Chromatography Forum

High every one ( Not Hi as we use High performance LC so we will be faithfull )

My question please : In HPLC grade methanol ( from Fisher Scientific ) you notice that there is absorbance at 210 nm around 0.17 A.U. wHAT i want to know is which this analyte or cmpd that absorb at this wavelength , because logically at the same wavelength you will see the same absorbance as peak when using same wavelength in UV detector .

If i have mass spectrometer i will see but unfortuantely i don't so could you please help with this , is there any paper talking about this as identification on methanol at 210 .
It is suppose to be at FIsher website something like mass spect for methanol but i am not able to locate it .
i will appreaciate any help

Thank you very much

Wesam

I think you are likely looking at the native absorbance of methanol...

λ 1 cm path, H2O reference
UV absorption
λ: 220 nm Amax: 0.3
λ: 230 nm Amax: 0.15
λ: 235 nm Amax: 0.1
λ: 240 nm Amax: 0.06
λ: 260 nm Amax: 0.02
λ: 290 nm Amax: 0.005
λ: 310 nm Amax: 0.003

While I do not know what impurities are commonly found in methanol, I do know that it is a good idea to purchase smaller quantities on a more frequent basis so that you get a variety of lot marks. When we have methods that use very low wavelengths, we scan samples of the various methanol lots on hand and pick the ones with the lowest absorbances in the 200-220nm range, marking them for use only with that specific assay.

Personally, I would tend to purchase B&J methanol if I had several low wavelength LC methods depending on it... (and I'd still scan them, using the ones with higher cutoffs for higher wavelength methods, wash bottles etc.).

I've seen the UV cutoff for methanol listed in various places as 205, 207, or 210 nm. If memory serves, UV cutoff is defined as the wavelength at which the absorbance for a 1-cm path drops to 1.0 .
In the case of methanol, the spectrum in the "end absorbance" region falls off very gradually (per AA's data). It doesn't take much in the way of carbonyl-bearing impurities to raise the cutoff, hence the necessity for screening lots.

What i see in my methanol bottle is that the higher absorbance is at 210 , but when i ask they say to me the cut off is 205 nm ? it seems illogical for me .

Do you thing i will able to identify this impurity in methanol at 210 when mass spect is used ? have you come across any paper did identification to see what is this at 210 nm .

Thank you for you all but i feel i am still loss

thanks

What about real time background correction? I think methanol signal do not exactly interfere with your signals, because your background absorbance is always corrected and subtracted from your sample chromatogram.
On the other hand methanol absorbance will be added to total absorbance detected by your detector, which may result in non-linear signals at high concentrations.

What i see in my methanol bottle is that the higher absorbance is at 210 , but when i ask they say to me the cut off is 205 nm ? it seems illogical for me .

Why illogical? Remember that UV spectra in solution are essentially continuous "blobs", and the definition of UV cutoff is arbitrary. If you have less than 1AU at 210, then you are above the cutoff.