36


February / March 2012


Protecting your GC Column


by Peter Morgan, Ruth Lewis, Tony Edge ThermoScientific, Runcorn, UK, WA7 1TA


The use of guard columns within chromatography, both HPLC and GC, is not as widely used as it could be. Many analysts see guard columns as an unnecessary fuss and an extra complication within the whole analytical arrangement. There are many reasons for this, and understanding these reasons gives an insight into how to improve the productivity of many commercial laboratories.


Within gas chromatography, the analysis of samples which have involatile components can lead to a gradual deposition of material at the inlet end of the column, which does not migrate from the column due to inherent high boiling points of these compounds. To try and remove these compounds will require a substantial period of time at very elevated temperatures and relies on the compound very slowly partitioning down the column. Generally speaking it is highly unlikely that it will be possible to bake the columns long enough and at a high enough temperature to effectively remove them, and to all intents and purposes these compounds remain irreversibly trapped at the head of column.


There is another source of possible contamination from samples containing semi-volatile components. These compounds will elute from the column if the correct temperature is applied for a long enough period, however due to their lack of volatility it may require several hours to effectively bake out the column to remove these contaminants. Also, it may not be practical to do this as there is potential to damage the column irrevocably, as at elevated temperatures stationary phase is lost and the column will start to lose structural integrity.


Contamination can come from a variety of sources, and the specific injection technique used will determine exactly the degree of contamination on the column. Clearly injecting samples directly onto the column or using large volumes will result in a greater degree of contamination of the column. Typically contamination arises from dirty matrices, such as biological fluids, soil, wastewater, food matrices, where there has


been minimal sample preparation.


The contaminants can cause a range of difficulties for the chromatographer, as they will interfere with the partitioning of the analytes, since the stationary phase effectively becomes the contamination. Depending on the nature of the contamination there may also exist a possibility of peak tailing due to non- desirable interactions occurring between analytes which contain hydroxyl or amine functionality. It is therefore very desirable to reduce these effects to a minimum.


For gas chromatography it is quite common to remove part of the inlet end of a GC column as this will remove the bulk of the involatile and many of the semi-volatile contaminants. Reducing the length of the column by 30 – 50cm will typically not affect the performance significantly, however repeatedly performing this operation will obviously have a detrimental effect of the ability of the column to effectively resolve between critical pairs. Another option is to use a guard column as was suggested at the start of this article. These can come in two formats, either as an integrated part of the column or as a separate entity which is then coupled to the main column using an appropriate sized coupling.


Separate guard columns will typically be a length of fused silica which can suffer from unwanted activity and introduce dead volume into the system, both of which can adversely affect the peak shape of the analytes. There is also the potential with some unions that small leaks can be introduced into the system which will have a detrimental effect on the chromatographic performance. Unlike with HPLC where the


coupling strength between a guard column and the main analytical column is governed by some form of screw thread, with many press fit couplings it depends substantially on the technical proficiency of the operator.


In this article two significant comparisons will be made. The first will investigate the effect of removing the front part of a column when there is an integrated guard present and when there is not one present. A separation of a series of PAH’s will be used to determine the effect that both of these approaches has on the integrity of the separation.


The second comparison will look at the difference observed with another test solution derived from toothpaste. In this example the difference between using an integrated guard column and the same analytical column that is fitted with a guard column using a press fitting to determine if there is any difference in performance.


Method


The first method uses a series of PAH’s as a marker of column performance. In both cases an equivalent analytical part of the column is used, allowing a direct comparison of the two approaches.


The PAH test mixture contained 16 test probes which were;


acenaphthene, acenaphthylene, anthracene, benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(ghi)perylene, chrysene, dibenzo(a,h)anthracene, fluoranthene, fluorine, indeno(1,2,3-cd)pyrene, naphthalene, phenanthrene, pyrene


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