A New Generation of GC Capillary Columns: SLB-5ms

By: Luigi Mondello and Rosaria Costa, Reporter EU Volume 20

Luigi Mondello and Rosaria Costa Dipartimento Farmaco-chimico, Facoltà di Farmacia, Università degli Studi di Messina, Viale Annunziata, 98168 – Messina.

The heart of a gas chromatographic system, the capillary column, has been since its introduction the object of considerable improvement in terms of analytical performance. Particular attention has been devoted to the production of low bleed columns, resistant to very high temperatures and especially suited to gas chromatography-mass spectrometry (GC-MS). The wide employment of mass spectrometers as detection systems in GC applications has increased the necessity of low bleed columns in order to increase sensitivity and to obtain “cleaner” spectra, thus allowing easier peak identification. For this reason, in the last years there has been a high interest towards the so called “MS” columns specifically designed for GC-MS benchtop systems. MS columns are characterised by high inertness, low bleed and higher temperature operating conditions.

It must be added that the need for high thermal stability columns (capable of operating above 300°C) to analyse complex mixtures containing high boiling VOCs has become a common issue in the petrochemical industry. The methods that are exploited in order to increase the stability of the stationary phase are as follows:

a) binding of the phase to the capillary surface (bonded phase) 
b) “cross-linking” the phase by the use of a radical initiator 
c) “Sol-gel” technology: the phase is encapsulated inside synthetic glass

Whatever technology is used, the final goal is always to reduce column bleed. The analytical sensitivity of a column is connected to the signal-to-noise ratio (S/N). Consequently, if the column bleed increases, the noise will increase as well.

Column bleed arises from the degradation of the stationary phase and is directly proportional either to the amount of stationary phase or to the column temperature. It is derived from the formation of cyclic (tri- and tetra-) polysiloxanes from thermal and oxidative degradation processes occurring in the stationary phase polymer. Due to a “backbiting” effect, these cyclic siloxanes are generated and, immediately after, separate from the stationary phase chain, thus producing noise in the detector signal.

Columns that exhibit low bleed have a longer lifetime and generally make quantitation and identification easier. It needs to be emphasised that a low level of bleed produces a better S/N ratio and avoids the formation of ion fragments unrelated to the matrix. Finally, a very low background greatly reduces the risk of contamination of the detector components, especially critical when using GC-MS.

Supelco has recently introduced a new low bleed GC capillary column, Supelco SLB-5ms, that has a significantly reduced bleed profile compared to conventional columns of the same 5% diphenyl/95% polydimethylsiloxane phase chemistry. SLB-5ms is the result of advances in the field of polymer chemistry and is characterized by improved cross-linking, a higher thermal stability and greater reproducibility.

Various parameters related to the column performance, such as resolution, analyte response, degree of bleeding and column life, have been accounted for during the development of this stationary phase. For the aforementioned reasons, SLB-5ms is particularly suited to GC-MS analysis. As can be seen in Figure 1, the SLB-5ms column produces an almost non existent baseline rise, even at the final temperature of the oven program. SLB-5ms also passes the bleed measurements test. There are different methods that enable the evaluation of column bleeding. It may be measured in terms of detector response or, more effectively, in terms of ng bleed/sec. Another way to estimate this factor is through the calculation of the percentage rate of loss of the total volume of stationary phase.

Figure 1. Bleed arising from SLB-5ms compared to conventional 5-ms column

Figure 2 shows an example of absolute column bleed measurement related to a conventional 5-type column. A standard solution of octamethylcyclotetrasiloxane (C8H24O4Si4, known as D4) is injected in programmed temperature (50°C @ 7°C/min to 325°C, held 10 min). A slice of area under the baseline at the maximum temperature is measured and this value is divided by the D4 peak area. Dividing the result obtained by the time of the slice width will provide the corresponding measure of the absolute bleeding in ng bleed/sec. This method allows to define the exact degree of bleeding at the maximum operational temperature. The analytical performance of the SLB-5ms still shows to be excellent even in the case of complex matrices such as essential oils and perfumes (Figure 3).

Figure 2. Absolute bleed measurement on conventional 5-ms column

Figure 3. Essential oils on SLB-5ms vs. conventional 5-ms column

In order to test the reliability of the SLB-5ms columns, we compared the Linear Retention Indices obtained for SLB-5ms with those calculated for the same compounds identified in perfume into the conventional 5-type column. Finally, we made a comparison of these values with those reported in our GC-MS library (FFNSC) dedicated to flavour and fragrance compounds (Table 1).

Table 1. Comparison of flavour and fragrance compounds Linear Retention Indices (LRI)

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