Get Empowered: Empower CDS User Question | Tip #250, PDA Resolution Differences

By November 3, 2021


Tip #250: Understanding the Difference in Resolution When Working with a Photodiode Array (PDA) Detector

Welcome back to Get Empowered! In the last tip-of-the-week blog for Empower™ Chromatography Data System (CDS), we learned how to create a column library (Tip #249).

In this week’s tip, we will answer an Empower CDS user question.

Q: In the instrument method for my ACQUITY™ PDA, I set the resolution to 1.2nm, however, when I look at the resolution in Spectrum Review, I see 0.6nm. Why the difference?

Great question. I will show you why.

Let’s get started.

Step 1

1.2nm is the lowest selectable value for Resolution in the ACQUITY PDA Instrument Method. This is the optical resolution (figure 1).

Figure 1

Step 2

The spectra tab within Spectrum Review displays a Resolution of 0.6nm. Additionally, when we look at the spectrum points tab, we see the distance between consecutive points on a spectrum is approximately 0.6nm. This is the digital resolution which in the case of the ACQUITY PDA is half of the optical resolution selected. (This PDA Detector goes up to 500nm.) (figure 2).

Figure 2

Step 3

The photodiode array pixels are approximately 50uM wide and spaced approximately 50uM apart. The slit is 100uM wide, so the resolved spectrum bandwidth for any given point is 100uM wide, which covers 2 pixels. Therefore, the value for each pixel is the sum of its own value plus half of each of the adjacent pixels (figure 3).

Figure 3

Step 4

In the case of the 2998 PDA Detector which goes up to 800nm, the digital resolution is 1.2nm because the slit is 50uM wide which is the width of one pixel (figure 4).

Figure 4

 

It’s that easy!

Final Note: This can be done with either the Pro or QuickStart interface.

 

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Next week in Tip #251: We will revisit the subject of Peak Purity in a new series using both ACQUITY PDA and ACQUITY QDa™.

 

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Categories: Empower Tips