Peak hopping mode

Q. What are the main differences between the scanning and peak-hopping mode of operation of a quadrupole mass analyser ... 

Two main modes of operation are possible. In the scanning mode, the whole profile of each of the masses is scanned. In the peak-hopping mode only several points (i.e. channels) over the peak profile are included in the data acquisition. The latter method is more rapid so is generally preferred, and is the method used here. 

The m/z value transmitted by a quadrupole mass analyzer can be scanned or switched very rapidly. The most common ways to acquire data with one of these devices are illustrated in fig. 19. The spectrum can simply be scanned continuously once, as shown in fig. 19a. In the peak-hopping mode (fig. 19b), the mass analyzer transmits a certain m/z value for a chosen time and then quickly switches to another value. This peak hopping is illustrated for three separate m/z positions spanning each of two peaks. This peak-hopping process is generally done repetitively with each mass peak monitored many times. The background can also be determined by hopping to an m/z position devoid of ions. 

Most mass analyzers for ICP/MS instruments are operated in a scanning mode. These mass analyzers operate as bandpass filters, passing a single mass-to-charge ratio at a time for detection by the ion collection system. The mass analyzers are scanned either by peak-hopping or by continuous scan. 

At lesser sensitivity requirements, or if a measurement of the peak shape is desired, a continuous scan of mass to record full peaks may be preferred. In modern instruments, control is really a digital process, so that the continuous scan mode is essentially peak hopping with a small mass interval—0.1 amu or less per step. Very high dynamic range instruments, in which the peak tails are to be measured, might use 50 or more steps per amu. 

Operation of a SIMS instrument resembles both that of an isotope ratio mass spectrometer and an electron microscope. Most SIMS instruments include an optical microscope so that the sample can be directly viewed during analysis, which allows for accurate positioning of the area of interest on the sample. Data can be in the standard mode used for other types of mass spectrometers in which ions are produced and the mass spectrum is analyzed by scanning or peak-hopping. This mode is sometimes called the microprobe mode in SIMS. Another application for SIMS is the acquisition of ion-images. This mode is called the microscope mode because the SIMS is operated as an ion microscope. 

Fig. 19. Scanning and data acquisition modes. Three successive cycles (indicated by 1, 2, 3) are shown for peak hopping and multichannel scanning. In peak hopping, each m/z position is typically monitored for 0.1 s. In multichannel scanning, each scan takes typically 0.01 s per m/z unit in the window observed.

In early theoretical studies the photodissociation of CH3I was treated as an effective two-mode system with CH3 considered as one particle. As with ICN, a new era began with ab initio calculations performed by Morokuma and coworkers on a reasonably high level of electronic structure theory. These authors determined six- and nine-dimensional adiabatic and diabatic PESs, which were used in classical surface hopping trajectory calculations. Hammerich et al. performed large-scale five-dimensional wave packet calculations using these PESs. As concluded by Eppink and Parker One caveat of the ab initio surfaces is that they do not successfully reproduce the experimental absorption spectrum, which peaks to the red side of and is broader than the spectrum predicted by the theoretical treatments.. .. Furthermore, the predicted I quantum yields of the most... 

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