Chromatography Forum

1. One trick to check for piston scratching (sapphire piston assumed) or other damage is to hold a laser pointer against the end of the piston and the piston will "light pipe" (glow), and show any imperfections in the surface of the piston as lines (scratches) or dots (chips).

2. While it usually isn't necessary with reversed-phase, you might consider using a seal wash on a regular basis. I believe that the 20AD has a flushing passage behind the check valves; just flush a few mL of water through to remove any water soluble stuff (buffer deposits, etc.), then chase it with a few mL of MeOH or IPA to remove the water. In extreme cases, you can add a continuous flush, but this rarely is required unless the ionic strength is very high (e.g., >10 mM).

3. Although it doesn't apply here with the 20AD operating at conventional <6000 psi / <400 bar pressures, with UHPLC, the higher pressures force the seal more tightly against the piston and the added friction can cause it to heat and fail early. In such cases, a continuous seal wash acts as a cooling stream to keep the seal from overheating and early failure.

-- John

Mostly , teflon seals ( yellow , white or colorless seals )are used for normal phase solvents ; such as hexane , iso-octane , ethyl acetate etc.
They are not very suitable for methanol and acetonitrile and leaking easily with those solvents.

Use gray or black seals for reverse phase.( High molecular weight polyethylene seals and graphite filled )

Bit of a mix-up here uzman. Yellow and colorless seals are based on UHMWPE ( ultra high molecular weight polyethylene ), and contain no PTFE ( Teflon ). White seals are unfilled PTFE. Gray seals are carbon / graphite fiber filled PTFE and black seals can be one of many blends containing carbon and or graphite in flake, powder or fiber form.

Generally ( and "generally" should be stressed ), gray or black for organic, and yellow or colorless for high percentage aqueous mobile phases. White seals are only ever used as back-end wash / flush seals. And MeOH and ACN are particularly "fun".

The trouble is that each pump / operating set up can throw up exceptions, and the only true rule is "what works is right".

The OP said that the seals were changed from graphite to teflon, but they are the same ( black in this case and graphite plus carbon fiber filled PTFE ( Teflon ) ), so I can't guess what the engineer was saying.

The case in hand would seem to me more likely to be check valve related though. The "off-set ball" design leaves them very susceptible to bouncing ( i.e. they can skip a beat ).

HTH