To make sure the you won't have a problem in the future with strings like 'SomeString1' or any other mix of characters including symbols especially in some of the collations available, etc, try the following (added test data to that which was already posted)...

WITH SomeData(SomeValue) AS

(

SELECT '1.51922e+015' UNION ALL

SELECT '1.53122e+015' UNION ALL

SELECT 'FMCIT' UNION ALL

SELECT 'ABCNP' UNION ALL

SELECT 'FMCPNG' UNION ALL

SELECT '1.62073e+015' UNION ALL

SELECT '1.6127e+015' UNION ALL

SELECT 'A+B' UNION ALL

SELECT 'A.B' UNION ALL

SELECT 'A B' --Space between letters

)

SELECT *

FROM someData

WHERE SomeValue NOT LIKE '%[^A-Za-z]%' COLLATE LATIN1_GENERAL_BIN

;