[Kate]

Hello.

I have 2 questions about the standard addition method:

1) I did an experiment (devised by a teacher) in which the standard was increasingly added to all the solutions being analyzed. But while researching about this method I found that in the first solution being analyzed, only the solvent and the sample should be present. So what I did is wrong and I'll get a different final result because of it?

2) I plotted the absorbances against the concentrations of the standards and then I extrapolated to y=0 to get the concentration of the analyte in the sample. But I don't understand why this works. Why do I do this and get the concentration of the analyte? Could someone explain this to me?

Thanks.

[kriggy]

2) I plotted the absorbances against the concentrations of the standards and then I extrapolated to y=0 to get the concentration of the analyte in the sample. But I don't understand why this works. Why do I do this and get the concentration of the analyte? Could someone explain this to me?

Thanks.

2) This seems weird to me. Shouldnt you use absorbance of unknown sample as y and then calculate x to get concentration?

[Archer]

Standard addition is used when the analyte conc. is below the limit of detection quantification of the instrument, i.e. when it is hidden or semi-obscured by the noise of the instrument.

By adding the standard to the analyte you are lifting it above the noise with a known amount of the same substance so you are reducing the interference of the instruments noise.

You have to add it to the blank so you get a true 'zero' for your analyte.

Does this make sense?

[Kate]

Standard addition is used when the analyte conc. is below the limit of detection quantification of the instrument, i.e. when it is hidden or semi-obscured by the noise of the instrument.

By adding the standard to the analyte you are lifting it above the noise with a known amount of the same substance so you are reducing the interference of the instruments noise.

You have to add it to the blank so you get a true 'zero' for your analyte.

Does this make sense?

Yes, it makes a LOT of sense now. Thanks.

So, in that case I don't have a true "zero" for my analyte. So the result I get when I set y=0 will be incorrect and the experiment is flawed?

One could assume that, in the presence of that missing solution, the variation of the signal would still be linear and so the result would be the same.

2) This seems weird to me. Shouldnt you use absorbance of unknown sample as y and then calculate x to get concentration?

You do that in a simple calibration method. This method is a little different.

http://en.wikipedia.org/wiki/Standard_addition

[Archer]

As I understand it you perform a standard calibration curve with out your analyte and then make up a calibration curve with your analyte. you can use a single point calibration but this makes the method less accurate (although considerably simpler).

The two results (whether it be a single point or your Y intercept) should differ by the analyte present.

This method is flawed if the uncertainty of the method (standard deviation etc.) Is higher than the analyte concentration. For this reason I have never found a use for standard addition.

I have only commented on my interpretation of standard addition method, I am not sure I fully understand the wiki explanation.

[Borek]

Standard addition is used when the analyte conc. is below the limit of detection quantification of the instrument, i.e. when it is hidden or semi-obscured by the noise of the instrument.

Perhaps it is one of possible uses, but not the basic one.

In a way standard addition means "preparing calibration curve in situ" - that is, not in a separate solution, but in the analyzed solution.

Unfortunately, I can't think of any better explanation than the one given in wiki.

[Kate]

Also, when the matrix interferes with the signal, you add the standards to the sample to correct the effect it has.

Thank you both for the help.