Negative LODs from Longerich, Howell, and Petke

amzipkin

Hi folks,

I just ran into (for me) a new problem. Iolite is sporadically spitting out negative LODs values for a few of my analytes (Na, Mn, Fe, and Ni) in a study of SiC. I am familiar with the Longerich method generating "infinitely low" LODs when there is no variation in the background, but negative values are a new one. All LOD calculation options in Iolite are doing the same thing for those ablations where this problem crops up. Any ideas? Thanks!

Bence

Hi amzipkin ,

The only thing I can think of is that you have negative count rates for your element of interest or IntStd.

Would you mind sending us an example experiment where this happens so that we can get to the bottom of this?

Many thanks,
Bence

amzipkin

Bence Thanks Bence; I just emailed you with an example project file in which this problem occurs.

Joe

amzipkin
and anyone else coming across this problem -

The issue here was that the sample/reference material selections were lower than the nearest baseline selection for certain channels / selections, therefore the sensitivity stored for the affected selections was negative and consequently the LOD math works out negative. I will add a check for < 0 sensitivity and present that in a more user friendly way.

All the best,

amzipkin

So, I have a new LOD-related question. I don't typically use semi-quantitative data reduction through the Trace Elements DRS but am currently working on a project where an internal standard is unavailable. Does semi-quant mode calculate LODs? I have ticked the boxes for all three LOD calculation methods but after crunching the results data table only shows dashes where the LOD values should be. I have tried re-running the DRS a few times, checked to make sure dwell times are present (and correct), made sure an index channel is selected (either total beam or the matrix element 29Si)...but the DRS will only calculate concentrations and uncertainties, not LODs. Is this something I just have been unaware of for semi-quant mode or do I have an actual problem? Thanks!

Joe

amzipkin

The LODs should be calculated for semi-quant as well. If you're able to share your io4 file to the support email, I'll have a look. You could check for the "Sensitivity" properties for each selection (Data/Selections browser, then click on a selection). If these > 0, you should be all set, or at least the problem is further down the line.

All the best,

Joe

amzipkin
Thanks for sending the file. I can see now that having the internal standard channel set to Si causes the DRS to skip calculating the sensitivity for it even in semi-quant mode. I've fixed that now. In the meantime, check "Use Internal Standards", change the internal element to something else, and uncheck "Use Internal Standards", and you should be good to go.

All the best,

amzipkin

Joe Thanks! So, will there be a bug fix for the Trace Elements DRS in the next update? And, for anyone else who may be interested, this issue is specific to the original Trace Elements DRS because I was able to run the Trace Elements Next DRS and get calculated LODs.

Joe

amzipkin
Yes, the fix will be in the next release, which we are putting out before the end of the year.