Yet since DADs are able to capture the full spectrum they can also be used in 3-D mode (time vs wavelength vs absorbance). This allows the researcher to better determine an unknown compound, for example, or to assess the purity of the sample. But, warns Franz, be careful to order 3-D-enabled software.
The frequency at which a DAD queries the sample can be adjusted to accommodate, for example, low- versus high-pressure separa- tions, or larger versus smaller expected bands. It’s certainly important to use a data rate that’s fast enough to adequately define the shape of peaks; Waters recommends at least 15 data points across the narrowest peak,1
but
faster means more data being collected and processed, which can slow the system. Most systems made today have a maximum data rate of at least 80 Hz, which Schulenberg- Schell says is “good enough” for even UHPLC, but 10-year-old DADs may not be able to meet those demands.
Table 1 – Purchasing considerations for diode array detectors
Lamps—deuterium or deuterium and tungsten? (This has implications for both spectral range and spectral resolution) Flow cell type, fl ow cell pathlength
Noise Drift
Dynamic range Data rate
Number of pixels/diodes (This has implications for spectral resolution) What to look for in a diode
array detector Although DADs can generally be purchased separately, “I would defi nitely see the detec- tor not just as an isolated box, but in the context of the entire system and the software,” says Franz. There’s not really an advantage in cherry-picking.
That being said, some systems come with op- tions, and available detectors may be part of the purchasing decision.
Cost and performance diff erences of (at least higher-end) DADs have been narrowing. Wavelength accuracy isn’t as important as it is in spectrometry, because in LC analysis the same slight deviation from exact wavelength
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