Order the result of a comma separated string generated with STUFF in sql server

 Sql query

DECLARE @Cols nvarchar(max);

SET @Cols = STUFF((SELECT ', ' + QUOTENAME(para_name) FROM t_WQ_Cont_list ORDER BY rpt_seq FOR XML PATH(''), TYPE).value('.', 'nvarchar(MAX)'),1,1,'')

SELECT @Cols;

///////// Result

 [Cl], [EC], [TDS], [Fe], [pH], [SO4]