Increasing the Stability of Liquid Stationary Phases for Improved Stability in High-Temperature GC

The attempt to stabilize stationary phases for use in gas chromatography is ongoing. This is because lower detection is increasingly required and, in order for the detection system to provide high performance, the risk of contamination must be minimized. Using stabilization technology, the breakdown of stationary phases was reduced, resulting in low-bleed stationary phases with tight specifications.

One of the parameters that initiates stationary phase breakdown is the internal surface of the GC capillary. Modifying the surface made it possible to stabilize stationary phases by a factor of 4–10 lower breakdown. This also resulted in higher operating temperatures for such phases as the UltiMetal column (Varian Inc., Middelburg, The Netherlands) for simulated distillation applications.

In the present study, arylene-stabilized 5% phenyl/95% methyl polydimethylsiloxane (PDMS) type phases were tested in a high-temperature GC using fused-silica columns that were deactivated in a way similar to the UltiMetal columns. The result is an extremely stable and low-bleed phase, the VF-5ht (Varian), which is capable of operating at up to temperatures of 400 °C.

High-temperature capillary columns

Various approaches were taken in the past for performing high-temperature separations. Columns with an aluminum outside coating were commercialized 15–20 years ago. In theory, these columns should have been very temperature stable and easy to handle. Practical experience with the aluminum-coated columns revealed, however, that they became fragile after repeated temperature programming. On the other hand, isothermal operations performed well. The difference in thermal expansion coefficient has been the main cause of this problem.

Figure 1 - Retention time stability following long-term, high-temperature exposure using UltiMetal column coated with 100% PDMS.

Alternative solutions were discovered in 1990,1 when a new deactivation technology was introduced for the deactivation of metal capillaries. The resulting columns could be used at up to very high temperatures. Figure 1 shows an example of such a column for a hydrocarbon sample running up to C100. The column was used for 200 runs up to 430 °C, and retention time for C100 was altered from 37.4 to 37.1 min. This meant that the majority of the stationary phase was rendered in the column. This is typical of UltiMetal Simdist columns, i.e., very low bleed and high stability. The columns are coated with 100% PDMS-type phases, providing the best match with boiling-point-type elution, as required for simulated distillation.

Figure 2 - Breakdown mechanism of siloxane stationary phases: formation of cyclic degradation products.

Surface stabilization

In order to create a surface that stabilizes stationary phases, the breakdown mechanism of stationary phases must be known. Figure 2 depicts the most common mechanism of siloxane breakdown: A terminal silanol attacks its own chain and a cyclic (volatile) degradation product is formed. This volatile degradation product can be detected by any sensitive detection system. There will be various ring sizes formed, which can be identified via MS by the different masses.

There are three mechanisms responsible for the formation of the terminal silanol group:

  1. Breakdown initiated by the silanol groups in the stationary phase. The greater the amount of stationary phase, the more breakdown (bleed) will be obtained, which is usually why the bleed increases linearly with the amount of stationary phase in the column.
  2. Breakdown by the silanols formed upon application. When a column is operated in a GC, there always will be water molecules entering the system. This occurs via the carrier gas, leaks, injection, and the septum. These water molecules will hydrolyze the siloxane chain and split the siloxane molecule, forming terminal silanol groups. This again initiates the breakdown and bleed. This can be controlled for different setups since it depends mainly on the purity of the carrier gas and how the system is put together and maintained.
  3. Breakdown initiated by the reactive surface silanol groups. The amount in these groups is determined by the surface and the surface deactivation.

Items 1 and 2 can be easily optimized/minimized using stationary phase stabilization technologies as well as gas filtration and thorough leak checking. Item 3 has to be optimized via the internal column surface.

Figure 3 - Surface silanol group transfers to stationary phase, forming a terminal silanol group.

Normal deactivation using siloxane deactivation reagents will always leave a number of silanol groups at the surface. These can react with the stationary phase, forming a surface bond, and the silanol group will then “jump” to the stationary phase, as shown in Figure 3. The degradation process reinitiates, as shown in Figure 2.

Figure 4 - Normalized bleed (bleed per mg stationary phase) for different suppliers.

The impact of the surface can be visualized if we calculate the bleed per milligram of stationary phase (normalized bleeding) based on the specifications of different suppliers. It is clear from Figure 4 that by decreasing the film thickness, the bleed per milligram of stationary phase increases exponentially. This is due to the impact of the fused-silica surface. Supplier 1 was more successful at deactivating the reactive surface silanol groups, resulting in a listing of lower-bleed specifications.

New deactivation of fused-silica surfaces

In addition to the standard siloxane deactivations of surfaces, a new deactivation was introduced in 1990.2–4 This deactivation was also applied to fused silica, which resulted in a new method for extending the temperature stability of stationary phases. By reducing the level of reactive silanol groups, this contribution to degradation has been minimized.

Figure 5 - Impact of column internal surface on stationary phase degradation.

When coating stabilized stationary phases, there is a significant improvement in thermal stability. Figure 5 shows a direct comparison of a similar layer of arylene-stabilized stationary phase, VF-5ms (Varian), coated on a fused-silica, deactivated surface, and the UltiMetal deactivation technology. The result is lower degradation of the stationary phase by a factor of almost 10. The phase stabilized using this technology is the VF-5ht. These phases can be used at much higher operating temperatures. Additionally, when working at lower temperatures, there is hardly any measurable bleed.

Column lifetime

In high-temperature analysis, column lifetime is often determined by degradation of the stationary phase. Lower degradation translates into increased lifetime. Furthermore, retention times will be more constant, which allows easier maintenance of integration parameters and calibration.

Although high-temperature phases that use fused silica are commercially available, they are stabilized mainly by polymer technologies. The effects of the fused-silica surface have not been optimized, as shown in Figure 6. A comparison was made of a 30 m × 0.25 mm fused-silica capillary coated with a 0.10-μm high-temperature phase. There is a significant difference in stationary phase degradation. For high-temperature analysis, the loss of phase usually determines the lifetime. This is significantly better for the VF- 5ht. When running different sample types, the difference in column bleed is always seen when the upper temperature is reached: The background of the VF-5ht is significantly lower.

Figure 6 - Comparison of existing high-temperature phases with similar siloxane composition.

Injection techniques

Most of the high-temperature applications deal with neutral components. It is important to have a representative sample on the column; an on-column injection technique is recommended for this. With on-column injection, the sample is introduced inside the capillary as a liquid. The method results in minimal discrimination, since all material injected onto the column will also elute from the column. To apply the on-column injection technique, it is important to have fused silica with a diameter of 0.32 mm in order to allow easy access of the syringe needle into the capillary. This diameter is standard for the VF-5ht phase. In addition, the phase is cross-linked and immobilized, meaning that direct contact with liquids will not change the column efficiency.

Split systems are also used for high-temperature operations. If the split system produces a reproducible discrimination, the technique will also produce acceptable data. Split injection provides more flexibility with sample types. Column diameters of 0.25 mm can be used, providing the highest separation power.

The programmed temperature vaporizer (PTV) is a more universal technique. It is growing in popularity and is applicable for high-temperature analysis as well.

Impact of oxygen/water

Oxygen is generally not a problem in GC when analysis temperatures are low. In some applications, air is used as the carrier gas at temperatures up to 130 °C. Above 130 °C, the impact of oxygen increases. When applications are run higher than 200 °C, oxygen must be removed.

Water can also be a problem at lower temperatures and must be eliminated. Because water forms terminal silanol groups, it is the main cause of stationary phase breakdown. As the reactivity increases with higher temperatures, the high-temperature applications require ultradry conditions. Therefore, leaks must be minimized and carrier gas must be absolutely pure, preferably filtered with special filters before the GC. Gas Clean filters (Varian) are highly recommended. With the indicator in the filter, the risk of water introduction is reduced to a minimum.

Figure 7 - Different classes of components that can be analyzed using the high-temperature column.

Loadability/inertness

The high-temperature columns are coated with thin films to have the lowest possible elution temperatures. The challenge to using thin films is always the loadability and the inertness of the resulting column. The VF-5ht is a good compromise. Generally, a wide range of compounds having different functionalities can be analyzed (see Figure 7). Highly polar compounds such as alcohols and free acids, however, will exhibit nonsymmetrical peak shapes. The majority of high-temperature applications involve neutral or slightly polar compounds. It is recommended that highly polar and heavy compounds be derivatized to improve chromatography and volatility. When using thin-film-coated columns, there is a constant risk of overloading, and samples need to be diluted to minimize the overloading effects.

Figure 8 - Analysis of biodiesel according to ASTM D6584.

Applications

The main application area for the VF-5ht phase is for the separation of high-boiling-point components/mixtures. ASTM D6584 describes the analysis of biodiesel, which requires high elution temperatures. Figure 8 shows the separation of biodiesel using the VF-5ht phase. Up to temperatures of 380 °C, virtually no bleed can be observed. On-column injection is used for this application to elute the heavy boilers with minimal discrimination. In this analysis in particular, it is important to have good quantification.

Figure 9 - Triglycerides of palm oil.

The same injection technique was used for the separation of triglycerides. Separation is by carbon number (see Figure 9).

Since the column has a stabilized polyimide outside coating, long-term operation at high temperatures will oxidize and weaken the protective coating. The degree of oxidation depends on the type of GC, position of the column in the GC, temperature, and the time exposed. Although the VF-5ht phase on this surface can withstand temperatures of 400 °C, it is recommended that the column be as short as possible at such high temperatures. High-resolution hydrocarbon separations can be done using the 0.25-mm-i.d. column, as demonstrated in Figure 10. The high efficiency of the columns is also shown. In addition, at higher temperatures, the VF-5ht phase remains very efficient.

Figure 10 - High-resolution separation of a wax sample.

Conclusion

The VF-5ht stationary phase offers extreme temperature stability, permitting efficient, high-temperature separations. The stability of the phase provides reproducible retention times to allow a reduced number of calibration runs. Column lifetime is significantly improved. It is important to ensure that the GC system is absolutely leak free, and carrier gas must be of high purity in order to gain the full benefit of these phases for high-temperature separations.

References

  1. Buyten, J.; Duvekot, C.; Peene, J.; Mussche, P. Eleventh International Symposium on Capillary Chromatography, Monterey, CA, May 14–17, 1990; 91.
  2. Buyten, J.; Duvekot, C.; Peene, J.; Mussche, P. Int. Chromatogr.
    Lab.
    1990, 2, 5.
  3. De Zeeuw, J. Chrompack News1990, 17(2), 3.
  4. Schaller, H. Fat Sci. Apr. 1991, 93, 510.
  5. High Temperature Gas Chromatography. Chrompack, no. 03551, 1990.

The authors are with Varian Inc., Herculesweg 8, 4330 EA Middelburg, The Netherlands; tel.: +31 118 671279; fax: +31 118 67235; e-mail: [email protected].