Hi,
I expect a concentration of 5 mg/L of Nitrate ions in a deionized water sample, containing only one solute i.e. NaNO3. How many points should I take and what respective concentrations to determine the conc. of unknown sample using the calibration curve?
Dear Tariq,
In our lab, we almost always use a five point calibration for all instruments. We've found that three point calibrations often lead to slope errors. After lots of experimentation, we settled on 5 points. You'll need to determine the detection limit for your method, but my guess is that you could use a 2 mg/L (but you will have to test this). The other issue is how "curved" the calibration curve is for the method as you want to calibrate the approximation of a straight line section. If you are fairly certain of the 5 mg/L concentration, then you could calibrate 2, 4, 6, 8, and 10 mg/L.
We have actually tested this on most of our instruments by calibrating standards over two to three orders of magnitude (0.1 to say 100 mg/l) then running those same standards as unknowns and graphing the results. You'll the "curve" in the calibration. You can also graph response vs concentration and see a similar result. This is a great exercise for students to better understand concept of a calibration "curve."
Sincerely,
Brannon
We also use at least 5 points of a curve, of course you can use more than 5, in our nitrates method we use 6 points 0.5, 1,5,10,15,20, 30 mg / L, but we also use a control of quality of 10 mg / L (the nitrate ion of this control comes from a solution other than the curve) you could use a 5 mg / L with that you would corroborate the concentration you expect.
I hope to help you a little.
There are basically two methods for measurement of analytes in environmental matrices i.e. calibration curve method or standard addition method. The first method is preferred when the standard and sample are comparable or equivalent with respect to their Physico-chemical parameters. When the matrix effect is anticipated, the standard addition method (SAM) is preferable. Typically, three-point calibration is practiced in analytical chemistry but five-point can be adopted with a rider that the analyte concentration within the calibration range of 20 to 80%. If the highly accurate measurement is desirable and the matrix difference is there, SAM is recommended. If required, more details may be sought.
In most of the UV Vis spectrophotometer, the nitrate is determined if standards calibration concentration is taken upto 25 mg/L , one may prepare standards within this range of nitrate, may be 3, 4 or 5. If SAM is done then also we must see that the range should not exceed 25 mg/L.
- A standard 0 must be included in the calibration curve because the instrumental signal is subjected to the same kind of error for all points. More the points will increase the accuracy of the unknown sample results. Also Please don't forget to take the reading of samples at 220 & 275 nm both specially for waste water samples.
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