A portable XRF is a cruder version of what you would use in the lab. It's good for finding out what's there in terms of elements (gold, silver, copper, etc.), but it's not as accurate. To get accurate results, the sample (especially the surface) has to be carefully prepared in a standard manner so that results can be compared to results for known standard samples. You can do that in a lab and get accurate results, but not in the field. Field results will be good for determining whether each element is a major, minor, or trace component. Someone with a lot of experience can probably do a bit better, but still not near the accuracy you could get by taking a sample back to the lab. Nevertheless, they're pretty neat little gadgets. And I would love to walk through the Sinker taking some readings.