Dear Mabrouk,
If you have doubts regarding performance of your MS, ask Shimadzu for performance verification compound, procedure and criteria. Usually your local Shimadzu engineer gladly helps or sometime even will perform this test for you. Check that instrument passes the test, this test was also performed at commissioning of the instrument. If you do not have this record, Shimadzu should. After you are confident in your instrument, build up evaluation further, try SS using well published analytes, if you use ES+, atrazins are a good mixture. Proceed to ppm level injection to check if you have a signal, then move to your range. 10 ppb is a problem for many pesticides, you need to concentrate. If analytes can not be ionized it is a problem too.
Alexandre.