Mass detectors, spectrophotometers

Two common detectors, which also are independent instruments, are Fourier transform infrared spectrophotometers (FT-IR) and mass spectrometers (MS). In GC-FT-IR, effluent from the column flows through an optical cell constructed... 

An HPLC detector is often a modified spectrophotometer equipped with a small flow cell, which monitors the concentration (or mass) of eluting sample components. A number of detectors used in HPLC are discussed below. Most applications utilize absorbance detectors such as UV/Vis or... 

The beams of reactant molecules A and B intersect in a small scattering volume V. The product molecule C is collected in the detector. The detector can be rotated around the scattering centre. Various devices may be inserted in the beam path, i.e. between reactants and scattering volume and between scattering volume and product species to measure velocity or other properties. The angular distribution of the scattered product can be measured by rotating the detector in the plane defined by two molecular beams. The mass spectrophotometer can also be set to measure a specific molecular mass so that the individual product molecules are detected. 

Peak purity tests are used to demonstrate that an observed chromatographic peak is attributable to a single component. Mass spectrometry is the most sensitive and accurate technique to use for peak purity evaluation because of the specific information derived from the analysis. However, a good number of HPLC methods use mobile phase conditions that are incompatible with mass spectrometry detection. In this case, PDA spectrophotometers using peak purity algorithms may be used to support the specificity of the method. Almost all commercially available diode array detectors are equipped with proprietary software that will perform these calculations. Although this technique is more universal in application to HPLC methods, the data provided is neither particularly... 

The identification of a compound using only its retention time is vulnerable to error. It is essential that a standard compound is injected in order to verify the retention time. As is the case in gas chromatography, more sophisticated detectors can be used. These detectors provide complementary information and can be installed at the end of the column. These can be other types of spectrophotometers or a mass spectrometer and they are used simultaneously as classical detectors (to obtain the chromatogram) or for identification purposes of the analytes (cf. Chapter 16). For example, the coupling of HPLC to NMR, which has long been considered impossible, has now been realised through the miniaturisation of the probes and the increased sensitivity of the NMR instruments (cf. Chapter 9). 

Flow injection analysis (FIA) is a robust method for automating complex chemical analyses (Ruzicka and Hansen, 1988). It is relatively simple and can be adapted for use with a variety of detectors, including spectrophotometers, fluorometers, mass spectrometers, and electrochemical analyzers. It has been used on board ships to determine dissolved nutrients (Johnson et al., 1985) and trace metals (Sakamoto-Arnold and Johnson, 1987 Elrod et al., 1991). Unsegmented continuous flow analysis (CFA) systems based on the principles of FIA can operate in situ over the entire range of depths found in the ocean (Johnson et al., 1986a, 1989). 

The mass spectrometer when used as a detector for GC is the only universal detector capable of providing structural data for unknown identification. By using a mass spectrometer to monitor a single ion or few characteristic ions of an analyte, the limits of detection are improved. The term mass selective detection can refer to a mass spectrophotometer performing selected ion monitoring (SIM) as opposed to operation in the normal scanning mode. Typical limits of detection for most compounds are less than 10 "l2 g of analyte. 

Auxiliary Instruments. Auxiliary instruments can be used on the fly as special detectors, or analytes can be trapped and taken to other instruments. Instruments that have been used with chromatography include the mass spectrometer (MS), the infrared spectrometer (IR), the nuclear magnetic resonance spectrometer (NMR), the polarograph, the fluorescence spectrophotometer, and the Raman spectrometer, among others. The two most popular ones are MS and IR, and they will be discussed in more detail in Chapter 11. In the beginning of this chapter we noted the utility iof GC/MS and LC/MS. 

The application of UV spectrophotometers to the analysis of styrene containing copolymers has been extensively reported in the literature. However, hypochromic effects and band shifts which result in deviations from Beer s Law have limited the use of UV detectors as mass or composition detectors in size exclusion chromatography applications. Deviations from Beer s Law for low conversion styrene-acrylonitrile copolymers in tetrahydrofuran have been experimentally investigated and compared with results previously reported in the literature. The behaviour of the extinction coefficient as a function of the copolymer composition is discussed in view of the information obtained from infrared and nuclear magnetic resonance measurements on the same polymers. As a result of this investigation, a quantitative correlation of the extinction coefficients of styrene-acrylonitrile copolymers with the length of the styrene sequences has been obtained which, in turn, allows for the use of UV spectrophotometers as sequence length detectors. 

When the detector itself is providing a second analytical method of analysis, qualifying it as bi-dimensional, identification becomes more certain. These can be a spectrophotometer used simultaneously as a classic detector (obtaining the chromatogram) and as an identification tool (obtaining the spectrum) for the separated species (cf section 16.5). Coupling an LC instrument with a mass... 

The ultra-violet spectra were recorded on a Hitachi U-2000 spectrophotometer, infrared spectra on a Perkin-Elmer 781 instrument, mass spectra on a HP-5995 GC-MS instrument and HPLC analysis was done on a HP-1050 HPLC instrument using a isocratic Beckmann pump and HP-1040 A diode array detector. Melting points were determined using liquid paraffin bath and thin layer chromatographic plates were prepared using TLC-grade Silica gel - G. 

The HPLC was a Du Pont 8800 quaternary solvent system with a Hewlett-Packard (HP) 1040A photodiode array detector (5-8). Flexible disks were used for data storage, and postrun data evaluation was performed by the detector s computer (HP 85). Samples were injected with a loop injector 10 p,L was used for analytical scale, and 200 jlL was used for preparative scale. Spectra were run on a spectrophotometer (Perkin-Elmer Lambda 3) for static absorbance and on a spectrofluo-rometer (Perkin-Elmer MPF-66) for fluorescence. Field-ionization mass spectra were obtained at a resolution of 45,000 (corresponding to a 0.01-daltons error for a mass of 450). 

The effluent from the gas-chromatograph part of the instrument can also be directed into an FT-IR instrument so that infrared spectra rather than mass spectra can be obtained. In that case, the infrared spectrophotometer acts as the detector for the gas chromatograph. 

Table 21.1 classifies popular LC detectors according to several criteria for purposes of comparison. At the present time, LC detectors are generally less sensitive than GC detectors, which can detect picograms of material under good conditions. Most LC detectors provide only limited structural information. However, spectrophotometers fitted with micro flow-cells can be used to obtain a stop-flow ultraviolet-or visible-absorption spectrum of an LC peak trapped in the flow cell. On-line coupling of liquid chromatographs with mass or infrared spectrometers offers sophisticated, but indeed expensive, detection/identification methods. Such systems have been described in the literature, but are quite limited by the solvents that can be used in the chromatography step. 

Mass spectrometry (MS) has played an increasingly important role in drug discovery including analytical characterization of potential drug molecules and metabolic identification. A mass spectrophotometer consists of three components (1) an ionization source, (2) a mass analyzer, and (3) a detector. Mass spectral analysis requires that the analyte be introduced into the mass spectrometer as a gaseous ion. 

After this brief review of theory, let us turn our attention to existing practice, as exemplified In environmental methods of analysis. Environmental methods of analysis employ many of the common analytical Instruments In analyzing a wide spectrum of chemicals In a variety of matrices. Instruments commonly used Include spectrophotometers (atomic absorption, visible. Inductively coupled plasma), gas chromatographs (with a variety of detectors. Including the mass spectrometer), and automatic analyzers. 

---