Carrier gases choosing

If the material whose single crystal we want is volatile or sublimable, then we may choose a vapor-method of crystal growth. These methods have been used for a variety of crystals including ZnS and CdS. In this method, a carrier- gas is most often used for material transport and for the sulfides, H2S is the gas of choice. The following shows a simple apparatus ... 

Most labs choose option 2 and install a series of traps that are located external to the GC unless there is some compelling reason to go with a ultrahigh- or research-grade purity carrier gas. A hydrocarbon trap is first... 

For the carrier gas flow rate which exhibited the highest efficiency, calculate the number of theoretical plates using equations from your text. In addition, for the optimum carrier flow rate, choose any pair of peaks and calculate the resolution for that pair. 

Capillary-column dimensions and the average carrier-gas linear velocity exert a strong influence on peak resolution and the speed of analysis. Chromatogra-phers can control a separation s characteristics by choosing these parameters as required to meet specific performance goals. In order to simplify the discussion and to provide a clear separation of the variables inflnences, we will hold the column temperature constant when considering the effects of changing the physical column parameters. The influence of colunm temperature is addressed later in this chapter. 

The choice of carrier gas will also be dependent on the type of columns you will use. For capillary columns the two most popular carrier gases are helium and hydrogen. When using packed columns, most analysts choose between nitrogen and helium. A van Deemter equation allows the comparison of efficiencies obtainable with carrier gases (4-9). The van Deemter equation (Equation 10.1) expresses the extent a component band spreads as it passes through the column in terms of physical constants and the velocity of the mobile phase ... 

The minimum value HETP is not the only consideration used in choosing a carrier gas. When considering the best gas for our analysis, we want to consider more than just the minimum HETP value. Figure 10.2 shows a theoretical plot of... 

In thermal-conductivity measurements, it is advisable to choose a carrier gas that differs maximally from the organic solutes (e.g., hydrogen or helium). The detector is a truly universal and simple device, but its sensitivity is marginal at best, submicrogram amounts are detected. The thermal-conductivity detector is most typically employed for the analysis of permanent gases and light hydrocarbons. 

---