I am trying to find the linear range for methyl methacrylate (MMA) monomer using a Restek-Rxi 5SilMS column and direct injection technique. I have observed some conflicting results while trying to adjust the split ratio and injection volume parameters.
First, I increased split ratio from 50:1 to 100:1 because of concerns for overloading the column. I was expecting the abundance of the MMA peak to decrease after increasing the split ratio, but the abundance was actually higher. I increased split ratio again to 200:1, but this time the relative abundance decreased. Am I right in assuming the relative abundance should decrease as the split ratio increases?
All MMA injections were 1 uL (1000 ug/mL). Red peak is 50:1 split, orange is 100:1 split, purple is 200:1 split.
With the split ratio at 200:1, I injected volumes of 0.2, 0.5, and 1 uL of MMA in methanol. The 0.2 uL injection showed the highest relative abundance (blue peak), and the 1 uL the lowest (purple peak). Why is this happening?
Blank runs between injections show there is no MMA left on the column. This is also interesting because in the past, it would take several runs of the column to restore it to background levels when we were using thermal desorption techniques. E.tune shows no leaks.