What is the difference between Inverse Gas Chromatography (iGC) and Analytical Gas Chromatography?

The main difference between analytical GC and inverse GC is that we invert the roles of the stationary and mobile phase. So in inverse gc, we have a column, typically a column. It could be a film, it could be a fiber, it could be a granule. There's lots of different morphologies we can work with, but in essence, we have a carrier gas, nitrogen or helium. And then we inject a probe molecule with known properties and measure the retention time of that probe molecule as it travels across the surface of the material of interest. We still use retention time as the fundamental measurement, but now we're going to see has this retention time changes with respect to temperature, flow rate, humidity and or the molecular size and or chemistry of the probe molecule. We can then use that to back calculate physical chemical properties about the surface. So with any inverse GC technique, since the material and the column is different every time you run a sample from a GC standpoint, we have to normalize that particular sample. And we do that with every experiment by using an inert molecule. In our instrument, we typically use methane. Other instruments may use argon or krypton as an inert molecule. So this measures the retention of dead volume for the column of interest or the sample of interest. And then all other probes are referenced from that dead volume. So we're looking at basically the net retention time, and we're looking at that net retention time and as that changes with either different molecule. So going from hexane to heptane to octane, which we'll be doing for surface energy experiments. In other types of experiments, the variable might be column temperature, column flow rate, cross sectional area of the probe molecule, or some other variable.