Using liquid standards/calibration curve for gas samples

[sas1995]

Hello, I am very new to chromatography so I am still learning how to obtain data. Our GC was bought on a budget so no training. I am wondering if a calibration curve using liquid standards can be used to determine gas sample concentrations if the gas samples are taken under ambient conditions.

Thanks!

[Consumer Products Guy]

Hello, I am very new to chromatography so I am still learning how to obtain data. Our GC was bought on a budget so no training.

Attend a training class. Would you try to fly an airplane with no training?

[sas1995]

Could you suggest one that would help me with this topic?

[drjames29]

Hello, I am very new to chromatography so I am still learning how to obtain data. Our GC was bought on a budget so no training.

Attend a training class. Would you try to fly an airplane with no training?

It seems as if Mr. "Consumer Products Guy" prefers to just remain a "consumer" of online information, rather than a contributor, Sorry sas.

While it's common to calibrate a GC instrument using liquid standards, applying this calibration to determine gas sample concentrations could cause potential issues and introduce uncertainty.

Gas standards are usually preferred for calibrating gas chromatographs because they closely mimic the state of the analytes in the samples. Using liquid standards to calibrate a GC instrument assumes that the compounds of interest will behave similarly in the gas phase as they do in the liquid phase, which might not always be the case.

If gas standards are not available or practical for your specific analytes, you may need to consider additional factors such as temperature, pressure, and the vapor pressure of the liquid standards to make adjustments. It's essential to understand the physical and chemical properties of the compounds you are analyzing and ensure that the calibration approach is appropriate for the specific conditions of your samples.

Hope this helps, best of luck.

[70 eV]

I would obtain gas standards, liquid is a messy business. We used to prep our internal standard via volatilization in an oven with a round bottom and we'd spike the gas into 6 cans at a time. The difference between the first can and the last can was noticeable and you usually ended up having to recalibrate.

We moved our internal over to a gas and it was more consistent. Liquid for the IS isn't the end of the world since it's just a reference but the math involved for liquid target analytes sounds like a pain in the ass.

[sas1995]

Thanks everyone!

[rb6banjo]

Gas-phase concentration is completely different than condensed phase (liquid or solid) concentration.

ppm (condensed phase) = µg/g (µg of analyte per gram of sample)

the density of water is pretty close to 1.0 g/mL so you'll also see it as µg/mL.

I like to think of gas phase concentration as:

ppm (gas phase) = µmole analyte/mole of ideal gas

Because the gas-phase calculations are generally performed using the ideal gas law, sometimes you'll see gas-phase ppm as µL/L. The ratios are the same. I prefer to think in terms of mass.

If you can share more about what you actually want to do, it'll help us help you more.

A wise contributor once said "the less you tell us, the less we can help you".

[AICMM]

sas1995,

Can I recommend you read another post, "calibration of liquid methanol in GC" (I thinkI got that right....) for some options for you to look at?

Best regards,

AICMM

[Peter Apps]

Depending on a host of factors that we know nothing of you could calibrate a gas analysis using liquid standards. The process would be to calibrate peak area vs mass or moles in your liquid standards, calculate mass or moles from the peak areas you get from your gas samples, and then calculate mass per volume or moles/moles from the volume of your gas sample. There is a lot that can go wrong with such an approach, and you need to avoid it if you can.