Spectrometer considerations

Mass spectrometer considerations and chromati raphy adjustments... 

Infrared-based techniques are used to identify molecules on the surface. IR radiation is used to excite vibrational modes in molecules in the gas phase or adsorbed on a surface. The transmitted or reflected IR spectrum can be analyzed in a spectrometer. Considerable improvement in the sensitivity can be achieved by use of Fourier transform infrared (FTIR) spectroscopy. Attenuated total reflection (the ATR-FTIR method) inside a crystal (germanium) of high refractive index can be used to further enhance the surface sensitivity (using the evanescent field). 

YJ2 COMPONENTS OF A MASS SPECTROMETER CONSIDERATIONS FOR SPACE APPLICATIONS... 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

The use of more complex or more costly articles of equipment, such as catalytic hydrogenation apparatus, autoclaves, polari-meters, ultraviolet absorption spectrometers, etc., has not been described, because the type of such apparatus employed indifferent laboratories varies considerably, and students must be taught the use of their own laboratory equipment. 

Modem mass spectrometers are used in a very wide variety of situations, so it is almost impossible to have a simple set of criteria that would determine whether a quadrupole or magnetic sector instmment would be best for any particular application. Nevertheless, some attempt is made here to address major considerations, mostly relating to cost. 

The previous discussion has concentrated on major factors likely to be important in choosing the best mass spectrometer for a given defined purpose. Clearly, there are likely to be other issues that need to be considered, and instrument price will be one of these. The major considerations are listed in Tables 39.1-4. 

A considerable amount of time is necessary to reach the point at which sample analyses can commence, and it is essential that the stability and reliability of the mass spectrometer be high to ensure maximum sample throughput during the limited time available between calibration checta. 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

A second important need for some guidance system lies in stray electric fields. Clearly, a sufficiently large potential arranged transversely to an ion beam can serve to deflect ions away from the intended direction. Such stray fields can be produced easily by sharp edges or points on the inside of a mass spectrometer and even more so in an ion guide itself. Considerable care is needed in the construction and design of mass spectrometers to reduce these effects to a minimum. 

Commercial spectrometers are usually bakeable, can reach ultrahigh-vacuum pressures of better than 10" Torr, and have fast-entry load-lock systems for inserting samples. The reason for the ultrahigh-vacuum design, which increases cost considerably, is that reactive surfaces, e.g., clean metals, contaminate rapidly in poor vacuum (1 atomic layer in 1 s at 10 Torr). If the purpose of the spectrometer is to always look at as-inserted samples, which are already contaminated, or to examine rather unreactive surfaces (e.g., polymers) vacuum conditions can be relaxed considerably. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

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