Frequently Asked Questions for Chirascan Instruments
Lamps
CD Measurements
- Why is the CD spectrum noisy in the far UV?
- Why is the amplitude of my CD spectrum wrong?
- How do I calibrate Chirascan to accurately measure CD?
- Why is my CD baseline so far from zero?
General
- Why is there a slow drift on my signal?
- Do I need to turn off the room lights when removing a detector?
What is the life of the lamp?
The Xenon lamp used in Chirascan degrades in performance by different amounts at different wavelengths. The best way to decide if the lamp needs replacing is to monitor its performance in the far UV, usually at around 180nm. As the lamp performance drops off, so the photomultipler high voltage with no cell in the cell holder will increase. By regularly checking the high voltage recorded for a purged system without a sample at 180nm and a 1nm bandwidth, the operator will be able to gauge any degradation in the lamp. Typically the lamp should be useful for up to 1000 hours.
How do I align the lamp?
The lamp is pre-aligned at the factory. Any attempt to align the lamp will result in a poorer performance.
Why is the CD spectrum noisy in the far UV?
The nitrogen purging may not be on, or of a high enough flow rate. The recommended flow rate is 1 Lmin-1, 3 Lmin-1 and 1 Lmin-1 for the lamp, monochromator and sample housing respectively. If the nitrogen flow is on at the recommended level and you are still having trouble getting adequate performance, check that the stainless steel purge tubes are firmly connected at both ends.
It is important to keep the absorbance of the sample relatively low (ideally around 0.8 AU) in order to ensure sufficient light reaching the detector. Thus it is important to reduce the buffer concentration sufficiently (e.g. 10 mM phosphate buffer). Chloride ions should be avoided.
Another means to reduce the absorbance is to reduce the pathlength of the cell. This will affect the amplitude however. Typical pathlengths for CD spectra are 0.1mm - 0.5mm in the far-UV and 1mm - 2mm in the near-UV. Generally the bandwidth is set to 1nm, although it can be increased to increase the intensity of light striking the sample. Note that doubling the bandwidth will quadruple the light intensity.
Why is the amplitude of my CD spectrum wrong?
It may be that the CD calibration value needs to be checked and reset. See Test Chemistry, CD calibration for Chirascan.
How do I calibrate the Chirascan to accurately measure CD?
See Test Chemistry, CD calibration for Chirascan.
Why is my CD baseline so far from zero?
The optical cell you are using may have a higher than normal birefringence or the detector may not be in its optimal position. Check that the dotted line on the detector body is aligned with the mark on the exterior of the detector mounting. In any case, it is important to measure a CD baseline to correct for any CD artefact introduced by birefringence in the cell or photomultiplier window.
Why is there a slow drift on my signal?
It is essential to allow the instrument to warm up after turning on the lamp. A half-hour period of stabilisation is recommended if starting from cold.
However, drift may be due to photochemistry. If your sample is photolabile, it may be necessary to reduce the bandwidth or use the attenuator to reduce the light intensity striking the sample. In extreme cases, you may need to restrict the short wavelength limit of your measurement.
Do I need to turn off the room lights when removing a detector?
No. The HT to the detectors is automatically cut when the lid of the instrument is open and the lid must be opened to release the detectors.
If you have a question that is not answered here please e-mail the Technical Support Department.