IF WE TAKE A MOBILE PHASE WITH A HIGH ABSORBANCE AND MY PRODUCT DOES NOT HAVING ANY ABSORBANCE IN UV REGION THEN CAN WE DO IT QUANTITATIVE ANALYSIS WITH THIS METHOD.

In some cases, yes.

If your analyte displaces the UV-absorbing component in the mobile phase, then the decrease in UV absorbance will be proportional to the concentration of analyte.

In practice, this works best when the UV-absorbing component of the mobile phase is present at relatively low concentration (e.g., a buffer). It is used occasionally for quantitation of inorganic ions using an HPLC system and UV detection; it goes by the name "indirect UV detection".

There are a number of pitfalls to be careful of if you try this approach:
- the analyte has to displace a significant fraction of the UV absorbing component, in order to get a reasonable signal-to-noise ratio. In practice this means that the UV absorber has to be present at a low concentration, or else the sample has to be injected at high concentration.
- The absorbance of the mobile phase must be within the linear range of the detector (usually, this means no more than about 1AU or so).
- The analyte and the UV absorber must displace one another. In practice, this means that the UV absorber must be the "strong" component of the mobile phase.

Bottom line: you can make it work, but it's not easy!

-- Tom Jupille / LC Resources Inc.

Right- indirect uv detection is also a well
established procedure w capillary ion analysis
wherein a UV absorbing ion (eg a benzoate) is
added to the bufer to produce a high background
absorbance. This approach can be quite sensitive.