ASTM E260 Standard Practice for Packed Column Gas Chromatography
11. Evaluation of Column Performance
11.1 Make a test mixture which contains a normal aliphatic hydrocarbon and the compound being analyzed, or one similar in structure. The aliphatic hydrocarbon peak should appear in the chromatogram near the second component in the test mixture. (Suggested aliphatic hydrocarbon mixtures are shown in Table 3.) The peaks of both components should be about the same size. The analytical conditions should be chosen so that both chromatographic peaks will appear about 4 to 6 times the retention time (distance) of the solvent peak. (If a selective detector is being used, choose a compound that will give a response to that detector.)
11.2 The different methods of determining column efficiency are shown in Eq 1-3. Figs. 4 and 5 show the measurements made on a chromatographic peak which are used to determine column efficiency.
N = 16 (tR/wb)2
N = 5.54 (tR/wh)2
H = L/N
where:
N = number of theoretical plates,
tR = retention time, or distance, measured in mm,
wb = width of the peak at base, measured in mm, (Determined by extrapolating, as shown in Fig. 4 and Fig. 5.)
wh = width of the peak at one-half the peak height, h, all measured in mm, (See Note 6.)
H = height equivalent to a theoretical plate, HETP, and
L = length of the column in millimetres, or in centimetres.
NOTE 6 - The peak width may be measured to the nearest 0.1 mm by a magnifying loupe fitted with a scale graduated in 0.1 mm increments. The peak to be so measured should be at least 10 mm wide; this is arranged by choosing an adequately large chart speed.
11.3 Eq 1 is often used. However, it involves an extrapolation of the baseline, shown in Fig. 4 and Fig. 5, which can be in error. The use of Eq 2 is preferred because the term, wh can be determined directly from the chromatogram without extrapolation. However, the width of the recorder pen line can be variable which can lead to difficulties in determining the true value of the peak width. Use a pen that writes with a sharp, thin line. The peak width should be measured from the leading edge of one line to the leading edge of the other line, as shown in Fig. 4. For accuracy, determine the peak width twice (see Fig. 4), and use the average value to calculate "N". To further minimize errors in determining the peak width, use a recorder chart speed that will give a minimum peak width of 4 to 5 cm. The peak height should be about 40 to 80% full-scale chart deflection. This can be achieved by either changing the detector sensitivity, or changing the sample size. However, be sure that the sample size does not exceed the capacity of the column.
11.4 The optimum efficiency occurs at the optimum carrier gas flow which can be determined by plotting the efficiency of the hydrocarbon peak versus carrier gas flow. General optimum values of the carrier gas flow rates are shown in Table 4, but should be determined for the particular column.
11.5 Calculate the efficiency of the second component. The results should be about the same for both components. If the efficiency of the second component is about 25 % lower than that of the hydrocarbon, the column might not be the best choice for analyzing the compound(s) of interest. A strong indication of nonsuitability is shown by comparatively greater tailing of the nonhydrocarbon component. Tailing can indicate strong interaction with the column packing (phase or support, or both), column tubing, or adsorption in the injection port or in lines leading from the column to the detector.
11.6 Typical efficiencies are shown in Table 5. Not all gas chromatographic packings are capable of excellent efficiencies. For example, porous polymers, Teflon supports, and some liquid phases, such as trifluoropropyl silicone can give efficiencies less than 500 plates per foot.
