The | operator is a logical OR.
So, in English your condition as you've written it is "if col is not ASSAY_PURITY OR col is not TOTAL SUM then..." Since col can't be both of those values at the same time that condition will always be true. You want an AND between them which is the & operator.
If( ((col != Column( "ASSAY_PURITY" )) & col != Column( "Total Sum" )),
If( (col[theRow] > target & !Is Missing( col[theRow] )),
result = "Fail"
)
);
-Jeff