I have made the suspension of FexOy nanoparticles in DI water and with known concentration checked in ICP-MS instrument. After the digestion with EPA method 3015A for the aqueous solution the sample is checked in ICP. I have taken the concentration of say 50ppm. When after digestion diluted it 50 times and then checked in ICP, I got 300 ppb approx. I have done the triplicate but I got the same measurement.
What is the mistake I am doing I am not able to recognize, or is their any other method to know the concentration. Please if there is any suggestion let me know.
Hello Dibyanshu:
I apologize for this trivial question/answer:
You mention that the concentration of your original FexOy nano particles is 50 ppm.
After digestion and dilution (50X) you consistently get 300 ppb Fe (presumably).
When you correct for the dilution factor, it turns out to be 300x50 = 15 ppm Fe in the original suspension..
Have you taken into account that the first material is in oxide form, and what you measure is only Fe ? Any other impurities?
Regards,
Nimal
Hi Dibyanshu,
You also need to check the linearity of you method
If you put 50ppm into the icp-ms maybe the calibration is not linear anymore at that high concentration
What internal standard are you using?
The diluted concentration maybe closer to the real concentration of your sample is the method has good linearity at 300ppb (15 ppm Fe)
Leonardo
Sir Leonardo,
Standards I have used for ICP was 5ppb, 25ppb, 50ppb, 75ppb and 100ppb. After digestion of my sample I have diluted it for 50 times . I am just checking the protocol, if it is working or not. So, I have taken the known sample of 50 ppm ( in which the iron contained was 36.5 ppm) and done the measurement . For which I got approx. 15ppm , I have done triplicate of this method but the result was same.
36.5ppm concentration but in measurement coming 15ppm is big difference.
Nimal Sir,
You are write that the nanoparticles I am using is Iron Oxide (FexOy ). In 50 ppm sample my Iron contained is 36.5ppm calculated, but still on measurement it is coming approx. 15ppm. I have done in triplicate but the results was almost same. The difference is very large instead of 36.5ppm it is coming 15ppm. How to solve this problem?
Hello Dibyanshu:
A couple of suggestions:
1. Are you sure, that your FexOy nano-particles are pure (even moisture)? Or, sure if the digestion is complete?
You can do a spike test to verify these.
Accurately add a known amount of Fe at a level comparable to what you measure(e.g. 15 ppm) to the original suspension . Digest this spiked solution along with a portion of the original suspension (preferably in replicates) and dilute both by 200 fold Measure by ICPMS. If you get 75 ppb (300ppb, if you diluted by 50X, as you already did) for the original and 150 ppb (600 ppb if you diluted by 50X) for the spiked solution, that means there is no problem with anything beyond the digestion. Then the problem should be one of the above.
2. You can also do a colorimetric test, as you are dealing with ppm levels (without dilution). If you don't have easy access to a colorimeter, you may be able to do a visual examination of the color with a couple of standards (at that level).
Best wishes,
Nimal
Thank you sir for your help. I will do it once and check my procedure.
Hello Dibyanshu:
Can you explain the EPA method sample preparation procedures you are followed..?
Adarsh Sir,
Its EPA 3015A method for aqueous solution in which ratio of sample and acid is 9:1. In this method for 45 ml sample 5 ml acid is used (i.e. HNO3:HCL:: 4:1).
There are chances that , samples may not be properly digested. You can cheak that possibilities also
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