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Calibration Curve Linearity Issues - Sodium

Question asked by agricult on Aug 8, 2020
Latest reply on Aug 13, 2020 by tmcsweeney

Hi, I'm doing so multi-element development work, constructing calibration curves for Fe, Zn, Mg, P, K, Ca, Na at different ranges. I have been using a home made multi-element standard for this work. I've been dividing the elements into groups based on their calibration ranges. So my process is I form the calibration curves, rerun some standards as samples, run a independent QC from a different manufacturer (Agilent Intelliquant cal. soln#1) and also a Seronorm matrix reference material. For Fe, Zn, Mg, P,K curves form okay at selected wave lengths and pass all the curve checks but the problem is Na. When I form curves for Na at 10-60 ppm or 20-120 ppm using lines 589.592 nm and 588.995 nm I run into problems.

For line 589.592 nm the independent QC is reading 30% above its actual known mid-point concentration on the curve (40ppm or 60ppn), while the reference material value seems okay but its uncertainty is quite wide so that isn't really useful. At the lower end of the curve it reads more closely to the true value which is telling me my curve isn't very linear. For line 588.995 nm the top part of the curve doesn't seem linear as the value returned is quite high compared to the expected value. 

 

This is suggesting two things to me:

 

1. I have a downward curvature of both curves, but more pronounced in 589.592 nm which is only evident when I use a independent standard at the middle and top areas of the curve.

 

or

 

2. My independent standard could be faulty and just contain more sodium than its stating but I don't think this is the case as this is a Agilent produced multi-element reference standard and it has been giving the expected value for all the other elements checked. 

 

I'm using a ICP-OES 5100

Measurement conditions: Radial view / Neb flow 0.60 l/min / RF power 1.2 / Viewing height 2 mm / Read time 10 sec

 

I'd appreciate anyone's feedback on what I could do to resolve this issue.

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