How can I consistently choose reasonable CSI parameters for spectra with no CSI?
8 February 2016 Comments Off on How can I consistently choose reasonable CSI parameters for spectra with no CSI? FAQ - Processing Spectra
In the absence of a supported CSI, you will need to set reasonable values for the position (chemical shift) and width of the lines when calibrating your spectrum with a CSI in Processor.
- Referencing the chemical shift to another compound requires knowledge of the expected chemical shift of the alternate compound. Zoom in to this known peak. Drag the x-axis by clicking on the ppm location this known peak is known to resonate at. A red line will appear at the ppm location. Drag the axis until the red line lines up with the known peak.
- To set the width, start by setting the CSI width to about 1.2 Hz. Most of the compounds in the Chenomx library were originally fit using spectra with about this linewidth, so this is a good starting point for many high-quality spectra. If you realize while analyzing the spectrum in Profiler that most compound signatures are considerably narrower or wider than the corresponding features in the spectrum, switch to Processor (use the Jump to Processor feature), adjust the CSI width accordingly, and switch back to Profiler (the Jump to Profiler feature lets you do this with the click of a button).
Remember, this technique will not allow accurate absolute quantification, but you will still be able to identify compound. Relationships among concentrations within the same sample will work (you should be able to tell that creatinine was twice the concentration of alanine in sample X), comparisons across multiple samples will work, so long as they were acquired at the same time with no changes to the NMR equipment.