Taking a Moment to Correct Fallacies
about DSM CD

From Competitor A: As recently as August 2002, our competition posted misleading and/or false information against dual beam CD spectroscopy. These "warnings" are basically the same ones they have relied upon for years. Today's discussion covers their three "favorite" arguments, as presented on the European Chirality Services (aka, Jasco Europe¹) web site, where facts and fallacies commingle, yet again.

They write:

1. "two linearly polarized beams at 90 degrees to each other"
2. "pass separately..." through sample and reference cells
3. "but matching of the detectors is obviously here the main point, a drawback not present in the old approaches..."

The truth:

1. The two beams are circularly polarized, 180 degrees out of phase with each other, and otherwise identical. We illustrate this simple detection mode graphically with the image at right. Click on small image to open a window with a larger view.

2. Our two beams of light pass separately through one sample cuvette. There is no reference cuvette. Instead, each beam of light acts as a reference for the other beam of light. Both beams of light originate from the same light source, are rigorously produced by one beam splitter, and pass through one PEM. Any non-sample induced difference between the beams is impossible. Thus, all non-sample induced differences subtract out. As WC Johnson explained it in 1996², “These data can be visualized as having the 50 kHz modulation and noise. The time average of these components is zero.” the ‘rectified’ 50 kHz component has a non-zero time average, whereas the time average of the noise is unaffected, remaining zero. The final step is to average the data. Again, the noise will average to zero and the result is the CD signal.

3. There is no effect of the detectors being matched well or matched poorly. This has been proven for years and has been explained in detail.

From Competitor B: Considering the “advantages” of the British Chirascan CD system with special comparison to the Olis DSM CD spectrophotometers.