Why you should care about column efficiency

If you would really, really like to obtain a perfect purification, then tweaking the resolution is the major way to go. I’ve already discussed the selectivity parameter of the resolution equation and here I zoom in on anther biggie factor, column efficiency. I don’t like a big stack of plates in my sink, but I love a big stack of theoretical plates in my chromatography column. Read on to see why.

My cousin just came back from a trip to Asia and was showing me some pictures of the Great Wall of China. Some of the steps looked half a meter long, so he had to really stretch his legs to climb over. I thought about how tiring and inefficient climbing a few of these large stairs would be compared to walking up many small steps.

Now you might think that the steps of the Great Wall of China have nothing to do with chromatography. But if you’ve been following the blog, you would know that my brain associates everything with chromatography. Indeed, these steps reminded me of column efficiency, or the theoretical plates of a chromatography column.

It has been awhile, but I’d really like to go back to resolving issues around resolution in chromatography . I’ve already discussed resolution in general. I’ve also gone into more details regarding selectivity and how the choice of stationary phase and mobile phase can influence resolution.

Then how about we take a look at the first term of the resolution equation now?

resolution equation, chromatography, selectivity, column efficiency

Column efficiency, or the number of theoretical plates, is an important factor that can help improve resolution. If all variables in the resolution equation are held constant, except for plate number, resolution is proportional to the square root of the number of theoretical plates.

Therefore, increasing the number of theoretical plates by four, increases the resolution by a factor 2.

Let me offer you some background information on this. In the original theoretical model of chromatography , Martin and Synge divided the chromatographic column into discrete sections, which they named theoretical plates. Each plate represents the theoretical distance required for one adsorption-desorption step of sample components between the stationary and mobile phase (see figure below). The number of theoretical plates is often used to establish the efficiency of a column. Plate numbers range from 100 to 106.

number of theoretical plates, column efficiency, adsorption, desorption, analyte, chromatography

The more theoretical plates available within a column, the more equilibrations between the stationary and mobile phases are possible and the better the quality of the separation.

Column efficiency can be calculated as follows:

column efficiency, theoretical plates, chromatography, equation
chromatography, baseline peak width, retention time, column efficiency

Where  tR is the retention time, w is the baseline peak width and b0.5 peak is the width at half height.

For a column with a given length L, a high number of plates corresponds to a lower distance between each plate, known as plate height (H). The plate height is often called ‘height equivalent to a theoretical plate’ (HETP) and can be expressed with the following formula:

column efficiency, theoretical plate, HETP, equation, chromatography

The plate height term measures how efficiently the column has been packed.

Just like I’d prefer many small steps than a few large ones at the Great Wall of China,

For high efficiency separations, the plate number (N) is high and the plate height (H) is low. 

All these theoretical calculations can be observed in practice on your chromatographs. In practice, the plate number (N) affects the peak dispersion on the column as the compound travels through it. More plates result in less dispersion, narrower, more efficient peaks and a better-quality separation.

Intuitively, the easiest way to improve the column efficiency would be to increase the length of the column.

Consequently, the plate number increases, the peaks become narrower, thus more efficient. But as the column length increases, the run time increases significantly too.

Thus, it is necessary to find out what other parameters can be altered to improve the column efficiency. How can you go about doing that? I will discuss some ideas in the next blog post, so make sure you drop by soon to get the full story.

And if you are a bigger fan of nature hikes than popular tourist attractions, don’t you worry. I can also associate natural sceneries with chromatography. See how mountain peaks inspired me to discuss selectivity, stationary phases and resolution in a previous blog post . Let’s see what else I can connect to chromatography in the future.

Till next time,

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