a) Do you still use high-resolution as a synonym for accurate mass? What is the true relationship between these terms?
High-resolution and accurate mass measurements are closely related to each other because the obtainable mass accuracy also depends on sufficiently resolved peaks. Nevertheless, they should not be confused, as performing a measurement at high resolution alone does not equally imply measuring the accurate mass.
b) What is mass defect?
The term mass defect is defined as the difference between the exact mass and the integer mass. It describes the fact that the exact mass of an isotope or a complete molecule is lower than the corresponding nominal mass. In case of 16O, for example, the isotopic mass is 15.994915 u, being 0.005085 u deficient as compared to the nominal value.
The term mass deficiency is also used to describe this deviation.
c) What enormously useful experiment is based on the existence of mass defect?
Most isotopes are more or less mass deficient with a tendency towards larger mass defects for the heavier isotopes. Thanks to these small deviations from integer values the accurate mass of an ion depends on its elemental composition, i.e., accurate mass measurements can be used to identify the elemental composition of an ion.