Development of a New Platform of Highly Inert Fused-Silica Capillary Columns

Gas chromatography technology has consistently improved sensitivity for difficult- to-analyze compounds. Increased sensitivity is especially important in trace analysis, where the response of components depends strongly on the inertness and background of the system. While the sensitivity of GC instruments and detection systems has been improving continuously, column performance has remained relatively constant over the last several years. In order to further increase sensitivity and make use of the instrument improvements, it is necessary to improve the column performance. This can be realized by 1) stationary phase stabilization, which reduces column bleed, and 2) better deactivation techniques, which improve column inertness. New deactivation technology has made it possible to make a more neutral column. As a result, more polar compounds as well as bases/acids can be analyzed with high response using standard-type stationary phases.

Rxi® column technology (Restek Corp., Bellefonte, PA) has been successfully applied with nonpolar phases such as 100% polydimethyl siloxane (PDMS) as well as 5–50% phenyl PDMS phases. A simple test was developed that provides an excellent indication of column quality.

Measurement of trace impurities

When measuring trace-level impurities, the sensitivity of the GC system needs to be optimized. Sensitivity is strongly influenced by the background of the system: A system with high background will always show higher noise levels and reduced sensitivity. To optimize sensitivity, the signal-to-noise ratio (S/N) must be maximized. A lower noise level will increase the S/N, allowing lower concentrations to be detected. There are two ways to maximize the S/N: 1) decrease the noise level (decrease N), and 2) increase the signal (increase S).

Decreasing the noise level

Many parameters need to be optimized to reduce noise levels. The most important are:

  • Carrier gas purity and the potential for leaks in lines
  • Injection/detection port liners and connections with possible leaks
  • Septa, needles, vials, and injection protocols
  • Transfer lines and temperatures
  • Stationary phase type, film, column length, and i.d.

By optimizing the GC system, not only is an increase in sensitivity obtained, but downtime is also reduced due to less detector contamination. Systems stabilize faster and, when using a mass spectrometric detector, the mass spectra will show better “match” factors because fewer contamination ions are formed.

Increasing the signal

Benefits are also gained from maximizing the signal. Signal can be increased by:

  • Concentrating the sample: This will take time and requires more sample purification since the matrix will also concentrate.
  • Injecting more volume onto the column: This is growing in popularity because the detection limits can be reduced by a factor of 20–100. There are many developments using large-volume injection techniques. Programmed temperature vaporizer (PTV) injection seems to be quite promising here.
  • Optimizing detector settings: Optimizing detector flows, temperatures, and voltages will maximize sensitivity.
  • Using inert liners and considering column position: Injection and detection port liners can be highly active. If the column is not positioned correctly in the detection port liner, the liner may adsorb polar/sensitive compounds.
  • Increasing gas velocity or temperature program rate: Using faster temperature programs or higher flow rate can elute a component faster, resulting in a smaller peak width. The peak area will be similar, which will result in a higher response.

A factor that is often forgotten is the inertness of the capillary column. The shape of the eluting peak greatly determines the height of the signal and therefore the sensitivity. Rxi column technology was developed to improve the inertness of existing capillary columns.

Reducing noise and increasing signal

Low-bleed stationary phases have been commercially available for some time. Through the use of arylene-type stabilizing groups, the mobility of siloxanes could be reduced significantly, resulting in more stable polymers. As a result, the breakdown reaction that is the basis of stationary phase degradation is less likely to occur.

With Rxi technology, the stabilization has been taken one step further. Endcapping the reactive silanol groups present in the polymer and on the fused-silica surface was the first step. Incorporation of systematic cross-bonds made it possible to link up the siloxane chains to thicker films, maintaining flexibility without cleavage at higher temperature. 

Figure 1 - Schematic representation of Rxi bonding process (cross-bonding, surface bonding, deactivation, and shielding for minimization of impact of [re]active silanols).

Additionally, surface deactivation was developed that allowed surface bonding of all Rxi polymers. The surface bonding makes the Rxi polymers extremely stable for mechanical attack of liquids in, for instance, splitless injections. This typically translates to longer column lifetime. Figure 1 shows a schematic of the bonding processes used to stabilize Rxi polymers. This deactivation has an important function in shielding any residual activity on the surface, creating a highly inert column. Polymers prepared with dimethyl, diphenyl, and arylene-type stabilization could be made highly inert using Rxi column technology.

Figure 2 - Bleed comparison of different types of 5% diphenyl and aryl phases: a) 0.25-μm film, b) 0.5-μm film.

Figure 2 shows a comparison of the degradation obtained on several commercially available stabilized or “low-bleed” phases. All columns tested were treated under identical conditions. Although most commercial columns have low-bleed characteristics, the Rxi columns clearly demonstrate very good performance regarding column bleed. This is particularly evident when using thicker films (Figure 2b): The stabilization resulting from the cross-bonds helps to keep the bleed low. The difference is even more pronounced when inertness is taken into consideration.