Document 4207Q258 V3.00 January 2018
Page 25 of 30
Chirascan Couette Cell LD Accessory User Manual
Note that any irregularities in the measuring system due to any slight bearing runout or cell eccentricity will
lead to slight variations the LD signal during a revolution of the rotor, and is desirable if possible to sample for
an integer, or half integer number of rotations. For this, the time-per-point set on the Pro-Data Chirascan control
page should correspond to the time per revolution of the rotor.
For example, if the rotational speed is 0.2 Rev/Sec, corresponding to a shear rate of 24 s
-1
, the time per
revolution is 5 seconds, and ideally this should be used as the time-per-point on the Pro-Data Chirascan control
page. If this is too long, then 2.5 seconds, equivalent to half a revolution, could be used for the time-per-point
(because the light passes twice through the rotor, positions 180° degrees apart tend to give a similar response,
so a half revolution is often enough for a baseline). At higher rotational speeds, where the sampling is over
many revolutions, this consideration is unimportant.
Enter the rate of change of velocity while increasing or decreasing to the set velocity, in the
Acceleration
box.
The motor can accelerate very rapidly, but it is best to approach the set speed relatively slowly, to avoid
shocking the sample.
The motor can be set to run clockwise by ticking the
Reverse
box.
To start the motor, click the
Start
button. When the Caption bar shows that the set velocity has been reached,
begin the data acquisition on the Pro-Data Chirascan control page.
To move to another velocity, enter the required velocity in the
Max velocity
box and click the
Start
button.
To stop the motor, click the Stop button. The motor will decelerate to zero at the rate entered in the Acceleration
box.
3.4.4 Taking a baseline.
The baseline in a linear dichroism experiment is considered to be the LD signal of the unperturbed sample.
Since the signal may vary slightly with angular position, and the LD spectrum is measured over a range of
wavelengths, the baseline must be taken in two dimensions: angular position and wavelength.
At Applied Photophysics we have found that the quickest and most reliable way to do this is to take three or
five spectra at equally spaced angular positions, i.e. at 120° or 72° intervals, and to average them to give a
global positional baseline. The reason for taking an odd number of spectra is that the light passes twice through
the rotor, so spectra taken at 180° intervals tend to be similar. The spectra can be measured over whatever
timescale is convenient, given that the longer the time taken the better the data quality will be, but, of course,
the longer the experiment will take.
During the baseline acquisition procedure, the rotor has to be moved from one position to the next, and this
should be done as slowly as is reasonably possible so that sample is minimally perturbed, for example at 0.02
rps.
