At typical column flow rates (0.1 - 2.0 mL/minute), the foreline pressure will be approximately 20 - 100 mTorr. If the foreline pressure is significantly above 100 Torr, or if the pressure you observe for a given column flow has increased over time, one the following may be the cause:
On MSDs equipped with a diffusion pump, column (carrier gas) flow should not exceed 2.0 mL/minute. On MSDs equipped with a turbomolecular pump, column (carrier gas) flow should not exceed 4.0 mL/minute. Reduce the column flow and retune the MSD.
If the oil level is low or the oil is contaminated, the foreline pump will not work effectively. This can increase both foreline and vacuum manifold pressure. Check the level of the oil in the foreline pump. If necessary, add foreline pump oil or replace the oil. Also check the condition (color) of the oil. The foreline pump oil gradually becomes less effective as it is subjected to heat, air, water, solvents, and samples. One symptom is a gradual darkening of the oil.
Check the foreline hose. Make sure it has not collapsed. Make sure it is not bent sharply or otherwise constricted.
Replace the foreline gauge. See the MSD maintenance documentation for instructions for replacing the foreline gauge.
Replace the foreline pump or contact your Agilent Technologies service representative.