Let's talk about lab water
Let's talk about lab water
There are no agreed definitions of the purity levels of “Purified Water” or “Ultrapure Water” but standards or guidelines which directly specify limits on impurity levels in purified water for specified types of application have been produced for many years. Some are general in nature but others relate to the concerns of particular industries, notably pharmaceutical and electronics. After some general comments, current standards are outlined. Many of the grades mentioned are summarised in the ELGA “PURE LABWATER GUIDE”.
I have been a member of committees that developed two such guidelines and users of these documents should not underestimate the lengthy discussions and debates that are involved. The issues that need to be considered include the implications of any changes to current standards, as these are often specified in analytical or other methodologies, practical application, unambiguity and robustness to remain relevant as laboratory practises evolve, as well as accuracy and acceptability. The result of such constraints and the time and effort involved is that standards evolve carefully and cautiously! No-one wants to rewrite them more often than needed.
For these reasons, most of the standards have not been revised for a number of years. This does not mean, necessarily, that they are, in any way, out of date. The standards are revisited periodically and, if there are sufficient reasons, they will be updated. However, sometimes inertia sets in and a standard that is clearly behind the times cannot generate enough support to be changed. A clear example is the ISO standard “Water for Laboratory Use” which was last updated in 1995 and uses analytical parameters from that era.
In addition to specifying purity limits for standards, all these documents contain extensive advice on how to produce, store and use purified water successfully. They also define starting water purity and the combination of treatment processes needed to achieve the required purity. Many indicate the purity of water needed for various types of application. They, therefore, are worth reading. They contain a lot of expertise!
There are a number of sets of standards or guidelines applied internationally which directly specify limits on impurity levels in purified water for specified types of application. Of these, the following apply to purified water in laboratories:
“Water for Laboratory Use”, International Organisation for Standardisation, ISO 3696: 1995 (1995)
Specifies 3 grades of water, with resistivity of >10, >1 and >0.2 Mohm.cm.
“Reagent Grade Water”, American Society for Testing and Materials (ASTM), D 1193-06 (2011)
Specifies 4 grades of water with resistivity of >18, >4, >1, >0.2 Mohm.cm and TOC of <50, <50, <200 ppb with each grade subdivided into 4 categories depending on microbial counts and endotoxin levels.
“Standard Guide for Bio-Applications Grade Water”, ASTM D 5196 – 06 (2013)
Bio-application grade water is intended for use in clinical, pharmaceutical, or biomedical applications, and has more stringent standards with respect to TOC.
“Preparation and Testing of Reagent Water in the Clinical Laboratory; Approved Guideline-Fourth Edition”, C3-A4. Clinical and Laboratory Standards Institute (CLSI) (2006)
Specifies Clinical Laboratory Reagent Water (CLRW) suitable for most routine testing (R >10 Mohm.cm). It also describes 4 other types of water for specific applications and Special Reagent Water (SRW) which is laboratory defined.
In each case, resistivity (and TOC) has been used to indicate purity levels; other parameters are usually included in the standards.
The pharmacopoeia specify a number of different grades of water for bulk use in various pharmaceutical applications. The same grades are often specified for laboratory testing. The most commonly used are Purified Water (PW) and Water for Injection (WFI). Neither is as pure in terms of resistivity or TOC as CLRW or ASTM type I. WFI includes tighter bacterial and endotoxin specifications.
United States Pharmacopoeia-National Formulary (USP-NF), www.usp.org/USPNF
European Pharmacopoeia, www.edqm.eu/.
Japan Pharmacopoeia (English Version), jpdb.nihs.go.jp/jp15e/.
ISPE Baseline Pharmaceutical Engineering Guide Series, Volume 4, Water and Steam Systems, 2nd edition / December 2011, chapter 9, Laboratory Water
Describes the ways to achieve the water purity needed for laboratory applications and the validation of the systems used.
The electronics industry requires extremely high water purity to minimise contamination with the limits set in the guidelines for many elements at ppt levels. Unlike the other standards, they often also include low limits on dissolved oxygen.
Ultrapure Water Monitoring Guidelines rev 2.0, Balazs Analytical Services (2006)
Guidelines for Ultrapure Water Used in Semiconductor Processing, SEMI F63-0213 (2013)
“Standard Guide for Ultrapure Water Used in the Electronics and Semiconductor Industries”, ASTM D5127 - 13 (2013)