This method does not use liquids or gels, but instead, it shoots the item to be tested with radiation and provides a readout on a screen. X-ray fluorescence is commonly called XRF. This method is by far the easiest and cleanest method of testing. Due to the technology, it is also the most expensive electronic testing method. And it works with silver too.
There are a number of suppliers of hand held XRF guns including Oxford Instruments, Olympus, Thermo Fisher Scientific and Bruker.