Liquid chromatography: choosing the system, the detector and the column

Specifying a liquid column chromatography system

1. Pressure class decides your columns. Conventional systems and the higher pressure class differ in the particle sizes they can run, and therefore in achievable speed and resolution. A method developed on one does not transfer unchanged to the other, so consider what methods must be run today and what must transfer in later.
2. Detector follows the analyte. Ultraviolet absorbance is the workhorse where the analyte has a chromophore. Diode array adds spectra and peak purity assessment. Refractive index handles analytes without a chromophore at the cost of gradient compatibility. Fluorescence gives sensitivity where the analyte allows. Mass spectrometry identifies as well as quantifies and is a different scale of purchase.
3. Gradient formation and dwell volume. High pressure and low pressure mixing differ in dwell volume, which shifts gradient methods between systems. If methods will move between instruments or come from elsewhere, the dwell volume is a transfer parameter and needs to be known rather than discovered.
4. Injector, carryover and sample capacity. Ask about carryover with your own difficult analyte rather than the specification figure, and check the autosampler's capacity, cooling and vial formats against how the laboratory actually queues work. Sample cooling matters for anything unstable at room temperature.
5. The data system outlasts the hardware. The chromatography data system holds raw data, processing methods and the audit trail, and it is where a regulated laboratory's integrity questions are answered. Choose it deliberately, and confirm how raw data is retained, who can reprocess and what the audit trail records.

Method development and transfer in high pressure liquid chromatography

Where a method arrives from elsewhere, the parameters that must travel are the column, the mobile phases, the gradient, the temperature, the flow, the injection volume and the dwell volume of the system it was developed on. Missing the last of these is the usual reason a transferred gradient method behaves differently.

Record the development decisions alongside the method. Reconstructing why a temperature or a gradient slope was chosen is hard later and is exactly what a reviewer asks.

HPLC instrumentation: consumables and maintenance

Columns, guard cartridges, mobile phase, seals, frits and lamps are the recurring spend, and pump seals and detector lamps are the parts whose decline appears as method drift rather than as failure. A maintenance schedule prevents more problems than troubleshooting solves.

Mobile phase preparation discipline is worth more than most instrument upgrades: filtered, degassed, freshly made and clearly labelled solvent removes a large share of baseline problems.

Column liquid chromatography: retention mechanism is the useful classification

Compounds are retained by hydrophobic interaction, by charge, by size, by specific affinity or by adsorption onto a polar surface, and each mechanism suits a different kind of analyte. Classifying by mechanism tells you which to try; classifying by instrument tells you nothing.

Most laboratories run one mechanism for almost everything and reach for a second when an analyte is too polar, too large or too similar to its neighbours. Knowing which mechanism answers that failure is the difference between a week of method development and a month.

Coupling to mass detection constrains the mobile phase

A mass detector needs a volatile mobile phase, so non-volatile buffers and ion-pairing reagents that work well with absorbance detection will foul the source. Volatile buffers at controlled pH are the usual substitution, and they change the separation.

Plan for that when a method is developed with absorbance detection and later has to move. Re-developing for volatile conditions afterwards is common, avoidable and usually discovered at the worst moment.

Solvent grade is part of the method

Solvents are sold in grades defined by what they contain rather than by what they are: absorbance at low wavelength, residue on evaporation, water content and, for mass detection, the absence of additives that suppress or add ions. A lower grade produces baseline noise and ghost peaks that look like sample.

Match the grade to the detection, buy in sizes that are used before they degrade, and record the grade and supplier in the method. Gradient-grade solvent is specified for gradient work precisely because impurities concentrate at the column head and elute as peaks.

Sample preparation on a bed rather than a column

Extraction on a small bed of stationary phase is chromatography used to clean up rather than to separate, and it is where most of the recovery and most of the matrix suppression in a method is decided. It is also where a method is most often left unoptimised.

Choose the sorbent by the same mechanism logic as a column, check recovery for your analyte at the concentrations you work at, and treat a change of sorbent lot as a change of method. Time spent here usually buys more than time spent on the gradient.

liquid chromatography mass spec, and what the pairing gives

Putting a mass spectrometer after a liquid chromatograph adds identity and selectivity to a separation that otherwise reports only retention and absorbance. The chromatography reduces the matrix reaching the source and separates isobaric compounds; the spectrometer reports mass, and with a tandem instrument a specific fragment transition, which is what makes quantitation at nanograms per litre possible. The cost is compatibility: the mobile phase must be volatile and free of non-volatile buffers and ion pairing agents, so a method developed for a UV detector often has to be rewritten before it can be coupled.

lc chromatography, and what the letters leave out

Liquid chromatography names the mobile phase and nothing else, so the useful specification adds three things: the mode, reversed phase, ion exchange, size exclusion or normal phase, which decides what separates; the pressure class, conventional or sub-two-micron, which decides the hardware; and the detector, which decides what is seen. A quote or a method that says only liquid chromatography is not comparable with another, and a request for one should name the analyte and the matrix so the mode can be chosen.

Common questions

What pressure class of HPLC systems should I buy?

It depends on the particle sizes your methods need. Higher pressure systems run finer particles for faster, sharper separations; conventional systems run established methods. Methods do not transfer unchanged between the two.

Which of the liquid chromatography detectors should I choose?

Ultraviolet or diode array for analytes with a chromophore, refractive index where there is none, fluorescence for sensitivity where applicable, and mass spectrometry where identification is needed. The analyte decides it.

Why does dwell volume matter on a liquid chromatography machine?

It sets how long a programmed gradient takes to reach the column, so a method developed on one system can behave differently on another. It is a transfer parameter and should be recorded with the method.

How important is the data system?

More than the hardware in the long run. It holds raw data, processing methods and the audit trail, and it is what a regulated laboratory is examined on. Choose it deliberately rather than accepting the bundled option.

Why does my method fail when moved to mass detection?

Usually the mobile phase. Non-volatile buffers and ion-pairing reagents foul the source, so volatile buffers have to be substituted, and that changes the separation. Plan for volatile conditions if the method will ever be coupled.

Browse by service class

• Compare Gene synthesis on advertised prices