I have a 7890 with a 5975 MSD and an AtomxXYZ purge and trap (Tekmar) interfaced to the front injection port. I have run dozens of samples using a pulsed split method. The split is 1:1 and the pulse is 50 psi for 10 minutes. In the middle of a sequence, the GC lost the ability to maintain the initial inlet pressure before initiating the pulse. In other words, the set pressure is 6.8 psi at an oven temp of 35° and the actual pressure drops until the GC goes into low pressure shutdown. When I change the split ratio to 2:1, The pressure holds as set. What could cause this to happen after many successful runs? It happened first in the middle of a sequence when I wasn't even in the lab.