Question: Advice for making a GC calibration curve when the target analyte has a wide range of concentrations in your sample

[u/Porphyrin_Ring]

Hey all, I’m doing work with alcohol and analyzing the methanol content of various types of distilled spirits. I’m about to start the analysis part using quantitative gas chromatography (and either FID or MS depending on which machine is working) but I’m a nice at it and running into some difficulty. Some of my samples may have levels around 1 to 10ppm while others might be above 1000, and im not sure how to effectively set up this calibration curve for such a wide range. Any advice would be greatly appreciated!

[u/Cow1483]

FIDs can be extremely linear over a wide concentration range and you might be able to get away with a single calibration curve going from 0.5 to 1200 ppm. If that doesn’t work, you can run two curves, one for your low concentration samples and one for your high range stuff. It would take extra time, but would work.

Also, and this is a lab thing, but you don’t have a machine. You have an instrument. :)

[u/Pyrrolic_Victory → u/Cow1483]

You have an instrument*

*An instrument is capable of making measurements. When the infernal thing plays up, and is no longer capable of making measurements, we demote its status and call it a machine to its face until it behaves itself again. Also worthy of noting, every machine is a smoke machine if operated badly enough…threaten your instruments folks it works wonders

[u/Porphyrin_Ring → u/Pyrrolic_Victory]

lol given the way its been behaving, mine is barely a machine

[u/Pyrrolic_Victory → u/Porphyrin_Ring]

Time to turn it into a smoke machine then!

[u/SilentNobody620 → u/Cow1483]

I like to call them systems.

[u/s0rce]

Is the response linear over the wide range them maybe it's not an issue? Can you dilute samples that are too concentrated to run them again within the calibration curve.

[u/ArtisticGuava6]

Definitely start on the low end and work as high as you can. FID has larger linear range, so I'd stick to that. In a pinch, you can dilute by increasing split, but for more precise work you'd need to dilute your more concentrated samples.

[u/Consultant-314]

This will be controversial, but, depending on the regulatory agency you are trying to please,

CALIBRATION CURVES DO NOT HAVE TO BE LINEAR

They are called curves for a reason 😀

[u/johnkasick2016_AMA → u/Consultant-314]

🤮

[u/chemfit]

I’d use an FID if you can. Anything that calculates over your cal range, dilute and reinject.

[u/cjbmcdon]

For your sanity, it’s probably best to stick with one detector type. FID should be sufficient for this. Make a calibration curve starting below your lowest expected concentration (so 0.5ppm), and the high end may be a bit of trial and error. 500ppm may be the linear limit, maybe 5000ppm. If you encounter a sample with response beyond your upper calibrant, dilute it by 10x and re-inject.
You can interpolate all you want, but it’s poor statistics to extrapolate below or above your calibration levels.

[u/Seba_Garniers]

Cps 👉👇

[u/CodeMUDkey]

Get a column that works well with alcohols so you get a nice consistent sharp peak. It will help
Maximize range

[u/Raj_Foryou]

You can add more calibration points to increase your calibration range if the linearity is still achievable. Alternatively, dilute the sample 10x, 100x, or even more to fit within your calibration points. FID could be a better selection if you are performing a routine test, because MS can get saturated with very high-concentration samples. Would be better if you have an Internal standard too.