In a regulated lab, working is not enough; you have to have shown that it works, in a form someone else can review. Qualification is the framework for that showing. It splits the question is this reliable into three smaller questions asked in order: was it installed correctly, does it operate as intended, and does it perform for my actual application. The shorthand is IQ, OQ, and PQ, and while the terms come from equipment qualification, the same three questions map cleanly onto qualifying a liquid handling method.
This is a plain-language walk through the three stages as they apply to pipetting, and where a validated liquid class sits inside the evidence you are expected to keep. The goal is not to turn you into a validation specialist but to let you see the logic, so that the paperwork feels like a description of good practice rather than an obstacle to it.
IQ: was it installed and configured correctly
Installation qualification asks whether the system was set up the way it was supposed to be. For a liquid handler that means the instrument is the specified model, the pumps and tips and labware match what the method assumes, the software is the intended version, and the physical setup, from levelling to the system liquid, is as documented. It is the least glamorous stage and the one people most want to skip, but a surprising share of downstream failures trace back to a quiet difference here: a different tip than the method was written for, a firmware version that changed a default.
OQ: does it operate within its specifications
Operational qualification asks whether the equipment does what it claims across its operating range, independent of any particular assay. For pipetting this is where the gravimetric checks live: dispense defined volumes across the working range and confirm the accuracy and precision meet the specification, following the measurement logic of ISO 8655. OQ is deliberately about the instrument's general capability rather than your specific liquid, so it usually uses water. Passing OQ says the machine can hit its numbers; it does not yet say your method will.
PQ: does it perform for my actual application
Performance qualification is where your liquid class earns its place. PQ asks whether the system delivers correctly for the real thing: your liquid, your volumes, your labware, under your routine conditions. This is the stage that a validated liquid class directly supports, because the class is precisely the record of how that liquid was made to deliver accurately on that instrument. A PQ for a viscous reagent is, in effect, a demonstration that the liquid class for that reagent produces the right volume with acceptable variation, run the way you will actually run it.
- Use the real liquid, or a validated surrogate, not water, because the whole point of PQ is application performance.
- Test across the volumes and labware you use in production, including the low-volume end where behavior is most sensitive.
- Run it under normal conditions and operators, since a qualification that only holds for the specialist on a good day is not a qualification of the routine.
- Define the acceptance criteria before you start, so the result is a verdict and not a negotiation.
Keeping the qualification alive
Qualification is not a one-time event that a certificate on the wall preserves forever. It has to be maintained through change control and requalification: when the instrument is serviced in a way that could affect delivery, when the method changes, or on a periodic schedule, you revisit the relevant stage. This is why a liquid class that is versioned and dated is so valuable in a regulated setting. It lets you point at the exact version that was qualified, and it makes a later change a visible event that triggers a look at whether requalification is due.
IQ, OQ, and PQ are three questions in order: installed right, operates right, performs right for my job. A validated liquid class is most of the answer to the third one, which is the one your results actually depend on.