We present a field mass spectrometer to analyse atmospheric pollutants and discuss the enhancement of its metrological performance. First of all, we describe the operating mode and the signal processing. Then, we deal with high voltage signals applied to the electrodes. We explain the characteristics of these signals which are quite different from those used in conventional ion trap mass spectrometers. We present the metrological parameters (mass range, sensitivity and mass resolution) of the analyser and the problems connected to the enhancement of its performance. The Fourier transform treatment leads to a very high sensitivity; the spectral signal-to-noise ratio is better than 4 for a mean number of ions of one. Resolution is limited by non-linearity of the confinement field. At present, the width at half maximum of the spectral peak is 0.052 u which leads to a mass resolution of 2500 for mass 132. To improve mass resolution, a more precise ion trap has been devised. To analyse heavier molecules, a new linear amplifier stage is being developed to supply a confinement AC voltage of a maximum amplitude of 1 kV.