Exploring the PDA Data view in the OMNISEC software
OMNISEC is known for its powerful multi-detection GPC/SEC analyses. The multiple detectors that work together to provide complete characterization of your samples are: refractive index (RI), light scattering, viscometer, and UV-Vis photodiode array (PDA) detectors. In this post I’m going to focus on the UV-Vis PDA detector and explore the data interface in the OMNISEC v10+ software.
What is the UV-Vis PDA detector?
Describing the UV-Vis PDA as a single detector might be underselling its capabilities. The UV-Vis PDA measures the absorption profile at 1024 wavelengths between 190-940 nm for each data slice. Another way to think about it is that every time UV-Vis PDA data is collected for a sample, 1024 chromatograms between 190-940 nm are recorded…that’s a lot of data!
How do you view that much data?
By using the PDA Data view! The PDA Data tab is located in the Detail View of the Analysis tab, and provides the following figures:
The UV-Vis spectrum map is at the top left, followed below by the UV-Vis chromatogram at a specified wavelength, and the UV-Vis absorption spectrum at a specified time point at the bottom left. On the right is all of the combined data presented in the 3D spectrograph. We’ll explore each image further in the sections below.
UV-Vis spectrum map
This image is the top-down view of the 3D spectrograph, and shows the wavelength (nm) against the retention volume (mL) for the chosen injection. The colors indicate the absorbance intensity, with blue being low intensity up to red for the most intense absorption. This plot is especially useful because you can click and hold the left mouse button and drag to expand an area of interest. When you do this, the 3D spectrograph zooms in accordingly.
UV-Vis chromatogram & absorption spectrum
I’m going to pair these together because they represent orthogonal 2D views of the 3D spectrograph. And more importantly, interacting with one will update the data presented in the other.
The UV-Vis chromatogram displays the elution profile detected at a specified wavelength. This is similar to the output of a single-wavelength UV detector. However, the advantage of having a full UV-Vis PDA is that when the slider control is moved to varying retention volumes, the UV-Vis spectrum window updates with the corresponding absorption spectrum for that particular retention volume data slice.
Likewise, moving the slider control across the UV-Vis spectrum in the bottom window will result in the corresponding chromatogram at that given wavelength to be displayed in the UV-Vis chromatogram window. Both of these actions are demonstrated below.
The positions of both control sliders, and thus the wavelength of the chromatogram and the retention volume of the UV-Vis absorption spectrum displayed can also be set using the fields at the top of the 3D spectrograph.
3D spectrograph
The 3D spectrograph displays all of the data obtained by the UV-Vis PDA in a three-dimensional plot. Two vertical planes are present; these represent the sliders in the UV-Vis chromatogram and UV-Vis spectrum windows. As mentioned above, the positions of these planes can be set using the Retention volume and Wavelength fields at the top of the 3D spectrograph window. These planes can be removed by clicking the Cursor button at the top right; clicking it again brings them back.
The 3D spectrograph perspective can be rotated by clicking and holding the left mouse button and dragging the cursor left or right. Additionally, the Start and Stop buttons at the top right of the window will start or stop a rotating animation of the plot, as shown below (at 3x, with the window maximized for clarity).
Using the PDA Data view to determine sample suitability
If you work with highly conjugated materials that produce nicely colored solutions, you may notice that the corresponding molecular weight values are a bit higher than expected. This happens if your sample exhibits any absorbance at 640 nm, which is the wavelength of the light source of the light scattering detector. If your sample absorbs the incident light, that energy is eventually emitted, and the light scattering detector cannot differentiate between scattered light and emitted light.
An example of this is shown below. What you observe is a stronger than normal, often off-scale, light scattering response (1), which leads to falsely high calculated molecular weight values (2).
To confirm this is indeed due to absorbance of your sample at 640 nm, simply click the PDA Data tab and set the wavelength to 640 nm (3). Check the UV-Vis spectrum at 640 nm in the bottom left window (4); if there is absorbance at 640 nm, as there clearly is in the example shown below, then the calculated molecular weight results from light scattering are not reliable. In this situation I suggest using universal calibration as an alternative.
Final thoughts
I hope this post helps you utilize the PDA Data view to maximize the characterization of your samples. And don’t forget, if you have a UV-Vis PDA and appropriate samples, compositional analysis is another way to use the UV-Vis PDA to gain even more insight into your samples. If you have any questions, please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.
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