Spectrometer Configurations

The dispersion in terms of v or Raman shift is not as constant, however, as noted in Section 8.1. Differentiation of the relationship between A. and v leads to Eq. (8.4), and combination with (8.3) yields (8.5)  

Since dv/dl depends on (v), it will vary both with Raman shift magnitude and with laser wavelength, as noted in Eq. (8.6) after substituting vr = vq — vj in 

Laser Wavelength Grating Density Focal Length (nm) (lines/mm) (mm) dv Tl (cm Vmm) for Av =  

The practical consequences of Eq. (8.6) lie in both resolution and spectral coverage. High dispersion (small dv/dl) yields high resolution (small Av range per pixel or slit width), according to  

In this case, dv is the Raman shift increment observable with a slit or pixel of width Wp. However, an array detector has a finite number of pixels, and the flat field of the spectrograph is of finite width, so there is a limit on the range 

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Mass spectrometer configuration. Multianalyzer instruments should be named for the analyzers in the sequence in which they are traversed by the ion beam, where B is a magnetic analyzer, E is an electrostatic analyzer, Q is a quadrupole analyzer, TOP is a time-of-flight analyzer, and ICR is an ion cyclotron resonance analyzer. For example BE mass spectrometer (reversed-geometry double-focusing instrument), BQ mass spectrometer (hybrid sector and quadrupole instrument), EBQ (double-focusing instrument followed by a quadrupole). 

Figure 12.1 shows the classic L-format of the most commonly used fluorescence spectrometer configuration which is topologically the same for the measurement of both steady-state spectra and lifetimes. The source and detector options of relevance to IR fluorescence measurements are discussed in Sections 12.3 and 12.4, respectively. The other optical components comprised of the lenses for focusing and collection and monochromators for wavelength selection contain few peculiarities in the near-IR as... 

Fig. 3.20 NSE data obtained from the incoherent scattering from a fully protonated PE melt at 509 K (M =190 kg/mol). The data close to the 0.1 ns boundary grey bar) are, due to technical difficulties at the range boundaries of the two spectrometer configurations, more uncertain than the bulk of the data points, as seen by the size of the error bars. Lines see text. (Reprinted with permission from [43]. Copyright 2003 The American Physical Society)...

Various mass spectrometer configurations have been used for the detection of explosives, such as ion traps, quadrupoles and time-of flight mass analyzers and combinations as MS/MS systems. The ionization method is usually APCI with corona discharge [24, 25]. An example is given in Figure 20, which shows the schematic diagram of an explosive mass spectrometer detector [25]. It is based on an ion trap mass analyzer, an APCI source with corona discharge and a counter-flow introduction (CFI) system. The direction of the sample gas flow introduced into the ion source is opposite to that of the ion flow produced by the ion source. 

Various mass spectrometer configurations have been used for the detection of explosives, such as ion traps, quadrupoles, and time-of-flight (TOF) analyzers and tandem mass spectrometer (MS/MS) combinations. Also, various modes of ionization have been employed, depending on the specific application in the detection of explosives. 

Fig. 1. Spectrometer configuration at NIST EBIT note that the EBIT source is located well inside the Rowland circle. The spectrometer is in the perpendicular orientation where the axis of the spectrometer is perpendicular to the long axis of the EBIT source. The detector arm moves vertically with changes in diffraction angle.

Once the number of elements to be analysed has been defined (a priori knowledge of the sample or qualitative analysis) the appropriate spectrometer configuration (choice of crystals) is adopted, the order in which the various elements are analysed is selected (certain elements such as alkalines are sensitive to the beam) and the spectrometers are calibrated. 

We will discuss SNR measurements for particular spectrometer configurations in Chapters 8 and 9, but throughout the general discussion in this chapter, we will abide by Eq. (4.1). There are a few situations where the ratio Sjdg yields a correct approximation of true SNR, and these will be noted as they occur. 

Flicker noise could indeed contribute, but it is difficult to address theoretically and may vary significantly for different spectrometer configurations and lasers. For a well-designed experiment, flicker noise can usually be reduced to a negligible level. 

The ion source used for the generation of biomolecular parent ions is critical, and only recently have the so-called soft ionization methods been developed.2 Electron-impact ionization sources fall into the category of hard sources, whereby the sample must be in the vapor phase initially, and the ionization process produces a very large number of fragments. Soft methods were introduced to overcome the problems associated with the thermal instability and involatility of macromolecular analytes. Soft ionization produces few fragments under relatively mild conditions. In Table 15.1 a comparison is shown between the three main soft ionization methods some of these values are strongly dependent on individual mass spectrometer configurations and the desired resolution. 

As Figure 4 shows, the intensify response is linear across two orders of magnitude from 5 ppm (pmol/mol) up to 500 ppm. The intensities shown are after 10,000 averages with a gate dirration of 4 ps. The error bars represent 95% confidence hmits on the data points. This graph shows that qirantitative determination of gas-phase concentrations is possible using the CP-FTMW technique down to a level of 5 ppm imder the current spectrometer configuration. At concentrations above 500 ppm the response becomes non-linear. We attribute this to cluster formation in the supersonic expansion. 

The detection limit of PC under the current spectrometer configuration was determined to be 5 ppm after 10,000 averages (22 min.). This sensitivity can be increased in two ways. Increasing the number of averages will increase the sensitivity by Vn where N is the number of averages. When the repetition rate of the experiment can... 

Pulse programs are written independently of a particular spin system or spectrometer configuration such as magnetic field strength or transmitter pulse power so that, for instance, the same HMQC pulse sequence can be used for a 13C/19F or a i N/lR... 

Depending on the type of sample and spectrometer configuration there are three main selective signal suppression methods ... 

Refer to Figures 19-48 to 19-51, p. 209, for the details of the rest of the mass spectrometer configuration. 

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