EPA 8270 by LVI GC-MS

[tangaloomaflyer]

I am running a cold splitless method for analysis of SVOC. I have seen a definite increase in column replacement and ion source cleaning. I suspect that the stationary phase is being damaged by the large volume of solvent passing throug the column and mass spec chamber. I am injecting 10 µl DCM extract with 10% toluene keeper. My liners are 900µl volume. Inlet starts at 70oC and ramps to 300oC, flow is constant splitless 1.2ml/min


Has anyone had similar experience? I would prefer to solvent vent but this is not possible given similar boiling points of analytes and solvent (DCM).

[jarodw]

we have set up our gc systems with backflushing and hardly have to clean the sources now. chopping the column (~1m) at a time should resolve most chromatographic problems? dont replace columns often unless damaged most likely due to high t exposure with air water leaks

[Bigbear]

Am I not seeing a decimal point? If you are injecting 10 ul with the conditions you note, you are over loading the liner by 250%!! This can cause lots of problems including caryover.
If you need 10 ul for sensitivity can you concentrate your extract more?
I do EPA 525 which include most of the 8270 compounds, the method incorporates a 1000 concentration .

[tangaloomaflyer]

No you read correct. I used to do a 1000 to 1 concentration, but now I go 100 to 1, which is why I need to inject 10µl.

How do you arrive at a figure of 250% overloading?

[Bigbear]

Agilent pressure/flow calculator. You can download it from their web site.
You input your GC conditions, liner and it does the rest.

[Peter Apps]

Solvent vapour is not likely to do any harm to columns (although it might crud up an ion source more quickly) but liquid solvent being blown along by carrier gas is a different story - and it will mess up peak shapes also.

You do not give your column programme starting temperature, but it needs to be slightly higher than the inlet starting temperature so that the DCM does not condense. If you cannot increase the starting temp of the column then decreae the starting temp of the inlet - which will also give a better chance of solvent venting.

Peter

[tangaloomaflyer]

Well, that explains a lot. The column start temp is 10degrees below the inlet start temp (we found that the inlet could not fully cool so it was hanging before the start of the next run. We are a high throughput lab so we have to reach setpoint for start of next run as quickly as possible.

Haven't experienced poor peak shapes, usually better than our 1µl injection programme. However erratic retention times of the early eluters is a big problem, and the high rate of column wear.

[tangaloomaflyer]

Also,

Column start temp is 60degrees and we are using an inactive fused silica pre-column (no stationary phase). There is 10% toluene keeper in the solvent so the aim was to vent all the DCM throughthe column while the toluene condenses in the pre-column until temp program begins

[Peter Apps]

The precolumn and the toluene may be why you see decent peak shapes despite possibly having liquid dichloromethane condensing.

Most PTV inlets (which I presume that you must have to do this kind of inlet programming) can be cooled by CO2 or liquid nitrogen - or fancy Peltier systems. For PTV in a high throughput lab I would say that active cooling is more or less essential. If you can get your PTV down to 40C and play a bit with gas flows you will be able to vent the dichlromethane while the toluene holds onto the analytes.

Peter

[tangaloomaflyer]

Cooling using a jet of directed compressed air. It's sufficient to cool the inlet in a reasonable time, but we find that (at least for some of our PTV inlets) that the inlet is heated by the oven so we can't get much lower than 10 degrees above the inlet. Something to re-visit though; many thanks for the advice.