Method development for tricky acetone-containing samples on 7900 ICP-MS

Question

Hello, I have been struggling with developing an acquisition method for acetone-containing samples on a 7900 ICP-MS with x-lens configuration. The samples were digested with acetone and then treated with NaOH before neutralizing and diluting with HCl and water. I'm having to use an HCl matrix (1-2%), and I'm seeing high signal variability (jumpy ISTDs) and high RSDs in triplicate (plus zero reproducibility). I've been playing around with He flows and the KED threshold, but I haven't been able to improve precision. Originally, I was using general plasma conditions in no gas and He modes, but I've now switched to HMI configuration (He only) that slows down sample uptake and dilutes the sample spray with Ar. This is an improvement, but not by much. The samples have high organics and nitrates, and I've read cooling the plasma can help with organics, especially volatile ones like acetone. I'm already diluting quite a bit (~100x), and I will try cooling to 1200W (from ~1500W), but does anyone have other suggestions?

Efstathios.Sianos · Answer

Hi TonyGT, I think your variance is matrix related. Could you please inform us of your acetone concentation on the sample vial located in the autosampler? Too much organic highly volatile concentration can indeed affect your plasma conditions. I suppose also you are using the standard nebulizer and the standard torch, delivered with the instrument. Keep also in mind that high organic content can enhance the signal of your low ionizable elements, like As, Se. So I would suggest also a matrix matched calibration.

Efstathios.Sianos · Answer

Hi again TonyGT, I cannot tell much without seeing the ISTD variance that you are reporting. Your matrix looks really complicated so at the beginning I would say, that this looks like a matrix-effect. Interference like viscosity changes, oveloading the plasma with too much sample, lowering your plasma ionization efficiency with too much elemental content or even the signal enhancement for the not easily ionizable elements like Se, As due to high Carbon content could affect your measurement stability. Try to dilute your samples e.g. 10 times with MilliQ water and then observe your measurement stability if it improves this will explain matrix interference.