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Levomoprolol is a beta-blocking agent used in the topical treatment of glaucoma. The necessity for comparing the plasma levels of a drug administered by the ophthalmic route with those obtained following systemic treatment requires increasingly sensitive methods in order to determine the low plasma concentrations produced by the administration of eye drops. On-line high-performance liquid chromatography-gas chromatography and concurrent solvent evaporation proved to be advantageous in the determination of levomoprolol in human plasma. Levomoprolol was determined by capillary gas chromatography (GC) with electron-capture detection (ECD) after solid-phase extraction from plasma and derivatization. Quantitation was based on the internal standard method. The detection limit of 0.2 ng/ml is 50 times lower than that obtained with previous GC methods involving on-column injection and ECD.
Pressurised hot water extraction (PHWE) was coupled on-line with liquid chromatography-gas chromatography (LC-GC) to determine brominated flame retardants in sediment samples. After extraction with pressurised hot water the analytes were adsorbed in a solid-phase trap. The trap was dried with nitrogen and the analytes were eluted to the LC column, where the extract was cleaned, concentrated and fractionated before transfer to the GC system. The fraction containing the brominated flame retardants was transferred to the GC system via an on-column interface. The PHWE-LC-GC method was linear from 0.0125 to 2.5 microg with limits of detection in the range 0.70-1.41 ng/g and limits of quantification 6.16-12.33 ng/g.
An approach based on on-line coupled liquid chromatography-gas chromatography (LC-GC) was developed for the rapid analysis of gamma-oryzanol in rice. Total lipids were extracted from rice and subjected to LC-GC without any prior purification. gamma-Oryzanol was pre-separated by HPLC from rice lipids and transferred on-line to GC analysis in order to separate its major constituents. 24-methylenecycloartanyl ferulate, cycloartenyl ferulate, campesteryl ferulate, beta-sitosteryl ferulate and campestanyl ferulate. The identities of the compounds were confirmed by off-line GC-MS analysis. Total gamma-oryzanol content could be quantified by HPLC-UV detection and the distribution of gamma-oryzanol constituents could be determined by on-line coupled GC analysis. The proposed methodology paves the way for high-throughput investigations providing information on natural variations in gamma-oryzanol content and its composition in different rice varieties.
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.
Pesticides in red wines were analysed by on-line coupled reversed-phase liquid chromatography-gas chromatography where a vaporiser/precolumn solvent split/gas discharge interface enabled direct transfer of aqueous eluent to the GC system. The LC part of the system provided sample clean-up and re-concentration, and the GC the final analytical step. The method developed allowed automated and quantitative analysis of the wine samples, where the only manual step was filtration. The limits of quantification were clearly below the maximum residue limits established for grapes, being lower than 10 micrograms l-1 for all pesticides studied.
The content of phytosterol oxidation products (POPs) in enriched and nonenriched commercial spreads was evaluated by thin-layer chromatography-gas chromatography (TLC-GC). Oxides of beta-sitosterol, campesterol, and stigmasterol were produced by thermo-oxidation (7-hydroxy, 7-keto, and epoxy derivatives) and chemical synthesis (triol derivatives), which were then separated and identified by TLC-GC. Their identification was further confirmed by GC-mass spectrometry (GC-MS). The total amounts of phytosterols found were 6.07 and 0.33 g/100 g of sample in phytosterol-enriched and nonenriched spread, respectively, whereas the total POPs contents were 45.60 and 13.31 mg/kg of sample in the enriched and nonenriched products. The main POPs found were the 7-keto derivatives of all phytosterols analyzed; 7-ketositosterol was the most abundant one (14.96 and 5.93 mg/kg of sample in phytosterol-enriched and nonenriched spread). No beta-epoxy and triol derivatives were detected in both types of samples. The enriched spread presented a lower phytosterol oxidation rate (0.07%) than the nonenriched one (0.41%).
Midazolam concentrations in patients' plasma was determined after extraction with high performance liquid chromatography (HPLC), gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). GC was selected for routine plasma assays in terms of selectivity, simplicity, precision, accuracy and sensitivity (0.02 microgram/mL); HPLC analysis was less sensitive (0.1 microgram/mL) than GC; GC/MS was used for analysis validation. Plasma protein binding of midazolam was determined by GC in patients' plasma after in vitro incubation with midazolam, ultrafiltration and extraction; 5% of the drug was unbound to plasma proteins. Midazolam distribution in lipoprotein fractions separated by ultracentrifugation of plasma obtained from patients on prolonged midazolam treatment was also assayed by GC.
A fully automated high-performance liquid chromatography-gas chromatography (HPLC-GC) network is described. A ten-port valve set up as a loop type LC-GC interface allowed the transfer of large LC effluent fractions into the gas chromatograph by concurrent solvent evaporation. The system performed highly efficient sample enrichment and clean up by LC and on-line GC separation with sensitive electron-capture detection. The efficiency of the system was demonstrated by application to the trace analysis of N-(3-chloro-2,6-dimethylphenyl)-N-(2-oxotetrahydrofuranyl)-2-me thoxyacetamide (CGA 80000) in various crops and soil samples. The residue level determined was 0.02 mg/kg for crop samples and 0.01 mg/kg for soil samples. The relative standard deviations of the calibration graphs were in the range 2-5%; the mean recovery was greater than 85%.
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 liquid chromatography-gas chromatography (LC-GC) has been applied to the analysis of steryl esters in cocoa butter. Separation of the steryl esters was achieved after on-line transfer to capillary GC. HPLC removes the large amount of triglycerides and pre-separates the components of interest, thus avoiding time-consuming sample preparation prior to GC analysis. The identities of the compounds were confirmed by GC-MS investigation of the collected HPLC fraction and by comparison of the mass spectra (chemical ionization using ammonia as ionization gas) to those of synthesized reference compounds. Using cholesteryl laurate as internal standard, steryl esters were quantified in commercial cocoa butter samples, the detection limit being 3 mg/kg and the quantification limit 10 mg/kg, respectively. Only slight differences in percentage distributions of steryl esters depending on the geographical origin of the material were observed. The patterns were shown to remain unchanged after deodorization. The method described might be a valuable tool for authenticity assessment of cocoa butter.
A multiresidue, automated and rapid method for the determination of pesticide residues in olive oil is presented. The method employs the through oven transfer adsorption-desorption interface for the on-line coupling of reversed-phase liquid chromatography and gas chromatography. In this fully automated system, olive oil is directly injected with no sample pre-treatment step other than filtration. Methanol-water is used as eluent in the liquid chromatography pre-separation step. The selected liquid chromatography fraction containing the pesticides is automatically transferred to the gas chromatography. The liquid chromatography column flow during elution is different from the flow during the transfer. Using a flame ionisation detector, pesticide detection limits varied from 0.1 to 0.3 mg/l.
A modified on-column interface is reported for the coupling of high-performance liquid chromatography with gas chromatography, incorporating an adapted, commercially available multidimensional gas chromatography switching system. Novel features include cryogenic analyte focusing, total solvent exclusion from the analytical column and independent carrier gas supplies to the analytical GC column and uncoated pre-column. The instrumentation was used for the determination of the veterinary anthelmintic drug levamisole in milk with analyte detection by both flame ionisation and nitrogen-phosphorus detectors. Detection limits for the assay were 2.2 micrograms l-1 and 0.4 micrograms l-1 by flame ionisation and nitrogen-phosphorus detectors, respectively. The assay was applied to a survey of fourteen milk samples from different dairy outlets.
Instrumental analytical and bioenzymatic methods were used to differentiate between species of the Actinobacillus-Haemophilus-Pasteurella group. Long-chain fatty acids were analysed directly with gas chromatography (GC) without derivatization. GC of trifluoroacetylated whole-cell methanolysates was a rapid method for differentiation. Cellular sugars were more suitable for differentiation than fatty acids. D-Glycero-D-mannoheptose, the major localization of which was lipopolysaccharide, distinguished H. aphrophilus from A. actinomycetemcomitans, H. paraphrophilus, H. influenzae type b, P. haemolytica, P. multocida, and P. ureae. GC of single colonies, which is a new chemotaxonomic method, was preferable to GC of liquid-grown cells. Lysozyme-and EDTA-induced bacteriolysis and reduction of methylene blue by cellular hydrogenase served as additional criteria for differentiation.
An automated method for analysis of unsaponifiable compounds in edible oils is presented. The method involves the on-line coupling of reversed-phase liquid chromatography and gas chromatography (LC-GC) using the through oven transfer adsorption desorption (TOTAD) interface. The oil is injected directly with no sample pretreatment step other than filtration. It may also be considered to dilute the oil sample. In the LC step, a short C4 column using a methanol/water eluent separates analytes from the other components of the oils, which are made up of mainly triglycerides. A LC fraction of up to 1.6 mL containing the analytes is transferred to GC at a flow rate of 0.1-2 mL/min. The TOTAD interface allows solvent venting and the introduction of the analytes into the GC column. The proposed fully automated method allows the analysis of different groups of compounds (free sterols, tocopherols, squalene, and erythrodiol and uvaol) in one chromatographic run or the analysis of these compounds in different groups. Sensitivity is more than necessary, and repeatability is good, the CV ranging from 3 to 12% for the full analysis.
A new application is proposed for the on-line coupling of reversed-phase liquid chromatography to gas chromatography (RPLC-GC) that allows the GC chirospecific analysis of gamma-lactones in fruits and commercially available fruit-containing products. The use of a programmed temperature vaporizer as an interface with the system makes the transfer of large volume fractions (i.e., 2520 microL) of aqueous eluents from LC to GC possible (speed of sample transfer, 1800 microL/min). Relative standard deviations obtained for the investigated lactones under the experimental conditions vary from 7 to 14%. The described system enlarges the LC-GC application field and overcomes the limitations reported thus far concerning the use of typical normal-phase eluents (i.e., the transfer of rather small volume fractions at low speeds of sample introduction).
An on-line combination of gel permeation chromatography and gas chromatography has been designed using either a laboratory-built or a commercially available LC-GC apparatus to determine organophosphorus pesticides in olive oil. Gel permeation chromatography was used for sample pretreatment, viz. to separate the low-molecular-mass pesticides from the higher-molecular-mass fat constituents of the oil. A mixture of n-decane and the azeotropic mixture of ethyl acetate and cyclohexane was found to give an adequate separation between the fat and the organophosphorus pesticides. The pesticide-containing fraction, monitored by a UV detector, was transferred on-line to the gas chromatograph using a loop-type interface. n-Decane (6%, v/v) was added to the eluent in order to widen the application range of the transfer technique towards more volatile pesticides. After solvent evaporation through the solvent vapour exit and subsequent GC separation, the compounds were selectively detected with a thermionic or a flame photometric detector. The set-up allowed the direct analysis of oil samples after dilution in the gel permeation chromatography eluent without further sample clean-up. Detection limits were about 5 and 10 micrograms/kg with the thermionic and the flame photometric detector, respectively, when using an injection volume of only 30 microliters of the 20-fold diluted oil. The total procedure was linear in the 0.01-10 mg/kg range for both detectors. For twenty organophosphorus pesticides, the relative standard deviations were 3-13% at the 20-60 micrograms/kg level.