I used graphite tube AAS for my Pb analysis in a fish sample. My procedure as below
1. Dip all glassware and plastic ware in detergent overnight and wash in 3 times using distilled water, then dip 10% HNO3 solution overnight and then again wash distilled water dried
2. Sample (fish) homogenized using domestic food blender
3. Take 1 g sample and add 10 mL HNO3 and run microwave program (Mars CEM XP-1500, closed vessels system), after digestion complete volume up to 50 mL using deinionized water.
4. Reagent blank-just add 10 mL HNO3 only. As well as spiked samples, use 0.5 mL of 1 ppm standard solution
5. Then read GT_AAS
I have few questions regarding the my results
1. Many times my reagent blank value is higher than the sample value (But, I thoroughly check the cross contamination, unable to find that)
2. Duplicate sample results are not similar (some time the deviation is 2, 3 times)
3. Hence, spike recovery not coming within the range
4. Some AAS reading end up with letter D, what is that?
But in same digested samples is checked for Hg and Cd also, blank, spike and everything show very good results.
Please be advise or comment my questions
Sir, Please make sure your Standard solution of Pb is properly prepared. Sometimes the stock solution not mixed properly and gave wrong concentration calibration curve. I am not sure what is going on but defiantly something is wrong. I have no idea why AAS reading end up with letter D. Just want to know the reason of this.
Dear Aliya,
Thanks for your reply, but calibration curve and solution is ok every time, the absorabance value is similar with AAS cook book (guide) values.
Dear sir.
Good day, just one question, Are you using any matrix modifier to avoid losses by volatilization in the furnace stages ?. I ask this question because I think that maybe you observe this effect is due not to the preparation itself, rather to the measurement technique.
Do not know your computer brand, so do not know the meaning of the letter D; but I would ask you this: do you use for background correction, zeeman effect or deuterium lamp ?.
For measurements we have made of Pb and other analytes, we often use palladium modifier; or magnesium nitrate hexahydrate and ammonium phosphate modifier; with them we have obtained excellent recoveries as well as we also have been very useful in the analysis of heavy metals in air particulate matter PM10 and PM2.5 specifically fractions.
Our microwave digestion protocols involve the use of hydrogen peroxide in the case of organic matrices, which has allowed us to analyze human hair even with acceptable recoveries.
I hope the above it you can be of some use.
Dear Sir
Please chech your blank preparatios and avoid any possible contaminations.
Also may be all this troubles could be due to something wrong in your Pb. hallow cathod lamp since you do not have any troubles with any other metal analysis.
If you can get good results for Hg and Cd in the same digested sample then obviously you have a contamination problem with lead. Check for contamination.
Dear Sir
I entirely agree with the last comment. You have a contaminantion problem with lead only. I think you should pay attention to your material decontamination. I will make some comments in relation to your procedure according to the steps:
(1) After the detergent, you do not need to wash with distilled water, you can use tap water, but after the HNO3, you HAVE To use DEIONIZED water, washing the material thoroughly and then drying it protected from dust.
(2) I hope you also remeber to decontaminate the blender;
(3) I hope you also remember to decontaminate all the material you use to treat and weigh the sample as well as the microwave vessels;
(4) Always use a good quality HNO3 (too low content of metals). In general, HNO3 contains metals. So you need to pay attention to their concentrations. In fact, you must pay attention to metal concentrations in all reagents you use;
(5)Do you use plastic cups for the readings?? Are they disposed after use? If you reuse such cups you must pay too much attention to their decontamination.
Regarding your questions, I think I have answer almost all of them, material contamination. As I do not know which equipment and electrical conditions you have, I can not say anything about the "D". You need to have a super stable electrical power. You also need to follow the Stabilized Temperature Platform Furnace (STPF) conditions for accurated analysis and among them you have to optimize pyrolysis and atomization temperatures and use a chemical modifier. Well, I hope I have helped you any way.
Please avoid cotamination wash with 10pecent nitric acid and then deionized water for your glass used
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