Mobile phase inert

As the mobile phase (inert gas) carries the chemicals through the column, repeated partitioning of molecules takes place between the mobile and the stationary phases, and molecules of same (or similar) chemical and physical nature are separated into groups. These groups reach the end of the column at different times. This process is called elution, and it can be compared to a marathon race all runners start at the same time, but reach the finish line at different times due to the differences in their athletic ability. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

A very helpful tool for manual application can be the employment of layers with a concentrating zone. The so-called concentrating or preadsorbent zone is a small part of the plate that is covered with an inert but highly porous adsorbent such as diatomaceous earth. Various precoated preparative layers with a preadsorbent zone are commercially available. The effect of the concentrating zone is depicted elsewhere in detail (see Chapter 3, Figure 3.4). In brief, the preadsorbent zone serves as a platform for manual application of any desired performance quality. When development starts, soluble components migrate with the mobile phase front and are... 

As mentioned before, there are two common types of GC gas-liquid chromatography (GLC) and gas-solid chromatography (GSC), depending on the physical state of the stationary phase. GSC is seldom used. In GLC the analyte is partitioned between the mobile phase (gas) and a liquid phase, which is retained on an inert solid support. The liquid phase should ideally possess a low volatility (so that it does not volatilise with the analyte), be thermally stable and chemically inert, and have favourable solvent characteristics. 

In lc there are other sorption mechanisms that can cause separation, depending on whether we choose to use a liquid or a solid as the stationary phase, or what kind of solid we use. Liquid-liquid chromatography (11c) uses a liquid stationary phase coated onto a finely divided inert solid support. Separation here is due to differences in the partition coefficients of solutes between the stationary liquid and the liquid mobile phase. In normal phase 11c the stationary phase is relatively polar and the mobile phase relatively non-polar, whilst... 

The fourth type of mechanism is exclusion although perhaps inclusion would be a better description. Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout. Separations occur because of variations in the extent to which the solute molecules can diffuse through an inert but porous stationary phase. This is normally a gel structure which has a small pore size and into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical size are excluded from the gel and move unhindered through the column or layer whilst smaller ones are retarded to an extent dependent on molecular size. 

A schematic diagram of a high performance liquid chromatograph is shown in Figure 4.30 and details of the components are discussed below. All materials which come into contact with the mobile phase are manufactured from stainless steel, PTFE, sapphire, ruby or glass for inertness. 

The mobile phase reservoir is made of an inert material, usually glass. There is usually a cap on the reservoir that is vented to allow air to enter as the fluid level drops. The purpose of the cap is to prevent particulate matter from falling into the reservoir. It is very important to prevent particulates from entering the flow stream. The tip of the tube immersed in the reservoir is fitted with a coarse metal filter. It functions as a filter in the event that particulates do find their way into the reservoir. It also serves as a sinker to keep the tip well under the surface of the liquid. In addition, in specially designed mobile phase reservoirs, this sinker/filter is placed into a well on the bottom of the reservoir so that it is completely immersed in solvent, even when the reservoir is running low. This avoids drawing air into the line under those conditions. These details are shown in Figure 13.3. 

Gas chromatography makes use, as the stationary phase, a glass or metal column fdled either with a powdered adsorbent or a non-volatile liquid coated on a non-adsorbent powder. The mobile-phase consists of an inert-gas loaded with the vapourised mixture of solutes flowing through the stationary phase at a suitable temperature. In the course of the passage of the vapour of the sample through the column, separation of the components of the sample occurs in two ways, namely ... 

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