Practical fast gas chromatography: methods,instrumentation and applications

Minimal time of operation in gas chromatography (GC) has been a research topic ever since the introduction of GC. Today, revived interest in fast GC is seen to be driven by applications, such as process control and high-throughput analysis, or by the desire to reduce the costs of operation and ownership in routine analysis. Numerous options exist for speeding up GC separations. Which option to select depends strongly on the application under study. The first step should always be to see if it is possible to trade resolution for time. If this approach fails, the concept of “resolution-normalised conditions” comes into play. An overview of options for faster chromatography and a system for classifying chromatograms are brought together to give guidelines for speeding up specific applications. The practical consequences of implementing these options are discussed and the instrumental requirements are outlined. Applications of the various methods for faster GC are given.     

Fast gas chromatography for pesticide residues analysis

The importance of method development in the area of pesticide residues analysis is apparent from legislative requirements continuously decreasing the maximum acceptable concentration levels in food and water. This covers also contribution in the science in the field of ultra-trace analysis of organic pollutants in complex mixtures. Analysis time is one of the most important aspects that should be considered in the choice of analytical methods for routine application. With this fact, fast gas chromatography (GC) has acquired a real importance in the pesticide residue analysis. This paper provides an overview of fast GC methods for analysis of pesticide residues in variety of matrices at ultra-trace concentration levels. Emphasis is put on the development in the last 6 years.     

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 microm) with normal-bore retention gap (1 m long, 0.32 mm i.d.) allowed maximal injection volume 8 microl for 4 mm i.d. liner used without any peak distortion when solvent recondensation in the retention gap was employed.     

Analysis of pesticide residues by fast gas chromatography in combination with negative chemical ionization mass spectrometry

A combination of fast GC with narrow-bore column and bench top quadrupole mass spectrometer (MS) detector in negative chemical ionization (NCI) mode (with methane as reagent gas) is set up and utilized for the ultratrace analysis of 25 selected pesticides. The observed pesticides, belonging to the endocrine disrupting chemicals (EDCs), were from different chemical classes. A comparative study with electron impact (EI) ionization was also carried out (both techniques in selected ion monitoring (SIM) mode). The programmed temperature vaporizer (PTV) injector in solvent vent mode and narrow-bore column (15 m × 0.15 mm I.D. × 0.15 μm film of 5% diphenyl 95% dimethylsiloxane stationary phase) were used for effective and fast separation. Heptachlor (HPT) as internal standard (I.S.) was applied for the comparison of results obtained from absolute and normalized peak areas. Non-fatty food matrices were investigated. Fruit (apple – matrix-matched standards; orange, strawberry, plum – real samples) and vegetable (lettuce – real sample) extracts were prepared by a quick and effective QuEChERS sample preparation technique. Very good results were obtained for the characterization of fast GC–NCI-MS method analysing EDCs pesticides. Analyte response was linear from 0.01 to 150 μg kg^?1 with the R² values in the range from 0.9936 to 1.0000 (calculated from absolute peak areas) and from 0.9956 to 1.0000 (calculated from peak areas normalized to HPT). Instrument limits of detection (LODs) and quantification (LOQs) were found at pg mL^?1 level and for the majority of analytes were up to three orders of magnitude lower for NCI compared to EI mode. In both ionization modes, repeatability of measurements expressed as relative standard deviation (RSDs) was less than 10% which is in very good agreement with the criterion of European Union.     

Fast gas chromatography for pesticide residues analysis using analyte protectants

Fast GC-MS with narrow-bore columns combined with effective sample preparation technique (QuEChERS method) was used for evaluation of various calibration approaches in pesticide residues analysis. In order to compare the performance of analyte protectants (APs) with matrix-matched standards calibration curves of selected pesticides were searched in terms of linearity of responses, repeatability of measurements and reached limit of quantifications utilizing the following calibration standards in the concentration range 1-500 ng mL(-1)(the equivalent sample concentration 1-500 microg kg(-1)): in neat solvent (acetonitrile) with/without addition of APs, matrix-matched standards with/without addition of APs. For APs results are in a good agreement with matrix-matched standards. To evaluate errors of determination of concentration synthetic samples at concentration level of pesticides 50 ng mL(-1) (50 microg kg(-1)) were analyzed and quantified using the above given standards. For less troublesome pesticides very good estimation of concentration was obtained utilizing APs, while for more troublesome pesticides such as methidathion, malathion, phosalone and deltamethrin significant overestimation reaching up to 80% occurred. According to presented results APs can be advantegously used for "easy" pesticides determination. For "difficult" pesticides an alternative calibration approach is required for samples potentially violating MRLs. An example of real sample measurement is shown. In this paper also the use of internal standards (triphenylphosphate (TPP) and heptachlor (HEPT)) for peak areas normalization is discussed in terms of repeatability of measurements and quantitative data obtained. TPP normalization provided slightly better results than the use of absolute peak areas measurements on the contrary to HEPT.     

Practical approaches to fast gas chromatography-mass spectrometry

Fast gas chromatography–mass spectrometry (GC–MS) has the potential to be a powerful tool in routine analytical laboratories by increasing sample throughput and improving laboratory efficiency. However, this potential has rarely been met in practice because other laboratory operations and sample preparation typically limit sample throughput, not the GC–MS analysis. The intent of this article is to critically review current approaches to fast analysis using GC–MS and to discuss practical considerations in addressing their advantages and disadvantages to meet particular application needs. The practical ways to speed the analytical process in GC and MS individually and in combination are presented, and the trade-offs and compromises in terms of sensitivity and/or selectivity are discussed. Also, the five main current approaches to fast GC–MS are described, which involve the use of: (1) short, microbore capillary GC columns; (2) fast temperature programming; (3) low-pressure GC–MS; (4) supersonic molecular beam for MS at high GC carrier gas flow; and (5) pressure-tunable GC–GC. Aspects of the different fast GC–MS approaches can be combined in some cases, and different mass analyzers may be used depending on the analytical needs. Thus, the capabilities and costs of quadrupole, ion trap, time-of-flight, and magnetic sector instruments are discussed with emphasis placed on speed. Furthermore, applications of fast GC–MS that appear in the literature are compiled and reviewed. At this time, the future usefulness of fast GC–MS depends to some extent upon improvement of existing approaches and commercialization of interesting new techniques, but moreover, a greater emphasis is needed to streamline overall laboratory operations and sample preparation procedures if fast GC–MS is to become implemented in routine applications.     

Practical limitations of capillary gas chromatography

Glass capillary gas chromatography is a high resolution separation method which allows the qualitative and quantitative analysis of even complex mixtures, which may contain many components–also isomeric–in a wide range of volatilities, polarities and concentrations. The principal limitation of gas chromatographic application is given by an insufficient volatility of the species to be separated. Elevated temperatures have to be applied if the application range is to be extended and to achieve steep peak profiles, i.e. low detection limits at high resolution. The use of elevated temperatures is limited, of course, by the temperature stability of both the solvent (stationary liquid and support) and the solutes. The problems of trace analysis for low volatility compounds at high resolution and its limitational parameters regarding sampling, separation and detection are discussed. The applicability of glass capillary columns in this field is influenced by the following parameters: tailing behaviour; irreversible adsorption of polar and decomposition of unstable solutes; thermal stability of stationary liquid (including the support deactivation); separation efficiency and sample capacity (film thickness). Multidimensional gas chromatography using capillary columns coupled either with a packed or another capilllary column for preseparations may be applied with advantage in the analysis of complex mixtures.     

Evaluation of low-pressure gas chromatography linked to ion-trap tandem mass spectrometry for the fast trace analysis of multiclass pesticide residues

A rapid multiresidue method for the analysis of 72 pesticides has been developed using a single injection with low-pressure gas chromatography/tandem mass spectrometry (LP-GC/MS/MS). The LP-GC/MS/MS method used a short capillary column of 10 m x 0.53 mm i.d. x 0.25 microm film thickness coupled with a 0.6 m x 0.10 mm i.d. restriction at the inlet end. Optimal LP-GC conditions were determined which achieved the fastest separation in MS/MS detection mode. Also MS/MS conditions were optimized in order to increase sensitivity and selectivity. The analytical parameters of the LP-GC/MS/MS method were compared with those obtained by GC/MS/MS using a conventional capillary column (30 m x 0.25 mm i.d. x 0.25 microm film thickness). Better precision and sensitivity values were obtained with the LP-GC/MS/MS approach. The limits of detection (LOD) of the compounds ranged from 0.1 to 14.1 microg L(-1) for LP-GC/MS/MS, lower than those obtained for conventional GC/MS/MS that ranged from 0.1 to 17.5 microg L(-1). The peak widths obtained with the short column in LP-GC are similar to those obtained using conventional capillary GC columns, and the peaks can be successfully identified by MS/MS detection with the conventional scan speed of ion-trap instruments. In addition, the analysis time was significantly reduced with LP-GC/MS/MS (32 min) versus GC/MS/MS (72 min), allowing the number of samples analyzed per day in a routine laboratory to be doubled.     

Evaluation of time-of-flight mass spectrometric detection for fast gas chromatography

Separations below 1 s of a mixture of organic compounds ranging from C5 to C8 have been performed to investigate the performance of a time-of-flight mass spectrometer in fast gas chromatography. The gaseous samples were focussed on a cold trap, and then injected after thermal desorption to obtain the required narrow input band-widths. Also, to obtain a very fast separation, a short narrow bore column was used, operated at above-optimum inlet pressures. With this system, it was possible to identify ten compounds within 500 ms, showing peak-widths (2.354sigma) as narrow as 12 ms. The spectral acquisition rate used for these analyses was 500 Hz. The quality of the recorded spectra and the comparison with library spectra was very high. Deconvolution algorithms offer the possibility of identifying overlapping peaks. It is shown that the spectral scan speed of the time-of-flight mass spectrometer is high enough for very fast separations.     

Rapid gas analyses for the investigation of spontaneous combustion using capillary gas chromatography

A simple and fast but sensitive and precise gas chromatographic method is described for the quantitative determination of O(2), N(2), CO, CO(2), C1 and C2-hydrocarbons for coal research. Gas analyses are necessary to obtain parameters for modelling spontaneous combustion and to predict long term coal behaviour. The method is based on a single PLOT-type capillary column in a single channel gas chromatograph. Using a micro-volume TCD coupled in series with a FID detector containing a capillary methanizer it is possible to determine high and trace level gases simultaneously. Trace quantities of CO and CO(2) can be determined with a single analysis and the detection limits are improved significantly using the capillary methanizer. The detection limit of the described method is approximately ten parts per million CO(2) and one part per million CO. Using the same instrument configuration the O(2)/N(2) ratios (major components), as parameter for coal reactivity, are also determined. The proposed approach is restricted to the determination of gases evolved during coal studies and the application to other gas mixtures is not considered.     

Day-dream on process and laboratory gas chromatography

Three main problems have to be solved in process gas chromatography: psychological, technical and economic. A simple concept already used with success for fifteen years by the author, called the Deferred Standard, is an answer for process GC and, to some extent, laboratory GC as well. That the injection of a pure compound in each analytical sequence is reproducible demonstrates the good working order of PGC. Accurate quantitative results may be obtained because the gas density balance is used as a calibrator detector for determining the relative response factors of the components of interest with respect to the Deferred Standard. By suppressing any calibration standards in process and laboratory chromatography, the Deferred Standard may reveal itself as a time and money saving technique.     

Multi-dimensional chromatography using on-line coupled microcolumn liquid chromatography—capillary gas chromatography for quantitative pesticide residue analysis

Highly selective separations can be achieved by utilizing different separation modes in a multi-dimensional (on-line coupled) chromatographic system. The application of such a system utilizing microcolumn liquid chromatography coupled on-line to capillary gas chromatography for the determination of 2-(1,1-dimethylethyl)-5-pyrimidinol, a pesticide metabolite, in various corn matrices at the ng g^?1 level is described. A comparative quantitative study with conventional manual sample pretreatments followed by capillary gas chromatography-mass spectrometry indicated equivalence between the two techniques. The system offers the advantages of reduced sample handling steps and analysis time, high potential for automation and adequate sensitivity.     

Comprehensive two-dimensional gas chromatography for fast screening of wash oils

Fast screening of wash oils is demonstrated using comprehensive two-dimensional gas chromatography (GCxGC). Wash oils are used in ethylene production plants to minimize compressor fouling. The composition of a wash oil determines its effectiveness in solubilizing heavy hydrocarbons. In particular, the relative amount of 1- and 2-ring aromatics is important. The presence of oxygenates is undesirable because of adverse effects to the process. It is shown that GCxGC is well suited for this application. Species in wash oils are separated and grouped into three bands: a nonpolar aliphatics band, 1- and 2-ring aromatics band, and polyaromatics band. For a given polar secondary column, the spacing between bands in the second dimension can be adjusted in a broad range by selecting a primary column and an oven-temperature-programming rate. Integration of GCxGC peaks is evaluated using a standard GC integration program and a new GCxGC integration program. Consistent results are obtained using both programs for well-separated GCxGC peaks with relative differences for individual peak ranging from 0.04% to 1.6%. Peak responses are integrated by the GCxGC software, and the relative amounts of aromatics content and aliphatics content are estimated by peak response percent with relative standard deviations ranging from 0.15% to 2.8% (n = 3).     

Clean-up methods for polycyclic aromatic hydrocarbons analyses by capillary gas chromatography

Summary Comparative results of pre-treatment methods for the extracts of polycyclic aromatic hydrocarbons (PAHs) from airborne particulate matter for quantitative determination by gas chromatography (GC) are presented. The first method included liquid-liquid extraction and column liquid adsorption chromatography. In the second procedure the extract was fractionated by liquid-liquid chromatography and liquid adsorption chromatography. In the last procedure two different solid phase extraction (SPE) cartridges examined makes it possible to separate PAHs and remove paraffinic compounds which is very important for the quantitative GC analysis of the PAHs. Recoveries of PAH standards and some their derivatives were determined and the contents of 15 of the most important PAHs, were compared. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Further applications for capillary gas chromatography in routine quantitative toxicological analyses

The use of capillary column gas chromatography for drug screening in forensic toxicology has become increasingly widespread. Screening procedures however are often lengthy and unsuitable for rapid confirmatory or quantitative applications. In order to develop a practical scheme for confirmatory/quantitative analysis, we have optimized a series of temperature profiles to allow the rapid quantitative determination of a wide range of acid/neutral and basic drugs in extracts from post mortem fluids and tissue. The appropriate profile is selected based on the retention index on a standard crosslinked methyl silicone column used to screen extracts. The use of a 5% phenyl methyl silicone phase allows complementary identification, and allows the separation of some pairs of compounds with identical retention indices in the screning procedure.     

Column selection and optimization for sulfur compound analyses by gas chromatography

The analysis of sulfur-containing compounds using fused silica capillary columns and the Sulfur Chemiluminescence Detector has been investigated. This combination of an inert chromatographic system and a high sensitivity, selective detector provides significant advantages for the analysis of low levels of sulfur compounds in complex matrices over existing techniques. Capillary columns coated with thick films (1–4 μm) of methyl silicone stationary phase permit separation of most sulfur containing compounds and, when used with sub-ambient column temperatures, these columns can be used for the separation of sulfur gases. The effects of stationary film thickness, column length, and internal diameter for the measurement of sulfur compounds in hydrocarbon matrices has been determined.     

Matrix-induced response enhancement in pesticide residue analysis by gas chromatography

The sample matrix can cause an enhancement in the observed chromatographic response for pesticide residues in a matrix extract compared with the same concentration in a matrix-free solution. The matrix increases the transfer of pesticides from hot vaporizing injectors by reducing the thermal stress for labile compounds and by masking active sites in the injector responsible for the adsorption or decomposition of polar pesticides. The use of different injector types and matrix simplification procedures can reduce matrix-induced enhancement but do not eliminate it. The most effective strategy is to use matrix-matched calibration standards or analyte protectants which equalize the response enhancement for calibration standards and sample extracts. From a practical point of view it is important that the method used to correct for matrix-induced enhancement is compatible with low system maintenance. The different approaches for correcting matrix-induced enhancement for calibration in pesticide residue analysis are discussed and compared in this review.