Chromatography Forum

First and foremost, our setup and relevant info:

Agilent 6850
Rtx-5 105m x .25mm x .25um (bonded polymethylsiloxyl, capillary GC column)
Injector port temp: 160C
Split Ratio: 20:1
Injection Size: 1uL
Constant pressure (44.14psi) - Flow = 2mL/min and Avg. Velocity at 25.
Packed, quartz wool, tapered liner
Analytes: C5 (hydrocarbons)

After pulling vacuum on our sample, we recovered approximately 6% of what we're calling "solvent lights" (meaning low molecular weight analytes that are covered on our chromatographs by the solvent peak--of course, we use a different solvent when looking at these peaks). When we GC the material prior to pulling vacuum, we're seeing it at about 3%. The discrepancy between these numbers has us thinking that we're possibly losing material at the injector port because it is so volatile (b.p.'s range between approximately Isoprene 34C - Cyclopentene 44C).

Thoughts?

You evacuate your product/sample, then extract with a solvent and do liquid injection. You have 6% of C5. When you don’t apply vacuum to the product you have 3% of C5.
How you calculate %.What is the precision of the method; is sample homogenous?

What are you using as solvent for the analysis of the C5 compounds ?

Also, putting the sample under vacuum is not an obvious step as a sample prep for volatile hydrocarbons.

Peter

In this case also injection technique might be important.

Is it manual or automatic injection?
Filled needle? Empty needle?
Cold needle? Hot needle?

@Alexandre: Not exactly that way, I should have been more specific. We take a crude C5 stream sample, vacuum distill it, and run a sample of the pot bottom after the vacuum distillation, the collected fraction, and the pre-distillation crude sample.

The comparison between the collected and crude is where we see the discrepancy (pot bottom has 0% volatiles after distillation). My 3% figure comes from area %, where their 6% probably comes from wt/wt....I am on the GC end of the project, so I'm not 100% sure about the distillation specifics or their calculations. I'll have to find out more explicitly what they are doing.

@Peter: We use toluene.

@R13: Automatic injection, not sure what you mean about filled vs. empty or cold vs. hot (though my inclination is to say "cold" as it is at room temp.)

Take a look in "Handbook of GC/MS: fundamentals and applications" p 95-96 for short explanations. Accessible trough Google Books.

I double checked with my coworker who is running the distillation and she said that they are, in fact, calculating percentages how I mentioned in an earlier post. Using GC area %'s for pot bottom and untreated and wt/wt %'s for the collected fraction.