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can any one help to understand about internal standard in ICP, what is a concentration of internal standard is to be used & how its intensity to be relate in spectrum.
in astm D 4951, what should be the concentration of internal standard & what will be the concentration of sample in diluents.

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Standards for calibration curve of cobalt and yttrium should be prepared in the same manner as for the other metals. It would be advantageous to prepare a low concentration curve first and run a sample(s) of the unknown concentration. If you have to dilute sample(s) you are testing to more than 100 times to fit it/them into the calibration curve, then it is better to establish new curve with a range of higher concentration standards of the element(s) of interest. For example, let’s assume you build a calibration curve for cobalt with standards containing 5, 10, 20, 50 ppb, and a 500 X dilution of tested sample reads 35ppb, then this sample contains close to 17.5 ppm of cobalt. The accuracy of this concentration depends on how accurate you were in making this dilution. In my opinion, rather than being “close to”, you would get more accurate result if you make new calibration curve with higher conc. standards (e.g. 2, 5, 10, 20, 50 ppm of cobalt) and rerun such sample without diluting it. The range of the standards (and control samples) for all elements of the interest is either set by the lab (most common practice) or it is arbitrary set by the analyst based on the concentration(s) range repeatedly seen at majority of tested samples.
Hi Sai, regarding internal standard concentrations , you need enough concentration to get a consistent intensity reading time and again. So the concentration will depend on the element, the line selected and instrument / method parameters. A higher dose may improve reproducibility but will cost more. However you should be able to get away with around 5 ppm on a reasonably sensitive line. You should see a RSD of less than 5% for internal standard when comparing sequential samples. I would suggest trying lower levels if 5ppm works out for you.

You may or may not be aware but many commercial labs do not use internal standards. They have real benefits but real disadvantages as well. On the disadvantage side, they add an extra step in the process, additional cost and if the dosing is not properly controlled, decrease the accuracy of results. On the positive side, if dosed correctly will improve the accuracy / repeatability of your results. They are not a magic wand though and some problems on ICP affect elements differently and what might cause a problem for some elements resulting in a loss of sensitivity will not affect other elements to the same degree. Therefore using an internal standard does not negate the need to run frequent QC samples.
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