Chem Age / Chemical age mapping of monazite

[Probeman]

Hi Ben,
I'm very curious to see how the MAN methods works for traces in monazite samples.  I also wonder if you could just utilize some of your REE phosphates as blank standards to test the trace element accuracy?  Of course if you have some chem age standard monazites that would also work, though as you pointed out above, the chem age is somewhat less directly connected to composition, since the chem age depends on the ratios of the three elements (U, Th, Pb), as opposed to the individual concentrations.  I've seen monazite compositions that vary a fair amount, but still produce roughly the same chem age.

Again Mike Jercinovic and Julien Allaz are the experts on these questions.
john

[Probeman]

For those who aren't familiar with the MAN method it may be interesting to appreciate that the MAN method often gives very slightly higher concentrations for trace elements in complex samples.  This is usually due to subtle off-peak interferences. Which of course the MAN method completely avoids because it doesn't measure off-peak backgrounds (unless of course one is utilizing the off-peak interpolated MAN method as originally suggested by Ben Hanson at Dow Corning):

http://probesoftware.com/smf/index.php?topic=987.msg6455#msg6455

In simple matrices where there are no signifcant off-peak interferences, the MAN and off-peak methods are usually within statistics.  Unless of course there is a curvature to the continuum in that region of the spectrometer as shown here in a figure from my Amer. Min 2011 paper:



Here's how I usually explain why off-peak measurements sometimes produce lower trace concentrations to my students:  generally the highest average peak intensity and lowest average background intensity is what we would consider most representative of the "true" net intensity. This is because one can usually only lose photons in the peak measurement (by not being quite on-peak), and one can generally only gain photons in the bgd measurement (by having off-peak interferences).

So finding the highest peak intensity and the lowest background is generally the goal. Of course this is just a "rule of thumb" as in the real world there are also sample and detector absorption edges, holes in the continuum and other continuum artifacts.  But the cool thing about the MAN background method is that it doesn't care about any of the off-peak interferences or continuum artifacts because one is always measuring *at* the on-peak position only!   

[BenjaminWade]

Hi John
Yes I will let you know how it goes. I will probably used my shared bkgd mnz package and compare it with that. Hopefully get a chance later this month to try it out again. I ran the single element phosphates as standards at the time and none of them as unknowns as I wasn't thinking about blank corrections, so unfortunately cant use the dataset I already have for the blank correction. Will do it next time though.

Cheers