these are the first measurements of Pb in a globular cluster. We chose the globular clusters M 13 and NGC 6752 because they exhibit the largest amplitude for the Al variation of all the well studied Galactic globular clusters and therefore offer the best opportunity to find abundance variations for Rb and Pb. Previous studies of M 13 include @cohen78 and @peterson80 who found large Na variations, @shetrone96a [@shetrone96b] who showed that the Mg isotope ratios were not constant, @cohen05 who discovered abundance variations in unevolved stars, as well as analyses by @kraft92 [@kraft97], @pila96, and @sneden04a. Previous studies of NGC 6752 include @cottrell81 who discovered the Na and Al enhancements, @ss91 who found C and N to systematically vary according to evolutionary status, @gratton01 and @grundahl02 who discovered O-Al variations in unevolved stars, @6752 (hereafter Y03) who found variations in Mg isotope ratios, @67522 (hereafter Y05) who presented evidence for slight abundance variations of Si, Y, Zr, and Ba, and @pasquini05 who measured Li in main sequence stars.

Observations and data reduction {#sec:data}
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The list of candidates included 5 giants in NGC 6752 previously studied in Y03 and Y05, 4 giants in M 13 previously studied by @shetrone96a 