Reduction of column bleed requires first of all a clear understanding of the term. Column bleeding results from thermal breakdown of the stationary phase at temperatures close to the upper temperature limit of the column. The level of bleed depends on the film thickness of the stationary phase layer on the adsorbent. Thick films columns are used to handle higher sample loads but at same time their bleed rates are higher. It is natural for all columns, irrespective of stationary phase film thickness, to bleed at high temperatures but excessive bleeding results in shortening of the useful lifespan of the columns.
Column bleeding is characterized by a steady rise in the baseline about 30 degrees C before the upper permissible temperature limit and reaches a plateau around the upper limit. The region is generally free of any distinct peaks. Column bleed needs to be discriminated from other similar bleedings such as septum bleed, column contamination and fouling of detector or liner. In the case of flame ionization detector white silica deposit in detector could result from column bleeding whereas if the deposit is black carbonaceous material it could be resulting from septum bleeding or contamination from analytes.
Presence of oxygen at high column temperatures contributes to increased column bleeding and results in rising baseline. It is essential to ensure freedom from leaks in the injector and making use of oxygen traps in the carrier gas lines.
The other common reason is conditioning of the column above the maximum temperature limit or operating at high temperatures under insufficient carrier gas flow rates or emptying of carrier gas tank during conditioning.
Septum bleeding should not be confused with column bleeding. Septum bleeding generally results in several distinct peaks in the rising baseline portion.
Steps to minimize column bleed
- Always operate columns at least 20 – 30 degrees C below the upper specified temperature limit
- Verify freedom of flow of carrier gas to column by immersing the detector end of the column in a vial filled with methanol and observing free formation of bubbles before conditioning the column
- Install oxygen and moisture traps in carrier gas lines and perform leak checks after column installation
- Condition columns as per the manufacturer recommendations before use
- Cap the open ends during storage
- Replace septa frequently to prevent damage due to coring
- In case of high degree of contamination by higher molecular weight samples rinsing of column with a suitable solvent is beneficial.
Low bleeding columns offer advantages such as extension of useful life of the column and improved detection sensitivity at elevated temperatures. The advantages are of greater significance in high sensitivity GC – MS analysis.