Instrument Type
Detector
Type:
Integer
Value Range:
Detector-dependent
Default:
Off
Related Commands:
Description:
With Dionex Photodiode Array Detectors, the reference wavelength is used to correct the absorption values of the wavelength that has been selected for the analysis.
Function:
If the absorption of the reference wavelength changes during the analysis, absorption values of the analysis wavelengths are adjusted up or down accordingly. The selected reference wavelength should be in a quiet area of the spectrum where little absorption occurs. Each change in the absorption then indicates substantially changed conditions; for example, a reduction of the lamp energy (lamp drift). Each change can be used to correct the absorption in the remaining wavelength range even during the analysis (the recorded signal is reduced or amplified accordingly, as necessary).
Reference wavelengths are especially useful for gradient analyses because, as the light intensity changes over time (due to the gradient), absorption values are adjusted. This minimizes baseline drift.
Peaks are not detected in the reference wavelength range, where the system is "blind" by definition. Should peaks be detected, nevertheless, select a different reference wavelength. You can deliberately do this to inhibit peaks.
The reference wavelength is freely selectable, not only for each channel of the detector, but also for a 3D field. Because the absorption of the reference wavelength is stored, it is possible to undo a correction later. The reference wavelength can be changed later by extracting the absorption values of the "new" reference wavelength from the 3D field and using them for correction.
If the reference wavelength is chosen in a light-deficient range, Dionex recommends that you average the signals of several photodiodes by increasing the Reference Bandwidth, thus improving the signal-to-noise ratio.
For Dionex Photodiode Array Detectors, you may change the reference wavelength after data acquisition. To do so, the PPA window has two options:
-
Extracting a channel at a wavelength other than the wavelength used for data acquisition: On the context menu, select Extract and then Chromatogram to file. Then, select a reference condition for the new channel.
-
Displaying the 3D field at a different reference wavelength: On the context menu, select Decoration to open the Peak Purity Decoration. Then, select the reference conditions on the General tab page.
Tip:
Do not confuse the reference wavelength with the Reference Channel used for Baseline Correction of Spectra.
Settings:
Details about reference wavelengths vary, depending on the type of detector:
Dionex UVD 170/340 Detectors
For the Dionex UVD 170/340 UV/PDA detectors, select a reference wavelength of 600 nm because relatively low absorption values are observed at this wavelength. In addition, the Dionex detectors have a special noise optimum at this value. However, when the reference wavelength is changed, the special noise optimum no longer applies.
Tip:
Use a Holmium Oxide Filter (spectra calibration) to reference the measured wavelength range; that is, to verify whether the actual nanometer value corresponds to the nominal value.
Dionex PDA, PDA-100, and PDA-3000 Detectors
By default, the reference wavelength is disabled for the PDA, PDA-100, and PDA-3000. With UV channels (¹ 3D Field), the absorption at the first scan after the autozero is used as a reference. This results in lower noise, but less drift compensation. The Reference_Wavelength=Off command in a program or on the control panel turns off the reference wavelength.
During collection of 3D Field data, the absorption after autozero is always used as a reference (as described above). After data collection, a reference wavelength can be applied to the data. In this case, the reference wavelength defaults to the highest wavelength in the analysis range. For example, if the analysis wavelength range is 190 - 350 nm, the default reference wavelength is 350 nm. If you select a different reference wavelength, the selected reference must be within the stored wavelength range.