Discussion Category:  Atomic Absorption

Replicate measurements with FIAS 400

I'm doing a mercury cold vapour analysis with a Perkin Elmer FIAS 400 equipped with an AS90 autosampler and Aanalyst 100 spectrometer. The reducing solution is 1.1% SnCl2 in dilute HCl. For all of my calibration standards and sample measurements the first sample is consistently higher than the rest by a factor of about 2. I've tried changing the fill time on the FIAS to 20s from 10s, but am getting the same results, so I don't think the problem is with the sample loop not filling completely on replicates subsequent to the first. Any other ideas?

 

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Asked by

VJM
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Ridha

Dear Sir

Mercury is difficult to determine by flame at low levels. The cold vapor method is much more sensitive because the flame background is eliminated. The mercury is freed from any organic matter by digesting the sample with  potassium permanganate and potassium persulfate in acid solution .This also oxidizes any chloride ion to free chlorine , eliminating it from the sample.The excess permanganate is removed with hydroxylamine hydrochlorine . Then the mercury salts are converted to free mercury by reaction by reaction with stannous chloride .

The free mercury has sufficient volatility so that it can be sparged out of solution . The sparged gas is passed through a quartz -windowed tube placed in the light path of the atomic absorbance spectrometer . For even more sensitive determination of mercury , the sparged gas may be passed throw a gold  plated  mesh. This collects the mercury as it amalgamates with gold .When the sparging is complete , the gold mesh is heated rapidly, releasing the mercury in a sharp peak. Detection limits of ng/l have been achieved with this method.

Ridha BEN OTHMAN
rdmekh@gmail.com

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VJM

Thank you, Ridha.  The cold vapour method is exactly what I am doing.  

My question is why the first replicate measurement on any given sample would be consistently higher than any subsequent replicates, which are all consistent with each other.  For example, a 10ppb Hg2+ solution might give absorbances of 0.11, 0.058, 0.060, 0.059.  Then right after this a 20ppb solution will give 0.23, 0.13, 0.14, 0.13 etc.  This happens with every calibration standard and sample that I run.   

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David_Banner

Hi VJM,

I don't have any experience with Hg cold vapour; I am running As via HGAAS with a set up similar to the one you described (with exception to the Aanalyst 400 we have).

I have had issues similar to yours in the past, and one of the things that I tried was to experiment with changing the FIAS cycle values. I see that you already changed the prefill time; have you tried increasing the Fill step time?

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VJM

Hi David,

Thanks for the thoughts.  I haven't changed the prefill time at all, just the fill step.  I'll try some fiddling with the other times and see if that helps.

Val

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Andrefelilo
I currently have the same problem, how did you fix it
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