ICP-MS Calibration Question

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ToxLabRat
ToxLabRat's picture
ICP-MS Calibration Question

 For those of you out there working in the environmental field, specifically those of you using EPA 200.8 and 6020 as well as those of you operating under NELAC certifications:

Are you using a linerar through zero calibration curve?  If not then what curve type are you using and what are the factors that made you use a curve other than a linear through zero (I.e. regulatory requirements)?

Thanks

skgndri
skgndri's picture
Yes we are using linear

Yes we are using linear through zero curve. But dont know why? Can you please explain.

Charity Schmidt
Charity Schmidt's picture
 I am looking at doing

 I am looking at doing updated methods on our ICP 9000 and we are previously using the linear through zero approach.  The correlation is better with the simple linear and so I am thinking of switching to it.  As far as I can read on the EPA 200.8 method it is not specified.  What did you think?

ToxLabRat
ToxLabRat's picture
It looks like everyone is

It looks like everyone is using linear through zero.  I have not run across a single person using simple linear or wieghted linear.  From what I can tell, the EPA does not specify what specific curve type you use other than to say it should be an external curve....so simple linear, linear thru zero and weighted linear all satisfy that requirement.  I did some additional research and it appears that many state branches of the EPA require linear thru zero calibration types so I am thinking that is why most people use such a curve, as it is the only true way to set your baseline to account for noise associated with your blanks.  Since my state is one that requires linear through zero, I kept my curves as such.  So I would suggest sticking with linear through zero unless you get information in writing from your state, EPA, ISO or NELAC auditors telling you otherwise.

Also, I have learned that linear through zero is also the curve type used in clinical labs as well as pharmaceutical labs as well, so I think that is another strong indicator that it is the preferred external curve type.  I think weighted linear and simple linear are more suited to geological analysis and special project work.

Charity Schmidt
Charity Schmidt's picture
 Thanks for the heads up.  I

 Thanks for the heads up.  I will verify that before going further.  If you look at the data points on the curve, I believe it is skewing linearity of the curve to force it through zero.  I would like to know why the states are forcing the linear through zero.  

Sven1982
Sven1982's picture
Actually the force through

Actually the force through zero calibration curves does not force it through zero but through the measured calibration blank (or the zero point after blank subtraction). So the force through zero approach is the most sensible if you are suffering from an actual blank and not purely baseline noise.

A simple linear curve is never a good option since your high standards will have a much higher impact on the slope of your calibration curve (and also your offset and thus your bias), unless your calibration curve is made across a relative small concentration range with equal concentration intervals.

A weighted linear curve gives the best results when a bias is obtained for the lowest calibration standard(s), and gives - mostly - similar results compared to the linear through zero, since the lowest point will have at least the same weight as the higher standards.