A cleanliness monitor for monitoring a cleanliness of a vacuum chamber. The cleanliness monitor may include a mass spectrometer, a molecule aggregation and release unit and an analyzer. The molecule aggregation and release unit is configured to (a) aggregate, during an aggregation period, organic molecules that are present in the vacuum chamber and (b) induce, during a release period, a release of a subset of the organic molecules towards the mass spectrometer. The mass spectrometer is configured to monitor an environment within the vacuum chamber and to generate detection signals indicative of a content of the environment; wherein a first subset of the detection signals is indicative of a presence of the subset of the organic molecules. The analyzer is configured to determine the cleanliness of the vacuum chamber based on the detection signals.