The portable ones used in geochemistry are usually imaging spectrometers, not mass-spec, unless I'm missing something. I.e. they're measuring something entirely

The portable ones used in geochemistry are usually imaging spectrometers, not mass-spec, unless I'm missing something. I.e. they're measuring something entirely different. Sure, there are portable mass spectrometers, in the sense of something you can put a sample in and also carry around. But they still require getting a sample, preparing it, and putting it in the machine. They're not something you can scan with. Especially in geochemistry, "scannability" / ability to process large sections of a core without manual sample prep is a big deal. You usually have hundreds of meters to kilometers of core (thin cylinder of rock) or a deep borehole that you're trying to extract information out of alongside other measurements + manual logging. Lots of things need destructive sampling, so you will do more things that actually take samples and process them, but non-destructive "scanning-style" methods are a huge help.