Hi, I am trying to determine the concentration of Mg in a natural water sample b

Question

Hi, I am trying to determine the concentration of Mg in a natural water sample by using a calibration plot after finding the absorbance using AAS. These are the values obtained:

0.3949

Now, the absorption is found in minus. Shall I assume this is 0 and change the data for my graph or should I keep the data and fix the axis at 0? The latter option gives a calibration curve looking like:

Secondly by rearranging the calibration equation Mg= (absorbance +0.0014) / 0.0395 , the concentrations can be found:E.g. absorbance of 0.0371 (found in water sample), the Mg concentration = 0.975 mg/L.

Should the blank of -0.0006 (or 0 in the case of changing the data) be subtracted?

Any help would be appreciated!

Mg Concentration Absorbance (mg/L) 0 -0.0006 2 0.0744 4 0.1656 6 0.2255 8 0.3165 10

0.3949

styles | | Paste Format a Format ClipboardB Font | Alignment . G Number :Ta Cells Editing Chart 1 Magnesium Calibration Curve 0.4 5n Absorbance = 0.0395 [Mg] -0.0014 R2 =0.9982 10 0.0 h 0 2 4 10 12 12 13 14 15 Magnesium concentration (mg/L)

Explanation / Answer

Hi,

You should correct the absorbance column by adding 0.0006 to each value since the absorbance you get at 0 concentration is -0.0006. This is to correct any interference that results in -0.0006 absorbance at 0 concentration. You can use the corrected values for plotting and getting the equation. Please see table below:

After plotting, here is the equation of the line: Absorbance=0.0395[Mg]-0.0008 and R2=0.9982

By rearranging, [Mg]=(Absorbance+0.0008)/0.0395

By using your example: Absorbance=0.0371

[Mg] = 0.959mg/L

Hope this helps :)

Mg Conc (mg/L) Absorbance 0 0 2 0.075 4 0.1662 6 0.2261 8 0.3171 10 0.3955

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