Advantages of a small-volume counter-current chromatography column

Counter-current chromatography (CCC) works with a support-free liquid stationary phase. This allows for preparative separations and purifications. However, there are serious technical constraints because of the need to keep a liquid stationary phase in a column. Centrifugal fields are used. A new commercial hydrodynamic 18 mL column made with a narrow-bore 0.8 mm Teflon tubing was evaluated by comparing it with older hydrodynamic CCC columns and a similar 19 mL column but made with 1.6 mm Teflon tubing. A small-volume CCC column allows for reliable and fast solute partition coefficient determination. When resolution is required, both high efficiency and liquid stationary phase retention are needed. Unfortunately, these two requirements bear technical contradictions. A column coiled with a narrow tubing bore will provide a high chromatographic efficiency while a column containing wider tubing bore will achieve higher stationary phase retention. In all cases, increasing the magnitude of the centrifugal field also increases the stationary phase retention. The solution is to build centrifuges able to produce high fields that will provide acceptable liquid phase retention with narrow-bore tubes. The new 18 mL 0.8 mm tubing bore column is able to rotate as fast as 2100 rpm generating a 240 × g field. The two older CCC columns cannot compete with the new one. However, the small 19 mL column with 1.6 mm bore tubing can be useful when fast results are desired without top resolution.     

High temperature gas chromatography: The development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar phases: Part 1

With the simultaneous development of blank aluminum clad flexible fused silica glass capillary tubing capable of withstanding temperatures up to 500°C, coincident with a series of special high temperature methyl polysiloxane polymers, it was possible to produce for the first time, long lived fused silica capillary columns containing thin films of thermostable stationary phases which could be maintained isothermally at 400425°C and temperature programmed to 425–440°C. The “bleed rate” here for a well conditioned column was 5 picoamperes or less. Under these circumstances, alkanes with carbon numbers in the C-90 to C-100 area were rapidly and efficiently eluted from these columns. By extrapolation here, one can easily detect certain compounds with boiling points in the 750°C range. Since this type of capillary column was found to possess certain favorable properties, it was thought that it will soon replace the packed column and will probably be more popular than the borosilicate capillary column for many high temperature applications. Moreover, evidence has now accumulated which leads us to further believe that the majority of analyses of “high molecular weight” compounds performed by Supercritical Fluid Chromatography (SFC), utilizing very narrow bore fused silica capillary columns at several hundred atmospheres, can be much more simply, much more rapidly, much more economically, and much more efficiently accomplished by gas chromatography utilizing this new generation of high temperature capillary columns.     

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

The 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.     

Deformation and degradation of polymers in ultra-high-pressure liquid chromatography

Ultra-high-pressure liquid chromatography (UHPLC) using columns packed with sub-2 μm particles has great potential for separations of many types of complex samples, including polymers. However, the application of UHPLC for the analysis of polymers meets some fundamental obstacles. Small particles and narrow bore tubing in combination with high pressures generate significant shear and extensional forces in UHPLC systems, which may affect polymer chains. At high stress conditions flexible macromolecules may become extended and eventually the chemical bonds in the molecules can break. Deformation and degradation of macromolecules will affect the peak retention and the peak shape in the chromatogram, which may cause errors in the obtained results (e.g. the calculated molecular-weight distributions). In the present work we explored the limitations of UHPLC for the analysis of polymers. Degradation and deformation of macromolecules were studied by collecting and re-injecting polymer peaks and by off-line two-dimensional liquid chromatography. Polystyrene standards with molecular weight of 4 MDa and larger were found to degrade at UHPLC conditions. However, for most polymers degradation could be avoided by using low linear velocities. No degradation of 3-MDa PS (and smaller) was observed at linear velocities up to 7 mm/s. The column frits were implicated as the main sources of polymer degradation. The extent of degradation was found to depend on the type of the column and on the column history. At high flow rates degradation was observed without a column being installed. We demonstrated that polymer deformation preceded degradation. Stretched polymers eluted from the column in slalom chromatography mode (elution order opposite to that in SEC or HDC). Under certain conditions we observed co-elution of large and small PS molecules though a convolution of slalom chromatography and hydrodynamic chromatography.     

Enantioselective analysis of methadone in saliva by liquid chromatography-mass spectrometry

Saliva was tested and evaluated as a biological matrix for methadone (Mtd) monitoring. Conventional method using a narrow bore C18 column, and an enantioselective method using a narrow bore alpha1-acid glycoprotein column, were developed using liquid chromatography coupled with a mass spectromeric (MS) detector. After optimisation of MS conditions by flow injection analysis, selected ion monitoring detection was used to enhance sensitivity. The total Mtd concentration and the enantiomeric ratio in saliva were validated using an experimental design. The methods were applied to samples provided by heroin addicts undergoing a Mtd treatment. Results on total Mtd determination showed a very poor correlation between saliva and serum, whereas the enantiomeric ratios of Mtd gave a very good one.     

Narrow Bore Ion Chromatography with Commercially Available Ion Chromatographic Equipment

Abstract

The use of narrow bore separator columns (inner diameter less than 1 μM) in high performance liquid chromatography has become a well established technique which has obvious advantages in specific situations over the use of traditional size separator columns. The work reported here investigates the use of traditional ion chromatographic equipment (essentially unmodified) for use in narrow bore ion chromatography. The only adjustments made to the equipment were some arrangements in the placement of valves and tubing in order to minimize extra-column dead volume. The columns used in this work were glass capillary columns of 1.0 mm inner diameter. A selected group of ion exchange resins were slurry packed into the column and the retention and separation characteristics of the narrow bore column were determined. The results of this investigation indicate that in certain cases the commercially available equipment is adequate for performing narrow bore ion chromatography. The distinct advantage of the use of narrow bore ion chromatography is the lower flow rates used in the chromatographic procedure with a corresponding decrease in eluant consumption, eluant cost, and eluant disposal.     

Development of a Dual Capillary Column GC Method for the Trace Determination of C2–C9 Hydrocarbons in Ambient Air

An analytical method based on a dual capillary gas chromatographic technique combining the advantages of GasPro PLOT and a non polar narrow bore WCOT column was developed for the analysis of air samples containing C2–C9 NMHCs. A refocusing step was not required due to the fast heating rate of the sample preconcentration trap and the resolving power of the PLOT column for C2 and C3 NMHCs. Water had to be removed from the air samples to avoid plugging of the columns if the initial GC oven temperature was below ambient temperature. To dry air samples, a scrubber and a cryogenic technique were employed. The interferences caused by carbon dioxide were reduced by purging the loaded sample preconcentration trap with helium. The dual column system was compared to a method employing a refocusing device and a single narrow bore WCOT column. Both systems provided a high degree of precision. However, the dual column approach was superior to the single column system due to better resolution of low molecular weight components.     

Conventional and narrow bore short capillary columns with cyclodextrin derivatives as chiral selectors to speed-up enantioselective gas chromatography and enantioselective gas chromatography-mass spectrometry analyses

The analysis of complex real-world samples of vegetable origin requires rapid and accurate routine methods, enabling laboratories to increase sample throughput and productivity while reducing analysis costs. This study examines shortening enantioselective-GC (ES-GC) analysis time following the approaches used in fast GC. ES-GC separations are due to a weak enantiomer-CD host-guest interaction and the separation is thermodynamically driven and strongly influenced by temperature. As a consequence, fast temperature rates can interfere with enantiomeric discrimination; thus the use of short and/or narrow bore columns is a possible approach to speeding-up ES-GC analyses. The performance of ES-GC with a conventional inner diameter (I.D.) column (25 m length x 0.25 mm I.D., 0.15 microm and 0.25 microm d(f)) coated with 30% of 2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl-beta-cyclodextrin in PS-086 is compared to those of conventional I.D. short column (5m length x 0.25 mm I.D., 0.15 microm d(f)) and of different length narrow bore columns (1, 2, 5 and 10 m long x 0.10 mm I.D., 0.10 microm d(f)) in analysing racemate standards of pesticides and in the flavour and fragrance field and real-world-samples. Short conventional I.D. columns gave shorter analysis time and comparable or lower resolutions with the racemate standards, depending mainly on analyte volatility. Narrow-bore columns were tested under different analysis conditions; they provided shorter analysis time and resolutions comparable to those of conventional I.D. ES columns. The narrow-bore columns offering the most effective compromise between separation efficiency and analysis time are the 5 and 2m columns; in combination with mass spectrometry as detector, applied to lavender and bergamot essential oil analyses, these reduced analysis time by a factor of at least three while separation of chiral markers remained unaltered.     

Rapid analysis of hydrocarbons and inert gases by a multidimensional gas chromatograph

A combination of a pressure switching system with multiple columns and photoionization detectors makes possible rapid analysis of a mixture of inorganic gases and hydrocarbons. Hydrocarbons are separated by a narrow bore capillary column. An alumina PLOT column is used for the separation of lower molecular weight hydrocarbons, especially C4 isomers, while a combination of a micro-packed column with Porapak N and a PLOT capillary column with Molecular Sieve 5A is used for the fast separation of inorganic gases. A photoionization detector is a powerful additional tool for organic gas analysis.     

Temperature dependence of the resolution of diastereomeric isoprenoids in capillary GC. Experiments with 50 μm, 100 μm,and 250 μm i.d. Columns

The resolution of the diastereoisomers of norpristane, pristane, and phytane was studied as a function of the column internal diameter and/or the residence time of the compounds in the column. Increasing the residence time in the column by operating the column at a lower temperature program rate enhances the resolution more than reducing the internal diameter of the column. Practical experience with ultra narrow bore columns is also presented.     

High temperature gas chromatography on narrow bore capillary columns

The use of high-temperature-stable, medium polarity glass capillary columns coated with immobilized PS-090 (a 20 % diphenyl-substituted, CH₃O-terminated polydimethylsiloxane) has made it possible to analyze routinely, and with good separation efficiency, high molecular weight compounds such as triglycerides and free base porphyrins. Cold on-column injection was used throughout this work to avoid discrimination against involatile compounds, and disposable (fused silica) retention gaps were used to protect the column against contamination with involatile material. On-column injection into narrow bore glass columns was achieved by using glass-to-silica connections to attach wider bore (0.2 mm i.d.) deactivated fused silica tubing to the columns.     

Stainless steel capillary columns for high temperature gas chromatography

A layer of elemental silicon has been deposited on the surface of stainless steel tubing by means of chemical vapor deposition (CVD). Two kinds of capillary column were prepared from the deactivated tubing: cross-linked, silanol-terminated polydime-thylsiloxane wall coated open tubular (WCOT) columns and molecular sieve 13X porous layer open tubular (PLOT) columns. Unlike fused silica capillary columns, stainless steel WCOT and PLOT columns can be operated at temperatures in excess of 400°C. High temperature simulated distillation has been performed successfully with a macro bore WCOT column and rapid PNA (paraffin, naphthene, and aromatic) analysis with a multidimensional gas solid chromatographic (GSC) system using PLOT columns.     

Two-dimensional liquid chromatography analysis of synthetic polymers using fast size exclusion chromatography at high column temperature

In recent years, two-dimensional liquid chromatography (2D-LC) has been used increasingly for the analysis of synthetic polymers. A 2D-LC analysis provides richer information than a single chromatography analysis at the cost of longer analysis time. The time required for a comprehensive 2D-LC analysis is essentially proportional to the analysis time of the second dimension separation. Many of 2D-LC analyses of synthetic polymers have employed size exclusion chromatography (SEC) for the second-dimension analysis due to the relatively short analysis time in addition to the wide use in the polymer analysis. Nonetheless, short SEC columns are often used for 2D-LC analyses to reduce the separation time, which inevitably deteriorates the resolution. In this study, we demonstrated that high temperature SEC can be employed as an efficient second-LC in the 2D-LC separation of synthetic polymers. By virtue of high temperature operation (low solvent viscosity and high diffusivity of the polymer molecules), a normal length SEC column can be used at high flow rate with little loss in resolution.     

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 C₈–C₂₈ 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×0.1 mm i. d.×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 C₁₀–C₂₈ n‐alkanes was achieved in less than 3 minutes.     

Preparation,evaluation, and comparison of wide bore (320 μm) and narrow bore (50 μm) cyanosilicone-coated capillary columns for gas chromatography

The preparation of wide bore (320 μm) and narrow bore (50 μm) fused silica capillary columns is described for immobilized cyanopropyl substituted silicones containing 60 and 88% substitution. The effect of high temperature deactivation with cyanopropylcyclosiloxanes was studied with a special test mixture. Curing was achieved with dicumyl peroxide or azo-tert-butane. The columns were evaluated and compared in terms of efficiency, activity, polarity, and temperature stability. Different coating methods were compared for the narrow bore columns. The activity of the 60% cyanopropyl columns that had been immobilized with dicumyl peroxide was significantly larger than for azo-tert-butane immobilized columns. The polarity of polar columns appeared to depend greatly on column temperature and is completely different for wide and narrow bore columns.     

The effect of column characteristics on the minimum analyte concentration and the minimum detectable amount in capillary gas chromatography

The need for faster and more efficient separations of complex mixtures of organic compounds by gas chromatography has led to the development of small inner diameter open tubular columns. Owing to their decreased plate height, extremely narrow peaks are obtained. When differently sized columns with equal plate numbers are compared, injection of a fixed amount of a solute will give the highest detector signals for the smallest bore columns. When P is defined as the ratio of the column inlet and outlet pressures, it can be seen from theory that under normalized chromatographic conditions the minimum detectable amount (Q_º) for a mass flow sensitive detector increases proportionally to the square of the column diameter for P = 1. In the situation of greater interest in the practice of open tubular gas chromatography where P is large, a linear relationship is derived between Q_º and the column diameter. It is a widespread misunderstanding, however, that narrow bore capillary columns should be used for this reason in trace analysis. If a fixed relative contribution of the injection band width to the overall peak variance is allowed, a decreased plate height drastically restricts the maximum sample volume to be injected. It is shown that the minimum analyte concentration in the injected sample (C_º) is inversely proportional to the column inner diameter when a mass flow sensitive detector is used. For actual concentrations less than C_º, sample preconcentration is required. The effect of peak resolution and selectivity of the stationary phase in relation to C_º and Q_º will be discussed as well. The validity of the given theory is experimentally investigated. Minimum analyte concentrations and minimum detectable amounts are compared using columns with different inner diameter.     

Fast-GC-conventional quadrupole mass spectrometry in essential oil analysis

This study reports on the compatibility of a conventional quadrupole MS (qMS) as detector for Fast-GC in terms of separation, identification, and quantitation when applied to the analysis of peppermint essential oil (e.o.), adopted as representative of this field. The influence of Fast-GC carried out on a 10 mx0.1 mm id narrow bore column with temperature programmes from 20 to 60 degrees C/min on the results of a qMS in total ion chromatograms (TIC) at different speeds (from 999 and 11 111 amu/s) and SIM modes was evaluated on ten differently abundant components characterizing peppermint e.o. Separation measure (S), peak capacity (n), and half height peak width were taken as separation parameters; match quality, number of scans per peak (NP), spectral skewing, and TIC area repeatability were used for identification. Quantitation was in SIM mode and NP, dwell time, SIM area repeatability and calibration curves, LOD, and LOQ of the selected components were measured. The results show that the peppermint e.o. markers can successfully be analysed qualitatively and quantitatively by F-GC-qMS up to temperature programmes of 60 degrees /min provided that a suitable scan speed is applied. Fast-GC-qMS reduces analysis time by a factor greater than ten and gives results that are qualitatively reliable and quantitatively comparable to those obtained by conventional GC-qMS.     

Comparison of Thermal Sweeper and Cryogenic Modulator Technology for Comprehensive Gas Chromatography

The two current technologies for achieving comprehensive gas chromatography (GC×GC) – the thermal sweeper and the cryogenic modulator – are compared in an interlaboratory study using a multicomponent semi‐volatile aromatic compound sample. The same column set (phases, film thickness, dimensions of columns) and conditions of oven temperature program were used. Carrier gas flow settings however were different for the data reported here. The thermal sweeper has a longer overall length due to the extra ca. 30 cm length of narrow bore tubing used for the modulator/accumulator section. Data reveal that the two methods behave in an analogous manner in respect of delivering GC×GC results, with key peak parameters of peak widths and symmetry measures showing good correlation. Retention time dissimilarity on the first dimension columns in the two systems arises from different flow rates used, however the second column retention is similar, and this is due to the resulting different elution temperatures that peaks elute on the first dimension in each system. Overall, the two approaches to GC×GC appear to produce equivalent results within the scope of the application studied. Each system does have its experimental limitations; the thermal sweeper has what may be called a ‘thick film effect’, where at high temperature it can be difficult to sufficiently trap the migrating bands in the accumulator column, and the pulsing of solutes in the cryogenic system may suffer from a ‘thick wall effect’ if a column with too thick a wall dimension is used at low oven temperature.     

大孔径毛细管气相色谱法快速测定饮料中的氯胺酮

采用固相萃取技术 ,以二十二烷为内标 ,FID检测器 ,建立了一种测定饮料中的氯胺酮的大孔径毛细管气相色谱新方法 .测定结果表明氯胺酮的线性范围为 0 0 2mg/L - 6 4mg/L ,其最低检测浓度为 0 0 2mg/L ,相对标准偏差为 2 6 % ,平均回收率为 85 6 % .该法简便 ,准确 ,重现性好 ,结果令人满意     

Sample enrichment in high speed narrow bore capillary gas chromatography

Reduction of the column diameter has proved to be a highly efficient tool to increase the speed of analysis. Unfortunately, the requirements for instrumental design with respect to sample input band width, low dead volume interfacing, and time constants of detection and registration systems are the more critical the smaller the inside diameter. Recently we reported input band widths as low as 1 ms [1] for gaseous samples at ppm concentration levels, without any preconcentration, in a study with narrow bore columns and thermal conductivity detection. In this study a simple versatile micro on-column cold trap/thermodesorption enrichment system for narrow bore columns is introduced and evaluated. The combination of considerable sample enrichment and preservation of the compatibility of the required input band width with column dimensions is critically examined. The process of thermodesorption (reinjection) which is the most critical step, is particularly emphasized. The system consists of a short aluminum coated fused silica or metal capillary with a low mass and a low cost electrical heating. Input band widths down to 1 ms are obtained without extreme demands on electrical power (300 watt). The potential of the system is illustrated with some extremely fast separations.