A Brief Definition of High-Temperature Liquid Chromatography

High-temperature liquid chromatography is really a fascinating topic. Nowadays, there is renewed interest in this technique which has long been talked about.

When liquid chromatography was still young, the sky seemed the limit and the capabilities of liquid chromatography were discussed with great enthusiasm. For a practitioner who was not born during the really "hot" period of chromatography, it appeared that there were no preconceptions about the boundaries of liquid chromatography. It was in the early days of HPLC that Hesse and Engelhardt stated that temperature programming should yield the same results as solvent programming, and even concluded that this procedure would have advantages over solvent gradient elution as the solvents need not be changed. Today, this statement seems to be highly innovative because temperature programming is regarded very difficult to implement in industrial applications, although a number of publications have demonstrated the feasibility of this approach. Since this initial enthusiasm, temperature has long been neglected in liquid chromatography and has not attracted much attention. However, in separation science it is like fashion: the trends of the former decades appear again and sometimes they are presented as if they were totally new. The same seems to be true for this topic, which in fact is not that new. Nevertheless, the instrumentation has improved a lot since the early days of HPLC. Therefore, it is worthwhile revisiting this old but still very new variant of liquid chromatography.

1.1 What is High-Temperature Liquid Chromatography?

Although the question seems to be very trivial, it is not easy to give an answer. Up until now, a definition of this technique does not exist although it has emerged as the topic of many scientific meetings and symposia. I will therefore try to outline what I understand to be high-temperature liquid chromatography. Looking through the literature, a range of terms has been used – some more obvious than others:

• Subcritical water chromatography

• Subcritical fluid chromatography

• Elevated-temperature liquid chromatography

• Superheated water chromatography

• Hot eluent liquid chromatography

• (Ultra) High-temperature liquid chromatography (HT-HPLC)

• Thermal aqueous liquid chromatography (TALC)

• and others.

It seems to be very difficult to select a temperature range and to assign this region to define high-temperature liquid chromatography, as it will be called throughout this monograph. This is immediately clear if we look at the terms given above. Subcritical fluid chromatography directly refers to the mobile phase. But at which temperature range is a liquid subcritical? Many scales are referred to water, which also plays an important role in liquid chromatography. Therefore, if water is taken as a reference, an upper temperature limit of up to 374 °C, which corresponds to the critical point of water, might be considered. Above this temperature, water becomes a supercritical fluid. But how can we distinguish high-temperature liquid chromatography from conventional or room-temperature liquid chromatography? Clearly, the subcritical region extends to the lower temperature range, which is also the domain of conventional HPLC. This contradiction can be solved when we look at the third expression, which is elevated-temperature liquid chromatography. This means that the temperature should be higher than ambient temperature. But when do we exceed the temperature limit beyond which the region of high-temperature liquid chromatography is entered? Is it 40, 50 or 60 °C? In a recent review article on high-temperature HPLC, Heinisch states that "High-temperature liquid chromatography (HTLC) is a term which refers to any separation carried out at temperatures above room temperature (typically within a range from 40 °C to 200 °C) with a mobile phase in a liquid state". Personally, I think that 40 °C is too low to speak about high-temperature liquid chromatography. In my opinion, we should look at the mobile phases we are using. Since I will exclusively talk about reversed-phase liquid chromatography (RP-HPLC) in this book, relevant binary solvent systems which are used in RP-HPLC will be considered.

Again, we could take water as a reference solvent and define the normal boiling point of water as the lower temperature limit for high-temperature HPLC. The normal boiling point of a liquid indicates at what temperature the liquid will turn into a gas at atmospheric pressure. Such a phase transition has to be avoided in the whole chromatographic system because it can lead to the immediate destruction of the column and strong detector noise as will be shown later on. However, two arguments speak against this temperature. First of all, this is a very high starting point, because temperatures above 100 °C are already considered extremely high in some fields of application. The second point is that normally binary mixtures of water and an organic co-solvent are used in RP-HPLC. Typically, a separation is carried out in solvent gradient mode, if very complex samples have to be analyzed containing polar and non-polar compounds. We usually start with a high water concentration and end with a high concentration of the organic modifier. Usually, methanol and acetonitrile are the most widely used organic co-solvents in reversed-phase HPLC. Now let's have a look at the normal boiling-point temperatures of these solvents, which I have listed in Table 1.1, along with some other solvents which might also be used as modifiers.

Whereas water has the highest boiling point due to strong hydrogen bonding, the boiling points for the other solvents are much lower. Acetone already starts to boil at 56 °C. For the much more common solvents methanol and tetrahydrofuran, the normal boiling-point temperatures are 65 °C and 66 °C, respectively. This means that from the perspective of the pure components, a much lower temperature limit would be appropriate if the boiling-point temperature is taken as a reference to define the lower temperature limit of high-temperature HPLC. In this case, a lower temperature limit of about 60 °C for high-temperature liquid chromatography would be appropriate. Adjusting the temperature above 60 °C then requires raising the outlet pressure of the column above the atmospheric pressure. Otherwise, a phase transition would be inevitable when a solvent gradient is run from pure water to pure acetone....