LC Technology common mistakes
LC Technology · Understand the traps and how to avoid them.
Memorizing retention times and resolution formulas as isolated facts but never practicing how change in flow rate, particle size, or mobile-phase strength shifts retention factor (k), selectivity (alpha), and plate number (N) together.
Candidates treat the fundamental equations as vocabulary to recall rather than as coupled relationships, so they cannot answer questions that require predicting the net effect of one changed variable on multiple chromatographic outputs.
What to do instead
Build a one-page map linking each variable (flow, temperature, %B, particle size, column length) to its effect on k, alpha, N, and resolution, and practice two-variable 'what if' scenarios, remembering that resolution depends on all three terms and that changing %B usually moves alpha and k together.
Confusing the roles of the guard column, inline filter, and analytical column when diagnosing pressure rise or peak distortion, and replacing the wrong component.
Candidates learn that 'guard columns protect the column' but never internalize that particulate and strong-solvent fouling accumulates at different points, so they cannot localize a problem from where symptoms appear.
What to do instead
Study the flow path component by component and link each to its failure signature: a clogged frit raises pressure without changing retention, a fouled guard column distorts early peaks, and a damaged bed causes tailing or splitting that persists after the guard is replaced. Practice deciding which part to swap first.
Treating system suitability acceptance criteria as a formality and misapplying the difference between theoretical plates, tailing factor, and resolution when reading a suitability report.
Candidates memorize that suitability 'must pass' but do not connect each parameter to what it measures, so they accept a report with failing tailing or misidentify which metric a limit applies to.
What to do instead
Learn the definition and calculation basis of each suitability parameter (plate count from peak width, tailing from the 5% peak-height points, resolution from the two retention times and average width) and practice interpreting a full report line by line against stated limits.
Assuming the mobile phase is the only source of pH or buffer problems and overlooking that aqueous buffer prepared at the wrong pH or concentration changes both retention and peak shape for ionizable analytes.
Candidates focus on organic modifier and gradient shape and treat buffer as background, so they miss questions where pH relative to analyte pKa controls ionization state and therefore retention and silanol interaction.
What to do instead
Study how analyte pKa versus mobile-phase pH governs the ionized versus neutral fraction, why buffers are chosen within roughly two pH units of the pKa, and how buffer concentration and ionic strength affect peak shape and mass-spec compatibility. Practice predicting retention shifts when pH crosses the pKa.
Reporting a result from a single injection or from peak height without verifying integration parameters, baseline placement, and whether the calibration model covers the sample concentration.
Candidates equate 'the software gave a number' with a valid result and do not question automatic integration or extrapolation beyond the calibration range.
What to do instead
Practice reviewing chromatograms for baseline assignment, integration start and end points, and peak purity, and always confirm the result falls within the calibrated range. Learn when to use linear versus quadratic fits and why forced-through-zero calibration can bias low-level results.
Describing sample preparation steps generically ('filter and inject') without specifying filter compatibility, membrane pore size, and adsorptive losses for the analytes of interest.
Candidates learn filtration as a routine step and do not consider that membrane material can adsorb hydrophobic analytes or leach extractables, or that pore size must be well below particle size and injection-port constraints.
What to do instead
Study filter selection by analyte polarity and solvent (e.g., PTFE versus nylon versus PVDF), the reason for pore sizes around 0.2 to 0.45 micrometers, and the risk of analyte loss or contamination. Practice choosing a filter and justifying it for a specific method.
Diagnosing a retention-time drift as a pump problem when the more likely causes are mobile-phase evaporation, column equilibration, or temperature fluctuation.
Candidates jump to the most visible hardware and skip the systematic check of mobile-phase preparation, degassing, equilibration volume, and column oven stability.
What to do instead
Adopt an ordered troubleshooting sequence: verify mobile-phase composition and sealed reservoirs, confirm sufficient column equilibration volumes, check oven and detector stability, then examine pump seals and check valves. Practice ruling out each cause with a specific observation before replacing parts.
Mixing up which detector is appropriate for a given analyte or mobile-phase constraint, such as using UV detection for a compound with no chromophore or running nonvolatile buffers into a mass spectrometer.
Candidates learn detector types as a list without linking them to analyte properties and mobile-phase compatibility, so they choose by habit rather than by requirement.
What to do instead
Build a detector selection table keyed to analyte properties (chromophore, ionizability, volatility, concentration) and mobile-phase constraints (volatile buffers for MS, UV cutoff for low-wavelength detection). Practice selecting a detector and defending the choice for sample analytes.
Treating regulatory requirements as a list of document names to memorize rather than understanding what each requires in practice, such as method validation parameters or system suitability justification.
Candidates prepare by recalling acronyms and titles and cannot answer questions that ask what a requirement means operationally or which validation element applies to a given change.
What to do instead
For each key requirement, write a one-sentence operational meaning and an example: what accuracy, precision, specificity, linearity, range, detection limit, quantitation limit, and robustness each demand in practice, and when revalidation is triggered by a method change.
Ignoring safety controls specific to LC work, such as solvent waste segregation, flammable mobile-phase storage, and pressurized-system hazards, and answering safety questions with generic lab-hygiene statements.
Candidates assume safety questions are common sense and do not study the specific hazards of organic solvents, high-pressure fittings, and hazardous waste streams generated by chromatography.
What to do instead
Study the concrete hazards: flammability and vapor exposure of common solvents, waste compatibility and labeling, pressure-relief and fitting-integrity checks before opening a pressurized system, and correct handling of needles and septa. Practice mapping each hazard to its control.