Standard addition method must be used when matrix is complex specially when work is on pesticides of the same class.

I guess you have to write more detailed what your problem really is.

I m studying on analysis and degradation of organochlorines and organophosphates in water, soil and vegetables matrices, technique m using GC-ECD-MS. In samples analysis and even in standard runs i find some pesticide response get too low compared to their concentration, for instance if i run mix of organochlorines 1 ppm say, the response of dicofol, alpha HCH go very low, i thought may be due to presence of other compounds like other isomers of HCH, DDTs, Endosulphans may the response is getting low. Does such lowering of response of one organochlorine pesticide get lower in presence of other organochlorines?

hmmm on an ECD you can get negative peaks from e.g. aliphates, but it need real high concentrations for that. Another halogenated compound that is co-eluting should increase the peak but newer decrease it, if the ECD is operated in a serious way. It might however be that the software is getting difficulties in findingthe baseline (chemstation e.g.). Otherwise no I do not think that could happen.

On the MS If you are using a quadrupol MS, I basically never have experienced that. On Ion traps things like that are imaginable, though not very often experienced in the way you describe it. Ion traps acumulate over a given time before ejecting the ions to the detector. the time they acumulate is dependent on the total amount of ions in the trap. so with a lot of matrix this happens, bt with just a few more standards it is hardly imaginable.

for more details, you need to tell us which instruments and which solvents you are actually using.

maybe a misunderstanding: are you using GC equipped with both an ECD and a mass spectrometer or are we talking negativie chemical ionisation mass spectrometry

The standards are made in n-hexane (Merck Chromatography grade)

GC-ECD used for quantitation and GC-MS separately for qualitative analysis.