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Page 92 Jenway Catalogue
Visible or UV/Visible?
Selecting a Spectrophotometer
UV-Visible spectroscopy is the measurement of the absorbance at a specific wavelength by a sample within the UV
or visible regions of the spectrum. It is one of the oldest analytical techniques and is used to identify the presence
and concentration of many molecular entities. Therefore the first question to ask is whether a visible or UV/visible
spectrophotometer is required? The near UV region is considered to be from 200 to 380 nm and the visible region
from 380 to 800 nm. Jenway have extended the ranges of some of the visible and UV/visible spectrophotometers up
to 1100 nm, which is considered to be in the NIR part of the spectrum.
Many applications require an option for spectrum scanning i.e. measuring the absorbance at each wavelength
within a region of the spectrum. However some laboratories only require scanning to ‘trouble shoot’ the occasional
problem. The cost of the instrument is significantly reduced if scanning is not required.
The majority of samples in UV/visible spectroscopy are liquid which are usually measured in 10mm glass or quartz
silica cuvettes depending upon the wavelength range required. Glass is only suitable for the visible region, whereas
quartz can be used in both the UV and visible region. A 10mm cuvette typically holds 2 to 3ml of sample. However
for applications such as DNA or protein assays where the amount of sample is limited Jenway offer micro cell holder
accessories and micro cuvettes that allow the accurate measurement of volumes down to 50μl. With the TrayCell
sample volumes as low as 2μl can be used. For extra sensitivity in sample measurement Jenway offer a range of long
path cell holders up to 100mm.
Jenway spectrophotometers are available with single, split-beam or double beam optics. Single beam
spectrophotometers are lower cost instruments, with the baseline to compensate for changes in lamp output
intensity and blank absorbance being stored prior to taking the sample measurement. The instrument automatically
compensates for the stored baseline when the sample is measured. In contrast double beam instruments record the
sample and reference simultaneously, resulting in the instrument electronics compensating for the reference in real
time. Double beam instruments tend to offer better stability in the UV region and are necessary when the blank or
sample solutions are unstable or transient.
UV/visible spectrophotometers generally use two light sources to ensure the spectral irradiance is of sufficient
intensity over the entire wavelength range. In these instruments the most common light sources used are tungsten
halogen lamps for the visible range and deuterium lamps for the UV wavelengths, with a source selector to switch
between the lamps. Jenway also offer a range of UV/visible and visible spectrophotometers that use a pulsed xenon
lamp. The pulsed xenon lamp offers a good intensity continuum over both the UV and visible parts of the spectrum
and has the added advantage of a much increased expected lifetime.
To speed up the time taken for sample processing and to reduce user error, modern laboratories often have the
requirement for spectrophotometers to save both methods and results. Jenway spectrophotometers offer a range
of storage media from internal memory storage through to removable media such as SD cards.
In commercial laboratories the need to increase sample throughput and reduce sample handling leads to the need
for automated sample introduction by using either a multi-cell changer or a sipper system. Multi-cell changers can
be used in manual or automatic operation and are often used for enzyme kinetic assays. Sippers improve sample
throughput by using a peristaltic pump to introduce precise volumes of sample into a flowcell.
Is scanning capability required?
What types of sample are being measured?
Optics system: single beam or double beam?
Which light source?
Is it a requirement to store methods and results?
Is it necessary to increase sample throughput?
Jenway Catalogue