Chromatography Forum

Hi, I am looking to create a calibration curve using different concentrations of gaseous toluene in N2, such that I can measure toluene concentration in biomass gasifier off-gas. I have tried injecting a fixed volume of liquid toluene in a fixed volume of N2 in a gaseous sampling bag (metallic bag), all at room temperature. When I go to measure my bag standard on the GC, the peak area of replicate samples decreases over time (sampled prepared on same day, with replicate GC runs performed within 1.5 hrs). Could anyone recommend a way of creating gaseous toluene standards for GC calibration?

I too have struggled with this. The best I've found is to create the standard in a glass bottle with a teflon-backed septum on top. I use 120 mL serum vials. Just search "serum bottle" with google and focus on images. Something like toluene is pretty volatile and will stay in the vapor phase, even at RT. If you make a concentrate in one serum vial by adding a small amount of liquid toluene to it, you can dilute it by taking a known volume of gas from the vial and then adding it to a sealed second vial, etc.

My biggest problem with this is that my additions generally don't match up with what I calculate. For instance, 0.1 µL of toluene at RT is roughly 0.94 µmole (use the face-value of the density from Sigma-Aldrich). In a 120 mL serum vial (4.90E-3 mole gas at 1 atm) that's about 192 ppm toluene (µmole/mole). A 1:120 dilution of that should be 1.6 ppm. It never seems to work out the way I expect. I have even struggled to make the same thing twice with this sort of procedure. Very frustrating. Since I usually have a certified gas calibration standard to which I can compare my standards, I can "certify" my preps that way. I'll prep something and then check it to my known standard. Actually, since I compare against a known standard anyway, I will usually just take a bit of the headspace of the reagent bottle and add it to the sealed serum vial. Saves a couple of dilution steps.

Seems to me that all of those tedlar-type bag things will absorb your analytes over time. I've never really had good experiences with them - especially for long-term type studies. Perhaps others have had good experiences with them?

Lisa_Canada,

Two suggestions. For a calibration curve I would suggest you invest in a gas sampling bulb. Keep it warm in an oven (watch that you don't melt teflon stopcocks) and add aliquots of toluene either as a neat or as a dilution is some other appropriate solvent. Add a couple of glass beads (or maybe teflon chips?) to the sampling bulb to affect mixing when you have spiked it. Clean it with copious quantities of compressed are when you are starting over.

Or, for low concentrations and lots of it, prepare it in an empty propane can. Add a known volume of toluene to propane can and back fill with compressed air to a known pressure. Getting the exact concentration is challenging but using it as a daily to check response and retention is wonderful. (Glass bulb to set up curve, propane can for daily might be a good way to do what you want to do.)

Depending on concentrations you are working at and the time you have available, permeation might be a viable option as well.

LEt me know if you want more info aicmm at flash (dot) net.

Best regards,

AICMM