Headspace Analysis Help!

[bearcla]

I'm stuck on figuring out how to calculate the ppm of a soil sample problem.
I need to calculate ppm from a 2 mL vial containing soil emitting a gas (use 1 mL air for problem) that 50 uL of headspace is taken from and injected into the GCMS.
The data is worked up and 10 ng is calculated to be the mass injected onto the column.

I just figured all that is needed is 10 ng per 50 uL which is 0.2 ppm, but I'm not sure if that's correct. Should I just take 10 ng per 1000 uL to get 0.01 ppm? Do I need to take into account the molar mass of the analyte being tested?

[rb6banjo]

This is a complicated one. We can probably help you better if we know more about what you're actually trying to accomplish.

Have you looked at some of the EPA methods that reference soil samples?

What you are calculating is the amount of your analyte in the gas phase above the sample - not necessarily what's in the sample. If the analyte you are trying to determine partitions into and from the soil according to Henry's Law, then what you are measuring in the gas phase is proportional to what's in the sample.

If you don't know how your analyte behaves in the soil, you need a more exhaustive method of extraction (dynamic headspace analysis) or solvent extraction to get it all out.

Also, gas-phase "parts per million" is not the same as condensed-phase parts per million. In the gas phase, ppm is:

microliters of analyte per liter of ideal gas OR µmole of analyte per mole of ideal gas

at whatever pressure (in your case atmospheric) and temperature your analysis takes place. From the ideal gas law, the volume and mole ratios are the same.

For your example, and let's say the analyte is toluene (MW = 92 g/mole):

0.010 µg toluene/0.050 mL x 22,400 mL/mole ideal gas x 1 µmole/92 µg toluene = 48.7 ppm (gas phase)

[James_Ball]

Are you trying to determine how much is in the soil, or how much evaporates(outgasses) from the soil.