Light hydrocarbons in water

[Conexus]

Hello!

I have a challenge and would like to know your experience. As a underground gas storage facility we have to determine the escaped amount of natural gas (C1 - C5). The sample we analyze is ground water (very high mineralization up to 100 g/L). The amount of combined hydrocarbons is up to 400 ml/L (standard conditions), that's what the outsourced laboratory says, but I don't trust their method...

The question is - what GC system should I use? As i have scoured the internet headspace seems the best, because of the high salt concentrations, but then again the LOQ is a concern. As this is just for our knowledge, the repeatability and accuracy can be lower, as it is with headspace. Maybe someone dose something like this in their laboratory, would be awesome if you could share the system, method and your opinion of the analysis.

Thank you and have a nice day!

[James_Ball]

I haven't analyzed these specific analytes but it sounds as if a GC/FID with a Purge and Trap would work best. FID will give good sensitivity for the analytes and Purge and Trap will give lower detection limits than Headspace.

We do everything from drinking water to ground water to waste sludge and soils on this and have good results.

[eg@aaclab]

FYI the official method for dissolved hydrocarbons in water is EPA RSK-175 which you can find online and that is used for fracking water all the time-to summarize samples are collected in VOA vials, shaken for 10 minutes, a few ml of Helium is injected into the sample using a syringe displacing the equal amount of dissolved gases into another syringe which is then injected into a GC-FID with a gas sampling loop setup for C1-C6+ hydrocarbons...this has been automated by a few companies as well as the purge and trap suggested above....

[Conexus]

Thank you for your answers, i took a look in to the methods, and asked my GC suppliers if the manufacturer has any of these modifications for our existing GC/TCD setup.

I have another question, as i have close to 0 experience with GC (only HPLC), do both of these sample preparations can maintain constant sample flow? As we have natural gas analysis system with 4 columns with variable valve/sample injections parameters from 1mL sample loop. Hope i made my thoughts understandable.

[James_Ball]

Most purge and trap samplers will route the flow to the inlet through the valve in the purge and trap and only switch the flow to pass through the trap when it desorbs, otherwise it is a normal flow to the instrument. There may be a slight pulse in flow when the valve changes but otherwise it is a steady constant flow of the carrier gas.