Optimizing sample pretreatment: temperature

In a recent semester I have been involved in several training activities. The most important of these have been the training day for water analyses laboratories organized by EKUK and the practical aspects of working in the laboratory by TÜ Katsekoda.

My lecture/seminar in the first training was about sample matrices as the source of uncertainty. In this lecture the main idea was to show how large uncertainty source may a matrix actually be. As I have been dealing with matrix effects since my masters thesis this is a very interesting point to me. On the practical aspects of working in the laboratory course my major topic was preparing samples for analyses.

Whether considering uncertainty or trueness it ends up with the understanding that a good sample pretreatment is a base for a good analytical method. It seems like an ongoing discussion on whether liquid-liquid extraction of solid phase extraction is a technique providing more possibilities for efficient sample prep. However, what I have often observed in the trainings and lectures is that one important parameter influencing sample extraction, that people do not come up with, is temperature.

Common knowledge is that higher temperatures are advantageous for extracting analytes from solid matrices and in these cases higher temperature helps to break the “bonds” between analyte and matrix. But what is not that often considered is the temperature effect on liquid-liquid extraction.

We know that water properties strongly depend on the temperature. Based on the NISTdatabase the dielectric constant of water ranges from 87.7 to 55.7. The latter value is more similar to acetonitrile then to “water” as we consider it under standard conditions. This means the enthalpy of solvatation for different species changes remarkably with temperature. For example Z. Congliang et al has observed that octanol-water distribution coefficient of an antibiotic sulfamethoxazole is reduced 5 times while working at 45 °C instead of room temperature. And for extracting the sulfamethoxazole from water with octanol the recovery would drop from 89% to 61%. Unfortunately data for lower temperatures are not available to evaluate if lower temperatures could result in higher recoveries. These effects are larger for compounds with low distribution coefficients and therefore with lower recoveries.

It is even interesting to consider how much could year-around room temperature fluctuation influence the extraction?