Inductively coupled plasma mass spectrometry, coincidence laser spectroscopy (ICP-MS-CLS), simulation of the transmission efficiency of a 3D quadrupole ion trap for cooling energetic ions from the ICP prior to optical detection

Abstract

A fundamental requirement of ICP-MS-CLS is a post mass separation ion beam with an energy spread of less than ∼0.2 eV to enable efficient pumping of fluorescent lines with a narrow band (Δν ∼ 100 kHz) laser for the optical detection step. The role of a 3D quadrupole ion trap for ion beam cooling, as an accessory for a new generation of ICP-MS-CLS (inductively coupled plasma mass spectrometry, coincidence laser spectroscopy) (B. L. Sharp, P. S. Goodall, L. M. Ignjatovic and H. Teng, J. Anal. At. Spectrom., 2007, 22, 1447) instrument, was investigated by means of numerical simulation. Whereas in-trap ion cooling is well established, it was found that extraction of cooled ions from the trap introduced additional broadening and a loss of transmission efficiency. Modelling was based on a beam of 20 eV mean energy with a spread of 3 eV which represents a worst case scenario for the ions derived from an ICP source. The efficiency of the cooling process (trapping + extraction), as well as the energy distributions of ions exiting the trap, were calculated and compared for several methods of extraction. Trapping efficiencies of ∼25% were obtained at buffer gas pressures up to 10 mTorr. The addition of virtual reflectrons to the trap resulted in an improvement in the energy distribution of the trapped ions by lowering the wings of the distribution, but did not significantly improve the efficiency. Zero field, rf field only, quadrupolar field and homogeneous field ion extraction were investigated to recover ions from the trap, and of these quadrupolar extraction was best. However, extraction efficiency and ion energy distribution were inversely related so that quadrupolar extraction at Uq = 10 V yielded an exit ion energy distribution of 0.5 eV half width, compared with a trapped width of 0.03 eV, but with only 10% transmission efficiency. Thus, it was demonstrated that a 3D trap can be used to cool energetic ions for post-trap mass or spectroscopic examination, but the low efficiency makes it unsuitable for use with ICP-MS-CLS.