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Author Topic: Flow injection analysis strange peaks  (Read 1112 times)

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Bromate

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Flow injection analysis strange peaks
« on: March 04, 2013, 07:35:41 PM »

I'm getting these strange peaks for total nitrogen by persulfate digestion.
Samples and standards are digested with potassium persulfate and placed in an autoclave and then run on the FIA. But every now and then we get these strange double peaks that go all the way through the run and can't figure out why. We checked the digestion reagent, buffer, color reagent, cadmium coloumn, flow rates.. everything and cant figure it out.
Please help.  ??? :-\ :'(


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Bromate

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Re: Flow injection analysis strange peaks
« Reply #1 on: March 07, 2013, 05:29:12 PM »

Anybody? I've been struggling for months now. There's no pattern to it, it will work sometimes and then it will do this. I'm checking back everyday to see if there's a answer. :(
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Arkcon

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Re: Flow injection analysis strange peaks
« Reply #2 on: March 07, 2013, 05:38:10 PM »

Sorry, but I've never used this particular instrument.  Pay attention after the weekend -- some very helpful instrumental analytical chemists check in over the weekends.  If nothing happens by Monday I'll consider emailing them for you.
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Bromate

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Re: Flow injection analysis strange peaks
« Reply #3 on: March 10, 2013, 08:18:39 PM »

Thanks very much!  :)
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Stepan

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Re: Flow injection analysis strange peaks
« Reply #4 on: March 11, 2013, 02:40:38 AM »

Do you mean that the problem is doubled peak? In GC injection this happens if  you have back-flash: part of the sample traveled along with stream and gave you first peak, part of the sample was pushed against the stream, and after some delay, returned back and traveled in right direction (second peak).

Possible cause: excessive pressure in the injector line, sudden reaction between the injected material and injector parts,  Injection missed the stream and injected sample into wrong area.

Bromate

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Re: Flow injection analysis strange peaks
« Reply #5 on: March 12, 2013, 04:18:36 PM »

Do you mean that the problem is doubled peak? In GC injection this happens if  you have back-flash: part of the sample traveled along with stream and gave you first peak, part of the sample was pushed against the stream, and after some delay, returned back and traveled in right direction (second peak).

Possible cause: excessive pressure in the injector line, sudden reaction between the injected material and injector parts,  Injection missed the stream and injected sample into wrong area.

Yes it's the double peak. The FIA is a low pressure system. Only using a peristaltic pump to push the reagents. The peak lasts about 10 seconds. I can see how the theory of the back flash may influence the peak. But i don't think that much of a dip would occur. That dip lasts about 3-5 seconds before climbing back up again for the second peak. 
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Stepan

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Re: Flow injection analysis strange peaks
« Reply #6 on: March 12, 2013, 04:45:03 PM »

Back flash is easy to confirm. It starts to disappear when you reduce injection volume. It can be caused also by a dead volume (cavity) near injection point. Cavity where part of the sample is captured. 

Bromate

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Re: Flow injection analysis strange peaks
« Reply #7 on: March 21, 2013, 10:07:57 PM »

 ;D ;D ;D ;D

Figured it out!

The persulphate used to digest the samples precipitates on the bottom of the sample tubes.
The sample probe travels right to the bottom of the tube to pick up the sample and the precipitate gets sucked up along with it.
This clogges the reagent lines and causes the buffer to run slower, thus creating the double peaks.
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