Chromatography Forum

i was told that it is fixed wavelength? What is the problem with variable wavelength UV detector or PDA?

Historically fixed wavelength detectors used transition lines, as such the wavelength accuracy was second to none; variable or DAD systems have to use a mechanical system to disperse the radiation so there is always the potential for mis-allignment.

Almost all modern DAD instruments re-calibrate every time the power is cycled so this is no longer a significant concern in most labs. A wavelength check is still useful for variable systems but in reality if a suitable wavelength was chosen when the method was defined (not near a sharp absorbance change or a highly pH sensitive area of the spectrum) you will be comparing standards with analytes and as such a small long term change will not impact on the results.

I didn't know that fixed-wavelength detectors were still being made.....I've been doing HPLC over 30 years and never have used one...

i was told that it is fixed wavelength?

Whoever told you that probably also thinks the most economical mode of transportation is a horse (the gas mileage is great!).

Even if one were still commercially available, a fixed wavelength detector would be the best choice only if two conditions were met:
- your compound has an absorbance maximum at or near the wavelength of the detector (most commonly, 254 nm)
- your compound has an absorbance spectrum with a lot of structure, such that spectral resolution is important (in fact, the solution spectra of most compounds shows very little fine structure).

The small addition I will make is that the photodiode array detector typically will be less sensitive than a variable wavelength detector. That said, on the newest instruments, differences in sensitivity between photodiode array and variable wavelength detectors are getting much smaller due to the use of "light pipes" and other advances in detector optics design.

Thanks so much!!! i really appreciate your help. this is the forum I heavy rely on.

The small addition I will make is that the photodiode array detector typically will be less sensitive than a variable wavelength detector. That said, on the newest instruments, differences in sensitivity between photodiode array and variable wavelength detectors are getting much smaller due to the use of "light pipes" and other advances in detector optics design.

When I see a publication or application with detector wavelength of 254nm, or using an internal standard when one would not seem necessary, that's a prime indicator of an old procedure developed on a fixed wavelength detector or using manual injection respectively.