Subcritical water chromatography: A green approach to high-temperature liquid chromatography

High-temperature liquid chromatography-isotope ratio mass spectrometry methodology for carbon isotope ratio determination of anabolic steroids in urine

BACKGROUND

Gas Chromatography Isotope Ratio Mass Spectrometry (GC-C-IRMS) has long been used in routine laboratories to determine the δ13C values of anabolic steroids in urine, differentiating between, e.g., endogenous and synthetic testosterone (T) in sports doping control. Until now, liquid chromatography (LC-IRMS) has not been used. The LC-IRMS setup doesn't allow organic solvents or modifiers in the mobile phase for δ13C determinations. Mid-to non-polar analytes such as steroids can be analysed in water heated to High Temperatures (HT, up to 200 °C) because at 200 °C has a similar polarity as 80/20 methanol/water at ambient temperature. In this work, we developed a method for steroids in urine, extending the application of the LC-IRMS to non-polar analytes in complex matrices.

RESULT

An HT-LC-IRMS method capable of determining the δ13C values of four steroids (i.e., testosterone (T), 5α-androstane-3α,17β-diol (ααβ), 5β-androstane-3α,17β-diol (βαβ) and pregnanetriol (PT)) in urine was developed and validated. Accuracy ranged from 0.23 ‰ (ααβ and βαβ) to 0.49 ‰ (T), and the detection limit was set at 10 ng mL-1 (T, ααβ+βαβ). The validation data and a comparison of authentic urine samples analysed with HT-LC-IRMS and GC-C-IRMS indicated a comparable performance between HT-LC-IRMS and GC-C-IRMS.

SIGNIFICANCE

HT-LC-IRMS can be used to determine δ13C values of anabolic steroids, extending the applicability of both HT-LC and LC-IRMS to non-polar substances determined in a complex matrix in routine laboratory practice.

Exploring the green frontier: Subcritical water chromatography for sustainable analytical practices

Water in the subcritical state is characterized by properties significantly different from water under standard conditions. These include low viscosity, low surface tension, and a much lower dielectric constant, increasing the solubility of nonpolar substances. For this reason, it can provide an alternative solvent and be used in chromatographic techniques-subcritical water chromatography (SBWC). SBWC appears to be one of the greenest analytical techniques until we unravel chromatography with pure water at room temperature. The versatility of SBWC is explored through its applications in the separation and analysis of a wide range of compounds, including pharmaceuticals, natural products, etc. The use of subcritical water as a mobile phase requires suitable stable stationary phases and special apparatus. Still, it makes it possible to conduct analyses without using organic solvents. When using this technique, it is important to remember that it suits the analysis of thermally stable substances. The following work is a critical review of developments in SBWC.

Fabrication process development and basic evaluation of eggshell-based column packing material for reversed-phase preparative separation

Purification of basic drugs in reversed-phase mode is often difficult, mainly due to adsorption of positively charged compounds to the silica gel-based stationary phase. Since this adsorption can be suppressed under alkaline condition, columns with alkali-resistance are required. In addition, compounds with acid-sensitive structures are sometimes degraded during separation on silica gel-based columns which exhibit acidity due to their surface structure. We prepared an alkali-resistant reversed-phase packing material, Eggshell-PMAcO based on eggshells modified with an amphiphilic copolymer, poly(maleic acid-alt-1-octadecene) (PMAcO). The height equivalent to a theoretical plate (HETP) of the Eggshell-PMAcO column was improved by surface treatment with ammonium acetate buffer (900 mM, pH = 3.7), which is an inexpensive reagent, and the retention behavior for hydrophobic compounds was compared to a typical ODS column based on silica gel, resulting in sufficient selectivity of the eggshell-based column for hydrophobic compounds, as indicated by the ratio of retention factors of pentylbenzene and butylbenzene (Eggshell-PMAcO column: 1.55, ODS column: 1.65). Column temperature-dependent retention behavior of naphthalene was investigated in the temperature range from 25 °C to 45 °C, followed by the calculation of thermodynamic parameters. There was little difference in the standard molar enthalpy (Eggshell-PMAcO: -19.6 kJ/mol, ODS: -21.7 kJ/mol). The absolute value of the standard free Gibbs energy for the Eggshell-PMAcO column was much smaller than that of the ODS column (Eggshell-PMAcO: -0.284 kJ/mol, ODS: -13.0 kJ/mol), indicating that the Eggshell-PMAcO column had a weaker retention strength for naphthalene than the ODS column mainly due to the large difference in the standard molar entropy (Eggshell-PMAcO: -64.9 J/mol K, ODS column: -29.2 J/mol K). The retention capacities for imipramine under neutral (water/methanol) and alkaline (0.1% triethylamine water/methanol) conditions were 0.2 mg and 5 mg, respectively, based on injection mass-dependent HETP, retention factor and symmetry factor. Finally, the prepared column was applied to the purification of a building block for nucleic acid drugs. This study demonstrated that reversed-phase columns, which can be fabricated from eggshells and an amphiphilic copolymer in an inexpensive and eco-friendly way, have the ability to purify basic compounds and acid-sensitive compounds.

Assessment of greenness for the determination of voriconazole in reported analytical methods

Analytical research with adverse environmental impact has caused a severe rise in concern about the ecological consequences of its strategies, most notably the use and emission of harmful solvents/reagents into the atmosphere. Nowadays, industries are searching for the best reproducible methods. Voriconazole is a second-generation azole derivative used effectively in the treatment of Candida and Aspergillus species infections and oropharyngeal candidiasis in AIDS patients. Recently it has become the drug of choice in treating mucormycosis in several countries, which raises the need for production in large quantities. The present review deals with various recent important analytical techniques used to estimate voriconazole and its combination in pharmaceutical formulations and biological fluids. The methods show their own unique way of analyzing voriconazole in different matrices with excellent linearity, detection, and quantification limits. Additionally, this article deals with methods and solvents analyzed for their impact on the environment. This is followed by estimating the degree of greenness of the methods using various available assessment tools like analytical eco-scale, national environmental method index, green analytical procedure index, and AGREE metrics to confirm the environmental impact. The scores obtained with the evaluation tools depict the quantum of greenness for the reported methods and provide an ideal approach adopted for VOR estimation. Very few methods are eco-friendly, which shows that there is a need for the budding analyst to develop methods based on green analytical principles to protect the environment.

Stationary Phases for Green Liquid Chromatography

Industrial research, including pharmaceutical research, is increasingly using liquid chromatography techniques. This involves the production of large quantities of hazardous and toxic organic waste. Therefore, it is essential at this point to focus interest on solutions proposed by so-called “green chemistry”. One such solution is the search for new methods or the use of new materials that will reduce waste. One of the most promising ideas is to perform chromatographic separation using pure water, without organic solvents, as a mobile phase. Such an approach requires novel stationary phases or specific chromatographic conditions, such as an elevated separation temperature. The following review paper aims to gather information on stationary phases used for separation under purely aqueous conditions at various temperatures.

Subcritical Water Extraction of Natural Products

Subcritical water refers to high-temperature and high-pressure water. A unique and useful characteristic of subcritical water is that its polarity can be dramatically decreased with increasing temperature. Therefore, subcritical water can behave similar to methanol or ethanol. This makes subcritical water a green extraction fluid used for a variety of organic species. This review focuses on the subcritical water extraction (SBWE) of natural products. The extracted materials include medicinal and seasoning herbs, vegetables, fruits, food by-products, algae, shrubs, tea leaves, grains, and seeds. A wide range of natural products such as alkaloids, carbohydrates, essential oil, flavonoids, glycosides, lignans, organic acids, polyphenolics, quinones, steroids, and terpenes have been extracted using subcritical water. Various SBWE systems and their advantages and drawbacks have also been discussed in this review. In addition, we have reviewed co-solvents including ethanol, methanol, salts, and ionic liquids used to assist SBWE. Other extraction techniques such as microwave and sonication combined with SBWE are also covered in this review. It is very clear that temperature has the most significant effect on SBWE efficiency, and thus, it can be optimized. The optimal temperature ranges from 130 to 240 °C for extracting the natural products mentioned above. This review can help readers learn more about the SBWE technology, especially for readers with an interest in the field of green extraction of natural products. The major advantage of SBWE of natural products is that water is nontoxic, and therefore, it is more suitable for the extraction of herbs, vegetables, and fruits. Another advantage is that no liquid waste disposal is required after SBWE. Compared with organic solvents, subcritical water not only has advantages in ecology, economy, and safety, but also its density, ion product, and dielectric constant can be adjusted by temperature. These tunable properties allow subcritical water to carry out class selective extractions such as extracting polar compounds at lower temperatures and less polar ingredients at higher temperatures. SBWE can mimic the traditional herbal decoction for preparing herbal medication and with higher extraction efficiency. Since SBWE employs high-temperature and high-pressure, great caution is needed for safe operation. Another challenge for application of SBWE is potential organic degradation under high temperature conditions. We highly recommend conducting analyte stability checks when carrying out SBWE. For analytes with poor SBWE efficiency, a small number of organic modifiers such as ethanol, surfactants, or ionic liquids may be added.

Subcritical Water Extraction of Salvia miltiorrhiza

In this work, a green extraction technique, subcritical water extraction (SBWE), was employed to extract active pharmaceutical ingredients (APIs) from an important Chinese medicinal herb, Salvia miltiorrhiza (danshen), at various temperatures. The APIs included tanshinone I, tanshinone IIA, protocatechualdehyde, caffeic acid, and ferulic acid. Traditional herbal decoction (THD) of Salvia miltiorrhiza was also carried out for comparison purposes. Reproduction assay of herbal extracts obtained by both SBWE and THD were then conducted on Caenorhabditis elegans so that SBWE conditions could be optimized for the purpose of developing efficacious herbal medicine from Salvia miltiorrhiza. The extraction efficiency was mostly enhanced with increasing extraction temperature. The quantity of tanshinone I in the herbal extract obtained by SBWE at 150 °C was 370-fold higher than that achieved by THD extraction. Reproduction evaluation revealed that the worm reproduction rate decreased and the reproduction inhibition rate increased with elevated SBWE temperatures. Most importantly, the reproduction inhibition rate of the SBWE herbal extracts obtained at all four temperatures investigated was higher than that of traditional herbal decoction extracts. The results of this work show that there are several benefits of subcritical water extraction of medicinal herbs over other existing herbal medicine preparation techniques. Compared to THD, the thousand-year-old and yet still popular herbal preparation method used in herbal medicine, subcritical water extraction is conducted in a closed system where no loss of volatile active pharmaceutical ingredients occurs, although analyte degradation may happen at higher temperatures. Temperature optimization in SBWE makes it possible to be more efficient in extracting APIs from medicinal herbs than the THD method. Compared to other industrial processes of producing herbal medicine, subcritical water extraction eliminates toxic organic solvents. Thus, subcritical water extraction is not only environmentally friendly but also produces safer herbal medicine for patients.

Stability and Extraction of Vanillin and Coumarin under Subcritical Water Conditions

In order to facilitate the development of the green subcritical water chromatography technique for vanillin and coumarin, the stability of the compounds under subcritical water conditions was investigated in this work. In addition, their extraction from natural products was also studied. The stability experiments were carried out by heating the mixtures of vanillin and water or coumarin and water at temperatures ranging from 100 °C to 250 °C, while subcritical water extractions (SBWE) of both analytes from vanilla beans and whole tonka beans were conducted at 100 °C to 200 °C. Analyte quantification for both stability and extraction studies was carried out by HPLC. After heating for 60 min, vanillin was found to be stable in water at temperatures up to 250 °C. While coumarin is also stable at lower temperatures such as 100 °C and 150 °C, it undergoes partial degradation after heating for 60 min at 200 °C and higher. The results of this stability study support green subcritical water chromatographic separation and extraction of vanillin and coumarin at temperatures up to 150 °C. The SBWE results revealed that the extraction efficiency of both analytes from vanilla beans and tonka beans is significantly improved with increasing temperature.

High-Temperature Liquid Chromatography and the Hyphenation with Mass Spectrometry Using High-Pressure Electrospray Ionization

Increasing the operating temperature of the liquid chromatography (LC) column has the same effect as reducing the diameter of the packing particles on minimizing the contribution of C-term in the van Deemter equation, flattening the curve of plate height vs. linear velocity in the high-speed region, thus allowing a fast LC analysis without the loss of plate count. While the use of smaller particles requires a higher pumping pressure, operating the column at higher temperature reduces the pressure due to lower liquid viscosity. At present, the adoption of high-temperature LC lags behind the ultra-high-pressure LC. Nevertheless, the availability of thermally stable columns has steadily improved and new innovations in this area have continued to emerge. This paper gives a brief review and updates on the recent advances in high-temperature liquid chromatography (HTLC). Recent efforts of hyphenating the capillary HTLC with mass spectrometry via a super-atmospheric pressure electrospray ionization is also reported.

Determination of liquid chromatography/flame ionization detection response factors for alcohols, ketones, and sugars

In the past, the main focus of flame ionization detector (FID) response studies was set on investigations of gas chromatography (GC) relevant analytes such as aliphatic hydrocarbons and selected functional groups. Only a few data are available for liquid chromatography (LC)/FID responses. Within this research paper, we present the FID response factors for a LC/FID system with an aqueous eluent as mobile phase. The study focus on the most common analytes of LC/FID studies in the past as well as several compounds that are not directly GC compatible because of their polarity. Furthermore, the range of substances was extended to isomers, poly-alcohols, and sugars to obtain more detailed information of the influence of hydroxyl groups on the recorded response. The data show a group-specific correlation of response factors with a correlation coefficient (R²) for, e.g., alcohols and ketones of 0.99. Constant contribution factors of functional groups as mentioned in several GC/FID response studies and prediction models were observed to a limited extent. Interactions of sugar analytes with water showed that transfer of GC/FID to LC/FID data cannot be done in general. The underlying mechanisms revealed several new aspects, which have to be taken into account for future response prediction models, especially of small molecules. Interactions between eluent and analytes show that LC/FID response prediction is more complex and requires more than simple addition of functional group contributions.

Hyphenation of high-temperature liquid chromatography with high-pressure electrospray ionization for subcritical water LC-ESI-MS

High-pressure electrospray ionization (HP-ESI) performed under super-atmospheric pressure allows a stable and efficient electrospray of pure aqueous and/or superheated solutions even under a μL min-1 flow rate regime. In this paper, we report the direct coupling of the HP-ESI source to high-temperature liquid chromatography (HT-LC) operated at ≤30 μL min-1 flow rates. In addition to ESI, the ion source functions as a back-pressure regulator to keep the mobile phase in the liquid phase when the column is heated to >100 °C. Under an ion source pressure of 7 bar, the LC column can be operated up to 160 °C. LC is performed under isocratic elution, and besides the isothermal mode, the temperature of the column can also be programmed to increase the selectivity while keeping the ion source at a constant temperature. For a given solution flow rate, the analytical time can be shortened by increasing the column temperature. HT-LC-ESI-MS using pure water as the mobile phase with a capillary column is also demonstrated.

Separation and Analysis of Aspirin and Metformin HCl Using Green Subcritical Water Chromatography

Organic solvents are widely used in pharmaceutical and chemical industry for chromatographic separations. In recent years, subcritical water chromatography (SBWC) has shown ability in replacing hazardous organic solvents used in traditional high-performance liquid chromatography (HPLC). In this work, a pain killer—aspirin—and an antidiabetic drug—metformin HCl—were successfully separated on an XBridge C18 column using no organic solvents in the subcritical water chromatography mobile phase. Both traditional HPLC and subcritical water chromatography were used for comparison purposes. SBWC separation of metformin HCl and aspirin were achieved at 95 °C and 125 °C, respectively. The recovery for both active pharmaceutical ingredients (APIs) obtained by SBWC is 99% in comparing with the stated content of each drug. The relative standard deviation is less than 1% for SBWC assays developed in this work. This level of accuracy and precision achieved by SBWC is the same as that resulted by the traditional HPLC analysis.

Subcritical Water Chromatography with Electrochemical Detection

Reverse phase liquid chromatography (RPLC) is a commonly used separation and analysis technique. RPLC typically employs mixtures of organic solvents and water or aqueous buffers as the mobile phase. With RPLC being used on a global scale, enormous quantities of organic solvents are consumed every day. In addition to the purchasing cost of the hazardous solvents, the issue of waste disposal is another concern. At ambient temperature, water is too polar to dissolve many organic substances. Therefore, although water is nontoxic it cannot be used to replace the mobile phase in RPLC since organic analytes will not be eluted. Subcritical water chromatography may be an alternative. The characteristics of water, such as polarity, surface tension, and viscosity, can be altered by manipulating water’s temperature, thus making it behave like an organic solvent. The aim of this study was to evaluate the feasibility of separation using water mobile phase and detection by an electrochemical (EC) detector. The classes of analytes studied were neurotransmitters/metabolites, nucleic acids/heterocyclic bases, and capsaicinoids. Both isothermal and temperature-programmed separations were carried out. The separation temperature ranged from 25 to 100 °C. For separations of all three classes of solutes, the retention time was decreased with increasing temperature, thus shortening the analysis time. The peaks also became narrower as temperature increased. The limit of detection of neurotransmitters/metabolites ranges from 0.112 to 0.224 ppm.

Characteristics of Subcritical Water Extraction and Kinetics of Pentacyclic Triterpenoids from Dry Loquat (Eriobotrya japonica) Leaves

In this study, pentacyclic triterpenoids (PTTs) were extracted from loquat leaves by subcritical water extraction (SWE) technique in a dynamic mode. The results revealed that PTTs yield increased up to 5.38±0.12 mg/g with the increasing temperature at 180 °C for corosolic acid (CA) and up to 7.20±0.11 mg/g at 200 °C for ursolic acid (UA) within experimental times. The optimum flow rates to obtain concentrated CA and UA extracts were found to be 33.33 and 41.67 mL/min, respectively. Furthermore, extraction temperatures strongly influenced the extraction rate as demonstrated by the rate constant of each temperature tested. Moreover, the kinetic rate constant decreased as the function of temperature indicating the yield of both CA and UA were significantly influenced by subcritical temperatures and extraction times.

A comparison of hydrocarbon and alkali metal response in the flame ionization detector used in subcritical water chromatography

The flame ionization detector (FID) response toward alkali metals and hydrocarbons was compared. Optimal hydrogen flame gas flow rates were found near 40 mL/min for hydrocarbon response and 80 mL/min for alkali response. While each displayed a linear FID response, alkali metals produced several orders of magnitude greater detector sensitivity than hydrocarbons. Of note, KCl, NaCl, LiCl, and ethanol yielded respective FID sensitivity of about 7500, 980, 130, and 1 mV/μg analyte. This was subsequently demonstrated to greatly alter the FID response of organic salts. For example, while formic acid is normally unresponsive in an FID, its potassium salt could be readily detected here at picogram levels. Conversely, this phenomenon also rendered the FID unsuitable for use with buffered mobile phases containing such salts. In particular, FID background and baseline noise levels for formic acid – sodium formate buffers were about 10 times larger than equivalent experiments with methanol–water and up to two orders of magnitude larger than pure water. Overall, the results show that alkali metals respond much stronger in the FID than do hydrocarbons. Accordingly, their presence in organic analytes or mobile phases must therefore be accounted for when using this detector, particularly in areas such as subcritical water chromatography where it is commonly employed.

Studies on the stability of preservatives under subcritical water conditions

OBJECTIVE

The goal of this work was to further validate the subcritical water chromatography (SBWC) methods for separation and analysis of preservatives through the evaluation of analyte stability in subcritical water.

METHODS

In this study, the degradation of preservatives was investigated at temperatures of 100-200°C using two different approaches. First, the peak areas obtained by SBWC at high temperatures were compared with those achieved using the traditional high-performance liquid chromatography (HPLC) at 25°C. In the second approach, several preservatives and water were loaded into a vessel and heated at high temperatures for 30 or 60 min. The heated mixtures were then analysed by GC/MS to determine the stability of preservatives.

RESULTS

The t- and F-test on the results of the first approach reveal that the peak areas achieved by HPLC and SBWC are not significantly different at the 95% confidence level, meaning that the preservatives studied are stable during the high-temperature SBWC runs. Although the results of the second approach show approximately 10% degradation of preservatives into mainly p-hydroxybenzoic acid and phenol at 200°C, the preservatives studied are stable at 100 and 150°C. This is in good agreement with the validation results obtained by the first approach.

CONCLUSION

The findings of this work confirm that SBWC methods at temperatures up to 150°C are reliable for separation and analysis of preservatives in cosmetic and other samples.

Green chiral HPLC study of the stability of Chiralcel OD under high temperature liquid chromatography and subcritical water conditions

We report here the study of the stability under subcritical water conditions of one of the most popular polysaccharide chiral stationary phase (CSP): Chiralcel OD. This CSP was used under high temperature and reversed phase conditions with acetonitrile and 2-propanol as modifier, respectively. The evolution of selectivity and resolution was investigated both in normal and reversed mode conditions with five racemates after packing, heating at 150 °C and separations of some racemic compounds under different high temperatures and mobile phase conditions. The results show that after using at high temperature and subcritical water conditions the selectivity was only moderately affected while the resolution fell dramatically especially in reversed mode due to the creation of a void at the head of the columns which reflects the dissolution of the silica matrix.

Industrial application of green chromatography - II. Separation and analysis of preservatives in skincare products using subcritical water chromatography

Several high-temperature liquid chromatography (HTLC) and subcritical water chromatography (SBWC) methods have been successfully developed in this study for separation and analysis of preservatives contained in Olay skincare creams. Efficient separation and quantitative analysis of preservatives have been achieved on four commercially available ZirChrom and Waters XBridge columns at temperatures ranging from 100 to 200°C. The quantification results obtained by both HTLC and SBWC methods developed for preservatives analysis are accurate and reproducible. A large number of replicate HTLC and SBWC runs also indicate no significant system building-up or interference for skincare cream analysis. Compared with traditional HPLC separation carried out at ambient temperature, the HTLC methods can save up to 90% methanol required in the HPLC mobile phase. However, the SBWC methods developed in this project completely eliminated the use of toxic organic solvents required in the HPLC mobile phase, thus saving a significant amount of money and making the environment greener. Although both homemade and commercial systems can accomplish SBWC separations, the SBWC methods using the commercial system for preservative analysis are recommended for industrial applications because they can be directly applied in industrial plant settings.

Maximizing the speed of separations for industrial problems

Recent improvement efforts in chromatography have provided great improvements in the rate of plate production, but less attention has been spent on optimizing the kinds of problems that are most often encountered in industry. When factors are not independent in their effects on the responses of a chromatographic separation, all adjustable factors must be considered in concert in seeking the best or optimum condition that solves the problem. This requires careful attention to specifying the goals, the adjustable factors, and the constraints required to make sure the outcome can actually be implemented. Strategies for optimizing assay and screening methods in the context of industrial needs are presented. Expanding the factor space of the system being investigated can lead to better outcomes. The prospect of adding column-outlet pressure control and expanding the mobile phase composition to include condensed gases or supercritical fluids is explored. Reversed-phase liquid chromatography, hydrophilic interaction chromatography, electrostatic repulsion hydrophilic interaction chromatography, and supercritical fluid chromatography are contiguous with regard to mobile phase characteristics. Adjustment of selectivity through instrument-controlled factors can benefit method development. Opportunities obtained by blending modifiers, varying temperature and pressure with compressible mobile phases, and controlling pH are discussed in the context of optimizing methods.

Investigation of polar organic solvents compatible with Corona Charged Aerosol Detection and their use for the determination of sugars by hydrophilic interaction liquid chromatography

A range of organic solvents (ethanol, isopropanol and acetone) has been investigated as alternatives to acetonitrile and methanol when used in conjunction with Corona Charged Aerosol Detection (Corona CAD). These solvents have been evaluated with regard to their effect on the response of the Corona CAD. Three dimensional response surfaces were constructed using raw data showing the relationship between detector response, analyte concentration and percentage of organic solvent in the mobile phase, using sucralose or quinine as the test analyte. The detector response was non-linear in terms of analyte concentration for all solvents tested. However, detector response varied in an approximately linear manner with percentage of organic solvent over the range 0-40% for ethanol or isopropanol and 0-80% for acetone and methanol. The chromatographic performance of the various solvents when used as aqueous-organic mobile phases was evaluated for isocratic and gradient separations of sugars and sugar alcohols by hydrophilic interaction liquid chromatography (HILIC) using an Asahipak NH2P-504E column coupled with Corona CAD detection. It was found that whilst acetonitrile provided the highest column efficiencies and lowest detection limits of the solvents studied, acetone also performed well and could be used to resolve the same number of analytes as was possible with acetonitrile. Typical efficiencies and detection limits of 5330 plates m(-1) and 1.25 μg mL(-1), respectively, were achieved when acetone was used as the organic modifier. Acetone was utilised successfully as an organic modifier in the HILIC separation of carbohydrates in a beer sample and also for a partially digested dextran sample.

Liquid chromatography-flame ionisation detection using a nebuliser/spray chamber interface. Part 2. Comparison of functional group responses

The application of a LC-nebuliser/spray chamber interface-flame ionisation detection has been demonstrated for the superheated water liquid chromatography of a wide range of aliphatic and aromatic analytes. The linearity and sensitivity of the response of volatile and involatile analytes have been compared. The response of the detector toward different analytes is similar to that in GC-FID and for volatile analytes was comparable to UV detection. However, the responses from involatile analytes, such as amino acids and carbohydrates, were poor and often lower than for a refractive index detector.

Separation of sunscreens in skincare creams using greener high-temperature liquid chromatography and subcritical water chromatography

In this study, high-temperature liquid chromatographic (HTLC) and subcritical water chromatographic (SBWC) separations of sunscreens contained in skincare creams were achieved at temperatures ranging from 90 to 250°C. The columns employed in this work include a ZirChrom-DiamondBond-C18, a XTerra MS C18 and a XBridge C18 column. The quantity of methanol consumed by the greener HTLC sunscreen methods developed in this project is significantly reduced although the HTLC separation at this stage is not as efficient as that achieved by traditional HPLC. SBWC separation of sunscreens was also achieved on the XTerra MS C18 and the XBridge C18 columns using pure water at 230-250°C. Methanol was eliminated in the SBWC methods developed in this study.

High-temperature reversed-phase liquid chromatography coupled to isotope ratio mass spectrometry

Compound-specific isotope analysis (CSIA) by liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS) has until now been based on ion-exchange separation. In this work, high-temperature reversed-phase liquid chromatography was coupled to, and for the first time carefully evaluated for, isotope ratio mass spectrometry (HT-LC/IRMS) with four different stationary phases. Under isothermal and temperature gradient conditions, the column bleed of XBridge C(18) (up to 180 °C), Acquity C(18) (up to 200 °C), Triart C(18) (up to 150 °C), and Zirchrom PBD (up to 150 °C) had no influence on the precision and accuracy of δ(13) C measurements, demonstrating the suitability of these columns for HT-LC/IRMS analysis. Increasing the temperature during the LC/IRMS analysis of caffeine on two C(18) columns was observed to result in shortened analysis time. The detection limit of HT-RPLC/IRMS obtained for caffeine was 30 mg L(-1) (corresponding to 12.4 nmol carbon on-column). Temperature-programmed LC/IRMS (i) accomplished complete separation of a mixture of caffeine derivatives and a mixture of phenols and (ii) did not affect the precision and accuracy of δ(13)C measurements compared with flow injection analysis without a column. With temperature-programmed LC/IRMS, some compounds that coelute at room temperature could be baseline resolved and analyzed for their individual δ(13)C values, leading to an important extension of the application range of CSIA.

Extending the applicability of pressurized hot water extraction to compounds exhibiting limited water solubility by pH control: curcumin from the turmeric rhizome

Pressurized hot water extraction (PHWE, also known as subcritical water extraction) is commonly considered to be an environmentally friendly extraction technique that could potentially replace traditional methods that use organic solvents. Unfortunately, the applicability of this technique is often limited by the very low water solubility of the target compounds, even at high temperatures. In this paper, the scope for broadening the applicability of PHWE by adjusting the pH of the water used in the extraction is demonstrated in the extraction of curcumin (which exhibits very limited water solubility) from untreated turmeric (Curcuma longa L.) rhizomes. Although poor extraction yields were obtained, even at high temperatures when using degassed water or neutral phosphate buffer as the extraction medium, yields exceeding those obtained by Soxhlet extraction were achieved using highly acidic pH buffers due to curcumin protonation. The influence of the temperature, pH, and buffer concentration on the extraction yield were investigated in detail by means of a series of designed experiments. Optimized conditions for the extraction of curcumin from turmeric by PHWE were estimated at 197 °C using 62 g/L buffer concentration at pH 1.6. The relationships between these variables were subjected to statistical analysis using response surface methodology.