Best way to clean a micromist nebulizer?

Hello,

To remove the possible blockage from the nebulizer, the first method would be to backflush the nebulizer with the cleaning tool in the reverse direction - so through the tip and out through the sample inlet.
Use a suitable cleaning agent e.g. methanol. Do not use excessive force when applying pressure to the syringe plunger.

Depending on what is causing the blockage, you may find numerous attempts may be required to remove the particle/s.

If this does not work and the blockage remains, try soaking the nebulizer overnight in concentrated nitric acid. Use a pipette to ensure there are no air bubbles trapped in the nebulizer capillary. After soaking overnight, then rinse with de-ionized water.

The following information on the Agilent Knowledge Portal can be a good reference for you:
Although this refers to the ICP-OEs nebulizer, the tips and recommendations provided here are also applicable to the ICP-MS nebulizer.
Cleaning the ICP-OES Nebulizer 

Finally as a reminder, NEVER sonicate or use a wire for cleaning a glass concentric nebulizer like the MicroMist.

Hope this helps you,
Cheers Eric