Eluent Degassing 217
Temperature
Temperature affects the solubility of gases in two ways:
• The higher the boiling point of a gas, the more soluble the gas is in a given eluent.
• Increasing the temperature of the gas-liquid solution affects the percentage of gas in
solution.
If the heat of solution results in:
• An exothermic reaction, the percentage of gas in solution decreases.
• An endothermic reaction, the percentage of gas in solution increases.
For example, the solubility of helium in water decreases with an increase in temperature,
but its solubility in benzene increases in direct proportion to the temperature.
Pressure
The mass of gas dissolved in a given volume of eluent is proportional to the partial
pressure of the gas in the vapor phase of the eluent. If the pressure of the gas decreases, the
amount of that gas in solution also decreases.
Dissolved Oxygen
Dissolved oxygen affects UV/Vis detector performance in several ways.
1
Oxygen
dissolved in eluents may form a UV-absorbing complex, the concentration of which is
considerably different in different eluents. The absorbance effect is particularly strong
with wavelengths below 260 nm. Therefore, any change in dissolved oxygen content may
affect a UV baseline considerably. This consequence is particularly evident in the eluent
tetrahydrofuran (THF).
Dissolved oxygen does not seem to affect the absolute sensitivity of a UV system, but
primarily causes baseline drift. This effect is especially noticeable during gradient
operation in which the dissolved oxygen content varies among the different eluents. As the
eluent composition changes, the dissolved oxygen content causes erratic baselines or even
peak-shaped artifacts in the baseline.
Dissolved oxygen in a fluorescence detector has quite a different effect. It results in a great
loss of sensitivity. Bowen and Williams have discussed the quenching of aromatic
hydrocarbons by dissolved oxygen in fluorescence detectors.
2
Parker and Barnes have
1. S.R. Bakalyar, M.B.T. Bradley, R. Hoganen, Journal of Chromatography, 158 (1978) 277.
2. E.J. Bowen and A.H. Williams, Trans. Faraday Soc., 35 (1939) 65.
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