In any instrument generatting results in form of gaussian peak there are two ways to define start and end. either automatically or manually. Better go for auto because in manual procedures the consistency cannot be maintained. 

The integration start point is the actual point in time along the chromatographic trace (SRM trace in MS) where the area under the curve for a peaks integration is first taken into account; the stop point is the last point in time along the chromatographic trace after which the area under the curve (or baseline) is no longer taken into account for the peaks area. Wada wants these points to be consistent so that the data are reliable. The links below discuss and demonstrate what it means to have consistent peak integrations manually or automatically.  

http://www.chromatographyonline.com/integration-problems

http://www.accutest.com/Collateral/Documents/English-US/Manual%20Integration%202012%20%5BCompatibility%20Mode%5D.pdf

http://www.dionex.com/en-us/webdocs/77494-PO-HPLC-LPN2297-01-Chrome.pdf