I have added a space between the number and the units ie 5 mg/l rather than 5mg/l. The reason I have done this is the distinguish between the number and the unit as in some cases it can be ambiguous. For example, in my thesis I have litres per minute expressed as l/min. If I have no space the number can be misinterpreted ie 2.5 l/min or 2.5l/min so the L can look the letter or the number one. I have seen the capital L being used for L/min and I beleive either can be used. I cannot quote the official ruling as journals can be specific as can uni regs for theses.