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.     

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.     

Assay of the nerve agent Soman in serum by capillary gas chromatography with nitrogen-phosphorus detection and splitless injection

Summary A procedure is described for the determination of the nerve agent Soman in serum. The nerve agent is separated from the serum on C₁₈ modified silica and then eluted with benzene. The concentrated sample, to which butylacetate is added, is injected splitless into a polyethylene glycol (CP Wax 57) coated fused silica column. Solvent trapping of the analytes occurs due to the added butyl-acetate. The detection is performed with a nitrogen-phosphorus detector (NPD). The determination limit of the method is 40 pg/ml.     

The influence of evaporative dynamics on precision in split capillary GC

The kinetics of sample evaporation was studied for four common injection liners at various temperatures. The rates of solvent and sample evaporation were measured. The sample distribution at the split point was probed by inserting two capillary columns in one injector. Greater homogeneity at the split point corresponded to higher precision (better correlation between the sample and internal standard peak areas). Evidence of aerosol formation using inverted cup inlet liners was seen. Packed column precision was better than capillary precision in each case, i.e., using straight split liners, inverted cup liners, and cold on-column injection. Capillary precision is best when the sample and internal standard elute close together, and may be improved by using solvents that vaporize slowly.     

An automated large volume on-column injection technique for capillary gas chromatography

Automated large volume (25-200 μl) on-column injections into a gas chromatograph with a capillary column were successfully performed by coupling a retention gap technique with an air actuated rotary valve. The linearity, injection precision, and carryover were evaluated. Slight boiling point discrimination was observed. This technique is compatible with commonly used chromatographic detectors (FID, ECD, MS) and conditions, while requiring very little instrument modification. The work is directed at the eventual reduction of manpower and turnaround time for sample collection and extraction, and Kuderna-Danish concentration when dealing with methods similar to EPA 625.     

Determination of cholesterol in fat and muscle of pig by HPLC and capillary gas chromatography with solvent venting injection

Two methods for determination of cholesterol in fat and muscle of pig were evaluated: extraction with chloroform:methanol (2:1, v/v) followed by saponification (method 1) and direct saponification (method 2). HPLC and GC were used to determine cholesterol concentrations. GC analysis was performed with a capillary column of 100 μm using a PTV injector in the modes of cold split and solvent venting. Cholesterol was analyzed without derivatization. Both methods of extraction did not present significant differences (p > 0.01). Sample analysis by GC with solvent venting injection and HPLC showed the lowest % r.s.d. but GC in the cold split mode allowed to obtain a shorter analysis time. Cholesterol concentrations obtained by HPLC were not statistically different from the results obtained by GC with solvent venting injection and were slightly lower than those previously reported. Cholesterol concentrations in fat and muscle tissues respectively ranged from 52 to 77 mg/100 g and from 55 to 65 mg/100 g.     

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.     

Splitless injection in capillary supercritical fluid chromatography

A splitless injection technique, allowing 0.5 μl injections on 50 μm i.d. columns, has been developed.     

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.     

Determination of the calcium entry blocker verapamil in plasma by capillary gas chromatography with on-column injection

A specific, sensitive, and accurate assay for quantitation of verapamil has been developed. The method involves a single extraction step with n-heptane, followed by evaporation at room temperature under nitrogen. 0.1 μl of the extract was injected into a capillary column coated with crosslinked 5% phenylmethylsilicone. The column separated verapamil, norverapamil, and its internal standard within 15 minutes using temperature programming from 90 to 290°C. The lower limit of detection was 1 ng/ml for verapamil. The calibration curve was linear in the concentration range (5–300 ng/ml). Plasma concentration data from dogs receiving intravenous verapamil infusion are presented.     

Increased on-column injection temperature for gas chromatography

A way of controlling the maximum allowable oven temperature during on-column injection by the column pressure drop is suggested. An arrangement using a restriction at the column outlet for adjustment of the column inlet pressure while maintaining the same column flow has been studied. Compared to non-restricted flow, substantially increased maximum oven temperatures were obtained during on-column injection. Injections at elevated temperatures resulted in an increased speed of analysis and decreased solute adsorption on the surface of the contaminated retention gap. The method is generally applicable to analysis of high boiling mixtures. In particular, with GC-AED, such an arrangement yields a higher sensitivity due to an increased solute interaction with the excitation plasma.