uv detection at 205 nm

[vijaykhetmalis]

Hi Friends, I have a question that is it O.K if i detect my drug(Levetiracetam) at 205 nm as others have suggest me to change this wavelength as it comes at cut off wavelength, Can anybody suggest me regarding this.

[shaun78]

if it comes at the UV cutoff for your mobile phase and you absolutely must use 205 nm as the detection wavelength, then you need to change your mobile phase.

Water and acetonitrile have a UV cut off of 190 nm. If you need to raise or lower the pH, then use acids and bases that have no UV band, such as HCl and NaOH.

You should not be developing a method at the UC cutoff of your mobile phase because you will see much noise in your baseling. This can cause problems with accuracy, precision, LOD, LOQ, etc...

[k66]

u better check other uv length by DAD - show where is the highest (top) signal response and take off 5 nm ( ex. 245 - 5 nm = 240 - optimum)
it's the best way
obviously sometimes it's impossible

[DR]

1) Not all lots of organic solvent are equal. Run a UV scan on a sample of each lot as it comes in. You will quickly find that the actual cutoff for some lots is helpfully lower than the 205nm specification.

2) At 205, even if you hand pick your solvent lots, you are going to have more baseline noise than you would at higher wavelengths because pretty well everything absorbs at 205nm.

3) If you are running gradients, you should probably take extra care to clean up your water before making your "A" phase. This will minimize system peaks late in the gradient. Search my posts for the word "Empore" for more details.

[HW Mueller]

The definition of "cut-off" for a 10mm cell path length seems to be: The wave length which gives an absorbance of ~1. Now, the detector´s detector will not "see black" at that absorbance, also, most likely, your "absorber" might be strongly diluted by water.
The 10mm cell cut-off for H2O is at 191nm, that of MeOH at 205nm. A mobile phase of, lets say, 80 H2O + 20 MeOH will be near an absorbance of 0.2. One can work with that.
On the other hand, being above the cut-off wavelength doesn´t mean that there is no absorbance.