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Extracolumn Effects
Nov 1, 2008
By: John W. Dolan
LCGC North America

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John W. Dolan
All the buzz lately about liquid chromatography (LC) columns packed with particles smaller than 3 μm often comes with warnings about extracolumn effects. For this month's LC Troubleshooting installment we will take a look at what these effects are and how they might influence the separations obtained from your LC system. I also will share a simple method that you can use to estimate the amount of extracolumn volume in your LC system.

What are Extracolumn Effects?




Extracolumn effects, also called extracolumn band broadening, are all the processes outside the column that increase the width of chromatographic peaks. One common way to describe extracolumn effects is

where σobs is the observed standard deviation of peak from the chromatogram, σcol is the standard deviation of the peak resulting from band spreading inside the column, and σec is the band spreading that takes place outside the column, also called the extracolumn volume. All peaks broaden as they pass through the column, and the longer the retention time, the broader the peaks (isocratic conditions assumed). Thus, we observe that peaks with small retention times are narrow and large retention times generate broader peaks. This is normal. Several different contributions make up σec, such as the injection solvent choice and injection volume, the connecting tubing volume (length and diameter), how well the fittings are assembled, the detector cell, the detector time constant, and the data collection rate. These various contributions add up as the square-root of the sum-of-squares of each contribution to give σec. It is easy to understand that if σec is small compared to σcol, there won't be much contribution to the overall band width, but if the column contribution to peak width is small, extracolumn effects can significantly increase the observed peak width. Fortunately, when using 150–250 mm x 4.6 mm columns packed with 5-μm diameter particles, extracolumn effects are rarely an issue if we take the simple precautions of using 0.007-in. i.d. connecting tubing in conveniently short lengths and making sure the connections are made properly. However, as we'll see in the following discussion, the story can change dramatically when we use conditions that generate small-volume peaks.

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Source: LCGC North America, 11/1/2008
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