Reflecting on the Influence of the Current State of Sample Preparation on GC, Part 1

Evaluation of common organic solvents for gas chromatographic analysis and stability of multiclass pesticide residues*1

Evaluation of common organic solvents for gas chromatographic analysis and stability of multiclass pesticide residues

Gas chromatographic determination of malonaldehyde formed by lipid peroxidation

A novel method for ultrashort capillary column gas chromatography (GC) analysis is introduced, which employs on-column injection and detection and rapid temperature programming. Using 10–20 cm long capillary columns, results showed that the method provides efficient and very rapid separations for relatively simple mixtures. Moreover, the on-column aspect of the method used here is demonstrated to avoid the extra column analyte degradation that can occur in traditional approaches to such separations. As a result, the developed method allows for the first time the GC analysis of some very large and (or) highly thermally labile analytes, such as polypeptides and drug molecules that are normally prone to decomposition. As an application, this method is further used to monitor pharmaceutical degradant formation as a function of temperature and was found to provide similar results to those obtained from conventional high-performance liquid chromatography analysis. Overall, the findings indicate that this ultrashort GC column approach could be useful in these areas and potentially others, where relatively simple GC analysis and universal flame ionization detection is desirable.

This presentation describes amino acid analysis with the gas chromatographic method and experimental conditions using the N-trifluoroacetyl n-butyl ester derivatives; the study we describe here was undertaken to compare gas chromatographic (GC) and ion-exchange chromatographic (IEC) analyses of amino acids in hydrolysates of 9 diverse sample types to gain insight into effects of these 2 chromatographic methods of analysis on variation in amino acid results. Our study showed that values for samples prepared by 2 separate laboratories using the same procedure were generally in good agreement when all of the hydrolysates were analyzed by a single laboratory using a single method of analysis. To compare results from gas chromatography with those from ion-exchange chromatography analyses were performed by 2 different laboratories on the same hydrolysates and on different hydrolysates prepared by the same method by both laboratories. The data demonstrate that GC and IEC can be expected to yield essentially identical results when applied to the same hydrolysate. Agreement is so close that interlaboratory differences in hydrolysate preparation of the same sample contribute as much to variation in amino acid results as does the method of analysis, a fact which should be noted in planning collaborative studies.

A gas chromatographic (GC) method has been developed for the simultaneous quantitation of fatty acids and sterols in orange juice, using a bonded phase fused silica capillary column of intermediate polarity, splitless automatic injection, and flame ionization detection. Sample preparation has been simplified by using 1 g C-18 adsorbent in a disposable minicolumn to extract 2 mL orange juice. Methylation of fatty acids and silylation of the sterols were carried out in the eluted extract (low polarity lipid fraction). The method precision was 7%; recoveries ranged from 83 to 113%. The precision of the injection technique was 2%. Seven major fatty acids and 5 sterols in orange juice were quantitated by the GC method and identified by GC/mass spectrometry. Quantitative data for several orange juice samples indicated that the levels of the compounds of interest were in the 1.3-72.0 mg/L range. The results demonstrate that bonded phase fused silica capillary GC has great versatility and potential for the quantitative determination of fatty acids and sterols.

Studies are being carried out by our laboratory with the overall goal to simplify gas chromatography (GC) methods, to make them more rugged, to simplify sample extract cleanup and to shorten GC analysis time. To simplify sample extract cleanup, the practicality of the use of commercially available disposable cartridges is investigated. These cartridges contain specified amounts of standardized adsorbents such as Florisil, alumina, and silica gel, or silica modified to contain active sites like amino, cyano and diol groups. Their use can result in considerable savings in time and materials. Also, use of such cartridges greatly reduces the chances of adsorbent material variation and minimizes operator error. Our efforts in GC analysis improvement focus on the use of capillary columns for all GC methods and on establishing identical GC conditions for both the primary and confirmatory column. This allows simultaneous analyses on the primary and the confirmatory column when dual-column gas chromatography are used which considerably reduces the time required for the GC analysis.

Background: Valproic acid is widely used as an anticonvulsant and mood-stabilizing drug in the treatment of epilepsy. Determination of serum valproic acid is required in epilepsy therapy for efficient control of seizures. Objectives: In the present study, the gas chromatography method was applied to determine serum valproic acid and resulting data were compared with those obtained by high performance liquid chromatography. Materials and Methods: Two methods were used: assayed patients' serum samples and serum quality control materials for laboratory diagnostics applied to high performance liquid chromatography (HPLC)and gas chromatography (GC). All chemicals had a grade suitable for analysis. Results: The obtained results in both systems were in agreement regarding linearity, correlation and recovery. The results revealed a lower GC limit of quantification (8 µg/mL), a better intra-assay coefficient of variation (CV) (3.86% - 3.99%) and inter-assay CV (6.14% - 6.42%) for all three levels of control materials. In addition, a shorter run time of the GC vs. the HPLC chromatographic step (6 minutes vs. 17 minutes) was observed. Conclusions: The present study reports a short run time of the chromatographic step as well as a good sensitivity for the GC method. Furthermore, the significance of this method is the simplicity in sample preparation step without the need for derivatization and specific detector ensuring cost-effectiveness for the GC analysis. In summary, in comparison with HPLC method, our obtained data illustrate a good performance of the GC analytical procedure and confirm its usefulness for the routine quantification of valproic acid level in patients' serum specimens.

Sample preparation and introduction techniques are very critical steps in gas chromatography analysis and particularly in the analysis of volatiles in solid samples. In these cases, they can be divided into two main categories: direct and indirect approaches, based on how the solid sample is treated, i.e. with and without dissolution (or extraction) of analytes from the solid sample. To enable routine application, coupling with sample preparation techniques (especially solid or solvent-based microextractions) is needed to achieve automation. Here, an overview of the most common sample introduction techniques for gas chromatography with their advantages and drawbacks is presented and discussed, including references to relevant examples. So, this review can serve as guidance for new users.

The carbon number distributions of 26 stock tank oils were determined by simulated distillation using gas chromatography and supercritical fluid chromatography using high-pressure CO2 as the carrier fluid. Carbon number distributions determined by the two methods were equivalent within normal experimental scatter. Comparison of the fraction of each crude oil eluted in the GC and SFC analyses indicates that for most oils, a significantly larger fraction of the oil elutes in an SFC analysis. In fact, essentially 100% of the hydrocarbons present in light oils elute from the column. In such cases, the internal standard analysis can be eliminated from SIMDIS without loss of accuracy. Another advantage of SIMDIS by SFC is that there is no danger of hydrocarbon decomposition at high temperature.

Chapter 21 Sampling and sample preparation for fragrance analysis

Leachate and soil samples collected from different tillage systems were analyzed for atrazine using gas chromatography (GC) and an enzyme‐linked immunosorbent assay (ELISA) based on magnetic particle technology. Solid‐phase extraction (SPE) was used to concentrate atrazine residues in leachate samples and soil extracts before GC analysis. Atrazine concentrations determined by GC ranged from 0.1 to 600 μg L‐1 for water samples and from 1.0 to 700 μg kg‐1 for soil samples. Atrazine concentrations in 92 leachate samples as determined by ELISA were well‐correlated (R= 0.97) with GC levels over the entire concentration range. Soil samples (215) were prepared and analyzed by three combinations of extraction/detection methods: 1)conventional extraction for GC / detection by GC analysis; 2)conventional extraction for GC / detection by ELISA analysis; 3)extraction for ELISA using a commercially available field kit / detection by ELISA analysis. Methanol (MeOH) in water was the common extractant. Although the initial comparison of soil extracts between the two different systems (Method 1 versus Method 3) was favorable (R = 0.97), two‐thirds of the samples contained levels below the lower threshold for atrazine detection by both methods and some extracts were perceived to provide unfavorable substrate conditions (> 10% MeOH). Elimination of these data points reduced the correlation value (R = 0.77). To determine possible sources of variability, the extraction and detection methods were examined separately. In a comparison of extraction methods (Method 2 versus Method 3), ELISA analysis of kit extracts underestimated (R = 0.71) atrazine levels compared to those conventionally extracted, suggesting that differences in extraction time between methods may have accounted for reduced kit efficiency. Where detection methods (Method 1 versus Method 2) were compared on specific extracts (< 10% MeOH), good agreement (R = 0.99) was achieved between ELISA and GC values, illustrating that control of extractant concentration is critical in using this assay for atrazine detection in soil.

The transformations of 1,2,3,4-tetrahydrodibenzothiophene (THDBT) were investigated with pure cultures of hydrocarbon-degrading bacteria. Metabolites were extracted from cultures with dichloromethane (DCM) and analyzed by gas chromatography (GC) with flame photometric, mass, and Fourier transform infrared detectors. Three 1-methylnaphthalene (1-MN)-utilizing Pseudomonas strains oxidized the sulfur atom of THDBT to give the sulfoxide and sulfone. They also degraded the benzene ring to yield 3-hydroxy-2-formyl-4,5,6,7-tetrahydrobenzothiophene. A cell suspension of a cyclohexane-degrading bacterium oxidized the alicyclic ring to give a hydroxy-substituted THDBT and a ketone, and it oxidized the aromatic ring to give a phenol, but no ring cleavage products were detected. GC analyses with an atomic emission detector, using the sulfur-selective mode, were used to quantify the transformation products from THDBT and dibenzothiophene (DBT). The cyclohexane degrader oxidized 19% of the THDBT to three metabolites. The cometabolism of THDBT and DBT by the three 1-MN-grown Pseudomonas strains resulted in a much greater depletion of the condensed thiophenes than could be accounted for in the metabolites detected by GC analysis, but there was no evidence of sulfate release from DBT. These 1-MN-grown strains transiently accumulated 3-hydroxy-2-formylbenzothiophene (HFBT) from DBT, but it was subsequently degraded. On the other hand, Pseudomonas strain BT1d, which was maintained on DBT as a sole carbon source, accumulated 52% of the sulfur from DBT as HFBT over 7 days, and, in total, 82% of the sulfur from DBT was accounted for by the GC method used. Lyophilization of cultures grown on 1-MN with DBT and methyl esterification of the residues gave improved recoveries of total sulfur over that obtained by DCM extraction and GC analysis. This suggested that the further degradation of HFBT by these cultures leads to the formation of organosulfur compounds that are too polar to be extracted with DCM. We believe that this is the first attempt to quantify the products of DBT degradation by the so-called Kodama pathway.

Soil fumigants are characterized by high vapor pressures and low boiling points, which determine that the commonly used solvent extraction-based methods are not suitable for sample preparation. The paper reports static headspace gas chromatography (GC) methods for the analysis of the fumigants 1,3-dichloropropene (1,3-D) and methyl isothiocyanate (MITC) in soil and water. Method optimization was achieved by modifying the sample matrix with concentrated salt solution and adjusting sample equilibration temperature and time on the headspace autosampler. The headspace GC methods showed1 order of magnitude greater GC response than a cold solvent extraction method under the same GC conditions and also gave cleaner chromatograms and more stable baselines. When applied to soil samples taken from a fumigated field, the headspacee GC method showed reproducibility similar to that of a solvent extraction method but had an improved detectability for deep-layer samples that contained low levels of 1,3-D.

Gas chromatography (GC) is a widely used analytical technique in many fields. Sample preparation is very important in GC analysis due to its time consumed and deviations produced. In the present paper, the progress of some typical sample preparation techniques in gas chromatography, including purge and trap, solid phase extraction, solid phase microextraction, liquid phase microextraction, microwave assisted extraction, ultrasonic-assisted extraction, etc., are reviewed.