Author Topic: Strategies for Improving Accuracy Using Trace Elements Standards  (Read 278 times)

Probeman

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This topic is for discussing trace element standards (whatever that term means to anyone), and how they might be utilized for improving the accuracy of our trace element analyses.

I'll present my thesis for improving accuracy of trace elements, but I would like to hear from others on their own strategies and of course we should discuss the advantages and disadvantages of these various approaches.

But first I think it's worth recalling that on the question of accuracy for trace elements we have the following items to consider:

1. The background correction:

Because the background correction is a subtraction, any error in the background is a direct error in the accuracy of the trace element. For example, a 100 PPM error in the interpolated background determination, say due to a small interfering peak on one of our off-peak positions or interpolation across an absorption edge, would cause a corresponding 100 PPM error in the net intensity of our trace element. So if we are measuring a 100 PPM trace level, we would obtain a result of around 0 PPM, and therefore an 100% relative error in our trace determination. The background correction is therefore usually the largest source of error in our trace element analyses.

2. The matrix correction:

On the other hand, the matrix correction is multiplicative. That is, a 1% relative error in the matrix correction is still a 1% error on our trace element analysis. For example if we are measuring a 100 PPM trace level, our 1 % error from the matrix correction results in a trace element value of 99 or 101 PPM depending on the direction of the matrix correction error.  Therefore, only a 1 PPM absolute error. So at trace levels, the matrix correction is generally a much smaller source of error than the background correction.

Now it should be mentioned up front that whatever (secondary) standards we are using to check the accuracy of our trace element measurements, we will still always be utilizing a (primary) standard that is usually a pure metal or a pure oxide. The advantage of a pure element or a pure oxide (primary) standard is that we can be confident of the accuracy based upon purity considerations only. For example if our Ti metal primary standard is 99.99% pure, its accuracy is known to 99.99% accuracy. Of course, in practice surface oxidation raises its ugly head with pure metals, so perhaps we want to use a pure synthetic TiO2 which is also relatively easy to obtain.

3. Next we can ask the question: what (secondary) standards can we be most confident with regard to accuracy of trace element measurements? I would propose that we can be very confident in the accuracy of so called zero blank standards. That is, standards that have a roughly similar matrix to our unknown, but do not contain any of the trace element in question. Of course if the blank matrix causes an interference correction on our trace element and our unknown matrix is similar, we can consider that the blank correction itself will correct for this interference (without the use of a separate interference correction). This is why we usually do not want to perform both a blank correction and an interference correction. Because we will get a double correction.

So why can we be so confident about the accuracy of zero blank standards?  Well for one thing, if the element in question is below the detection limit of our measurement, then the accuracy of that zero measurement is very high. In fact the accuracy of the zero blank is equal to the variance of the blank measurement itself.  In addition, when our blank is below the detection limit, the homogeneity of our blank standard will likewise be equal to our measurement precision. It will be very difficult to obtain or synthesize a non-zero trace element standard with such accuracy and homogeneity.

That is because the accuracy (and homogeneity) of a non-zero trace element has to be determined.  And how is that determined? Only by other techniques, say ICP-MS, which also has its own systematic errors. That is, if our ICP-MS reports 100 PPM, how sure can we be that it is actually 100 PPM and not say 95 or 104 PPM?  Do we have another opinion?  If so, what are the systematic errors of that method? And if these two methods give different results, do we simply assume that these systematic errors are random and therefore can simply be averaged?

Therefore I conclude that while it might be nice to have a high accuracy non-zero trace element standard, in practice this is a very difficult thing to achieve. On the other hand, if the trace level of our blank standard (say also determined by ICP-MS) is always below 1 PPM, then we can easily have confidence that our trace accuracy equal to our blank measurement precision. And synthesis of very pure (homogeneous) materials is quite easy compared to doped materials.

But let's discuss it because I'm sure there are many strategies for trace element characterization depending on the materials.

john
« Last Edit: July 20, 2017, 09:27:17 am by Probeman »
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Probeman

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Re: Strategies for Improving Accuracy Using Trace Elements Standards
« Reply #1 on: July 20, 2017, 09:21:50 am »
I'm bumping this topic because I fixed some typos and mistakes in the paragraph on background corrections.
john
The only stupid question is the one not asked!