Several companies have developed hand held units to aid in screening for RoHS compliance
Let me start off by saying that I think such a unit would be very helpful; however one needs to be a little cautious about what it can and can't do.
To the best of my understanding, these units use safe x-ray sources to fluoresce characteristic x-rays from materials. By analyzing the energy of the fluoresced x-rays the unit can determine what elements are present in the material being analyzed and approximate the element's concentration. The caveats are:
1. The units can only measure things as small as millimeters in size, but not sub-millimeter. For example, they can not accurately measure the elements in a single small solder joint.
2. The units will measure all elements in their field of view. So, if a solder joint is sitting on PCB material it will measure the elements in the solder joint AND the PCB material. So there is an "averaging effect" which is dependant on what is in the field of view.
3. Considering points 1 and 2 it is difficult for these unit to measure say 0.01% cadmium in a screw plating, or 0.1% lead in a sub millimeter solder joint.
4. The unit cannot tell the difference between hexavalent (banned) chromium and trivalent (acceptable) chromium.
5. All banned flame retardants contain bromine, which the unit can detect, but some acceptable flame retardants also contain bromine. These devices can't tell the difference between the two.
So what is the bottom line? These units can likely tell if a product is in gross violation of RoHS, e.g. it has tin-lead solder, or several percent of mercury, or cadmium, even in a physically small sample. The unit could also be used to screen alloys or even components. Therefore, they can tell if a product grossly violates RoHS. However, they cannot confirm that a product complies.
Based on my interpretation of the EU's "in homogeneous material" definition, it is likely that the only way one can demonstrate that a solder joint on an 0.4 mm PQFP is RoHS-compliant is to carefully excise the solder joint and analyze it with atomic absorption spectrophotometry. The only way to analyze CrVI vs. Cr III on screw platings is with a technique like ion chromatography. And the only way to analyze for banned flame retardants is by using something like infra-red spectrophotometry.
In conclusion, these devices have limitations, can produce false positives when testing materials for banned substances, and can't discern critical differences between similar substances like Cr VI vs Cr III and brominated flame retardents.
However, I still think such a unit can be helpful and I would be interested in performing tests using such a device. Fast screening of objects grossly violating RoHS is an important task!