I have experienced a problem regarding the estimation of arsenic(III) in AAS. I placed some water (MiliQ) samples treated with functionalized Multi-Walled Carbon Nanotubes (f-MWCNT) in HG-AAS. Arsenic was converted to AsH3 by NaBH4 and HCl solution withing the machine. Surprisingly the AAS is showing higher values of As even greater than the concentration of As before treatment with f-MWCNT. This problem does not occur for other arsenic solutions.
AAS can be species-dependent and is also pH-dependent. Which chemistry, pH, and expected As-species do your calibration standards have? It is probably different than the final As-species that are determined 3-4 higher than they should. The sensitivity of the AAS is sometimes strongly dependent on the sample matrix, analyte speciation, and pH. Have you tried digestion of your samples? If you know the speciation of your sample (of As ions/complexes), its pH, and solution matrix, you can simply calibrate the instrument with As standards of similar chemistry. You can also try digestion (to remove organics or change the analyte speciation) of your samples to get them in a comparable state with the calibration standards. Finally, dont forget that kinetics can play a role in speciation, which means that precipitation, the formation of certain As species, or interfering ions/complexes in either the calibration standards or the samples are time-dependent.
Could provide the determined value? Maybe, the f-MWCNT you used was polluted by As, or some other reasons, such as experimental error.
There is no chance of arsenic contamination in the f-MWCNT. I have carried out EDS, FTIR and XRD study and the result I got did not suggest any contamination of As within f-MWCNT. After carrying out the adsorption study, the solution was filtered with 200nm pore-sized CN-membrane filter paper.
The AAS showed 3-4 times higher concentration than the initial conc. of As. And it happens with these samples again and again!
AAS can be species-dependent and is also pH-dependent. Which chemistry, pH, and expected As-species do your calibration standards have? It is probably different than the final As-species that are determined 3-4 higher than they should. The sensitivity of the AAS is sometimes strongly dependent on the sample matrix, analyte speciation, and pH. Have you tried digestion of your samples? If you know the speciation of your sample (of As ions/complexes), its pH, and solution matrix, you can simply calibrate the instrument with As standards of similar chemistry. You can also try digestion (to remove organics or change the analyte speciation) of your samples to get them in a comparable state with the calibration standards. Finally, dont forget that kinetics can play a role in speciation, which means that precipitation, the formation of certain As species, or interfering ions/complexes in either the calibration standards or the samples are time-dependent.
Les causes peuvent être diverses. Il y a peut être des organe arséniés ou il y a des effets de matrice.
Des solutions:
- effectuer le dosage sur des solutions de plus en plus diluées;
- employer la méthode des ajouts dosés.
cordialement;
pierre Benoit
we have also experienced the same problem with Chromium.....later we found out we have to get the proper calibration graph where the values should fall in the sample range values.....may be its one of the reason......
La courbe de calibration n'est peut être pas linéaire, pour des faibles valeurs. Depuis que nous employons l'ICP, nous n'avons plus ce problème, tou étant dissocié à la température du plasma d'Argon.
Cordialement.
AAS is not proper method to detect of Arsenic, you can use another instrument such as ICP for this.
Les raies d'émission de As, sont peu intenses. D'où la nécessité de travailler en torche horizontale, en augmentant le temps d'injection. Cela permet ainsi d'éviter la spectre de masse, onéreuse, derrière l'ICP.
Any voice or vibration coming around will increase the concentration value, you get from the machine. This machine is very sensitive to vibration thats the reason it is preferred to place it in a separate place. Follow absorption values you get from the machine and get the reading when it is stable. I also get sometimes unexpected high concentrations from the machine, if you are sure that this is not what you expect, you should let system to clean itself with distilled water for a while and then feed your sample again. AAS-HVG is a suitable method for measurement of As however it is quite difficult!
Thank you all for your precious comments. We would surely keep these in our mind when we will work on it next time.
Using HVG and prior preparation with KI & HCl before 20 min , the said problem has not been found during As analysis on AAS.
As to get best result of Arsenic analysis, it is better to go for ICP-MS. Compare the result with Arsenic standards. Also observed with AAS and compare the results.
I have tried to analyse my external proficiency sample especially dry sample for As using AAS and failed. I don't have ICP or HG sys. Why AAS work perfectly for Pb n Cd but not As.?
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