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.     

Injection temperature effects using On-column and split sampling in capillary gas chromatography

The effect of sample injection temperature on quantitation is examined for on-column and conventional split modes of sampling in capillary gas chromatography. Discrimination effects can be observed even with on-column injection if the injection temperature is too far above the boiling point of the solvent (or that of a major low boiling constituent(. This is attributed to higher boiling components being left behind in the syringe needle, and a set of simulation experiments are described to illustrate this effect. Various discrimination patterns using conventional split injection were observed, depending on temperature of injection. Apart from syringe needle effects, discrimination is probably due to the preferential venting of higher boiling components as liquid sample droplets, which can have a lifetime greater than the time of transit to the splitter. With such a two-phase system, involving variable droplet size, the flow distribution in the splitter will be critical to uniform sampling. The use of combination on-column/split sampling, with the appropriate temperature control to provide sample uniformity to the splitter is discussed as an advantageous alternative to conventional split sampling.     

The use of precolumns for solvent focusing in capillary column gas chromatography

In trace analysis by capillary GC it is often desirable to use larger than normal injection volumes to obtain sufficient sensitivity. This, however, results in a wider solvent peak and tailing, and may reduce column efficiency. This paper describes the use of a short length of a capillary precolumn coated with a stationary phase of polarity similar to that of the sample solvent and a film thickness greater than that of the analytical column; provided the right combination of polarities of injection solvent and liquid phases are used, the precolumn focuses the solvent band, thereby enabling the maximum injection volume to be increased without measurably reducing efficiency. Typical precolumn dimensions are 1 m length, 0.32 mm i.d., and 0.5 μm stationary phase film thickness. Using a precolumn increases the maximum injection volume up to 8 or 10 μl, or three times that appropriate for a conventional analytical column, with little or no loss in efficiency.     

Application of the “cooled needle” technique to split and splitless sampling onto capillary columns

Cooled needle sampling using syringes was applied to splitless injection and to simulated distillation analyses. Slight changes of the construction of the previous device are also described. The changes concern the temperature profile within the injector and especially the vaporization insert. Even with the low carrier gas flow through the injector during splitless injection, discrimination by component volatility can be avoided. Precision and accuracy of simulated distillation data obtained with split sampling also can be improved by the cooled needle technique.     

The solvent venting injection technique in capillary supercritical fluid chromatography

A solvent venting technique for injection of volumes up to 1 μl on 50 μm i.d. SFC columns has been compared to direct injection methods. The peak broadening and peak splitting observed with direct injection have been examined and found to be related to the starting pressure, the column temperature, and the sample solvent, in addition to the sample volume. The solvent venting technique removed peak splitting and improved the column efficiency. With a proper selection of experimental conditions, the sample recovery was 100%. The major part of the solvent was eluted in 15–20 s. Several applications have been demonstrated.     

High precision trace analysis in capillary gas-liquid chromatography using an inlet based on the static solvent effect

A sample inlet, based on the static solvent effect has been designed and constructed. The inlet can handle samples up to 20 μl which allows for the detection, using an F. I. D., of constituents in the parts per billion range. The reproducibility of the inlet in terms of peak percentage area is around 1% relative standard deviation over a range of chemical types.     

Approaches to optimisation of precision in capillary electrophoresis

Summary A study has been performed to obtain insight into the relative importance of critical factors affecting the repeatability of hydrodynamic injections in CE. Precision was measured for repeated analysis of a test mixture containing two acidic compounds.The use of an internal standard was clearly shown to improve precision especially when peak area precision was poor. It is suggested that precision is maximised by employing a combination of a constant temperature, an appropriate electrolyte system, an internal standard, long injection times, and high sample concentrations. Other factors are discussed, but are classified as having only a minor impact.     

Distorted solvent peaks in capillary GC: A symptom of porous column surfaces

Many uncoated precolumns, and to a lesser extent separation columns, strongly retain some of the solvent. Retarded release of such solvent elevates the baseline after the solvent peak and causes ugly “humps” of eluted solvent as the temperature is increased. The problem is probably a result of retention by a porous surface, e.g. surfaces obtained by leaching or hydrothermal treatment of capillaries prior to silylation. It is assumed that other problems with capillary columns can be explained by the same mechanism, including: a kind of adsorption including apolar compounds, and “ghost” peaks as well as “memory” effects. Fused silica capillaries are superior to glass, but even for these special procedures are required in order to achieve thorough deactivation of the internal surface without introducing porosity.     

Conditions for accurate split injection of samples with wide boiling point range

The effect of injection temperature, carrier gas flow rate, geometry of the glass insert, and column temperature program on the precision and accuracy of split injections was measured. Three types of injection techniques were compared: injection into a hot isothermal injector, isothermal injection with the injector at the solvent boiling point temperature, and programmed injection temperature. The last of these techniques produced the best accuracy and precision of analysis. Conditions for complete sample trapping at the beginning of programmed temperature analysis are described.     

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.     

Aspects of the capillary GC analysis of all-trans- and 13-cis-acitretin

The capillary gas chromatographic (CGC) analysis of the der-matological drug trans-acitretin (Neotigason^R) and its cis metabolite is described. Separation of the methyl ester derivatives can be achieved on a 90% biscyanopropylsiloxane phase. The importance of using cold on-column injection and short, thin film capillary columns is discussed. For patients treated with the prodrug of acitretin, etretinate (Tigason^R), i.e. the ethyl ester of Neotigason, three compounds have to be separated. Selectivity tuning is required for successful CGC separation. An alternative can be found in the selectivity of ion monitoring mass spectroscopy. Analysis of plasma samples involves liquid-liquid extraction, a derivatization step, and HPLC purification.     

Automated optimization of split ratio in capillary gas chromatography

An electronic gas-flow controller system called Advanced Flow Control (AFC), which controls not only carrier gas pressure at the column inlet but also the total gas flow including the split flow, was designed and evaluated. BASIC programming of repetitive analyses of standard mixtures under varied split ratios and pressure programs allowed automated optimization of those conditions for the desired column loading and resolution between adjacent peaks.     

Basic aspects of stop-flow split injection

Summary Major concerns in the development of the stop-flow split injection are discussed. Split ratio fluctuation caused by the pressure wave, solvent recondensation and gas viscosity change can be substantially diminished through the formation of a nearly uniform gaseous sample plug. Instrumental variables exerting influence on the accuracy of quantitation were studied. Higher injector temperature and the use of a carrier gas with higher heat conductivity benefit the quick vaporization of the sample liquid, and thus, help in the formation of a sample plug with more unformity.     

Enantiomer separations by capillary GC on modified gyclodextrins

Three new chiral selectors, 6-tert-butyldimethylsilyl-2,3-diethyl-a-cyclodextrin, 6-tert-butyldimethylsilyl-2,3-diethyl- and dipropyl-β-cyclodextrin (TBDE-α-CD, TBDE-β-CD, TBDP-β-CD) were synthesized and tested as chiral stationary phases in capillary gas chromatography. TBDE-β-CD in particular showed a high enan-tioselectivity for test chiral compounds due to good solubility in a polar polysiloxane (OV-1701). Enantioselectivity obtained with TBDE-β-CD was compared with that of 6-tert-butyldimethylsilyl-2,3-di-O-methyl-β-cyclodextrin (TBDM-β-CD). Better enantiose-lectivity was obtained with TBDE-P-CD than with TBDM-β-CD for the test chiral compounds studied. This is probably due to greater effect of the increased hydrophobicity of TBDE-β-CD which favors inclusion of the analytes than the effect of increased steric hindrance. With TBDP-β-CD the less polar lactones are well separated due most likely to increased hydrophobicity of the propyl groups while the more polar are not well resolved. For TBDP-β-CD it is likely that the unfavorable steric hindrance is predominant over the favorable hydrophobicity of the propyl groups, thus hindering the formation of inclusion complexes of the alcohols with TBDP-β-CD. TBDE-α-CD was also a valuable chiral selector for the separation of small chiral molecules such as simple secondary alcohols and nitro-substituted alcohols.