Shimadzu Autosampler inconsistent aspiration at pressure

[JC_GCLC]

On our Shimadzu LCMSMS 8030, we use their autosampler, SIL-20AC HT. A year ago I successfully set up a PFAS quantitation method using 1ul sample size. The method hasn't been used in a year and the instrument is used sparingly (about 2-5 times/week) for a nominal mass checks by organic synthesis researchers who need to verify their purified compounds using a simple Flow Inject (bypassing columns, using autosampler with 0.1 or 0.2 ul injection), +/- scan method.

I've run into an autosampler problem setting up the PFAS method this year. It seems as though the autosampler doesn't aspirate 1ul samples WHEN a high pressure (aka thru column, pressure @4Kpsi) method is used. It seems to aspirate small samples OK when a Flow Inject (No column, pressure @100psi) method is used. (A 10ul sample seems to give me decent MRM peaks of my standards for my PFAS method but going to that big of a sample size is going to be problematic with actual samples I'm afraid)

What could be going on?

Below is some data of the mass difference after taking sets of TEN injections of water with sample size and conditions described:

1ul - through column (@4500psi) - mass diff = 0.0010g
1ul - bypass column (200psi) - mass diff = 0.0127g

then repeat

1ul - through column (@4500psi) - mass diff = 0.0036g
1ul - bypass column (200psi) - mass diff = 0.0126g

then
10ul - through column (@4500psi) - mass diff = 0.0891g

[TylerSmith123]

Hi JC,

Could you describe the column and conditions you're running in?

Additionally, when you're talking about the mass difference data, is the mass difference based off what it is supposed to be, ie the conc. you injected? If that is the case wouldn't the system be working worse without the high-pressure or am I interpreting this wrong? You seem to have 1 milligram of difference with the column in line one, and then 12.7 mg without the column?

I'm just trying to gather more information because it could be a lot of things, do you know the procedures for when the organic chemists use the instrument? Does the needle-wash solution need to be replenished and has it been consistently replenished or switched lately? Have you replaced your injection needle recently, there may be a blockage from an organic synthesis product?

Anyway, sorry I couldn't be more help. Hopefully with a little more info someone can lend a better hand

[JC_GCLC]

TO CLARIFY:
1ul - through column (@4500psi) - mass diff = 0.0010g: PROPER INJECTIONS would produce a mass diff of @0.0100g so it's NOT picking up 90% of my sample!

1ul - bypass column (200psi) - mass diff = 0.0127g: PROPER INJECTIONS would produce a mass diff of @0.0100g

then repeat

1ul - through column (@4500psi) - mass diff = 0.0036g: PROPER INJECTIONS would produce a mass diff of @0.0100g -- again, not aspirating enough sample from the vial.
1ul - bypass column (200psi) - mass diff = 0.0126g: PROPER INJECTIONS would produce a mass diff of @0.0100g

then
10ul - through column (@4500psi) - mass diff = 0.0891g

: PROPER INJECTIONS would produce a mass diff of @0.1000g

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****RESOLUTION*****: I replaced the rotor on the High Pressure Valve AND THE PROBLEM is GONE!!!