Hi all,

I've had a few questions recently about negative concentrations coming from the 3D Trace Elements DRS. The short answer for this is when the measured intensity is less than the intercept. The longer answer is below.

If you think of a traditional calibration something like this:

In the example above you can see that the fit line does not go through the origin (0,0). Any measured count rates that fall between the intercept and 0 CPS will have negative concentrations (in the area highlighted with the red arrow).

In equation form, concentration is calculated via:

[x] = (r - b) / m

where [x] is the concentration of element x, r is the baseline-subtracted count rate, b is the intercept and m is the slope (also referred to as sensitivity).

Seeing as we never have negative sensitivity (no negative slopes in LA-ICP-MS calibration curves... at least I hope never!) if (r - b) is negative, we'll get a negative concentration.

One way to avoid this is to force the calibration curve through the origin, which you can do in 3D Trace Elements using the (0,0) button. However, a negative concentration is telling you that either the count rate is indistinguishable from background, OR, that you have a poor calibration curve, or both. Which one it is will depend on your reference materials (RMs), the fit calculated, and the sample being measured.

In any case, if I find negative concentrations, the first place I look is the Block Plot:

I chose Ca in the example above because it shows a pretty common feature: both the RMs in this example have high Ca concentrations and we have relatively little spread in Ca concentrations. The intercept in this example is 1,590 baseline-subtracted CPS. If there was a sample with 1000 CPS, it would have a negative concentration. Forcing through 0,0 in this case makes sense because of the lack of spread of Ca values in our RMs.

If the Block plot doesn't look right, it could also be how your RM blocks have been detected. You can adjust this (manually if needed) by clicking the wrench icon in the top centre of the Block Plot (next to the block count).

I hope this helps. Please let me know if you have any questions.

Kind regards,
Bence