That compressed gas duster is pure r134a, the same refrigerant that's used in automotive refrigeration systems. Lotta people are surprised by this, normally associating r134a with bad nasty flammable ozone depleting baby killing substances. They call it by it's chemical name on the back of the can which avoids this issue.

The resulting white frost could be any # of things, ranging from frozen carbon dioxide distilled from the air to frozen moisture and anything else present.

Careful, it'll give you frostbite real quick. It's non-flammable though and unlike freon, isn't too much of a suffocation hazard. You don't wanna displace the oxygen you're breathing with anything though, even water will kill ya.

- Keman

I guess you ran out of dust? ;-)
Dry ice sublimes at -78.5C, so either you are getting temperatures a lot colder than what you are measuring or that is some other substance.

Possibly IR from the room was also reflecting off of the surface of the frost. Interesting - your reading was almost exactly halfway between room temperature and dry ice temperature (just from guesstimation, I think that is pretty much what you'd expect if your reading came from reflected IR; a little colder since nothing is perfectly reflective). I'm not sure what the IR reflective properties of recently condensed dry ice frost are, what wavelength your thermometer uses, and I'm not sure how thick a layer you produced... Try freezing something else at the same time and hold it over the area of the desk you are freezing to shield it from the room's radiant heat while you measure, and you should get a lower reading if it is reflected IR. If not, then you know it is from the desktop itself.