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qiucongjiao2008

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5.3. Possible ways to push the practical limits
Several approaches have been investigated that could be considered to improve the performance level of unidimensional HPLC columns. They are discussed in more detail in the following subsections. The most important of them is the development of high-temperature HPLC [77,86,87,223]. This trend was under way for several years now, although its origin is far older
[78,102]. It aims at shortening analysis times by decreasing the mobile phase viscosity. The second idea discussed below was far less successful. Microwave irradiation seems to accelerate the mass transfer kinetics, hence could increase the column efﬁciency [224,225]. The gain, however, is limited. Finally, it is possible to harness the conventional preparative process of recycling chromatography that has been successfully used to resolve
pairs of compounds, notably enantiomers, that have a low separation factor. This process consists in recycling the column eluent separation through the column. Because complex mixtures with a wide range of retention factors are now considered, not a binary mixture, recycling would not be used. In this new implementation, the entire column eluate would be pumped on-line
into a second column. A suitable pump would raise the eluate pressure to send it through the next column. The chromatograph would work like a pipe-line with intermediate pumps that would re-compress the mobile phase.
   5.3.1. High temperature HPLC
   Most of the past attempts at designing and making high efﬁciency columns (and an instrument to operate them) have been carried out while operating these columns at room temperature,under conditions that are otherwise quite conventional. The use of high-temperature HPLC was suggested by Antia and Horv´ ath[78] long ago. They showed that analyses were considerably accelerated and that the column efﬁciency was improved when the column was operated at high temperatures. They made a theo-
retical investigation of the effects of sample decomposition on the column efﬁciency and suggested that those effects should be alleviated by the high speed of the separations achieved. Finally,they discussed the small changes that should bemade to classical equipment to operate columns at temperatures in excess of the normal boiling point of the mobile phase. However, experimental difﬁculties have slowed down the diffusion of this method,particularly the thermal stability of the stationary phases that
are used in RPLC. It is important that the temperature gradient across chromatographic columns be very low, no more than a few ◦C. A greater radial thermal gradient will cause a large loss in efﬁciency.Depending on the phenomenon that this radial temperature gradient triggers, a radial gradient of the mobile phase velocity, a radial gradient of the organic modiﬁer concentration,or a radial gradient of the retention factors of the analytes, the loss in column efﬁciency is proportional to the column diameter
to the power four to six. This, obviously, means that columns packed with ﬁne particles and operated with a high inlet pressure, at a fast mobile phase velocity must have a narrow bore in order to permit the radial leak of the heat generated in the column. Alternately, the column wall could be made to have as low a heat conductivity as possible. Insulating the column wall
with a thick layer of plastic foam could help to achieve this goal but, unfortunately with such columns, the temperature would increase rapidly along the column and, with long columns, could reach a level incompatible with the production of the separation power desired.
   In recent years, conventionalHPLChas been conductedmore aften at high temperatures, for several reasons. The most important of them is that viscosity decreases rapidly with increasing temperature and that, everything else being constant, the hold-up time decreases in proportion to the viscosity of the mobile phase (see Eq. (37); note that Eqs. (30) and (34) cannot be used for the purpose of this discussion because ηDm is nearly con-
stant,merely increasing in proportion to T) [77,78]. The fact that the mass transfer kinetics is also enhanced by an increase in the column temperature helps as well.
   Keywork to develop high-temperatureHPLCwas undertaken by Carr and coworkers [86,87],mostly to accelerate the analyses made in the second dimension of LC × LC separation schemes. In this method, the eluate of a ﬁrst column is injected periodi-cally into a second column onwhich the fractions eluted fromthe ﬁrst column are separated after a different retention mechanism.This approach has the potential of considerably increasing the separation power available to the analysts. However, it makes
a stringent request, that the fractions that are collected at the exit of the ﬁrst dimension column and are then injected into the second dimension column must have a volume not exceeding cally into a second column onwhich the fractions eluted fromthe ﬁrst column are separated after a different retention mechanism. This approach has the potential of considerably increasing the separation power available to the analysts. However, it makes a stringent request, that the fractions that are collected at the exit of the ﬁrst dimension column and are then injected into the
second dimension column must have a volume not exceeding the standard deviation (in volume unit) of the bands eluted from the ﬁrst column [226]. The longer the ﬁrst dimension column,the less drastic this constraint. Obviously, carrying out the second separation at high temperature makes it easier to meet the requirement.

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