What is the system detection range?
Chirascan systems with a photomultiplier detector have an operating range of 163-950 nm. This range is extended to 163-1150 nm in systems using a solid state avalanche photodiode detector.
Why do I need to purge with nitrogen?
The far UV output of the standard 150W Xe lamp interacts with oxygen to form ozone. Ozone rapidly attacks optical surfaces, resulting in a significant reduction in light throughput and increasing signal noise. Purging with high purity nitrogen eliminates oxygen thereby preventing the formation of ozone. Effective purging with nitrogen extends the lifetime of the optics, ensures absorbent molecules, such as water vapour, are removed from the light path, thereby providing optimal light throughput across the entire wavelength range.
How can I view the absorbance spectrum with my CD spectrum?
Ensure that the absorbance option is ticked in CD mode. Collect or load a background measurement covering the wavelength range, bandwidth and step-size to enable calculation of the absorbance signal during a CD scan.
What is the optimum absorbance of a sample for CD analysis?
An absorbance of approximately 0.9 AU should ensure an optimal signal:noise level. In general, any CD data measured with an absorbance exceeding 2 AU are unreliable as sample and buffer are absorbing >99% of the light.
Buffer components may be interfering with my CD analysis, what can I do?
CD analysis in the far UV region, typically 260-180 nm, can be affected by high absorbance of certain buffer components at the lower wavelengths. To reduce the effect of highly absorbing buffers and enable measurements further into the UV before a 2 AU limit is reached use a flow cell or cuvette with a shorter pathlength.
Is it better to use multiple repeat scans or an increased sampling time-per-point to reduce noise?
Noise levels in CD spectra can be reduced by increasing the sampling time-per-point setting or averaging repeat scans - each approach yields the same result. Repeat scans produce a series of faster scans that enables rapid identification and correction of inappropriate settings, but does require data processing to average the final spectrum. Increasing the time-per-point setting produces a single spectrum that requires no further data processing.
Why is the CD background not zero?
The CD spectrum background yields a signal close to zero, but may reveal minor features caused by birefringent properties of the optics and detector. The baseline should not change over time and is subtracted during normal processing of CD data i.e. buffer spectrum subtraction.
How often should I replace the lamp?
The recommended lamp lifetime is approximately 1,000 hours. As lamps age, they become less stable, less intense and more difficult to ignite. It is very important that lamps are not operated beyond the manufacturers maximum lifetime (normally 2000h).
How do I align the lamp?
There is no need to realign the lamp. A pre-aligned lamp assembly facilitates rapid lamp replacement. This ensures that the lamp alignment is always optimal and greatly simplifies the lamp exchange procedure.
What is the purpose of the auto-shutter?
The auto-shutter protects the monochromator from exposure to far UV light when the system is idle. The shutter opens before a measurement starts and closes when measurement is complete.
What is the height of the measuring beam?
The distance between the base of the cuvette holder (single cell or Chirascan 6-cell) and the optical centre line, the mid-point of the measuring beam, is 15mm. Note that the single cell holder, supplied as standard, includes a removable spacer which elevates the cuvette by 6.5mm and can allow the sample volume to be reduced.