Low Concentration Detection

[JaceAssured]

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?

[rb6banjo]

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.

[AICMM]

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

[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

[Consumer Products Guy]

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

Curious: are you able to explain "why"?

[MSCHemist]

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.

[LindseyPyron]

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

[AICMM]

LindseyPyron,

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

Best regards,

AICMM

[LindseyPyron]

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.

[JaceAssured]

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?

[judibird0516]

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

[Bigbear]

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.

[CharapitsaS]

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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