Detector, atomic spectrometer

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. 

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 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]. 

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. 

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. 

Other detectors, such as the flame ionization detector, atomic emission detector, Eourier transform infrared spectrometer and nuclear magnetic resonance detectors, have been used in GC for the... 

For indirect determinations without precipitate dissolution, the sample and reagent flows are transferred directly to the detector. For atomic spectromet-ric detectors which require an optimum uptake, the combined flows from the sample and reagent (ani ometimes the diluent) should be such that the detector will not be excessively stan ed as to deteriorate its performance. For photometric detectors, the flow-rates are more flexible. 

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. 

A unique feature of TRMS is that both identities and quantities of ions may be studied in the time domain. For an engineer, the relationship between conventional MS measurements and TRMS is like the relationship between multimeters and oscilloscopes multimeters only indicate average electric properties while oscilloscopes measure and record fast changes in electrical signals over time. By analogy, TRMS uses a high-performance chemical detector (mass spectrometer) to study the dynamic phenomena of atoms, molecules, and ions during reactions. 

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. 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

Schematic diagram of a muitichannei atomic emission spectrometer, showing the arrangement of muitipie exit siits and detectors for the simuitaneous anaiysis of severai eiements.

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. 

---