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

Applied Photophysics, Ltd is circulating results of an “APL Market Survey about CD Spectroscopy.” Among their conclusions, not surprisingly, is that their Chirascan is “[better] than other CD spectrometers.”

This is an incorrect claim, explained here using their own arguments.

They state (incorrectly) that “data of equivalent quality take less time to measure on Chirascan than on other CD spectrometers.” We direct your attention to our 32 millisecond CD spectrum which is achievable only with the Olis DSM 1000 CD spectrophotometer. This is our most extreme example of the “less time” an Olis DSM CD will take to collect CD spectra. Under most conditions, each Olis dual beam CD spectrophotometer will collect scans more quickly, usually with lower noise, than all single beam CD spectrophotometers.

Applied Photophysics states that their application of a time constant after the raw data are collected is “uniquely among CD spectrometers.” Olis DSM CD spectrophotometers use no time constant settings. One needs a lock-in amplifier to have a time constant setting. Our digital subtractive method CD spectrophotometers use no lock-in amplifier, so we have no time constant settings, just as we have no amplitude or sensitivity settings. We employ digital filtering before or after the raw data are collected.

Applied Photophysics states a bandwidth advantage “... the bandwidth can be set higher than on other CD spectrometers. This is of particular benefit in the far-UV where, with entrance and exist slits set at twice the bandwidth possible in other spectrometers, a theoretical four-fold increase in the light flux can be achieved.”

Not only is this statement incorrect in that slit widths set bandwidth, but it is incorrect in presuming that the three Olis DSM CD spectrophotometers cannot achieve any desired bandwidth. The entrance and exit slits can be from 0.1 to 20 nm and other widths can be used (by special arrangement). The Olis DSM 17 CD has computerized control over slit widths, a feature used routinely in absorbance work and infrequently in CD.

Applied Photophysics states “better linear polarization.” Using two polarizers instead of one does has the measurable and detectable effect —ironically—of a loss of light (otherwise, all CD spectrophotometers would be designed with two polarizers). Any improved “linear polarization” effect that they realize is very likely undetectable when collecting CD.

Applied Photophysics concludes that their “Chirascan optical train is an F/7 design, which is considerably shorter than that found on other CD spectrometer, and ... allows twice as much light ... than with, for example, a more conventional F/10 design. Hence ... Chirascan is able to utilize more light from the source than other CD spectrometers.”

This statement is nonsense, in that the F# is a ratio of focal length to aperture, not focal length. And, while it is generally true that shorter focal lengths result in more light throughput (all else being the same) the Olis DSM 20 CD uses an F/4.2, the Olis DSM 17 uses an F/8, and the Olis DSM 1000 CD uses an F/4.4 double monochromator, making the “considerably shorter than other CD spectrometer” statement about the F/7 Chirascan untrue.

Thus, using the “Advantage” statement issued by Applied Photophysics, it is clear that there is nothing new, nothing unique, and nothing superior about their recently introduced single beam CD spectrometer. It is another “me-too” single beam instrument, albeit in a very large box (“Chirascan requires a minimum bench space of 1.5m x 0.6m; space for a PC and its peripherals will also be required.”)

Uncontestable and unmatched advantages of the Olis DSM CD spectrophotometers include:

1. Absolute CD collected by definition: CD is derived by manipulating raw digital data.
2. Linearity to 10,000 millidegrees (and beyond)
3. Use as a true dual beam absorbance spectrophotometer
4. Rapid-scanning to 62 CD scans per second^*
5. Rapid-scanning to 1000 absorbance scans per second^*
6. Windows®-based Olis software which includes modern secondary structure determination algorithms, modern 3D fits for temperature, time, hyperbaric pressure, and concentration dependent scans, direct output to EXCEL® or ASCII files, and modern facilities of Windows compatiable software.
7. Flat baselines
8. No drift
9. Independent nitrogen regulation for monochromator, sample compartment, and lamp housing, for minimal nitrogen use.
10. Minimalization of the effect of lamp instability on the signal, producing sensitivity of CD signals to 10^-6 AU.
11. Wavelength extensions in the NIR to 1700 nm and 2600 nm.
12. Confirmed use with high pressure cells, easily reversible 1.4 Tesla magnets, and other exciting accessories.