October 2014 For Bromine, you want to use the K edge. the L edges are all down in the 1-2 keV range, where the main beam will never reach the crystal because the air absorption is so high. In general, you seldom get a situation where both the K and L edges are "accessible". You take a hit in flux as you go to higher and higher energy, and you have to deal with more and more absorption effects as you go to lower and lower energy. As a rule of thumb I'd advise against going below ~5 keV or above 18 keV at 8.3.1. There are no elements with K and L edges lying in this range. The first element where you might have to make a choice is tin, with an L-edge at 4.4 keV and the K-edge at 29.2 keV. If you are at a beamline that can reach 29.2 keV, then I'd say do that! 4.4 keV is still quite annoying to deal with in air, since half of your scattered x-rays will be absorbed in the 100 mm air gap to the detector. You will also loose half your signal going through 100 microns of protein crystal. You can work in helium or vacuum, like the new Diamond beamline is proposing, but most of us in the industry are watching patiently to see if it is worth the effort. If you'd like to try something below 7235 eV at 8.3.1, let me know, you may need to do a few tricks to clean up the "third harmonic" (a monochromator artefact). But generally if you have an edge between 4 and 7 keV its best to just collect at 7235 eV. The detector calibration is optimal here, and you still "catch" the tails of the edges down there at the slightly higher energy. you can look up air absorption on the CXRO website here: http://henke.lbl.gov/optical_constants/gastrn2.html Protein crystal is made of almost exactly the same stuff as air, but 1000x denser, so 1 micron of crystal is equivalent to 1 mm of air. Hope this helps! -James