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High temperature liquid chromatography – a brief review about an emerging technique


by Thorsten Teutenberg, Institut für Energie- und Umwelttechnik e. V (IUTA), Bliersheimer Straße 60, 47229 Duisburg, Germany teutenberg@iuta.de


This review is focused to present a general overview about high-temperature liquid chromatography. It starts with a brief definition and then explains the necessary requirements to make use of this emerging technique. Also, the advantages of high-temperature liquid chromatography such as the reduction in the mobile phase’s viscosity and the possibility to replace toxic organic solvents with water are outlined. Furthermore, the influence of temperature on selectivity is demonstrated. This means that temperature gradients can be integrated into method development to optimize the resolution of critical peak pairs.


In the last few years, there is renewed interest to explore the full potential of temperature in liquid chromatographic separations. Why is this? Although it might sound curious, temperature can be regarded as a universal parameter in liquid chromatography. Temperature influences almost every other parameter which can be used to optimize a separation in terms of speed and resolution [1,2]


. However, this is


only one aspect. There are some special hyphenation techniques which rely on the use of a water-only mobile phase [3-6]


. In this


case, temperature is the only option to change the solvent properties of water, which becomes more like an organic solvent with increasing temperature [7]


. Before going into


further detail, it is useful to define the practical temperature range in high- temperature liquid chromatography.


Until yet, a definition of high-temperature liquid chromatography does not exist although this technique has emerged as the topic of many scientific meetings and symposia. First of all, the lower temperature range needs to be defined. In this respect, the boiling point of the mobile phase should be considered. Methanol and tetrahydrofuran, which are widely used in reversed-phase liquid chromatography (RP-


HPLC), start to boil at 65 and 66 °C, respectively. Hence, the domain of high- temperature HPLC is entered at around 60 °C because increasing the temperature requires increasing the outlet pressure above the atmospheric pressure to prevent a phase transition of the mobile phase in the HPLC system. The upper temperature can then be defined as the point where every mobile phase which is used in reversed phase HPLC will turn into a supercritical fluid. While most of the organic solvents will become a supercritical fluid around 200 to 250 °C, the highest critical temperature is observed for water at 374 °C.


Now that the domain of high-temperature HPLC has been defined, which extends from 60 to 374 °C, the question of what is the useful temperature range needs to be addressed. Although in some fields of application, temperatures as high as 60 °C will not be tolerated and are considered too high to be used, the application of temperatures as high as 370 °C with a pure water mobile phase has been reported in the literature [8]


. Even if the complete


temperature range can be used for high- temperature HPLC, a limited temperature range is currently available for routine analysis. The reason is that the technical


requirements dictate the real upper limit. Most conventional LC heating systems are only capable to raising the temperature to 80 °C. Although it is possible with every chromatographic system which is equipped with a column oven to enter the domain of high-temperature liquid chromatography, the region cannot be exploited further. Therefore, some instrument manufacturers have developed special heating systems which can generate temperatures as high as 200 °C [9-11]


. The second aspect which has to


be considered is the stability of the stationary phase. When HPLC was young, silica based reversed phase materials tended to rapidly degrade even at moderately elevated temperatures around 80 °C. It is therefore not surprising that alternative materials with an enhanced temperature stability like polystyrene-divinylbenzene (PS-DVB) or polymer-coated metal oxide stationary phases were already being examined at the end of the 1980s. Although silica-based phases have long lagged behind, they are now catching up in terms of stability at high temperatures. In some cases, they are even more stable than their metal oxide based counterparts. From many recent studies it can be deduced that temperatures as high as 200 °C will not lead to an immediate


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