LC-MS/MS is a spectacularly sensitive way to measure a compound, and that sensitivity cuts both ways. The instrument will faithfully report whatever is actually in the vial, including the small pipetting errors that crept in while you were building the calibration standards and adding the internal standard. In quantitative mass spec, the analytical run is rarely the weak link. The sample preparation is, and most of that preparation is small, precise liquid transfers of demanding liquids. Get those right and the chromatography is almost boring; get them wrong and no amount of instrument tuning will save the numbers.
This is a look at the liquid handling that decides whether an LC-MS/MS assay is trustworthy, focused on the places where the volumes are small and the liquids are difficult. The chemistry varies by assay; the pipetting problems are remarkably consistent.
The internal standard is where precision matters most
Most quantitative LC-MS/MS relies on an internal standard, often an isotopically labelled version of the analyte, added to every sample and standard. The whole method leans on the assumption that the same amount of internal standard went into every tube. If that transfer varies, the ratio the quantitation depends on varies with it, and the error propagates into every result. This is the single transfer to make as precise as you possibly can. A small, consistent volume delivered identically across the plate is worth more here than almost anywhere else in the lab.
Low volumes behave differently, so treat them differently
Standards and internal standards are often added in the low microliter range, where pipetting stops being forgiving. At these volumes the liquid retained on and in the tip is a large fraction of what you meant to deliver, so the details of how the tip enters and leaves the liquid dominate the result.
- Match the tip to the volume; a large tip delivering a tiny volume magnifies the retained-liquid problem. Small volumes want small tips.
- Tune the class for the low-volume regime specifically, rather than reusing settings validated at larger volumes where the physics is gentler.
- Pay attention to how the tip withdraws from the liquid, because at these volumes a clinging last drop is a measurable error, not a rounding detail.
Organic solvents fight the pipette
LC-MS/MS sample prep is full of methanol, acetonitrile, and other volatile organic solvents, and they are actively awkward to pipette. They have low surface tension, so they wet and drip readily, and they are volatile, so they evaporate and build vapor pressure inside the tip that pushes liquid out unbidden. The handling has to account for both: enough trailing air to hold a low-surface-tension liquid in, enough blowout to clear a volatile one cleanly, and settling times kept short so the liquid neither drips nor evaporates while the tip hesitates. Pre-wetting the tip, by aspirating and dispensing the solvent once before the real transfer, saturates the tip environment and noticeably steadies the delivery.
Serial dilutions and the carryover trap
Calibration curves are usually built by serial dilution, and serial dilution is unforgiving of two things. The first is compounding error: a small inaccuracy at the top of the series is carried down and amplified at every step, so the accuracy of the first transfer matters out of proportion to its size. The second is carryover. Any technique that draws a little extra and pushes it back, useful for a single accurate transfer, becomes a route for a high concentration to contaminate a lower one down the series. For dilution series, favor fresh tips between levels and drop the over-aspirate tricks that quietly move material where you do not want it.
A mass spectrometer measures your pipetting as faithfully as it measures your analyte. In LC-MS/MS, the calibration curve is a pipetting artifact first and a chemical one second.