BTEX determination, DB-VRX, split/splitless

Discussions about GC and other "gas phase" separation techniques.

10 posts Page 1 of 1
Hi,
please, do you know to recommend me GC/MSD parameters to determinate BTEX using DB-VRX column without purge&trap injection?
Thx a lot :)

L.
If you're being held to some sort of regulatory standard, i don't see how you're going to get around not having some sort of sampling that involves concentration of the analytes (e.g., purge-and-trap).

The only thing i can recommend for this that doesn't involve purge-and-trap is SPME (or something like it, HiSorb, Retek's "Arrow", etc.).
You might also use a micro extraction into hexane or pentane, using about 35ml of the water sample and 2ml of the solvent, shaken in the 40ml VOA vial, then draw off the solvent and inject.
The past is there to guide us into the future, not to dwell in.
James Ball is right :) I use microextraction in 2 ml hexane and 1 l water(sample) ;)


rb6banjo wrote:
If you're being held to some sort of regulatory standard, i don't see how you're going to get around not having some sort of sampling that involves concentration of the analytes (e.g., purge-and-trap).

The only thing i can recommend for this that doesn't involve purge-and-trap is SPME (or something like it, HiSorb, Retek's "Arrow", etc.).
In a 1 L sep funnel? Recoveries are good? Intuitively, seems like not enough organic phase to get good distribution between phases.
rb6banjo wrote:
In a 1 L sep funnel? Recoveries are good? Intuitively, seems like not enough organic phase to get good distribution between phases.


I agree, it will take a lot of shaking to get 2ml of hexane in contact with all of the water in a 1l sample. What type of detection limits are you needing to meet?

https://www.epa.gov/sites/production/fi ... s/3511.pdf

This method gives good recovery for waters spiked as low as 9ug/L using FID detection. MS should give better sensitivity since it can see through the hydrocarbons and pull out the aromatics better than with FID.

For GC conditions I would start with oven at 35-40C hold 1 minute with splitless injection 1ul opening the split vent valve at 1minute. Ramp 10 degrees per minute until everything is eluted, or to within 20C of the max temperature. Inlet temperature I would start with 200C and adjust to get the best peak shapes and sensitivity. Set a long solvent delay on the first injection and watch the vacuum gauge if you have one for the solvent peak to elute and set the delay just after that.
The past is there to guide us into the future, not to dwell in.
In 1 l - something like conical flask with narrow neck connected with another smaller Erlenmeyer flask, shaking for 5 minutes, recoveries are really good (suprisingly).

rb6banjo wrote:
In a 1 L sep funnel? Recoveries are good? Intuitively, seems like not enough organic phase to get good distribution between phases.
I use 3511 to do my extractions for DRO in a 40mL VOA for 20g soil sample. I put them on a rotator, then vortexer, then sonication, then centrifuge. I get great surrogate and spike recoveries for soils with an MDL of 5 ppm DRO for a 5uL splitless LVSI injection and GC-FID.

I don't see why this could not work just as well for BTEX if you used pest grade hexane or pentane. If you had an extraction solvent volume of 2 mL and 30 mL of water and managed to do a 5uL splitless LVSI injection, you might be able to hit an MDL of ~7 ppb. My routine MDL for Benzene is 0.48 ppb (P&T GC-MS) so 14 times that is ~7 ppb if GC-FID has similar detection limits.

For MS, I don't think the MS would thank you for the large solvent volumes. But split injections could work. I am splitting 40:1 for P&T GC-MS.

If acetone is not a target, it greatly enhances recovery e.g. (2 mL each of hexane and acetone). I figured this out when I found that hexane would not clean teflon bombs used for digestion of oily samples for metals. But Acetone:hexane or acetone and water alone will clean them right out. No wonder when hexane:Acetone is an approved solvent combination for soxlet extractions. Acetone partitions into the water so its not in the final volume you use for dilution calculations. From experience I've found that even though its in the aqueous phase, it still contributes to extracting complex organics.
We used to do BTEX/GRO on and GC-FID/PID instrument by splitting the column to dual detectors PID and FID. The PID will get that BTEX signal and ignore the more saturated hydrocarbons.
LALman wrote:
We used to do BTEX/GRO on and GC-FID/PID instrument by splitting the column to dual detectors PID and FID. The PID will get that BTEX signal and ignore the more saturated hydrocarbons.


I was doing this but not splitting the flow. I plumbed the outlet of the PID back down into the oven with some 0.53 silcosteel tubing and into the FID inlet that way I didn't lose any sensitivity at the split. It worked great.
The past is there to guide us into the future, not to dwell in.
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