GC carryover

Discussions about GC and other "gas phase" separation techniques.

10 posts Page 1 of 1
Hi, Everyone,
This is my first time to post my question regarding very strange GC carry-over I met in testing, hope I will get your kindly suggestion and help.

Method: Test acetic acid in propionic anhydrate by GC
Sample preparation: Hydrolyze the sample with water by steam bath heating, then dilute with acetone.
Standard preparation: Appropriate volume of acid, add the same volume of water as sample preparation.
Instrument: GC 6890N, Column: Glass column: supelco 10% SP1200 1% H3PO4

Unexpected chromatograms:

1. After 6 injections of standard, the very high carryover of acetic acid could be observed in the chromatogram of the first injection of sample, almost as high as standard, then rapidly disappeared in second and afterward sample injections.

2. if I inserted a blank injection of acetone immediately after the six injections of standard, the chromatogram of blank is very clean, no any carry-over, however, then I injected a sample after blank, the high carryover came again.

3. I have changed the septum, syringe, re-connected column, and not improve.

Please, anyone, have any idea can share, I will appreciate very much.
Many thanks.
1997
The PA (propionic acid) induced desorption of acetic acid, so carryover appeared.

Tips:

1. Standards and blank must contained PA at the same level as samples (after hydrolysis)

2. Syringe wash solution also must contained PA

Best wishes, Alex
Welcome to the forum.

I addition to the advice about adding PA to standards, it is not clear form your description whether you dilute your standards with water or with water plus acetone. You should use water plus acetone - samples and standards need to be as similar as possible.

Peter
Peter Apps
Peter Apps wrote:
Welcome to the forum.

I addition to the advice about adding PA to standards, it is not clear form your description whether you dilute your standards with water or with water plus acetone. You should use water plus acetone - samples and standards need to be as similar as possible.

Peter

Hi,Peter, Thanks for the input.

Standard is prepared by diluting acetic acid (20mg) in 2 ml of water then adding acetone to volume in 50 ml volume flask.

My sample is prepared by dilute 4ml of Propionic anhydrate in 2 ml of water then heating by the water bath to hydrolysis, then diluting with acetone to 50 ml.

The matrix of standard and sample are different due to sample contains propionic acid.

I tried water: Acetone (2:48) mimic sample diluent as blank instead of my first 100% acetone, it didn't work to wash out the carry-over as well.
Please feel free to share your thoughts. Many thanks.
Best,
My1997
sav wrote:
The PA (propionic acid) induced desorption of acetic acid, so carryover appeared.

Tips:

1. Standards and blank must contained PA at the same level as samples (after hydrolysis)

2. Syringe wash solution also must contained PA

Best wishes, Alex

Hi, Alex,

Thank you very much for your tips. Great to know the new knowledge to me.

I am curious why acetone cannot wash the acetic acid?

How PA induces the desorption of acetic acid?
Where did the acetic acid hide in the GC system?

I totally have no idea about these. :) Will this absorption of acetic acid result in the wrong quantification? You know, I spiked a serial of recovery sample with different concentration, the recovery and linearity of spiked sample both are good. I cannot explain.

Could you please share me more your experience.
Thank you very much.
Best regards.
My1997
My1997 wrote:
I am curious why acetone cannot wash the acetic acid?
It can washed out AA when adsorption is not included.

My1997 wrote:
How PA induces the desorption of acetic acid?
PA also have carboxyl group, so it displace of AA from adsorption sites.

My1997 wrote:
Where did the acetic acid hide in the GC system?
Syringe, septa (vials and inlet), inlet needle guide, top part of inlet (etc.)

My1997 wrote:
Will this absorption of acetic acid result in the wrong quantification?
Yes
What detector are you using?

Gasman
GasMan wrote:
What detector are you using?

Gasman

FID, any suggestions?
sav wrote:
My1997 wrote:
I am curious why acetone cannot wash the acetic acid?
It can washed out AA when adsorption is not included.

My1997 wrote:
How PA induces the desorption of acetic acid?
PA also have carboxyl group, so it displace of AA from adsorption sites.

My1997 wrote:
Where did the acetic acid hide in the GC system?
Syringe, septa (vials and inlet), inlet needle guide, top part of inlet (etc.)

My1997 wrote:
Will this absorption of acetic acid result in the wrong quantification?
Yes

Sav,
thanks for your providing so many informative replies.
Regards.
The only reason I asked is that I have seen odd results with fatty acids when using the TCD on the 6890.

Gasman
10 posts Page 1 of 1

Who is online

In total there are 14 users online :: 0 registered, 0 hidden and 14 guests (based on users active over the past 5 minutes)
Most users ever online was 188 on Wed Nov 02, 2016 2:58 am

Users browsing this forum: No registered users and 14 guests