Sample not volatilizing completly in GC-FID purity check?

[rodrigaoman]

Hello, people.
I need to determine purity of a high purity commercial chemical (N,N'-diethyl-N,N'-diphenylurea - molecular mass: 268; boiling point: 330 ºC). I'm using the mass balance approach (100 % - total impurities) to do it.
I'm using GC-FID (DB-5 30x25x0,25 - split 1:50 and 1:25 - inlet and detector temp 250 ºC) to get chromatographic purity. I can see 3 impurities.

Impurity 1: Retention Time 6.8 min
Impurity 2: Retention Time 7.1 min
Impurity 3: Retention Time 32.9 min
N,N'-diethyl-N,N'-diphenylurea: Retention Time 33.3 min


When I went to GC-MS to try to identify the impurities by making use of the library it came out:

Impurity 1: ethylbenzene
Impurity 2: p-xylene
Impurity 3: non identified by the library, but molecular peak was 254 (it seems to be related to N,N'-diethyl-N,N'-diphenylurea - very close retention times - and similar molecular mass - impurity 3 and main compound molecular mass has a diffence of 14 which indicates a CH2 absence in the impurity, so I guess impurity 3 is N-ethyl,N'-methyl-N,N'-diphenylurea. Furthermore, the fragmentation of this compound fits the real GC fragmentation). N-ethyl,N'-methyl-N,N'-diphenylurea boiling point is 364.6.

N,N'-diethyl-N,N'-diphenylurea DSC purity experiment came out 99.83 mol% purity (very high purity).
The impurities are present in a very small concentration. This is why I prepared 7 mg/mL solutions for the GC-FID analysis and the injection volume was 1 microliter (split ratio 1:50 or 1:25). I need to come with high concentration to see the impurities.

The problem is: After the first run, I I ran a solvent blank (methanol), and a little bit of N,N'-diethyl-N,N'-diphenylurea (retention time 33,3 min) appears, like a ghost peak. Even though the peak is so small compared to the previous run, after a second run of the real sample, the small peak would contribute to the peak area of this new run. As I need to make very accurate measurements, I have to get rid of this very small amount of N,N'-diethyl-N,N'-diphenylurea from previous run.

The small peak disappears after 1-2 solvent blank runs or if I condition the inlet using a very high temperatyre (350-370 ºC).

What are the suggestions to get rid of the "ghost" peak? It seems N,N'-diethyl-N,N'-diphenylurea doesn't volatilize completly and part of it remains in the inlet and goes out in the next 2 runs.
I use an inlet temperature of 250 º C. Should I try to raise it? Won't it cause problems to the other peaks, like ethylbenzene and p-xylene. At the very end, I'll do peak area normalization and the peaks have to represent the real amount of each one in my real sample.

I appreciate your contributions.

Best regards,
Rodrigo

[Peter Apps]

Hi Rodrigo

As long as you use a high temperature septum, and your main compound and impurities do not decompose there should be no problem in using a higher inlet temperature.

If you do not want to raise the temperature then you could try increasing the gas flow through the inlet after injection by programming the split ratio.

Sharp ghost peaks are quite often due to cold trapping at the top of the column or inadequate syringe washing, so you need to be sure that enither of these is contributing to the problem.

Peter

[rodrigaoman]

Hi Rodrigo

As long as you use a high temperature septum, and your main compound and impurities do not decompose there should be no problem in using a higher inlet temperature.

If you do not want to raise the temperature then you could try increasing the gas flow through the inlet after injection by programming the split ratio.

Sharp ghost peaks are quite often due to cold trapping at the top of the column or inadequate syringe washing, so you need to be sure that enither of these is contributing to the problem.

Peter

My temperature program is:
40-220 ºC at 5 ºC/min
220-300 ºC at 40 ºC/min
hold 300 ºC for 7 min

My septa can stand 400 ºC.

Syringe has adequate washing. I have checked it.

Is there an ideal detector temperature?
How one chooses dedector temperature?

I'll try incresing inlet temperature, like 350 ºC.
lol

Thannks for your contribution.

[Peter Apps]

Hi Rodrigo

Detectors are usually set 10 - 20 C hotter than the maximum of the column temperature programme - the main requirement is not to have a negative temperature gradient at the end of the column which would broaden the peaks.

I forget to mention - if you have carryover in the inlet you might have some septum fragments or other crud in there.

Peter