Empower Tip: Review Window and the Processing Method

We covered some aspects of using the System Suitability option in earlier tips (Tip 27 reviewed flagging failing results and Tip 28, Tip 29, Tip 30 & Tip 31 covered calculating signal-to-noise ratio). I’ve had a number of questions about some of the other features within the System Suitability option and one of those is, “What are statistical moments and how does it help me?” Statistical Moments are mathematical ways of evaluating peak shape. Some chromatography data systems (including Empower) make these calculations so that you can compare results across different systems.

We begin by bringing a Sample Set into Review, open a Processing Method and Integrate/Calibrate or Integrate/Quantitate a channel.

Go to the Processing Method window, click the Suitability tab and look towards the bottom of the screen. There you will find the Calculate Peak Statistical Moments check box. Check the box.

Return to the Main Window, click Integrate/Calibrate or Integrate/Quantitate. The Statistical Moments are hidden peak fields. Go to the Table Properties and scroll down to locate the fields. Remove the check marks next to the fields and click OK.

The Statistical Moments are now visible in the Peaks table.

So what do these numbers mean?

The Zeroth Moment is equal to peak area.

The First Moment is a measure of the center of gravity of the peak. If the First Moment equals the retention time of the peak, the peak is a Gaussian or symmetrical. If the Zeroth Moment is less than the retention time, the peak is fronting. If it is greater than the retention time it is tailing.

The First and Second Moments can be used to calculate plate count.

The Third Moment is a measure of the vertical symmetry or skew of the peak. If the Third Moment is equal to 0, the peak is Gaussian or symmetrical. If the Third Moment is negative the peak is fronting. If the Third Moment is positive it is tailing.

The Fourth Moment is a measure of vertical flattening or stretching of the peak. If the Fourth Moment is equal to 0, the peak is Gaussian or symmetrical. If the Fourth Moment is negative the peak is flattened or compressed. If the Fourth Moment is positive it is stretched or taller.

For example, the first peak has a USP Tailing of 1.0 and the First and Third Moments indicate a Gaussian peak. The second peak has a USP Tailing of 1.3 and the First and Third Moments indicate a tailing peak.

It’s that easy!