D-8 Solvent Considerations
Dissolved oxygen affects UV-VIS detector performance in several ways
1
.
Oxygen dissolved in solvents can form a UV absorbing complex, the amount of
which is considerably different in different solvents. The effect is particularly
strong with wavelengths below 260 nm. Therefore, any change in dissolved
oxygen content can affect a UV baseline considerably. This phenomenon is
particularly evident in the solvent 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 where the dissolved oxygen content varies between the different
solvents and, as the composition changes, causes erratic baselines or even
peak-shaped artifacts on the baseline.
Dissolved oxygen in a fluorescence detector has quite a different effect. It
causes a tremendous loss of sensitivity. Bowen and Williams
2
have discussed
the quenching of aromatic hydrocarbons by dissolved oxygen in fluorescence
detectors. Parker and Barnes
3
have reported a 95% reduction in sensitivity of
the fluorescence of borate-benzoin complex in air equilibrated ethanol. The
oxygen quenching varies with different types of compounds, and aromatic
hydrocarbons, aliphatic aldehydes, and ketones are especially susceptible.
If for any reason the characteristics of the solvent change, the precision and
accuracy of the solvent delivery system can be adversely affected. This can
cause variations in both peak retention and, to some extent, peak height or
area.
Solvent degassing methods
Sparging, or bubbling a gas through solvent, reduces the partial pressure of
the unwanted gas at the surface of the solvent. This removes unwanted gas
from solution and saturates the solvent with the second gas. Sparging with
helium removes background absorbance on a UV detector and the quenching
phenomenon caused by dissolved oxygen on a fluorescence detector.
Helium sparging combines the convenience of short initial degassing time
required, ease of maintaining the solvent condition during operation, and
complete control within the framework of the Waters Prep systems.
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.
3. C. A. Parker and W. J. Barnes, Trans. Faraday Soc., 82 (1957) 606.
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