[milica]

Hi, I am analyst working in one food analysis laboratory. I have certain problems with arsenic analysis by HGAAS. I am using microwave digestion (nitric acid, hydrogen peroxide). I suppose that all the arsenic from sample is in +5 oxidation state after digestion. But, when I do hydride generation AAS, I can not measure any of it. I know that  As in +3 ox. state make hydride better than +5. My calibration solutions are made of As+3. Would it be good idea to take As+5 calibration standards and then try to measure arsenic in samples digested by MW?
I hope that you could understand my confusing post  . Thanks in forward if someone can answer and help me.

[JGK]

Sorry, can't help you, our lab is using ICP-OES for all our metal analyses.

[milica]

Anyway, thanks, I hope there is somebody who uses HGAAS  , or maybe GFAAS, because I have that alternative, but as you know, sensitivity is not so good, it is o.k. for seafood where you can expect high concentration of As. But, I am interested, do you use hydride generation prior to ICP-OES? I am not so familiar with this technique.

[JGK]

No, we dont use hydride generation, the majority of samples we analyze are oils, by direct aspiration.

[jruckle]

Do you run your cal standards through the same microwave digestion method as the samples? If not doing so may help.

[cil-baha]

Hi,
It appears that your acid digestion is insufficient for As to be recovered from the matrix.  The very dangerous HF acid could be included with your HNO3 for digestion - read MSDS for HF before using, and have Calcium gluconate gel in the lab and at your bedside.  Dilute before contacting instrument and glassware.  Food is high in organic content, so you should include H2O2 as well.
Googling the literature shows that "The HNO3/H2O2 mixture was found to be the most suitable medium for sample digestion."

sciencedirect.com link

[cil-baha]

...whereas other lit. suggest increasing the microwave energy.
Or you have some other physical/technical problem.

[cil-baha]

What exactly is your sample?
Matrix effects seen - have you done a standard additions method to validate your method?
Is sample high in Fat? Defatten your sample --> http://www.fsis.usda.gov/PDF/CLG_TM_4_01.pdf

[cil-baha]

or are you using the Mercury line to look for Arsenic?  You should use the Arsenic line.
Ensure that the Arsenic line is used when using the HGAAS