Separation of metal complexes by on-line coupled LC-GC

The on-line coupled LC-GC technique was applied to the analysis of several metal chelates of N,N-diethyldithiocarbamic acid. A 10-port valve interface was used to couple the LC and GC instruments. Conditions during sample transfer into the GC gave fully concurrent solvent evaporation. The chelates investigated were separated with a short apolar fused silica column. LC preseparation was made with cyano or amino phases using a hexane/dichloromethane mixture as eluent. On-line LC-GC combination seems to be very suitable for the separation of the metal chelates studied.     

Screening of four beta-blockers and codeine in urine by on-line coupled RPLC-GC with on-line derivatization

On-line coupled reversed phase liquid chromatography-capillary gas chromatography (RPLC-GC) was used in the separation of four derivatized beta-blockers and codeine in urine. Sample clean-up was accomplished in the LC part and compounds were separated in the GC part. After the LC column the aqueous phase was switched to organic solvent by on-line liquid-liquid extraction and the two phases were separated in a sandwich-type phase separator. The organic extract was then transferred to a loop-type LC-GC interface. Beta-blockers were derivatized on-line in the interface before GC analysis. Concurrent eluent evaporation was used during the introduction of the sample fraction, and excess of solvent vapors was removed via an early vapor exit. The sample pretreatment was minimal; the only manual pretreatment step was the filtration of the urine sample.     

Applications of coupled LC-GC: A review

Although coupled liquid chromatographygas chromatography (LC-GC) was first demonstrated ten years ago, only in the last few years has there been a sudden surge of interest in the technique. Approximately 70% of the total number of LC-GC applications have been published in the last two years (1987–88) alone. This review categorizes LC-GC publications into four main application areas: fossil fuels, foods, environmental samples, biologiical/pharmaceutical samples, and miscellaneous samples. Multidimensional separations carried out using other coupled-column chromatographic techniques (such as supercritical fluid chromatography (SFC) with GC, and on-line trace enrichment-GC) have also been included in this review.     

On-line sample treatment—capillary gas chromatography

Summary Sample pretreatment is often the bottleneck of a tracelevel analytical procedure. In order to increase performance, increasing attention is therefore being devoted to combining sample pretreatment on-line with the separation technique that has to be used. In the present review, a variety of procedures in use today for sample treatment coupled on-line to capillary gas chromatography (GC) is briefly discussed. Special attention is devoted to coupled-column techniques such as SPE-GC and LC-GC (SPE, solid-phase extraction; LC, column liquid chromatography) which are topics of much current interest, also because of their frequent use in so-called hyphenated systems.     

Pressurised hot water extraction coupled on-line with liquid chromatography-gas chromatography for the determination of brominated flame retardants in sediment samples

Pressurised hot water extraction (PHWE) was coupled on-line with liquid chromatography-gas chromatography (LC-GC) to determine brominated flame retardants in sediment samples. After extraction with pressurised hot water the analytes were adsorbed in a solid-phase trap. The trap was dried with nitrogen and the analytes were eluted to the LC column, where the extract was cleaned, concentrated and fractionated before transfer to the GC system. The fraction containing the brominated flame retardants was transferred to the GC system via an on-column interface. The PHWE-LC-GC method was linear from 0.0125 to 2.5 microg with limits of detection in the range 0.70-1.41 ng/g and limits of quantification 6.16-12.33 ng/g.     

New on-line concentrator for LC-GC: Analysis of PAHs with LC-GC

A new, versatile, and low cost on-line LC-GC interface has been devloped for the fast and reliable introduction of large volume samples into a cappillary GC column without using the conventional retention gap. PAHs in soot were analyzed by on-line normal phase HPLC-capillary GC. A glass, vial-shaped on-line concentrator provides a zone for solvent evaporation and sample concentration. Large volumes of HPLC eluate can be concentrated with the on-line concentrator and then transferred directly into the cappillary column. Trace levels (< 10 ppb) of PAH compounds can be efficiently concentrated with the on-line concentrator and determined without loss or contamination.     

Determination of p,p′-DDE and PCBs in adipose tissues using high-performance liquid chromatography coupled on-line to capillary GC – the EURAMIC study

Long-term exposure to fat-soluble xenobiotics is assessed by the concentration of DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethyl-ene], the persistent metabolite of DDT, in subcutaneous fat, aspirated from the buttocks of breast cancer patients and age-matched controls, from five European centers collaborating in a case-control study on breast cancer. In such studies using sample material of living subjects only small amounts of samples can be made available for analysis. In this particular study the only sample material available for the analysis of DDE were aliquots of the aspirates that were originally analyzed for fatty acids. Due to the small sample quantities available, e.g. aliquots of 200–800 p. 1, on-line LC-GC is most convenient because a major part of the sample can be used in the analytical procedure. In the LC-GC procedure 50 μl of sample was injected on the LC column resulting in a 180 μl fraction containing the analytes of interest. The LC fraction was transferred to a capillary GC with electron capture detector by means of partially concurrent solvent evaporation. This way, sample handling is minimized thus reducing losses and preventing contamination. The feasibility of the on-line LC-GC system for the analysis of DDE is demonstrated with the analysis of 634 adipose tissue extracts in different series. The validity of this approach, using samples already analyzed for another parameter, with LC-GC is clearly demonstrated by the fact that in over 97% of the samples DDE can be quantified. Thus rendering a meaningful data-set for further epidemiological evaluation. The DDE levels found ranged from 0.99–3.13 μg/g adipose tissue.     

The use of combined LC-GC for the analysis of fuel products: A review

A review of the analysis of petroleum fuels, oils, tars and engine exhaust particulate extracts by on-line coupled LC-GC is presented.     

Fat content for nutritional labeling by supercritical fluid extraction and an on-line lipase catalyzed reaction

A method using sequential supercritical fluid extraction (SFE) and enzymatic transesterification has been developed for the rapid determination of total nutritional fat content in meat samples. SFE conditions of 12.16 MPa and 50°C were utilized to extract lipid species from the sample matrix. The enzymatic transesterification of the lipids by methanol was catalyzed by an immobilized lipase isolated from Candida antarctica. Conversion of the triglycerides to fatty acid methyl esters was monitored by supercritical fluid chromatography, while the fatty acid content of the extract was determined by capillary gas chromatography (GC). Total fat, saturated fat and monounsaturated fat contents were calculated from the GC data and compared to values from traditional extraction and lipid determination methods. Both off-line SFE and automated SFE followed by on-line GC analysis using two different instruments were utilized in this study. The enzymatic-based SFE method gave comparable results to the organic solvent extraction-based method followed by conventional BF₃-catalyzed esterification.     

Determination of acylglycerols in vegetable oil methyl esters by on-line normal phase LC-GC

An on-line LC-GC method for the analysis of mono-, di-, and triacylglycerols in vegetable oil methyl esters has been developed. The concentrations of these components have turned out to be key parameters for the quality of diesel fuel substitutes. Separation of all classes of acylglycerols from the fatty acid methyl ester matrix is achieved by LC after acetylation of the hydroxyl groups. The acylglycerol fraction is transferred on-line to GC, using the loop-type interface and concurrent eluent evaporation. Quantification of mono-, di-, and triacylglycerols is performed by combining external calibration with internal standardization. Both recovery of the procedure and reproducibility of the quantitative results are evaluated.     

An empirical retention model for the optimization of on-line normal-phase LC-GC for the multi-analyte determination of hydrophobic compounds in fatty samples

Summary Normal-phase LC (NPLC) is a powerful method for the clean-up of fatty samples in the determination of organochlorine pesticides (OCPs). The injected sample deactivates the stationary phase and the triglyceride matrix therefore serves as a polarity modifier in the NPLC separation. Thus, the amount of sample injected is the key to both selectivity and sensitivity in matrixmodified LC coupled to capillary GC. In coupled LC-GC the NPLC separation becomes particularly critical because only a limited amount of the LC eluent can be transferred to the GC and the triglyceride matrix must be prevented from entering the GC, because it degrades the performance of the injector and the column. In previous applications method development was seriously hampered by these boundary conditions and tedious and lengthy trial-and-error experiments were required to determine suitable experimental conditions. In this study an empirical model was developed that describes the NPLC separation process in terms of column dimensions and fat loadability. The output is given as the probability of achieving successful LC-GC analysis of a particular set of analytes, thus furnishing a useful tool for the development of new applications in the field of exposure assessment and analysis of residues of apolar compounds in fatty samples. The limitations of current procedures—maximum transfer volumes and minimal separation—are also discussed.     

Headspace Solid-Phase Microextraction GC–MS for Rapid Rice Aroma Analysis Using Optimization Tools

A key feature of rice acceptance by consumers is closely related to its aroma. A few decades of research on rice aroma indicated associated difficulties which arise from its complicated volatile composition. Our investigation seeks to resolve this highly complicated aroma profile using an experimental design for headspace solid-phase microextraction GC–MS. The Plackett–Burman methodology was used as a factor screening method for the headspace solid-phase microextraction procedure and GC–MS analysis, and a central composite design was implemented as an optimization methodology for both steps. Optimization of the extraction procedure and GC–MS analysis leads to a highly resolved rice aroma profile resulting in 66 new constituents. A total of 123 constituents were identified by implementing the procedure on Champa rice from the south of Iran.     

Offline LC-GC x GC in combination with rapid-scanning quadrupole mass spectrometry

The present research is focused on the offline combination of normal-phase LC to double-oven GC x GC-quadrupole MS. Initially, a diesel sample was subjected to automated LC x GC in order to define the elution windows of four fractions, viz., saturated hydrocarbons, monocyclic aromatics, dicyclic aromatics, tri- + tetracyclic aromatics; each fraction was collected exploiting the LC system in a further analysis and subjected to large-volume-injection-GC x GC analysis using an apolar-polar column combination. The GC x GC operational conditions were tuned in relation to the specific separation requirements of each heart-cut. The main benefits of what can be defined as offline LC-GC x GC were: (i) the high first-dimension LC selectivity; (ii) the injection of high sample amounts in the GC x GC system, enabling the detection and quantification of a series of low-amount diesel constituents; (iii) improved GC x GC operational conditions for each heart-cut with respect to direct GC x GC.     

Application of coupled LC-GC to the analysis of the polar fraction of diesel particulate matter

Emissions from diesel engines contain mutagenic compounds, many of which are derivatives of polycyclic aromatic hydrocarbons. The complexity of extracts of particulate matter requires multi-stage chromatographic analysis performed off-line. In this study, coupled LC-GC is presented as an alternative method for the analysis of the polar fraction of extracts of diesel particulate matter. Preseparation of the sample was achieved by normal phase HPLC and fractions were transferred to the GC through an on-column interface. Through selective transfer, extensive characterization of the extract is possible and chromatograms can be greatly simplified, thus aiding identification.     

在线液相-气相二维色谱高灵敏测定卷烟主流烟气中的4-（N-甲基亚硝胺基）-1-（3-吡啶基）-1-丁酮

建立了在线液相-气相二维色谱测定卷烟主流烟气中4-(N-甲基亚硝胺基)-1-(3-吡啶基)-1-丁酮(NNK)的方法。 NNK 的分析在在线凝胶气质联用仪上进行,采用自行装填的微型碱性氧化铝柱,并把仪器上的凝胶柱换成氧化铝柱,用于 NNK 的分析。捕集有主流烟气总粒相物的剑桥滤片用二氯甲烷提取,以 D4-NNK 为内标,提取液经微型氧化铝柱分离,含 NNK 的部位切割进入气相色谱,排干溶剂后启动气相色谱升温经毛细管柱进行分离,用质谱检测。本方法将烟气国标方法 NNK 测定中的氧化铝柱色谱净化和气相色谱-质谱分析在线连接起来,可不经样品前处理净化直接进样分析;每次进样可达40μL,是常规气相色谱-质谱分析最大进样量(2.0μL)的20倍,显著提高了分析灵敏度。方法线性范围达1.2~120 ng/ mL,相关系数为r=0.9998,回收率为93.9%~96.0%;检出限和定量限分别为0.25 ng/ mL 和0.9 ng/ mL,样品分析结果与中国烟草总公司企业标准方法进行对比,结果相符合。     

Polycyclic aromatic hydrocarbons in edible fats and oils: occurrence and analytical methods

This review deals with analytical methods for polycyclic aromatic hydrocarbon (PAH) determination in oils and fats. The data reported in the introduction deal with PAH dietary intake from this group of food and contamination levels recently found by some authors in different vegetable oils, stressing the importance of establishing a method suitable for routine analyses. Traditional sample preparation relies on tedious, time-consuming procedures. They generally consist of an extraction step (liquid-liquid partition, caffeine complexation, saponification) followed by one or more purification procedures (column chromatography, thin-layer chromatography, solid-phase extraction). The analytical determination is usually carried out by HPLC and spectrofluorometric detection, or through high-resolution capillary GC coupled to flame ionisation detection or mass spectrometry. LC is a valid alternative to the traditional sample preparation, and off-line LC-LC allows performing an accurate PAH analysis in less than 2 h. Also supercritical fluid extraction, allowing performing both extraction and clean-up in one combined step, is a promising technique. Hyphenated techniques such as LC-GC and LC-LC-GC seem to be very promising. A completely on-line method for alkylated PAH determination in oils or lipidic extracts contaminated with mineral oil involves a two-dimensional LC-step with intermediate eluent evaporation and GC transfer through a vaporiser/overflow interface.     

On-line reversed-phase liquid chromatography-capillary gas chromatography using a programmed temperature vaporizer as interface

An on-line reversed-phase LC-GC method involving the use of a Programmed Temperature Vaporizer (PTV) as interface is used for the rapid analysis of sterols. The experimental procedure requires only the use of commercial HPLC and GC equipments and a Rheodyne valve. The LC preseparation can be performed satisfactorily even with aqueous eluents containing up to 25% water. Factorial experimental design is applied to investigate the sensitivity achievable in the analysis with some variables (adsorbent material used in the PTV, particle size and plug length) being considered at two different levels. As a result, the sensitivty can be increased by a factor as large as 35 merely by performing the analysis under the experimental conditions giving the highest response. The use of the proposed configuration allows the transfer of up to 1050 μL from LC into the GC system, 1400 μL/min being the speed of sample introduction. The proposed procedure was succesfully applied to the analysis of real samples.     

Comprehensive multi-dimensional chromatographic studies on the separation of saturated hydrocarbon ring structures in petrochemical samples

Characterization of complex petrochemical samples has been a classical subject of comprehensive two-dimensional (2D) gas chromatography (GC x GC). Macroscopic properties of these samples can be described accurately by separation of compounds in classes of identical molecular functionality. Ring structures in the carbon backbone of these compounds, which can be divided in saturated and unsaturated, are amongst the foremost functionalities affecting samples properties. Unfortunately, GC x GC tuned for separation of both saturated and unsaturated ring structures is likely to result in convoluted chromatograms when a distribution of both molecular properties is present in the sample. An independent liquid chromatographic (LC) separation preceding GC x GC could be used to resolve the mixture based on unsaturated rings, allowing saturated rings to be resolved separately in the GC x GC separation. This three-dimensional separation (abbreviated LC-GC x GC) was performed after rigorous evaluation of LC as part of a multidimensional separation using LC x GC. Group-type separation was achieved using this separation for components with either saturated or unsaturated rings. Results of this separation were used to compare information obtained by GC x GC with LC-GC x GC.     

Comparison of two different multidimensional liquid–gas chromatography interfaces for determination of mineral oil saturated hydrocarbons in foodstuffs

This investigation focused on direct comparison of two popular multidimensional liquid–gas chromatography (LC–GC) systems, the Y-interface (retention gap approach) and the syringe-based interface (programmed temperature vaporizer approach). Such transfer devices are structurally very different, and could potentially have a substantial effect on the outcome of a specific application. In this work the application was a topic of much current interest, determination of mineral oil saturated hydrocarbon (MOSH) contamination of a series of food products (rice, pasta, icing sugar, olive oil); the final results were then compared. The two LC–GC methods developed were validated for linearity over the calibration range, analyte discrimination, precision, accuracy, and limits of detection and quantification. No significant differences were found between the two approaches.

Comparison of three spectrophotometric methods for the determination of gamma-oryzanol in rice bran oil

A comparison of the results obtained by applying three spectrophotometric methods (at fixed wavelength, second-derivative and multicomponent analysis) to the determination of gamma-oryzanol in rice bran oil is reported.At fixed wavelength the results are more accurate when using isopropyl alcohol, rather than n-heptane, to dilute the oil samples, because the absorption bands of gamma-oryzanol are red-shifted and the absorbance, measured at lambda(max)=327 nm, is less affected by the interference of the oil "matrix" (lambda(max)=314 nm in n-heptane).However, to obtain accurate results also in oils with a low content of gamma-oryzanol, it is necessary to perform the analysis using second-derivative ((2)D330.365) or multicomponent (lambda=310-360 nm) methods. The first one fully removes the interference of oil matrix whilst the second, which needs a specific computational program to process the spectrophotometric data, furnishes evidence the presence of some unexpected interference in the analysis and/or standards which are not representative of the analysed samples, from the square root of the sum of the squared differences at each point between the linear combination of the standards and the unknown spectra (RMS error).Finally, some aspects of the chemical, spectroscopic (UV, IR) and thermoanalytical (TG, DSC) behaviour of gamma-oryzanol and the values of the parameters which enable "computation" of its UV spectra are reported.