Having problems with SQL in 'select by attribute'

Hey all,

I have a table which I imported from excel with concentrations of iron in parts per billion. There are thousands of samples.

I am trying to select all samples that exceed 300, however when i try: "Iron" > '300' , non-numeric values in the table such as ' < 10 ' and 'M' still remain, and even some numeric values like '60' which is clearly not greater than 300.

any help would be appreciated

Thanks!