than one microliter (1 μL). In such a small volume, even the smallest bubble will displace a significant amount of the sample volume and cause low measurement values.
It's easy to visualize a microliter since it's the same size as a one‐millimeter cube (1 mm3). A volume of one thousand microliters is equal to one milliliter (1 mL) and to one cubic centimeter (1 cm3), commonly called a cc.
Another challenge with liquid samples is getting a complete and instant vaporization without making the sample too hot, lest it start to react or decompose. The small volume is helpful, and most process liquids quickly vaporize without significant decay.
The column
The separating device
The chromatographic column is the heart of any gas chromatograph. It separates the analytes from the other sample components, and from each other, so the detector can measure them individually.
Figure 1.4 pictures some typical chromatographic columns.
Figure 1.4 Typical Gas Chromatographic Columns.
Source: Ohio Valley Specialty Company, Inc. Reproduced with permission.
The carrier gas carries the injected sample molecules into the column, where they touch the selected stationary phase. It's the contact with the stationary phase that causes separation. The stationary phase delays the sample molecules − some more than others − so different components end up with different transit times through the column. Each component emerges from the column after its own characteristic retention time.
It takes time
A chromatographic separation takes time. In most process applications, the analysis time is from one to ten minutes, depending on the complexity of the analyzed mixture. Some complex separations take longer.
It's important to realize that separation is just a prelude to analysis. The enormous power of the gas chromatograph comes from its ability to physically separate almost any chosen component from all other components, and then to measure it. Other analytical techniques attempt to measure the concentration of one substance in the presence of all the other substances, a goal that few accomplish well. Gas chromatographs separate the analytes first, and then measure them individually. When properly designed, a process gas chromatograph may be the only process analyzer that doesn't suffer interference from other stuff in the sample.
Of course, it's possible for two or more components to have about the same retention time in a column, so a column might not separate every component from every other component present in the sample. The task of a PGC column system is to separate the measured components from all the others. It's neither necessary nor desirable to separate everything.
Multiple columns
The choice of separating column is always the key to a successful analysis. In practice, it's difficult to achieve the desired separation using just one column, so process gas chromatographs usually employ multiple columns to achieve the necessary separation in the shortest possible time.
With multiple columns, certain partially separated components from one column must flow into another column to achieve the desired separation. To divert flows between columns, an online gas chromatograph typically uses one or more column valves that are usually similar in design to its sample injection valve. Figure 1.5 shows an example of a simple column system.
For simplicity, the figure shows a rotary valve that rotates 90° when actuated, thereby flushing later peaks to vent. Other valves have a similar function.
Figure 1.5 A Simple Column Switching System.
Intercolumn valves must not leak. They must also have very low internal volume and smooth flow paths, lest separated components start to remix. For the same reason, a PGC typically employs
PGCs are individually configured for a particular application. During this procedure, known as application engineering, the application engineer chooses a column system to perform the desired separation and decides on the stationary phase needed for each column. Refer to Chapter 9 for a review of some standard column configurations and the function of each column.
SCI-FILE: On Column Types
Introduction to SCI‐FILEs
The more theoretical and mathematical content of the book resides in separate segments called SCI‐FILEs. These contain optional reading that may or may not be part of a course of study.
Each SCI‐FILE is a supplement to the main text that you can safely omit if not of immediate interest. Treat them as reference sources to consult when needed.
Two kinds of column
The stationary phase must be secure inside the column so it doesn't move. The packed column and the open‐tubular column differ per the method they use to anchor the stationary phase in place.
Packed columns
A packed column most often uses several meters of ⅛‐inch o.d. stainless steel tubing, although early PGCs used larger diameters, and some PGCs now employ
In the traditional packed column, the packing is a granular porous solid with particles about the same size as granulated sugar. These particles pack tightly together inside the tube so that any sample molecules moving with the carrier gas are in intimate contact with them.
The type of column so produced depends on the role of the solid particles:
An active‐solid column contains solid particles having a large activated surface area to selectively adsorb certain molecules from the sample gas.Since the stationary phase is solid, this technique is gas‐solid chromatography (GSC).
A liquid‐phase column contains solid particles having a coating of non‐volatile liquid to selectively dissolve certain molecules from the sample gas.Since the stationary phase is liquid, this technique is gas‐liquid chromatography (GLC).
Many columns now use proprietary stationary phases, often made from specialized polymer material. These columns don't easily fit into the old classifications
