Sample Introduction

The most common method of sample introduction is by means of a microsyringe through a septum-sealed inlet. Microsyringes are useful for introducing liquid or gas samples, a technique developed by N.H. Ray in 1954. For quantitative work, gases are normally introduced by loop sampling valves, which are highly reproducible and readily automated. Solid samples may be introduced after dissolution in a suitable solvent. Direct introduction of solids is seldom used with open-tubular columns.

The limited sample capacity and low carrier gas flow rates associated with open-tubular columns makes sample introduction much more difficult than for packed columns. A thermostatted flash vaporization chamber in which the evaporated sample is mixed with carrier gas and divided between a stream entering the column (carrier gas flow) and a stream vented to waste (split flow) was the first practical solution to this problem. Split injection discriminates against high boiling compounds (bp > 250°C) owing to selective vaporization. Quantitative analysis of wide boiling range mixtures is difficult, and for the analysis of samples present in a dilute solution detectability is limited by the small amount of sample transferred to the column.

The so-called splitless injection technique was devised to overcome some of the deficiencies of split injection for the analysis of mixtures of compounds in a solvent (such as frequently occurs in environmental studies) through the transfer of relatively large volumes to the column. The gas flow through a splitless injector is relatively low, and the sample is introduced into the column over a comparatively long time

(30-60 s), relying on cold trapping and/or solvent effects to refocus the compounds at the head of the column. The importance of these refocusing mechanisms was not fully understood at first but splitless injection did demonstrate the possibility of performing trace analyses with open-tubular columns. It is also easy in practice to convert an injector from split to splitless operation by the operation of valves and minor hardware modifications.

The programmed-temperature vaporization (PTV) injector overcame many of the problems observed with the hot split and splitless injector. The PTV injector is designed to allow rapid heating and cooling and the sample is introduced at a low temperature. A rapid rise in temperature after introduction ensures rapid volatilization of the highest boiling sample components. The PTV injector may be used in both split and splitless modes and the accuracy and precision approach those obtained by cold on-column injection.

The production of wide-bore silica columns in the early 1980s allowed introduction of the syringe needle directly into the column and the use of immobilized phases eliminated the problem of removal of the stationary phase by large volumes of liquid sample. In cold on-column injection the sample is introduced as a liquid into the column inlet where it is subsequently vaporized. Discrimination based on volatility differences has been virtually eliminated and the risk of sample decomposition minimized. With secondary cooling of the injector, the oven temperature can be kept well above the boiling point of the solvent while maintaining the column inlet at a much lower temperature. This is important for using on-column injection in high temperature gas chromatography. Dirty samples present a problem owing to contamination of the sample introduction zone, which leads to poor chromatography and unreliable quantitation.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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