Gas Chromatography


Durability of Supelco GC Capillary Columns
The Proof: Durability

The extreme durability of SLBms columns is achieved as a result of Unique Advances in Polymer Synthesis and the Innovative Manufacturing Processes that we established for these columns.

Durability of a column is reflected in the published maximum temperature. A durable column allows short analysis times and long column life to be achieved, resulting in savings of both time and money.

Durability = High Maximum Temperature = Short Analysis Times

The type of capillary column that most readers may be familiar with consists of a thin film of liquid stationary phase coated on the inner wall of fused silica tubing. This type of column is termed fused silica open tubular (FSOT). The separation process for FSOT columns takes place through gas-liquid chromatography (GLC); the separation of analytes due to the differences in their partitioning rates between a gas phase (the carrier gas) and a liquid phase (the stationary phase). In GLC, the gas chromatograph (GC) oven temperature can be used to affect partitioning, and hence retention. At lower oven temperatures, partitioning is toward the stationary phase. At higher oven temperatures, partitioning is toward the gas phase. Analyte boiling points must also be considered. Analytes with higher boiling points will tend to require more time to elute from the column than analytes with lower boiling points.

One of the many benefits of GC is the sheer number of compounds that can be separated in a single analysis. However, analysis of analytes with a wide range of boiling points can lead to long analysis times. One of the factors influencing analysis time is the maximum allowable operating temperature (MAOT) of the column. When compared to columns with lower MAOTs, a column with a higher MAOT offers more flexibility with regards to the temperature program and final temperature used to elute the analytes.

The two chromatograms shown in Figure 1 can be used to illustrate this point. The last eluting peak, benzo(g,h,i)perylene with a 500 °C boiling point, requires a high final oven temperature to elute with good peak shape and in a reasonable amount of time. By increasing the final oven temperature from 330 °C to 350 °C, partitioning of the analyte is driven towards the gas phase, resulting in less retention and a shorter analysis time. This is observed by a total analysis time of less than 19 minutes. The benefit to the analyst is that more billable samples can be analyzed in a given period of time. In addition, the shorter retention of later eluting peaks will result in less band broadening.

Figure 1. The Benefit of Higher Maximum Temperature
For the complete US EPA Method 8270D instrument conditions and peak identifications, refer to Application Report 398 (136kb pdf) (350 °C chromatogram) and Application Report 391 (132kb pdf) (330 °C chromatogram).

Durability = Quick Baseline Stabilization = Short Installation Times

After installation of a new column, a quick purge and conditioning are required prior to use. The purge is performed to remove any moisture and/or oxygen that may have entered the column during installation. The conditioning procedure will stabilize the baseline. The recommended installation procedure for a 30 m x 0.25 mm I.D., 0.25 µm SLB-5ms column is:
1. Install the column and set the desired flow rate or linear velocity.
2. Purge the column at ambient temperature for 15 minutes.
3. Program oven at 15 °C/min. to 325 °C and hold for a minimum of 30 minutes.
4. Cool the GC oven to ambient temperature and re-snug the fittings.

Non-MS columns may require a minimum hold time of several hours at the final oven temperature before baseline stability is achieved. Because the SLB-5ms conditioning procedure requires a much shorter hold at the final oven temperature, instrument downtime for column change-out is minimized.

Durability = Minimal Phase Loss = Long Column Life

As carrier gas passes through any capillary column at elevated temperature, phase is continuously being degraded, creating column bleed. Elevated temperatures hasten this degradation, seen as a baseline rise when using oven temperature programs. At some point, enough phase will have degraded so that resolution and retention are no longer acceptable. It is at this time that the column must be replaced.

To test column life, we cycled an SLB-5ms column through a rigorous oven temperature program that ended with a four hour hold at 360 °C. This is 20 degrees above the published maximum isothermal temperature limit. While it is never recommended to operate above a column’s MAOT, 360 °C was selected in an attempt to expedite degradation in column performance to prove our point. A 30 m x 0.25 mm I.D., 1.0 µm dimension was selected over a 30 m x 0.25 mm I.D., 0.25 µm dimension for this test because a column with a higher film thickness is more susceptible to phase damage from elevated temperature than a column with a lower film thickness. The column was evaluated for key performance parameters at the beginning of each cycle.

Figure 2 shows the isothermal portions of the chromatograms from the analyses of a column evaluation test mix from the first and the 20th cycle. The bottom chromatogram was generated prior to the start of the test, and the top after the 20th cycle (after the column had been exposed to 360 °C for a total of 76 hours). As expected, there was slight decrease in retention, but no change in peak shape, response, or resolution. Additionally, column bleed remained at a low level, indicating good phase stability.

Figure 2. Column Evaluation Test Mix, Before and After Column Life Test