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19861005
第62楼2010/10/05
兄弟讲的不错，我仔细看了一下，学了不少知识
plexu(plexu) 发表:Myth 4: High Temperature Always Leads to Better Separations
False. As the temperature increases, the viscosity of the mobile phase decreases and therefore the rate of solute mass transfer should increase thereby offering better chromatographic efficiency. True, but besides the column efficiency term (H), temperature also can affect the retention factor (k) and selectivity (α). The impact on these terms can result in improved resolution (which is really what we are concerned about in chromatography) or detract from it. Retention usually decreases as the temperature increases because, being a thermodynamic parameter, the analyte prefers to remain in the mobile phase and are eluted sooner from the column. However, different chemical species might have different degrees of change in their retention versus temperature. Stated more correctly, their van''t Hoff plots (ln k vs. 1/T where the temperature T is measured in kelvins) might display different slopes; in other words, their α values can change. In addition, high temperature can cause a low k peak to be eluted so quickly that it can be eluted at or near t , the unretained peak retention time, and therefore be difficult to quantitate. .
Take the example of analgesics shown in Figure 2. This series of chromatograms shows the separation of seven analgesics at column temperatures of 20–90 °C. One can note several features of the chromatograms. First, the retention of all peaks is decreased with higher temperatures and the peaks do become narrower indicating improved efficiency. Second, one of the analgesic compounds, salicylic acid, has a much greater retention change with temperature relative to peaks 5 and 6, the closest eluted compounds. In fact, the elution order is reversed after increasing the temperature from 20 to 40 °C. At the intermediate 30 °C temperature, the salicylic acid and peak number 6, phenacetin, are coeluted. So in this case, temperatures greater than 40 °C resulted in a shorter separation time for the entire separation yet the elution order was changed from the lower temperatures.
Of course, a side benefit of operating at higher temperature is the decrease in column operating pressure that allows one to use higher flow rates or smaller particles.
Another experimental parameter that can cause column performance problems at higher temperature is the possible thermal mismatch of the incoming mobile phase. For example, if the column is heated to 60 °C and the incoming solvent is at room temperature, the incoming cooler solvent can cause peaks to be distorted because of the differences in the temperature that the solutes can experience in the initial part of the column. It is recommended that one preheats the mobile phase when using higher column temperatures
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