If the KOH recirculation pump fails for a period of time (i.e., 20 minutes), must the test be invalidated for Method 0061?

YES!  Remember the function of Method 0061, Cr +6 sampling train, is to minimize the reactivity and conversion of Cr +6 to Cr +3 during sampling.  We learned in lecture that organics, oxidants and other chemicals can cause this conversion to occur in the impingers.  Consequently, in Method 0061 we inject the KOH at the outlet of the nozzle to quickly capture and stabalize the Cr +6 molecule before it has a chance to react with other chemicals.  This allows us to “fix” the Cr +6 molecule immediately when extracted from the source.  If we allow the pump to not circulate the 0.1 N KOH solution to the front of the probe, then the Cr +6 molecule has an opportunity to change before reaching the impingers.  Thus we aspirate the KOH into the front of the probe to capture the Cr +6 molecule.