Beam-type instrument

Unlike beam-type instruments, mass separation in a QIT is achieved by storing the ions in the trapping space and by manipulating their motion in time rather than in space. This task is accomplished with an oscillating electric field that is created within the boundaries of a three-electrode structure. The mass spectrum is acquired by changing the applied rf field to eject ions sequentially from the trapping field. 

Fig. 2.2 Hypothetical distribution of internal energies of ions formed in a typical beam-type instrument. Ions with dissociation rate constants <10 s" remain intact on the timescale of the mass spectrometer (10 s). Those with rate constants > 10 s" are unstable ions and fragment in the ion source, while the intermediate situation (10 10 s" ) represents metastable ions...

Automatic instruments are usually reflection-type instruments that measure the deflection of a beam of light as it passes from one medium, into the sample and then is reflected back to a detector. The angle at which the light beam exits the medium is related to the refractive index of the sample. Automated instruments are calibrated with standard substances of precisely known refractive index prior to use. 

Like much instrumentation working in the IR/visible/UV region of the spectrum, most modern UV/visible spectrometers are of the dual-beam type, since this eliminates fluctuations in the radiation source. The principle of this has been described in detail in Section 3.2. Radiation from the source is split into two by a beam splitter, and one beam is passed through the sample cell (as in a single beam instrument). The other beam passes through a reference cell, which is identical to the sample cell, but contains none of the analyte... 

Figure 10.9 Sequence for obtaining a pseudo-double beam spectrum with a Fourier transform infrared spectrometer. The apparatus records and memorizes two spectra, which represent the variations of 7q (the blank) and I (sample) as a function of the wavenumber (these are emission spectra 1 and 2). Next the conventional spectrum is calculated, identical to that of an instrument of the double beam type, by calculating the ratio T = 1/4 = fW for each wavenumber. Strong atmospheric absorption (COj and HjO), which is present along the optical path is eliminated in this way. The illustrations correspond to the spectrum of a polystyrene film.

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