Understanding the difference in Resolution when working with a Photodiode Array Detector - Tip250

OBJECTIVE or GOAL

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?

ENVIRONMENT

• Empower
• PDA Detector
• Photodiode Array Detector
• Empower Tip of the Week #250

PROCEDURE

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

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).

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).

ADDITIONAL INFORMATION

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