Respected,
I was doing analysis of aluminium, present in filtrate of dust water sample. The filtrate samples are solution of dust water collected. i have run the samples parallel to standard solutions of different concentrations ( i.e. 0.25 ppm, 0.5 ppm, 0.75 ppm, 1.0 ppm,1.25 ppm, 1.5 ppm and 1.75 ppm). About 90 samples were analysed with these concentrations of standard solutions. The results showed for some sample negative values and for some higher than standard concentrations. usually negative values lies from 1.0 ppm and higher values are up to 4.0 ppm. What could possibly be done to get all values with in range as well as positive value.
1) should I change the standard solutions concentrations? if so then what could be the concentration?
2) if still the values become negative should I consider as zero?
Kindly, help me to get through this.
Thanking You.
Hello Zagra,
Many times its happen due to problem with standard solution. If its not your specific requirement, you can make standard solution of 1ppm, 3ppm, 5ppm and 7ppm, and make sure the initial graph showing concentration of standard solution is not much deviated than the actual entered valued (i. e. 1ppm, 3ppm, 5ppm and 7ppm ).
Dear Zahra Mohammad Ilyas,
It seems there is some problem in your standard solutions. Always use same grade of reagents and chemicals to make standard and digestion of any sample. You should calibrate your instrument with blank first and then with standards. As Mr Rahul said , you also increase the level of standard solution.
Good luck.
I am grateful to Mr. Rahul and Mr. Pankaj. Really appreciated for the reply. i will try this out and give up the update. For Zinc, Lead and Cadmium can i use same concentration of standard solutions?
thank you once again.
Hello Dear Zagra,
You can do the same way for Zinc, Lead and Cadmium also, unless you measuring below ppm level.
Hello zahra
It seems that your standard are not well prepared as 1.75 ppm is your highest std and you got results upto 4ppm. First, you should check that you get a calibration curve as per std which is given in the book of your aas instrument. Filter your standard solution before using in aas. Also the spectra wavelength on which you are performing your experiment is as per your standard. Negative values comes when concentration is below your least standard in the solution. After analysis of your sample always check the conc of known sample to get a confirmation of the results.If you are preparing standard as mention like 0.25 ppm always dilute it with lowest concentration of metal solution otherwise there are chances of error.Also know how much amount you have to dillute the samples and prepare std as per your dillution.
Did you digest the samples? Al will normally need heat and acid to be soluble.
Is your standard response linear? Have you estimated the detection limit? Have you run a blank?
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