Questions about PTV inlet & backflash (contamination)

[HarleyD]

Hi,

For my internship i'm validating a GCMS method for the analysis of PAH and PCB in construction materials. The method in question is already being employed for soil samples.

Im using an Agilent 7890B in solvent vent mode with a 5190-2297 2mm splitless liner. The original method i'm supposed to validate uses an injection volume of 3 uL, the extraction solvent used is petroleum ether. According to solvent expansion calculations this volume would cause significant backflash. My question is: does the use of solvent vent mode allow for a larger expansion volume or does the liner volume set the limit for this? It's strange to me that they have 9 GCMS' running this method if the injection volume would surely cause backflash.

Now to continue my story, i accidentally used an injection volume of 6 uL for a control sample, i'm pretty sure this did in fact cause backflash. Ever since all my petroleum ether blanks show 3 peaks that are very constant in terms of retention time but vary in height and area. I have baked out the oven at 300 C several times and replaced the liner, septa and cleaned the inlet. This eliminated one peak which i believe to be naphtalene (m/z 128). two peaks remain with m/z values (257 & 285) that don't correspond to any of my analytes. Aside from these peaks the most abundant m/z values are 207 and 281, which are common for column bleed and septum contamination.

Since the retention times are so reproducable i think the contamination might be coming from the gas inlets due to backflash or the needle (which i don't think is likely with all the washing steps used). I'm hoping anyone has a clue as to what else could be the source of contamination. But mostly i would like to know how to go about cleaning the inlet and carrier gas lines in case of backflash.

[James_Ball]

Hi,

For my internship i'm validating a GCMS method for the analysis of PAH and PCB in construction materials. The method in question is already being employed for soil samples.

Im using an Agilent 7890B in solvent vent mode with a 5190-2297 2mm splitless liner. The original method i'm supposed to validate uses an injection volume of 3 uL, the extraction solvent used is petroleum ether. According to solvent expansion calculations this volume would cause significant backflash. My question is: does the use of solvent vent mode allow for a larger expansion volume or does the liner volume set the limit for this? It's strange to me that they have 9 GCMS' running this method if the injection volume would surely cause backflash.

Now to continue my story, i accidentally used an injection volume of 6 uL for a control sample, i'm pretty sure this did in fact cause backflash. Ever since all my petroleum ether blanks show 3 peaks that are very constant in terms of retention time but vary in height and area. I have baked out the oven at 300 C several times and replaced the liner, septa and cleaned the inlet. This eliminated one peak which i believe to be naphtalene (m/z 128). two peaks remain with m/z values (257 & 285) that don't correspond to any of my analytes. Aside from these peaks the most abundant m/z values are 207 and 281, which are common for column bleed and septum contamination.

Since the retention times are so reproducable i think the contamination might be coming from the gas inlets due to backflash or the needle (which i don't think is likely with all the washing steps used). I'm hoping anyone has a clue as to what else could be the source of contamination. But mostly i would like to know how to go about cleaning the inlet and carrier gas lines in case of backflash.

If you follow the inlet lines back to the controller you will find they are attached to a block that is held onto the controller with a screw in the middle. If you remove the block there will be two tiny O-rings that seal it, don't loose those. You should be able to pass some solvent through the lines to the inlet cap and flush out anything from the backflash.

With the solvent vent, there should always be a flow of about 100ml/minute through the inlet which will sweep away the solvent which is injected slowly at a rate that it evaporates as quickly as it is injected, so even with a larger injection it shouldn't backflash, especially since the inlet should be at a low temperature when doing the solvent vent injection.

Another thing to clean is the split vent line, it is the copper line form the side of the inlet weldment to the controller in the rear. If it gets contaminated with high boiling compounds it can cause some carryover at times. You can either remove the copper line and flush it with solvent then dry with a flow of nitrogen, or just replace the copper line(you will need new ferrules if you do that).

http://www.agilent.com/cs/library/userm ... 041007.pdf

That will give a little background on the inlet you are using and maybe help you understand the working of it.

https://blog.restek.com/?p=633

This will also show some techniques where they do large volume injections with a normal split/splitless injection port.