Rest Services - using filters

Hi,

I am trying to use Rest Services on SharePoint 2013 to query data on a list. I am trying to filter the number of records returned based a column called "Supplier Name". If I use the following syntax:

https://<my url>/ts/ss/_api/web/lists/getbytitle('Contract%20Library')/items?$select=ID&$filter=substringof('RFX',Supplier Name)

I get an error "The expression "substringof('RFX',Supplier Name)" is not valid.:

However, if I filter on the ID value like so, it works:

https://<my url>/ts/ss/_api/web/lists/getbytitle('Contract%20Library')/items?$select=ID&$filter=substringof('123',ID)

Can someone help me with the syntax to filter on the "Supplier Name" column please?

thanks,

Sherazad