Empower Tip: Review Window and the Processing Method

In this tip, we will discuss another option for Deriving Channels when working with PDA data, called RatioPlot.

RatioPlot plots the ratio of absorbance at one wavelength to absorbance at another wavelength. Prior to the development of the Photodiode Array detector, and chromatography software with peak purity algorithms, RatioPlot was used to determine peak purity (spectral purity). The theory is, if the ratio of absorbance is constant across a peak then it is spectrally pure. The challenge is knowing which wavelengths to select.

Let me show you how it is done.

We begin by bringing a channel of PDA data into Review and open the Method Set.

Go to the Method Set window and Derive a Channel, selecting RatioPlot.

Decide on the wavelengths and enter those values followed by the Minimum Value. The Minimum Value is the minimum response allowed for numerator and denominator.

Save the Derived Channel and select a generic processing method.

Return to the main window and apply the Method Set. The RatioPlot is displayed.

If we zoom in on the top of the peak between 3 and 4 minutes we see the RatioPlot is flat, i.e., the ratio is constant across the peak which indicates the peak is spectrally pure.

If we zoom in on the top of the peak between 1.3 and 1.5 minutes we see it has a bit of a rounded shape, i.e., the ratio is not constant across the peak which indicates the peak is not spectrally pure. As it turns out, there is an impurity in the front of the peak which could easily be determined using the peak purity function in the processing method. Stay tuned for a future tip on that.

It’s that easy!