Spectral dynamic range

As discussed in detail in ref. 21, a major problem in FT/ICR is the limited mass spectral dynamic range (factor of - 1000 1). Briefly, it is not easy to detect fewer than - 100 ions at easily managed chamber pressures (>/IO"9 torr) alternatively, the spectrum can be distorted by Coulomb forces when more than - 100,000 ions are present. 

Let us consider what spectral dynamic range can be accommodated by a 16-bit... 

Spectral width, dynamic range, resolution and sensitivity are expected to be pushed toward further limits. An emerging advancement in NMR spectroscopy is the DOSY technique (Section 5.4.1.1) which offers a separation capability as a function of the rates of steady state diffusion of molecules in solution. 

Table 8.25 Principal spectral emission lines, minimum detectable limits, and linear dynamic range of some commonly monitored elements in GC-AED...

Wavelength database libraries of >32000 analytical lines can be used for fast screening of the echellogram. Such databases allow the analyst to choose the best line(s) for minimum interferences, maximum sensitivity and best dynamic range. Further extension of the wavelength range (from 120 to 785 nm) is desirable for alkali metals, Cl, Br, Ga, Ge, In, B, Bi, Pb and Sn, and would allow measurement of several emission lines in a multivariate approach to spectral interpretation [185]. 

The aim of this tutorial is to present briefly some of the optoelectronic elements, which can be used for designing of the interfaces for FOCS. More details on optoelectronic devices can be found in the references6 8. Special attention is placed on the appropriate spectral matching of these elements in order to obtain a sensor exhibiting optimal measuring properties such as dynamic range, sensitivity etc. 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

The spectral response curves of various types of photometers are usually fairly flat or smoothly rounded and do not normally create programming problems, but they must be known so that you do not stray beyond the usable dynamic range. 

Nuclear magnetic resonance traditionally has had low sensitivity and spectral resolution. It can provide rigorous quantification of analytes, but not accurate qualitative identification. Individual resonances may be sensitive to the chemical and physical environment of the molecule, which then requires appropriate preparation of samples. The process also has little dynamic range, in contrast to GC-MS. 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

Figure 15.7—Optical diagrams for a spectrophotometer with an echelle grating. Model PU 7000 optical system (reproduced by permission of Philips). All spectral lines are captured, which allows a more complete study of the sample. The dynamic range of these instruments is still lower than that of a PMT.

---