Identify Water Quality Needed
Tap water commonly contains substances that may be classified as particles, ions/inorganic compounds, organic molecules, microorganisms, and dissolved gases. Depending on how the water is used in the laboratory, some of these contaminants may or may not have an influence on experimental results.
For example, in liquid chromatography, the water used in the mobile phase should be devoid of organic molecules for good quality baselines. However, the presence of organic molecules in the water would have less impact on applications such as glassware rinsing, autoclaving, or microbiology. Before setting up a new laboratory or even starting a new experiment, it is important to identify the specific water quality needed.
Various organizations and regulatory bodies have defined water quality standards. As they all differ slightly it is difficult to define clearly the types of laboratory-grade water available. The most common classifications are:
• Type I. This water (also known as ultrapure) contains extremely low levels of ions (resistivity 18.2 MW.cm), organic molecules, bacteria, and particles. Usually, it is produced by combining purification technologies such as ion-exchange resins, activated carbon, reverse osmosis, ultraviolet photo-oxidation, filtration processes, and electrodeionization. It is used not only for analytical methods such as high-pressure liquid chromatography (HPLC), gas chromatography, and inductively-coupled plasma mass spectrometry (ICP-MS), but also to prepare reagents for molecular biology and cell culture.
• Type II. This water (also known as pure) may contain a small amount of ions (resistivity above 5 MW.cm), organic molecules (up to 50 ppb of total organic carbon), and possibly a small amount of bacteria. It is used commonly in preparing everyday reagents and buffers.
• Type III. This water usually is generated by reverse-osmosis or ion-exchange. It is used for less sensitive applications such as qualitative analyses, glassware rinsing, and water baths.
Based on their specific areas of activity, laboratories may use one or more of these types of water. Figure 1 illustrates the combination of purification technologies used in an ultrapure water purification system.