Problem: Costant area decrase

[alemaggot]

Hi guys.

Today I had a problem with GC.
I did launch this sequence with head space method:

6 reply of solution one +

6 reply of solution two +

6 reply of solution trhee

The first twelve injection would be about the same area, but this was not happened. The first injection is about 35000 area unit, the second 10000, the tirdh 9000 ecc ecc. The eighteen injection is 6000 area/unit

The area is decrase at each injection, about proportionally.

Can you help me?

[Axar]

Were all six injections performed from one vial?

[alemaggot]

Were all six injections performed from one vial?

No! 18 injection -- 18 vials

[chromatographer1]

I think the Forum can help, alemaggot.

You have a problem in that the concentration is decreasing in your vial or your GC hardware is becoming less sensitive with time.

To say: My area counts are decreasing is not giving us much to work with.

Can you help and give us some information?

What the analyte decreasing in area?

What matrix is it found in?

What dissolution solvent in which you dissolve the matrix?

What vial septa are you using?

How long do you heat the vials, what temperature?

best wishes,

Rod

[alemaggot]

I think the Forum can help, alemaggot.

You have a problem in that the concentration is decreasing in your vial or your GC hardware is becoming less sensitive with time.

To say: My area counts are decreasing is not giving us much to work with.

Can you help and give us some information?

What the analyte decreasing in area? - Acetid acid

What matrix is it found in? - Solid

What dissolution solvent in which you dissolve the matrix? - Water

What vial septa are you using? - Maybe PTFE/teflon

How long do you heat the vials, what temperature? - 30 minuts at 80°C

best wishes,

Rod

I've quote your messagge. I color my respound in red

Thanks!

[chromatographer1]

Acetic acid can react with many matrices, whatever your mystery solid matrix is, is no exception. This will cause loss of analyte.

Any volatile component of your mystery solid matrix which might remain in your sample path can be a source of active sites for the free acid. This will cause loss of analyte.

Search this forum and you will see many analysts have trouble with acetic acid by HS.

Ion chromatography is the preferred methodology for measuring acetic acid content.

Direct injection of an acidified dissolved matrix is often the preferred method. Dirty, yes, but with maintenance on a daily, even an hourly basis may allow your analysis to be performed.

Any metal in your flow path will inhibit, if not prevent the analysis of acetic acid.

Search the forum for more details.

best wishes,

Rod

[aldehyde]

Water will also influence the equilibrium of acid and conjugate base, causing problems. this dissertation: http://scholar.lib.vt.edu/theses/availa ... TDWANG.PDF

has a good amount of information. it is certainly doable, but matrix effects can cause major method specific issues and you must derivitize.

[alemaggot]

Water will also influence the equilibrium of acid and conjugate base, causing problems. this dissertation: http://scholar.lib.vt.edu/theses/availa ... TDWANG.PDF

has a good amount of information. it is certainly doable, but matrix effects can cause major method specific issues and you must derivitize.

Thank You!

I've read this document. I must buy BSTFA to make it.

But I must search how to prepare BSTFA solution. When I'll can do this try I'll write all that I'll found in my other topic "Method for sodium acetate determination".

See us in that topic!

thanks