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Topic: False positive result on nitrous oxide flame of AAS  (Read 5678 times)

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Offline vtdpada

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False positive result on nitrous oxide flame of AAS
« on: October 31, 2013, 02:48:46 AM »
I have compare C2H2-air and C2H2-N2O flame AAS on determination of calcium. Both use same range of std to plot calibration curve. (2-6ppm Ca, w/o KCl or other releasing agent) When i measure the blank sample that contains phosphate, KCl and LaCl3 mixture, C2H2-N2O flame give false positive result, around 0.5ppm. When i measure another blank sample with phosphste, KCl and EDTA. C2H2-N2O flame also give 0.5ppm false positive. But both above mentioned blank samples would not give false positive when measured by C2H2-air flame. What is the reason?

Offline Archer

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Re: False positive result on nitrous oxide flame of AAS
« Reply #1 on: October 31, 2013, 03:16:54 AM »
This is difficult to follow.

You have a blank which contains a phosphate, what kind of phosphate?

When you run this blank you get a reading for calcium with N2O as the oxidant.

When you run the above with EDTA you also get a reading for calcium

However when running air as the oxidant you don't get a reading for either with or without EDTA.

Is this correct?
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Offline Archer

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Re: False positive result on nitrous oxide flame of AAS
« Reply #2 on: October 31, 2013, 03:28:14 AM »
I have compare C2H2-air and C2H2-N2O flame AAS on determination of calcium. Both use same range of std to plot calibration curve. (2-6ppm Ca, w/o KCl or other releasing agent) When i measure the blank sample that contains phosphate, KCl and LaCl3 mixture, C2H2-N2O flame give false positive result, around 0.5ppm. When i measure another blank sample with phosphste, KCl and EDTA. C2H2-N2O flame also give 0.5ppm false positive. But both above mentioned blank samples would not give false positive when measured by C2H2-air flame. What is the reason?

Could you also provide all the concentrations of the compounds in your blank samples please.

Do you know the significance of using different oxidants or why one adds EDTA or LaCl3 to a sample?
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

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Offline vtdpada

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Re: False positive result on nitrous oxide flame of AAS
« Reply #3 on: October 31, 2013, 04:42:38 AM »
Dear Sir
Thank you so much for your reply & sorry for confusion caused.
Below is more accurate description:

Blank A-20ppm phosphate + 0.5% LaCl3 + 0.2% KCl
Blank B-100ppm phosphate + 0.005M EDTA + 0.2% KCl

Phosphate source is NaH2PO4

Blank A----N2O give 0.547ppm,  air give almost zero
Blank B----N2O give 0.426ppm, air give almost zero

Both used same set of working std (2, 4, 6ppm) to plot 2 calibration curve.
One for air, one for N2O. Std prepared from 1000ppm stock calcium soltion

All above mentioned std & blanks are diluted from 1% HNO3

I know LaCl3 & EDTA are used for prevent calcium and phosphate form unvolatile complex. Purpose of N2O flame is to provide sufficient thermal energy to breakdown calcium phosphate complex. Therefore if phosphate is the only known interference, EDTA and LaCl3 are not required if N2O-Acetylene flame used.

Offline vtdpada

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Re: False positive result on nitrous oxide flame of AAS
« Reply #4 on: October 31, 2013, 09:23:24 AM »
Sorry, the phosphate source should be H3PO4
&
EDTA is buffered in pH9

Offline Archer

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Re: False positive result on nitrous oxide flame of AAS
« Reply #5 on: October 31, 2013, 04:43:25 PM »
I am a bit stumped on this one, have you looked at possible sources of contamination?

What value does a deionised water blank provide?

You could try each component individually to see if any of these are giving a signal for Ca.

There are colourmetric methods for calcium, I don't have the references to hand but I can supply these in a day or two.
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline vtdpada

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Re: False positive result on nitrous oxide flame of AAS
« Reply #6 on: October 31, 2013, 08:11:56 PM »
Dear Sir,

That is the teaching experiment, in fact I don't have second chance to do that.
The only thing I can do is to explain this observation in my lab report.
Thanks again for you help.

Offline Archer

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Re: False positive result on nitrous oxide flame of AAS
« Reply #7 on: November 01, 2013, 02:16:33 AM »
Ah, ok.

You have quite a mixture of concentrations. You have %, ppm & Molar, try converting everything to Molar or mMolar.

See how these numbers look.
“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline marquis

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Re: False positive result on nitrous oxide flame of AAS
« Reply #8 on: November 01, 2013, 10:58:35 AM »
A couple of ideas that may or may not work.

Background noise was always a problem when running calcium with
a nitrous oxide acetylene flame. I lost sensitivity with nitrous oxide
because of the background noise. It was better to run air acetylene
in my application.

Can you try a different wavelength?  I think you are using 422 nm.
Are there any other wavelengths that have the sensitivity you need?
See if that makes any difference.

Good luck and hope this helps.

Offline scwilson

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Re: False positive result on nitrous oxide flame of AAS
« Reply #9 on: November 02, 2013, 07:49:58 AM »
Naturally, we can't be sure without some extra experimentation, but, personally, I think you've already alluded to the answer. Additionally, your data certainly seems to be trending towards a quite reasonable explanation. Your data makes perfect sense, in my opinion.

I'm going to try "guiding" you towards a few thoughts without just stating my thoughts explicitly, since this is your lab assignment, after all ; )

You mentioned that a nitrous/acetylene flame has a greater thermal energy than an air/acetylene flame. This is ABSOLUTELY correct. You also mention that the purpose of the KCl and lanthanum salt and EDTA, etc. are to act as "releasing agents," which are designed to prevent formation of stable complexes with calcium. Obviously, these stable complexes are undesirable, since they don't always decompose in a flame to yield calcium in its ground state. Our FAA/ES can NOT see calcium unless it is in the ground state when we try to measure it. Would a nitrous/acetylene flame more efficiently decompose these stable complexes compared to an air/acetylene flame? One well-documented FAA/ES interference for caclium is phosphate. (Source: http://www.files.chem.vt.edu/chem-ed/spec/atomic/interfer.html). Notice that BOTH of your blanks have phosphate, and remember that even DI water will have at least SOME calcium in it. Additionally, your added phosphate, KCl, and EDTA will, certainly, have SOME calcium in them, as well. If this phosphate formed complexes with calcium in your blank, what would happen if the air/acetylene flame did not have enough energy to decompose these complexes? What sort of signal would we measure? If your nitrous/acetylene flame has MORE thermal energy, would it be able to decompose some or ALL of the calcium-phosphate complexes? And, would that INCREASE or DECREASE your signal?

Also, notice that blank B had MORE phosphate in it than blank A, and remember that phosphate is a known interference for calcium. And your signal ended up decreasing. If we had more phosphate in this blank, what would happen to the calcium, and would we expect the signal to increase or decrease?

Lastly, I have one final suggestion. Check the slopes for each calibration curve--for the calibration curve using the air/acetylene flame and for the calibration curve using the nitrous/acetylene flame. Is the slope for the nitrous/acetylene curve larger? If so, by how much? The slope of a calibration curve is a measure of the sensitivity of a technique. The slope tells us how much change in signal we got for a given change in concentration. A really large slope means our method is very sensitive, because for a very small change in concentration, we get a very large change in signal, which means that it is easy to tell even small differences in concentrations apart from each other. The sensitivity, in this example, would help us to evaluate if the nitrous/acetylene flame was more effective at decomposing interferences that remained present in the air/acetylene flame.

I hope this helps. Everyone, please feel free to correct me, if you think I veered off into the wrong direction.

Offline vtdpada

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Re: False positive result on nitrous oxide flame of AAS
« Reply #10 on: November 02, 2013, 01:01:32 PM »
Hi marquis,
Yes, I used 422nm. But I don't have chance to repeat the expt.
Yes, I know the sensitivity depend on S/N ratio. Thanks.

Hi Wilson,
Thank you for your details reply.You not just guiding me, in fact you almost answered everything.
The only concern for me is 0.5ppm is really a large signal if only explained by contamination.
You are correct, the slope of N2O calbration curve was 3 time higher than air flame.

I also want to share my alternative thinking (may be totally non-sense).
For Blank A, the signal may be due to molecular absorption of LaPO4. Since melting point of LaPO4 is around 2000*C. C2H2-N2O flame have ability to volatile it but cannot decompose it so cause molecular absorption that overlap 422nm. But in C2H2-air flame, LaPO4 cannot be volatile so cannot absorb radiation.
But for Blank B, obvious cannot explained by absorption of EDTA since both flame have ability to breakdown it.

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