Atoms spectrometer

In addition to conventional aspiration, using a nebulizer and spray chamber, samples may be introduced in to atomic spectrometers in a number of different ways. This may be because a knowledge of speciation (i.e. the organometallic form or oxidation state of an element) is required, to introduce the sample while minimizing interferences, to increase sample transport efficiency to the atom cell or when there is a limited amount of sample available. 

The calibration of atomic spectrometers can be handled much easier than that of conventional IC detectors using the large dynamic range of ICP techniques. Those simple off-line calibrations had been used for ICP-AES and ICP-MS in on-line preconcentration applications. With its ability to decide between isotopes the ICP-MS is well suited for isotope dilution analysis (IDMS), a calibration tool which increases the accuracy, the results and saves time due to reduced calibration work. The use of IDMS in combination with on-line coupling methods allows a significant speedup of the usually to IDMS applied time consuming separation processes. 

It should be also pointed out that the robustness of electrothermal atomization enables one to avoid the use of high dilution factors. The sensitivity of ET-AAS to a high percentage of dissolved salts, major elements and/or acids is relatively controllable. Manipulation can be also reduced. Calibration is therefore possible at concentrations where contamination phenomena can be better mastered. Multielement atomic spectrometers have additional advantages of saving time and resources by quantifying simultaneously Cd and Pb. 

The choice of the food sample preparation procedure has an impact on the performance of the quantitation technique used, the behavior of the sought-after element and the amenability of the sample matrix to proper digestion. A convenient way to introduce solid (liquid) material into the measurement cell of an atomic spectrometer is to prepare a suspension or a slurry. 

Nebulization is a physical process widely used in analytical chemistry for introducing samples into atomic spectrometers [66], Ultrasonic nebulizers are the most effective devices for this operation. Rather than a step preceding sample preparation, nebulization is a sample preparation operation and so close to detection that the nebulizer is a component of flame and plasma spectrometers that influences their efficiency. This warrants separate discussion on ultrasonic nebulizers in Chapter 8. 

Slurries can be inserted into an atomic spectrometer by hand, via an autosampler or through a flow injection manifold. When autosampler cups are used to weigh the solid sample, the maximum volume of slurry that can be inserted is limited to the cup volume (about 2 ml), which in turn dictates the maximum amount of material that can be suspended. Once the solid sample and the liquid are brought into contact, the solid must distribute evenly in the volume of liquid. 

Liquid nebulization as a means of obtaining aerosols is commonly used for activities such as drug administration, hair spraying or perfume application [1,2]. Direct nebulization of a liquid phase containing the target analytes has been widely used in analytical chemistry for sample insertion into some detection systems (particularly atomic spectrometers). The main purpose of analytical nebulizers is to insert the maximum possible amount of sample, and hence of analyte, in the form of aerosol consisting of very small droplets, into the detection system. 

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]. 

The relatively high cost of commercial USNs has promoted the development of custom-made prototypes. Thus, some authors have modified commercial humidifiers to obtain batch USNs for use with atomic spectrometers. However, these custom-made nebulizers are impractical and feature a severely limited throughput. Thus, Yeon et al. used a humidifier to make a continuous USN for ICP-AES with a configuration similar to that of the CETAC units [23]. The prototype exhibited improved sensitivity relative to a pneumatic nebulizer, but also impaired precision owing to signal instability. 

Applications of atomic spectrometers with ultrasonic nebulization for sample insertion... 

The widespread use of USNn for sample insertion in atomic spectrometers is apparent from the number of reported applications. As noted earlier, the atomic techniques benefiting to the greatest extent from USN are plasma-based techniques [4,19]. By contrast, FAAS- and ETAAS-based detectors have scarcely been used with USNn [9-11]. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

Separation equipment such as HPLC or CE has been coupled to atomic spectrometers via USNs to enable the determination of inorganic and organometallic analytes. Thus, antimony speciation analysis — inorganic Sb(lll) and Sb(V), as well as TMSbCl2 — was accomplished by separation by anion exchange chromatography and subsequent insertion of the eluent into the plasma of an ICP-MS instrument using USNn [28]. 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

In order to successfully utilize the LC/AAS combination, both the chromatograph and the spectrometer must be optimized, which has been discussed in some detail in a number of publications [39 1], It has been claimed [39] that the poor sensitivity that has been obtained from the LC/AAS system, relative to that from the atomic spectrometer alone, was due to the dispersion that takes place in the column. 

Ultrasonic slurry sampling is also discussed in Chapter 8 alongside other modes of solid sample insertion into atomic spectrometers. 

Once hydrides are formed, they must be driven to the detector (usually an atomic spectrometer but also, occasionally, a gas chromatograph) under optimal conditions as regards concentration and the absence of species unfriendly to the chromatographic column. 

Devices for solid sample treatment prior to introduction into atomic spectrometers electrothermal devices and glow-discharge sources... 

This section is devoted to the types of devices most frequently used for both liquid and solid sampling prior to introduction into atomic spectrometers [12-14]. Atomic techniques and mass spectrometry make massive use of electrothermal devices, the maturity of which has been endorsed by lUPAC, which has included it in its Nomenclature, Symbols, Units and their Usage in Spectrochemical Analysis. XII. Terms Related to Electrothermal Atomization , published in 1992 and subsequently reprinted in Spectro-chimica Acta [1]. 

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