Thermal desorption GC/MS questions

[Jae Hwan Lee]

Hi, I have questions for thermal desorption GC/MS.

My equipment informations were as follows:
1. Adsorbent: Tenax TA 1.5g Packed in S.S tube.
2. Thermal Desorber:JTD-505H:Japan Analytical Industry
3. GC/MS: Agilent 6890/5973N
4. GC Column: HP-5MS(30m x 0.25 x 0.25um film thickness)
5. 1st desorption temp: 250degC for 10min
6. Cryofocusing with LN2 (-40degC, 10min)
7. 2nd desorption temp: 250degC for 1min
8. GC temperature programming: Initial temp. 50℃(5min holding), 15℃/min to 300℃(10min holding).
9. MS scan: 35~550amu
10. Sample volume: 5L Volume
11. Sampling site: Flat TV Fabrication
12. GC Carrier gas: Helium

When I have chromatogram using above conditions,
there are extraordinary GC peak was shown at 0.926min retention time.

If I double click this peak, MS fragmentation pattern was like this.
Library search result was "Heptafluorobutyric anhydride".

But, I couldn't belive Library search result at that time.
So, I bought Heptafluorobutyric anhydride standard from Supelco.
After injection into Tenax TA adsorbent tube, analyzed standard with same
method.
The results showed something diffrent. Retention time and MS fragmentation was something different.
In other words, this peak is not heptafluorogutyric anhydried because of different retention time.

My questions are belows;
1. If Heptafluorobutyric anahydride(m.w 410) is right, why retention time shows 0.96min zone? You know, m.w 410 is a little bit higher, so retention time may be shown at latter zone.

2. Did you have experience this phenomena?
3. Could you tell me your thinking?
4. Is this peak from thermal degradation products?
5. Is this peak ghost peak?

Please help me. Thanks.

Best regards.

[Peter Apps]

The mystery peak is eluting at the dead time of the column - in other words it is not retained at all. Either it is a ghost peak or it has a very low molecular weight.

You need to run a blank - desorb from a trap that has not had sample loaded onto it. Does the peak still appear ?

Also try holding the column temprature at its maximum for an extra 10 min - if the peak is a ghost it will now elute towards the end of the run.

Peter

[danicrd]

Probably the peak came from desorbption of tenax trap.
If you try a run with second desorbption temp as zero (so, no desorbption in effect) probably the peak disappear.
Tenax traps tend to decompose at high temp.
You can try changing trap with carboxen based trap (depend of your application) that have better performance compared to tenax.

[MikeD]

My only comment is that 1.5 g of Tenax is a lot of Tenax to get thermally clean at 250 degC. 0.2 g is sufficient for applications where Tenax is suitable, so the thermal desorber design is rather strange to my eyes. What is the actual intended substance to be sampled and measured ?

[AICMM]

Along MikeD's line of comment, you have a big trap, a thin film column and a large sample volume. You have a mis-match. If you want lights (big trap) then the column film is not thick enough and your starting temp is too high. If you want heavies, then the trap is too big and the column, initial temp, and sampling volume should be fine. Right now you have a mis-match.

Also, starting at 35 amu you are probably going to get a large CO2 peak. Might want to start at 50 amu, especially if looking at heavies.

Best regards.