Should there be a space between a number and its unit? This is a matter of style, and when in doubt, you should follow the style of whatever publication or organization you are preparing for. Having said that, I think there is some consensus for the style given in the Chicago Manual of Style, which is used by many publications in the biological and other physical sciences. Namely, the Manual recommends that there should be a space between the numeral and the unit:
- “250mM imidazole” WRONG
- “250 mM imidazole” RIGHT
The only exception to this rule is the use of degree, minutes, and seconds for angles and longitude:
- 12 ° 45 ´ 54 ´´ WRONG
- 12°45´54´´ RIGHT
Note that other uses of the degree symbol do not fall under this exception. In particular, when specifying degrees Celsius, put the degree symbol next to the “C”, not the numeral:
- “37° C” WRONG
- “37 ° C” WRONG
- “37 °C” RIGHT