In a nitrogen evaporation instrument, temperature and gas flow rate are the two major operational parameters that determine the rate at which the solvent is removed from a system. However, it is too large of an assumption to presume that both a higher temperature and gas flow rate will produce the most effective removal of solvent. Adjusting the bath temperature can have varying effects on the analyte, and it is important to remember that a higher temperature is not always optimal. In many cases it may be necessary to sacrifice time in favor of sample integrity. The investigator must determine the optimal combination of bath temperature and gas flow rate that will remove the unwanted solvent without degrading the sample. This article will focus on the variables to consider when choosing a bath temperature and a subsequent article will follow with considerations for gas flow.
Temperature is the operational parameter most often used to control the evaporation rate of the solvent from the sample. An increase in the bath temperature increases the evaporation rate of the solvent. An operational bath temperature approximately 10o C below the boiling point of the solvent being removed is often considered optimal. In some cases, however, the analyte or substance of interest may be susceptible to thermal degradation at or below the boiling point of the solvent in which it is contained, thus limiting the operational temperature range. In theory, reducing the operational bath temperature to lower the possibility of thermal degradation is a sound approach. However, it must be weighed against the possibility that longer exposure at a lower temperature may be just as detrimental as a shorter exposure time at a higher temperature. In multi-component or complex systems, some of the analytes or substances of interest may be partially or moderately volatile. At longer exposure times, these substances may exhibit measurable decreases in concentration which may not be as pronounced at a shorter exposure time at a higher temperature.
In addition to thermal degradation, there is also a very real possibility of chemical reactions between the sample and the other substances within the sample’s solution. Kinetically, the reaction rate of a system may increase or decrease with temperature depending on the reactants and reaction conditions. This being the case, it is possible for a longer exposure time at a lower temperature to result in greater damage to the sample than a shorter exposure time at a higher temperature. Another important aspect of the reaction kinetics is the effect of reactant concentration. In a chemical reaction, the reaction rate may also be related to the concentration of one or more of the components of the system. As solvent is removed from the sample, the concentration of the sample and other nonvolatile reactants or components increases, and it is possible for the reaction rate of intra-sample reactions to increase as solvent is removed. In this case, a longer exposure time at a higher temperature may result in a significant increase in the reaction products in the sample. Increasing the temperature to shorten the exposure time may increase or decrease the degradation or composition of the sample, while increased exposure time will always increase the effect of any degradative or intra-sample reaction processes.
As an example, consider a hypothetical system (System #1) in which the analyte is a polymer synthesized in toluene and quenched with methanol. The quenched reaction mixture is then transferred to a separatory funnel and extracted or washed with multiple portions of aqueous alkali, and the sample solution is then transferred to a pre-weighed sample tube. The reaction vessel is then rinsed with two portions of fresh toluene. The rinsed portions of toluene are then transferred to the same separatory funnel, washed as above and transferred to the pre-weighed sample tube. This tube is then mounted in an Organomation N-EVAP nitrogen evaporator to remove the solvent in order to isolate and quantify the polymer sample.
In System # 1, the sample solution contains toluene with a boiling point (BP) of 111o C, water with a BP of 100o C, methanol with a BP of 65o C, residual monomer with a BP of 60o C, and a non-volatile polymer (the target analyte or substance of interest) with a boiling point of "x". Based on the known composition of the sample solution, the water bath temperature is set as close to the boiling point of toluene, the highest boiling and predominant component, as possible. The N-EVAP nitrogen evaporator water bath is set at 90o C and the nitrogen flow at 8 LPM. Under these operating conditions, the solvent removal will take less than 60 minutes and the weight of the polymer in the sample will be determined by a simple back weighing of the sample tube. In this instance, a high temperature and a high gas flow rate achieved the desired result quickly and efficiently. The polymer is non-volatile and thermally stable at 90o C, so we quickly quantify the amount of polymer formed and have a representative sample of the analyte for further study and analysis.
A second hypothetical system (System #2) contains a different monomer and polymer, but is subjected to the same processing and analysis as System #1. In this case, we find no polymer at all; a significantly different result from that in System #1. We are certain that the polymer is non-volatile, so even if it thermally degrades there should be some residue. However, there is no evidence that any polymerization occurred. If, however, the experiment is repeated and the solvent is removed at 50o C with a gas flow of 8 LPM (taking over 180 minutes) we find a polymeric residue, which can be quantified just as in System #1. The explanation for the observed result in System #2 is that the polymer is thermally unstable at the higher temperature and depolymerizes or “unzips,” thereby reverting back to monomer which is volatile and evaporates with the toluene and other volatile components of the system.
In a third hypothetical system (System #3), the monomer has a BP of 95o C and a vapor pressure of 42 mmHg at 25o C. In this case, the solvent was removed from two separate samples at temperatures of 50 and 95o C, respectively, with a gas flow of 8 LPM. At 95o C, the polymer was isolated in about 60 minutes and quantified without any sign of thermal degradation. However, the weight of the polymer was lower than the theoretical or calculated weight for the system indicating that, in this case, the reaction did not proceed to 100% conversion, and that the toluene and the unreacted monomer were removed. On the other hand, at 50o C, the results were markedly different with a weight of isolated residue in the sample tube being above the theoretical or calculated value after 180 minutes. In this case, an HPLC analysis of the residue showed that the unreacted monomer evaporated at a slower rate than the toluene and therefore was present in the residue accounting for the added weight.
The number of hypothetical examples is infinite, and in each case there is doubtlessly an actual or equivalent system. For any given system, the optimal operational parameters necessary for the safe and efficient isolation of the subject analyte or substance of interest are variable. It is important to note the properties of each part of the sample solution. In many cases it may be necessary to sacrifice time in favor of sample integrity. However, temperature alone is not always the answer, the decrease in the evaporation rate with a decrease in temperature may, in some cases, be mitigated by increasing the gas flow rate. The effect of gas flow rate on evaporation time will be discussed in an article to follow.