A released gas in mobile phases during analysis causes noisy baselines and pressure fluctuations which affect sensitivity and reproducibility. It also severely compromises the use of fluorescence-, refractive-index- or UV-detectors (particularly at wavelengths below 210 nm). As a consequence effective mobile phase degassing must be employed for many analyses.

But what is effective mobile phase degassing?

1. Ultrasonication

This common degassing method does not give the required results, as with this procedure only the super saturated gases are removed.

1. Reflux method

The elaborate reflux method [described by J.N. Brown et al., J. Chromatogr. 204 (1981) 65] is the most efficient method for polar solvents, while the results are similar for nonpolar eluents to the more common methods below.

1. Vacuum degassing

Simple vacuum degassing, where eluent bottles are subject to a vacuum source prior to use, is not the most efficient, but in a number of analytical HPLC systems online degassers are incorporated. In these devices, the mobile phase is pumped through a tube that serves as a membrane. A vacuum at the outside extracts dissolved gases through the polymeric material. The devices are nearly maintenance-free and can be used for nearly all commonly used eluents. However, mixtures containing highly volatile components may lead to some trouble. For example, the concentration of tetrahydrofuran can be reduced significantly by vacuum degassing, which results in longer retention times in RP-HPLC.

1. Helium sparging

Sparging with helium is to be recommended as the most simple, yet effective method. However, the cost of helium and its use in high pressure gas bottles have to be carefully monitored. Also a possible contamination of the laboratory atmosphere has to be kept in mind.
Similar to vacuum degassing a change in the composition of a premixed mobile phase over time can occur, due to the difference in the evaporation rates of mobile phase components. 

Table: Pros and Cons of methods for mobile phase degassing

For further information please contact Dr. Wolfgang Schräder, YMC Europe GmbH, Phone: +49 (0)2064 427-290, Fax: +49 (0)2064 427-222 or visit www.ymc.de