Report
Ion Traps: What Do They Hold for Elemental Mass Analysis?
T
he sensitivity and isotopic information afforded by MS have made it the technique of choice for trace multielement and isotopic analysis. The simplicity of the spectra facilitates qualitative analysis, and the intensity of the ion signal provides an indirect means of quantification. Magnetic-sector based analyzers and linear quadrupoles have historically been the mainstays for elemental analysis. Sector-based instruments provide moderate mass-resolving power that helps combat polyatomic interferences These instruments also have low detector background noise which improves trace element detection Although linear quadniDole mass analyzers do not possess the low detection limits of Qprtor*; thpv provide a less evnensive annroarb to elemental analysis when isnharic interferern'AC a r e n n t p r p c p n t o r c a n K p rc*nAi}\7 s M i h t r a c t e r l
While sector-based and linear quadrupole mass analyzers were being developed for elemental and isotopic analysis, the development of time-of-flight and ion trap devices was being driven by the needs of organic mass spectrometrists. Ion traps, in particular, have received much attention because of their unique capabilities. Fourier transform ion cyclotron resonance (FTICR) MS for example has developed into a viable analytical technique in part because it provides high mass-resolving power for the analysis of biological molecules (2) In a similar vein the need for a sensitive inexpensive detector for GC led to the development of the fiuadmnole ion trap mass unprfrometer C? -?^
Much of the development in ion trap
Douglas C. Duckworth Christopher M. Barshick Oak Ridge National Laboratory
Ion traps represent a radical change in thinking and a unique opportunity for the elemental mass spectrometrist to utilize these unique capabilities. Analytical Chemistry News & Features, November 1, 1991 709 A
Report
fines ions radially, while electrostatic potentials applied to the two end-plate electrodes trap the ions axially. Ions trapped by the magnetic and electrostatic field are characterized by three motions—a simple "trapping" motion that confines the ions between the two end-plate electrodes, magnetron motion, and cyclotron motion (5). Of primary importance in the detection of stored ions is cyclotron motion. As ions are resonantly excited to larger cyclotron orbits by resonant radio frequency (rf) signals sent to the excitation electrodes the coherent packets of ions induce an image current on the detection electrodes The current is converted to voltage amplified dieitized and stored as a transient resnonse The FT of this transient converts the signal
Figure 1. Fundamental components of inorganic ion trap MS.
into its fr^rni^nrv pftmnnnents;
is related MS occurred in the late 1970s and early 1980s. Thus the inorganic community entered the ion trap arena in the early 1990s with the groundwork already laid. Clearly, prior efforts in sampling externally generated ions from laser desorption, electrospray, thermospray, and atmosphericsampling glow discharge sources (1,4) facilitated the interfacing of glow discharge (GD) and inductively coupled plasma (ICP) ion sources to traps, allowing the inorganic MS community to start using these mass analyzers for elemental analysis. Figure 1 shows the fundamental components of an ion trap MS instrument Hon source transport optics and mass analyzer) The solid lines represent demonstrated combinationQ- the dashpH line is an altpmative that
should work but has yet to
either a continuous or discontinuous (pulsed) beam of ions, the opportunity to utilize unique capabilities. Fundamentals ICR mass analyzers and quadrupole ion traps are similar devices; stored ions precess in the trapping fields with a frequency dependent on the mass-to-charge ratio (ml2). Table 1 shows other characteristics of the two devices, many of which are comparable. At some point, however, similarities between the two break down because of differences in geometry, the nature of the trapping fields, operating pressures, and the method of ion detection. In an ICR, ions are constrained spatially by a combination of electric and magnetic fields. A homogeneous magnetic field con-
to the
ion cyclotron motion by
co = qB/m
(1)
in which q is the ion's charge, B is the magnetic field strength, and m is the ion's mass. Extremely high mass resolution can be achieved because the basis for mass measurement lies in measuring frequency, which can be done with great precision. It should also be noted that mass resolving power is inversely proportional to the ion's m/z (6, 7)) a fortunate circumstance for elemental analysis because the entire periodic table is covered in the first 250 Da. A thorough treatment of the fundamentals of ion trapping storage and detection in the ICR be found in several excellent reviews (1 5 8)
hp put into
practice No dedicated instrument is commerrially available for elemental ion trap MS This Report focuses on the analytical applications possible when GD and ICP ion sources are interfaced to ICRs and quadrupole ion traps and illustrates their promising attributes. We should point out that these analyzers are not the panaceas of elemental MS; several limitations still exist, primarily because of the limited storage capacity and thus the limited linear dynamic range of these devices. Ion traps do, however, represent a radical change in thinking and offer the elemental trometrist accustomed to mass analysis of 710 A
Table 1. Characteristics of the FT-ICR and quadrupole ion trap. Characteristic
FT-ICR
Quadrupole ion trap
Trapping fields Detection scheme Geometries
Electrostatic/magnetic Image current Cubic, rectangular, cylindrical, hyperbolic 1 ( r 6 - 1 0 - 9 Torr 200,000-200,000,000 (fwhh) 106 10 4 -10 6 -15% ±0.4-±3.0%RSD
Electrodynamic Electron multiplier Hyperbolic
Operating pressure Mass resolving power Abundance sensitivity Storage capacity Quantification accuracy Isotope ratio precision MS/MS Cost
Analytical Chemistry News & Features, November 1, 1998
+
10" 3 Torr 200-2000 (fwhh) 106 10"-10 6 Unknown ±0.8 - ±3.0% RSD ++
$$$
$
Quadrupole ion traps use an oscillating quadrupolarfieldto store the ions within the trap. The most common quadrupole ion trap configuration uses two end-cap electrodes and aringelectrode. In common practice, the endcaps are grounded, and a trapping potential with a nominal 1-MHz frequency ii spplied to the erng electrodee For a given trap radius and operating frequency, there exists a set of rf and dc voltages that result in stable radial and axial trajectories. As the rf amplitude is increased ions of increasing m/z come into resonance with the trapping signal and are ejected through the end-cap electrode; it is at this point that the ions are detected by an electron multiplier This mechanism for mass analvsis termed mass selective instabilitv wa
