Chromatography Forum

Hello everyone.

I am currently using an Agilent 7890B GC-FID and my boss wants me to be able to measure ethanol concentrations in water in the ppb range. Currently, my limit of quantification is ~30 ppm. Is there anyway I can make my limit of quantification into the ppb range by changing method parameters or will new equipment (such as a purge and trap) be needed?

Current Method Parameters:
Injection Volume = 0.5 uL
Inlet Temperature = 220 C
Septum Purge Flow = 3 mL/min
Inlet Pressure = 7.5429 psi
Split Injection Ratio= 100:1
Oven Initial Temp = 50 C with a 7 C/min ramp to 100 C
FID Temperature = 250 C
Air Flow = 400 mL/min
H2 Flow = 30 mL/min
N2 Flow = 25 mL/min

If new equipment is needed, any suggestions?

P&T likely won't work for EtOH. Headspace SPME might be the best way to go with this. We just talked about this a couple of weeks ago:

viewtopic.php?f=2&t=24929

I was able to see 10 ppm with SPME. With the S/N ratio I measured on 10 ppm, I think I could detect less than 1 ppm. Perhaps if I added salt to the matrix, I could push it lower. 10's of ppb might be a big stretch for this analyte.

JaceAssured,

In my humble opinion, low ppb on an FID is not going to happen with a 0.5 uL injection. We are talking pg on column, below what an FID can be expected to see.
You have to have some way of increasing the overall mass on column to get to a point where the FID can reasonably detect it.

Best regards,

AICMM

JaceAssured,

One more thing I just thought of. You might explore Restek's 1,4-dioxane notes (with large volume injection I believe) for inspiration.

Best regards,

AICMM

my boss wants me to be able to measure ethanol concentrations in water in the ppb range.

Curious: are you able to explain "why"?

Pulsed splitless or pulsed low split also if you turn down your inlet temp to minimum and your pulse pressure to maximum you can inject a larger volume without risking backflash.

A retention gap may be necessary to help keep the peaks focused. Also I usually start at 40 deg C when doing ethanol though it increases the GC cooling time.

BTW what is your matrix/solvent?
edit never mind water. In that case I would not go above 0.5ul and definitely use a retention gap to spare the analytical column and watch that the water doesn't extinguish the hydrogen flame on the FID. Static headspace or SPME would be better for this analysis.

I did a Cognac Oil (grape extract) for residual ethanol last week though I used a GC/MS with SIM ions of 45 and 46. I had no need to go below 1ppm though as my residual ethanol requests have ranged from 50ppm to 3000ppm.

We consistently run ETOH down to ~50ppb by GC/MS. I would expect you to get close to the same PQL by FID.

LindseyPyron,

By SIM or scan? By direct injection or P&T?

Best regards,

AICMM

I went back and pulled some data we ran for a customer in California by purge and trap. This was run on a 5973 MS. Low standard was a 5 ppb standard in full scan with a 10 ml sample size. If you want a copy of the calibration curve cut me an email to lpyron@estanalytical.com and I will get you a copy.

Thanks for all the replies everyone. We recently found a paper that had method parameters that let us get down to ~100 ppb using our current setup (Direct Injection,GC-FID). We are still considering P&T because of its superior LOQ but it is a large investment.

We are currently having an issue with carryover though and we have changed all inlet consumables (syringe, septum, liner, gold seal) and I now suspect it may be due to the split vent trap. Is there any way to test for that specifically? Can I remove the trap entirely and run a sample without harming the machine in any way as a check?

Jace,

I'm trying to detect ppb concentrations of ethanol in water too, what paper did you find that had the method you are using to obtain ~100ppb ethanol concentrations?

thanks

As someone already mentioned, if you are injecting water you need to lower the injector temp to just above 100, maybe 125. you could also try a lower split. Also use a 4mm liner.
No test I know for split vent trap, just change it.

Jace,

You wrote "We recently found a paper that had method parameters that let us get down to ~100 ppb using our current setup (Direct Injection,GC-FID)".
Would you like send me copy of this paper on e-mail svcharapitsa@tut.by .

Thanks,
Siarhei Charapitsa


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