GB1302193A - - Google Patents

Abstract

1302193 Mass spectrometers SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ NV 16 April 1970 [18 April 1969] 20005/69 Heading H1D In a mass spectrometer of the time-of flight type, a beam of ions is passed through a pair of electrostatic deflection plates 21, Fig. 2, the field between which is periodically stepped between two discrete values. Ions as shown at a 0 which have passed through the plates before the step waveform is applied follow the path a 1 , a 2 to one side of a slit 4 in an output diaphragm, whereas ions as shown at e 0 which enter the field as the waveform is applied are deflected to the other side of the slit along the path e 1 , e 2 ; only ions which lie within the region c 0 , b 0 , d 0 , c<SP>1</SP> 0 at the time of the step pass through the slit to an output electron multiplier (not shown). The solid lines in Fig. 2 illustrate the shape of the beam at various instants, and the ions in the selected region separate according to their mass/charge ratio in the drift space, which may or may not include an electric field, so that the masses may be displayed as a function of time on an oscilloscope synchronised with the step wave. The end of the step wave could cause a second spectrum, and this may be displayed and may be prevented from interfering with the first spectrum by using a long enough pulse; alternatively the second spectrum may be suppressed by providing a second pair of deflection plates inclined e.g. at 90 degrees to the first and supplied with the same waveform, or by triggering the oscilloscope only on the leading edge of the pulse. For adequate resolution the rise time of the pulse should be less than half the transit time of the heaviest ions through the deflection plates. The width of the slit 4 influences the sensitivity and resolution and may be adjustable or may vary along its length so that a desired width may be selected by adjustment of the direct voltage across the second pair of deflection plates.