What Does Column Efficiency Mean?

Column efficiency is one of the many terms bandied about in relation to chromatography – and it can be integral to the success of analysis and quality of results. But what exactly does it mean? Read on as we take a closer look.

What is column efficiency?

It goes without saying that efficiency is a measure of how efficient something is – in other words, how well it does its job. For chromatography columns, efficiency refers to the quality of separation. Given that each peak represents a component in the sample, you’re looking for how many peaks can be separated.

High resolution results in narrow peaks taking up less space, meaning that more peaks can be separated. This is the basic definition of an efficient column. It’s expressed using the plate count theory.

What is the plate count?

Plate count is the (theoretical) idea that columns comprise several, separate plates. During analysis, the sample moves through the column from one plate to the next, interacting with the stationary phase at each step.

Based on Martin & Synge’s plate theory, the formula for calculating column efficiency is: N = 16 (t/w)²

• N is the theoretical number of plates
• t is retention time
• w is peak width

What affects column efficiency?

The plate count formula provides some clear ways to increase the theoretical number plates and thereby increase efficiency. Firstly, increase retention time. This is done by increasing column length, so that it takes longer for the mobile phase and analyte to pass through.

Next, there’s the peak width at the base. This is the distance between the points where a peak begins and ends on a chromatogram. Generally speaking, this is determined by chromatographic resolution. High resolution results in lower peak widths, while low resolution causes wider peaks.

There are a number of factors that contribute to resolution, including column length, particle size, temperature and volume. However, it can simply be down to columns losing their resolution power after continual use. There are some things you can do to mitigate this issue, outlined in our guide to extending the life of a column:

• Conditioning
• Preparation
• Repacking
• Guarding
• Heating
• Sealing

Troubleshooting peak shape issues

Broad peaks are just one of the many peak shape issues, with fronting, tailing, split peaks, ghost peaks and no peaks also making the list. Fortunately, many of them can be overcome by troubleshooting to reduce instrument downtime and get systems back up and running, as discussed in the article ‘Gas Chromatography Troubleshooting Part I – Peak Shape Issues’.