Atomic spectroscopy-chromatograph

Whereas some instrumental methods produce just one signal per component, N = n, e.g., chromatographic methods, some other methods such as atomic spectroscopy generate much more signals as required, N n. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

The development of solvent-impregnated resins and extraction-chromatographic procedures has enabled the automation of radiochemical separations for analytical radionuclide determinations. These separations provide preconcentration from simple matrices like groundwater and separation from complex matrixes such as dissolved sediments, dissolved spent fuel, or nuclear-waste materials. Most of the published work has been carried out using fluidic systems to couple column-based separations to on-line detection, but robotic methods also appear to be very promising. Many approaches to fluidic automation have been used, from individual FI and SI systems to commercial FI sample-introduction systems for atomic spectroscopies. 

Once the sample preparation is complete, the analysis is carried out by an instrument of choice. A variety of instruments are used for different types of analysis, depending on the information to be acquired for example, chromatography for organic analysis, atomic spectroscopy for metal analysis, capillary electrophoresis for DNA sequencing, and electron microscopy for small structures. Common analytical instrumentation and the sample preparation associated with them are listed in Table 1.1. The sample preparation depends on the analytical techniques to be employed and their capabilities. For instance, only a few microliters can be injected into a gas chromatograph. So in the example of the analysis of pesticides in fish liver, the ultimate product is a solution of a few microliters that can be injected into a gas chromatograph. Sampling, sample preservation, and sample preparation are... 

A comprehensive review of directly coupled gas chromatography-atomic spectroscopy applications has been published [128]. This review list over 100 references classified according to the detection technique and is highly recommended. Another excellent review outlines the advances in interfacing and plasma detection [130]. A review of the gas chromatographic detection of selected trace elements (mercury, lead, tin, selenium, and arsenic) has been published. This article reviews the many different detection methods available including atomic emission techniques [131]. 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

Chapter 5 covers ultraviolet-visible spectroscopy and Chapter 6, on immunoassay techniques, emphasizes the wide array of new methodologies that do not use radioisotopes. Chapter 7 discusses one of the most novel techniques for chromatographic separation of molecules—capillary electrophoresis—and its widespread applications to pharmaceuticals. Chapter 8, Atomic Spectroscopy, and Chapter 9, Luminescence Spectroscopy, contain current information on these important technologies. 

The book edited by Harrison and Rapso-manikis (1989) on environmental analysis using chromatography interfaced with atomic spectroscopy has chapters on basic principles of chromatography and AAS, interfaces between liquid chromatography and AAS and determination of individual elements. The book by Kebbekus and Mitra (1998) contains a chapter devoted to chromatographic methods, a discussion of which is also included in chapters on methods for air, water and solid sample analyses. 

A very recent volume edited by Berthed (2002) is on countercurrent chromatography - the support-free liquid stationary phase. Ebdon et al. (1987) review directly coupled liquid chromatogramphy-atomic spectroscopy. The review by Uden (1995) on element-specific chromatographic detection by atomic absorption, plasma atomic emission and plasma mass spectrometry covers the principles and applications of contemporary methods of element selective chromatographic detection utilizing AA, AES and MS. Flame and furnace are considered for GC and HPLC, while MIP emission is considered for GC and ICPAES for HPLC. Combinations of GC and HPLC with both MIPAES and ICPAES are covered and supercritical fluid chromatographic (SFC) and field flow fractionation (FFF) are also considered. 

Elemental speciation using mass spectrometry in conjunction with ICPAES is a latest advance in atomic spectroscopy, which is becoming popular in analytical research labs. Mason et al. ExxonMobil Research and Engineering) show how linking ICP-MS to various liquid chromatographic techniques has enabled determination of ppm levels of metals in hydrocarbons to ppb level measurements in refinery effluent streams. Hyphenated ICP-MS techniques were used to provide speciation information on nickel and vanadium in crude oils and assist in development of bioremediation options for selenium removal in wastewater treatment plants. Similar ICP-MS technique without sample demineralization was used by Lienemann, et al. Institut Francais du Petrole) to determine the trace and ultra-trace amounts of metals in crude oils and fractions. 

Amongst the wide range of sample introduction methods available for atomic emission spectroscopy, chromatographic methods are most popular as they transform a complex mixture into a time-resolved separated analyte stream [49]. 

An ion chromatograph using a suppressed IC mode of operation can be viewed as a moderate-pressure-performance liquid chromatograph. A schematic of the essential components of an IC is shown in Fig. 4.59. Because the trace concentration levels of various inorganic anions is of most interest to TEQA, we will focus our discussions on how anions are measured. Alkali and alkaline-earth metal ions and the ammonium ion are common applications of IC. In fact, NH4 can only be measured chromatographically by cation IC The transition metal ions can also be separated and detected, however, atomic spectroscopy, to be discussed after we complete IC, is the predominant TEQA determinative technique. We will not discuss the separation and detection of cations here. 

Optimization has a significant role in analytical science. There are many reasons for finding an optimum. For example, it may be important to maximize the extraction efficiency of a compound from a matrix there may be a large number of factors involved in the extraction procedure. Other examples involve improving chromatographic separations and optimizing the factors that influence signal intensity in atomic spectroscopy. 

Uden P (1989) Chromatographic detection by atomic plasma emission spectroscopy. In Harrison R, Rapsomanikis S (eds) Environmental Analysis Using Chromatography Interfaced with Atomic Spectroscopy. Ellis Horwood Ltd., Chichester, pp 96-126. 

The need for the determination of metallic constituents or impurities in pharmaceutical products has, historically, been addressed by ion chromatographic methods or various wet-bench methods (e.g. the USP heavy metals test). As the popularity of atomic spectroscopy has increased, and the equipment has become more affordable, spectroscopy-based techniques have been routinely employed to solve analytical problems in the pharmaceutical industry. Table 1 provides examples of metal determinations in pharmaceutical matrices, using spectroscopic techniques, and the reasons why these analyses are important. Flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry... 

Revised references on cosmetic analysis have been classified into five gronps according to the analytical technique used (Figure 2.2.6), namely, chromatographic and related techniques (such as electrophoresis) (69%), molecular spectroscopy (15%), electrochemical measurements (8%), atomic spectroscopy (5%) and others (3%). 

Four modes of atomic spectroscopy have been interfaced for chromatographic detection, fiame emission (FES), atomic absorption (AAS), atomic fluorescence (AFS), and atomic plasma emission (APES) [2-3]. In contrast to AAS, APES can accomplish simultaneous multielement determination, while giving a good dynamic measurement range and high sensitivities and selectivities over background elements in many cases. 

PC. Uden. Atomic spectral chromatographic detection. In Element Specific Chromatographic Detection by Atomic Emission Spectroscopy, Ed. by PC. Uden, ACS Symposium Series, American Chemical Society, Washington, DC, 1990, in press. PC. Uden, Y. Yoo, T. Wang, and Z. Cheng. Element-selective gas chromatographic detection by atomic plasma emission spectroscopy. Review and developments. J. Chromatogr., 468, 319 (1989). 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

The strong selectivity of A A -dialkyl A-benzoylthiourea toward platinum metals has been favorably exploited to determine noble metals (Rh, Pd, Pt, and Au) in samples of ore and rocks by graphite fnmace atomic absorption spectroscopy (GFAAS) and UV detection after liquid chromatographic separation on silica HPTLC plates [23]. The results are presented in Table 14.3. 

P.C. Uden (ed.), Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy, ACS Symposium Series, Vol. 479, American Chemical Society, Washington, DC (1992). 

AAS = atomic absorption spectroscopy CdS04 = cadmium sulfate GC/ECD = electrochemical gas chromatographic detection GC/FPD = gas chromatography with flame photometric detection HC1 = hydrochloric acid H2S = hydrogen sulfide NaOH = sodium hydroxide NR = not reported PAS = photoacoustic spectroscopy... 

---