Spectrometer resolution

The minimum detection limit, MDL, of an isolated peak on a uniform background is proportional to the square root of the FWHM. So a 20% reduction in spectrometer resolution will produce about a 10% improvement in MDL. If there is peak overlap, however, then it can be shown that a 20% improvement in resolution can reduce the interference between overlapping peaks by a factor of 3, which gives about a 50% improvement in MDL. 

Equipment technology and processing software for FTIR are very robust and provide a high degree of reliability. Concerns arise for only the most demanding applications. For quantitative work on an isolated feature in the spectrum, the rule of thumb is that the spectrometer resolution be one-tenth the width of the band. FTIR instruments routinely meet that requirement for solids. 

Detection limits for a particular sample depend on a number of parameters, including observation height in the plasma, applied power, gas flow rates, spectrometer resolution, integration time, the sample introduction system, and sample-induced background or spectral overlaps. ... 

Describes how spectrometer resolution affects detection limits in the presence and absence of spectral overlaps. 

Spectrometer Resolution (cm-1) Stray light Type Focal length (m) Gratings ... 

Figure 4.13 Theoretical molecular-ion region of the mass spectrum of a molecule of composition C35H48NgOnS at a mass spectrometer resolution of 1500.

The resolution of most mass spectrometers (the ability to separate ions of similar m/z values - see Section 3.3 above) in routine use is sufficient to allow the separation of the ions containing the individual isotopes if low-molecular-weight compounds (<1000 Da) are being studied. This is illustrated in Figure 4.13 which shows the molecular-ion region of a compound having the molecular formula C35H48N8O11S determined with a mass spectrometer resolution of 1500. The masses of the isotopes present in this molecule are shown in Table 4.2. 

If, however, we consider a protein of modest size, such as aprotin with a molecular formula of C284H432N84O79S7 at a similar mass spectrometer resolution, the molecular-ion region of its mass spectrum, shown in Figure 4.14, does not show the individual isotopic contributions, a resolution around 5000 being required for these to be evident (Figure 4.15). 

Figure 4.17 Theoretical mass spectrum of the 7+ charge state of aprotin (molecular weight, 6507) at a mass spectrometer resolution of 5000.

Optical emision spectra nowadays are simply measured using a fiber optic cable that directs the plasma light to a monochromator, which is coupled to a photodetector. By rotating the prism in the monochromator a wavelength scan of the emitted light can be obtained. Alternatively, an optical multichannel analyzer can be used to record (parts of) an emission spectrum simultaneously, allowing for much faster acquisition. A spectrometer resolution of about 0.1 nm is needed to identify species. 

From a detailed analysis of the spectrum of Fig. 13, including a comparison with the calculated partial 5 f density of states (broadened to take into account life-time effects and spectrometer resolution), it was concluded that the results could not be fully under-... 

X-ray sources on most instruments use either Ka peaks of magnesium (1254 eV) or aluminum (1487 eV). The Ka peaks of light elements have a smaller full width at half maximum (FWHM), 0.7-0.9 eV, than those from the more energetic heavy elements, such as copper and molybdenum. That is, they are almost monochromatic and yield more narrow photoelectron lines, which are a measure of the spectrometer resolution. These two sources are suitable particularly because the x-rays produced have sufficient energy to excite electrons below 1000 eV, the region where most of the useful photoelectron peaks occur. 

Figure 23.3 RAIRS spectra acquired at 104 K for 5 L exposure of CO on (a) clean Mo2C (b) Mo2C pre-exposed to 1.8 L 02. Spectrometer resolution is 4 cm"1.

The mass spectra of the gases evolved from the deuterated SWNT sample heated in vacuum were measured with the MI 1201V mass spectrometer. Gas ionization in the ion source of the spectrometer was produced with a 70-eV electron beam. To obtain the gas phase, the sample was placed in a quartz ampoule of a pyrolyzer that was connected to the injection system of the mass spectrometer through a fine control valve. Then the ampoule was evacuated to a pressure of about 2-x 10-5 Pa in order to remove the surface and weakly bound impurities from the sample. After the evacuation, the ampoule was isolated from the vacuum system and the sample was heated to 550°C in five steps. At each step, the sample was kept at a fixed temperature for 3 h then the fine control valve was open and the mass-spectrometric analysis of the gas collected in the ampoule was performed. After the analysis, the quartz ampoule was again evacuated, the valve was closed, and the sample was heated to the next temperature. The measurements were carried out over the range 1 < m/z < 90, where m is the atomic mass and z is the ion charge. The spectrometer resolution of about 0.08% ensured a reliable determination of the gas-phase components. 

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