in fig1 b i e peaks 9 10 are delayed until the tic drops below the threshold 7 the resulting measured mass chromatogram i e after removal of the lockmass peaks is shown in fig2 b when compared with fig1 c it can be seen that the narrow peak 1 is present in the chromatogram and the low intensity peak 2 has not been degraded it will be appreciated that according to various embodiments the degradation and or loss of mass spectral data can advantageously be avoided or at least the degree to which the mass spectral data is degraded and or lost can be minimised in particular the problem of particularly important useful or interesting e g particularly abundant analyte ions being displaced by lockmass ions can be avoided according to an embodiment the instrument may be configured so as to only perform a calibration when the mass spectral data is increasing or decreasing for example in an embodiment the instrument may be configured so that the calibration lockmass correction is only performed on the falling edge of an eluting peak e g once the tic has fallen below a threshold 7 intensity according to another embodiment instead of monitoring the tic the intensity of one or more ion species of interest may be monitored it will be appreciated that monitoring the tic or.