Detector, atomic spectrometer system

A flame emission spectrometer therefore consists of an atom source, a monochromator and detector and is therefore simpler instrumentally than the corresponding atomic absorption system. Particular developments engendered by atomic absorption have restimulated interest in flame emission spectrometry after a dormant period. Chief of these is the use of the nitrous oxide—acetylene flame which is sufficiently hot to stimulate thermal atomic-emission from a wide range of metal elements. 

The optical detection systems used in MIPs are the same as those used for other atomic spectrometers and can be either single or multichannel. Fourier transform-based spectrometers have also been used. Conventional optical systems are best designed if the plasma is viewed from the exit of the discharge tube, as is possible with the TMqio type cavity, rather than through the walls of the discharge tube, which become etched. The commercially available AED uses a computer-controlled silicon photodiode array detector which has multielement detection capability over segments of spectra. In recent years, MIP sources have also been investigated as ion sources for mass spectrometry. 

The association of a spectrometer with the liquid chromatograph is usually for the purpose of structure elucidation of the eluted solute, a procedure that will be discussed in a later chapter. The association of tui atomic spectrometer with the liquid chromatograph, in contrast, is almost exclusively for the specific detection of the metalic and semi-metalic elements. The atomic spectrometer is a highly specific detector, and for element detection perhaps more so than the electrochemical detector. However, in general, a flame atomic absorption spectrometric (AAS) system is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed then multi-element detection is possible. The inductively coupled plasma spectrometer can also, under some circumstances, provide multi-element detection but all three instruments are extremely expensive particularly in terms of an LC detector. It follows that most LC/AAS combinations employ a flame atomic absorption spectrometer or occasionally an atomic spectrometer fitted with a graphite furnace. Furthermore the spectrometer is usually set to monitor one element only, throughout the development of any given separation. 

There have been a number of reviews in the literature on the identification of metal species by LC/AAS (40-42) but to successfully utilize the combination, both the LC and the spectrometer system have to be optimized and this has also been the subject of a number of publications (43-45). It has been claimed (44) that the poor sensitivity obtained from the LC/AAS system relative, to that obtained from the atomic spectrometer alone, was due to the dispersion that takes place in the column. Although substantially true, this misunderstanding arises from the fact that the spectroseopist views the chromatograph as just another sampling device and not as a separation system. The point of interfacing a liquid chromatograph with an atomic spectrometer is to achieve a separation before detection and consequently, the important dispersion characteristics are not those that occur in the column but those that occur in the interfaces between the detector and the spectrometer and in the spectrometer itself. 

Atomic emission spectroscopy is one of the most useful and commonly used techniques for analyses of metals and nonmetals providing rapid, sensitive results for analytes in a wide variety of sample matrices. Elements in a sample are excited during their residence in an analytical plasma, and the light emitted from these excited atoms and ions is then collected, separated and detected to produce an emission spectrum. The instrumental components which comprise an atomic emission system include (1) an excitation source, (2) a spectrometer, (3) a detector, and (4) some form of signal and data processing. The methods discussed will include (1) sample introduction, (2) line selection, and (3) spectral interferences and correction techniques. 

For the determination of isotope ratios, the precision of TOF-ICP-MS has been studied in a preliminary comparison with other mass spectrometer systems [643]. Typical isotope ratio precisions of 0.05% were obtained, thus overtaking sector field mass spectrometry with sequential detection, for which values of 0.1-0.3% for Cu/ Cu in Antarctic snow samples have been reported [644]. Similar results were obtained by Becker et al. [645] for Mg and Ca in biological samples (0.4-0.5%). In principle, the features of TOF-ICP-MS may be superior to those of sequential sector field or quadrupole mass spectrometry however, true parallel detection of the signals, as is possible with multi-collector systems or array detector mass spectrometry, may be the definitive solution, as shown by Hirata et al. [646]. Here, the use of detectors which allow true parallel measurement of the signals within the relevant mass range, just as the CCDs do for optical atomic spectrometry, may be the ultimate solution and bring about the final breakthrough for ICP-MS isotope ratio measurements as is required in isotope dilution mass spectrometry. 

Atomic Absorption Spectroscopy. Mercury, separated from a measured sample, may be passed as vapor iato a closed system between an ultraviolet lamp and a photocell detector or iato the light path of an atomic absorption spectrometer. Ground-state atoms ia the vapor attenuate the light decreasiag the current output of the photocell ia an amount proportional to the concentration of the mercury. The light absorption can be measured at 253.7 nm and compared to estabUshed caUbrated standards (21). A mercury concentration of 0.1 ppb can be measured by atomic absorption. 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

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