Hi all,

I have two different questions I hope it’s ok to put them into one topic.
My first question is about negative trace element concentrations. I’m analysing sulphide droplets by line scans which sometimes occur as inclusion or next to a mineral. The line scan starts in the surrounding material (e.g., glass), then crosses the droplet and again ends in the surrounding material (e.g., oxide). I’m using the signals of the surrounding material for signal correction and to determine the “surrounding material corrected” element concentrations in the sulphides. Most of the elements seem to be ok but some calculated values are negative using this method. However, I’m also getting two different types of negative values. The first ones are values that should be near or below the detection limit, as they occur for elements that are difficult to detect due to their low abundance (e.g., PGE’s). This is also visible in the time series window, as the signal is not or only with spikes above the baseline. Could it be possible that I’m getting negative values if the LA-ICP-MS had problems in detecting such low abundant elements? Or do you think it is something else and the concentrations should at least be a positive value below the detection limit?
And the second type of negative value is due to the surrounding material (that’s what I have thought so far) because sometimes the surrounding material (for instance an oxide on one side of the droplet) displays higher cps values for some elements compared to the sulphide regarding the input signals in the time series window. By using the glass (on the other side of the droplet) and the oxide signal for signal correction I’m also getting negative element concentrations. This is also the case if I’m only using the glass for the correction, which has a lower content of chalcophile elements compared to the sulphide. My question for this issue is if you have an idea of what I’m doing wrong, or what I could try to avoid negative values if the surrounding material also incorporates the element of interest?
After having these issues, I tried the Sills software (it’s not my object to compare iolite with another program). By using this software, I did not get any negative value. I guess this is due to the function, that I can type in an internal standard for the matrix and the inclusion in sills. What I did in iolite was, select an area before and after the inclusion (as a baseline group) and separately (as a sample group) the sulphide as the inclusion. Doing this, it is not possible (and maybe not necessary?) to define an internal standard for the baseline group, is it? I hope that was explained understandably... However, I think I still fail to understand how iolite is doing the signal correction for inclusions causing my problems in understanding the negative values.

And the second question is more a general issue. Sometimes I come across the problem that I’m only getting 0’s for all my element concentrations after crunching the data in the trace element DRS window (interestingly this happens only for one line scan per measurement day, all other line scans measured on the same day are ok). The input signal in the time series window just looks like it should but after crunching the data it is creating an output channel that lies on the 0 line. Also, the result window and exported data display 0 for all the elements of this one line scan. The problem could be solved for one line by renaming the original csv file. However, this worked for this line-scan only and not for all the other lines with 0’s. Furthermore, the labelling of my samples appears not to be a problem (at least it’s not the only one I guess) as all the other line scans show appropriate element concentrations.

Many thanks in advance and I would appreciate any suggestion.