[owsik]

Hello everyone, it's my first topic.

I'm the middle of my Final Year Chemistry project, and I'm really stuck.

I'm trying to calculate the concentration of nitrogen oxide (ppm) that I have managed to collect during the sampling period.
I have used UMEx200 as a sampling device (9 of them) and I.C. auto sampler as analyzing method.

I will use just one reading to show you what I did:

Machine output for my first reading was 0.345 mg/L and exposure time for this one was 7 h (420 min).
Firstly I have follow the manufacturer manual on ''how to calculate concentration'' (you can get this manual from SKC websitehttp://www.skcinc.com/instructions/40108.pdf). With the difference that ''my'' manual says 17.3 ml/min for Sampling Rate

Manual says that Concentration (μg/ml) nitrogen dioxide = Concentration (μg/ml) nitrite
And I know that mg/L conversion factor to μg/ml = 1 ( so no conversion is needed).

Next, I did substract blank from sample readings:
0.345 μg/ml  - 0.041 μg/ml  = 0.304 μg/ml

Next step is to calculate Total mass of nitrogen:
Concentration nitrogen dioxide (μg/ml) x Desorption volume (in my case was 4 ml since that is how much I've used to extract sample).
0.304 μg/ml  x 4 ml = 1.216 μg

Next step is to calculate Volume of air:

Volume of air (liters)= [Time(minutes) x (Sampling rate) ] / 1000
Volume of air (liters)= (420 min x 17.3 ml/min) / 1000 = 7.266 L

Next, we just calculate concentration in ppm:

Concentration (ppm)= [Mass(mg) x 24450 ] / [ Air volume (L) x Molecular weight (46.01) ]

And this is the place where I don't really know what they expect me to do, hence if I just plug 1.216 μg result would be:

Concentration(ppm)= [1.216 μg x 24450] / [7.266 L x 46.01]
Concentration = 88.933 ppm(?)

I know that formula states (mg) for Mass, so I did converted 1.216 μg to 0.001216 mg and then plugged it to formula:

Concentration(ppm) = [0.001216 mg x 24450] / [7.266 L x 46.01]
Concentration = 0.08893 ppm (?)

Them results are really confusing in both cases because in both cases its below/over the sensitivity of the device, which is 0.4 to 8ppm according to manufacturer manual.

I did look for other ways to calculate the concentration, and I did find different formula (which is even more confusing).

ppm NO₂ = (Molar Volume x μg/mLNO₂^-x Solution Volume x Conversion) / Formula weight x Air volume

Molar weight: 24.45 (25⁰C and 760 mmHg)

They use Conversion  depending on concentration, simplified version for below 10 ppm NO₂ :

ppm nitrogen dioxide= (μg/mLNO₂^- x 4 mL x 0.843) / Air volume (L)

For Above 10ppm NO₂:

ppm nitrogen dioxide= (μg/mLNO₂^- x 4 mL x 1.0633) / Air volume (L)


It is really confusing for me, and for ''below 10 ppm'' I've managed to get: 0.1410 ppm
and for ''above 10 ppm'' I have: 0.1779 ppm.

Which is still below/above the sensitivity.

What I think I'm doing wrong is probably some unit conversion, or (more possible) my English is just not good enough and ''Validated Concentration Range for NO₂ 0.4 to 8 ppm'' is not a sensitivity but something different.

But I have exactly the same kind of problem with different sampling device (gradko RAM) and I'm just helpless.
Only Diffusion tubes reading were easily to convert to ppm.

Please, someone with better ability to think, could help me.

Regards
Owsik.

[Borek]

ppm of what in what?

[owsik]

Mass of nitrogen oxide in given volume of air (mg/L) i.e. content of Nitrogen Oxide that sampling device was exposed to.

[Borek]

ppm is typically mass over mass - what is mass of the air?

[owsik]

ppm as mg/L
How much of nitrogen in Liter of air?
This units are just what they are asking me to do in manual, so I'm trying to follow the script.
I know that mostly (when it comes down to air pollution) units are μg/m³ - that's the unit commonly used in academic journals and by DEFRA.

[owsik]

I see no one can come up with any good idea.

What if I would like to calculate the output directly in μm/m³

[scwilson]

I'm not terribly familiar with analyses of gases, but I'm assuming that you can trust the manual in order to do your calculations.

I believe I see your "problem." The manual states that your instrument has a "Validated Concentration Range" of 0.4 to 8 ppm (or 400 ppb to 8 ppm). It states below that, that the instrument's "Detection Limits" are anywhere from 5 to 170 ppb, depending on how long your sample is treated. In your case, you treated your sample for 7 hours, so your detection limit should be close to 5 ppb. For reference, 1 ppb = 1 nanogram per gram or 1 nanogram per mL, etc.

Try reading up on "detection limits" and "limits of quantitation." That will help you to understand what's going on here more easily.

Now, I'm going to "give" you what I think is the "answer" to your problem, because I think that some of what I'm about to say can't be found in a textbook and relies partially on experience. Before I continue, please understand that I don't KNOW for a fact everything I'm about to say, but I DO think that it's insightful.

Okay, for an 8-hour treatment, the instrument CAN see concentrations AT 5 ppb. That's why it's the "detection limit." The instrument can detect a signal at that concentration. However, when you're at the limit of detection, your signal is so small that it's imprecise and, often, not very accurate. Sort of like seeing a car a mile ahead of you on the freeway; you can see that car and know it's a car, but it would be very hard to try and guess the make and model of that car. All you can say is that you saw it, but you can't say much more than that. So, in your case, an ~90 ppb (or 0.09 ppm) result isn't impossible. It is, indeed, possible to detect concentrations that low using your experimental conditions (assuming you performed everything correctly).

Now, the manufacturer uses a term called a "Validated Concentration Range." This isn't a textbook-type term, so we have to guess what they mean by this. Typically, something that has been "validated" has been proven accurate by comparing the results with a reliable and approved method/instrument. So, to me, it sounds like their "validated concentration range" is the range of concentrations on their instrument that they have proven to give accurate results (as long as the instrument is used as directed). Concentrations above or below that range can still be measured; they just may or may not be accurate.

So, the instrument is probably SENSITIVE enough to see concentrations as low as 5 ppb. It was, therefore, sensitive enough to measure your 90 ppb sample. However, the instrument hasn't proven to be acceptably accurate for a concentration that low.

[owsik]

Interesting, but simultaneously I was running diffusion tubes and results for diffusion tubes at the same environments are much way higher ( expected).