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- Posts: 10
- Joined: Mon Nov 07, 2011 4:58 pm
I really hope someone can help as I'm ready to just throw in the proverbial towl. *Disclaimer - I'm not a chemist nor do I pretend to be one, I really am not trying to be obtuse on purpose 
*
We are doing headspace analysis for dissolved gas in water. I don't understand how to do the calculations once I have my data from my GC, to report how much of each gas is in the water. My previous background has been just passive diffusive sampling, with spreadsheets already developed by the PI (ie I was pretty much taught to just inject samples and copy/paste results, with minimum explanation on chromatography operation), so this type of work is new to me. I'm having a hard time getting help (my first PI is now gone, and one person here has done this but not often so we're both struggling to figure this out), and I've tried to find references, but its confusing when I see so many different ways to calculate everything.
Here's what I'm doing, if you need more info let me know. We have 40ml vials of groundwater, which I create a 4ml headspace by displacing 4ml of water into an empty syringe, by injecting 4ml of Argon gas (which is my carrier gas). Vial is shaken, then a sample is pulled and injected into my GC which has a 0.5ml sampling loop; results are on a TCD and its set up to report results as ppm. From what I've talked about with the person here who has done similar stuff before, partial pressure is apparently not an issue with the calculations, due to the way I sample the gas and the way the GC is set up.
So Q#1 - Response factor. I know I need to get a response factor to adjust my results (or that's what I've been previously taught to do), which again I see various ways to do this, so I'm not sure which is right?
RF = Area of Standard GC reports/Known Conc of Gas
RF = Known conc of Gas/Area of the Std the GC reports
RF = (Known Conc of Standard * Volume injected)/Area result for Std reported by GC
Q#2 - Converting ppm results into other values (namely umol)
The equation setup the person here gave me, bases the values on umol of gas (and he's not sure how to convert this either). I tried to look up how to convert ppm to umol but again, see various ways to do so. The one that makes sense to me is 1ppm = 1umol/mol. I'm not entirely sure how this works though - would it be 1ppm of gas X = 1umol of gas X in 1mol of the entire amount of gas injected? or because my GC sample loop is 0.5ml, are the reported ppm based on that? Or am I entirely on the wrong track and need to do something else?
Q#3 - Calculating gas in water.
From what I've talked to the person here and read a little about, it looks like I have to find the amount of gas in the headspace, plus the amount in the liquid phase? The equations I was given to do this was
Gas phase = umol of gas x (vol of headspace/vol of gas sample)
Total gas = (umol of gas in gas phase) (1+ [alpha V(l)/V(g)])
Where alpha = (1.7023x10^7)/(K-1)(18), K being the Henry’s constant; and V(l) = vol of liquid in vial and V(g) vol of gas in vial
Does this look right? Or are there other ways to do it?
I've tried to do a few calculations but they're not working out to sensible answers. Pretty sure the method for sample processing is ok, but somewhere in the data processing I think something is going wrong.
Sorry this is so long
Thanks for reading and I appreciate any help!
* We are doing headspace analysis for dissolved gas in water. I don't understand how to do the calculations once I have my data from my GC, to report how much of each gas is in the water. My previous background has been just passive diffusive sampling, with spreadsheets already developed by the PI (ie I was pretty much taught to just inject samples and copy/paste results, with minimum explanation on chromatography operation), so this type of work is new to me. I'm having a hard time getting help (my first PI is now gone, and one person here has done this but not often so we're both struggling to figure this out), and I've tried to find references, but its confusing when I see so many different ways to calculate everything.
Here's what I'm doing, if you need more info let me know. We have 40ml vials of groundwater, which I create a 4ml headspace by displacing 4ml of water into an empty syringe, by injecting 4ml of Argon gas (which is my carrier gas). Vial is shaken, then a sample is pulled and injected into my GC which has a 0.5ml sampling loop; results are on a TCD and its set up to report results as ppm. From what I've talked about with the person here who has done similar stuff before, partial pressure is apparently not an issue with the calculations, due to the way I sample the gas and the way the GC is set up.
So Q#1 - Response factor. I know I need to get a response factor to adjust my results (or that's what I've been previously taught to do), which again I see various ways to do this, so I'm not sure which is right?
RF = Area of Standard GC reports/Known Conc of Gas
RF = Known conc of Gas/Area of the Std the GC reports
RF = (Known Conc of Standard * Volume injected)/Area result for Std reported by GC
Q#2 - Converting ppm results into other values (namely umol)
The equation setup the person here gave me, bases the values on umol of gas (and he's not sure how to convert this either). I tried to look up how to convert ppm to umol but again, see various ways to do so. The one that makes sense to me is 1ppm = 1umol/mol. I'm not entirely sure how this works though - would it be 1ppm of gas X = 1umol of gas X in 1mol of the entire amount of gas injected? or because my GC sample loop is 0.5ml, are the reported ppm based on that? Or am I entirely on the wrong track and need to do something else?
Q#3 - Calculating gas in water.
From what I've talked to the person here and read a little about, it looks like I have to find the amount of gas in the headspace, plus the amount in the liquid phase? The equations I was given to do this was
Gas phase = umol of gas x (vol of headspace/vol of gas sample)
Total gas = (umol of gas in gas phase) (1+ [alpha V(l)/V(g)])
Where alpha = (1.7023x10^7)/(K-1)(18), K being the Henry’s constant; and V(l) = vol of liquid in vial and V(g) vol of gas in vial
Does this look right? Or are there other ways to do it?
I've tried to do a few calculations but they're not working out to sensible answers. Pretty sure the method for sample processing is ok, but somewhere in the data processing I think something is going wrong.
Sorry this is so long
Thanks for reading and I appreciate any help!
