While the reasoning for isokinetic sampling is, in theory, correct, there are many instances where the effect of non-isokinetic sampling is negligible. Calculations show that a particle with an aerodynamic diameter of fewer than 2 microns has virtually no inertia at normal duct velocities and non-isokinetic sampling will have no effect on the measured concentration. Even sampling 4-micron diameter particles at velocities of 50% higher or lower than the gas stream will only cause a measurement error of 2 or 3 percent.
If you are sampling downstream of a control device and formal emissions or EPA-type testing is not dictated by law or contract, it may be worthwhile to consider eliminating the constraints of traditional isokinetic sampling. It will save time, money, and effort.