Operation of a quadrupole ion trap mass spectrometer to achieve high mass/charge ratios

Abstract

One of the principal limitations of the commercial quadrupole ion trap mass spectrometer is the relatively low limiting value of mass/charge ratio (650 Da per charge). This constraint limits applications of desorption ionization techniques which can produce ions of many thousand Da per charge. Several techniques for extending the mass/charge range of the quadrupole ion trap are presented. These include (i) the use of smaller electrodes, (ii) operation at lower radio frequencies, and (iii) resonance ion ejection using a voltage of appropriate frequency, applied symmetrically across the end-cap electrodes during the mass scan. The performance of each of these methods is compared using external ionization of alkali halide salts with Cs + bombardment. Special attention is given to the effects of scan rate on resolution and a method of reducing the scan rate without loss of data is described. Mass measurement accuracy is studied in some detail, the mass shifts which occur using resonance ejection are characterized and this information is used to correct mass assignments. The relative merits of the three methods of mass range extension are assessed. Field inhomogeneities in the particular smaller electrodes used here apparently cause some loss of performance in these devices making them the least successful of the methods. Frequency reduction gives excellent results over a limited range of m/z values. However, resonance ejection can be used alone to achieve an even greater m/z extension and is the (single) method of choice. A combination of modest size and frequency reduction with axial modulation is probably an ideal solution for high mass biological mass spectrometry. Performance at high mass is illustrated by recording data on CsI clusters and on peptides. These experiments show that mass/charge measurements are accurate to better than 0.1% without external calibration, that a mass resolution of 3000 FWHM is achieved (unit resolution at 50% valley to 3000), and that a mass/charge range in excess of 70 000 Da per charge is accessible.