When you make up fresh mobile phase you probably assign an expiry date based on the fact that you know the solvent mixture will be subject to change over time but what you may not consider is the storage of the solvents prior to mobile phase preparation.
In the case of water, this can have significant detrimental effects on your chromatography. The primary reason is that water is a great solvent. It can absorb contaminants via leaching from the container in which it is stored, and it can also absorb contaminants, such as airborne bacteria, from exposure to the atmosphere. These contaminants are likely to increase the total organic carbon (TOC) in the water which in turn can lead to chromatographic problems such as: noisy and drifting baselines; extra (unexpected) peaks, often referred to as 'ghost' peaks, and in extreme cases problems with retention times and peak shapes.
The extent and impact of these effects will depend on the nature of the HPLC method that you are using. For some methods you may not see any detrimental effects and for others it can be absolutely critical. Contaminants are much more likely to show up, and thus create problems, if you are using UV detection at low wavelengths, in the area of ~ 210nm, or sensitive mass spectrometry. In these cases it would be worth investigating the effects of water storage and source (e.g., lab purification system vs. bought in bottled water) as part of robustness in method development so that the problem and issues are understood fully before the method is in routine use.
To prevent these problems (or reduce them as much as possible) the following is advised:
- Don't store water, get it fresh before use.
- Discard the first 1-2 litres from the water purification system.
- Use glass containers for water.
- Don't attach plastic tubing to the delivery point of your water purification system.
- Look after your water purification system, maintain it well and don't ignore warning lights!
- For critical methods, investigate the effect of water storage (and source) as part of method development.