Chromatography Forum

I could use some clear heads, as I have been beating mine for the last 6 weeks or so.
We are trying to qualify a new headspace blood alcohol method usin a Tekmar HT3 ( loop only) and an Agilent 6890 fid.
using a DB-624 column 30M X 0.25 X 1.4 @ 2 cc/min. The Headspace has a 1 ml loop and I am splitting @ 60:1.
THe headspace is set for constant heat for 20 min @ 65C.

My compounds are Acetaldehhyde, MeOH, ETOH, Acetone, IPA, t-butanol ( surrogate), and n-propanol ( IS).

What I have been fighting is I get a 40% or so drop for all compounds between injection 1 and 2, then from 2 on the areas stay consistant. Nothing I've tried has made any significant improvement. Those changes have been: rebuilt lower part of the injection port, swapped EPC's, different column ( same phase but 60M), new FID jet. Tekmar sent me a loaner and data is still bad, but pattern is different.

We have been running the method on a Tekmar 7000, and Agilent 5890 using most of the same parameters and it is rock steady. This setup uses a 30 X 0.45 X 2.55 DB 624 at about 7 cc/min using a packed inlet. The Tekmar has a 100 ul sample loop.

What am I missing??

Please clarify.

Your first injection peak areas are the same as the later injections but injection 2 is 40% lower than the others?

Or are the first two injections 40% lower than the following injections?

Rod

Sorry. My first injecton is 40% or so higher than all those that follow.

You are laying down a layer of phase, probably water or a mix of volatiles in blood, in your loop, transfer line and injector.

These are probably absorbing the analytes of interest (alcohols).

Or, are you somehow treating the first vial differently than the others? timing injection heating crimping, etc

best wishes,

Rod

Logical, but when I tried setting my gas saver to 100 cc/min @ 0.5 min of run time I get the pattern described.
I ran a set of 6 vials ( 2-7) with vial one empty. there was roughly a -9% diff in areas between vial 2 + 3.
My bad, you may be on to something. When I repeated the above experiment using water in vial 1 , there was a +17% diff. between vials 2+3!
Guess the next experiment is increasing my transfer line temp.

and don't forget the injection port temperature.

best wishes,

Rod