For dark matter particles with spin up to 1/2, the non-relativistic effective field theory provides 28 coupling strengths for the dark matter-nucleon interactions. The standard assumption of collaborations is that dark matter couples equally to protons and neutrons. Since this is not necessarily true, we developed a method to derive limits taking the interference among operators into account. Using this method and data provided from XENON1T and PICO60, the relaxation of the published cross-section limits can be up to four orders of magnitude. Further we extended the developed method which now allows to do a combined analysis of several experiments. The combination of experiments can strengthen the limits by up to four orders of magnitude.