RE: Distilled Water as Sheath Revisited

Julie Oughton wrote:

>I've been monitoring this thread and feel compelled to weigh in on this
>issue. I've been using distilled water in our benchtop analyzers for
>almost 10 years with no known problems. We use house distilled water
>that is filtered through a Milli-Q Ultrapure water system that produces
>18.2 megohm-cm water. So, we experience little, if any, "noise" from our
>water.

The noise we're talking about here isn't coming from anything the 
Milli-Q cartridges remove, or from particles, which are removed by 
the 0.22 um membrane filter at the output of the Milli-Q; it is 
coming from the small refractive index difference between water and 
saline, as Dan Rosson's experiment (he also used a 0.22 um filter) 
appears to show. Such noise is only likely to be noticeable when you 
are trying to measure scatter from submicron particles.

If you don't use high-quality, filtered distilled water as a sheath, 
you may well get big enough particles in the sheath to produce 
scatter noise, but that type of noise will come from junk in a saline 
sheath as well as from junk in a water sheath.

People who routinely work with really small particles use 0.1 um 
filters, but the pressure drop across those is typically big enough 
so that you can't mount them inline in the flow system.

-Howard