by
danko » Fri Apr 16, 2010 9:20 am
Hi Hans,
Glad you asked
OK, that’s how it works:
The working wavelength (beam) is still split just like on any other detector/spectrophotometer. The role of this particular design is meant to correct for lamp energy fluctuation and to give the possibility of zeroing the signal. So, the measuring beam goes through the flow-cell whilst the reference (the split) beam is redirected round the flow cell and to the PMT. These two beams are of the same wavelength and in fact originate from the same source/lamp. The reason that they don’t cancel each other and thus cancelling the analyte’s absorption is precisely the fact that the reference beam does not traverse the flow-cell. That’s the traditional referencing model. In the case of Agilent’s 1100 and 1200 detector models there is a second reference beam at a different wavelength to the analytical ditto. This reference beam traverses the flow-cell as well, but because it has a different wavelength than the analytical one (and does not overlap it) it doesn’t cancel the analyte’s absorbance and thus the peak is not affected.
Finally because effect like light scattering and refraction are less dependant on the wavelength compared to the molecular absorption there is a reasonable chance for the fluctuations caused by gradient and other artifacts to be detected both at the working wavelength and the one chosen for the reference wavelength (regardless of their origin – lamp wise).
So, the software compares these two signals and sets the resultant/derived signal to zero, thus cancelling the detected fluctuations (e.g. baseline rise or whatever due gradient etc.)
Hope the above is not too sleep-inducing
Best Regards