When related to chemical analysis, the "matrix" refers to all the other components of a particular sample, other than the analyte or compound you are interested in.
A matrix can be referred to in general terms such as soil, water and air, however in some cases it is important to provide additional detail for comparison to the appropriate guideline or regulation.
For example, it is often important to distinguish between surface water and ground water, or an ambient air sample verses a sub-slab sample.
The make up of a matrix can have a considerable effect on the chemical analysis. This effect is often referred to as matrix effects or matrix interference.