Saturday, August 3, 2013
Selecting the best HPLC wavelength(s) to monitor during an analysis method for use in quantitation and/or purity determination requires careful attention. Here is the basic procedure to use:
Step 1. Create the Method. To determine which UV/VIS detector wavelength(s) should be chosen for the analysis of your sample, you will first need to create a general HPLC Method which retains and resolves the compound(s) of interest on the column (goal is a k prime of >2.0, less than 10.0). Be sure and utilize a scanning diode array detector in full scan mode (often referred to as a photo-diode array detector, PDA or DAD) to scan all relevant wavelengths of your samples (e.g. 210 to 450nm). Note: Your choice of mobile phase and detector settings will effect the S/N values.
Step 2. Determine the lambda max of the sample's spectra using the Data analysis software. Once you have completed the analysis, review the spectral data to determine which prominent peak wavelengths have the maximum signal to noise (S/N) ratio. These “peaks” can be used as the individual wavelengths for integration and purity determination. By sure and double check that any detector options which use a “reference” wavelength are turned ‘OFF’ when running these methods (more info on “reference wavelengths” can be found on this blog in another post).
Step 3. Edit the method to use the discreet wavelengths found in step 2. Whenever you run a real sample, continue to use the full scanning mode of the detector so you will know about any other components which absorb at wavelength far away from or near the peak wavelengths. These compounds can add or subtract signal from the main peak making it appear to be more or less concentrated (or more or less pure) than it actually is. If you only monitored the sample with a single wavelength detector, then you would miss this vital information and make errors in your purity or concentration determinations.
Conclusion. Using a multi-wavelength, scanning HPLC detector such as a DAD is one of the most important tools you can use to create accurate and reliable chromatography methods. Learning how to correctly use and set up the detector parameters and options is the second most important thing as incorrect settings can cause false or misleading results. Only after you have developed a reliable and scientific method with good sample retention can you begin to provide accurate integration and concentration values and/or make UV/VIS "purity" determinations.