Everyone wants to eat fresh food as opposed to stale food. No one wants to drink skunked beer. So, we have born-on dates on certain foods and foodstuff. This allows us to judge whether or not we want to accept it and drink/eat it.
Carrier gas purity is a big issue in Gas Chromatography. We have to remember that helium is the mobile phase that carries the analytes that are going to be analyzed. It pressurizes the system to “push” the injected sample through the column. Ideally, the carrier phase is totally inert which means that it does not interact with the stationary phase or the analytes of interest. We can’t have it interact with either so as to change the sample or the result. Although helium is not the very best carrier gas to use per the Van Deemter equations, it has become more or less the dominant carrier gas used in forensic testing laboratories when GC is used. Helium is typically delivered in a laboratory and stored in a cylinder.
Again, the carrier gas must be totally inert which means that it does not interact with the stationary phase or the analytes of interest. For testing of VOC’s by GC-FID, helium meets this 2 criterion. It is totally inert which means that it does not interact with the stationary phase or the analytes of interest.
What if this were not so?
What if the carrier gas is not pure and does not contain only helium but instead contains things that are not inert but contains things that interact with the stationary phase or the analytes of interest and are even detected?
The validity of our results may be compromised.
This is a real problem.
Gas manufacturers place out on the market for laboratories to buy different purities.
- high purity (99.998% min. purity)
- ultra-high purity (99.999% min. purity)
- research grade (99.9999% min. purity)
But are you in fact buying that truly that level of pure gas?
The gas manufacturers measure purity not as it is actually contained in the cylinder. It is measured BEFORE it gets placed into the cylinder. The manufacturer measures for oxygen, moisture, total hydrocarbons, carbon monoxide, nitrogen, and others while it is being delivered to the cylinder. Again, no one at the manufacturer tests the purity after it is actually placed in the cylinder.
Customers purchase the gas and rent the container (This is called “demurrage”).
There will always be some level of hydrocarbons in the cylinder as the gas is never ever 100% pure even when it is delivered for the very first time.
Every time the cylinder is emptied through use, it needs to be refilled. The gas companies simply hook the cylinder up to a vacuum and run the vacuum through a cycle 3 times. Helium is very, very light, therefore, it is first in time to be removed and is very easy to remove through the vacuum. Hydrocarbons are much heavier by comparison and therefore will be the last in order to be removed and requires more vacuum to remove. These three cycles is simply not enough to remove all the hydrocarbons. As this process of emptying and filling happens over time with the incomplete “draining” (clearing) of hydrocarbons each time, hydrocarbons will build up in increasing concentration over time. So, it is important to note the cylinder “born on date” to gauge the potential of increased hydrocarbons over time in the cylinder and therefore introduced into the sample that is going to be analyzed.
Hydrocarbons will interact with the sample and the stationary phase. Remember that a Flame Ionization Detector will burn all hydrocarbons and therefore detect it. Therefore, it is a potential source for contamination and cast doubt on the validity of the reported result.