BUCHI ELS Detector C-650 - Manual (Page 40)

C-650 Operation Manual, Version B 6 Operation 40
NOTE
If the mobile phase used is not easily volatile, such as DMSO or DMF, temperature should be
increased to allow correct evaporation process.
The temperature can be adjusted during the method optimization process.
If you suspect that the compound of interest is thermally labile or semi-volatile, a lower tem-
perature could be used to improve the sensitivity by reducing the thermal decomposition or
evaporation. For a given flow rate and solvent, there is, however, a point at which the noise in the
chromatogram is dramatically increased because not all of the mobile phase is vaporized.
As an example, consider the analysis of caffeine with evaporation temperatures of 30 °C and
60 °C (Fig 6.26) [the conditions for the separation are – Column: ODS KromasilTM (5 µm, 30×
2.1 mm), Sample: 4 µL (10 mg/L) Caffeine]. Eluent: Water, 0.2 mL/min, temperature as indicated).
It is clear that the use of a low temperature provides significantly better sensitivity for volatile and
thermally sensitive compounds.
Fig 6.26: Chromatogram of caffeine at various temperatures
The minimum temperature that can be used is dependent on the flow rate, ambient temperature
and the nature of the mobile phase.
6.4.2 Optimizing the mobile phase
Particulate matter in the mobile phase will increase the background noise.
The purity of the solvent is a critical issue in the noise. When filtering the solvent, verify that it
does not extract any contaminant from the filter.
The purity of the solvent is critical for a low background noise. The sensitivity is inversely propor-
tional to the solvent noise.
In most cases, distilled water and HPLC grade solvents are satisfactory. When you are compar-
ing solvents, the most critical parameter is the Residue After Evaporation; this parameter should
be less than 1 ppm to maximize the sensitivity of the detector.
As an example, consider the analysis of a sample in a pure water mobile phase and a polluted
water mobile phase. It is clear that the use of an insufficient quality solvent can dramatically de-
crease your S/N ratio (sensitivity).