Читать книгу Process Gas Chromatographs - Tony Waters - Страница 74

To evaluate various measures of column performance, we'll need to measure the peak retention time and the peak width. Some computer programs will make these measurements for you, but you won't get the significance of them until you learn to do it on a chromatogram.

Figure 3.4 illustrates the most important measurements. Measure the variables in seconds or in millimeters. Many of our optimization parameters are ratios, so the units cancel out.

You can make simple measurements from a chromatogram to discover how well the columns are performing. The data most often collected are shown in this illustrative chromatogram above, and are further discussed in the text.

Figure 3.4 Typical Chromatogram Measurements.

It's not possible to measure the actual peak width at the baseline because the peak gradually fades away. Instead, draw tangent lines along the flat sides of the peak and extend the baseline across the width of the peak. Chromatographers call this procedure triangulating the peak.

Some chromatographers prefer to measure the width of the peak at half its height. This is often easier to do than triangulating the base width, and perhaps more accurate, since no triangulation errors occur. The two width measurements are related, and either of them can be used to evaluate column performance.

Make the following measurements:

 Measure the holdup time (tM) from the injection time mark to the apex of the air peak (you can triangulate the air peak if you wish).

 Measure the retention time (tR) of each component peak from the injection time mark to the intersection of its tangent lines.

 Measure the base width (wb) of each component peak between the intersections of the tangent lines with the extended baseline.

 Alternatively, measure the width at half height (w0.5) of each component peak.

For clarity of display, Figure 3.4 shows a single wide peak. You may have many peaks, and most likely they will be narrower than that. It's difficult to measure the width of a narrow peak. To obtain a good measurement, you may have to expand the time base on a computer display, or increase the chart speed on a recorder.