Upper stability island of the quadrupole mass filter with amplitude modulation of the applied voltages

Abstract

Modulation of the voltages applied to a quadrupole mass filter (QMF), either RF or RF and DC, leads to splitting of the stability region into islands of stability. The ion optical properties, such as transmission, resolving power and peak tails of the first upper stability islands have been investigated by numerical simulation of ion trajectories. The dependence of the location of this island on the amplitude of the modulation and the parameter ϭ /⍀ ϭ Q/P where is modulation frequency, ⍀ is main angular radio frequency, and Q and P are integers, is calculated in detail. Different methods of adjusting the QMF resolution are examined. It is found that operation at the upper and lower tips of the stability islands created by amplitude modulation of the RF voltage is preferred, because of the technical simplicity of this method and a reduction of the required separation time. Amplitude modulation of the RF and DC potentials applied to a quadrupole is an example of parametric excitation. The theory of parametric resonance of different orders of the ion oscillations in quadrupole RF fields is given in [5] and investigated experimentally in [6]. Mass separation in upper stable island was used previously to modify and improve the peak shape [7]. The general problem of the calculation of stability diagrams for ion motion in periodic RF quadrupole fields is reviewed in [8]. The resonant excitation of the ion oscillations by a weak auxiliary RF signal produces strong changes in the Mathieu stability diagram [5][6][7][8][9][10]. The excitation causes instability bands, along the iso-␤ lines. As a result, the original stability region splits into islands of stability. Amplitude modulation of the main trapping voltage also produces instability bands on the stability diagram [4,8].In this paper we describe a study of the upper stable island caused by modulation of the RF or DC and RF voltages by a small auxiliary signal. The location of the stability islands for different values of the amplitude of modulation and the ratio ϭ /⍀ are given in detail. Different methods of adjusting the resolution are examined. We show that operation at the upper and lower tips of the islands created by amplitude modulation of the RF voltage is preferred because of the technical simplicity and a reduction of the required separation time. We show that a rod set that is constructed with round rods and operated with modulation of the voltages can give higher performance than an ideal quadrupole field without modulation.

Mass analysis with islands of stability with linear quadrupoles incorporating higher order multipole fields

“…Addition of these field distortions might be expected to cause conventional mass analysis, done by scanning the applied direct current (DC) and radio frequency (rf) voltages so that ions cross the tip of the first stability diagram [1], to deteriorate. Nevertheless, mass analysis with quadrupoles with added octopole [13] or hexapole [14] fields has been found to be possible in some cases, provided the positive DC is applied to the x rods so that the Mathieu parameter a, (eq 5 below) is positive for positive ions.For mass analysis, linear quadrupoles can also be operated with islands of stability [4,[15][16][17][18][19][20]. Islands of stability, separated by bands of instability, are formed in a stability region when auxiliary quadrupole excitaAddress reprint requests to D. J. Douglas,…”

“…For mass analysis, linear quadrupoles can also be operated with islands of stability [4,[15][16][17][18][19][20]. Islands of stability, separated by bands of instability, are formed in a stability region when auxiliary quadrupole excita-tion with a properly chosen frequency and amplitude is added to the quadrupole.…”

Mass analysis in islands of stability with linear quadrupoles with added octopole fields

“…Ions within the quadrupole are excited by dipole or quadrupole excitation, gain sufficient kinetic energy to overcome a potential barrier at the quadrupole exit, and are ejected [3, 16]. Preliminary experiments show that this method can be used for mass analysis with a linear quadrupole that has an added octopole field [17].For operation as mass filters, linear quadrupoles can also be operated with "islands of stability" formed by parametric excitation of ions produced by excitation with auxiliary quadrupole RF or by amplitude modulation of the RF voltage [19]. In a mass filter with a pure quadrupole field, the potential, V (x, y, t), is…”

“…For operation as mass filters, linear quadrupoles can also be operated with "islands of stability" formed by parametric excitation of ions produced by excitation with auxiliary quadrupole RF or by amplitude modulation of the RF voltage [19]. In a mass filter with a pure quadrupole field, the potential, V (x, y, t), is…”

“…The stability diagram forms bands of instability along iso-␤ lines, with ␤ u values given by eq 8. Islands of stability are formed between the bands of instability [18,19]. Ions of a particular mass to charge ratio (m/z) can be mass analyzed by adjusting the applied RF and DC voltages to place ions at the tip of an island.…”