RSD% of phthalates on GC/MS not good

[thohry]

Hi all,

Recently, the reproducibility of our phthalates analysis get worse.
One strange thing is that, the lighter the compound, the worst the RSD.
We have 8 phthalate from DIBP to DIDP and the RSD% are : 14.6; 15.7; 10.4; 11.0; 10.1; 6.9; 5.3 and 4.6 respectively. Before, all are below 5%.
I have replaced new liner, syringe, septum, clean the source.. but it did not help (just a litle bit).
Anyone have any idea on this ?
Thanks in advance.

[Peter Apps]

Hi Thohry

With 400 posts you have been around long enough to know that the less you tell us the more we can't help.

If you don't tell us what you are doing (details of methods) how do you expect us to work out what is going wrong ?

Peter

[thohry]

Sorry about missing infos.
The GC conditions are:
Inlet temp : 280 oC
Mode : splitless, purge flow : 50ml/min, purge time: 2min
Column HP5 (30mx0.25x0.25), flow : 1ml/min
Oven: initial: 80 (1min) increased to 270 at 30oC/min then immediately to 300 oC at 10 oC/min and holds for 5 min.
MSD (SIM mode).

I also thought of the column, but the peak shapes are rather good.
Could it be the electronics or some kinds of hardware of the GC itself ?

Thanks

[Consumer Products Guy]

I'd change the inlet gold-plated thing, clean the inlet, maybe break off 3 cm off the inlet end of the column, re-install column.

[James_Ball]

Also clean the split vent line. If it is Agilent you can replace the split vent line with new 1/8" copper line instead of trying to clean it. Especially in splitless, and with 2 minute splitless you can get adsorption of analytes in the split vent line if there is some heavy boiling residue in it.

[thohry]

Thanks all for suggestions.
I have almost done all the things relating to the inlet : change the syringe, the liner, the gold seal, trim the column .. Perhaps only without replacing the split vent line.
The solution by now is using benzyl benzoate as the internal standard. It helps, but still I am not satisfied. I think, there should be still something wrong with the instrument: the EPC for the inlet, the autosampler... because, previously, it was quite OK without ISTD.
Any more ideas ?

Thanks all again.

[Peter Apps]

Is there anything you are not telling us ? hint; sample composition, injection volume, type of inlet liner .......

Peter

[tkubowicz]

Hello

I know that it could sound a little silly but maybe it is problem with analyte evaporation - I'd check liner and glass wool position (if you have that type). I remember interesting results for different glass wool position.
It could have big impact for light compounds.

I'd suggest to check different method for different compounds just to make sure it is instrument problem not method.

Regards

Tomasz Kubowicz

[Peter Apps]

Hello

I know that it could sound a little silly but maybe it is problem with analyte evaporation - I'd check liner and glass wool position (if you have that type). I remember interesting results for different glass wool position.
It could have big impact for light compounds.

I'd suggest to check different method for different compounds just to make sure it is instrument problem not method.

Regards

Tomasz Kubowicz

Not silly at all Tomasz - anything that affects light and heavy analytes differently is very likely to be due to inlet discrimination. But, as usual, we are just guessing because thory has not provided the information we need to give useful answers.

Peter

[tkubowicz]

Hello

I've noticed that your purge time is 2 min. I think is too long. Did you optimize it empirically? Normally you should use the peak area of an early eluting peak to determine analyte transfer time.

Regards

Tomasz Kubowicz