LC Technology cheat sheet

Chemical hazard classes drive handling rules: flammable solvents (acetonitrile, methanol, THF) require grounded containers, bonding during transfer, and storage away from oxidizers; corrosives (TFA, formic acid, phosphoric acid) require secondary containment and acid-resistant PPE.

Peroxide-forming solvents (THF, diethyl ether, dioxane, isopropanol) must be dated on receipt and tested or discarded before the labeled expiration; test for peroxides before distillation or rotary evaporation.

HPLC waste is typically classified as hazardous waste when it contains ignitable solvents at ≥24% alcohol by volume or listed analytes; segregate aqueous and organic waste streams and label with accumulation start date.

Aqueous mobile phases containing buffers or modifiers above threshold concentrations (e.g., sodium azide, phosphate) may be regulated as corrosive or toxic; check local discharge limits before drain disposal.

Required safety documentation includes SDS access, chemical inventory, waste manifests, and training records; the laboratory chemical hygiene plan must address SOPs, engineering controls, and PPE for each method.

Fume hood face velocity is typically maintained at 0.3–0.5 m/s (60–100 ft/min); verify with an anemometer and confirm sash height markers before use.

Compressed gas cylinders (N2, He, H2) must be secured with chains or brackets at one-third and two-thirds height; helium and nitrogen regulators are not interchangeable with hydrogen service.

High-pressure LC components: never open a fitting under pressure; depressurize, and treat any solvent line as potentially pressurized until the pump is off and the system vented.

Retention factor k = (tR − t0)/t0; values of 1–10 give practical retention without excessive band broadening.

Selectivity α = k2/k1 (ratio of retention factors for two adjacent peaks); resolution Rs = 2(tR2 − tR1)/(w1 + w2), with baseline resolution typically requiring Rs ≥ 1.5.

The resolution equation: Rs = (√N/4)·((α − 1)/α)·(k/(1 + k)); N improves resolution only with the square root, so selectivity is the most efficient lever.

Plate number N = 16(tR/w)² = 5.54(tR/w1/2)²; reduced plate height h = H/dp, with h ≈ 2–3 indicating a well-packed column.

Van Deemter: H = A + B/u + C·u; the A term (eddy diffusion) dominates with larger particles, B (longitudinal diffusion) at low flow, C (mass transfer) at high flow.

Reversed-phase retention increases with analyte hydrophobicity and decreases with higher organic content; ionizable analytes shift retention with pH, so buffer pH should be at least ±1 unit from analyte pKa.

Gradient retention is described by the linear solvent strength model: log k = log kw − S·φ; gradient peak capacity depends on gradient time, flow, and column dead volume.

Normal-phase and HILIC retain polar analytes; HILIC requires a high-organic starting condition (≥80% acetonitrile) and an aqueous-rich injection solvent to avoid peak distortion.

A binary or quaternary pump delivers solvent at constant flow; degassing (helium sparge or in-line vacuum) prevents bubble formation that causes retention-time drift and pump noise.

Autosamplers with a loop or split-loop design determine injection precision; carryover is reduced by needle wash with a strong solvent followed by a weak solvent rinse.

Column ovens control temperature to ±0.1–0.5 °C; retention and selectivity change with temperature, and a stable oven prevents baseline drift.

UV-Vis detectors use deuterium (UV) and tungsten (visible) lamps; a 1–10 µL flow cell with path length 1–10 mm determines sensitivity.

Photodiode array detectors acquire full spectra for peak purity assessment; a purity threshold based on spectral match and absorbance ratio flags co-elution.

Mass spectrometers (single quad, triple quad, QTOF) require ionization source choice (ESI for polar/ionic, APCI for nonpolar); ESI response depends on mobile phase composition and pH.

Refractive index detectors are universal but incompatible with gradients; fluorescence detectors offer high sensitivity for derivatized or naturally fluorescent analytes.

The injector, column, and detector are connected with low-dead-volume fittings; extra tubing volume after the column causes band broadening and reduced resolution.

Start with reversed-phase C18 and a water/acetonitrile or water/methanol gradient; screen 5–95% organic over 10–20 column volumes to map analyte retention.

Buffer selection: phosphate (pH 2–8) for UV transparency at low wavelengths is limited; ammonium formate/acetate (pH 3–7) is preferred for MS compatibility.

Adjust selectivity first (solvent, pH, column chemistry), then optimize gradient slope and flow, then temperature; resolution gains from column length cost run time.

For ionizable analytes, screen pH at least 2 units above and below the pKa; the unionized form usually retains more strongly in reversed phase.

Method transfer scaling: keep linear velocity constant when changing column diameter (scale flow by d²), and keep gradient volume constant when changing column length or flow.

Forced degradation (acid, base, oxidation, heat, light) demonstrates specificity and peak purity; mass balance should be assessed to detect co-eluting degradants.

System suitability criteria typically include resolution ≥ 1.5–2.0, tailing factor ≤ 2.0, RSD of replicate injections ≤ 2.0%, and theoretical plates above a method-defined minimum.

Method validation per ICH Q2(R1) covers accuracy, precision, specificity, linearity, range, LOD/LOQ, robustness; report the acceptance criteria and the statistical basis for each.

Injection volume should be small relative to column volume (typically ≤ 1–2% of column volume) to avoid volume overload and peak distortion; use a weaker injection solvent than the mobile phase.

Protein precipitation with acetonitrile or methanol (≥3:1 solvent:sample) removes protein before injection; centrifuge and filter to protect the column.

Solid-phase extraction steps: condition, equilibrate, load, wash, elute; match sorbent chemistry to analyte polarity and keep flow controlled to avoid channeling.

Liquid-liquid extraction depends on partition coefficient; salting-out and pH adjustment shift analytes into the organic phase, and evaporation/reconstitution concentrates samples.

Filters: 0.22 µm or 0.45 µm syringe filters remove particulates; check for analyte adsorption on membrane material (e.g., nylon binds some compounds).

Internal standard selection: stable-isotope-labeled analog is preferred for LC-MS; a structurally similar compound is used when the labeled standard is unavailable.

Carryover testing: inject blank after high-concentration sample; carryover should be below the LOQ of the method, typically ≤20% of LOQ response.

Sample stability studies should cover bench-top, refrigerated, freeze-thaw, and autosampler residence time; autosampler temperature affects volatile or degradable analytes.

Peak area is the primary quantitation response; peak height is used when peaks are broad or when integration of area is unreliable.

Calibration models: external standard for clean matrices, internal standard for variable injection or extraction recovery, standard addition for matrix-matched quantitation.

Linearity is assessed with a calibration curve of at least 5–6 levels; the correlation coefficient alone is insufficient-examine residual plots for curvature.

LOD and LOQ are estimated from the standard deviation of the response and slope: LOD = 3.3σ/S, LOQ = 10σ/S, or from signal-to-noise of 3:1 and 10:1.

Integration parameters (slope sensitivity, peak width, baseline treatment) must be fixed in the method; manual integration changes require documented justification.

Peak purity and spectral confirmation via PDA or MS help distinguish co-eluting impurities; report the qualifier/quantifier ion ratio for MS methods.

Data integrity requires audit trails, unique user accounts, and controlled electronic records (ALCOA+); raw data must be retained and traceable to the reported result.

Signal-to-noise is calculated as 2H/h, where H is peak height and h is peak-to-peak baseline noise; baseline noise should be measured over a representative segment.

Baseline drift: check mobile-phase degassing, lamp aging, column equilibration, and temperature stability; a drifting UV baseline often indicates a failing lamp or air bubble.

Ghost peaks: trace to carryover, contaminated mobile phase, or previous injections; run a blank gradient and replace solvent/bottle filters.

High backpressure: locate the restriction by disconnecting the column and measuring system pressure; common causes are a clogged frit, guard column, or inline filter.

Retention-time drift: verify mobile-phase pH and composition, column temperature, and pump seals; gradient drift can indicate proportioning valve failure.

Peak splitting or fronting: check injection solvent strength, column void at the inlet, and sample overload; tailing often indicates silanol interactions or a partially blocked frit.

Leaks: inspect fittings, pump seals, and injection valve; torque fittings to manufacturer specification and replace ferrules when reconnecting.

Pump maintenance includes seal replacement at defined intervals and checking check-valve function; a noisy pump trace often points to a failing check valve or air in the head.

Column care: store in the recommended solvent, avoid pH and temperature extremes, and use guard columns; record injection counts and column history for traceability.