Simultaneous determination of amino acids in different teas using supercritical fluid chromatography coupled with single quadrupole mass spectrometry

The Impact of Water as an Additive on the Elution of Some Basic Organic Compounds in Supercritical Fluid Chromatography

In this study, water was used as an additive in the methanol-modified carbon dioxide-based eluent for the elution of some basic organic compounds from a hybrid silica column via supercritical fluid chromatography (SFC). The experiments were applied to sulfonamides, propranolol, and other organic nitrogen compounds involving aromatic rings from different classes of amine, pyrimidine, and purine with different pKa values (the pKa values for the studied analytes range from 4.6 to 10.4). The results revealed different responses to the different percentages of water addition. Adding 1~2% of water to the modifier (methanol) led to a positive effect manifested by more symmetrical peak shapes and reduced retention times for most compounds. The key factor for this improvement in the properties of chromatographic peaks is due to the adsorption of water on the silanol groups of the stationary phase, consequently resembling the phenomena observed in hydrophilic interaction liquid chromatography (HILIC). Moreover, the availability of hydrogen bond acceptor and donor sites in the analyte structure is an important factor to be considered when adding water as an additive to the modifier for improving the chromatographic peaks. However, introducing water in an amount higher than 3% resulted in perturbed chromatographic signals. It was also found that water as an additive alone could not successfully elute propranolol from the hybrid silica column with an acceptable peak shape; thus, the addition of a strong base such as amine salts was also necessary. The proposed use of a particular amount of water in the mobile phase could have a positive effect compared to the same mobile phase without water, improving the chromatographic peak properties of the elution of some basic organic compounds from the hybrid silica column.

Rapid profiling of cytokinins using supercritical fluid chromatography coupled with tandem mass spectrometry

BACKGROUND

The determination of plant hormones is still a very challenging analytical discipline, mainly due to their low concentration in complex plant matrices. Therefore, the involvement of very sensitive high-throughput techniques is required. Cytokinins (CKs) are semi-polar basic plant hormones regulating plant growth and development. Modern methods for CK determination are currently based on ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), which enables the separation of CK isomeric forms occurring endogenously in plants. Here, ultra-high performance supercritical fluid chromatography coupled with tandem mass spectrometry (UHPSFC-MS/MS) was used for the simultaneous determination of 37 CK metabolites.

RESULTS

The chromatographic conditions were tested on three different columns with various retention mechanisms. Hybrid silica modified with 2-picolylamine was selected as the stationary phase. Several parameters such as column temperature, back pressure regulation, mobile phase composition and make-up solvent were investigated to achieve efficient separation of CK isomers and reasonable sensitivity. Compared to UHPLC-MS/MS, a 9-min chromatographic analysis using a mobile phase of supercritical CO2 and 5 mM ammonia in methanol represents a three-fold acceleration of total run time. The quantification limit of UHPSFC-MS/MS method was in the range of 0.03-0.19 fmol per injection and the method validation showed high accuracy and precision (below 15 % for most analytes). The method was finally applied to the complex plant matrix of the model plant Arabidopsis thaliana and the obtained profiles of CK metabolites were compared with the results from the conventional UHPLC-MS/MS method.

SIGNIFICANCE

The presented work offers a novel approach for quantification of endogenous CKs in plants. Compared to the conventional UHPLC-MS/MS, the total run time is shorter and the matrix effect lower for the key CK metabolites. This approach opens the opportunity to utilize UHPSFC-MS/MS instrumentation for targeted plant hormonomics including other plant hormone families.

Applicability of supercritical fluid chromatography for oligonucleotide analysis: A proof-of-concept study

We evaluated the suitability of supercritical fluid chromatography (SFC) for oligonucleotide analysis using 4-mer oligonucleotides with various phosphorothioate (PS) contents as model compounds. Column screening showed that the diol-modified column was able to separate sequences with different PS contents. Optimization of the column body and additives allowed us to analyze polar oligonucleotides using SFC. Various sequences were also analyzed using the optimized method. A good peak shape was obtained when the guanine plus cytosine content of the analyte was two or less in the 4-mer oligonucleotides. Furthermore, we found that the retention times of the selected sequences were positively correlated with polar surface areas, indicating that oligonucleotides interact with polar stationary phases. In contrast, more hydrophobic full PS sequences were retained more strongly in the diol column than the full phosphodiester (PO) sequences. This suggests that the diol column has unique selectivity for PO and PS linkages. These results indicate that SFC is potentially applicable to oligonucleotide analysis with a separation mechanism that is different from that of ion-pair reversed-phase liquid chromatography.

Evaluation of control of additive concentration in gradient analysis of supercritical fluid chromatography-coupled to tandem mass spectrometry

Mobile phase additives are used to improve retention behavior in chromatography. In supercritical fluid chromatography (SFC), for which supercritical fluid carbon dioxide (SF-CO2) is used as the main mobile phase, additives can only be added into the modifier. For that reason, when gradient analysis is performed by changing the modifier ratio to SF-CO2, the additive concentration in the mobile phase increases in parallel with the modifier ratio. In a preliminary study performed using the conventional SFC system, ammonium acetate was necessary to improve the peak shape of a polar steroid, dehydroepiandrosterone sulfate (DHEA-S), while the peak intensity of a non-polar steroid, progesterone, decreased by 78% compared to that in the absence of the additive in mobile phase when gradient elution was performed. Since ammonium acetate had both favorable and unfavorable effects on sensitive and simultaneous analysis of these two steroid compounds, a compromise between these effects had to be sought. A three-pump configuration of SFC was developed by adding a pump unit to SFC instrument, which enabled control of the additive concentration independently of the modifier ratio, for the purpose of investigating the additive effect in detail using both steroids as model compounds. The putative cause of the decrease in peak intensity of progesterone was excessively elevated additive concentration in gradient analysis. When the additive concentration in the mobile phase was controlled to ensure that it did not increase during gradient analysis, the peak intensities of progesterone, cortisol, corticosterone, and testosterone were 55%, 40%, 25%, and 17% higher than when the additive concentration was not controlled, respectively. On the other hand, the peak intensity of DHEA-S was almost identical between the conditions, with an increase of 2% with three-pump instrument. The three-pump configuration showed the potential to solve problems relating to the use of modifier additives by keeping their concentration constant in gradient SFC analysis.

Tea (Camellia sinensis (L.) Kuntze) as an emerging source of protein and bioactive peptides: A narrative review

Tea residues represent one of the major agricultural wastes that are generated after the processing of tea. They account for 21-28% of crude protein and are often discarded without the extraction of valuable proteins. Due to various bioactivity and functional properties, tea proteins are an excellent alternative to other plant-based proteins for usage as food supplements at a higher dosage. Moreover, their good gelation capacity is ideal for the manufacturing of dairy products, jellies, condensation protein, gelatin gel, bread, etc. The current study is the first to comprehend various tea protein extraction methods and their amino acid profile. The preparation of tea protein bioactive peptides and hydrolysates are summarized. Several functional properties (solubility, foaming capacity, emulsification, water/oil absorption capacity) and bioactivities (antioxidant, antihypertensive, antidiabetic) of tea proteins are emphasized.

Drug Adulteration Analysis Based on complexation with CD and Metal Ions Using Ion Mobility Spectrometry

Drug adulteration and contamination are serious threats to human health therefore, their accurate monitoring is very important. Allopurinol (Alp) and theophylline (Thp) are commonly used drugs for the treatment of gout and bronchitis, while their isomers hypoxanthine (Hyt) and theobromine (Thm) have no effect and affect the efficacy of the drug. In this work, the drug isomers of Alp/Hyt and Thp/Thm are simply mixed with α-, β-, γ-cyclodextrin (CD) and metal ions and separated using trapped ion mobility spectrometry-mass spectrometry (TIMS-MS). TIMS-MS results showed that Alp/Hyt and Thp/Thm isomers could interact with CD and metal ions and form corresponding binary or ternary complexes to achieve their TIMS separation. Different metal ions and CDs showed different separation effect for the isomers, among which Alp and Hyt could be successfully distinguished from the complexes of [Alp/Hyt+γ-CD + Cu-H]⁺ with separation resolution (R _P-P) of 1.51; whereas Thp and Thm could be baseline separated by [Thp/Thm+γ-CD + Ca-H]⁺ with R _P-P of 1.96. Besides, chemical calculations revealed that the complexes were in the inclusion forms, and microscopic interactions were somewhat different, making their mobility separation. Moreover, relative and absolute quantification was investigated with an internal standard to determine the precise isomers content, and good linearity (R ² > 0.99) was obtained. Finally, the method was applied for the adulteration detection where different drugs and urine were analyzed. In addition, due to the advantages of fast speed, simple operation, high sensitivity, and no chromatographic separation required, the proposed method provides an effective strategy for the drug adulteration detection of isomers.

Development and validation of an efficient HILIC-QQQ-MS/MS method for quantitative and comparative profiling of 45 hydrophilic compounds in four types of tea (Camellia sentences)

Hydrophilic constituents are significant for the taste and nutrition of tea, but their simultaneous quantification remains challenging due to the lack of efficient methods. Based on the hydrophilic interaction chromatography coupled with triple quadrupole-tandem mass spectrometry, this work developed and validated an efficient (8.5 min per run), sensitive (LOQ: 0.002-0.493 μg/mL) and accurate method. This method was successfully used to determine the contents of 45 hydrophilic constituents in Yunnan large-leaf tea. Umami amino acids and umami-enhanced nucleotides generally exhibited higher content in green tea and Pu-erh raw tea. By contrast, a few number of amino acids (e.g., proline and γ-aminobutyric acid) and most alkaloids and nucleosides showed significantly higher contents in black tea or Pu-erh ripen tea. By performing the orthogonal partial least squares discriminant analysis, classification models for distinguishing four types of tea, and green tea from Pu-erh raw tea were established.

Predicting the grades of Astragali radix using mass spectrometry-based metabolomics and machine learning

Astragali radix (AR, the dried root of Astragalus) is a popular herbal remedy in both China and the United States. The commercially available AR is commonly classified into premium graded (PG) and ungraded (UG) ones only according to the appearance. To uncover novel sensitive and specific markers for AR grading, we took the integrated mass spectrometry-based untargeted and targeted metabolomics approaches to characterize chemical features of PG and UG samples in a discovery set (n=16 batches). A series of five differential compounds were screened out by univariate statistical analysis, including arginine, calycosin, ononin, formononetin, and astragaloside Ⅳ, most of which were observed to be accumulated in PG samples except for astragaloside Ⅳ. Then, we performed machine learning on the quantification data of five compounds and constructed a logistic regression prediction model. Finally, the external validation in an independent validation set of AR (n=20 batches) verified that the five compounds, as well as the model, had strong capability to distinguish the two grades of AR, with the prediction accuracy > 90%. Our findings present a panel of meaningful candidate markers that would significantly catalyze the innovation in AR grading.

Emerging Separation Techniques in Supercritical Fluid Chromatography

Supercritical fluid chromatography (SFC) has unique separative characteristics distinguished from those of HPLC and gas chromatography. At present, SFC is widely used and there are many applications in various biological, medical, and pharmaceutical fields. In this review, we focus on recently developed novel techniques related to SFC separation including: new column stationary phases, microfluidics, two-dimensional separation, and gas-liquid separation. In addition, we discuss the application of SFC using a water-containing modifier to biological molecules such as amino acids, peptides, and small proteins that had been challenging analytes.

Antioxidant and antibacterial insights into the leaves, leaf tea and medicinal roots from Astragalus membranaceus (Fisch.) Bge

Used as traditional Chinese medicine, Astragalus membranaceus (Fisch.) Bge. (A. membranaceus) roots are also used as tonic food material in a wide range of applications, while the leaves are left in the field, unused. Therefore, comprehensively exploring and utilizing the leaves will inevitably reduce the associated resource waste and environment pollution. In this study, the plant leaves were processed into tea using green tea processing technology. Bioactive components, antioxidant and antibacterial activities of the Leaf Tea (LT) and Dry Leaves (DL) were studied, and compared to that of the Dry Roots (DR). The results showed that the polysaccharides content (POL) in the DR (20.44%) was twice as high as the DL (10.18%) and LT (8.68%). However, the DL contained 36.85% more water-soluble extracts (WSE), 35.09% more ethanol-soluble extracts (ESE), 409.63% more total flavonoid content (TFC), 221.01% more total phenolic content (TPC) and 94.34% more proteins, and the LT contained 26.21% more WSE, 40.64% more ESE, 326.93% more TFC, 191.90% more TPC and 37.71% more proteins. The total amino acid (AA) content in the DR was 8.89%, while in that of the DL and LT were 24.18% and 28.96% respectively, nearly 3-times higher than that of the DR. The antioxidant activity of DR was much lower than those of DL and LT, both of which had antioxidant activity closer to that of Vitamin C (VC) and the antioxidant activities were even stronger when the optimal concentration was reached. Except for Aspergillus niger and Staphylococcus aureus, the DL and DR exhibited inhibition activities to Salmonella, Bacillus subtilis, Escherichia coli and yeast, while the LT had antimicrobial activities against all the strains except for A. niger. In summary, compared with the most commonly used DR, the DL and LT from A. membranaceus contained higher bioactive components, and stronger antioxidant and antimicrobial activities. Producing leaf tea may be an appropriate way to economically and reasonably utilize the plant leaves which are by-products.

[Preparation of Fe3O4@BA-MOF magnetic solid-phase extraction material and its application to the detection of pesticide residues in tea]

Tea is one of the most popular beverages worldwide, and its quality is often affected by the excessive pesticide residues during production. During the detection of pesticide residues in tea by chromatography-mass spectrometry and other methods, a strong matrix effect attributed to tea polyphenols and pigments is observed, which seriously impacts the analysis results. In this study, Fe3O4 magnetic nanoparticles and boric acid-functionalized metal organic framework (BA-MOF) materials were combined to prepare a highly efficient adsorbent Fe3O4@BA-MOF for capturing tea polyphenols and pigments. An effective analysis method for pesticide residues in tea samples in combination with gas chromatography-mass spectrometry was established. The introduction of boronic acid ligands into the metal organic framework, as the recognition site of cis-diols, enhanced the polyphenol capture ability. Adsorption of the pigment in the matrix was achieved through π-π interactions between the MOF ligand and the pigment. This new material has significant advantages such as rapid magnetic separation, large surface area, and abundant functional sites. Fe3O4@BA-MOF was prepared by employing simple conditions and characterized by Fouriertransform infrared spectroscopy, scanning electron microscopy, and X-ray diffractometry to identify its functional groups and morphology. After investigating the adsorption effect of different doses of Fe3O4@BA-MOF adsorbents (5, 10, 30, 50, and 80 mg) on tea polyphenols, 50 mg of the adsorbent was added to the tea matrix and shaken thoroughly. The tea polyphenol content in the matrix solution was determined using an ultraviolet spectrophotometer. The polyphenols were reduced by 74.58% within 5 min. The effect of solution pH (2.0, 4.0, 6.0, and 7.0) on the adsorption efficiency was investigated, and pH 7.0 was chosen as the optimal condition. By adjusting the pH of the solution, Fe3O4@BA-MOF could be recycled, and it maintained the excellent adsorption performance after four cycles of use. The introduction of Fe3O4 magnetic nanoparticles led to rapid magnetic response characteristics during sample pretreatment and improved the pretreatment efficiency. In the actual application of tea pesticide detection, after Fe3O4@BA-MOF pretreatment, the average recovery rates of the ten pesticides were in the range of 75.8%-138.6%, and the RSD was in the range of 0.5%-18.7% (n=3). The Fe3O4@BA-MOF nanocomposite prepared by introducing the boric acid ligand into the MOF structure and incorporating Fe3O4 magnetic nanoparticles could specifically adsorb the tea polyphenol matrix. When applied to the detection of pesticide residues in tea, it purifies the matrix and improves the detection efficiency, thus being suitable for the detection and analysis of pesticides in tea.

Supercritical fluid chromatography using methacrylate-based monolithic column for the separation of polar analytes

A poly(N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine-co-ethylene dimethacrylate) monolith was prepared in a 100 × 2.0 mm id stainless-steel column and was investigated for supercritical fluid chromatography. Optimization of its porosity was performed by changing the conditions of polymerization. Then, the chemical group of this column was confirmed by Fourier transform infrared spectroscopy and elemental analysis. The morphology was characterized by scanning electron microscopy. The prepared column showed good repeatability based on the retention factor of adenine, uracil and cytosine to calculate their run-to-run, day-to-day, column-to-column, and batch-to-batch relative standard deviations. Those values were less than 1.9% (n = 10), 6.5% (n = 3), 5.6% (n = 3), and 1.7% (n = 3), respectively. In addition, the column was found to be stable within 3 and 10 days with relative standard deviations less than 6.5 and 8.5%, respectively. These results indicated that the columns showed good reproducibility and stability. Compared with liquid chromatogaphy, supercritical fluid chromatography provided better kinetic performance and selectivity. Finally, several neutral, acid, and basic polar analytes were utilized to test its application. The results demonstrated that the prepared column exhibited a good separation performance and showed great potential in supercritical fluid chromatography.

Development of a novel method for polar metabolite profiling by supercritical fluid chromatography/tandem mass spectrometry

Supercritical carbon dioxide (scCO₂), the main fluid in the mobile phase for supercritical fluid chromatography (SFC), is non-polar. The majority of polar compounds are little soluble in scCO₂, thereby rendering them poor candidates for achieving separation by carbon dioxide-based SFC. There is no reported method for the comprehensive analysis of hydrophilic metabolites by SFC with mobile phases comprising a high CO₂ ratio. In this study, we investigated the effect of additives in the modifier for enabling the application of SFC to profile diverse polar compounds for metabolomics. Eleven types of columns were screened by using proteinogenic amino acids as the model compounds. The addition of water and acids (formic acid and trifluoroacetic acid (TFA)) to the modifier was also investigated to improve the solubility of the polar compounds and mitigate the unfavorable interaction between the stationary phase and the polar compounds. A significant improvement in the peak shapes of the amino acids was observed upon addition of TFA. The CO₂/modifier ratio and TFA concentration in the mobile phases were investigated using the CROWNPAK CR-I (+) column, which showed the best performance during the column-screening. The CO₂/methanol/water/TFA ratio of 70/27/3/0.15 (v/v/v/v) was determined as the optimized mobile phase composition. Furthermore, the applicability of the optimized analytical method to other polar compounds was examined; 100 cationic and amphoteric compounds with predicted logP_ow values that ranged from -5.9 to 1.7 could be simultaneously analyzed without derivatization. Anionic compounds such as organic acids, phosphates, and sugars were excluded from the target analytes. Most of the previously reported SFC methods for analyzing polar compounds employ a gradient elution and require the use of high modifier ratios at 40% or more. In the proposed method, the use of water and TFA enabled the rapid and simultaneous analysis under isocratic elution within 10 min, even with a high CO₂ ratio of 70%. Additionally, a rat serum extract was analyzed using the optimized conditions, and 43 polar metabolites were successfully detected. This result demonstrates the applicability of the SFC/tandem mass spectrometry method to real samples.

Chromatographic analysis of biomolecules with pressurized carbon dioxide mobile phases - A review

Biomolecules like proteins, peptides and nucleic acids widely emerge in pharmaceutical applications, either as synthetic active pharmaceutical ingredients, or from natural products as in traditional Chinese medicine. Liquid-phase chromatographic methods (LC) are widely employed for the analysis and/or purification of such molecules. On another hand, to answer the ever-increasing requests from scientists involved in biomolecules projects, other chromatographic methods emerge as useful complements to LC. In particular, there is a growing interest for chromatography with a mobile phase comprising pressurized carbon dioxide, which can be named either (i) supercritical (or subcritical) fluid chromatography (SFC) when CO₂ is the major constituent of the mobile phase, or (ii) enhanced fluidity liquid chromatography (EFLC) when hydro-organic or purely organic solvents are the major constituents of the mobile phase. Despite the low polarity of CO₂, supposedly inadequate to solubilize such biomolecules, SFC and EFLC were both employed in many occasions for this purpose. This paper specifically reviews the literature related to the SFC/EFLC analysis of free amino acids, peptides, proteins, nucleobases, nucleosides and nucleotides. The analytical conditions employed for specific molecular families are presented, with a focus on the nature of the stationary phase and the mobile phase composition. We also discuss the potential benefits of combining SFC/EFLC to LC in a single gradient elution, a method sometimes designated as unified chromatography (UC). Finally, detection issues are presented, and more particularly hyphenation to mass spectrometry.

Analytical strategy coupled to chemometrics to differentiate Camellia sinensis tea types based on phenolic composition, alkaloids, and amino acids

Catechins, amino acids, and alkaloids are primary chemical components of tea and play a crucial role in determining tea quality. Their composition and content largely vary among different types of tea. In this study, a convenient chemical classification method was developed for six Camellia sinensis tea types (white, green, oolong, black, dark, and yellow) based on the quantification of their major components. Twenty-one free amino acids, 6 catechins, 2 alkaloids, and gallic acid in 24 teas were quantified using ultra-high-performance liquid chromatography (UHPLC). The total catechin contents in these tea samples ranged from 10.96 to 95.67 mg/g, while total free amino acid content ranged from 2.63 to 25.89 mg/g. Theanine (Thea) was the most abundant amino acid in all tea varieties. Catechin and amino acid levels in tea were markedly reduced upon fermentation of tea. Furthermore, high-temperature processing (roasting) during tea production induced degradation and epimerization of catechins, yielding epimerized catechins, simple catechins, and gallic acid. Principal component analysis revealed that major ester-catechins (EGCG and ECG), major amino acids (Thea), and major alkaloids (caffeine) are potential factors for distinguishing different types of tea. Linear discriminant analysis showed that 100% of teas were correctly classified in which (+)-catechin, ECG, EGC, gallic acid, GABA, cysteine, lysine, and threonine were the most discriminating compounds. This study shows that quantification of the major tea components combined with chemometric analysis, can serve as a simple, convenient, and reliable approach for classifying tea according to fermentation level. PRACTICAL APPLICATION: Different Camellia sinensis tea types can be produced worldwide but it is still challenging to know which chemical markers can be used to trace their production. in this paper we used a targeted methodology to classify six tea types (white, green, oolong, black, dark, and yellow) based on phenolic composition, alkaloids, and amino acids. The main chemical markers responsible for the discrimination were pinpointed with the use of chemometric tools.