The analysis of aroma/flavor compounds in green tea using ice concentration linked with extractive stirrer

Ultratrace analysis of per- and polyfluoroalkyl substances in drinking water using ice concentration linked with extractive stirrer and high performance liquid chromatography - tandem mass spectrometry

Detection of drinking water contaminants is vital to the protection of human health. One group of contaminants that have recently generated serious concerns over health risks are per- and polyfluoroalkyl substances (PFAS). These compounds are very bio-persistent, leading to their detection in all types of water sources, including drinking water. While analysis of drinking water for PFAS is important, it is currently arduous to detect ultratrace levels of these contaminants. Specifically, current ultratrace PFAS analysis methods are difficult, costly, require large sample volumes, and consume relatively large volumes of organic solvent. In the present work, an analytical method using Ice Concentration Linked with Extractive Stirrer (ICECLES) and high performance liquid chromatography-tandem quadrupole mass spectrometry (HPLC-MS/MS), was developed and validated to provide simple and ultratrace analysis of drinking water for 14 PFAS. The method featured a relatively low sample volume requirement (10 mL), automated extraction, minimal matrix effects, and minimal organic solvent use (i.e., the method requires only 50 µL of methanol per sample). The method produced a wide linear range of 0.5 to 500 ng/L, ultratrace limits of detection (0.05 to 0.3 ng/L), and good accuracy and precision (i.e., 87 to 108% accuracy and ≤19% relative standard deviation as a measure of precision). This method was tested on drinking water samples from across the United States and detected at least one PFAS compound in 52 of the 53 drinking water samples tested. Perfluorohexanoic acid (PFHxA), perfluorooctanoic acid (PFOA), and perfluoroheptanoic acid (PFHpA) were detected in 89, 96, and 77% of the samples tested with maximum concentrations of 268 ng/L for PFHxA, 213 ng/L for PFOA, and 75.7 ng/L for PFHpA. Additionally, perfluorononanoic acid, perfluorodecanoic acid, and perfluoroheptanoic acid were each detected in at least one drinking water sample at concentrations > 20 ng/L. The availability of the method presented here allows ultratrace detection of PFAS while circumventing many of the disadvantages of current methods.

Stir bar sorptive extraction and its application

Recent progress of stir bar sorptive extraction (SBSE) in the past six years is reviewed. The preparation methods including electrodeposition, self-assembly, solvent exchange, physical magnetic adsorption and electrostatic spinning, for the coated stir bar are summarized and compared, specifically for a specific material for coatings fabrication, e.g., carbon-based materials and metal organic frameworks. The emerging materials (e.g., graphene, graphene oxide, carbon nanotubes, monolith, metal-organic frameworks and porous organic polymers) applied for coated stir bar fabrication are one of the focus of this review, along with their respective advantages in extraction process and application in trace analysis. The development and application of extraction apparatus of SBSE are also involved. Based on these information, the development status and prospects of SBSE as an efficient sample pretreatment technique in real sample analysis are discussed.

Enhanced extraction using a combination of stir bar sorptive extraction and thin film-solid phase microextraction

Sorptive extraction techniques have experienced increased popularity, but they face limitations in dynamic range and sensitivity. In this study, a new method combining stir bar sorptive extraction (SBSE) and thin-film solid-phase microextraction (TFSPME) was developed, and optimization for extraction temperature (70 °C) and time (120 min) was carried out. Polydimethylsiloxane (PDMS)-coated SBSE and PDMS/carboxen (PDMS/CAR)-coated TFSPME were used, and both headspace and direct immersion extraction modes were also studied. Using 40 selected volatile compounds, the combined method generally gave a wider linearity range with lower minimum limits (2 to 3 orders), satisfactory coefficient of determination (R²>0.980), and improved sensitivity when compared to SBSE-only or TFSPME-only techniques. Furthermore, despite the combined use of two extraction devices, the repeatability (<13.1 %) and reproducibility (<13.4 %) of the combined method were comparable to SBSE-only or TFSPME-only results. Higher recoveries of up to 20% were also achieved by the combined method. Compared to the conventional SBSE method, the new method provided superior performance in terms of dynamic range and sensitivity for compounds of various polarities. In conclusion, this study provided insights on the suitability of the various extraction methods for compounds of different chemical properties which could aid in future applications for volatiles analysis in food, biological, and environmental sectors.

Ultratrace analysis of atrazine in soil using Ice Concentration Linked with Extractive Stirrer and High Performance Liquid Chromatography-Tandem Mass Spectrometry

Atrazine is a widely-used pesticide with a relatively long half-life in the environment. This leads to persistent soil contamination with the potential of migration to ground and surface waters. Analysis of atrazine in soil is difficult due to the inherent complexity of soil as a sample matrix. Moreover, the moderate hydrophobicity of atrazine makes it difficult to extract into typical sorbent phases during sample preparation. Therefore, a method for the ultratrace determination of atrazine in soil using Ice Concentration Linked with Extractive Stirrer (ICECLES) and high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was developed to address these issues. For the method, soil samples (10 g) were initially extracted with methanol:water (8:2, v:v), followed by solvent exchange to 100% water. The samples then underwent ICECLES with back-extraction into 100% methanol prior to HPLC-MS/MS analysis. The ICECLES-HPLC-MS/MS method produced a wide linear range of 10 to 1000 ng/kg, featured excellent limits of quantification and detection of 10 and 5 ng/kg, respectively, and good accuracy (100 ± 12%) and precision (≤9.6% relative standard deviation). This method was tested on field soil samples and provided ultratrace detection of atrazine. With this method, previously unachievable low parts per trillion (ppt) detection of atrazine in soil is now possible.

Nutrition and aroma challenges of green tea product as affected by emerging superfine grinding and traditional extraction

Green tea, superfine green tea powder (SGTP), and tea extract were prepared to determine their chemical components and antioxidant activity. The nutrition and aroma challenges of green tea during traditional extract technique and superfine grinding were profiled in this study. The traditional extract technique took advantage over superfine grinding in L-theanine and glutamic acid (Glu) preservation, but not in other 16 essential amino acids. SGTP preserved a maximum of elements from green tea, whereas tea extract greatly changed the elements ratio. Tea extract contained higher VB2 and VC contents and doubled the tea polyphenol (TP) content in comparison with green tea and SGTP. Additionally, tea extract contained more favorable aroma compounds and maintained stronger antioxidant activity in comparison with green tea and SGTP. This study profiled an important basis for the comprehensive utilization of green tea resources by consumers and manufacturers.

Characterization of the key aroma compounds in Longjing tea using stir bar sorptive extraction (SBSE) combined with gas chromatography-mass spectrometry (GC-MS), gas chromatography-olfactometry (GC-O), odor activity value (OAV), and aroma recombination

Longjing tea is the most famous premium green tea, and is regarded as the national tea in China, with its attractive aroma contributing as a prime factor for its general acceptability; however, its key aroma compounds are essentially unknown. In the present study, volatile compounds from Longjing tea were extracted and examined using stir bar sorptive extraction (SBSE) combined with gas chromatography-mass spectrometry (GC-MS). Data obtained from the present study revealed that 151 volatile compounds from 16 different chemical classes were identified by GC-MS analysis. Enols (8096 µg/kg), alkanes (6744 µg/kg), aldehydes (6442 µg/kg), and esters (6161 µg/kg) were the four major chemical classes and accounted for 54% of the total content of volatile compounds. Geraniol (6736 µg/kg) was the most abundant volatile compound in Longjing tea, followed by hexanal (1876 µg/kg) and β-ionone (1837 µg/kg). Moreover, 14 volatile compounds were distinguished as the key aroma compounds of Longjing tea based on gas chromatography-olfactometry (GC-O) analysis, odor activity value (OAV) calculations, and a preliminary aroma recombination experiment, including 2-methyl butyraldehyde, dimethyl sulfoxide, heptanal, benzaldehyde, 1-octen-3-ol, (E, E)-2,4-heptadienal, benzeneacetaldehyde, linalool oxide I, (E, E)-3,5-octadien-2-one, linalool, nonanal, methyl salicylate, geraniol, and β-ionone. This is the first comprehensive report describing the aroma characterizations and the key aroma compounds in Longjing tea using SBSE/GC-MS. The findings from this study contribute to the scientific elucidation of the chemical basis for the aromatic qualities of Longjing tea.

Novel sample preparation approaches in gas chromatographic analysis: Promising ideas

The development of sample preparation procedures is still a dynamic process despite a number of already proposed techniques. The main challenge in this research field is to fully replace classical procedures like liquid-liquid extraction and solid-phase extraction in gas chromatographic analysis. Some progress has been already achieved for the last 20 years when miniaturized techniques were incorporated in ISO standards. The current review is focused on novel approaches in sample treatment that appeared since 2010. It includes research studies describing non-conventional instrumental design available to inspire future progress in the field. A combination of a few extraction principles and supporting with additional treatment are the main core suggested for improvement of sample preparation efficiency. This requires good compatibility of extraction media, assessment of multiple experimental parameters, and potential automatization possibilities.

Studies on the Origin of Carbons in Aroma Compounds from [13C₆]Glucose -Cysteine-(E)-2-Nonenal Model Reaction Systems

The thermal degradation of lipid oxidation products with amino acids and reducing sugars is known to be important for the characteristic aroma generation in both meat and meat-like process flavorings. SPME(solid phase microextraction)/GC-MS was used to analyze the volatiles produced from a solution of [¹³C₆]glucose, cysteine, and lipid degradation product- (E)-2-nonenal, heated at 130 °C for 90 min. Analysis of the mass spectra showed that the resulting 2-butyl-thiophene and 5-butyldihydro-2(3H)-furanone were ¹³C₆-labeled and hence stemmed from glucose. Glucose and (E)-2-nonenal were equally important for the formation of 2-pentylfuran, whether cysteine was present in the reaction or not. 2-Furanmethanol, (E)-2-(1-pentenyl)-furan, 2-hexanoylfuran, ethanethiol, 5-methyl-2(5H)-thiophenone, 1-methyl-5-mercaptotetrazole, 4-pentyl-pyridine, 2-pentyl-thiophene, and 2-mercaptopropanoic acid were virtually ¹³C₁-¹³C₄ labeled, suggesting an origin from both glucose and cysteine and/or (E)-2-nonenal carbons. Thus, the relative contribution of aldehyde to the C-skeleton of a particular aroma compound changed substantially when both glucose and cysteine were involved in its formation.

Recent Developments of Stir Bar Sorptive Extraction for Food Applications: Extension to Polar Solutes

Stir bar sorptive extraction (SBSE) is a miniaturized and solvent-less sample preparation method for extraction and concentration of organic compounds from aqueous samples. The method is based on sorptive extraction, whereby the solutes are extracted into a polymer, such as polydimethylsiloxane (PDMS), coated on a stir bar. Using an apolar PDMS coating, SBSE provides high recoveries for apolar solutes; however, SBSE recoveries for polar solutes are low. Although several more polar coatings for SBSE were developed, these extraction phases are mostly not compatible with thermal desorption (TD) and/or have inferior performance characteristics related to robustness, bleeding, stability, etc. compared to PDMS. In this perspective, two recently introduced SBSE approaches are described that can be used to extend the applicability of a PDMS coating to more polar solutes: (1) SBSE with freeze concentration [ice concentration linked with extractive stirrer (ICECLES)], which is based on the concentration of analytes by gradually reducing the phase ratio (sample/extraction phase), and (2) SBSE using a solvent-swollen PDMS [solvent-assisted SBSE (SA-SBSE)], which is based on a combination of polarity modification and volume increase by PDMS phase swelling using certain types of solvents while maintaining the original characteristics of the PDMS phase.