On-line liquid backflush—a new technique for GC analysis of samples containing nonvolatile material

Deteriorated performance in capillary GC is often caused by the deposition of nonvolatile components in capillary GC inlets. In many cases this is a limiting factor in trace analysis of complex samples. Volatile solutes of the sample interact with the high boiling material, which leads to broadened and unsymmetrical peaks. This paper describes a novel technique which utilizes independent temperature programs of a retention gap and the column as well as a liquid backflush of the retention gap. Samples that normally contaminate the retention gap after a few injections can be repeatedly injected over extended periods of time, while the chromatographic resolution is retained.     

Modification of a packed GC injector to a versatile capillary injector

A modification of a packed GC injector to a capillary injector is presented using a simple device which is connected to the GC column by an adsorption-free connector. It permits injections of large sample volumes up to 500 μl hexane solutions on normal 0.3 mm i.d. capillary columns. No special requirements for the stationary phase are necessary. Involatile residues remain inside the device which can be exchanged easily. High performance of separation and quantification is achieved.     

Automated at-column injection into narrow bore capillary GC columns

An injector designed for automatic direct liquid injection into narrow bore capillary GC columns has been constructed and evaluated. The tip of the syringe needle is aligned with, and positioned close to, the column entrance in a small, pressurized cavity: when the sample is dispensed it is immediately forced into the column by the action of the surrounding carrier gas. A standard autosampler equipped with a standard stainless steel syringe needle was utilized for at-column sample transfer into 100 μm i.d. columns. RSD values for n-alkanes were between 0.1 and 0.3% for relative area counts and approximately 1% for absolute area counts.     

Sample introduction in high speed capillary gas chromatography; input band width and detection limits

The speed of analysis in capillary gas chromatography can be substantially increased by reduction of the column inner diameter. However, special demands are then posed upon instrumental design. In particular, the sampling system is highly critical because it has to be capable of delivering extremely small injection band widths which must be compatible with the column inside diameter. This study focuses on the evaluation of two potentially suitable sample introduction systems with respect to input band width and detection limits and their compatibility with small bore (≦ 100 μm) columns in capillary gas chromatography. One of them allows liquid on-column injection, based on liquid splitting, of only a few nl onto small bore (≦ 100 ?m) fused silica columns. For gases, input band widths as low as 1 ms are obtained with this system. The other one is part of a miniaturized gas chromatograph with extremely low dead volume interfaces and detector volumes. It allows input band widths for gases of a few ms. Without any preconcentration ppm concentrations are measured in gaseous samples with a 80 ?m thick film capillary column. It will be shown that a further reduction of the minimum detectable amount and analysis time is possible with this equipment.     

Increased speed of steroid analysis by capillary GC and GC-MS; effects of sample pretreatment and sample introdution

Quantitative analysis of steroids and their metabolites in urine samples calls for increased speed of sample clean-up, of the derivatization procedure, and of separation. A fast procedure for sample pretreatment, which can be performed within 8 hours, is introduced and evaluated. It is shown that use of fast pretreatment in combination with narrow bore columns, which are compatible with existing instrumentation, can considerably increase, laboratory throughput. The effect of different sample introduction techniques (e.g. splitless, on-column, and moving needle) on column efficiency and resolution is demonstrated and discussed.     

Advances in the preparation and evaluation of GC capillary columns

The preparation of (glass) capillary columns of high quality is still not without difficulties. Six problems in this context are highlighted. Column material choice(l), glass wall modification(2) and coating(3) are considered to have reached a satisfactory level. Deactivation(4) is still in a state of flux. Quantitation(5) is the least satisfactory aspect of capillary GC. Scope expansion(6) will be helped by the introduction of new phases of high thermal stability and by the adoption of solutions exclusively applicable to capillary GC. Evaluation of (glass) capillary GC columns is a rapidly developing field. Hopefully the number and complexity of test or polarity mixtures will not expand much further and it will be possible to make a clear and generally acceptable choice. The preparation and evaluation of ever-improving capillary columns is important to an ever-widening group of chromatographers. Our opinion on selected topics in this already vast field is summarized in the following overview.     

Total steroid profiling with capillary GC. Details on sample preparation

The accuracy and reproducibility of steroid and steroid conjugate determinations using capillary GC are evaluated. It is possible to analyze many unconjugated steroids, steroid sulfates, and some steroid glucuronides simultaneously, simply by using group fractionation based on successive hydrolysis and solvent extraction steps. However, a reliable determination of corticosteroid glucuronides can only be obtained after previous isolation of the glucuronides as a group, with ion exchange chromatography. In complex chromatograms, the combined use of nitrogen specific detection and flame ionization detection facilitates peak identification.     

Evaluation and optimisation of a splitter injector for capillary chromatography

Summary The repeatability and discrimination of a home-made splitter injector, operated under various conditions, towards a C-6 to C-24 test sample were investigated. Other different test samples were analysed to prove the integrity of the injector. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Potential and limitations of differently designed programmed-temperature injector liners for large volume sample introduction in capillary GC

Programmed temperature sample introduction is an attractive technique both for the introduction of large volumes of dilute samples in capillary gas chromatography and for LC-GC interfacing. In this study a new type of liner containing a porous bed of sintered glass is introduced and evaluated. Solute recoveries achieved with this liner are compared with those obtained from liners packed with Tenax TA and Thermotrap TA. An open liner containing baffles and a small plug of silanized glass wool in its upper section was used as a reference. The maximum speed of sample introduction allowable using the new liner is ca one order of magnitude higher than for the standard liner. For similar liner operating conditions the trapping efficiency is substantially improved: at a liner temperature of ?30°C components with a volatility lower than or equal to that of n-tridecane are trapped quantitatively. Liners packed with trapping materials such as Tenax TA or Thermotrap TA also provide significantly improved trapping efficiencies, even when used at relatively high temperatures.     

PTV vapor overflow — principles of a solvent evaporation technique for introducing large volumes in capillary GC

The concept of a GC solvent evaporation technique is outlined that involves a modified Programmed Temperature Vaporizing (PTV) injector. The vapor overflow technique is intended for introducing samples in large volumes of solvent by syringe injection of strongly diluted samples or by coupled LC-GC. The liquid is introduced into a packed vaporizing chamber kept above the solvent boiling point at a pressure which is near or below ambient. The carrier gas is essentially switched off. Evaporation and discharge of the solvent vapors occurs by expansion of the vapors, driven by the solvent vapor pressure. For transferring the sapmple into the column, the carrier gas is switched on again and the vaporizing chamber heated. Compared to PTV solvent split injection, vapor overflow offers the following advantages: It automatically optimizes operational parameters, therefore facilitating its application. Losses of volatile materials are minimized by a minimal flow rate through the injector. Vapor overflow is a promising technique for transferring watercontaining eluents in coupled LC-GC since no wettability is required and leaching of pre-column surfaces is avoided.     

Glass wool in the inserts of split injectors for capillary GC?

It has been reported that glass wool packed tightly into the glass liner of a vaporizing injector used in the splitting mode considerably reduces the standard deviation of the results obtained, because of improved evaporation of the sample prior to reaching the split point at the capillary column entrance. This finding could not be reproduced on using the same sample composition as reported in the literature, i.e. methanol/2-ethyl-1-hexanol (1:1). The standard deviations obtained were between 3 and 10% (depending on the conditions selected) and were not influenced significantly by the introduction of glass wool. The peak area ratio (methanol/2-ethyl-1-hexanol) was found to depend on a number of parameters, such as: injector temperature; glass liner internal diameter; syringe handling technique; the relative position of the syringe needle exit and capillary column entrance; the sample volume injected; and the packing of the glass liner. Generally, the area ratio deviated further from the correct one (determined by cold on-column sampling) when the glass liner was packed with glass wool. On the basis of the results, it is speculated that either a complete evaporation of the sample should be achieved (which appears to be impossible under the conditions we used) or, alternatively, the sample should be given the least possible opportunity to evaporate, thus allowing it to enter the column in the form of droplets. The results were worse in terms of precision and accuracy the greater the partition of sample components between the liquid (droplet) and vapor phase. It is concluded that the use of evaporation aids such as glass wool cannot be generally recommended.     

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.