The ultrapure water required for successful LC-MS requires close attention to all stages of water production, handling and equipment maintenance.
Ultra High-Performance Liquid-Chromatography-Mass Spectrometry, commonly known as LC-MS, which requires top-quality salts and organic solvents for mobile phase preparation, also needs the highest purity water, which is perhaps sometimes not always appreciated.
Natural and tap waters contain major classes of contaminants, e.g. inorganic ions, organic molecules, particulates, colloids and bacteria.
Merck Millipore water purification systems deliver either Type 1, “ultrapure/Milli-Q water, Type 2 pure water/Elix water or Type 3 laboratory water.
To avoid interference Type 1 water should always be used for standards, blanks, samples, container rinsing and as a component of the mobile phase.
Problems caused by poor water purity include ghost peaks and interferences, baseline drifting, decreased sensitivity and resolution, poor repeatability and reduced column life.
To prevent such problems water quality monitoring is required, preferably by resistivity and total oxidisable carbon (TOC) monitoring.
Resistivity monitoring is particularly important if ions or metals are being analysed and a value of 18.2 M?-cm should be aimed for to prevent mass spectrometry signal reduction and adduct generation.
TOC monitoring measure organic purity and low levels (<5ppb) can be achieved through UV photooxidation, combating problems like ghost peaks and system noise.
The highest purity water is actually produced by a combination of technologies e.g. a ’pre-treatment pack’ is followed by reversed osmosis, electrodeionisation, germicidal UV lamp, ion-exchange resins, synthetic activated carbon, and UV photooxidation.
As such ultrapure water is a good solvent it is prone to contamination from the atmosphere and containers and hence should ideally be used immediately.
If short-term storage is unavoidable borosilicate bottles rather than plastic containers should be used for the minimum period.
A point-of-use filter will also prevent back contamination and provide a final filter directly before delivery. Even so it may be necessary to discard several litres of water prior to use for the most rigorous experiments.
Maintenance of every part of your purification system is vital to ensure the delivery of optimum purity water and prolong the equipment’s lifetime.
Replacement dates on components should always be adhered to and calibration of monitoring equipment, such as the resistivity and TOC monitors, is also vital.
Only by rigorous attention to water quality can a negative impact on LC-MS experiments and analyses be prevented.