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( they are usually lipopolysaccharides from the degradation of gram negative 
bacteria ). They are measured by either injecting a sample into test rabbits
and measuring body temperature increase or by the more sensitive Limulus
Amebocyte Lysate (LAL) test. The internal membrane of an ultrafiltration
system has a pore size of <0.005um. This will remove most particles, 
colloidal silica, and high MW organics such as pyrogens, down to about 
10,000MW. These are usually for cell-culture and DNA research, and are 
located at the point of use, however the ultrafiltration unit has to be 
regularly sanitized to prevent microbial growth.
 
Ultraviolet irradiation can be used as a bactericide (254nm) or to destroy
organics by photo-oxidation (185nm). The water is exposed to UV for periods 
up to 30 minutes, and the UV interacts with dissolved oxygen to produce 
ozone. The ozone promotes hydroxyl radical formation, which result in the 
destruction of organic material. Usually a high intensity, quartz mercury 
vapour lamp is used, and is followed by an ion exchange and organic scavenger
cartridge to collect decomposition products. The product water is very low in
total organic carbon.

Dissolved gases can be removed by passing the water through a vacuum 
degassing module that utilises an inert, gas-permeable membrane surrounded 
by a vacuum to remove dissolved gases from the water.

The purest laboratory water is usually obtained after passing through a 
system that can include reverse osmosis or distillation of the feed water, 
followed by activated carbon to remove chlorine and organics. The water is 
passed through ion exchange resins to remove inorganic ions, through a 
UV oxidation stage, followed by a combined ion exchange and organic scavenger 
cartridge, and finally through a 0.22um filter. An additional stage of vacuum
degassing to remove dissolved gases may be added for some applications - such
as for semiconductor production. 

These pure water systems are regarded as " point-of-use ", because it is 
extremely difficult to prevent the reintroduction of contamination during
storage and distribution. The water is commonly known as " 18.3 Megohm " 
water, because it has a specific resistance greater than 18.3 Megohm-cm 
at 25C. It also contains < 5 ppb of total organic carbon, < 10 ppb of total 
dissolved solids, and < 1 colony forming unit / mL of micro-organisms.
  
Details of laboratory and industrial water-purification processes are 
available in the catalogues of equipment suppliers such as Barnstead [16]
and Millipore [17].  

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