Diode Array Detector (DAD)
Definition
A UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a single wavelength. Lets the analyst do peak-purity checks and library-match each peak against a stored UV spectrum, which is the second-tier identification on top of retention time.
- Spectral range
- 190 to 800 nanometers (ultraviolet and visible light)
- Response time
- Full spectrum captured in under one second
- Typical configuration
- 1024 silicon photodiodes in a linear array
Common questions
What does a diode array detector actually measure?+
It measures the full UV-Vis light spectrum (190 to 800 nanometers) that gets absorbed by a compound as it comes out of an HPLC column. Instead of looking at just one wavelength, it captures the entire spectrum in under a second.
Why is knowing the full spectrum useful in forensic HPLC?+
The spectrum acts as a second fingerprint for identifying a peak. The analyst can check if a peak is pure (single compound) or mixed, and can compare the recorded spectrum against a library of known drug or poison spectra. This makes the identification much more reliable than retention time alone.
How many photodiodes does a typical DAD have?+
A standard DAD has roughly 1024 silicon photodiodes arranged in a line. Each one captures a narrow slice of the spectrum, so together they build the complete picture.
Related terms
- Absorbance (A)
- The base-10 logarithm of the ratio of incident to transmitted light intensity, A = log10(I0/I). Dimensionless. Linear with concentration in the working...
- Bandpass and Slit Width
- The wavelength interval the monochromator delivers to the sample. Routine forensic UV-Vis runs at 1 nm or 2 nm bandpass; pharmacopoeial methods...
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Densitometric Scanning
- The HPTLC quantitation step. After the developed plate dries, a Camag TLC Scanner 4 sweeps a narrow slit of UV or visible...
- Isocratic vs Gradient Elution
- Isocratic means the mobile-phase composition stays constant through the run; simple, reproducible, but limited resolving power for wide-polarity mixtures. Gradient means the...
- Method Validation Parameters
- The numbers a forensic method must hit before it is considered fit for routine use: linearity R² > 0.999 over the working...
- Molar Absorptivity (ε)
- The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and...
- Photomultiplier Tube (PMT)
- Cascaded electron-multiplication detector behind the monochromator slit on a dispersive scanning instrument. Reads one wavelength at a time at very high sensitivity.
- Reverse-Phase Chromatography
- Liquid chromatography with a non-polar stationary phase (most commonly octadecyl-bonded silica, called C18) and a polar mobile phase (water mixed with methanol...
- Sub-2 µM Particle Column
- A UHPLC column packed with 1.6 to 1.8 µm silica particles instead of the conventional 5 µm. Smaller particles give sharper peaks...
Explained in these topics
- HPLC, UHPLC and HPTLC for Forensic AnalysisA UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a single wavelength. Lets the analyst do peak-...
- UV-Vis Spectrophotometry and the Beer-Lambert LawA linear array of typically 1024 silicon photodiodes that captures the entire 190 to 800 nm spectrum simultaneously in under a second. Standard on every modern...