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Optimization in liquid chromatography - Guiochon G.

Guiochon G. Optimization in liquid chromatography - Horvath, 1980. - 344 p.
Download (direct link): optimizationiquidchromatography1980.pdf
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results in large band volumes that are not affected by the detector and
the connecting tube, whereas the narrow bands obtained at the small
flowrate through narrow columns have small volumes and can readily be
disturbed by extrncolumn effects.
In this section we discuss the contributions of the detector time con-
44
George* Guloclion
1ABI.K VIII
EquadoM Specifying Minimum Performances of
Equipment and Sampling Conditions
Equation Equation number
, = -*JL 63
" Vn
Kh * " ^ "(1 + *') hd,Vfi 67
r*l < 69U1 + kWlNd* ~ 70

Vn 75
stant, detector cell volume, connecting tubes, and sampling device on
band spreading as well as the effect of detection limits. The discussion
of these phenomena in gas chromatography by Sternberg (45) is also
applicable to extracolumn band spreading in liquid chromatography. A
detailed experimental and theoretical study has been published recently
(57). The equations used in the following discussion are given in Table
VIII for easier reference.
A. Detector Time Constant
' This is still the most serious problem at present, as the time constant
of ^nost detectors ranges from 0.5 to 3 sec including the time constant
of the detector electronics to which the analyst has little access. Only
a few commercial detectors have a time constant less than 0.5 sec whereas
a few detectors, which are used mostly to build inexpensive homemade
liquid chromatographs, have a time constant well in excess of 3 sec.
The effect of the detector time constant on the apparent efficiency
depends only on the time width of the bands. It has been shown by Sch-
mauch (41) and by Me William and Bolton (42) that the ?aiOa recorded with
a detector having a time constant r is wider than the actual profile by a
factor (1 + r/<rt), where <rt is the time standard deviation of the
profile, provided this factor is less than about 1.2. Moreover, the peak
height becomes smaller although the peak area remains unchanged. The Hist
mo ment (retention time) of a peak increases by o and the retention time
of the
Optimization in Liquid Chromatography 45
pcuk muxiinum by a somewhat smaller amount. If the decrease in apparent
efficiency is required to be smaller than a*, the detector time constant
should be smaller than the maximum permissible time constant rM that is
evaluated by
* vfe ;
1631
Equation (63) (ef. Table OO) shows th&t if this requirement is fulfilled,
the relative increase in retehtion time is less that) ?/s/N. j If we
accept an efficiency loss of a few percent, we cab tolerate a 1%
contribution for each part of the equipment and let a* = 0.01. Then Eq.
(63) shows that in order to obtain N = 5000 plates and 4*5 min, the time
constant should be less than 0.4 sec, which is quite if demanding
requirement in view of the performance of currently available commercial
liquid chromatographs. . ;
In most cases the time constant of the detector is due to'the slowness
of the electronics', this is especially true for optical detectoii. There
would be no technical problem to reduce the time constant o 20-50 msec,
although the noise level is expected to increase somewhat; With such fast
detectors computer data acquisition becomes necessary, as recorders with
a time constant less than 0.5 sec are rare and expensive. Since they are
difficult to maintain, they are impractical.
The time needed for the mobile phase to sweep the cell Volume is not
included into the time constant since the contribution of thii volume
will be discussed below. 1
If the detector is not linear, the recorded peak is smaller and
broader than the actual peak. It has been shown that if the detectbr
response can be expressed by
o = kC (I - aO )
(64)
the peak variance increases by a factor of 0.4 ah/k, where h is the
actual peak height (43). This effect can be important especially; with a
UV detector. }
The contribution of slow detector response can be neglected when the
base peak width is at least 40 times larger than r [cf. Eq. |63)]. In
practice it is difficult to correct for such distortion because the time
constant concept is only an approximation. It is not very reproducible^
and is sensitive to changes in the characteristics of the various
elements of the electronics. Furthermore, detectors, amplifiers, and
recorders are not first-oider syslems e ml (heir response is only
approximated by an exponential function (44). The response time is
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