calibration/method questions

[KimSchaaf]

We are currently running a mix of various acrylic monomers for a residual monomer analysis by GC/FID. We have been having increasing issues with getting a good reproducible calibration week to week using an internal standard method. We typically prepare our mixtures with ~0.04g of each monomer using isobutyl acrylate as our ISTD in ~10mL of acetone. This is the way our company has been doing the analysis for years. We are using a 5%phenyl phase column (30m x 0.25mm x 1micron) (sometimes DB-5 sometimes Rtx-5). We are injecting 1 microliter of sample onto the column with a split of 35:1.

I was wondering if anyone out there had suggestions for alternative ways to do this analysis? Most of our components are within a spec of +/-10% on the relative response factor (as referenced to Isobutyl Acrylate).

We are looking at monomers such as Ethyl Acrylate, Methyl Methacrylate, Allyl Methacrylate, Butyl Acrylate, Butyl Methacrylate, Hydroxyethyl Acrylate, Hydroxyethyl Methacrylate, Styrene, Hydroxpropyl Acrylate, Hydroxypropyl Methacrylate, Hydroxybutyl Acrylate, t-butylaminoethyl methacrylate, ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, isobornyl acrylate, isobornyl methacrylate, glycidyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, lauryl methacrylate. There are some others that we look at on a less routine basis. But this at least gives you an idea of our routine monomers that go into our standard calibration mix. And yes, we do put all of the same mixture (weighing each standard in order of increasing volatility (less volatile to more volatile).

Thanks in advance for any advice/suggestions.

[Don_Hilton]

In what way is the calibration irreproducable - scatter in the plotted curve? Disagreement with past calibration?

[KimSchaaf]

Several of the later eluting components do not fall within our response factor spec. We have a spec of +/-10% off of an accepted response factor that we had generated previously after several round robin analyses of those mixes of monomers.

[Don_Hilton]

Changes in response factor would suggest either a change in actual concentration of analytes in the mixtures or a change in the chromatograph. (It would be interesting to know if this is increased variability of results, with the points scattering to either side of the expected value or if the results are trending, in which way.)

A few questions come to mind:

So, how old are the monomers being used to create the calibration curve and what has been used to certify their qualty? If you have opened new bottles of monomers, was the change in compouds related to changes in response?

How does the chromatography look? Has the peak shape or retention time changed for the later eluting compunds? How old is the inlet liner, seal plate, and septum? Have you changed supplier or type of liner in use? How old is the GC column and if you have changed it, what effect was seen on the calibration?

When was maintanance last done on the FID - such as cleaning the jet? Again, any change noted when that was done?

Any change in split rate? And how long has it been since the charcoal filter on the split vent line has been replaced?