Food analysis laboratory needs for purified water tend to be modest in quantity but demanding in quality. Food analytical methods call for two general water quality grades, pure and ultrapure. To meet purified water needs, cost-conscious laboratories consuming up to 15 liters of water per day must choose from among several options:
Explore this issueJune/July 2005
- Purchasing and maintaining separate purification systems for each grade of water used;
- Acquiring a single, ultrapure water system and absorb the capital and maintenance costs of producing very high-quality water for less-than-critical applications;
- Buying bottled water that roughly corresponds to pure and ultrapure standards;
- Obtaining an all-purpose water system that supplies water on demand at the appropriate purity level for every assay.
The decision ultimately is based on quality, convenience and cost. Generally, small in size, food laboratories often combine R&D with quality-related work. The assays lab techs conduct can include quality testing of raw materials and finished products. The goal is to determine food safety requirements, nutritional analysis, quality assurance/quality control and product development.
Water quality needs to be considered for optimal results. Operations requiring purified water include most analytical methods (titration, HPLC, ion analysis, colorimetric assays), microbiological tests, quality control, growth media, washing and rinses, column analytical and preparative chromatography, etc.
The two major classes of laboratory water as pure and ultrapure. Pure water has been distilled, deionized or treated by reverse osmosis. It is used in non-critical applications, where detection limits are high or in situations in which contaminants from water purity fall well within the error limits of the method used.
Ultrapure water takes pure water and adds purification steps that kill adventitious pathogens as well as removes trace ions and organics. Not every application that calls for ultrapure water demands removal of all remaining contaminants up to ultrapure standards. Specific analytical methods may only require depletion of certain ions, bacterial products or organics.
The need to remove organic contaminants is the most common reason for specifying ultrapure water. Organics interfere with analytical HPLC by altering peak resolution and integration, introducing ghost peaks, and affecting stationary phase chemical selectivity. About 70 and 80 percent of HPLC performance problems are directly attributable to water quality. For analytical HPLC, organic contaminants easily can achieve on-column concentrations equal to those of target analytes. In preparative chromatography, organics may concentrate on columns and co-elute with product.
Analytical-grade ultrapure water should comply with ASTM Specification D1193, which stipulates that water be freshly drawn and used within 8 hours of production. ASTM D1193 also specifies acceptable levels for dissolved total organic carbon (TOC) at less than 50 ppb, and microbiological contamination to less than 10 colony-forming units per liter.
Standards for ultrapure water are significantly higher than for pure water, specifically with respect to removal of organic contaminants, which include free-standing hydrocarbons, halogenated organics, detergents and bacterial products.
Ultraviolet-mediated photo-oxidation is the most efficient, cost-effective means of achieving removal of environmental and pathogen-related organics. UV photo-oxidation employs a dual-wavelength, low-pressure mercury UV lamp in quartz sleeves and dual-wavelength (185 and 254 nm) irradiation. This wavelength combination generates hydroxyl radicals from dissolved oxygen and water. Hydroxyl and secondary free radicals react with and break up small organic molecules, the end products of which are water and CO2.
Bottled water is perhaps the least cost-effective solution. Ultrapure HPLC-grade bottled water costs close to $20 per liter – far higher than the cost of water produced by an ultrapure water system.
A jug of distilled water costs between $1 to $2.50 per gallon in supermarkets and pharmacies. While this water is certainly of very high quality for ordinary household chores, using it in a laboratory setting is not advisable. It’s impossible to determine the pedigree of bottled water that is not reagent grade. Plastic containers used to store jug water may leach chemicals into water. And once the bottle is opened all bets are off since water absorbs gases and chemicals present in laboratory air.