Instead of a space between the units, use a nonbreaking space
(Ctrl+Shift+spacebar).

--
Regards,
Jay Freedman
Microsoft Word MVP FAQ: http://word.mvps.org
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Imad wrote:
Dear all,

I am having a difficulty with scientific units when writing some
technical reports. The problem is that sometimes a unit such as (mg
mm-3 or mg L-1) with -3 and -1 as superscripts is being splitted at
the end of a sentence leaving half of the unit (e.g. mg) at the end
of the sentence and the other half at the begining of the following
line (e.g. L-1) which looks very ugly. Sometimes you can change the
word ordering to avoid this but it does not works like that all the
time. Any suggestions?

Many Thanks,
Imad