The Alkalis are the most tempestuous of elements. Willing to drop their unfilled S shell at the drop of a hat, they shamelessly lure anions away from more respectable metals, giving up their unpaired electron like a thermal ionization source in heat.
They are also filthy little atoms, dissolving into almost any aqueous solution, and easily evaporating under fairly reducing conditions. This, along with their ubiquity, makes them infamous contaminants in ICP mass spectrometry, and their background concentrations under standard conditions can be equivalent to thousands of ppm.
Special analytical techniques, such as soft extraction, have been developed to get around the pernicious easy-going electron-induced ionization of alkali caused by premature charge separation. But these techniques lead to reduced sensitivity, uneven backgrounds, and higher detection limits.
An alternative method, which has kept me out of trouble for the past six months, is to prevent these ionic Jezebels from getting into the cones in the first place. Although this “absence-based” protocol sounds easy, actually quarantining the gas expansion region of our machine from the alkali has been a bit of a chore, and only last month (and just in time for the conference) have I had any luck getting results from this technique.
On Tuesday, I finally managed to kick some group one ass. For some planetary post-Goldschmidt visitors, I let them open fire on lunar minerals with a 15 ppb detection limit for Rb on a 70 micron spot and minimal laser power. Now, the moon isn’t exactly alkali heaven. A dry, degassed, volatile-depleted wasteland, it has one of the lowest assumed mantle K/Th ratios of any known planetary body. So even in the compatible phases Rb is going to be low. But thanks to my trusty new tuning glass, half a year of assiduous record keeping, and a bit of good old-fashioned luck, these guys have a chance at getting something other than “bdl” to put in their papers when they’re done.
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