Ask the Expert-Answer

Q: What is bandwidth and how does it affect my measurements? – Puerto Rico

A: Thermo Scientific: When light passes through a spectrophotometer’s monochromator, a Gaussian intensity distribution of wavelengths emerges from the slit. This can be visualized as a peak. The spectral bandwidth is defined as the full width of this band of light at one half of the peak maximum. The spectral bandwidth of a spectrophotometer is related to the physical slit-width of the monochromator and, therefore, to the resolution capabilities of the instrument.

Generally speaking, using a narrower bandwidth can improve resolution of closely spaced peaks for samples exhibiting more than one absorbance peak across the absorption spectrum. However, reducing bandwidth also leads to a reduction in energy through the system, resulting in a corresponding decrease in the signal-to-noise ratio (S/N) of the peak. A reduction in the S/N can result in decreased data precision and repeatability, accuracy of the results, detection and quantitation limits and overall method robustness. Consequently, it is necessary to balance the need to resolve spectral features of the sample with the requirement to keep the noise at an acceptable level. That being said, a bandwidth that is appropriate for one analysis may not be optimal for another.

Q: I am using a stainless steel fiber optic probe for remote sampling. How can I get more repeatable measurements of my sample that are closer to those I get using a standard cuvette method? – Spain

A: Thermo Scientific: Fiber optics do reduce the energy throughput of a system; therefore, it is important to make sure that the system is properly optimized. This can have some affect on the repeatability of the measurements and may also lead to a misunderstanding of the effects of the dynamic range. No stainless steel or other types of machined probe materials, such as Torlon, have a highly accurate path length as it is impossible to machine them to very tight tolerances. These types of probes are very good for solutions which are not extremely aggressive and where the user requires a relative reading to a standard or can tolerate a slight offset in absolute photometric accuracy. If absolute photometric accuracy is a requirement, a VERSA probe is a better alternative. The tolerances of its quartz probe tip are equal to that of a cuvette. Also, as a general rule of thumb when performing measurements in a fixed wavelength or Quant Mode, a signal averaging time of 3 seconds will greatly improve the precision of the measurement.

Q: Can my analysts look at and analyze data generated on the spectrophotometer’s PC workstation on a computer at another location? If so, will this affect 21 CFR Compliance? – Wisconsin

A: Thermo Scientific: Data files obtained using our VISION software can be opened and analyzed on any computer that has the software installed. When 21CFR Part 11 Compliance is desired, Thermo Security Administration can be installed on the network allowing user access levels to be maintained at all networked computer stations used.