A leak detector for detection of leaks from an object to be tested by detection of a tracer gas, the leak detector including: an inlet of gases; a pumping device; a pumping line connecting the inlet of the gases to the pumping device; a device for detection of tracer gas fitted to the pumping line, as bypass of the pumping device, the device for detection of tracer gas including a probe configured to ionize the gas molecules and to measure current of the ionized gas molecules, and a separating membrane arranged upstream of the probe. The separating membrane is a monoatomic layer of graphene deposited on a gas-permeable substrate of the device for detection of tracer gas including selection orifices, the greatest dimension of which is less than 5 angstroms, made in the monoatomic layer of graphene.