How to count individual molecules?
We need a method that triggers an electrical signal which can be counted quickly and automatically whenever a molecule hits the detector. In such a case we are in a good position to measure the intensity over time. In order to see the quantum interference fringes we also need a good spatial resolution (some 10 nano meters). In the KTDLI, this is provided by the third grating. More on that later.
Since molecules might fragment in the source or the detector we select molecules with the expected mass.
Therefore we use a quadrupole mass spectrometer (QMS) that detects particles up to 16,000 atomic mass units. Molecules are ionized by electron impact; a hot wire filament emits electrons (70 eV) so as to release at least one electron from the passing molecules. Typically this is achieved with every thousand to one in ten thousand molecules .
In order to differentiate between one specific type of molecules and other and their fragments they are sent through a quadrupole mass filter. This creates a two-dimensional radio-frequency quadrupole field which selectively transmits particles with a specific mass-to-charge ratio. All other molecules are deflected and are not detected.
Behind the mass filter the molecular ions are accelerated to a metal surface by a strong electric potential of several kilo-Volts. There, they eject several electrons which are enhanced by an electron multiplier to enable the measurement of an electrical pulse. These pulses are then digitized and counted.
The mass spectrometer can scan a mass range step-by-step to record a mass spectrum. The electrical fields are adjusted at the quadrupole for each mass and the molecules are counted. Thereby, we know how many particles of which mass reach the detector.
If we want to count only particles of one specific mass, the electric fields of the filter are fixed and the whole measurement time is used solely to count molecules with the chosen mass.
Experimental challenge: Identify molecules
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