Ion-trap frequency

Finally, the stability of the source for the DC voltage U applied to the end-cap electrodes is obviously an important parameter because the ion-trap frequency is related directly to U through Equations 10.1 and 10.5. With the DC supply at hand, the relative drift of the ion-trap frequency is below 10 over approximately 15 minutes (see Figure 10.15 below). As discussed in Section 10.5.7, a slow drift is non-critical for many experiments where a reference measurement and a measurement of C0+ can be made within a short time interval. 

In this laboratory, the resolution is dominated by the spot size resolution of the imaging system which is about 3 pm HWHM, while the amplitude for a " Ca ion is less than 2 pm at a temperature of 5 mK and an ion-trap frequency of 2n x 100 kHz. 

In a longer reaction sequence, for example either of bi-molecular reactions as described in Section 10.4.2.1, or photoreactions as described in Section 10.4.2.3, or a combination thereof, a drift of the ion-trap frequency would compromise the mass measurements for the different reaction steps. A linear drift can be corrected for by measuring the ion-trap frequency before and after the reaction sequence, provided that the necessary trap loading after the sequence does not give rise to significant shifts of the ion-trap frequency. Measurement of co+ and co during the... 

The uncertainties on the measured frequencies, either cOi and co+ or co+ and co, translate to an uncertainty on the mass m2 of the unknown ion determined from Equation 10.10. As shown in Figure 10.16, the relative uncertainty depends on the chosen reference measurement as well as the mass ratio p. For a mass ratio larger than 1.25, it appears to be advantageous to determine the mass from co+ and co, however, the difficulties in making a precise determination of cd are sufficiently severe that a mass measurement based on cOi and co+ is generally preferable, provided that error sources, which are more severe for mass measurements based on o), and C0+, for example, drift of the ion-trap frequency and RF-induced secular potentials, are sufficiently small. 

Prestage, J.D. Extended linear ion trap frequency standard apparatus, US Patent 1995, 5,420,549. 

Tjoelker, R.L. Prestage, R.L. Maleki, L. The JPL Hg+ extended linear ion trap. Frequency standard Status, stability, and accuracy prospects. Proc. Precise Time and Time Interval, PTTI1996, 26, 235-243. 

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

Other types of mass spectrometer may use point, array, or both types of collector. The time-of-flight (TOF) instrument uses a special multichannel plate collector an ion trap can record ion arrivals either sequentially in time or all at once a Fourier-transform ion cyclotron resonance (FTICR) instrument can record ion arrivals in either time or frequency domains which are interconvertible (by the Fourier-transform technique). 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

The reactions of the gold anion Au- and of the di- and triatomic gold cluster monoanions Au2- and Au3- with CO were studied in a radio-frequency octopole ion trap experiment at cryogenic temperatures. Au- shows no affinity for CO, but the two cluster anions absorb up to two CO molecules. Particular stability has been ascribed to [Au3(CO)2]-, for which the binding energy has been estimated from thermolysis rate coefficients.292... 

Fig. 17.8 Illustration of the trapping principle in an ion trap. The effect of rotating the saddle potential in part (a) is a pseudopotential well illustrated in part (b). For particles with suitable mass (and charge) the particle motion in the pseudopotential is indicated by the black line. The motion is a combination of a secular motion in the pseudopotential well and a small amplitude micromotion at the frequency of rotation mf. If the particle motion is damped, the particle can come to rest at the bottom of the well...

As a result of this excitation step, the net coherent ion motion produces a time-dependent signal on the receiver plates, termed the image current , which represents aU ions in the FT-ICR cell. The image current is converted to a voltage, ampMed, digitized, and Fourier transformed to yield a frequency spectrum that contains complete information about frequencies and abundances of all ions trapped in the cell. A mass spectrum can then be determined by converting frequency into mass because frequency can be measured precisely, the mass of an ion can be determined to one part in 10 or better. 

Linear quadmpole ion trap LIT Continuous ion beam and trapped ions storage and eventually separation in linear radio frequency quad-rapole field due to stability of trajectories... 

Ion trapping devices are sensitive to overload because of the detrimental effects of coulombic repulsion on ion trajectories. The maximum number of ions that can be stored in a QTT is about 10 -10, but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a sub-type of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution. [160,161] During the RF scan, the modulation voltage with a fixed amplitude and frequency is applied between the end caps. Its frequency is chosen slightly below V2 of the fundamental RF frequency, because for Pz < 1, e.g., = 0.98, we have z = (0 + 0.98/2) = 0.49 x... 

Tolmachev, A.V. Udseth, HR. Smith, R.D. Radial Stratification of Ions As a Function of Mass to Charge Ratio in Collisional Cooling Radio Frequency Multipoles Used As Ion Guides or Ion Traps. Rapid Commun. Mass Spectrom. 2000,74, 1907-1913. 

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