Spectrometer Design

For FT-IR spectrometers that do not require the use of a J-stop, a simple off-axis paraboloidal segment is used to collect and collimate the beam from the source. For higher-resolution instruments, a slightly more complicated optical arrangement is 

Some sampling accessories [e.g., infrared microscopes (see Chapter 14) and gas chromatography interfaces (see Chapter 23)] are too big to fit in even the largest sample compartment. In these cases, the accessories are mounted outside the instrument and the modulated beam is directed to them by means of a switch-able mirror (often known as di flip mirror). It is very rare that the beam is returned to the detector mounted inside the instrument, so a second detector is usually a component of accessories that are mounted outside the spectrometer in the external beam. Several contemporary instruments are also equipped with flip mirrors to direct the beam to different possible internal beam paths, in one or more of which a different accessory may be mounted permanently. In this way, the user can switch rapidly from one type of measurement to another, making optimal use of a single spectrometer. 

Water vapor in the beam of FT-IR spectrometers is a major source of interference. Thus, the optical bench should ideally either be purged with dry air or evacuated completely. Some low-resolution spectrometers are hermetically sealed and desiccated, so that the only humid air is in the sample compartment. These instruments are adequate if the spectra of conventional films, KBr disks, and solutions 

The presence of carbon dioxide is usually not quite as detrimental as water vapor because CO2 absorbs in less important spectral regions for most applications. Nonetheless, the best spectra are acquired by purging the instrument with C02-free air. Some regenerating driers produce C02-free air. The flow rate of the purge gas through the spectrometer should be enough to reduce the level of H2O and CO2 to an acceptably low level within about a minute after the sample has been loaded into the spectrometer. However, the flow rate should not be too high, because microphonic noise can be introduced into the spectrum. 

In general, the best way to eliminate the effect of atmospheric H2O and CO2 is always to remove these molecules before the measurement of any spectrum. If there are stiU some features caused by the absorption of water vapor in the spectrum, they may sometimes be eliminated by subtraction of a reference spectrum of water vapor measured with the same resolution and apodization from the spectrum. However, it should be remembered that changes in either the optical throughput or the alignment between the measurement of the reference and sample spectra will cause a wavenumber shift (see Section 2.6), leading to the same type of derivative-shaped residual lines as discussed above. 

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The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

Advances in computer and spectrometer design have made possible an NOE experiment which does not rely on spectrum subtraction. This is some-... 

Although many spectrometer designs have been produced over the years, the vast majority of laboratory instruments are based on the simplified block diagram shown in Figure 1.4. Plane-polarized microwaves are generated by the klystron tube and the power level adjusted with the Attenuator. The Circulator... 

Four different laboratories have built IR kinetic spectrometers for use with organometallic compounds. A fundamental feature of all these spectrometers is that the detector is AC coupled. This means that the spectrometers only measure changes in IR absorption. Thus, in the time-resolved IR spectrum, bands due to parent compounds destroyed by the flash appear as negative absorptions, bands due to photoproducts appear as positive absorptions, and static IR absorptions, due to solvents, for example, do not register at all. The important features of these spectrometers are listed in Fig. 2. Since three spectrometers have a line-tunable CO laser as the monochromatic light source, we begin with the CO laser. Then we look in more detail at spectrometers designed for gas phase and solution experiments. 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

Werle, P., R. Muecke, and F. Slemr, Development of a Prototype IR-FM Absorption Spectrometer Design Criteria and System Performance, in Monitoring of Gaseous Pollutants by Tunable Diode Lasers (R. Grisar, H. Boettner, M. Tacke and G. Restelli, Eds.), pp. 169-182, Kluwer Academic, Dordrecht, The Netherlands, 1992. 

Electric or magnetic field that does not change in time.1 Mass spectrometer designed for mass spectrometry/mass spectrometry.1... 

Secondary Ion Mass Spectrometer Design Considerations for Organic and Inorganic Analysis... 

Interferometry. A detailed description of spectrometer designs, of which there are several now commercially available, is beyond the scope of this book. There are several fine texts to which workers new to the field, or interested non - practitioners, may refer.(1 - 3). 

Thus far, only one high-field DNP system operating at 9.2 T and 260 GHz has been reported to experimentally realize DNP enhancements of solution-state systems. The spectrometer design and ESR spectra were presented by Denysenkov et al.,134 and the first DNP data were presented by Prandolini et al.79 This system gave E = 3.0 for4-hydroxy-TEMPO in... 

Most isotope ratio measurements have been performed using sector mass spectrometers. Some work has been reported, notably by Heumann [35], in which a quadrupole-based system was used. Instruments used for measurement of isotope ratios are most often dedicated to that purpose. In most instances only a relatively small mass range needs to be monitored, just enough to encompass the isotopes of the analyte element. Without the ability to scan the entire elemental mass range [usually from mlz = 6 (Li) through mfc = 238 (U) for elemental analysis], mass spectrometers designed to measure isotope ratios cannot readily be adapted for other purposes. See Chapter 2 for a discussion of instrumentation required for elemental analysis of solid materials and Chapter 3 for a treatment of the in-strumenation needed for elemental analysis of solutions. 

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