Chromatography Forum

Hi
This is my first post to this forum so please excuse me if miss someting.

We are setting up a GC system to check residual solvent, however we are having some headaches.
We have made a standard solution consisting of 0,1% ethanol and 0,01% acetonitrile. Analysing this solution with our system, we are finding that the peaks from ethanol are behaving much more erratic than acetonitrile. In Our last run we had six samples, yielding a 16% RSD for ethanol and 7% for acetonitrile. This is a very typical result. We have recently had service on the system, and both injector and FID has been replaced. Switching to a new column had no effect. What are we missing. Why are ethanol giving so poor precision of the peak-area of ethanol?

The system is GC-FID
1:3 split
140 oC injector, 250 oC detektor.
program;
5 min @ 50 oC
10 degrees/min to 130 oC
2,7 ml/min helium flow

Head Space PAL system
10 ml head Space vial
4950 µl water 50 µl sample
45 min @ 80 oC incubation
90 oC Needle temp
1 ml injection volume

Syring flushed With helium between injections

It's been a while for my doing HS blood alcohol, but as I remember I started my oven at 35 or 40. I would bump your injector to 200 or so.
We injected 100 ul of sample without splitting.

You are probably getting a pressure pulse as you inject - a 1ml sample volume is 25% of the total flow and would need an injection time of several seconds to avoid a pulse. Either reduce the injection volume or increase the split ratio and see what happens.

Peter

The injection speed is 1000 µl/sec. Could this be increased to say, 330µl/sec? I've tried to up the split to 1:5 without much effect. RSD for ethanol is very poor. We used to have much higher flow, i.e. 10 ml/min, however no other effect than lower retention times.

The injection speed is 1000 µl/sec. Could this be increased to say, 330µl/sec? I've tried to up the split to 1:5 without much effect. RSD for ethanol is very poor. We used to have much higher flow, i.e. 10 ml/min, however no other effect than lower retention times.

1000 ul/sec is 60 ml/min so you are definitely generating a pressure pulse that causes variability in transfer of sample to the column.

Increase the split to 20:1 or decrease the injection volume to 100 ul.

Peter

Thanks for the input
Increasing the injector to 200 oC cant be done, as we are using the PhEur method B for our system. Playing around with the split is OK.
I adjusted the split to 1:10 (an also reduced injection speed to 300 µl/sec), however the answers are much the same.

Running nine injections of the same test solution gave
5.0% RSD for THF (mean 46,7 uV.Min),
11.1% RSD for EtOH (mean 266.1 uV.Min)
5.3% RSD for AcCN (mean 38.3 uV.Min)

Which all are below the PhEur requirement of 15% RSD, but still not what I would call good.
Which leads me to the quiestion: What RSD can be demanded of such a system? Is RSD below 5% for repeated injections to much to ask for, for a PAL-CTC GC-FID system?