I am trying to plot a graph with R^2 > 0.8 with increasing values of Cadmium chelate from 0,2,4,6,8,10 mg/l . According to a paper I have read Cadmium-Alizarin Chelate is used in this method so that is what I will be using as well . They have used H2SO4 solution just like me. The values that they have got are uniformly increasing for 0,2,4,6,8,10. My values however show uniform increase from 2-10 mg/l but a steep increase from 0-2 mg/l ,,,...
What can i infer from this ? When I plot a graph for these values the R^2 value that I get is 0.66 which is not enough ? (needs at least 0.8)
My blank (0 mg/l) is without the Cadmium-Alizarin Chelate
0 ---- 0.015
2 ---- 0.304
4 ---- 0.371
6 ----0.38
8 ----0.39
10 ----0.42
You don't include 0 mg/L in the calibration curve. You infer this value by the intercept of the linear function fitted to the data.
Camilo Morais Thank you Sir for your response
1) I would like to also know why however there is a steeper rise from 0-2 than there is from 2-4 or 4-6 or 6-8 , 8-10 ?.
2)Also could you please cite a source saying that we can infer 0 mg/l in standard calibration curve so that I can include it in my report ? I have searched for one but I'm not getting it perhaps I'm searching for it wrong ?
Hi Derek James Mathew
What I mean is that concentration cannot be measured as zero, because there is always uncertainty in concentration measurements. The absorbance at 0 mg/L could be extrapolated from the linear fitting equation assuming a linear relationship at low concentration.
Please verify if the experiments were performed correctly, that is, if the blank or background used to set the instrument software is the actual blank (solution without analyte). If yes, you can calculate the limit of detection (LOD) and limit of quantification (LOQ) of the method (somewhere between 0-2). You can expect a linear response after LOQ.
Some textbook material: Skoog (Principles of Instrumental Analysis).
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