grating, from where that rainbow is directed to a one-dimensional array of pixels. “Because nothing is moving you can calibrate it,” Bain explains, “and calculate which pixel number corresponds to what wavelength.”
UV-VIS specs will typically use either a tungsten lamp to produce visible light combined with a deuterium (D2
) lamp for UV, or a xenon flash lamp to cover both spectra. Tungsten and D2 bulbs need time to warm up, and
have relatively short lives, requiring replacement once or twice per year (which can be done by the user). Xenon lamps, on the other hand, are instant-on and last for years, but the instrument needs to be sent back to the factory for recalibration when they are replaced.
A split-beam or double-beam instrument utilizes a second (reference) pathway for light to travel—bypassing the sample—allowing the instru- ment to keep track of and correct for a change in lamp intensity over time. In a true double-beam setup, a cuvette with sample matrix can be placed in the path. The change of absorbance of that matrix can also be tracked and corrected for, a boon for kinetic experiments, for example.
Because xenon lamp intensity varies slightly from flash to flash, it is imperative to use it with a reference beam “to get really good stray light control and stability,” says Bain.
Most research-grade instruments utilize either a photomultiplier tube (PMT) or a photodiode array (PDA) detector. The former tend to be more expensive but are considerably more sensitive to very low light levels, as would be found when querying very highly absorbent samples. PDAs, on the other hand, will capture the entire spectrum at once, and can extend the range of the instrument to about 1100 nm (in the near-IR [NIR] range). Other detectors, such as indium gallium arsenide (InGaAs) and lead sulfide (PbS), are also sometimes used by UV-VIS specs to detect even further into the NIR.
Not to be outdone, detectors can also capture the entire spectrum (or selected parts of it) by allowing the monochromator to scan stepwise, recording the intensity of output at each wavelength.
Purchasing considerations
Double-beam or single-beam instrument A high-end research instrument will likely be a double-beam, with two monochromators (the second helps to further reduce stray light); have one or more PMTs; be more than a meter wide (the long distance between optical components also helps to control stray light); and cost more than $30,000. It may have the ability to measure up to 8 or 10 AU (absorbance units—meaning 1/108
to 1/1010 of the light gets through). But “not many people or applications require that high level of performance,” says Bain.
On the other end, an entry-level, nonscanning, single-beam instrument can have a small footprint and cost under $3000, notes Bain. But he warns that instruments at the very low end may not have the photo- metric range to accurately measure over about 1.5 AU, which is not high enough for a typical analytical lab. Photometric range may be listed in the instrument’s specifications as “photometric linearity,” “absorbance range,” or the like.
(800) 458-5226
info@julabo.com www.julabo.com AMERICAN LABORATORY • 9 • MAY 2014
2014-04_PRESTO-W9x_3x9_AmLab.indd 1 17.04.2014 11:57:27
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