Liquid chromatography is a family of modes rather than one technique, and the mode is chosen by what distinguishes the analytes: hydrophobicity, charge, size, or a specific interaction. The detector is chosen by what the analyte does, which is the part most method problems come from.
hplc and gc, and which instrument a sample needs
The choice behind hplc and gc is volatility and thermal stability: gas chromatography needs an analyte that will vaporise without decomposing, which excludes most biomolecules and anything ionic, while liquid chromatography handles them and gives up some resolving power. Derivatisation can move a sample from one to the other, at the cost of a reaction step in every analysis.
An hplc conductivity detector, and a conductivity detector hplc method
An hplc conductivity detector measures the ionic content of the eluent, which is what makes ion chromatography possible, and it sees nothing that does not carry charge. Suppression of the eluent's own conductivity is what gives sensitivity, so the suppressor is part of the method rather than an accessory. Temperature control matters because conductivity moves with it.
An ri detector hplc setup and its limits
An ri detector hplc method is used where the analyte has no chromophore, notably sugars and polymers, and the price is low sensitivity and no gradient, since the refractive index of a changing mobile phase swamps the signal. Temperature and flow stability dominate the noise. A charged aerosol or mass detector is the modern alternative where sensitivity matters.
An hplc solvent filter and what it protects
An hplc solvent filter sits on the inlet line and keeps particulates out of the pump and the column, and it is a consumable rather than a fixture: a clogged frit shows as a falling flow rate, cavitation or a noisy baseline. Degassing and filtering the mobile phase are the two cheapest habits that extend a column's life, and both are skipped more often than any other step.
A normal phase chromatography column and the water problem
A normal phase chromatography column has a polar surface eluted with a non polar solvent, which is the reverse of the usual arrangement and is chosen for isomers and very lipophilic compounds. Traces of water in the solvent change retention, so dryness and long equilibration are the practical costs, and retention drifts until the column has settled.
An hplc column selection guide, and how to use one
An hplc column selection guide narrows chemistry by analyte class, then particle and pore size by the resolution and pressure available, then dimensions by run time and load. What it cannot do is choose between two similar phases for a specific critical pair, which is a short experiment rather than a reading exercise. Pore size is the line that decides whether a large molecule enters at all.
A liquid chromatography lab and what it is built around
A liquid chromatography lab is organised around solvent handling and waste rather than the instruments: ventilated storage, dedicated waste with compatible containers, a water system at the right grade, and bench space for mobile phase preparation. Column and consumable stock control is what keeps methods running, since a missing frit stops an instrument as effectively as a failed pump.
Questions people ask about types of liquid chromatography
When is gas chromatography the right choice?
When the analyte vaporises without decomposing. Ionic and most biological samples need liquid chromatography.
Why can a refractive index detector not run a gradient?
The mobile phase's own refractive index changes, which swamps the analyte signal.
What is the cheapest way to extend column life?
Filter and degas the mobile phase, and keep an inlet filter as a consumable.