Chromatography Forum

hi,

it seems that everytime I run standards on my headspace theres always one vial that seems to not pick up anything. there should be two peaks coming out at 12.5 and 13min. this is frustrating because this is usually a system suitability vial which will obviously invalidate my run.

I use 10ml amber screw top vials and I always make sure there nice and tight. 6ul of standard is placed into 3ml of sample.

any suggestions to help/stop this occurrence. im not going to talk about method parameters as they cant be changed anyway and it is an isolated incident.

Which is it;

"everytime I run standards"

or

"it is an isolated incident".

What hardware are you using ?

Peter

Are there any regularities? For instance is it always 1st or 2nd vial?
Finding regularities is a good starting point.

it seems more often than not it will happen every time to one vial anytime I perform an analysis.

so yes I would say its everytime I run standards for a new analysis run.

headspace sampler is agilent G1888 with agilent 7890A GC.

if theres any parameters that you require to know obviously I will provide

thanks

Hello

If you don't see peaks at all I would say there is nothing going to GC from HS sampler.
In Agilent G1888 you have two small valves: pressurization valve and vent valve - you should check them first. It is not difficult to diagnose which one (or both) is not working however if you've never done it it could be tricky.
If valve(s) are not working properly (small leak) random problems like yours can happen.

Regards

Tomasz Kubowicz

Let's try to be clear what is going on;

You make up a standard vial by diluting 6ul of standard into 3ml of sample, and when you run it there are no peaks ?

There are never any peaks from standards made up like this. ?

Is this standard vial always at the same place in the sequence ?

Do you make up standards with higher concentrations than this, and do you see peaks when you run them ?

It is a waste of time getting details on methods and hardware a little bit at a time - so go ahead and tell us everything, including what is on the standards and its concentration.

Peter

validated method mr apps. below parameters cannot be changed. no, the vial that gives no response seems to be quite random.

I just ran another sequence and my DSS vial this time decided to show no peaks. concentrations for the two peaks in the DSS was 0.5ppm.

the working standards are around 19ppm. stocks are made up in a mixture of water and dimethylacetamide. sometimes the ws shows no peaks.

vial pressure : 16psi
vial oven temp:80C
loop temp:130C
transfer line temp:140C
agitation: high
vial equilibration:20min
vial pressurization:0.4min
loop fill:0.2min
loop equilibration:0.2min

column flow: 5ml/min
split:1:1

to reiterate it happens to one vial out of every 30 vials or so and it can be anywhere in the sequence (dss, system suitability, bracketing std).

If you are spiking vials one at a time then human error has to be a possibility. Make up in bulk enough of a high standard to charge a full sequence of vials, and then divide it between the vials. Then you know you have analyte in every vial. Run.

Or retrieve the dud vial, put a new cap on it, and run it again.

A leaky cap is another possibility - but I would expect at least some peaks.

Do you ever get chromatograms with no peaks (no peaks at all, not just no peaks for the analytes) when you run samples ?

Peter

...
column flow: 5ml/min
split:1:1
...

Just curious, what is your column ID ?

Do you inspect the rim of each vial. We recently had some 40ml VOA vials where a few per case would have chips in the rim, which would not allow the septa to seal when doing headspace in the vial. Very small chips but enough to interfere with the seal.

Hi dblux,

column using is db-5, 30M x 0.53mm i.d.

thanks james ball I will have a look closer at each vial I prep.

If the problem is only with standards and not with samples then it is unlikely to be anything to do with the vials. Unless peak-free chromatograms have only happened a few times.

Peter