Detector fluorescence spectrometry

Auger electron spectroscopy Phosphorous/nitrogen-selective alkali/flame ionisation detector Atomic force microscopy Atomic fluorescence spectrometry All-glass heated inlet system... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

The basic theory, principles, sensitivity, and application of fluorescence spectrometry (fluorometry) were discussed in Chapter 8. Like the UV absorption detector described above, the HPLC fluorescence detector is based on the design and application of its parent instrument, in this case the fluorometer. You should review Section 8.5 for more information about the fundamentals of the fluorescence technique. 

Environmental Protection Agency (USA) emergency response guidebook electron spin resonance field desorption mass spectrometry flow injection analysis flame ionization detection/detector fluorescence detection/detector Xylenol Orange-ferric complex final ozonide... 

A number of techniques have been used for the speciation of arsenic compounds. The most important has been the formation of volatile hydrides of several species, separation by gas chromatography and detection by AAS. HPLC has been used to separate arsenic species. Several types of detectors have been studied for the determination of arsenic species in the column effluent. These have included AAS both off- and on-line, ICPAES and ICP-MS. An important comparative study of coupled chromatography-atomic spectrometry methods for the determination of arsenic was published (Ebdon et al., 1988). Both GC and HPLC were used as separative methods, and the detectors were FAAS, flame atomic fluorescence spectrometry (FAFS) and ICPAES. The conclusions were (1) that hydride generation and cryogenic trapping with GC-FAAS was the most... 

The interfacing of GC with MI fluorescence spectrometry also has been accomplished for fluorometric examination of GC effluents, the conventional photomultiplier tube detector is replaced by a SIT vidicon to facilitate rapid acquisition of MI fluorescence spectra of individual GC fractions (30). 

As noted earlier, USNs have been employed for sample insertion into atomic spectrometers suoh as flame atomio absorption spectrometry (FAAS) [9,10], electrothermal atomic absorption speotrometry (ETAAS) [11], atomic fluorescence spectrometry (AFS) [12,13], induotively ooupled plasma-atomic emission spectrometry (ICP-AES) [14,15], inductively coupled plasma-mass spectrometry (ICP-MS) [16,17] and microwave induced plasma-atomic emission spectrometry (MIP-AES) [18,19]. Most of the applications of ultrasonic nebulization (USNn) involve plasma-based detectors, the high sensitivity, selectivity, precision, resolution and throughput have fostered their implementation in routine laboratories despite their high cost [4]. 

There exist several different detectors suitable for detecting the analytes after the chromatographic separation. Some commercial detectors used in LC are ultraviolet (UV) detectors, fluorescence detektors, electrochemical detectors, refractive index (RI) detectors and mass spectrometry (MS) detectors. The choice of detector depends on the sample and the purpose of the analysis. 

Besides the fractionation of metalloids through SEPs, a series of methods has been proposed for the determination of individual species in the various oxidation states (Gong et al., 2002 Kahakachchi et al., 2004). The most popular detectors for metalloid speciation are inductively coupled plasma-mass spectrometry (ICP-MS) and atomic fluorescence spectrometry (AES), especially after liquid chromatographic separation and hydride formation, which are increasingly replacing atomic absorption spectrometry (AAS). Speciation analysis of pollutants in terrestrial environments is, however, beyond our scope in this chapter. 

Both molecular and atomic detectors have been used in combination with SCF extractors for monitoring purposes. Thus, the techniques used in combination with SFE are infrared spectroscopy, spectrophotometry, fluorescence spectrometry, thermal lens spectrometry, atomic absorption and atomic emission spectroscopies, mass spectrometry, nuclear magnetic resonance spectroscopy, voltammetry, and piezoelectric measurements. 

Figure 12.8 The two mtegories of detectors used for energy dispersive X-ray fluorescence spectrometry. (a) Proportional counter used in pulse mode (b) Cooled Si/Li diode detector using Peltier effect (XR detector by Amptek Inc.) (c) Functioning principle of a scintillation detector containing a large size reverse polarized semi-conductor crystal. Each incident photon generates a variable number of electron-hole pairs. The very high quantum yield enables the use of low power primary sources of X-rays (a few watts or radio-isotopic sources).

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

More recently for ultratrace determination and speciation of antimony compounds the so-called hyphenated instrumental techniques have been applied which combine adequate separation devices with suitable element-specific detectors. They include high-performance liquid chromatography (HPLC) connected on-line with heated graphite furnace (HGF) AAS (HPLC-HGF-AAS), hydride-generation atomic fluorescence spectrometry (HPLC-HG-AFS) or inductively coupled plasma (ICP) mass spectrometry (MS) (HPLC-ICP-MS) capillary electrophoresis (CE) connected to inductively coupled plasma mass spectrometry (CE-ICP-MS) and gas chromatography (GC) coupled with the same detectors as with HPLC. Reliable speciation of antimony compounds is still hampered by such problems as extractability of the element, preservation of its species information, and availability of Sb standard compounds (Nash et al. 2000, Krachler etal. 2001). Variants of anodic stripping voltammetry for speciation of antimony have also been applied (Quentel and Eilella 2002). 

Distillation was performed on a sample of 300 mg by addition of 0.5 mL H2SO4 (9 mol L ) in 20% KCl in H2O at a temperature of 145 °C distillation recovery was ca. 90%. Derivatization was by addition of 1% NaBEt4 in acetic acid. Separation was by gas liquid chromatography (column of 0.5 m length, internal diameter of 4 mm Chromosorb W AW-DMSC 60-80 mesh, loaded with 15% OV-3 stationary phase temperature of the injector of 500 °C, detector temperature at 20 °C column temperature of 100 °C He as carrier gas at 40 mL min ). Detection was by cold vapour atomic fluorescence spectrometry. Calibration was by calibration graph and standard additions using MeHgCl calibrant in Milli-Q water. 

Atomic fluorescence spectrometry is certainly an excellent detector for the determination of arsenic compounds. Necessary low detection limits can be obtained only after hydride generation. This is certainly a drawback as the nonhydride-forming arsenic compounds have to be converted into the volatile hydride-forming ones. 

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