Development of the transfer techniques for on-line high-performance liquid chromatography-capillary gas chromatography

SLE Single-Step Purification and HPLC Isolation Method for Sterols and Triterpenic Dialcohols Analysis from Olive Oil

The unsaponifiable fraction of oils and fats constitutes a very small fraction but it is an essential part of the healthy properties of some specific oils. It is a complex fraction formed by a large number of minor compounds and it is a source of information to characterize and authenticate the oil sample. Specially, the composition of sterols of any oil or fat is a distinctive feature of itself and, therefore, it has become a useful tool for detecting contaminants and adulterants in oils. A new supported liquid extraction (SLE) technique for the analysis and characterization of the unsaponifiable fraction of fats and oils is proposed. The SLE system includes, as a stationary phase, a combination of adsorbent materials which allow a highly purified unsaponifiable matter ready to be isolated by high performance liquid chromatography (HPLC) and quantified by gas chromatography (GC). This method ensures the removal of fatty acids, avoiding possible interferences and making the analysis of sterols and triterpenic dialcohols easier. The procedure uses a small sample size (0.2 g), reduces the volume of solvents and reagents, and reduces the handling of samples subjected to analytical control. All this is achieved without losing either precision—a relative standard deviation of each compound lower than the reference value (≤16.4%)—or recovery, being for all compounds higher than 88.00%. Therefore, this new technique represents a significant economic and time saving in business control laboratories, a larger productivity and enhancement of working safety.

On-line coupled reversed phase liquid chromatography and gas chromatography: a new sealing design for the TOTAD interface

Total elimination of the eluent resulting from the pre-separation step is the critical point when coupling LC to GC. As a helium flow is applied during transfer to eliminate the solvent, the interface used for linking the two chromatographic systems must be properly sealed to prevent gas leaks and to achieve an effective evaporation of the eluent. The aim of this work was to improve the performance of the Through Oven Transfer Adsorption Desorption (TOTAD) interface to remove the eluent coming from LC by modifying the way in which the injector sealing system is held in place. As with the original design, the new approach makes it possible to transfer high volumes at a high rate, but the proposed modification also simplifies the experimental work because the displacement risk of the sealing system is reduced. Analyses of an ester mixture by RPLC-GC were performed to confirm the applicability of the system modification. In this work, volumes of up to 5 ml, at flow rates as high as 2 ml/min, were transferred from LC to GC with almost complete solvent removal even when working in reversed phase mode in the LC step.

A quick method for identifying radiolytic hydrocarbons in low-fat-containing food

BACKGROUND

As radiation-induced alterations of the lipid fraction of foods are related to their initial fat content, concentrations of fat degradation products used as irradiation markers are expected to be lower when irradiating low-fat-containing foods. Thus the sensitivity required when applying analytical methods for identifying irradiation markers in foods eventually depends on their respective amounts of fat. The aim of this study was to perform the qualitative analysis of characteristic hydrocarbons resulting from irradiation of samples with a fat content as low as 25 g kg(-1).

RESULTS

A rapid extraction using a small amount of ethyl acetate was the unique sample pretreatment required to accomplish the analysis of radiolytic markers by using on-line coupling of reverse phase liquid chromatography and gas chromatography with mass spectrometry detection (RPLC/GC/MS). Efficient elimination of the large volumes (up to 2170 µL) directly transferred from LC to GC was achieved by optimising the operation mode of the through-oven transfer adsorption/desorption system used as interface.

CONCLUSION

The reported procedure allowed confirmation, in less than 65 min, of the occurrence of up to five irradiation markers, namely n-pentadecane, 1-hexadecene, 1,7-hexadecadiene, n-heptadecane and 8-heptadecene, in cooked ham irradiated at doses as low as 2 kGy.

Enantiomeric analysis of β-pinene and limonene by direct coupling of reversed phase liquid chromatography and gas chromatography using absorbents as packing materials

A method based on the use of absorbents as packing materials inside the interface of the online coupling between RPLC and GC is proposed for the enantiomeric analysis of beta-pinene and limonene in essential oils. For that purpose, a comparison of the RSD, detection limit and recovery provided by two absorbents and one adsorbent is included in this study. The results found in this work proved the validity of absorbents as packing materials in online RPLC-GC to determine minor compounds in complex matrices. In particular, PDMS seemed to be specially useful to analyse nonpolar compounds, such as beta-pinene and limonene, since it provided higher sensitivity for this kind of compounds. The developed method was applied to the evaluation of the natural and non-natural character of commercial essential oils by means of the determination of the enantiomeric composition of beta-pinene and limonene.

Direct analysis of pesticide residues in olive oil by on-line reversed phase liquid chromatography-gas chromatography using an automated through oven transfer adsorption desorption (TOTAD) interface

A fully automated on-line reversed phase liquid chromatography-gas chromatography system is described. The system uses a prototype of the automated through oven transfer adsorption desorption interface. The system is demonstrated by presenting a new rapid method for the determination of pesticide residue in olive oil, which is injected directly with no sample pretreatment step other than filtration. Methanol:water is used as the eluent in the LC preseparation step, while the LC fraction containing the pesticide is automatically transferred to the gas chromatograph. Detection limits of pesticides varied from 0.18 to 0.44 mg/L when a flame ionization detector was used. As an example, relative standard deviation and linear calibration are presented for terbutryne.

On-line coupled liquid chromatography-gas chromatography

On-line coupled liquid chromatography-gas chromatography (LC-GC) is a powerful technique that combines the best features of LC and GC and is ideal for the analysis of complex samples. This review describes the unique features of on-line coupled LC-GC. The different interfaces and evaporation techniques are presented, along with their advantages and disadvantages. Guidelines are given for selecting a suitable LC-GC technique and representative applications are noted.

Solid-phase extraction coupling to capillary electrophoresis with emphasis on environmental analysis

This article reviews the status of solid-phase extraction (SPE) coupled with capillary electrophoresis (CE). It focuses on some of the organic pollutants which have captured the interest of analytical chemists--phenols, surfactants, dyes, polynuclear aromatic hydrocarbons (PAHs), aromatic and aliphatic amines, aromatic acids and aromatic sulfonic acids--and, in particular, on monitoring pesticides from different sources. It shows that the coupling of SPE to CE has considerable potential in the analysis of environmental pollutants.

Efficiency through combining high-performance liquid chromatography and high resolution gas chromatography: progress 1995-1999

Progress during the last 5 years in on-line LC-GC and related techniques is reviewed. In normal-phase LC-GC, the wire interface proved to have advantages over the loop type interface. Further investigations on the solvent evaporation process in an uncoated precolumn under conditions of an early vapour exit revealed that the rules for the transfer by the retention gap techniques must be modified. For reversed-phase LC-GC, approaches with a phase transfer compete with direct evaporation. Eluents were extracted into a bed of Tenax located in a programmed-temperature vaporiser and thermally desorbed. Direct evaporation is possible when a hot vaporising chamber is used and solvent/solute separation occurs in a separate compartment, a coated precolumn possibly in combination with packed beds. As a future strategy, LC-GC transfer techniques should be adjusted to those of large volume injection and involve a single device. It is believed that on-column injection/transfer is the choice. This requires that concurrent evaporation in LC-GC is performed by the on-column interface.

Determination of the enantiomeric composition of gamma-lactones in edible oils by on-line coupled high performance liquid chromatography and gas chromatography

A new method is proposed for the determination of the enantiomeric composition of gamma-lactones in different vegetable edible oils (i. e., olive oil, almond oil, hazelnut oil, peanut oil, and walnut oil), and its potential for authenticity control is underlined for a limited number of samples. The method is based on the direct injection (i.e., without requiring a sample pretreatment step) in on-line coupled reversed phase liquid chromatography to gas chromatography (RPLC-GC) using a chiral stationary phase in the GC-step. Different experimental values for both speed of sample introduction into GC and volume of the transferred fraction are considered to improve the recoveries obtained. Relative standard deviations lower than 10% and detection limits ranging from 0.06 to 0.22 mg/L were achieved for the investigated gamma-lactones.

Programmed temperature vaporiser-based injection in capillary gas chromatography

The application of programmed temperature vaporisation (PTV) in capillary gas chromatographic analysis is reviewed. The development of the different strategies as well as the state of the art are described. As the analytes are normally enriched in the PTV insert, the quoted papers are subdivided depending on whether the enrichment was carried out from organic solvents, from water or from gaseous media. Furthermore, the possibilities of PTVs for on-line coupling with sample preparation methods or other separation techniques and their use as thermoreactors are mentioned.

On-line combination of aqueous-sample preparation and capillary gas chromatography

Methods currently in use to combine the preparation of aqueous samples on-line with capillary gas chromatography (GC) comprise heartcut-orientated reversed-phase liquid chromatography-GC and analyte-isolation-orientated analyte extraction-GC. These approaches either use techniques in which water is directly introduced onto the GC column, or an indirect approach in which water is eliminated, i.e., by solid-phase extraction, solid-phase microextraction or liquid-liquid extraction, prior to introduction of the analytes onto the GC column. The latter type of approach is much more successful and user-friendly, and many applications have been reported.

Evaluation of the programmed temperature vaporiser for large-volume injection of biological samples in gas chromatography

The use of a programmed temperature vaporiser (PTV) with a packed liner was evaluated for the injection of large volumes (up to 100 microl) of plasma extracts in a gas chromatograph. Solvent purity, which is essential when large volumes are injected into the GC system, was determined. Special attention was paid to the purity of the solvents used for the solid-phase extraction (SPE) procedure. For this SPE method, ethyl acetate was used as the extraction and reconstitution solvent, and thus the purity of the ethyl acetate was critical, especially when a non-selective GC detector was applied. The liquid capacity and inertness of different packed liners were investigated. The liner packed with ATAS "A" (modified Chromosorb-based material with special treatment) was found to be the most suitable for the analysis of the tested drugs. Good linearity in response for variations in volume and concentration was observed. A comparison was made between the applicability of flame ionisation detection (FID) and mass-selective detection (MSD). When 50-microl volumes of plasma extracts were injected with the PTV, the detection limits for secobarbital, lidocaine, phenobarbital and diazepam were about 50-times lower than when 1-microl volumes were injected. The detection limits of the tested compounds in plasma for injection of 50-100 microl plasma extract are 5-10 ng/ml for GC-FID whereas plasma concentrations of 250 pg/ml can be detected using the selected ion monitoring (SIM) mode of a MSD. For non-selective GC-FID, the background from a 50-microl injection was substantially larger than with 1-microl injection as a result of co-injected plasma matrix components and solvent impurities. These background effects were less with GC-MSD in the total ion current mode and virtually absent with GC-MSD in the SIM mode.

On-line combination of aqueous-sample preparation and capillary gas chromatography

An overview is presented of methods currently in use to combine the preparation of aqueous samples on-line with capillary gas chromatography. Two approaches can be distinguished: heartcut-orientated reversed-phase liquid chromatography-gas chromatography (GC) and analyte-isolation-orientated analyte extraction-GC. These approaches either use techniques in which water is directly introduced onto the GC column, or an indirect approach in which water is eliminated, i.e., by solid-phase extraction, solid-phase microextraction or liquid-liquid extraction, prior to introduction of the analytes onto the GC column. The latter type of approach is much more successful and user friendly, and many applications have been reported.

On-line coupling of solid-phase extraction to gas chromatography with mass spectrometric detection to determine pesticides in water

A group of pesticides with different chemical structures was determined in water by on-line coupling of solid-phase extraction to gas chromatography with mass spectrometric detection through an on-column interface. A 10 mm x 2 mm I.D. precolumn packed with PLRP-S was selected for the solid-phase extraction process. The parameters affecting the transfer of the analytes from the precolumn to the GC system (e.g. flow-rate, temperature and solvent vapor exit time) were optimized. An organic modifier was added to the sample before the extraction process to avoid adsorption problems. The use of the MS detector under selected ion monitoring acquisition enabled the analytes to be quantified at sub microgram-per-litre levels preconcentrating only 10 ml of sample, and the limits of detection (S/N = 3) were between 2 and 20 ng l-1. The method was applied to the determination of the pesticides in tap and river water, and molinate was determined in Ebro river water.