Peak, variance

As already mentioned, there are two so called "dead volumes" that are important in both theoretical studies and practical chromatographic measurements, namely, the kinetic dead volume and the thermodynamic dead volume. The kinetic dead volume is used to calculate linear mobUe phase velocities and capacity ratios in studies of peak variance. The thermodynamic dead volume is relevant in the collection of retention data and, in particular, data for constructing vant Hoff curves. 

The use of 5 pm particles permits the use of much longer columns due to the increased permeability. This, in turn, permits the use of much larger sample volumes. In fact, for a column 20 cm long, eluting a solute at a (k ) of 5, the maximum sample volume that can be used without increasing the peak variance by more than 10% will... 

Now, from the Plate Theory (Chapter 6) the peak variance in volume units (Oy) is... 

Assuming the peak variance is increased by 5% due to the sample volume, (Vi) will... 

Column type Peak variance (o) Calibration slope (D) Specific resolution (R,p)... 

Traditionally, column efficiency or plate counts in column chromatography were used to quantify how well a column was performing. This does not tell the entire story for GPC, however, because the ability of a column set to separate peaks is dependent on the molecular weight of the molecules one is trying to separate. We, therefore, chose both column efficiency and a parameter that we simply refer to as D a, where Di is the slope of the relationship between the log of the molecular weight of the narrow molecular weight polystyrene standards and the elution volume, and tris simply the band-broadening parameter (4), i.e., the square root of the peak variance. 

As already stated any sample placed on a column will have a finite volume, and the variance of the injected sample will contribute directly to the final peak variance. It follows that the maximum volume of sample that can be placed on the column must be limited, or the column efficiency will be seriously reduced. Consider a volume Vi, injected onto a column. Normal LC injections will start initially as a rectangular distribution and the variance of the eluted peak will be the sum of the variances of the injected sample plus the normal variance of the eluted peak. 

It is generally accepted that the maximum increase in band width that can be tolerated due to any extraneous dispersion process is a 5% increase in standard deviation (or a 10% increase in peak variance). 

Increase In peak variance) was the maximum extra-column dispersion that could be tolerated without serious loss In resolution. This criteria is now generally accepted. 

Graphs of Peak Variance against Flow Rate for Coiled and Serpentine Tubes... 

When measuring the recorded peak variance we must keep in mind that the observed width of a peak consists of two parts the width due to the distribution process in the column and the undesirable dispersion outside the column (injector, connecting tubing, detector, everything that comes from the instrument). From the theorem of variance additivity of independent processes we can write... 

A reduction parameter 8 of 0.091 corresponds to a 10 % increase in peak variance. The dependence of the peak width on the injected plug length for alkyl and aryl benzoates in packed CEC was investigated experimentally [30]. At low injection plug... 

Second moment measures peak variance and hence [Pg.16]

The sensor volume of a detector can cause dispersion and contribute to the peak variance in two ways. Firstly there will be dispersion resulting from the viscous flow of fluid through the cell sensor volume, which will furnish a variance similar in form to that from cylindrical connecting tubes. Secondly, there will be a peak spreading which results from the finite volume of the sensor. If the sensor has a significant volume, it will not measure the instantaneous concentration... 

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