"DKafkewitz" <dkafkewitz at aol.com> wrote in message
news:20031015145344.09624.00000978 at mb-m25.aol.com...
> Light scattering varies inversely with wavelength. The shorter the wave
the
> more the scatter. Hence 440 is more sensitive than 660. But absorbance of
the
> light by the medium is also important. Colorless media abosrb no visible
> light: that is why they are colorless. Yellow media absorb in the shorter
> range. Therefore you use a longer wave to minimize the false reading of
> Absorbance rather than scattering.
>If you set zero on a sterile blank of the same medium, or use a double beam
with a sterile blank, you cancel the effect of the medium unless the culture
is producing a pigment. We always used 430nm with our bacteria because it
gave a longer linear area of ODs with our spectometers, cutting out the
amount of dilution needed. One common mistake I've seen students make is to
dilute with water rather than sterile medium, thereby introducing another
variable.
Lesley Robertson
http://www.beijerinck.bt.tudelft.nl
