GC carrier gases, do you really have a choice?12 Aug 2013
With the ongoing helium shortage, there are still plenty of analysts looking for alternative GC carrier gases. So what other options are out there, and are they really viable? In this post, I will briefly discuss the two most common alternatives (hydrogen and nitrogen), and when they really can’t be used as an alternative to helium.
Let’s start with nitrogen. This inert gas was seldom considered because in order to obtain maximum capillary column efficiency, carrier gas linear velocities (flow rates) needed to be much slower than either helium or hydrogen, which led to longer sample analysis times. Longer analysis times meant fewer samples getting analyzed which led to less revenue. As a result, using nitrogen as a carrier gas was usually reserved for packed columns.
In contrast, hydrogen was used more often than nitrogen when using capillary columns because carrier gas linear velocities (flow rates) could be much higher (approximately 4X nitrogen and 2X helium) and still maintain capillary column efficiency. As a result, shorter analysis times were possible. However, hydrogen has several drawbacks of its own, with the two most common being safety and reactivity (hydrogen is not an inert gas).
It’s not surprising that helium became the preferred GC carrier gas. It is inert, safe, and provides high column efficiency at moderate linear velocities (flow rates).
So why did I title this post “GC carrier gases, do you really have a choice?” It’s because, in some cases, you really don’t have a choice when it comes to choosing a carrier gas. For example:
1. Many EPA methods list helium as the carrier gas, so if you want to follow the method as written, you don’t have a choice. The same is true for many other methods, including those from OSHA and NIOSH.
2. Depending upon the GC detector, you may not have a choice. This is especially true with “universal” detectors. Consider a helium ionization detector (HID), a pulsed discharge detector (PDD), a pulsed discharge helium ionization detector (PDHID), and a discharge ionization detector (DID). These all need helium to operate properly. Before you try using a carrier gas other than helium, you should contact the detector manufacturer.
3. Continuing on the topic of universal detectors, you may not have a choice of carrier gas if using a TCD (thermal conductivity detector). Consider the following:
a. You cannot choose a carrier gas which is a target compound. For example, you cannot use nitrogen as the carrier gas if nitrogen is a target compound. In other words, the TCD will (in theory) detect every compound (if the concentration is high enough) except the carrier gas.
b. You should choose a carrier gas which is the same as the balance gas (in your calibration gas standard and/or samples). For example, if your balance gas is helium, you should use helium as your carrier gas. By doing so, the TCD will not detect/chromatograph the huge balance gas peak (which is almost always a good thing).
c. The carrier gas you choose for an analysis will affect target compound responses. Valco (VICI) has an excellent Carrier Gas Selection section in their TCD manual (pages 7 & 8). I pasted their table on thermal conductivity values below. The important thing to remember is that, generally speaking, the larger the difference in thermal conductivity values between the carrier gas and target compounds, the more sensitivity for the target compounds.
Gas / Thermal Conductivity
5. Finally, if you are using the most popular GC universal detector, mass spec, a variety of reported issues may prevent you from using any carrier gas other than helium. I covered several of these issues in a post I wrote last year. It’s a confusing time for GC/MS analysts
If your analysis fits into one of the examples listed above, you may not have a choice. However, if none of the examples listed above are relevant to your analysis, then you probably do have a choice when it comes to carrier gas, and you should explore your options as they may lead to a stable gas supply and/or saving money.