The Detector

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5. The Detector

After connecting the column into the detector it is time to prepare it for the analysis. It is important to remember that the detector is the eyes of the system. The baseline is actually the detector's response to the mobile phase flowing out of the column constantly, even before the injection and following it. It can serve as an indication to the status of the column or the solvents. If the column is polluted or the solvents are not appropriate for HPLC work the baseline is not stable. If the solvents are not appropriate for HPLC they can contain contaminations from the process of their production step, so that it is impossible to use them for gradients due to adsorption and release of the contaminations all the time while the composition of the mobile phase is changed.

The most abundant detector for HPLC is a variable wavelength UV detector. The wavelength is selected in advance and used throughout the injections.

The selected wavelength should be appropriate for the specific solutes used in the analysis. For example, wavelength range for aromatic compounds with polar substituents is 240-260 nm.
If the compound does not have any absorption in the UV range it is sometimes necessary to derivatize it with a UV-VIS absorbing functional group.
Attempts to use wavelength range of 210-220 nm for the UV detection can be very frustrating due to problems with absorption of the mobile phase itself.
Many UV-VIS detectors it is possible to use two wavelengths in parallel, which can be helpful during development of a new method or during adjustment of existing method to new compounds. For example, one can try to work in two ranges simultaneously, one wavelength selected from the range of 210-220 nm and the other from the 240-260 nm simultaneously in order to detect as many substances as possible in the sample.
In a diode-array detector a full UV-VIS spectrum is collected every acquisition point; therefore, it is frequently used for method development and validation. The optimal wavelength can be chosen, after screening all the spectra of the peaks in the chromatogram.

For more information:

http://www.forumsci.co.il/HPLC/topics.html#Detectors

http://hplc.chem.shu.edu/NEW/HPLC_Book/Detectors/detc_tot.html

http://www.forumsci.co.il/HPLC/2detect2004.pdf

Introduction

1-3: Solvent delivery system

4. The column

5 - Detector

6- Injector

7-Gradient and isocratic

8-Data Processing