Separation Times, Vol. 22, Issue 1 

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Will my selectivity be affected if I change my column from a standard HP-5ms or DB-5ms column to the new Ultra Inert columns?

By Simon Jones and Mark Sinnott
Agilent Application Engineers

Anytime the polymer or manufacturing process of a column is modified, the resulting chromatography may be affected in a number of ways including analyte response, peak shape, retention order and resolution.

Any such change necessitates re-validation and the additional work required can make switching columns not worth the effort. But revalidation would not be necessary if we can demonstrate that the selectivity between two columns is the same.

In order to compare column selectivity, we must look at each column’s retention index (RI). We could simply compare the QC test chromatograms provided with each column. However, Agilent J&W Ultra Inert Capillary GC columns are QC tested with a different test mixture, so it is not possible to make this direct comparison. Instead, we must test the columns under identical conditions.

The RI is a measure of the retention of a solute relative to normal alkanes at a given temperature on a particular stationary phase. Its value corresponds to the relative retention that the particular solute has to an alkane of a particular carbon number. The temperature chosen for this comparison also has an effect on RI. Therefore, tests performed at different isothermal temperatures will further exploit potential differences in selectivity. The probes used in this example were chosen to demonstrate the similarities in the selectivity of solutes with different functionality, and to have relatively high retention. This ensures a high degree of interaction with the stationary phase which further exploits selectivity differences.

For example, the HP-5ms data in Table 1 shows that 1-methylnaphthalene with a value of 1326.5 will elute slightly after a normal alkane containing 13 carbons. If we now look at the same RI in Table 2 at 180°C, 1-methylnaphthalene is retained more closely to an alkane with 14 carbons. In contrast, when we compare the RI for 1-undecanol at both temperatures, its relative retention is nearly identical.

When we compare the RI of a compound using two different columns, it is important to test under isothermal conditions. This eliminates any effect on RI due to differences in the length and diameter of the tested columns. Isothermal analysis also minimizes the differences between inter-laboratory testing. If the thermal profiles of any two gas chromatographic systems differ even slightly, the position of an analyte at a given time and temperature may also be different, and this would ultimately affect its RI.

Table 1 illustrates RIs obtained from tests performed isothermally (125°C) on standard HP-5ms and DB-5ms columns compared to their Ultra Inert counterparts.

Table 1 – Retention indices measure at 125°C and 39 cm/s He.

As is illustrated, the RIs for the standard -5ms phases compared to the Ultra Inert columns are nearly identical within their respective HP/DB families. We performed a second test at 180°C with a different test mix to further scrutinize potential selectivity differences. This new test mix incorporated the same components used in the low temperature RI measurements with additional analytes to test the selectivity at this elevated temperature, as shown in Table 2.

Table 2 – Retention indices measure at 180°C and 35 cm/s He.

	Figure 1. Chromatogram Comparison of DB-5ms and DB-5ms Ultra Inert Test Mix at 180°C. (Click here to see this image larger.)

Table 2 shows that the RI’s for both 1-methylhaphthalene and 1-undecanol have changed with the new temperature and velocity, as would be expected. When compared between the standard -5ms and the Ultra Inert -5ms phases, the RIs remain essentially identical as was seen in Table 1. This is further illustrated if we compare the chromatograms of the high temperature test mix on the DB-5ms and DB-5ms Ultra Inert columns (Figure 1). The list of compounds illustrated here demonstrate the similarities in selectivity between the DB-5ms and DB-5ms Ultra Inert columns. The elution orders, retention times, as well as resolution between peaks are nearly identical between the two chromatograms, even when looking at peaks 3 and 4.

The Agilent J&W Ultra Inert HP-5ms and Ultra Inert DB-5ms columns are designed to add an extra level of inertness to your analysis without the need for method revalidation. As is illustrated here, selectivity remains the same between the non-Inert, and Inert -5ms phases. To research more applications for Ultra Inert in your lab, visit our product page and read the available Application Notes.