Peak hopping

Illustration of the difference between scanning and peak-hopping mass analysis. In (a) there are up to 100 data points defining eaeh peak, whereas in (b) there are only five data points per peak. 

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. 

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. 

Figure 9.31 (a) Transmission quadrupole mass spectrometer. Rods A and B are tied together electrically, as are Rods C and D. The two pairs of rods, AB and CD, are connected both to a source of direct potential and a variable RF excitation such that the RF voltages are 180° out of phase, (b) The geometry of the rods, (c) Sequential detection of elements by peak hopping. 

The method should be set up for Tm, and aU U isotopes of interest from U, U, U, and U. Use five replicates of 1000 passes with 0.010 s dwell time (fast peak hopping) on m and the selected combination of U, U, U, and U isotopes. Since Q-ICP-MS instrumentation does not possess the sensitivity of a magnetic sector instrument as described in Protocol 1, accurate counting of lower-abundance isotopes, U, U, and U, may not be possible unless the uranium is concentrated on the TRU cartridge from a sufficient volume, or unless a desolvating introduction system is used. 

Continuous scanning of (a part of) the mass spectrum or peak hopping or peak jumping is accomplished by changing these voltages in such a way that the U/V ratio remains constant. The spectral peak width is constant over the mass range (mass resolution, on the other hand, varies as a function of the mass considered). 

Also, there is the peak hopping approach, in which the quadrupole power supply is driven to a discrete position on the peak (normally the maximum point), allowed to settle, and a measurement is taken for a fixed amount of time. This is represented in Figure 12.5. 

FIGURE 21.2 There are typically two approaches to peak quantitation— peak hopping (usually at peak maximum) and multichannel scanning (across the full width of the peak). 

It is also important to ranember that peak measurement protocol will also have an impact on detection capability. As mentioned in Chapter 12, there are basically two approaches to measuring an isotopic signal in ICP-MS. There is the multichannel scanning approach, which uses a continuous smooth ramp of 1-20 channels per mass across the peak profile, and peak-hopping approach, where the mass analyzer power supply is driven to a discrete position on the peak, allowed to settle, and a measurement taken for a fixed period of time. This is usually at the peak maximum but can be as many points as the operator selects. This process is simplistically shown in Figure 21.2. 

FIGURE 21.3 Good mass stability is critical for single-point, peak-hopping quantitation. 

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