5. Now originally, when I first began presenting these trace element measurements using the MAN correction, several colleagues took me aside to say that I was missing something. And what they said was: it is all well and good to use the MAN background correction for trace elements, but that I must include the precision of the MAN regression in the sensitivity calculation for detection limits.
But here is the interesting thing: that would be completely true only *if* we were to re-measure the MAN calibration curve for every point or pixel, but in fact we don't. Generally we measure the MAN calibration curve maybe once every probe session, and maybe even less often than that, as the continuum intensities for a given emission line for a given spectrometer/crystal are extremely stable (unlike on-peak intensities!), and therefore the uncertainty in the MAN regression curve is basically an accuracy issue, not a precision issue. Which can be corrected using the blank correction!
If you need more convincing on this point, please check the MAN trace paper where we compared calculated sentivities with and without the MAN regression precision, with the actual observed variance in the background intensities, and the results are clear that we should not be including the MAN regression precision in the sensitivity for detection limits.
Is this making more sense? I hope so, because I certainly agree all this is rather unintuitive, as both reviewers of the MAN trace paper mentioned their reviews, even as they were convinced (I wonder who you are?).
6. The advantage of the MAN correction (over say the Nth point method) is that as the composition of the unknown changes, the calculation of average Z changes, and therefore the regressed MAN background intensity adapts automatically. So you never re-measure the background, but your background correction is always accurate.
7. Now in the case of trace element measurements, we have to deal with the fact that the accuracy of the MAN method becomes equal to the concentration below 100-200 PPM, due to various continuum artifacts, drift, etc. But by utilizing a standard for the blank correction, which has a known zero concentration, or even a known *non-zero* concentration of the element of interest, we convert that offset (measured under similar conditions as the unknown), from a concentration to a virtual intensity, which is then matrix corrected for the actual unknown composition (which is essentially no correction at all if the standard for the blank correction is a close matrix match to the unknown composition). But this means that the standard used for the blank correction doesn't really need to be a close matrix match to the unknown!
8. Now I typically tell people, yes, use the MAN and blank correction together for trace element measurements in simple matrices where one has a close matrix match to the unknown, e.g., synthetic SiO2, ZrSiO4, etc., but use the traditional off-peak (or even better use the multi-point background method), for complex materials such as traces in amphiboles, feldspars or monazite. But, if we consider the fact that because the blank intensity offset is automatically corrected for the difference in matrix correction between the blank standard and the unknown, this might not even be necessary to have a close matrix match.
9. Do I have any evidence for the above claim? Not a lot and we should perform some careful measurements using the MAN and blank correction on some (standard) complex materials to see how well it performs. But I do have this:
http://pages.uoregon.edu/epmalab/reports/Withers%20hydrous%20glass.pdfThis was an attempt to measure water in glasses by measuring oxygen directly (as first described by B. Nash), then calculating the amount of oxygen from cation stoichiometry, and subtracting them, resulting in an excess (or a deficit) of oxygen, which can then be converted into water or hydroxyl.
10. Now in the above example of measuring oxygen in hydrous glasses, even after I used the best MACs, corrected for peaks shape, and dealt with the intensity changes over time (TDI), my oxygen accuracy wasn't quite as good as I would have liked. So then I thought: wait, I have the blank correction! And remember, the element in the standard used for the blank correction doesn't have to be zero, it can be non-zero as well. And because the blank offset is matrix corrected from the blank standard to the unklnown, I really don't need an exact matrix match.
So I ran the NIST mineral glasses as a blank standard for oxygen (that is I measured it as an unknown under the same conditions as my hydrous glasses), but from photometry of the NIST glasses, I know the ferric/ferrous ratio and hence the total oxygen, I can "blank" correct the oxygen measurement in my unknown hydrous glasses. I guess probably should not be calling this a "blank" correction, since the oxygen in the NIST glass is around 44 wt. %!
11. Now, I'm not claiming that one should try to measure water in glasses by direct measurement of oxygen- it is a very difficult measurement to make, but it does demonstrate that the standard used for the blank correction might not need to be a close matrix match to the unknown, even in the case of oxygen which of course has a very large matrix correction!
Just some ideas floating around in my head this afternoon...
john