How do you develop a method for GC?
I’ll give you three quick trips for GC method development.
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TIMESTAMPS
00:11 Intro and Analogy
00:58 #1 Look for a method in a column catalog
01:28 #2 Choose a good standard column
02:15 #3 Try all the temperatures in the universe
Hi there, it is Lee Polite again from Axion labs and today I am going to answer one of the toughest questions in the GC world. That is, “how do you develop a method for GC?”
You may have heard my analogy before but I think of this like cooking.
Like, how do you come up with a recipe for a cake? I mean I could buy a box of cake mix, I could follow all of the directions, but I wonder, who decided how much flour? How many eggs? How much sugar? Do you put the flour and the eggs in the oven first and then you mix them, or do you mix them first and then put them in the oven? I mean who came up with that recipe?
It’s the same thing in GC: Who chose the right column length, diameter, film thickness, stationary phase, flow rate, injector temperature, detector temperature?
It sounds daunting! But the reality is that it is a lot simpler than it sounds.
So let me give you three quick tricks for developing GC methods.
Trick #1: Look in a column catalog. I know that sounds like a cop out, I know that sounds like I’m cheating here, but do not reinvent the wheel.
Column catalogs contain tens of thousands of applications that work on GC columns. So open up the Agilent catalog, open up the Phenomenex catalog and look for benzene toluene xylene for example, look for the application and that will give you the correct temperatures and flows and everything you need.
Trick 2: Let's develop a method from scratch. Let's have this total unknown and we have no idea what conditions to use. So the first thing that I tell people is to use a really good standard column.
In the GC world we like the DB5. That is 5% phenyl 95% methyl. Everyone makes one of those.
Restek’s is called the RTX5, Phenomenex’s is called ZB5, Agilent’s is called DB5.
So choose a 5 column. It is our go to column in the GC world. It separates most things.
I like to have a 30 meter column, 0.25 0.25 is my favorite. That’s 0.25 millimeter diameter, 0.25 micron film thickness. That is not that important, but what I am getting at is to choose one good generic very long column.
Trick #3 - And then, we are going to inject our sample and we are going to run a temperature program. And what I tell people in my class is that we are going to try all the temperatures in the universe.
Now what does that mean?
What is the lowest temperature you can use on a GC? Anyone know? It is pretty much room temperature. You can’t go much below room temperature unless you have cryogenics on it. So room temperature - I call about 40 degrees [Celsius] - that is the lowest temperature we can use. What is the highest temperature we can use on a GC? It’s determined by the column. The column will tell you the maximum temperature. So let's say we’re going to go from 40 degrees to 325 degrees and we’re going to do that at 20 degrees a minute. That’s a fairly fast rate, but what we’re going to do is do a scan from 40 all the way to 325 and simply see where the peaks come off. If all the peaks come off between 250 and 325, well then there is no need to start at 40 degrees.
So one good temperature program from room temperature to column max. Hold the column max for an extra 10 minutes and simply see where the peaks come off. That is method development is GC. It is a lot easier than it sounds.
Now what I would love to know from you guys is what works for you. What types of columns do you like use., Any tricks in method development? Anything that I missed out. I love to learn new things and I learn from you. So go ahead and chime in.