Calibration issues Al, Na & Si


I'm having some persistent calibration issues for an ICP-OES analyzing Al, Ca, Fe, K, Na, Ni, P, Si, V & Zn in the range of 0.05 - 1 ug/mL.

The following lines (Al 237.312, Si 185.005 and Na 589.592  nm) keep failing the calibrations due to lowered measured concentration in the highest standard (should be 1 ppm but is measured at about 0.75-0.80).

The standards are prepared using a multi-element standard containing all 10 elements at the same concentration, they are all prepared in 9% HNO3, matched with the acid content of digested samples measured with this method.
The concentrations are as follows:

0.05 ppm
0.10 ppm
0.5 ppm
1.0 ppm

For the lines mentioned above, the highest standard repeatedly falls well below the actual concentration, the calibration standards were prepared 3 times, all with comparable results. I suspect the issue to either stem from the sample introduction or the optics, as all other elements form valid calibrations over the whole range.

What could be the usual culprits in this case? The pump tubing has just been replaced, as well as the axial pre-optics window. Both the nebulizer and spray chamber have been cleaned somewhat recently, but will give them a clean next week to be sure. I was wondering if the torch could also have an influence on this. Although it is cleaned every month or so with aqua regia, there are soms signs of devitrification and an unknown brown stain that does not come of with acid or alkaline detergent.

Any tips, pointers and idea's as to what may be causing this issue are more than welcome!

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