If an analysis shows an unusually high baseline, check for column bleed or other contamination.
Column bleed occurs primarily during temperature programmed runs. Check for ions characteristic of column bleed in the mass spectra. If the ions are present, remove the column from the GC/MSD interface and condition it.
See the MSD maintenance documentation for instructions for conditioning a capillary column.
Contamination can come from the injection port, saturated traps, or bad carrier gas. Check for ions characteristic of contamination in the mass spectra. If contamination ions are present:
Replace the injection port septum, liner, and seal.
Replace the carrier gas supply.
Replace the moisture trap.
Service the chemical trap.
Replace the oxygen trap.
Set the carrier gas flow.
Purge the entire system overnight.
See the documentation supplied with your GC for instructions for injection port and trap maintenance.
Reduce the electron multiplier voltage by 50 or 100 volts. If performance improves, continue to adjust the electron multiplier voltage until you find the optimum setting.