Detector apparent dispersion

Apparent Dispersion from Detector Sensor Volume... 

In practice, experimental peaks can be affected by extracolumn retention and dispersion factors associated with the injector, connections, and any detector. For hnear chromatography conditions, the apparent response parameters are related to their corresponding true column value by... 

The importance of the extra column dispersion now becomes apparent, as equation (26) shows that the minimum detectable mass increases linearly with the extra column dispersion. It is also becomes obvious that it is of little use designing a detector for increased sensitivity (Xd) if this is achieved (as is often the case) at the expense of increased extra column dispersion (oe). Conversely, if the chromatographic system is designed to have very low extra column dispersion, a proportional reduction in the minimum detectable mass will be achieved even if the... 

A more subtle effect of positioning occurs with dispersive spectrometers, when the distribution of light across the slit is altered by sample motion. A change in this distribution causes a shift in position at the detector, yielding a small apparent frequency shift. This frequency jitter can amount to a wavenumber or more and sometimes affects spectral subtraction or background correction. 

These measurements clearly show that convective flow Is the major origin of the dispersion observed at the output of a loop Injector. However, these measurements were made In a time domain which Is close to that where dispersion by diffusion should start to become apparent according to hydrodynamic theory. This Is apparent In the data obtained closest to the wall where the dispersion Is greatest. Solution linear velocity Is lower In this region giving greater time for diffusion. The small size of the detector allows the radial concentrations to be probed with little perturbation of the stream. This Is evidenced by the ability to reconstruct the bulk behavior from the individual radial measuresients. 

As an example of how molecular weight and poly-dispersity are apparent from such a method, we show to the right the overlay of four GPC traces from various samples of poly (methylmethacrylate). Here the x axis is the elution volume and the y axis is the response of a refractive index detector. The four samples were formed by continued irradiation of the polymer by y rays. Trace 0 was prior to irradiation, and trace 3 was after the highest dose of irradiation. 

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