Are you saying that the field is a text field with both alpha and numeric values inside?
If that is the case, then you would need to either select for maximum based on another field, or extract numeric data from this field to use for selecting a highest value. A text field is just text, there is no inherent hierarchy.
You could pull the numeric portion of that field out using LEFT () or RIGHT() and use that to find the MAXIMUM () value, assuming that the numeric portion of the field indicates a highest value.
