Tip #28: Calculating signal-to-noise ratio

In the last tip-of-the-week post for Empower Software, we explored how to use the System Suitability option to flag failing results. The next several tips will address how to calculate signal-to-noise ratio.

Just about everyone wants to calculate signal-to-noise (S/N) ratio and Empower Software can make the calculation in several different ways.  Let’s begin by looking at the simple calculation of peak height divided by noise.

• Part 1, calculating S/N using peak height divided by noise (#28)
• Part 2, calculating S/N from within a chromatogram (#29)
• Part 3, calculating S/N from a blank injection (#30)
• Part 4, using custom calculations to determine S/N (#31)

Step 1

Bring raw data (Channels, Injections, or Sample Set) into Review and open the processing method. Click the Processing Parameters tool (Figure 1).

Figure 1

Step 2

Click the Suitability tab and check the box that says Calculate Suitability Results. Enter the Void Volume Time; an estimate is fine (Figure 2).

figure 2

Step 3

Select how you want noise calculated from the Noise Value for S/N drop-down list (Figure 3). Peak to Peak Noise is a popular choice.

Figure 3

Step 4

Select where in the chromatogram you want noise calculated by clicking the Noise and Drift tab. Click Yes to the message that appears (Figure 4).

Figure 4

Step 5

Select Calculate Detector Noise and Drift (Figure 5).

Figure 5

Step 6

Enter a time region in the chromatogram that is free of peaks (Figure 6). This time region must be equal to or greater than the Segment Width, and must contain at least 60 data points. The Segment Width is used for calculating average noise and drift values and it must contain at least 30 data points.

Figure 6

Step 7

Return to the Main Window and click Integrate followed by either Calibrate or Quantitate (Figure 7).

figure 7

Step 8

Signal-to-noise is a hidden field by default so let’s add it to our Peaks table. Right-click on the Peaks table and select Table Properties (Figure 8).

Figure 8

Step 9

On the Columns tab, find s/n and remove the check mark to make it visible in the table (Figure 9).  Click OK.

Figure 9

Step 10

The signal-to-noise ratio is displayed for every integrated peak (Figure 10).

Figure 10

Step 11

To keep the S/N field in the peaks table, select Save Preferences from the File menu (Figure 11).

Figure 11

Step 12

Finally, save all changes to the Processing Method (Figure 12).

Figure 12

It’s that easy!

Final notes:

1. You must have the System Suitability option and enable it in the project.
2. This procedure can be followed for both the QuickStart and Pro Interfaces.
3. Since peak height is in microvolts and detector noise is in plot units, Empower automatically corrects for the difference in units by applying a Scale to Microvolts factor.

Please rate this Empower Tip of the Week

Next week in Empower Tip #29: Calculating Signal-to-Noise Ratio (Part 2)

More resources:

• Want to learn more? Check out our Empower training courses and eLearning
• See all of our tips for using Empower CDS Software
• Comments? Connect with at #EmpowerTips or @WatersCorp on Twitter.
• Submit an Empower 3 Review with Select Science.  Write a review.

Do you want Empower Tips sent to your inbox every week?

Questions? Tips of your own? Let us know!