Detector, atomic spectrometer selection

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 selective detectors discussed in the previous sections often do not provide enough information to elucidate with 100% probability the nature of the eluting solutes. For this reason, data with selective detectors can be erratic. The future in this respect definitely belongs to the spectroscopic detectors that allow. selective recognition of the separated compounds. Today, the hyphenated techniques CGC-mass spectroscopy (CGC-MS), CGC-Fourier transform infrared spectroscopy (CGC-FTIR), and CGC-atomic emission detection (CGC - AED) are the most powerful analytical techniques available. They provide sensitive and selective quantitation of target compounds and structural elucidation or identification of unknowns. The applicability and ease of use of the hyphenated techniques were greatly increased by the introduction of fused silica wall coaled open tubular columns. The main reason for this is that because of the low flows of capillary columns, no special interfaces are required and columns are connected directly to the different spectrometers. The introduction of relatively inexpensive benchtop hyphenated systems has enabled many laboratories to acquire such instrumentation, which in turn has expanded their applicability ever further. 

FIA star 5010 Modular, semi- or fully automatic operation. May be operated with process controller microprocessor. Can be set up in various combinations with 5017 sampler and superflow software which is designed to run on IBM PC/XT computer 60-180 samples h Dialysis for in-line sample preparation and in-line solvent extraction.Thermostat to speed up reactions. Spectrophotometer (400-700nm) or photometer can be connected to any flow through detector, e.g. UV/visible, inductively coupled plasma, atomic absorption spectrometer and ion-selective electrodes... 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

The general construction of an atomic absorption spectrometer, which need not be at all complicated, is shown schematically in Fig. 1. The most important components are the light source (A), which emits the characteristic narrow-line spectrum of the element of interest an absorption cell or atom reservoir in which the atoms of the sample to be analysed are formed by thermal molecular dissociation, most commonly by a flame (B) a monochromator (C) for the spectral dispersion of the light into its component wavelengths with an exit slit of variable width to permit selection and isolation of the analytical wavelength a photomultiplier detector (D) whose function it is to convert photons of light into an electrical signal which may be amplified (E) and eventually displayed to the operator on the instruments readout, (F). 

The components of an atomic absorption spectrometer are a radiation souree, an atomization cell, a sample introduction system, a method of wavelength selection and a detector (Fig. 27.2). 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

The most common type of emission spectrometer in use today (inductively coupled plasma-optical emission spectroscopy, or ICP-OES) atomizes a sample by passing an electric current into a gas plasma that contains the sample. In these optical emission methods, the sample is heated to high temperature. At this temperature the individual elements glow with their representative colors, e.g., red for potassium, yellow for sodium. The light from the sample is focused on a monochrometer to select a wavelength appropriate for the element of interest. That light at the correct wavelength is focused on a detector that measures its intensity (Fig. 4.8). 

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