workflows not triggering for Null condition

Gurus,
I have a picklist -[<plData_Corruption_Type_ITAG>]  - with 3 values. I need to trigger an email when the value of the picklist is changed from null to any of the 3 values.
I have tried the following in a workflow :
1). IfNull(PRE('<plData_Corruption_Type_ITAG>'),'Null') <> IfNull([<ltStatus_Comments_ITAG>],'Null') AND [<plOrg_ITAG>]=LookupValue("OCC_CUST_LOV_SR_19", "SCD") AND PRE('<plData_Corruption_Type_ITAG>') <> [<plData_Corruption_Type_ITAG>]

2) [<plOrg_ITAG>]=LookupValue("OCC_CUST_LOV_SR_19", "SCD") AND  IfNull([<plData_Corruption_Type_ITAG>],'Null') <> 'Null'  AND PRE('<plData_Corruption_Type_ITAG>') <> [<plData_Corruption_Type_ITAG>]