calibration liquid methanol in GC

[samira123]

Hi all,
I am using GC 7890A. I want to calibrate the GC for methanol. however, I have pure liquid methanol. How can I make the liquid methanol into vapor and make different concentrations of sample to inject GC?
I know that our GC is working on methanol.


thanks

[Consumer Products Guy]

Hi all,
I want to calibrate the GC for methanol.

Over 4 decades experience, and I have no idea what this means.

I would make up methanol solutions in water, and inject SMALL volumes (under 0.5 microlitres) into the GC, as water vaporizes a ton.

[rb6banjo]

I agree. That's a cryptic post for sure.

I think what you're getting at is how to make standards. I have to do this periodically looking for methanol in spirits. I looked at the density of methanol as a function of temperature and it really doesn't change a great deal in the neighborhood of what we consider as room temperature. I use the method of standard addition to calibrate. I add somewhere in the neighborhood of 0.5-2 µL of neat methanol to 4 mL of sample. Using the density of methanol, that gets me in the 100-400 ppm range and I inject that. I only inject the needle volume of sample as anything more than that causes my EPC to go crazy and precision suffers.

[AICMM]

samira123,

There are several alternatives to making methanol as a gas standard. Easiest is to inject a known volume of methanol, either as a neat or as a dilution, depending on the desired concentration, into a warm gas sampling bulb of known volume. Your can then draw with a syringe and inject.

Alternatively, you can do the same thing with a tedlar bag and add a known volume of gas like nitroge.

You can also set up a syringe pump to pump a small amount of methanol into a vaporizor and add gas flow but I suspect this is more than you are bargaining for.

Best regards,

AICMM

[samira123]

thanks, I think it is a good solution.

samira123,

There are several alternatives to making methanol as a gas standard. Easiest is to inject a known volume of methanol, either as a neat or as a dilution, depending on the desired concentration, into a warm gas sampling bulb of known volume. Your can then draw with a syringe and inject.

Alternatively, you can do the same thing with a tedlar bag and add a known volume of gas like nitroge.

You can also set up a syringe pump to pump a small amount of methanol into a vaporizor and add gas flow but I suspect this is more than you are bargaining for.

Best regards,

AICMM

[AICMM]

Samira123,

I realized I left our permeation tubes. If you have a lot of time and patience and a really, really good analytical balance this is an option. There is a very easy way to make these if you have the time to calibrate it your self or you can spend the money to buy one from KinTek or Owlstone.

I case you are interested.....

BEst regards,

AICMM

[AICMM]

RB6Banjo,

Curious, why standard addition? I have other questions about spirits analysis if you care to ping me at aicmm at flash dot net.

Best regards,

AICMM

[rb6banjo]

It's no big secret. I just find it easier. Since there's no matrix effect (direct injection), I just calibrate in the matrix because adding methanol to the sample is easy. I plotted some theoretical data of density vs. temperature for MeOH. It turns out that the density doesn't change enough at my range of lab temperatures to make an appreciable difference in the concentrations of my additions.

I just ran a set of 4 different samples. I calibrated in the sample with the lowest concentration of MeOH then used that data to calculate the others from the chromatographic data.

For me, the key to making this analysis reliable was injection of only the needle barrel full of sample. Inlet pressure stays much more stable during injection. LOD might not be as good as other methods but it's given me good results down to 5 ppm or so.