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liquid chromatography column - Scott R.P.W.

Scott R.P.W. liquid chromatography column - John Wiley & Sons, 2001. - 144 p.
Download (direct link): liquidchromatographycolumntheory2001.djvu
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Thus if (D) is the voltage output from the detector, equation (18) can be put in the form:-
, mt') Bx m-'i
+ pXge
Where, (C) is a constant,
(a) is the response of the detector to solute (A), and (p) is the response of the detector to solute (B).
Equation (19) was examined by Scott and Reese (15) employing mixtures of nitrobenzene and fully deuterated nitrobenzene as the test sample. Their retention times were 8.927 min and 9.061 min respectively giving a difference of 8.04 seconds. The separation ratio of the two solutes was
1 fhA f tkiA frAI\t /Nf fhn \1/ |1) '
I iv u ic ci i iticnt ico vi me 11 vi it cji iv cai yvi uvuo vi ti ic ^/ccjiw vv ci
5908 and 3670 theoretical plates respectively. The detector was, not surprisingly, found to have the same response to both solutes, i.e. = p. Thus inserting these values in equation (19),:-
A range of concentrations of the two substances were inserted in equation
(20) and a curve constructed relating retention time of the composite peak (calculated by means of a computer) to mixture composition. The results are shown as the curve in figure (7)
Figure 7
Graph of Retention Time of Composite Peak against Composition
of Mixture
The retention time of a series of mixtures of nitrobenzene and deuterated nitrobenzene were determined by Scott and Reese and are shown plotted as points in figure (7). It is seen that close agreement is obtained between the experimental points and the theoretical curve The procedure described is an interesting alternative for the analysis of mixtures of closely eluted solutes, to the difficult and tedious construction of columns of extremely high efficiencies. With great care and the use of modern sophisticated computer programs the required accuracy can be easily obtained, and it is suprising that this approach is not used more often.
A Theoretical Treatment of the Heat of Absorption Detector
The Heat of Adsorption Detector, devised by Claxton (16) in 1958 has been investigated by a number of workers (17,18,19) but although once commercially available, has not been extensively employed as an LC detector. One reason for this is the curious and apparently unpredictable shape of the temperature-time curve that results from the detection of the usual Gaussian or Poisson concentration peak profile. The shape of the curve changes with detector geometry, the operating conditions of the chromatograph, the retention volume of the solute and for closely eluted peaks, it produces a complex curve that is extremely difficult to interpret.
The detector consists of a small plug of adsorbent, usually silica gel, through which the eluent passes after leaving the column. Embedded in the silica gel is either a thermocouple or a thermistor which continually monitors the temperature of the adsorbent and its immediate surroundings When a solute eluted from the column enters the cell, some is adsorbed onto the surface of the silica gel, the heat of absorption is evolved and the temperature rises. When the peak maximum has passed through the cell, the solute desorbs from the silica, absorbing heat with a consequent fall in cell temperature. The output of the temperature measuring device will thus, first record a rise in temperature, and subsequently a decrease in temperature relative to its surroundings. The form of the temperature profile will depend on the heat loss during the adsorption/desorption process and the relative plate capacities of the detector cell and the associated column. The elution curve equation, as derived from the plate theory, can be used to obtain an expression for the change of temperature of the detector cell with the volume of mobile phase flowing through it.
Consider a small cell, containing the adsorbent, situated at the end of the column through which the column eluent passes. It is assumed that the eluent is brought to a constant temperature by a suitable heat exchanger situated between the cell and the column and that the exchanger does not contribute significantly to band dispersion. Let the cell have internal and external radii of (|) and () respectively and length (1). The following postulated will be made:-
1/ The flow of mobile phase through the cell is constant.
2/ The temperature of the cell surroundings is constant.
3/ The cell is of sufficient size that solute equilibrium between the two phases can be assumed.
4/ The cell Itself does not contribute significantly to band dispersion. Consider the heat balance of the cell,
{(Heat Capacity of Cell) X (Change in Temperature)} = (Heat Evolved in Cell)
- (Heat Convected from Cell by Mobile Phase) - (Heat Conducted from Cell)
The volume flow of mobile phase will be measured in 'plate volumes' (v) of the attached column and the column plate volume will be designated as (c0) for solute (a). Let a volume dv of mobile phase pass through the cell carrying solute that is absorbed onto the silica with the evolution of heat, and let the resulting temperature change be (d0).
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