DSM Acquisition Modes

Acquisition Modes for Three Olis DSM Models

Computer Calculated Time per Datum with Scanning

All three Olis DSM CD spectrophotometers support ‘variable time per datum’ where the computer is allowed to determine how much signal averaging is needed per datum.
The advantage of this mode is that the best S/N is obtained in the minimal length of time, since the computer collects and signal averages at each datum for exactly the optimal length of time.
Using the common protein BSA spectrum of 260 nm to 180 nm as the example, the computer will determine that 0.5, 1, or 2 seconds is sufficient in the wavelength region above around 200 nm.
As the light level drops in the 200-180 nm region, the computer will calculate increasingly longer periods of time per datum, such that tens of seconds will be spent collect the final data points.
The scan will be collected with effectively identical noise levels from one end of the scan to the other.
The only potential disadvantage of this mode is that too much time can be spent at the final data point. If this happens, one can interrupt data collection to truncate signal averaging at that datum while retaining all data to that point.

Bovine Serum Albumin

Variable Time/Datum
Equivalent S/N

~30 minute total time,
1 to 200 sec/datum

Operator-Specified Time Per Datum with Scanning

All three Olis DSM CD spectrophotometers can support fixed time per datum readings wherein up to 1000 reading per second (1 kHz) are made and signal averaged per retained datum before moving to the next wavelength.
This time can be any number of seconds from one millisecond to hundreds of seconds.
How much time is between data point (wavelengths) is variable based on the speed at which the gratings/prisms move to the next wavelength, but this time is under one second in all three models.
The advantage of this mode is an exactly known length of time to make the measurement.
This mode works best when scanning a region across which there is approximately equal light levels.
This mode is recommended when survey scans are being done, such that the best signal to noise at the lowest wavelengths is of less interest than getting an informative scan in a short time.
The disadvantage of this mode is the likelihood that too much time will be spent at easy wavelengths or that too little time will be spent at tough wavelengths, depending on the time entered.

Bovine Serum Albumin

Constant Time/Datum
Variable S/N

~1 minute total time,
0.5 sec/datum

Unique to the Olis DSM 1000 CD is the ability to collect a CD scan every 16 milliseconds. Rapid-scanning is used for fast kinetics and quick scans.
In the fast kinetics mode, one collects a series of ‘snapshot’ scans of a sample undergoing changes induced by photolysis, mixing, or other process. Each of these 16 millisecond scans is a 2D data point.
These ‘2D points acquired as a function of time’ will be used to reconstruct the kinetic event by processing with the Olis GlobalWorks fitting algorithms to calculate the starting and ending spectra, any transient spectral intermediates, and the kinetics (observed rate constants) of the reaction.
Rapid-scanning is also useful for making quick scans.
Ideally, the spectral range will be above 190 nm—such that there is useful light available—and the CD signal will be multiple millidegrees—such that there is a meaningful signal to be detected!
For easy CD cases, spend subseconds to get perfectly respectable results.

All three Olis DSM CD spectrophotometers can support high-speed data acquisition in a kinetic mode, such that 1000 reading per second (1 kHz) are made and signal averaged per retained datum.
Up to three “fixed times per datum” are available across a time course.


Tue, May 13, 2008