Phoenix perchlorate kerfuffle

A. J. S. Rayl has an excellent article, with just the right balance of humor and fact, on the Phoenix perchlorate business here. I was waiting for her to post before I added any commentary about it. I'll just add a few other random facts that I got from the press briefing and from email and phone conversations afterward. (If you haven't checked out the recording of my Ustream presentation today, watching that might make the following discussion a bit clearer.)

One of the more surprising things I heard in Tuesday's press briefing was TEGA instrument lead Bill Boynton saying that they had inconclusive evidence of the presence of perchlorate in their first sample because they hadn't been looking for chlorine during their heating cycle because they hadn't expected to see it. I had an instinctive negative reaction to this remark: how on Earth could you possibly send a mass spectrometer to Mars and then not look for some masses? There's actually a very good reason, and I'll try to explain it.

I usually think of TEGA as a mass spectrometer, which is an instrument that takes in gases and then splits them up by their mass and spits out a spectrum, basically a graph of mass of a molecule versus a count. Certain molecules or atoms produce peaks at certain points. Oxygen -- one of the things they'd be looking for -- would produce peaks at masses of 32 daltons (for molecular oxygen, O2) and 16 daltons (for a neutral oxygen atom). To "see" chlorine, they'd have to be "looking" for a mass of 35 daltons. I always assumed that mass spectrometers just kind of took a continuous count of all different masses of molecules, but that's not how TEGA works. Boynton told me that TEGA actually has to scan across the masses, and a full scan across its entire mass range takes 5 minutes. That's not too long, right?

Yes and no. TEGA isn't just a mass spectrometer. Before and during its mass spectrometry run, it also does something called Differential Scanning Calorimetry. To do mass spectrometery of solid samples, the first thing you need to do is to make a gas out of your solid, and to do that TEGA heats the solid sample to 1000 degrees Celsius. But before it does that, first it does calorimetry on two earlier heating cycles to lower temperatures.

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