Quadrupole atom traps

Almost any type of analyzer could be used to separate isotopes, so their ratios of abundances can be measured. In practice, the type of analyzer employed will depend on the resolution needed to differentiate among a range of isotopes. When the isotopes are locked into multielement ions, it becomes difficult to separate all of the possible isotopes. For example, an ion of composition CgHijOj will actually consist of many compositions if all of the isotopes ( C, C, H, H, 0, O, and 0) are considered. To resolve all of these isotopic compositions before measurement of their abundances is difficult. For low-molecular-mass ions (HjO, COj) or for atomic ions (Ca, Cl), the problems are not so severe. Therefore, most accurate isotope ratio measurements are made on low-molecular-mass species, and resolution of these even with simple analyzers is not difficult. The most widely used analyzers are based on magnets, quadrupoles, ion traps, and time-of-flight instruments. 

The obvious benefit of the quadrupole ion trap is that it is an ion storage device. Therefore, ions can be both accumulated and stored for extended periods. Accumulation can occur over a continuous ionization event or over multiple pulsed ionization periods. When used with pulsed ionization sources, duty cycle, defined in terms of sample utilization, can be as high as 100%. Because a broad range of atomic ions can be stored simultaneously, the quadrupole ion trap is a promising analyzer for transient peak analysis. 

Figure 2 Schematic diagram illustrating the configuration of the quadrupole ion trap for laser ablation inside the ion trap. Samples inserted through the ring electrode are atomized and ionized in the trapping volume by the laser beam, entering the ring electrode from 180° relative to the sample. (From Ref. 30.)...

Other gas-phase ion chemical reactions were explored by using oxygen introduced into the octopole collision cell product ions were injected into and analyzed by the quadrupole ion trap [55]. Reaction of oxygen with Y+, Zr+, and Xe+ was found to proceed rapidly, forming YO+, ZrO+, and Xe neutrals, effectively eliminating interferences by the atomic ions. These ions are particularly difficult... 

To increase the sensitivity the remaining ions are sent into a quadrupole ion trap where they accumulate and are slowed down through collisions with rare gas atoms Helium is injected into the trap through a leak valve to keep the pressure in the trap at 1x10 Torr. Figure 8 shows the quadrupole and ion trap assembly. 

Lascr-microprt)be mass speciromelers arc used for the study of solid surfaces. Ablation of the surface is accomplished with a high-power, pulsed laser, usually a Nd-YAG laser. After frequency quadrupling, the Nd-YA(i laser can produce 266-nm radiation focused to a spot as small as 0.5 pm. The power density of the radiation within this spot can be as high as 10 to 10" W/cm On ablation of the surface a small fraction of the atoms arc ionized. The ions produced are accel eraied and then analyzed, usually by limc-of-nighi mass spectrometry. In some cases laser microprobes have been combined with quadrupole ion traps and with Fourier transform mas speciromelers. Laser-microprobe tandem mass speclromeiry is also reeeiv-... 

FIGURE 2.3 Representativemassspectrafortheieactionofdeprotonated2-deoxy-5-cytidine monophosphate (5-dCMP) with D2O in a quadrupole-ion trap. Reaction times are (a) 0 ms, (b) 250 ms, (c) 2000 ms, and (d) 10,000 ms. Peaks are labeled as D , where n equals the number of incorporated denterinm atoms. (Reproduced from Chipuk, J.E. Brodbelt, J.S. J. Am. Soc. Mass Spectrom. 2007,18, 724-736. With permission from Elsevier.)... 

Part 2. Ion Conformation and Structure presents discussions of structural characterization of proteins and peptides using quadrupole ion trap mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and the novel method known as traveling wave ion mobility mass spectrometry. In addition to the observation of collective fluctuations of the molecular substructures within biomolecules, the organization of atoms in small ion clusters is investigated using electron diffraction. 

While not thus far exploited for LLE, it is interesting to note experiments (Bogan 2002, 2004) in which charged droplets ( 0.27 xL) are produced from a piezo-electric atomizer and are levitated in the electric field of a modified quadrupole ion trap (Section 6.4.5) operated at atmospheric pressure. Thus far the main analytical application of this technique (Bogan 2004) has involved wall-free preparation of micrometer-sized sample spots for fmol detection limits of proteins by MALDI-MS, but extension to LLE in such levitated droplets is a possibility. 

Sometimes the observation of ion-helium clusters in a trap is simply used as a qualitative indication that the ions are cold, without providing a precise temperature determination. For example, Jasik et al. [61] observed that the hydrocarbon dication can be clustered with as many as five helium atoms in their wire-based quadrupole ion trap cooled to 4 K. 

One direct way to close the chaimel whereby atoms escape from the central region of a quadrupole magnetic trap is to displace the potential (Fig. 6.9(a)) in the symmetry... 

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