Chromatography Forum

I apologize in advance, I have very little experience with running a GC.

I have to run a series of samples for a calculation that are further diluted as they go. The "A" sample runs fine and the peaks I'm expecting are there. Same with "B", but anything after that is so diluted that I'm not picking up much of anything. The large peaks are there but the smaller ones are gone after "B" Is there anyway to configure the GC or software to be more sensitive and pick up these peaks that I know should be there?

GC
Agilent 6890N with a FID with Agilent OpneLab Chemstation.

I'm sorry but that's just the way dilution goes. At some point, "the solution to pollution is dilution" takes over.

You can try to change your inlet conditions (lower split flow, splitless) to get the smaller ones in the diluted sample but it will have an affect (likely adverse) on the the larger ones.

Likely solution is to have 2 methods - one for the larger-concentration materials and one for the smaller.

Like I said I'm not very knowledgeable on the subject. 10 grams of sample are added to a vial with 10 grams of THF, one gram of that is added to a vial of THF, and so on and so on. By the third or fourth I'm only registering a THF peak.

Thanks for the advice.

1) did you tried to reduce the split ratio or go splitless ?
2) did you tried to inject more volume ?
3) are you sure about your temp parameters for injector (dont burn your sample ! decrease temp ?) and your fid (burn your sample ! increase temp ?) ?

had the problem once, i decreased injector temp from 250C to 200C, peaks came back ! i was probably decomposing my sample in a way too hot injector

Remember Rod's rule - "The more you don't tell us, the more we can't help you."

could you give us details of the 6890 FID settings. Specifically detector temperature, air and hydrogen flow rates plus make-up gas.

Remember Rod's rule - "The more you don't tell us, the more we can't help you."

kpotter,

A FID is sensitive to about 1-10 ng on column of material. At 1 mg/ml you have a long way to go before getting there.

So, three key questions. 1) As mentioned, what is your split ratio? 2) How many components (and how low are the minor components) are in you original sample? 3) What is your injection volume?

At 1 mg/mL and 1 uL and a 10:1 split, you are still putting 100ug of material on column - which is a ton. But if your minor component is 1% then only 1 ug of that component. Only a few 10X dilutions later and it will be gone. Higher split ratio's make that situation worse in a hurry.

Best regards,

AICMM

Look for the low responding analytes in the first dilution, and the higher ones in the later dilutions. If the large peaks interfere in the first dilution then dilute until they do not interfere but that will be the limit of your detection.