Changes of Volatile Organic Compounds of Different Flesh Texture Pears during Shelf Life Based on Headspace Solid-Phase Microextraction with Gas Chromatography-Mass Spectrometry.

Aroma is an important sensory factor in evaluating the quality of pear fruits. This study used headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) to analyze the volatile organic compounds (VOCs) of three crispy pears and five soft pears during shelf life, and the changes in soluble solids content (SSC) were analyzed. The results showed that the SSC of the soft pears such as Nanguoli, Jingbaili and Louis was always higher than that of the crispy pears throughout shelf life. A total of 160 VOCs were detected in the eight pear varieties. Orthogonal partial least squares discriminant analysis (OPLS-DA) and hierarchical cluster analysis (HCA) combined with predictor variable importance projection (VIP) showed that the eight pear varieties could be obviously classified into six groups according to the differences in their VOCs, and 31 differential VOCs were screened out, which could be used to differentiate between pears with different flesh textures. The results of clustering heat map analysis showed that, with the extension of shelf life, the content of each different VOC did not change much in crispy pears, whereas the difference in soft pears was larger. This study confirmed the potential of determining the optimal shelf life of different pear varieties about aroma evaluation and studying the mechanism of differences in VOCs in the future.

Monitoring Volatile Organic Compounds in Different Pear Cultivars during Storage Using HS-SPME with GC-MS.

Aroma, which plays an essential role in food perception and acceptability, depends on various mixture of volatile organic compounds (VOCs). Meanwhile, as a field of metabolomics, VOC analysis is highly important for aroma improvement and discrimination purposes. In this work, VOCs in pear fruits were determined via headspace solid-phase micro-extraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) to study variations among different cultivars and storage stages. In 12 cultivars of pear fruits, a total of 121 VOCs were quantified, including 40 esters, 32 alcohols, 16 aldehydes, 13 alkenes, 11 ketones, 4 acids, and 5 other compounds. The types and amounts of VOCs in different cultivars varied dramatically, which were in the range of 13-71 and 3.63-55.65 mg/kg FW (fresh weight), respectively. The Dr. Guyot cultivar showed the highest level of VOCs, both in type and amount. After 21 days storage at 4 °C, total concentration of VOCs increased from initial levels of 50.76 to 101.33 mg/kg FW. Storage at 20 °C made a larger contribution to production for VOCs than that at 4 °C, resulting in the maximum content of VOCs (117.96 mg/kg FW) in fruit after 14 days storage at 4 °C plus 7 days at 20 °C. During storage, the content of esters showed a gradual increase, while the content of alcohols and aldehydes decreased. Based on the results presented, related alcohols were recognized as the intermediates of conversion from aldehydes to esters.

Determination of polychlorinated biphenyls in tea using gas chromatography-tandem mass spectrometry combined with dispersive solid phase extraction.

A gas chromatography-tandem mass spectrometry method was developed for simultaneous determination of 38 polychlorinated biphenyls (PCBs) in tea. Sample preparation was based on a dispersive solid phase extraction procedure through an extraction of target compounds. An appropriate amount of polyvinylpolypyrrolidone was directly added in tea extractions to effectively remove polyphenols, and then tea extracts were cleaned up with primary secondary amine, florisil and graphitised carbon black. The method was validated, and linearity with correlation coefficients higher than 0.99 was obtained. Satisfactory recoveries at 2, 10, 50, and 100 µg kg-1 ranged from 71% to 117% with a maximum relative standard deviation of 23%, except for PCB 81, 77, 126 and 169, of which recoveries were in the range of 32%-63%. Limits of quantitation for PCBs were 2 or 10 µg kg-1, which was set as the lowest validated and spiked level meeting the acceptable accuracy and precision.

Dissipation pattern and safety evaluation of cartap and its metabolites during tea planting, tea manufacturing and brewing.

There are few studies for risk assessment of cartap and its metabolites, although cartap is easily transformed into metabolites which could induce higher toxicity. This study aimed to investigate the dissipation pattern of cartap and its metabolites during tea planting, manufacturing and brewing for evaluating the safety of cartap pesticide. Cartap metabolites were identified using Q-Exactive Orbitrap mass spectrometry. Half-lives of cartap in fresh tea leaves ranged from 0.49 to 0.59 days. Cartap decreased rapidly with time, and it was degraded into nereistoxin and cartap monothiol during tea production chain. Cartap monothiol residues dissipated rapidly by 98% in three days during tea planting. Nereistoxin had a longer residual period than cartap and it dominated the total residue in made tea after tea manufacturing. Transfer rates of nereistoxin during tea brewing ranged from 78.24% to 121.56%. Therefore, we suggested sum of cartap and nereistoxin residues as maximum residual limits in tea.

Simultaneous determination of cartap and its metabolite in tea using hydrophilic interaction chromatography tandem mass spectrometry and the combination of dispersive solid phase extraction and solid phase extraction.

Risk assessment of cartap residue in tea should include the exposure of cartap and its metabolite due to rapid degradation of cartap into nereistoxin. Herein, a reliable method for determination of cartap and nereistoxin in tea was developed by hydrophilic interaction chromatography tandem mass spectrometry. Target compounds were extracted with water containing 1% formic acid and 5 mM ammonium formate. The use of dichloromethane effectively removed caffeine. Tea extracts were cleaned up by dispersive adsorbents of octadecylsilane and strong anion exchanger, then further purified using hydrophilic lipophilic balanced solid phase extraction cartridge. Isotopic internal standard was employed to calibrate the loss of analytes during sample preparation and compensate matrix effects. Method validation illustrated excellent linearity, with correlation coefficients (R2) higher than 0.999. Satisfactory recoveries of target compounds spiked in green tea, black tea and oolong tea ranged from 87.6% to 119.9% with intra- and inter-day precisions below 20%. Limits of quantification of cartap and nereistoxin were 10.0 µg kg-1, and limits of detection were 2.0 µg kg-1 for cartap and 4.0 µg kg-1 for nereistoxin. The developed method was applied to determine cartap and nereistoxin in thirty tea samples.

Identification of new interferences leached from plastic microcentrifuge tubes in electrospray ionization mass spectrometry.

RATIONALE: The incredible sensitivity of the modern mass spectrometry instrument enables scientists to detect a large number of molecules ranging from small organic compounds to biological macromolecules. However, the same sensitivity often throws up challenges with respect to background interferences and contaminants. The identification and source of these contaminants is very important for reducing background contamination and ensuring the accuracy of the analysis results. METHODS: The interfering compounds were analyzed by high-performance liquid chromatography coupled with a hybrid quadrupole-orbitrap mass spectrometer. The structural analysis was conducted by obtaining the accurate masses of precursors and their fragment ions. The retention time and MS/MS spectrum of one of the interfering compounds (N-lauryldiethanolamine) were compared with an authentic standard to reach an unequivocal structural assignment. RESULTS: The interferences (m/z 274 and 318 in positive mode) were observed during the analysis of herbicides in tea samples by electrospray ionization mass spectrometry (ESI-MS). Their structures were identified to be N-lauryldiethanolamine and N-(2-hydroxyethyl)-N-(2-(2-hydroxyethoxy)ethyl)dodecylamine by fragmentation interpretation and further confirmed by a standard compound. These interferences were found to be leached from the plastic microcentrifuge tubes used during sample pretreatment. The plastic tubes from two of the five suppliers tested were found to contain these two interferences. CONCLUSIONS: In this work, we presented an example about the observation, identification and source of interferences in ESI-MS. The N-lauryldiethanolamine and other ethoxylated aliphatic alkylamines are common plastic antistatic agents. They possess high proton affinity so that they show a strong response in ESI positive mode. In order to avoid their interference during mass spectrometric analysis we need to choose plastic tubes (or other plastic materials) that do not contain such antistatic agents.

A method based on precolumn derivatization and ultra high performance liquid chromatography with high-resolution mass spectrometry for the simultaneous determination of phthalimide and phthalic acid in tea.

Phthalimide can be formed from either the degradation of folpet and phosmet, or reaction of phthalic anhydride with primary amino groups. Consequently, the sum of phthalimide and folpet, expressed as folpet-residue definition, is highly prone to false-positive levels of folpet in tea. An analytical method is thus urgently needed to investigate the residue level and source of phthalimide in tea. In this work, we developed an accurate method of determining phthalimide and phthalic acid (the indicator of phthalic anhydride) by acetonitrile extraction and 3-bromopropyltrimethylammonium bromide derivatization coupled with ultra high performance liquid chromatography and high-resolution mass spectrometry. The method was validated, and linearity (correlation coefficients > 0.99) was obtained. Satisfactory recoveries at 10, 20, 50, and 100 µg/kg ranged from 76 to 117%, and the intra- and interday accuracies were <23%. The limit of quantification for phthalimide and phthalic acid was 10 µg/kg. The developed method was further successfully used to determine phthalimide and phthalic acid in some tea samples. The positive rate of phthalimide and phthalic acid detected in the tea samples ranged from 30-75 and 50-90%, respectively.

Enantioselectivity and residue analysis of fipronil in tea (Camellia sinensis) by ultra-performance liquid chromatography Orbitrap mass spectrometry.

This study aimed to determine the residues of fipronil, metabolites, and enantiomers in tea (Camellia sinensis) during tea planting and green tea manufacture. An AD-RH chiral column was used to separate the fipronil enantiomers. During tea planting, the half-lives of the sum of fipronil and metabolites were similar to those of fipronil, which were 2.37 and 3.88 days for tea shoots and mature leaves, respectively. The residues of fipronil and its metabolites increased 2.3-3.6-fold during green tea manufacture. The values for the processing factors of fipronil and metabolites ranged from 1.0 to 2.1. A slightly significant enantioselectivity of (R)- and (S)-fipronil was observed during tea planting and green tea manufacturing. The residue pattern indicated that fipronil should not be applied in tea gardens due to its long persistence. The maximum residual limits of fipronil and metabolites at 2 µg kg-1 were considered optimal.

Development and validation of an ultra performance liquid chromatography Q-Exactive Orbitrap mass spectrometry for the determination of fipronil and its metabolites in tea and chrysanthemum.

A fast, sensitive and reliable method for the determination of fipronil and its metabolites in tea and chrysanthemum was developed using a modified QuEChERS technique and an ultra performance liquid chromatography Q-Exactive Orbitrap mass spectrometry. The mixture of adsorbents containing primary secondary amine (PSA), octadecylsilane (C18) and carbon nanotubes (CNTs), was used as QuEChERS adsorbents. The use of mass resolution at 70000 full width at half maximum (FWHM) and narrow mass windows at 5 ppm achieved high selectivity and repeatability. Satisfactory linearity with correlative coefficient (R2) higher than 0.996 was achieved for all compounds. Recoveries at three levels (2, 10 and 50 µg kg-1) ranged from 86% to 112%, while the intra- and inter-day accuracies were less than 15%. Limits of quantification for fipronil and its metabolites were 2 µg kg-1, which fulfils the requirement of maximum residue limits formulated by European Union and Japan.

Residue pattern of polycyclic aromatic hydrocarbons during green tea manufacturing and their transfer rates during tea brewing.

Residues of polycyclic aromatic hydrocarbons (PAHs) in green tea and tea infusion were determined using gas chromatography-tandem mass spectrometry to study their dissipation pattern during green tea processing and infusion. Concentration and evaporation of PAHs during tea processing were the key factors affecting PAH residue content in product intermediates and in green tea. PAH residues in tea leaves increased by 2.4-3.1 times during the manufacture of green tea using the electric heating model. After correction to dry weight, PAH residue concentrations decreased by 33.5-48.4% during green tea processing because of PAH evaporation. Moreover, spreading and drying reduced PAH concentrations. The transfer rates of PAH residues from green tea to infusion varied from 4.6% to 7.2%, and PAH leaching was higher in the first infusion than in the second infusion. These results are useful for assessing exposure to PAHs from green tea and in formulating controls for the maximum residue level of PAHs in green tea.

Neutral losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]+ ions of methoxyfenozide and tebufenozide via intramolecular rearrangement in electrospray ionization tandem mass spectrometry.

RATIONALE: Electrospray ionization (ESI) tandem mass spectrometry can be applied to determine structural information about organic compounds. The [M + Na]+ ion is one of the major precursor ions in ESI mass spectrometry, but its fragmentation mechanism study is still insufficient. This study reveals the interesting fragmentation reactions of the [M + Na]+ ions of methoxyfenozide and tebufenozide. METHODS: The fragmentations of the [M + Na]+ , [M + Li]+ , and [M + H]+ ions of methoxyfenozide and tebufenozide were studied using a hybrid quadrupole-orbitrap mass spectrometer and an ion trap mass spectrometer. A hydrogen/deuterium (H/D)-exchange experiment in the amide group of methoxyfenozide allowed for the confirmation of the fragmentation mechanism. Density functional theory (DFT) calculations were performed for a further understanding of the fragmentation mechanism of the [M + Na]+ ion of methoxyfenozide. RESULTS: Neutral losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]+ ions of methoxyfenozide and tebufenozide were observed as the major fragmentation pathways. In contrast, similar fragmentations were not observed or minor pathways in the fragmentation of the [M + Li]+ and [M + H]+ ions of methoxyfenozide and tebufenozide. In addition, a minor product ion resulting from loss of NaOH was identified, which was the first reported example in the fragmentation of sodiated compounds in mass spectrometry. CONCLUSIONS: Losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]+ ions of methoxyfenozide and tebufenozide are proposed to be formed through an intramolecular rearrangement reaction, which is supported by DFT calculations. An H/D-exchange experiment confirms that the carboxyl hydrogen of benzoic acid and the hydrogen of NaOH exclusively derive from the amide hydrogen of the precursor ion. This study enriches our knowledge on the Na+ -induced fragmentation reactions. Copyright © 2016 John Wiley & Sons, Ltd.

Multiresidue Method for the Rapid Determination of Pesticide Residues in Tea Using Ultra Performance Liquid Chromatography Orbitrap High Resolution Mass Spectrometry and In-Syringe Dispersive Solid Phase Extraction.

A method based on in-syringe dispersive solid phase extraction (IS-D-SPE) and ultra performance liquid chromatography Orbitrap high resolution mass spectrometry for the multiresidue analysis of 117 pesticides in tea was developed. Full scan mode was acquired over an m/z range of 100-800 with Orbitrap resolution at 70000, followed by full scan/dd-MS2 mode for confirmation. The identification criteria of retention time and mass accuracy tolerance was ±0.20 min and ±5.0 ppm, respectively. MS/MS fragment ions obtained dd-MS2 were necessary to identify the pesticides with the same molecular mass weight. The IS-D-SPE technique involved a mixture of 200 mg PSA, 100 mg C18, and 15 mg multiwalled carbon nanotubes for the cleanup of tea matrix. Good linearity (R2 > 0.99) for 117 pesticides was obtained. Satisfactory recoveries in the range of 70-120% were obtained for 105 pesticides, while intraday and interday precisions were below 20%. Limits of quantification were generally 10 µg kg-1. Finally, this method was employed to analyze 117 pesticides in 70 tea samples.

Occurrence and Residue Pattern of Phthalate Esters in Fresh Tea Leaves and during Tea Manufacturing and Brewing.

The residues of 16 phthalate esters (PAEs) in fresh tea leaves and made tea were determined via gas chromatography-tandem mass spectrometry to study their distribution and degradation characteristics during tea planting and processing. Five PAEs were detected in all fresh tea leaves, and higher concentrations were detected in mature leaves. The distribution of PAEs in fresh tea leaves ranged from 69.7 to 2244.0 µg/kg. The degradative percentages of ∑5PAEs during green tea manufacturing ranged from 61 to 63% and were significantly influenced by the drying process. The transfer rates of PAEs-D4 ranged from 5.2 to 100.6%. PAEs with a high water solubility showed the highest transfer coefficient in the range of 91.8-100.6%, whereas PAEs with a high log Kow showed a low leaching efficiency below 11.9%. These results benefit the risk evaluation and establishment of a maximum residue limit for PAEs in tea.