Retention Gap Research Articles

A Large Volume Injection Procedure for GC-MS Determination of PAHs and PCBs

A simple on-column injection system for large volume of liquid samples for the GC-MS determination of traces of PAHs and PCBs has been investigated. A deactivated fused silica capillary 20 m × 0.53 mm I.D. and 2 meters of an HP5 column (0.53 mm,1 μm film thickness) were used as retention gaps. Injection volumes of 80 μL for PAHs and 90 μL for PCBs, allow determination of 5–50 ng L−1 PAHs and 11–44 ng L−1 PCBs in hexane solution with an RSD of < 10%. The method has been used for the determination of PCBs and PAHs in soil sample.

Search on ruggedness of fast gas chromatography–mass spectrometry in pesticide residues analysis

In this study, suitability of fast gas chromatography–mass spectrometry (GC–MS) on a narrow-bore column with a programmed temperature vaporizer for the analysis of pesticide residues in non-fatty food was evaluated. The main objectives were ruggedness and stability of chromatographic system with regards to co-extractives injected. The chromatographic matrix induced response enhancement was found to be strongly dependent on the concentration of residues and is reaching up to 700% compared to the pesticides solutions in a neat solvent. However, the responses of pesticides in matrix-matched standards at different concentration levels do not significantly change during 130 injections. Response enhancement/or decrease is influenced by the sample preparation technique. External calibration with matrix-matched calibration standards should, therefore, provide results with good precision also at the concentration level of 0.005 mg kg −1. Special attention is given to the performance of the chromatographic column and retention gap with regards to peak widths, peak tailing and different sample preparation methods. During approximately 460 matrix sample injections, the performance of the analytical column was acceptable. GC–MS set-up with 0.15 mm i.d. column can be successfully utilized for the pesticide residues analysis.

Practical aspects of splitless injection of semivolatile compounds in fast gas chromatography

Possibilities and practical aspects of implementation of splitless injection of larger volumes for fast GC purposes utilizing narrow-bore column, hydrogen as carrier gas, fast temperature programming under programmed flow conditions and commercial instrumentation were searched. As a model sample semivolatile compounds of a broad range of volatility and polarity (7 n-alkanes and 19 pesticides) were chosen. Peak shapes, peak broadening and peak areas and its repeatability were evaluated under various experimental set-ups (liner/injection technique combinations). Various factors, such as liner design, injection technique, retention gap length, compound volatility and polarity, the solvent used, initial oven temperature influenced compound focusation and/or maximal injection volume. Combination of analytical column (CP-Sil 13 CB 25 m long, 0.15 mm i.d., film thickness 0.4 μm) with normal-bore retention gap (1 m long, 0.32 mm i.d.) allowed maximal injection volume 8 μl for 4 mm i.d. liner used without any peak distortion when solvent recondensation in the retention gap was employed.

Influence of the injection technique and the column system on gas chromatographic determination of polybrominated diphenyl ethers

In this paper, we present an investigation of the influence of the gas chromatographic separation system on the determination of polybrominated diphenyl ethers (PBDEs). Capillary columns, retention gaps and press-fit connectors, as well as different injection techniques have been evaluated with respect to yield and repeatability. The split/splitless injection has been optimized and compared to on-column injection, the septum equipped temperature programmable injector (SPI) and the programmable temperature vaporizing (PTV) injector. Furthermore, a comparison of the different operational modes of the PTV injector is presented. The results show that there are large variations in the yield of PBDEs depending on the column and the injection systems. Especially the high molecular weight BDE congeners can be subject to severe discrimination. Unfavorable conditions can lead to a complete loss of nona and deca substituted BDE congeners.

Forensic analysis of hair surface components using off‐line supercritical fluid extraction and large volume injection

AbstractThe analysis of organic material on the surface of human hair may indicate various subject characteristics such as age, race, gender, or the use of hair products. In addition, comparisons of hair chemical composition may be used to assess the potential for a common origin between a known and unknown sample. However, evidentiary hair samples are often extremely small, necessitating highly sensitive extraction and analysis techniques. In this communication, off‐line supercritical fluid extraction (SFE) followed by gas chromatography/mass spectrometry (GC/MS) using large volume injection (LVI) is demonstrated as a potential method for extracting and analyzing hair surface components. The retention gap technique was used to achieve large volume injections, allowing up to 30 μL of the off‐line extracts to be analyzed. Subsequent chemical analysis by GC/MS identified various natural components on the hair surface such as saturated and unsaturated free fatty acids, squalene, and cholesterol. Overall, off‐line SFE was quite efficient, allowing the measurement of surface components of sample amounts ranging from 0.22 mg to 0.24 mg. In addition, derivatization of free fatty acids using a silylating agent increased both chromatographic resolution and sensitivity. Results from two hair samples are discussed to demonstrate the technique.

Factors influencing chromatographic data in fast gas chromatography with on‐column injection

AbstractThe use of larger volume injection with on‐column injection and fast GC commercial instrumentation was evaluated with the model mixture of n‐alkanes of a broad range of volatility (C10–C28). The presented configuration allows introduction of 40–80‐fold larger sample volumes without any distortion of peak shapes compared to “usual” fast GC set‐ups using narrow‐bore columns. A normal‐bore retention gap (1–5 m×0.32 mm ID) was coupled to a narrow‐bore (5 m×0.1 mm ID×0.4 μm film thickness) analytical column using a standard press‐fit connector. The connection was tight and reliable, and hence suitable for hydrogen as carrier gas. The effect of pre‐column and analytical column connector, injection volume, pre‐column length, column inlet pressure, and analyte volatility on peak shape, peak broadening, and focusing are discussed. The precision of chromatographic data measurements and peak capacity under optimised temperature programmed conditions for fast separations with large volume injection were found to be very good. The presented fast GC set‐up with on‐column injection extends the applicability of the technique to trace analysis.

Role of the retaining precolumn in large-volume on-column injections of volatiles to gas chromatography

In the present study the retaining precolumn, which is commonly used in a set-up for large-volume on-column injections, or when solid-phase extraction (SPE) or liquid chromatography is coupled to gas chromatography (GC), was removed after varying its length from the standard length of 3 m down to zero. A dramatic increase of the evaporation rate of the injected organic solvent was obtained from a typical value of 100 μl/min up to 300 μl/min. The increased evaporation rate allowed (i) injection of a larger volume in the same retention gap, (ii) faster injection/transfer of the organic solvent and (iii) reduction of the transfer temperature. As volatile compounds under partially concurrent solvent evaporation conditions are easily lost once the organic solvent has been removed via a solvent-vapour exit (SVE), the parameters for large-volume injection, i.e. the evaporation rate and injection speed, were optimised using accurate measurements of the real flow-rate of the carrier gas into the GC system. All these options have been evaluated over the last 4 years. In order to demonstrate that omitting the retaining precolumn had no effect on the application range of the on-column interface, analytes as volatile as benzene were injected into GC–MS using 50–200 μl of n-pentane solutions. Contaminants were extracted from river water and wastewater into n-pentane using in-vial liquid–liquid extraction. The detection limits for benzene, toluene, ethylbenzene and m-xylene were ∼10 ng/l. To obtain optimum results the SVE had to be closed 1 s before the end of evaporation. Several brands of n-pentane were analysed to check for the presence of benzene. Most of them contained interfering compounds and benzene at the low μg/l level and therefore had to be cleaned by means of column chromatography. As another example C 8–C 17 alkylphenones were extracted from wastewater with n-hexane. Detection limits were 10–40 ng/l.

Use of deactivated fused-silica capillary precolumns in pesticide analysis by gas chromatography with electron-capture detection

The advantages and disadvantages of coupling a retention gap of fused-silica between the injection port and the chromatographic column are discussed. The influence on the peak width and height of several factors such as the solvent ( n-hexane, acetone, ethyl acetate and methanol), the gap (length, inner diameter, deactivation mode), the injection volume and the pesticide concentration has been examined. Those factors have very different incidences so, it is not possible to extract a general recommendation about the use of gaps. For this reason, checking its viability in each particular case is more advisable.

Development of an Analytical Marker for the Prediction of Biological Activity of Bitumen Fumes and Bitumens Using the FIA-DMSO Set-Up. Experimental Set-Up and Correlation with Mutagenicity and Carcinogenicity

During the hot application of bitumen-containing materials, as, for instance, in road paving and roofing, fumes are emitted which contain trace amounts of polycyclic aromatic compounds (PACs). The PAC levels are very low, and from the available epidemiological studies, there is no evidence that human exposure to bitumen or its fumes results in any cancer risk to the workforce. For comparative purposes we are trying to identify a laboratory “marker” for bitumen fume condensates (BFCs) which will give a correlation with biological activity and hence carcinogenic potential. Previously it was found that the determination of total 3–6-ring PACs by DMSO extraction followed by gas chromatographic analysis, whose use is well established for mineral oils, showed promise as a marker for BFCs as well. Because standard DMSO extractions require about 4 g of sample, whereas generally only one mg or less of bitumen fume can be sampled, we developed a micro extraction technique, using Flow Injection Analysis (FIA). The FIA-DMSO extraction was coupled with normal phase liquid chromatography (NPLC) followed by gas chromatography with flame ionisation detection. Only 9 μl of sample solution in cyclohexane (at least 2% m/v) is injected into a cyclohexane stream and extracted with DMSO in the FIA coil. A 50 μl cut from the DMSO stream is introduced into a tetrahydrofurane (THF)/n-octane (50/50) stream to the NPLC-column. A 608 μl fraction of the THF/octane stream after the NPLC column, comprising the time window in which the standard EPA 2–6 ring PAC mixture elutes, is directly introduced into the retention gap. After evaporation of most of the solvent in the retention gap, the concentrated analyte is swept into the GC for the quantitative analysis using the FID. The system is selective towards the biologically most active fraction, the unsubstituted and lightly substituted PACs. The 3–6-ring PAC content by FIA-DMSO correlates well with the Mutagenicity Index (MI) of the Mobil modified Ames test (R2= 0.927), these data have been measured on a wide range of materials ranging from bitumen fume condensates, distillate luboils to heavy residual materials such as bitumens. The residual products have to be freed from the heaviest molecules by pentane precipitation. The 3–6-ring PAC content also correlates with carcinogenicity in mouse skin-painting bioassays. This technique enables us to assess the carcinogenic hazard of potential new feedstocks, processing routes and products in an early stage of development.

Large Volume Injection in Fast Gas Chromatography with On-Column Injector

In this work a fast gas chromatography set-up with on-column injection was optimized and evaluated with a model mixture of C 8 -C 28 n-alkanes. Usual injection volumes when using narrow-bore (e.g., 0.1 mm i.d.) analytical columns are ca. 0.1 μL. The presented configuration allows introduction of 10-30-fold larger sample volumes without any distortion of peak shapes. In the set-up a normal-bore retention gap (1 m × 0.32 mm i.d.) was coupled to a narrow-bore (4.8 m x 0.1 mm i.d. x 0.4 μm film thickness) analytical column using a low dead volume column connector. The effects of the experimental conditions such as inlet pressure, sample volume, initial injection temperature, and oven temperature on a peak focusing are discussed. H-u curves for helium and hydrogen are used to compare their suitability for high speed gas chromatography and to show the dependence of separation efficiency on the carrier gas velocity at high inlet pressures. In the fast gas chromatography system a baseline separation of C 10 -C 28 n-alkanes was achieved in less than 3 minutes.

Standard on-column injection: A fast and simple method for large volume injection in medium-resolution gas chromatography

In gas chromatography (GC) large volume injection of samples containing very high molecular weight analytes presents unique problems. Standard programmed-temperature vaporizing or on-column large volume injection systems cannot be used, mainly due to instrumental difficulties. A new, simple solution to this problem is presented. If short, wide-bore columns are used at carrier gas velocities above optimum, the cloud of vapor formed upon evaporation of the solvent can be rapidly discharged from the system by the column flow alone. Hence, in medium-resolution GC, large sample volumes (e.g., 50 μL) can be injected using a standard on-column injector. Neither a retention gap nor a solvent vapor exit is needed. The absence of column connectors makes this setup extremely rugged. The optimization of the direct on-column large volume injection is described. The applicability of the new technique for the injection of large volumes of triacylglycerides and heavy mineral oil fractions is shown. It is demonstrated that the new technique is relatively insensitive toward solvent composition. This makes the new method eminently suited for combination with gradient liquid chromatography preseparations. © 2000 John Wiley & Sons, Inc. J Micro Sep 12: 523–529, 2000

Pulsed Splitless Injection and the Extent of Matrix Effects in the Analysis of Pesticides

The applicability of pulsed splitless injection to the gas chromatographic analysis of pesticide residues in fruit and vegetables has been evaluated. 22 pesticides belonging to different chemical classes, including those known to be liable to matrix induced response enhancement, were selected for the study. The parameters of pressure pulse have been tested for optimum performance of injection. Application of the pressure pulse was found to decrease matrix effects during analyses of real samples. Further decline of matrix effects was obtained using higher sample injection volumes. The installation of a deactivated retention gap was necessary to obtain good peak shapes with injection volumes exceeding 1 μL of sample. Up to 4 μL was then injected without peak distortion and consequent loss of resolution. Using 4 μL pulsed splitless injection, matrix effects were almost completely eliminated even at very low concentration levels of analytes. The highest matrix effects observed for tested compounds at the lowest concentration level tested were in the range of 110–122%.

Optimization of large-volume on-column injection conditions in gas chromatography by monitoring the actual carrier gas flow

The change of the evaporation rate of the solvent during injection and evaporation is the most critical aspect during optimization of large-volume on-column injection conditions in gas chromatography. The change is caused by the pressure drop along the retention gap when using an early solvent vapour exit (SVE) and can be described by a mathematical model. Four procedures for the optimization of the injection conditions were compared. It was found that different procedures often yield different evaporation rates, which may also depend on the injection speeds used during optimization. For optimization of a new set-up, i.e. if little is known about the optimal injection conditions, the evaporation rate should be determined by increasing the injection time at a fixed injection speed, injection temperature and head pressure; subsequently, an appropriate injection speed can be calculated. If a mere re-optimization is required as e.g. after the exchange of the retention gap, adjusting the evaporation rate to the injection speed by varying the injection temperature at a constant injection speed is the preferred procedure. With both methods, optimization can be achieved by means of 2–5 injections of pure solvent and monitoring the helium carrier gas flow. That is, optimization of the injection conditions takes less than 1 h. When using this strategy, analytes as volatile as monochlorobenzene can be determined in aqueous samples by in-vial liquid–liquid extraction–gas chromatography–mass spectrometry. Closing the SVE at the very end of solvent evaporation results in a considerable increase of the capacity of the retention gap compared to closing the SVE before all solvent is evaporated.

Automated On-Line GPC-GC-FPD Involving Co-Solvent Trapping and the On-Column Interface for the Determination of Organophosphorus Pesticides in Olive Oils

The robustness of an automated on-line GPC-GC-FPD method for the determination of Organophosphorus (OP) Pesticides in olive oil has been significantly improved, now allowing a routine operation. The original GPC-GC transfer technique employed a loop type interface with an early vapor exit and co-solvent trapping. Shooting, caused by boiling delay and malfunctioning of the solvent vapor exit during the transfer of the 3 mL solvent fraction to the retention gap, resulted in insufficient focusing of the OP-pesticides. In this interfacing concept, the GPC fraction is introduced directly into the carrier gas stream during a certain time interval using the Dualchrom pressure- and flow regulation, resulting in a very stable transfer. The interface now basically corresponds to an on-column interface with flow regulation. Via a Pyrex glass door behind the GC-oven door it can be seen that the liquid flow is broken up into alternating gas-liquid segments by the carrier gas flow during transfer. This avoids shooting in the retention gap and broadens the temperature range for successful transfer. By using a smaller sized GPC column the solvent fraction transferred was reduced to 1.3 mL. Methyl acetate/cyclopentane and the co-solvent n-nonane were selected for their lower boiling points. The modified method is very robust and has been applied to the analysis of 28 different OP-pesticides in olive oil with an overall detection limit of 0.002 mg/kg. Routinely 24 h of unattended operation is possible with a cycle time of 75 min.

Automated On-Line GPC-GC-FPD Involving Co-Solvent Trapping and the On-Column Interface for the Determination of Organophosphorus Pesticides in Olive Oils

The robustness of an automated on-line GPC-GC-FPD method for the determination of Organophosphorus (OP) Pesticides in olive oil has been significantly improved, now allowing a routine operation. The original GPC-GC transfer technique employed a loop type interface with an early vapor exit and co-solvent trapping. Shooting, caused by boiling delay and malfunctioning of the solvent vapor exit during the transfer of the 3 mL solvent fraction to the retention gap, resulted in insufficient focusing of the OP-pesticides. In this interfacing concept, the GPC fraction is introduced directly into the carrier gas stream during a certain time interval using the Dualchrom pressure- and flow regulation, resulting in a very stable transfer. The interface now basically corresponds to an on-column interface with flow regulation. Via a Pyrex glass door behind the GC-oven door it can be seen that the liquid flow is broken up into alternating gas-liquid segments by the carrier gas flow during transfer. This avoids shooting in the retention gap and broadens the temperature range for successful transfer. By using a smaller sized GPC column the solvent fraction transferred was reduced to 1.3 mL. Methyl acetate/cyclopentane and the co-solvent n-nonane were selected for their lower boiling points. The modified method is very robust and has been applied to the analysis of 28 different OP-pesticides in olive oil with an overall detection limit of 0.002 mg/kg. Routinely 24 h of unattended operation is possible with a cycle time of 75 min.

Monitoring the actual carrier gas flow during large-volume on-column injections in gas chromatography as a means to automate closure of the solvent vapour exit

Monitoring of the helium flow into a gas chromatograph (GC) by means of an electronic flow meter has been used to optimize and control large-volume on-column injections. The nature of the observed carrier gas flow-rate profiles is discussed in some detail. A rather strong dependence of the evaporation rate on the injection speed was found for injections into a 0.32 mm I.D. retention gap, which can be attributed to the pressure drop along the retention gap when using a solvent vapour exit (SVE). The variation of the evaporation rate with the injection speed was found to be less critical for a 0.53 mm I.D. retention gap. The carrier gas flow-rate profile during the actual injection was used to effect the automated closure of the SVE precisely at the end of the evaporation process. Retention gaps of 0.53 mm I.D. were preferred over 0.32 mm I.D. retention gaps, as 0.53 mm I.D. retention gaps allowed a clearer detection of the end of the evaporation process. Compared with the conventional procedure which involves closure of the SVE at a predetermined time, the present approach is more robust and hardly any optimization is required; it did not cause losses of volatile analytes. The procedure considerably simplifies the use of large-volume on-column injections. Large-volume injections of alkanes were used to study the potential of the large-volume injection–GC system.

Use of a presolvent to include volatile organic analytes in the application range of on-line solid-phase extraction–gas chromatography–mass spectrometry

The application range of the on-line solid-phase extraction–gas chromatographic (SPE–GC) analysis of aqueous samples has been extended to volatile analytes. In the new set-up, after conventional aqueous-sample loading and drying of the SPE cartridge with nitrogen gas, 30–50 μl of an organic solvent, the so-called presolvent, such as methyl acetate or ethyl acetate are introduced into the retention gap prior to the actual desorption to ensure that a solvent film is already present in the retention gap when the introduction of the analyte-containing desorption solvent starts. This procedure allows the recovery of analytes as volatile as monochlorobenzene and xylene. Aspects such as the type of retaining precolumn, and the type and amount of presolvent have been studied systematically to explain the performance of the novel set-up. Actually, when using 50 μl of presolvent, the use of a retaining precolumn did not have any significant influence on the recovery of the volatile analytes. The modified SPE–GC procedure was tested by analysing 10 ml of river Rhine water spiked at the 0.5 μg/l level with about 80 microcontaminants covering a wide range of volatility. The test compounds included chlorobenzenes, substituted and nonsubstituted aromatic compounds, anilines and phenolic compounds and organonitrogen and organophosphorus pesticides. The system performance in terms of recovery (typically 70–115% at the 0.5 μg/l level) and repeatability (R.S.D. values typically 1–9%; n=7) was satisfactory, even for monochlorobenzene, the most volatile analyte of the test mixture. Low recoveries due to early breakthrough (polar analytes) or adsorption to the tubing (apolar analytes) were observed for a few analytes only. The detection limits in SPE–GC–MS using full-scan acquisition generally were 20–50 ng/l.

Integrated SFE/SFC/MS System for the Analysis of Pesticides in Animal Tissues

The development of an integrated system for an on-line supercritical fluid extraction/cleanup/supercritical fluid chromatography/mass spectrometry analysis (SFE/SFC/MS) of thermally labile carbamate pesticides from beef and chicken meat samples is discussed. A key to the system is the mixed stationary phase (7% diol in C18) used in the cleanup column to separate target analytes from the extracted lipid matrix interferences. The detection limits of the system at 2:1 signal to noise ratio were 200 and 175 ppb for bendiocarb and carbaryl, respectively. The reproducibility of the present system was not totally favorable due to trapping problems, resulting from frequent clogging of either the cryogenic retention gap between the extraction cell and the SFC or the actual restrictor in the retention gap. Operating in an integrated mode, 53% of the experiments were completed without complications connected to extracting and detecting the analyte. Keywords: Carbamates; pesticides; SFE; SFC; integrated system; on-li...

In-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of anilines in tap, surface, and wastewater

In this study miniaturized liquid–liquid extraction carried out in an autosampler vial was used to determine a series of anilines in tap, surface, and industrial wastewater. The analytes were extracted from the aqueous phase into n-hexane and 100 μl of the extract, i.e., 10–20%, was injected into the gas chromatography (GC) system using an on-column injector in combination with a deactivated uncoated fused silica capillary (retention gap) and an early solvent vapor exit. A mass spectrometer was used for detection and identification. The parameters for large-volume injection, i.e., the evaporation rate, speed of injection, and injection temperature, were optimized using accurate measurement of the actual flow rate of the carrier gas into the GC system. This measurement was used as input for a solvent vapor-exit controller, which was equipped with a microprocessor for determination of the endpoint of the evaporation process. The optimum conditions for an injection volume of 100 μL of n-hexane are reported. Extraction with a water-to-n-hexane-ratio (1:1, v/v) gave quantitative recoveries except for aniline (46%). The addition of sodium chloride improved the extraction yield for aniline about 10%. Linear calibration curves were obtained in the 0.01–1-μg/L concentration range, relative standard deviations were less than 10%, and detection limits typically were 4–20 ng/L for GC–mass spectrometry in the multiple-ion-detection mode. The total procedure allowed the analysis of unfiltered water samples; that is, no sample treatment was necessary. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 581–588, 1998

In‐vial liquid–liquid extraction with subsequent large‐volume on‐column injection into GC–MS for the determination of anilines in tap, surface, and wastewater

In this study miniaturized liquid–liquid extraction carried out in an autosampler vial was used to determine a series of anilines in tap, surface, and industrial wastewater. The analytes were extracted from the aqueous phase into n-hexane and 100 μl of the extract, i.e., 10–20%, was injected into the gas chromatography (GC) system using an on-column injector in combination with a deactivated uncoated fused silica capillary (retention gap) and an early solvent vapor exit. A mass spectrometer was used for detection and identification. The parameters for large-volume injection, i.e., the evaporation rate, speed of injection, and injection temperature, were optimized using accurate measurement of the actual flow rate of the carrier gas into the GC system. This measurement was used as input for a solvent vapor-exit controller, which was equipped with a microprocessor for determination of the endpoint of the evaporation process. The optimum conditions for an injection volume of 100 μL of n-hexane are reported. Extraction with a water-to-n-hexane-ratio (1:1, v/v) gave quantitative recoveries except for aniline (46%). The addition of sodium chloride improved the extraction yield for aniline about 10%. Linear calibration curves were obtained in the 0.01–1-μg/L concentration range, relative standard deviations were less than 10%, and detection limits typically were 4–20 ng/L for GC–mass spectrometry in the multiple-ion-detection mode. The total procedure allowed the analysis of unfiltered water samples; that is, no sample treatment was necessary. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 581–588, 1998