Retention Gap Research Articles

Automated derivatization for on‐line solid‐phase extraction‐gas chromatography; phenolic compounds

AbstractAn automated system for derivaatization was coupled on‐line with solid‐phase extraction‐gas‐chromatography (SPE‐GC). The system was optimized for the determination of phenol and chlorinated phenols in aqueous samples. The test analytes were acetylated with acetic anhydride; proper buffering of the sample was a critical factor. Next, the phenol acetates were enriched on a SPE cartridge and transferred to a GC; two appraoaches were studied. In the first approach, the derivatives were enriched on disposable C18 cartridges (ASPEC type) and desorbed with methylacetate. Aan aliquot of the final eluate was injected on‐line the GC by means of a loop‐type interface. In the second approach, trace enrichment was performed on 10 × 2 mm i.d. LC‐type precolumn packed with polystyrenedivinylbenzene copolymer (PLRP‐S) this precolumn was dried with a mitrogen purge and the phenol acetates were desorbed with ethyl acetate which was injectedon‐line into the retention gap of the GC under partially concurrent solvent evaporation (PCSE) conditions. The Derivatization‐SPE‐GC system which was based on the loop‐type interface has the advantage of simplicity and easy operation, the main drawback is the impossibility to determine phenol acetates which elute prior to trichlorophenol acetates. With the derivatization‐SPE‐GC approach using PCSE‐based desorption, even the most volatile analyte of the test series, phenol acetate, can be determined successfully. The entire procedure, including the derivatization step, was fully automated and integrated in one set‐up. The precision data for the integrated on‐line derivatization‐SP‐FID system were fully satisfactory, with RSD values of 1–12 % at the 1 μg/1 level. When a sample volume of 2.2 ml was analyzed, The detection limits for the chlorinated phenol acetates were in the 0.1–0.3 μg/1 range.

Supercritical fluid chromatography–mass spectrometry of thyme extracts (Thymus vulgaris L.)

Capillary supercritical fluid chromatography–mass spectrometry (SFC–MS) has been applied to the analysis of essential oils and commercial supercritical carbon dioxide extracts of thyme (Thymus vulgaris L.). A new SFC–MS interface and an injection technique using a retention gap have been applied successfully to separate and identify components of the extracts.

Is the Hippocampus Involved in Temporal Discrimination and the Memory of Short Intervals?

Rats with lesions to the hippocampus proper and the subiculum were tested for timing behavior and temporal memory. Using the peak procedure, they were trained to discriminate a 40 s interval and a retention gap tested the memory for time. Results were interpreted within the theoretical framework of the internal clock and with respect to current theories on hippocampal function. Timing behavior was unaffected by either lesion and no shifts in the temporal discrimination functions were observed. The lesions also failed to show a deficit in the memory for temporal events. For all groups, the retention gap increased the mean peak time by the time of the gap. This indicated that all rats used the stop rule which required the use of working memory. Thus, it was concluded that the hippocampus is neither necessary for accurate timing behavior nor for the memory of temporal events.

On‐line coupling of liquid chromatography and capillary gas chromatography for the determination of a trace pyrethroid insecticide in fruit extracts

AbstractThe following paper presents a fast, selective, sensitive, and automated procedure for the residue analysis of the pyrethroid insecticide acrinathrin in various fruit samples by on‐line combination of Liquid Chromatography (LC) with capillary Gas Chromatography (GC) and an Electron‐Capture Detector (ECD). The loop‐type interface and Fully Concurrent Solvent Evaporation (FCSE) were chosen due to their ease of use and low volatility of acrinathrin. Commercially available LC diol columns with small internal diameters (2.1 mm) provided the selectivity required for the separation of the solute from matrix components and permitted the transfer of fraction volumes up to 500 μl into the retention gap, located inside the gas chromatograph. Optimization of the operating conditions with standard solutions showed the influence of the GC initial transfer temperature on the response of acrinathrin and the quantitative nature of the transfer was established by liquid scintillation counting. The linearity and precision were studied with the aid of calibration plots, whereas quantitative recoveries of the analyte (at least 95 %) were determined in spiked fruits. Compared to the corresponding off‐line technique, the on‐line approach allows us to reach lower detection limits (ppb level when 20 μl LC injections are performed) with excellent repeatability between 0.6 and 6.6 %. Moreover, the integrated analytical set‐up is robust, less time‐consuming and subsequently brings a marked improvement in the sample preparation step of complex biological matrices. Finally, the versatility of the system renders it highly suitable for the determination of other similar pyrethroids in order to develop a multiresidue method.

Large-volume injection in residue analysis with capillary gas chromatography using a conventional autosampler and injection by programmed-temperature vaporization with solvent venting

Large-volume injection in capillary gas chromatography may be used to compensate for the limited detection sensitivity of selective detectors when further evaporation of sample extracts is not feasible owing to increasing losses of volatile solutes or because the analytical method can no longer be performed automatically. Large-volume injection using programmed-temperature vaporization (PTV) with solvent venting has proved its worth as an on-line pre-chromatographic sample concentration technique that permits the reproducible analysis of pesticide residues over a wide volatility range. Automated sampling and injection of a 12.5-μl volume of toluene extract using an HP 7673A autosampler with a 25-μl Hamilton syringe is described. The various parameters, including design of injector inlet, speed of injection, retention gap and initial column temperature, are discussed and the optimization procedure for split flow-rate, solvent evaporation temperature, solvent venting time and splitless or transfer time is reported. A test mixture containing C 12, C 14 and C 16 hydrocarbons, 1,7-dibromoheptane, 3,4-dichloroaniline, pentachlorobenzene, hexachlorobenzene and the pesticides heptenophos, propachlor, naled, lindane, heptachlor, aldrin, tetrachlorvinphos, dieldrin, p,p′-DDE and endrin in toluene was used, representing moderately volatile to high-boiling solutes and thermolabile compounds. Complete recovery of all compounds in the test mixture was achieved with the method developed. No degradation of the thermolabile pesticides naled and endrin was observed. The precisions of quantitative determinations of 12.5-μl autosampler injections at different concentration levels were good to excellent down to concentration levels representing 1–2.5 ng per pesticide injected when applying atomic emission detection (AED). Data on standard deviations and the wide dynamic range of all test solutes are reported.

Large-volume injection in capillary gas chromatography using a programmed-temperature vaporizing injector in the on-column or solvent-vent injection mode

The use of the programmed-temperature vaporizing (PTV) injector as a versatile interface for large-volume injection in capillary GC is proposed. By using a special on-column insert, in addition to large-volume solvent-vent injection, also large-volume on-column injection can be performed with the PTV injector. For this, a set-up consisting of a retention gap, a retaining precolumn and an early solvent vapour exit is installed between the on-column insert and the analytical column. The performance of this set-up for large-volume injection was evaluated and found to be similar to that of large-volume injection using a conventional on-column injector. The proposed instrument hence allows PTV and on-column large-volume sampling to be performed using only one injector. This greatly reduces instrument costs for large-volume sampling equipment. Guidelines are given for method development for large-volume sampling.

An improved approach for on‐line solid‐phase extraction‐gas chromatography

An improved setup for interfacing solid-phase extraction (SPE) and gas chromatography (GC) was constructed. Its working mechanism resembles that of the loop-type interface, but due to the use of small desorption volumes (50–100 μL) in combination with a long 0.53 mm i.d. retention gap, the application range also includes volatile analytes. The interface is equipped with a 6-port and a 14-port valve. The 6-port valve is used to divert the carrier gas either to the GC system or to the 14-port valve, on which a liquid chromatography (LC) precolumn and two loops for storing organic solvent are mounted. After loading the sample on the LC precolumn and drying by nitrogen purging, both valves are switched simultaneously. The solvent in the first loop effects phase swelling of the retaining precolumn in the GC system, which increases the application range at the volatile end. The solvent in the second loop desorbs the analytes from the stationary phase in the LC precolumn and transfers them to the GC system. The on-line SPE-GC approach was tested by analyzing a mixture of pesticides in 10-mL water samples using mass selective detection. System performance in terms of recovery, repeatability, linearity, and robustness was satisfactory. The detection limits in the full-scan mode generally were 0.1 μg/L or better. With postrun ion extraction, the detection limits were in the 2–20 ng/L range. For the purpose of target analysis, SPE-GC-mass spectroscopy using selected ion monitoring (SIM) was applied to the analysis of several classes of compounds, including chlorophenols, chloroanilines, and nitromusk compounds. The detection limits in the SIM mode were down to 0.2 ng/L. The procedure was extended to the analysis of diluted sediment extracts in which several microcontaminants were detected and identified. © 1996 John Wiley & Sons, Inc.

Repetitive liquid injection system for inverse gas chromatography

A hybrid injection system composed of a closed-loop vaporization chamber with a gas sampling valve is described. The system allows liquid injection into the vaporization loop with subsequent multiple injections of the vaporized solute(s) via the gas sampling valve (GSV). Very small amounts of probe solutes can be injected without a dilution solvent, and the vaporization loop acts as a “retention gap” for capillary columns. The use of a pneumatically controlled valve injector provides very accurate and reproducible injection volumes at precise time intervals. Combination of precisely timed injections with temperature programming of the column oven produces continuous chromatograms and retention data at controlled temperature increments. The proposed injection system eliminates the need for split injectors; it is cleaner than normal in-line injectors because nonvolatile samples cannot reach the column; and the instrumentation is easily automated. The major disadvantages are the restriction that the samples must be volatile at temperatures lower than the upper temperature limit of the valve rotor; the GSV is susceptible to both contamination and mechanical failure; the sample can be exposed to metal components of the valve; and initial distribution of the sample throughout the closed-loop vaporization chamber may be slow.

Quantitative Open-Tubular Supercritical Fluid Chromatography Using Direct Injection onto a Retention Gap

Injection in open-tubular supercritical fluid chromatography is usually accomplished using a dynamic-flow-splitting or time-splitting technique. These techniques limit the effective injection volumes to low-nanoliter levels. Sample volumes can be increased significantly without broadening the detected peaks, and injection precision improved by using direct injection and a retention gap. The required instrument modifications are simple and very inexpensive. The initial oven temperature and pressure must be set with respect to the phase behavior of the binary mixture formed by the mobile phase and the sample solvent. Relative standard deviations of peak areas and peak heights (comparing the same peak among repeated injections) are in the range of 0.6-1.8% for well-behaved solutes using injection volumes of 0.1 and 0.5 μL

An on‐line coupled HPLC‐HRGC system for the quantitative characterization of oil fractions in the middle distillate range

AbstractA commercially available HPLC‐HRGC instrument was adapted to enable the full quantitative characterization of oil fractions in the range 150–400°C. The HPLC separates the sample into five fractions: saturates, mono‐aromatics, di‐aromatics, fluorenes + dibenzothiophenes, and tri‐aromatics. These fractions are subsequently quantitatively transferred via an on‐column interface to the HRGC part of the system. This transfer is performed into an empty, wide‐bore precolumn (retention gap), which enables a refocussing effect of the compounds of interest in the tail of the HPLC mobile phase, which acts as a solvent in the GC. A solvent vapor exit in the system enables the vent of the major part of this solvent. The final HRGC separation provides a carbon distribution of the saturates and the mono‐aromatics and enables the identification of a number of compounds in the other three fractions. It is demonstrated that the described method is quantitative for the hydrocarbons, but also for sulphur‐containing compounds in only those samples that have a low sulphur content. © 1995 John Wiley & Sons, Inc.

Large‐volume on‐column injections for gas chromatography

AbstractLarge‐volume injection is an attractive means to improve detection limits when analyzing dilute sample extracts. There is now no need to achieve enrichment by means of solvent evaporation, which makes the total analytical procedure less reliable and less robust due to loss of volatile compounds. In this study on‐column injection was applied for large‐volume injection using an on‐column injector in combination with a deactivated uncoated fused silica capillary (retention gap) and an early solvent vapor exit. Two strategies are described. The first one relies on the programmable speed of an infusion pump, which is adapted such that it lies just above the evaporation rate of the extraction solvent used. With the second procedure, which can be used for a commercially available autosampler, the evaporation speed is adjusted to the fixed speed of injection by varying the temperature during injection. During optimization the additional solvent evaporation time was determined. The optimum conditions for an injection volume of 100 μL of either n‐hexane or ethyl acetate are reported. Large‐volume injection was used for simplified and miniaturized solid‐phase extraction procedures. The procedures were used to determine various organic micropollutants, such as organophosphorus pesticides and triazine herbicides, in tap and surface water extracts, using a flame photometric, and a mass spectrometric detector. For both methods good recovery for most of the analytes (70–87%), linear calibration curves (0.01‐5 μg/L range), and relative standard deviations of less than 10% were obtained. Detection limits typically were in the range of 1–10 ng/L for GC‐FPD and 10–150 ng/L for GC‐MS. When working in the time‐scheduled multiple ion detection mode detection limits in GC‐MS were one order of magnitude lower. © 1995 John Wiley & Sons, Inc.

On-line automated sample preparation-capillary gas chromatography for the analysis of plasma samples

An automated sample preparation module, (the automated sample preparation with extraction columns, ASPEC), was interfaced with a capillary gas chromatograph (GC) by means of an on-column interface. The system was optimised for the determination of the antidepressant trazodone in plasma. The clean-up of untreated plasma was carried out with disposable C18 cartridges, using several washing steps. The analyte was desorbed with methanol, the extract was diluted on-line with buffer and preconcentrated on a PLRP-S trapping column. The trapping column was dried by purging with nitrogen gas. Desorption (phase switching) of the trapping column was carried out with ethyl acetate which was introduced into a retention gap using partially concurrent solvent evaporation conditions. Trazodone was determined by GC with flame ionisation detection (FID). With a 1 ml sample, this resulted in a detection limit for trazodone in plasma of 3 ng/ml. The system showed good linearity and repeatability in the range 0.01–1 μg/ml, thus covering the range of pharmacokinetic/dynamic-to-therapeutic concentrations of trazodone in plasma.

Increased on‐column injection temperature for gas chromatography

AbstractA way of controlling the maximum allowable oven temperature during on‐column injection by the column pressure drop is suggested. An arrangement using a restriction at the column outlet for adjustment of the column inlet pressure while maintaining the same column flow has been studied. Compared to non‐restricted flow, substantially increased maximum oven temperatures were obtained during on‐column injection. Injections at elevated temperatures resulted in an increased speed of analysis and decreased solute adsorption on the surface of the contaminated retention gap. The method is generally applicable to analysis of high boiling mixtures. In particular, with GC‐AED, such an arrangement yields a higher sensitivity due to an increased solute interaction with the excitation plasma.

Determination of neutral chlorinated extractable organic compounds in water samples using large volume on‐column injection capillary gas chomatography–mass spectrometry

AbstractAn automated large volume on‐column injection technique (LOCI) for capillary gas chromatography – mass spectrometry, GC‐MS, with solvent divert and heated retention gap technology has been used to simplify the method for determining sixteen neutral chlorinated extractable compounds (NCEC) in industrial plant process water samples at a level of 10 ng/L. According to the simplified procedure, 30 mL of water sample is extracted with 5 mL hexane, and 100 μL of the extract analyzed by combining large volume on‐column injection with mass‐selective detection operated in the selected ion monitoring mode. This combination has enabled simplification of the sample preparation and analysis procedures and provided a fast, labor‐saving technique for analyzing water samples. Simplifications include significant reduction of sample and eluent volumes during extraction, and elimination of the Kuderna‐Danish evaporative concentration step. The latter simplification has not only resulted in significant time savings, both during the analysis and for subsequent glassware cleaning; analyte losses and the introduction of contaminants and artifacts which may occur during this step have also been eliminated.System behavior, e.g. linearity of detector response, method detection limit, and reproducibility, were evaluated for all sixteen analytes using spiked water samples in the lower ng/L range. The response of the mass detector is linear in the range 5–20000 ppt for all sixteen NCEC investigated. Injection of 100 μL of sample using LOCI makes it possible to achieve an instrument detection limit in the ppt range. The calculated method detection limits are similar to those of the instrument precision and are lower than the required method detection limit of 10 ppt.Extraction efficiency has been maximized by optimizing sample‐to‐solvent ratio and extraction time. The extraction efficiency seems to be independent of sample‐to‐solvent ratio in the range of 2. 5 to 6 and increasing the extraction time from 25 to 120 minutes resulted in no significant increase in efficiency. Extraction efficiency also seems to be independent of the concentration of analyte in the water samples. The simplified procedure presented here was automated by means of a laboratory robot and used to generate all the extraction data reported in this manuscript.

Reversed‐phase liquid chromatography coupled on‐line with capillary gas chromatography II. Use of a solvent vapor exit to increase introduction volumes and introduction rates into the gas chromatograph

AbstractA large‐volume introduction system for capillary gas chromatography (GC) is described which can be coupled on‐line with reversed‐phase liquid chromatography (LC) using 2 mm i.d. LC columns. An on‐column interface with a special retention gap, coated with a very thin film (0.02 μm) of Carbowax was used. Insertion of a solvent vapor exit before the analytical gas chromatography column allowed the solvent evaporation rate for an azeotropic acetonitrile/water mixture (84:16, v/v) to increase from 20 to 175 μL min−1 at 91°C and 150 kPa inlet pressure. A volume of 200 μL of this mixture containing phenanthrene and the potential basic drug eltoprazine, could be injected into the retention gap at introduction rates up to 200 μL min−1, under partially concurrent solvent evaporation conditions. Repeatability (CV = 2.4%) and linearity (at least 10–1000 ng mL−1) were satisfactory. The Carbowax‐coated retention gap has a relatively large retention power for the polar solute. This means that the solvent can be completely evaporated through the vapor exit without loss of analyte. With apolar solutes, losses will occur if evaporation is unduly prolonged. In order to allow the use of a nitrogen‐phosphorus detector, a special pressure‐balanced carrier gas circuit was designed and tested for total solvent‐free performance of the detector. With this set‐up, 150 μL injections of eltoprazine in the acetonitrile/water azeotrope were made at an introduction rate of 200 μL min−1. No traces of acetonitrile were detected, while analyte detectability was 20‐fold higher than with the FID.Finally, the practicability of reversed‐phase LC‐GC was demonstrated by on‐line heart‐cutting of the phenanthrene‐containing fraction from a 2 mm i.d. LC column using acetonitrile/water (84:16, v/v) as eluent, and analyzing it by capillary GC‐FID.

On-line trace-level enrichment gas chromatography of triazine herbicides, organophosphorus pesticides, and organosulfur compounds from drinking and surface waters.

A simple, selective and precise procedure for the analysis of water samples by on-line solid phase extraction gas chromatography (SPE-GC) is presented. The determination of several triazines, organophosphorus pesticides, sulfur containing compounds in tap water was performed by SPE-GC using polymer-packed precolumns and flame ionization (FID), nitrogen-phosphorus (NPD) or flame photometric (FPD) detection. A cartridge packed with silica was inserted between the precolumn and the gas chromatograph in order to eliminate trace amounts of water present in the ethyl acetate used as desorption solvent. Incorporation of a drying step allowed the retention gap to be used for the analysis of ca. 100 samples without significant deterioration of the chromatographic peaks. Analyte recovery was at least 72% when 10 ml of tap-water sample were analysed. The detection limits for all analytes in tap water were lower than 0.1 microgram l-1, with all detectors used. Although with NPD and FPD, selectivity and sensitivity were markedly better. The SPE-GC-NPD system has also been used for the analysis of surface-water samples from different European rivers.

Evaluation of solvent venting with a gas purge for the introduction of microliter sample volumes in supercritical fluid chromatography

AbstractPeak broadening and sample recovery are important considerations when working with large sample volume injections in capillary SFC. To measure possible contribution to peak broadening in the venting valve, a tee coupling was inserted between the retention gap and the column. A standard sample containing glyceryl esters was injected. Based on time variance measurements, the venting valve did not lead to measurable band broadening. Recovery was found to be dependent on the length of the retention gap, the CO2/He ratio in the purge gas, and the temperature of the retention gap. Cryofocusing was needed to increase the recoveries of volatile n‐alkanes. To increase the recoveries of glyceryl esters, an enhanced CO2/He ratio was needed.

Independently temperature programmed uncoated precolumn: A concept for improved sample introduction in capillary GC

AbstractIn this paper, a GC sample introduction technique is described in which an uncoated precolumn and the analytical capillary column are individually temperature programmed. The influence of different rates of temperature programming of the retention gap on band broadening of a series of n‐alkanes has been investigated for different lengths of the precolumn. It is shown that extra column band broadening of high boiling solutes is reduced when the temperature of the precolumn is ramped at a faster rate than the analytical column. This benefit is particularly pronounced when long retention gaps are employed.The technique extends the useful lifetime of the analytical column when complex real‐world samples with a wide volatility range are analyzed. This is due to the fact that the final temperature of the precolumn can be kept below the final temperature of the analytical column. Thus, the column is not contaminated with high boiling non‐elutable solutes.

Fully automated water analyzer based on on‐line solid phase extraction‐gas chromatography

AbstractAn automated procedure for the analysis of aqueous samples by membrane disk extraction coupled on‐line to capillary gas chromatography (GC) is presented. Organophosphorus pesticides are preconcentrated from aqueous samples on three 0.5 mm thick, 4.0 mm diameter XAD‐2 membrane extraction disks which are mounted in a polymer holder for use in a PROSPEKT sample processor. The layers are dried by a stream of nitrogen (30 min at ambient temperature). Desorption of the analytes is carried out with ethyl acetate which is directly introduced into a retention gap under partially concurrent solvent evaporation conditions, using an early solvent vapor exit. The method is a fully software‐controlled automated system, and includes sample preparation, sample transfer and GC analysis. The final analysis is carried out by GC with nitrogen‐phosphorus (NPD) or mass‐selective detection. The technique is applied to the determination of a series of organophosphorus pesticides in tap water and water from European rivers. With a sample volume of only 2.5 mL, the detection limits achieved with the NPD were 20–50 ng L−1 in tap water and 20–100 ng L−1 in river water.

On‐line liquid backflush—a new technique for GC analysis of samples containing nonvolatile material

AbstractDeteriorated performance in capillary GC is often caused by the deposition of nonvolatile components in capillary GC inlets. In many cases this is a limiting factor in trace analysis of complex samples. Volatile solutes of the sample interact with the high boiling material, which leads to broadened and unsymmetrical peaks. This paper describes a novel technique which utilizes independent temperature programs of a retention gap and the column as well as a liquid backflush of the retention gap. Samples that normally contaminate the retention gap after a few injections can be repeatedly injected over extended periods of time, while the chromatographic resolution is retained.