Beware of solubility effects! As your substance is badly soluble in water, you might have partly undissolved substance, depending on concentration and water/methanol ratio. In this case light scattering from undissolved matter will contribute to your absorbance reading (unstructured band, strongly increasing towards short wavelengths)

I seem to recall a colorimetric method for Methanol 40-50 years ago.  But GC -FID replaced it.

Thank you Peter Kapusta. 

My question is actually focused on how to get accurate result when using UV-Vis spec to measure the absorbance of a material (for example, acetaminophen) in  methanol/water mixture.  I have read some couple of articles on methanol/water mixture used for dissolving acetaminophen but my question is will the methanol/water mixture influence the absorbance reading of the acetaminophen? 

As I recall Acetaminophen absorbs quite well at 248 nm (?) and Methanol/Water does not interfere.

Thank you so much Bruce Neagle.

Beware of solubility effects! As your substance is badly soluble in water, you might have partly undissolved substance, depending on concentration and water/methanol ratio. In this case light scattering from undissolved matter will contribute to your absorbance reading (unstructured band, strongly increasing towards short wavelengths)

Mr. Oginni,

In the same way as you measure unknown solute. You should treat the CH3OH as solute, while the water as solvent, studying "solute"-solvent interactions.

Usually there are solvent effects on spectra of dissolved compounds. You should check this in advance with known content of solute, i.e. compare spectra in pure methanol and in solvent mixtures safely above water content with visual precipitation.

It seems that you are not familiar with internet searching for your research and Google scholar!!

The answerers gave you very good answer and valuable comments.

launch google scholar and insert :

acetaminophen solubility

Among a lot of hits, you'll  find the joined pdf.

http://ci.nii.ac.jp/els/110003624627.pdf?id=ART0004131043&type=pdf&lang=en&host=cinii&order_no=&ppv_type=0&lang_sw=&no=1479149497&cp=

please  read the joined pdf and picture  about co-solvents (p.e. ethanol/water) and solubility and extin.coeff. of acetaminophen.

Please note that methanol is very toxic and you must work with a lab hood .

According to the very good answers of the answerers, you must use methanol (spectrocopy grade)  and dry it prior to use.

Water must bidistilled.

What a kind of spectrometer are you using ? (single or dual beam or diode arrays).

PLEASE insert  a UV-Vis spectrum in your next answer.

JRG

drying of methanol for use in mixtures with water seems unnecessary, however, to get it clean from aromatic contamination it is a good strategy to dry it over CaO and distil it, use the middle fraction, by controlling different fractions with UV spectroscopy.

If you want to check solvent effect then run acetaminophen dissolved in pure methanol and compare it with blank methanol absorbance in UV..then do same with mixture...calculate solubility difference in both cases..but keep in mind that your standard curve in UV should be in both e.g. in just methanol and in mixture of methanol water..

In some cases, no difference is observed either you prepare your standard curve in organic or organic aqueous mixture.. 

Beware that there are many grades of Methanol and typically it contains impurities that absorb strongly around 200-230 nm. Some HPLC-grade variants have low absorbance (e.g. 0.1 at 210 nm) in this range:

https://www.shimadzu.com/an/hplc/support/lib/lctalk/35/35lab.html