Ion trap capacity

The GC/MS applications discussed above demonstrate clearly that ion traps provide excellent data for applications, despite a history of poor performance in early ion trap designs. Qualitative and quantitative analysis in heavy matrices are possible because increased ion trapping capacity and ion population control is available in modern instrumentation. The technology has been accepted for use with major USEPA methods as a routine analytical tool for challenging environmental samples. The analyzer is versatile, because scan modes such as MS/MS, liquid Cl, hybrid Cl, and full scan mass spectrometry can be performed on the same instrument. This instrumental versatility reduces cost and increases specificity by providing more information about the molecules under study. 

Mordehai, A. Miller, B. Bai, J. Brekenfeld, A. Baessmann, C. Schubert, M. Hosea, K. Improved 3D ion trap-ion detector coupling and techniques for evaluating exact ion trap capacity. Proc. 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, 2004. 

Ion trap capacities A simple model for the comparison of ion trap capacities based on the volumes of the trapped ion clouds subjected to collisional focusing in each instrument yields... 

What is one of the significant advantages of a linear (2D) ion trap mass analyzer when compared to a 3D (Paul) ion trap (increased ion capacity, which leads to improved sensitivity). 

The commercially available stand-alone LITs, marketed under the name LTQ, are made of four hyperbolic cross-sectional rods (Fig. 1.25). Since ions are trapped in an axial mode as opposed to central trapping on 3D ion traps, LTQs have been successfully coupled with Orbitrap and FTICR for achieving high-resolution capabilities (Peterman et al., 2005 Sanders et al., 2006) (Chapter 5). Functional improvements in 2D traps over 3D traps include 15 times increase in ion storage capacity, 3 times faster scanning, and over 50% improvement in detection efficiency and trapping efficiency. 

In recent years, linear (or 2D) ion trap mass spectrometers have been developed to improve detection limits over the traditional ion traps (Hager, 2002 Schwartz et al., 2002). In addition, LITs have greater ion storage capacities that increase the number of ions that can be trapped, and hence detected, without space charge effects. 

Figure 2.33 represents such a linear trap. The two detectors allow the use of all the ions expelled from the trap. Trapping efficiency is in the range 55-70 % while it is only 5 % in the Paul ion trap. Unit resolution is achieved at 16 700 Th s 1 scan rate. At 27 Th s 1, Am = 0.05 is observed at m/z 1520, corresponding to a resolution of 30000 FWHM. The ion capacity is about 20 000, 40 times more than in the Paul ion trap. 

Obviously, an RF-only multipole device by itself has a charge-capacity limitation. This was investigated by Tolmachev et al. [40]. It was found that the charge-capacity limit is only determined by the number of poles and the RF voltage, but not by the mass and/or charge of the ions. The recently introduced linear ion traps take advantage of these features (Ch. 2.4.2). 

The 3D quadrupole ion trap suffers from a severe limitation. If the number of trapped ions is too high, the electrical field due to the Vcos iot potential is overlapped by that due to the ion cloud. The result is a drop in instrumental performances, particularly in mass resolution and linear response. To avoid this undesired phenomenon, a preliminary scan (not seen by the ion trap user) is performed and the ionization time (or the ion injection time) is optimized, thus confining the optimum number of ions inside the trap (see Fig. 2.18). This prescan leads to a well-controlled instrumental setup but, of course, it limits the sensitivity of the instrument. To overcome this problem, two different approaches can be employed (1) increase the ion storage capacity of the trap by increasing the electric field strength (2) increase the inner volume of the trap, so as to obtain a less dense ion cloud, which results in a decrease of space charge effects. 

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