Fig. 1 Contamination of fused silica columns can be easily removed by cutting-off the first section

Most of the time the impurities accumulate in the first meter(s) of the column. As the separation process in the capillary starts here, the unwanted interactions that take place will impact immediate the initial band width.  By cutting off the contaminated section, the sample will be introduced in a “clean” section and we obtain similar separation as before..  See fig 1.

Many users choose for connecting a guard column in front of the analytical column.

The guard column protects the analytical column for contamination, which dramatically increases the column life time. Such a guard column can be trimmed when polluted and eventually be replaced.  Depending on the application  guard columns have a life time of 1 week up to 6 months.

Deactivated fused silica tubing can be used as guard column and can purchased per meter of which a defined length. Usually this is 2-5 meters. Disadvantage of cutting parts of the guard column is that the column becomes shorter resulting in change of retention times. If similar length is cut every time, the change of retention becomes predictable. One can also replace the guard entirely when contaminated. This way retention times will always be similar.

Fig. 2 cutting fused silica: slide in one direction with a ceramic wafer

Bigger challenge is to make the coupling. Best generic results are obtained using a universal PressTight^® connector. Make a 90-degree cut, (fig 2), by using a ceramic wafer and let the wafer slide one-way along your nail,  wet the column and guard ends with some methanol and push it into the PressTight . By pushing it, an optical “ring” is observed (see fig.3) which is the actual seal made by the polyimide and the surface.

http://blog.restek.com/?p=4168

A more elegant solution that eliminates column coupling is to use Integra-Guard® columns

Fig. 3 When making the connecting a "brown" ring must appear.

Integra-Guard^® Columns

Integrated Guard columns have been first introduced by Restek^® Corporation around 1990 and have been widely used since. Integrated guard columns are prepared by coating only the last section of the column. This technique is also called “segment” coating.

The advantages are clear. As there is no coupling present we have all the advantages of having “no coupling”..

-          No connection to make, saves lots of time

-          No leaks, improved stability and more accurate data

-          No dead volumes/ activity or thermal mass

-          Easy in maintenance; integrated solution

Fig. $ Making Integrated guard column: No coupling at all, used widely with Rxi-5Sil MS

Such Integra-Guard® columns are typically 5 – 10 m in length and can be made for most standard stationary phases and mostl column diameters. Important is to make a clear mark on the column where the guard column starts. Not every stationary phase can be used as an Integra-Guard® solution. The column needs to be deactivated first, followed by a second deposition of the stationary phase layer.  There are different phases that can be manufactured with an Integra-Guard® guard column.   Working with integrated guard columns allows the user to use cut a piece periodically from the inlet, without the trouble of making a coupling. Integra guard solutions are avialable for a series of phases: Besides the Rxi-5Sil MS see also:

http://www.restek.com/pdfs/59441.pdf

For operation of GC columns a carrier gas is required.  In the first blog of this series we zoomed into the importance of using clean carrier gas.  We also have a choice on type of carrier gas. Mostly Helium is used, because of its high optimal velocity and inertness but that gas is loosing its attractiveness for several reasons.

Helium is becoming more and more expensive, increasing cost per analysis. This will not change as the supply of helium is also not secured.

One of the main sources of helium in Amarillo, Texas, will be drying up.  We have to consider alternatives. http://www.blm.gov/wo/st/en/info/newsroom/2007/january/NR0701_2.html

Alternatives are available for many applications using Hydrogen. One has to deal with safety aspects, but its not as dangerous as people always seem to think. See for details our blog on hydrogen: http://blog.restek.com/?p=3520 risks are very low and we can take precautions.

Helium has also disadvantages.  It’s my personnel experience that I found that columns used under hydrogen show a more lasting inertness then used under helium I always blamed low level residues of sulfur compounds to be responsible for that, but I never could proof that.

A big challenge all must know is that helium diffuses through the fused silica. That means that if you take a column out of the GC, in 16 hrs there is no helium present anymore and depending on how good you seal the column, it will be completely evacuated or filled with lab-air, see http://blog.restek.com/?p=568 .

We know that Restek columns filled with lab-air, have no problem when kept at ambient temperature, but you need to be aware of this. That means also, when you install the column in GC you need to flush very well before heating as there is air (meaning water and oxygen)present.  Because air contains water, this is also entering the column. For alumina column this means high temperature conditioning as the water will deactivate the alumina.

Fig 1. Leak detection will assure correct column installation and reveil possible septum or leaks in the gas lines. Its an investment that is earned back in very short time

Whatever gas we use, leaks and gas purity are always a big challenge.  The higher the temperature of the  injector, column and detector, the more problems we can anticipate. Leak detection can be done nowadays with sophisticated instruments like a leak detector. See fig.1. Such a leak detector is measuring on the principle of gas conductivity and is very sensitive for H2 and helium. Its clean and it will give an audio or visual alarm if a leak is detected.  Such a device is also practical for checking the septum seal as the septum is probably the biggest leak in the system.(but many do not realize that).

I also list the injector and detector on purpose as many seem to forget that in this part we have high temperatures and activation can be very fast. See the extreme data generated by activity in a 6890 FID.

When you do not use the column and want to keep it in the GC, always maintain a positive flow through the column and keep it at a temperature of 80-90 C.  I have seen many times that when the flows are turned off, back diffusion of air can occur even into the gas lines. Keep the split flow to a minimum value of 5-10 m/min to save carrier gas.