found to show much difference in the concentration of citric acid and malic acid. CONCLUSION: A green analytical method has been developed for the simultaneous determination of CCAs by LC/MS analysis, and the identification of old rice samples from new ones was easily carried out according to their CCA content for the first time. The results indicated that the described method has powerful potential for the accurate identification of old rice samples from new ones.

Diastereomer recognition of three pairs of tetracyclines by electrospray ionization mass spectrometry.

RATIONALE: Stereoisomer profiling is always a difficult issue. Based on the difference between diastereomers, usually because of steric hindrance, isomers can be differentiated by mass spectrometry (MS), although it is often not an easy task. In the current study, tetracycline, chlortetracycline and doxycycline could be distinguished from their respective 4-epimers by MS. METHODS: The electrospray ionization tandem mass spectrometry (ESI-MSn ) analyses were carried out on a Bruker 3000plus ion trap mass spectrometer. For MS/MS experiments, the collision energy was set between 0.18 and 0.45 V to perform energy-resolved mass spectrometry (ERMS). Test solutions were prepared in methanol/water (90:10, v/v) at a concentration of 10 µg/mL. RESULTS: Compared with the collision-induced dissociation (CID) spectrum of protonated tetracycline, the most abundant peak changed from m/z 427 to m/z 410 for 4-epitetracycline. For chlortetracycline and its 4-epimer, differences in relative abundance were observed too. In the CID spectrum of a fragment ion of doxycycline, the abundance of m/z 154 was relatively higher than for the 4-epimer, showing the same trend as in the CID spectra of the other two pairs of tetracyclines. CONCLUSIONS: The CID spectra of tetracycline and chlortetracycline were different from those of their 4-epimers. The CID spectra of protonated doxycycline and its 4-epimer showed only a subtle difference, but the m/z 154 fragment ion in the CID spectra of the fragment ion at m/z 428 offers the possibility to differentiate both epimers.

Determination of trace fluoride in water samples by silylation and gas chromatography/mass spectrometry analysis.

RATIONALE: Due to the ubiquity of fluorides and the small gap between a safe dose and a harmful one, it is necessary to develop a robust analytical method for determination of fluoride ions in various water samples with complex matrices. METHODS: Silylation of the fluoride ion was carried out by treatment with hydrochloric acid and phenyldimethylchlorosilane at room temperature. The formed phenyldimethylfluorosilane was detected by gas chromatography/mass spectrometry (GC/MS). RESULTS: Under the optimized conditions, linearity in the analytical method ranged from 0.050 to 5.0 µg/mL for the fluoride ion with R2 >0.9999. Reasonable reproducibility was obtained with the intraday relative standard deviation (RSD) (N = 5) of 2.04% and interday RSD (N = 5) of 3.75% at the concentration of 0.10 µg/mL. The developed method has been successfully applied to determine the fluoride ion in real water samples, including waste water samples, with the recovery between 81.12% and 113.04%. CONCLUSIONS: A robust method for the determination of the fluoride ion has been developed by silylation with phenyldimethylchlorosilane and GC/MS analysis. The established method showed good anti-interference and precision, and it has been applied for determination of the fluoride ion in various water samples.

Quantification of phytic acid in baby foods by derivatization with (trimethylsilyl)diazomethane and liquid chromatography-mass spectrometry analysis.

RATIONALE: Phytic acid (PA) is both a naturally occurring nutrient and a widely used food additive for conferring antioxidant properties to food. PA can be found in baby foods and it is essential to monitor PA content due to its anti-nutritional properties when present in excess. Current methods for determining PA content are unsatisfactory because interference from inositol phosphates and inorganic phosphates complicates PA quantification. METHODS: Baby foods were extracted using aqueous HCl, and the extractant was subjected to derivatization with (trimethylsilyl)diazomethane after de-metalation using a cation exchange resin. The PA derivative was quantified using liquid chromatography-mass spectrometry (LC/MS/MS) with a multi-response monitoring mode (m/z 829 to 451). RESULTS: The linearity of the developed analytical method ranged from 10 to 1000 ng/mL for PA with R2 > 0.999. Reasonable reproducibility was obtained with an intraday relative standard deviation (RSD; N = 5) of 4.5% and an interday RSD (N = 5) of 5.7% at a concentration of 10 ng/mL. The developed method was successfully applied to determine PA content in various baby foods, with PA recovery between 90.6% and 119.8%. CONCLUSIONS: A robust and sensitive method for the determination of PA in baby foods has been developed by methyl esterification with (trimethylsilyl)diazomethane and using LC/MS/MS analysis. The established method showed good anti-interference and precision, and it has been applied for the determination of PA in various baby foods.

Determination of phytic acid in wheat products by complete methyl esterification and liquid chromatography-mass spectrometry analysis.

Phytic acid, the principal storage form of phosphorus in wheat, plays both beneficial and antinutrient functions for human being, and its analytical method still needs further development. In this work, we have developed a new method for the determination of phytic acid in wheat products based on derivatization with (trimethylsilyl)diazomethane in combination with liquid chromatography-mass spectrometry analysis. Methyl esterification greatly decreased the polarity and the acidity of phytic acid, and thus the corresponding derivative can be easily analyzed by liquid chromatography-mass spectrometry under common conditions. Furthermore, treatment with cation exchange resin removed the polyvalent metal ions in the solutions, and thus derivatization of phytic acid can be achieved efficiently and completely. The standard curve for phytic acid has been well established in the linear range of 0.5-100 ng/mL with squared correlation coefficient more than 0.999 and the quantification limit of 0.25 ng/mL. The phytic acid content varies greatly in different wheat products, ranging from 153.5 to 17299.0 µg/g.

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters.

RATIONALE: In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]+ ) and/or cationized molecules (e.g., [M + Na]+ ). The formation of specific [M - H]+ cations in the API process is of significant interest for further investigation. METHODS: The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level. RESULTS: The anomalous [M - H]+ ion and the regular [M + Na]+ ion were both observed using ESI-MS. Interestingly, no [M + H]+ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]+ rather than [M + H]+ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]+ ion can thermodynamically and kinetically undergo facile H2 elimination to generate [1 - H]+ . CONCLUSIONS: The favorable formation of [M - H]+ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]+ ion formed easily underwent H2 elimination to produce the [1 - H]+ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]+ ion.

Differentiation of isomeric cresols by silylation in combination with gas chromatography/mass spectrometry analysis.

RATIONALE: m-Cresol is listed as a priority controlled contaminant in many countries, but it is very difficult to accurately determine isomeric cresols due to their incomplete chromatographic separation on commercially available chromatographic columns and their nearly identical mass spectra. METHODS: Silylation of isomeric cresols was carried out by treatment with N-methyl-N-(trimethylsilyl)trifluoroacetamide. The formed trimethyl(tolyloxy)silanes were analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level. RESULTS: The derivatives of three isomeric cresols and six isomeric xylenols have been completely separated on an HP-5MS capillary column within a GC run of only 10 minutes. In addition, the derivative o-cresol can be very easily differentiated from its isomers due to its characteristic base peak ion at m/z 91 in electron ionization (EI)-MS. DFT calculation results indicated that the formation of the abundant fragment ion at m/z 91 is attributed to a facile dissociation pathway involving the shift of a neighboring phenylmethyl hydrogen atom in EI-MS of trimethyl(o-tolyloxy)silane. CONCLUSIONS: Silylation provides a promising solution for simultaneous determination of isomeric cresols and isomeric xylenols.

Pd-Catalyzed Enantioselective Ring Opening/Cross-Coupling and Cyclopropanation of Cyclobutanones.

A palladium-catalyzed enantioselective sequential ring-opening/cross-coupling of cyclobutanones is disclosed that provides chiral indanones bearing C3-quaternary stereocenters. The reaction process involves palladium-catalyzed nucleophilic addition of cyclobutanones and aryl halides, enantioselective ß-carbon elimination, and intermolecular trapping of a transient σ-alkylpalladium complex with boronic acids. Alternatively, an intramolecular cyclopropanation is realized through C-H bond functionalization in the absence of external coupling reagents, affording chiral cyclopropane-fused-indanones in good yields and enantioselectivity.

Differentiation of isomeric methylanilines by imidization and gas chromatography/mass spectrometry analysis.

RATIONALE: Carcinogenic o-methylaniline is one of the banned aromatic amines in azo dyes, but it is very difficult to distinguish it from its noncarcinogenic isomers due to their identical retention time on chromatography and similar mass spectra. METHODS: Imidization of the isomeric methylanilines was carried out by treatment with benzaldehyde under mild conditions. The formed derivatives were analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out on the Gaussian 03 program by using the density functional theory method at the B3LYP/6-311+G(d,p) level. RESULTS: Imidization of methylanilines occurred easily and gave rise to the corresponding N-methylbenzylidene benzenamines. The isomeric derivatives were completely separated by GC, and thus the three isomeric methylanilines could be determined simultaneously. Due to the ortho effect, the derivative from o-methylaniline has a characteristic fragment ion at m/z 118 with a stable bicyclic structure, and it could be easily differentiated from the meta- and para-isomers in electron ionization mass spectrometry. CONCLUSIONS: These results provided a promising solution for simultaneous determination of isomeric methylanilines.

Simultaneous determination of isomeric substituted anilines by imidization with benzaldehyde and gas chromatography-mass spectrometry.

The chromatographic separation of several isomeric anilines is a challenging issue. Herein, a simple method for the simultaneous determination of four groups of isomeric primary aromatic amines, including chloroanilines, methylanilines, methoxylanilines, and dimethylanilines, was presented. In this method, all of the 15 primary aromatic amines were easily transformed into the corresponding imine derivative by treatment with benzaldehyde under mild conditions. The formed isomeric imine derivatives were completely separated on a commercial capillary gas chromatography column. The effects of several derivatization parameters were investigated and optimized. Linearity in the optimized method ranged from 0.050 to 50 µg/mL with the squared correlation coefficients (R2 ) between 0.9981 and 0.9999. Reasonable reproducibility was obtained, with the intraday relative standard deviation (N = 5) ranging from 0.89 to 4.57% and interday relative standard deviation ranging from 2.26 to 7.69% at the concentration of 5.0 µg/mL. The developed method has been successfully applied to determine these isomeric aromatic amines in real samples.

Differentiation of isomeric haloanilines by tosylation in combination with electrospray ionization mass spectrometry.

Differentiation of the isomeric haloanilines still remains a challenging and necessary analytic task due to their identical retention time in chromatography and similar mass spectra. In this work, p-tosylation of haloanilines by reaction of haloanilines with p-toluenesulfonyl chloride resulted in the corresponding N-tosyl haloanilines. Fragmentation of protonated N-tosyl haloanilines in electrospray ionization tandem mass spectrometry (ESI-MS/MS) mainly resulted in tosyl cation, haloaniline radical cation, and halohydroxyaniline radical cation. The MS/MS of the three group isomeric derivatives showed significant difference in abundance distribution of these product ions, respectively. Theoretical calculations showed that the stability of the ion-neutral complex (INC) is a key factor influencing the relative intensity of the product ions. The three group isomeric derivatives were also separated by high performance liquid chromatograph (HPLC) at conventional conditions. p-Tosylation combined tandem MS (or HPLC) technique were carried out to realize the differentiation of isomeric haloanilines.

Two competing ionization processes in ESI-MS analysis of N-(1,3-diphenylallyl)benzenamines: formation of the unusual [M-H]+ ion versus the regular [M+H]+ ion.

A series of N-(1,3-diphenylallyl)benzenamine derivatives (M) were investigated by electrospray ionization mass spectrometry in the positive-ion mode. Both the anomalous [M-H]+ and the regular [M+H]+ were observed in the ESI mass spectra. The occurrence of [M-H]+ has been supported by accurate mass spectrometry, liquid chromatography mass spectrometry, and tandem mass spectrometry analysis. Calculation results indicated that formation of [M-H]+ is attributed to the ion-molecule reaction of M with the protonated ESI solvent molecule (e.g. CH3OH2+) via hydride abstraction from a tertiary Csp3-H. The competing ionization processes leading to [M-H]+ or [M+H]+ were significantly affected by the concentration of formic acid in the electrospray ionization solvent and the proton affinity of the N atom.

Rapid determination of ginkgolic acids in Ginkgo biloba kernels and leaves by direct analysis in real time-mass spectrometry.

A novel method based on direct analysis in real time integrated with mass spectrometry was established and applied into rapid determination of ginkgolic acids in Ginkgo biloba kernels and leaves. Instrument parameter settings were optimized to obtain the sensitive and accurate determination of ginkgolic acids. At the sample introduction speed of 0.2 mm/s, high intensity of [M-H]- ions for ginkgolic acids were observed in the negative ion mode by utilization of high-purity helium gas at 450°C. Two microliters of methanol extract of G. biloba kernels or leaves dropped on the surface of Quick-Strip module was analyzed after solvent evaporated to dryness. A series of standard solutions of ginkgolic acid 13:0 in the range of 2-50 mg/L were analyzed with a correlation coefficient r = 0.9981 and relative standard deviation (n = 5) from 12.5 to 13.7%. The limit of detection was 0.5 mg/L. The results of direct analysis in real time-mass spectrometry were in agreement with those observed by thermochemolysis gas chromatography. The proposed method demonstrated significant potential in the application of the high-throughput screening and rapid analysis for ginkgolic acids in dietary supplements.

Dissociation of protonated N-(3-phenyl-2H-chromen-2-ylidene)-benzenesulfonamide in the gas phase: cyclization via sulfonyl cation transfer.

RATIONALE: In the tandem mass spectrometry of protonated N-(3-phenyl-2H-chromen-2-ylidene)benzenesulfonamides, the precursor ions have been observed to undergo gas-phase dissociation via two competing channels: (a) the predominant channel involves migration of the sulfonyl cation to the phenyl C atom and the subsequent loss of benzenesulfinic acid along with cyclization reaction, and (b) the minor one involves dissociation of the precursor ion to give an ion/neutral complex of [sulfonyl cation/imine], followed by decomposition to afford sulfonyl cation or the INC-mediated electron transfer to give an imine radical cation. METHODS: The proposed reaction channels have been supported by theoretical calculations and D-labeling experiments. RESULTS: The gas-phase cyclization reaction originating from the N- to C-sulfonyl cation transfer has been first reported to the best of our knowledge. CONCLUSIONS: For the substituted sulfonamides, the presence of electron-donating groups (R(2) -) at the C-ring effectively facilitates the reaction channel of cyclization reaction, whereas that of electron-withdrawing groups inhibits this pathway.

Two competitive INC-mediated reactions in the gas-phase fragmentation of protonated indolyl benzo[b]carbazoles.

RATIONALE: Ion-neutral complexes (INCs) are intermediates extensively existing in gas-phase ionic reactions. METHODS: Tandem mass spectrometry (MS(n) ) analysis of indolyl benzo[b]carbazoles was performed on an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer in positive ion mode. RESULTS: Two competing INC-mediated reactions were obtained in the fragmentation of protonated indolyl benzo[b]-carbazoles. CONCLUSIONS: This study enriches our knowledge of the important roles of INCs in the dissociation of indole compounds in the gas phase. Copyright © 2016 John Wiley & Sons, Ltd.

Isomeric differentiation of chloroanilines by gas chromatography-mass spectrometry in combination with tosylation.

p-Chloroaniline is one of the banned aromatic amines in azo dyes, but it is very difficult to distinguish it from its isomers due to their identical retention time in chromatography and similar mass spectra. In this work, derivatization of the isomeric chloroanilines was carried out to yield the corresponding N-tosyl chloroanilines, which were completely separated by gas chromatography and also possessed clearly different electron ionization mass spectra. Thus, the three isomers could be differentiated and determined at the same time. Density functional theory calculation results indicated that the effect of the substituent pattern in electron ionization mass spectrometry is mainly due to the difference in the stability of the product ion (P2) at m/z 126, originating from the loss of tosyl radical from the precursor ion.

Two competing ionization processes in electrospray mass spectrometry of indolyl benzo[b]carbazoles: formation of M⁺• versus [M + H]⁺.

RATIONALE: Ionization in electrospray ionization mass spectrometry (ESI-MS) mainly occurs as a result of acid-base reactions or coordination with metal cations. Formation of the radical cation M(+•) in the ESI process has attracted our interest to perform further investigation. METHODS: A series of indolyl benzo[b]carbazoles were investigated using a quadrupole ion trap mass spectrometer equipped with an ESI source or an atmospheric pressure chemical ionization (APCI) source in the positive-ion mode. Theoretical calculations were performed using the density functional theory (DFT) method at the B3LYP/6-31G(d) level. RESULTS: Both the radical ion M(+•) and the protonated molecule [M + H](+) were obtained by ESI-MS analysis of indolyl benzo[b]carbazoles, while only [M + H](+) was observed in the APCI-MS analysis. The relative intensities of M(+•) and [M + H](+) were significantly affected by several ESI operating parameters and the nature of the substituents. CONCLUSIONS: Formation of M(+•) and [M + H](+) was rationalized as two competing ionization processes in the ESI-MS analysis of indolyl benzo[b]carbazoles.

Observation of the intermediates of in-source aldolization reaction in electrospray ionization mass spectrometry analysis of heteroaromatic aldehydes.

Electrospray ionization mass spectrometry (ESI-MS) analyses of 2-(1,2,4-triazole-1-yl)-6-methyl-3- quinolinecarboxaldehyde were carried out by using an ion trap mass spectrometer in a positive-ion mode. Interestingly, several unusual [M + 15](+), [M + 33](+), and [M + 47](+) ions were observed with a high abundance in the ESI-MS spectrum when methanol was used as the ESI solvent. However, only the protonated molecule was obtained with acetonitrile as the ESI solvent. These unusual ions have been proposed as the intermediates of an aldolization reaction occurring in the ESI source, which have been validated by a tandem mass spectrometry experiment, high-performance liquid chromatography/mass spectrometry analysis, and theoretical calculations. A full understanding of this reaction can contribute to the avoidance of analysis errors in the ESI-MS analysis of unknown heteroaromatic aldehydes.

HPLC/QTOF-MS/MS application to investigate phenolic constituents from Ficus pandurata H. aerial roots.

Ficus pandurata H. aerial roots are used as a traditional Chinese medicine for the treatment of uarthritis, indigestion and hyperuricemia. However, the bioactive constituents responsible for the pharmacological effects of F. pandurata H. are unclear. A simple and efficient HPLC/QTOF-MS/MS (high-performance liquid chromatography/electrospray ionization with quadrupole time-of-flight tandem mass spectrometry) method was established to detect and identify active constituents in the n-butanol extract of F. pandurata H. aerial roots. Chemical constituents were separated and investigated by HPLC/QTOF-MS/MS in the negative-ion mode. Thirty-seven compounds, including hydroxycinnamic acid derivatives, hydroxybenzoic acid derivatives, hydroquinone glycosides, flavonoid glycosides, etc., were identified or tentatively characterized in the n-butanol extract of F. pandurata H. aerial roots by comparing the UV spectra, accurate mass spectra and fragmentation pathways and retrieving the reference literatures. Moreover, the flavonoid trisaccharides and hydroxybenzoic acid derivatives were tentatively characterized in F. pandurata H. for the first time. The analytical tool used here is very valuable in the rapid separation and identification of the multiple and minor constituents in the n-butanol extract of F. pandurata H. aerial roots.

Fragmentation reactions of N-benzyltetrahydroquinolines in electrospray ionization mass spectrometry: the roles of ion/neutral complex intermediates.

RATIONALE: Electrospray ionization mass spectrometry (ESI-MS) combined with the collision-induced dissociation (CID) technique has assumed increasing importance as an invaluable tool for the structural analysis of organic and biological molecules. However, general rules for elucidating the fragmentation behaviors of charged molecules in the gas phase are still lacking. Therefore, explorations on the mechanistic information are desirable at all times. METHODS: CID experiments of protonated N-benzyltetrahydroquinolines were carried out on ESI ion trap mass spectrometer and accurate mass measurements were performed on a high-resolution ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in positive ion mode. RESULTS: An ion/neutral complex, [RC6H4CH2(+)/tetrahydroquinoline], resulting from cleavage of the C-N bond induced by the positive charge brought in by protonation, was proposed to be the intermediate to elucidate the fragmentation reactions. For all the compounds investigated, benzyl cation transfer, electron transfer and hydride transfer reactions mediated by the complex were observed. Moreover, for the compound substituted by a methyl group at the para-position of the benzylic phenyl ring, proton transfer reaction via the complex also occurs. CONCLUSIONS: This study is a case for better understanding the intriguing roles of ion/neutral complexes in gas-phase fragmentation reactions and enriching the knowledge about the gas-phase chemistry of the benzyl cation. In addition, it provides useful information for researchers working on analysis or structural elucidation of complicated compounds which contain the N-benzyltetrahydroquinoline substructure.