Water purity and lab glassware washers: Just how "pure" does it have to be?

Hands dispensing RO or DI water

Water purity supplied to a laboratory glassware washer for rinsing has been the subject of debate in regards to the end result of producing clean glassware. It should be noted that the term ‘deionized water’ does not reference any specific water purity level. The deionization of water produces water within a wide range of purity levels. The deionization process can be used to make medium grade water at 10-20 microsiemens of conductivity, or, it can be used in polishing systems to produce water with a 16-18 megohm resistivity level. There are four points to consider when selecting the purity level of the water used for rinsing in a stainless steel laboratory glassware washer:

  1. All laboratory glassware washers are made with stainless steel interiors and start their rinse cycles with contaminated tap water saturated with laboratory soil and detergent. Using ultra-pure (18 megohm) water for rinsing will not significantly reduce this contaminant load any more than using lower quality purified rinse water. The 18 megohm water will be quickly contaminated as it fills a glassware washer’s stainless steel sump and the resulting purity of the rinse water will never approach 18 megohms.
  2. While laboratory Type I water polishing systems actively produce 18 megohm water, storing this water and maintaining its purity is extremely complicated and costly. For example, carbon dioxide, in normal air, will re-contaminate exposed 18 megohm water, reducing its purity.
  3. Deionization filters used in laboratory polishing systems designed for producing Type I water for analytical requirements, will be quickly exhausted due to the higher water usage rate in multiple rinse cycles available in laboratory glassware washers.
  4. At very high temperatures and over a long period of time, 18-megohm water can corrode the stainless interior of a glassware washer.
Given the limitations of initial glassware washer water cleanliness levels, along with the cost of making and storing large volumes of 18 megohm water, it's far more economical to use medium-purity water such as RO water, or deionized 1-20 microsiemen water for washer rinse cycles. Medium-purity water systems can make and store large volumes of water at lower costs than 18 megohm water production systems. The laboratory glassware cleaned in a glassware washer with medium grade purified water will be acceptable for most wet-bench chemical analysis. For ultra-low (ppb levels) elemental or organic chemical analysis, further glassware processing is required.
Aggressive acid baths, which aren't recommended in a glassware washer, and subsequent rinsing with 18 megohm water, can clean glassware to the same ionic concentration levels as found in 18 megohm water. For organic chemical removal, glassware must often be baked-off in an oven. Labconco has completed third party testing on glassware cleaning efficiency using 1 micro ohm/microsiemen purified water rinses in the FlaskScrubber® Glassware Washer. Chemical contaminant carry-over tests have been completed using analytical test methods EPA 200 series metals, EPA 524.2 volatile organics, EPA 525.1 semivolatile organics and EPA 8270 semi-volatile organic compounds. The results measured the remaining contaminants at the parts-per-billion level. The significance of this test data will vary based on application and analysis needs. 
Results for analytes tested are available from Labconco Corporation. For Labconco’s SteamScrubber® and FlaskScrubber Glassware Washers, we recommend using our WaterPro® RO  Station. The internal 17 Liter storage tank supplies up to 2 out of the potential 6 RO water rinses. If the washer will be programmed for 6 rinses in pure water, we recommend the addition of our 70 Liter RO water storage tank. Labconco’s glassware washers use 12.9 liters of water for each fill. Two fills of tap water are required for detergent washes and up to 6 rinses with tap water or pure water are available, requiring 12.9 liters each.
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