Most items may have more than one standard analysis method and in part based on your lab capabilities and equipment.  It has been to long for me to tell you, but the methods are typically published and updated with time, often specific to individual heavy metals.  Years ago, we would weigh a dried sample in a ceramic boat, and put into the gas chromatograph, and for some things that released a measurable signal.  Other things we had to dissolve the ashes in t boat in acid solution, and maybe a few more steps to do flame emission or atomic absorption spectrophotometer results.  Others chemical there were colorimetric tests and analysis with colorimeter.  Hopefullyy there is a book now

In our lab, we use HNO3 at 180 degree (celsius) for 6 hours to digest dry plant material.

Hi Noomene,

1. Nowadays, many researchers use Flame Atomic Absorption Spectrophotometer to measure Heavy metals conc. in plants. [see movie in Youtube: https://www.youtube.com/watch?v=-fCX8OFBO-A ]

2. I also attached 3 research papers for your references. The first paper (pages 2-4, yellow highlighted) has a detail description of how to collect, prepare and treat the plant samples before use for Flame Atomic Absorption Spectrophotometer measurement.

thank you