How are your calibration standards prepared? Are you doing headspace analysis? What target compounds are you looking at?

If you are just doing headspace analysis (voa vials ) you would want your calibration standard to be stored in a cooler in an ice bath. That way you have a controlled temperature (0-1 degree C) and at this constant temperature you will be able to calculate the concentration in the headspace measured in ppbv (this is the critical step). So prep a standard at 10 ug/L into a 40 mL VOA vial (Dilute stock methanol VOA std into reagent water in 40 mL VOA vial with no headspace) store in ice bath. This is stable for up to 1 week. When ready to inject pull std from ice bath pierce septum with syringe and remove 10 mL of the liquid, this will give you 10 mL of headspace. Place back in ice bath upside down (septum down) - let it equilibrate for 30 min. After equilibration inject 200 uL of headspace onto GC. This will be your calibration standard and you can calculate ppb/v based on these standard conditions. Once calibrated you would then just inject 200 uL of your sample (like soil gas etc.) and calculate concentration based on your calibration standard.

As you know ambient air temperature can vary widely and this is why you need to store the cal std at known stable temperature (on ice).

see section 9.2.5.7 --->

https://www3.epa.gov/ttnamti1/files/ambient/airtox/to-15r.pdf

Thank you, Peter Philbrook for answering the question. The doubt is not cleared yet. Let me explain the query in detail.

Actually, we want to measure the dissolved concentration of methane and carbon dioxide in water in the field.

So basically, we have two terms:

1. dissolved gas concentration in water: we will estimate this by using the headspace technique and also taking the ambient air samples to correct the concentration in the headspace.

2. Dissolved gas concentration in water at equilibrium with the atmosphere: This will be estimated using Henry's law and also calculating Henry's constant for that purpose.

In both cases, we are using Henry,s law; in the first case, we have a water sample in the syringe and a small headspace over that water sample which also has ambient air mixed, we allow it to equilibrate and test the headspace sample in GC and correct the concentration using the ambient air gas concentration for say methane.

using the partial pressure of total headspace that we obtained from GC calculation we also estimate the gas dissolved in a water sample

and by adding up the gas concentration in Headspace and the gas concentration in the water sample, we will get the total dissolved concentration at a particular time.

While in the second case, it is a kind of general statement like at specific temperature and pressure what maximum amount of particular gas can be dissolved in the water. So I have doubt in this how do we calculate this. If we are just calculatimg the gas concentration in ambient air by testing the sample in GC and then calculating partial pressure because according to henry's law, the concentration dissolved in a vol of sample is equal to the partial pressure of gas above it. Here also comes the Henry's constant which is different at different temperature and pressure.

DOubt: So the doubt is how do we calculate dissolved gas concentration at equillibrium with the partial pressure?

The document below might be useful -

Thank you Peter Philbrook