THE QUARTZ CRYSTAL MICROBALANCE 397 f 10 = 1 2 C 1 L 1 (9.6) Here f 10 is the resonance frequency of the main resonance (n = 1), the dissipation factor D is the inverse Q factor of the equivalent circuit, and R 1 , L 1 , and C 1 denote elements of the lumped circuit. Schematics of the QCM-D and the equivalent electrical circuit are shown on Figure 9.13. 3.6. SOME DISADVANTAGES OF THE QCM Despite the above-mentioned advantages of AT-cut quartz resonators, these devices also have some weak points. In particular, they suffer from high sensitivity to mechanical strains and vibrations and mechani- cal stress caused by temperature variations or temperature gradients. Another problem is related to the electrode and mounting geometry, in the sense that the associated mechanical strains may contribute to variations in the measured frequency shifts. More detailed consideration shows that the measured change in resonance frequency is different in the central area of the resonator (r = 0) and in the electrode region (r = R) due to the mass contribution of the electrodes. The active area of the QCM is about half of the disk diameter. Some improvements in this field include the use of other metals for electrodes (e.g., gold) and of larger-area electrodes. Another characteristic feature is the differential mass sensitivity, which demonstrates a maximum in the central part of the resonator surface and drops off in the edge