Please, in order to be more specific, you should give more details. What matrix? what is the content you will determine??

I guess ICP-OES or ICP-MS may be the best choice, if the matrix is aqueous or mineral.

Regards

Dear Anibal, the matrix is acidic aqueous solution containing rare earth elements (La, Sm, Eu, Ce, ...), other techniques no ICP. Many thanks 

I think you might need to be a little more specific still, in order to get a complete and useful answer. Anibal mentioned these points as well.

- Does the solution contain any elements other than rare-earth elements? This depends of course on what was dissolved.

- What is the expected order of magnitude of the concentration? A few percent? A few hundred ppm? A few ppm? Less? This makes a huge difference in terms of suitable methods.

- Is lanthanum the only element you are interested in or do you need an analysis of the full rare-earth element group?

- Is sample size a limiting factor? How much is there?

If the solution does not contain large quantities of interfering elements and the concentrations are high enough, I would suggest trying XRF (either by EDS or WDS). If lanthanum is the only element you need, then the spectral interferences are not so bad. If you need the whole suite, it will be more challenging.

Thank you Axel for your answer. Lanthanum element is in aqueous acidic medium containing some rare earth elements such as Sm, Eu, Ce in the nearly same order of magnitude and I have to do routine analysis, without ICP/AES since my apparatus is Shutdown. Concentrations are in the range 1 - 200ppm.