Calculating the headspace concentration

Basic questions from students; resources for projects and reports.

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Hi there,

I´m writing a risk assesment for the method I´m going to use in my bachelor thesis. Therefor I have to calculate how much of my analyte (NDMA) is introduced in the GC-MS if I inject 1 mL of the gas in the headspace.

So i know the concentration in the liquid, but i have no clue how to calculate the concentration in the gas phase. Maybe someone here can help me.
Not an easy exercise. You have to first measure the partition coefficient for NDMA and the solvent (water?) as the NDMA moves into the headspace. It will be temperature dependent.

If you really need to know how much is in the gas phase, it would be easiest to do if you had a gaseous standard of NDMA. I doubt it exists but you might be able to get someone to make it for you. Then you could get a response factor for NDMA in the gas which would correlate directly with what you inject from the headspace above your sample.

If the goal is to determine the amount of NDMA in a condensed-phase sample, why do you care how much NDMA is in the gas? Knowing that it partitions into the headspace in a predictable manner and using that to get at the concentration in the liquid phase (Henry's Law) is all you really need. I do a lot of headspace analysis and I've really never had to know exactly how much of an analyte is in the gas phase. Most people want to know how much of the analyte is in the condensed phase (liquid, polymer, etc.).
rb6banjo wrote:

If the goal is to determine the amount of NDMA in a condensed-phase sample, why do you care how much NDMA is in the gas? Knowing that it partitions into the headspace in a predictable manner and using that to get at the concentration in the liquid phase (Henry's Law) is all you really need. I do a lot of headspace analysis and I've really never had to know exactly how much of an analyte is in the gas phase. Most people want to know how much of the analyte is in the condensed phase (liquid, polymer, etc.).


Thanks for your reply.
I know that normally everybody is interested in the concentraiton in the condensed phase. But for my risk assignment I need to know how much (amount) of my analyte (nitrosamines) gets into the system if I inject 1 mL of the headspace gas phase. I need it to calculate how high the concentration of nitrosamines in the room would be, if there was a leakage. So for the worst case scenario I have to calculate the maximum concentration in the workspace if all I put into the GC/MS system would get in the surrounding to know if the legally permitted concentrations are exceeded or not. And from this calculations I can then say how high the maximum concentration of the samples can be, without exceeding the legal limits in case of a failure.
Try a Henry's law calculation using data from:
https://www.epa.gov/sites/production/fi ... _final.pdf

It's good at 20 degrees.
If you need a refresher on Henry's law try:
https://www.thoughtco.com/henrys-law-ex ... lem-609500
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