Calculating ratios

RomainT

Hi guys,

I'm trying to put togehter a Pb isotope DRS to process triple quad Pb isotope data. In doing so I have noticed that the ratios (say 206Pb/204Pb here for example) calculated in Iolite differ from the ratios one would calculate by simply dividing the Pb206_CPS intensity by Pb204_CPS intensity as reported in intermediate channels.

I'm struggling to understand why, so any help would be much appreciated!
Thanks!

PS: I can forward the .io4 and .py files if that helps.

Joe

RomainT

Unless I am misunderstanding the problem, I believe this is likely that the mean of A(i) / mean of B(i) != mean of A(i)/B(i). Consider the data below:

Where I've just generated random numbers between 0.8 and 1.2 for A and between 0.4 and 0.6 for B. The ranges do not matter. You can see that the average of the individual A/B values is 1.79439. But if we calculate the average A / average B it is 0.90800 / 0.51100 = 1.77691. Is this the type of thing you might be seeing?

All the best,

RomainT

Many thanks Joe, I guess this may be what's happening.

Just to double check, here is a screen grab of some BCR-2G Pb isotope analyses, with the 206Pb/204Pb ratios calculated in Iolite, and manually by dividing (mean Pb206_CPS) / (mean Pb204_CPS):

BCR-2G

What do you think?

Many thanks
Romain

Joe

RomainT

I think that is probably it. I can double check your code if you like? If yes, you can post it here or email it to support.

All the best,

RomainT

That would be awesome, thanks Joe. I've attached the .py file.

Cheers
Romain

pb-isotopes.py

11kB