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
Steve Reimer wrote:
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


Thank you! I´ll give it a try.
I also wonder if you could use the "full evaporation" technique to get at it. In essence, you'd be creating your own gas-phase standard of ndma. Basically, you add a known amount of your ndma standard to a vial and seal it. Heat the vial above the boiling point of everything in the vial (solvent must boil less than the ndma in this case). Use a heated gas-phase syringe to make your injection. In theory, if you have 22 µg of ndma in a 22 mL headspace vial and you inject 1 mL of gas from the vial, then you'd be injecting 1 µg of ndma into the gc.

That's how you could get your response factor for gas-phase ndma. You can't have any cold spots and everything has to be gaseous.
.. but the biggest likely failure is the vial getting broken (user drops it, autosampler has a wobble and throws it on the floor or rams something into it) - which will release the entire contents of the vial. I would be inclined to assess this risk first. If this risk is tolerable, then the smaller risk of release of just a bit of the headspace would probably also be tolerable, except for the fact you won't notice it happening, and it could become a chronic issue. And it's not the only chronic risk: if you have old vials with damaged septa hanging around in the lab, they may also be leaking their contents slowly.
Naively, can you just take a vial with known standard, analyse it on two successive days, and see how much the signal decreases? If the signal on day 2 is 90% of that on day 1 (relative to a reliable standard or whatever) then somehow, 10% of the total content of the vial has gone missing. If you know the total content of the vial at the outset (which you do, because it's a standard) then you know the maximum that could have entered the lab from that vial; you can divide by the air-flow to work out the average concentration it would have created in the atmosphere (or divide by the volume of the lab if you want to know the situation if the air-handling fails). This process assesses the worst-case scenario of having leaky vials and a GC that releases its waste into the lab; you'd need to scale up for the number of vials you typically have stacked up in the autosampler or in preparation.
I might be being completely naïve about this...
Thank you for all your replies.

Maybe I let out some information or was unclear because I´m no native english speaker.
It´s the risk assignment for the lab where I´m writing my bachelor thesis. They want to know, how much of the cancerogeneous compounds that enter the meassuring system during one run could escape in the case of a e.g. broken transferline. Therefore I have to calculate which sampling method brings in the most amount of analyte into the system. We have a limit of 1000 ppm which is the maximum concentration allowed to use. I know that this is way higher, than the concentrations needed.
But given, that I measure a 1000 ppm solution, with which sampling method do I introduce the biggest amount of analyte into the system under standard conditoins. For the liquid phase sample methods it´s easy. The calculations for SPME and SBSE are done by a colleague. But I struggle with the calculation for head space sampling.
When we have the calculated values for all used sampling methods, we will choose the one that intruduces the highest amount of analyte into the system and calculate with this amount and the volume of the lab, the possible workplace concentrations if something goes wrong. If we have this value, we can take a look at the legal limits and can than say "Well, 1000 ppm might be the maximum allowed concentration, but maybe we should only use 200 ppm solutions, because if something is broken, we exceed the legal limits."

This has to be a strictly theoretical calculation and therefore I have to calculate the amount introduced when 1 mL of head space gas is analyzed.
Do you need to use headspace analysis ?

If not, just say that the parameter (headspace concentration) required to assess risk is not available and that this method should not be used.

Peter
Peter Apps
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