Contaminant ions observed in LC-MS runs that use preslit PTFE or silicone septa vial caps - WKB12032

SYMPTOMS

• Contaminant ions observed in LC-MS runs that use preslit PTFE or silicone septa vial caps

• Contaminant ions are observed when doing replicate injections; the first injection is clean
• Contaminant ions are generally siloxanes

ENVIRONMENT

• LC-MS with Waters vial caps (186005666CV or 186000327C)

CAUSE

After the first injection, the solvent on the outside of the needle reacts with the outside (top) and slit of the septum. 

FIX or WORKAROUND

The following are better vial caps to use:

186004169 - Polyethylene septumless vial cap

186007198 - PTFE septum in cap

186005826 - PTFE silicone (the lowest bleed PTFE Sil septum Waters supplies)

ADDITIONAL INFORMATION

When using 186004169 or 186007198, the hole in the cap will stay open after piercing, so samples may evaporate over time.