Empower Tip: Review Window and the Processing Method

In this tip, we will create a Processing Method for determining Peak Purity. (To understand how the math for Peak Purity works, refer to article: Detecting Coeluted Impurities by Spectral Comparison)

STEP 1 Begin by deriving a wavelength of interest, create a PDA Processing Method and set up the peak detection/integration parameters as well as any Component parameters. Add this information to your Method Set. (Refer to Tip 34 for how to extract a wavelength of interest)

On the Purity tab in the Processing Method there are several critical parameters.

Spectral Contrast Parameters: The wavelength range should be set to start above the UV cutoff of the mobile phase and end where the components have absorbance. The noise interval should be set as per the discussion in Tip 135.

Purity Parameters: Tick the ‘Purity Enabled’ box and set the ‘Active Peak Region %’ (the default is 100). Empower determines Peak Purity from the spectra across a chromatographic peak from peak start (or liftoff) to peak end (or touchdown). If the signal to noise is poor or if the peaks are not baseline separated, you may have to lower the value. The Threshold criteria needs to be set to calculate the Purity Threshold and the default is Auto Threshold. (Please go to the Waters Knowledge Base for more detailed information on Threshold Criteria.)

Apply the Method Set and review the results in the Peaks table under the chromatogram. If the Purity Angle is less than the Purity Threshold, this is one piece of information that indicates the peak is spectrally pure.

Before we make a definitive conclusion, it is best to check the Purity Plot. Go to the Results window within Review and click the ‘Purity Plot’ tab at the bottom of the window. If the purity line (green) is below the threshold line (blue) across the peak and the reported Purity Angle is less than the Purity Threshold, then we can conclude the peak is spectrally pure.

I would recommend zooming on the bottom of the Purity Plot to get a better look at the two lines.

It is also recommended you check that you selected an appropriate noise interval. Back in the main window of Review, select ‘Spectrum Review, Display Noise Spectrum’. The noise spectrum can be seen underneath the spectra for the other peaks. The noise spectrum should be random in shape. If the noise spectrum has a shape like the spectra for any of your peaks of interest, then you need to go back to MaxPlot and select a different noise interval. Otherwise, you will have a high threshold and peaks with impurities will pass peak purity.

It’s that easy!