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standard additions methods "spiking"


liambob1
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So Im having a bit of trouble remembering how standard additions methods work and after going over my notes its still unclear. I know you have a sample of unknown concentration in a complex matrix so you add a sample of known concentration of the same type as the unknown and then somehow extrapolate the unknown concentration. I know its very simple but I cant seem to remember. If I remember correctly I did an experiment involving this and the concentration of iron in water. Can someone help clear this up?

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Well...you normally take a known amount of your unknown concentration material and then add a known amount of "spike". Then you measure with whatever technique you are using, and then work backwards.

 

For example, when I want to know the concetration/level of impurity of a sample I've made, I take a known amount of the mixture, add a known amount of dioxane and then get a 1H NMR. I can then compare the intergrals of the dioxane peak to that of my sample. Since I now how much dioxane i added and how much of the mixture I added, I can work out what level if impurity I have.

 

Another method, if you are doing LC-MS or HPLC, is to run your unknow sample to get a baseline chromatogram. And then add some authentic standard of those compounds and see whch peak gets bigger.

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"Another method, if you are doing LC-MS or HPLC, is to run your unknow sample to get a baseline chromatogram. And then add some authentic standard of those compounds and see whch peak gets bigger."

If you do that with known amounts of analyte then you are actually using the "standard additions" method.

 

NMR is a very useful method. It has one aspect that makes it particularly easy co calibrate- all protons give the same response.

That's not true for most analytical techniques where the response factor is different for different materials.

So I could use dioxane as a calibrant for an NMR, even if I was measuring toluene. But if I tried doing that with a UV detector or a FID I would get the wrong answer.

 

This should tell you a lot about the method (and why it's not always as good as people think it is)

http://www.rsc.org/images/brief37_tcm18-149213.pdf

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Yes sorry, maybe I didn't make it clear in my post. What ever you use to measure the sample would depend on the techique you were going to use. Dioxane is useful for NMR because all the protons are equivalent, but it would be useless for UV.

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as far as I remember we prepared 5 standards each containing 2,4,6,8 and 10 mls of a 100 ppm stock and then added 25 mls of the sample of unknown concentration ran each in an AA obtained a graph and extrapolated the line to give the concentration of the unknown sample, does this sound correct?. I still have the data somewhere if it would help

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What exactly have you plotted here?

 

And no, where the line crosses the y-axis tells you the absorbance of your mixture when there is none of your solute present.

 

What I expected to happen, was that you have an unknown concentration of x which you wished to know the concetration of. So you would have made up some samples of known concentrations of x and then measured the absorbance (which should be your blue points). They should then give you a nice straight line as the absorbance will be depending on the concentration of x.

 

So now that you have your graph and straight line, you need to measure the absorbance of the unknown concentration of x. Once you have that, you will then be able to use you straight line to correlate the absorbance on the y-axis) to a concentration (on the x-axis).

 

Does that help?

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If you don't know what he has plotted then you can't really comment on it.

The plot is the traditional plot for the standard additions method. It's a plot of instrumental response vs. added concentration.

The measured concentration is, as the OP says, the value of the concentration axis where the plotted line crosses it.

 

"What I expected to happen, was that you have an unknown concentration of x which you wished to know the concetration of. So you would have made up some samples of known concentrations of x and then measured the absorbance "

Why?

He already said he's doing this by standard additions

 

"Does that help? "

Probably not.

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If you don't know what he has plotted then you can't really comment on it.

The plot is the traditional plot for the standard additions method. It's a plot of instrumental response vs. added concentration.

The measured concentration is, as the OP says, the value of the concentration axis where the plotted line crosses it.

 

"What I expected to happen, was that you have an unknown concentration of x which you wished to know the concetration of. So you would have made up some samples of known concentrations of x and then measured the absorbance "

Why?

He already said he's doing this by standard additions

 

"Does that help? "

Probably not.

 

So I was right? Essentially what the methods consist of is making up standards of increasing concentration and then adding a definite amount of the sample of unknown concentration then plot the original concentration vs the instrument response and extrapolate the graph to get the unknown concentration? If thats right it an awful lot simpler than I thought.

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Yes, you were pretty much right as far as I can see. Did the site from the RSC help?

 

It did indeed, I've been trying to join there and there must be a way for Irishmen to do so since there is a branch here but I cant seem to find it

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