The wavelength range of PDS detectors is 200-800 nm compared to 125-430 nm for VGA-101. It is easy to think that the PDA’s larger wavelength range is an advantage over the VUV detection range.
However, it is not the size of the wavelength range that is important. It is part of the UV spectrum being analyzed. The majority of compounds absorb in the 125-240 nm range meaning that the VUV has a distinct advantage.
A PDA on the left and a GC-VUV Detector on the right
Advantages of VUV Detectors
As shown in Figure 1, the three groups of electrons that may absorb in the UV range are sigma (σ), pi (π), and non-bonding (n). Figure 2 shows how each of these absorbs light at different wavelengths. Non-bonding (n) electrons absorb around 200-780 nm, pi (π) electrons absorb around 150-250 nm, and sigma (σ) electrons absorb at <150 nm.
This means that all three groups of electrons can be seen with a VGA-101 and a lot more spectral detail can be garnered than with a PDA, which is only able to see n electrons and some π electrons. Spectral fingerprinting of individual compounds is possible with VUV’s spectral detail, even if they are isomers.
Limitations of PDA Detectors
PDA detectors are generally paired with HPLC, which relies on liquid solvents to push everything through. It wouldn’t be possible for the PDA detector to see any sample molecules as these solvents start to absorb under 200 nm. As such, when it comes to spectral matching, PDA detectors have a blind spot. On the other hand, the VUV detector was designed specifically to identify compounds by their spectral fingerprints.
Figure 1. Two representations of a 2-Buten-1-ol molecule are shown in (a) and (b). In organic molecules, σ electrons make up single covalent bonds, π make up double/triple bonds, and n electrons are not used for bonding. The different types of electrons are color-coded in (b): blue corresponds to σ electrons, red to π electrons, and green to n electrons.
Figure 2. An electron can absorb energy from light if a photon has sufficient energy to promote the electron to a higher energy level. Sigma (σ), pi (π), and non-bonding (n) electrons each absorb light within different wavelength ranges because they require different amounts of energy to be promoted.
This information has been sourced, reviewed and adapted from materials provided by VUV Analytics.
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