Element-specific detectors

TABLE 4.1 Experimental Conditions of the Chip-Based CE-ICP System  

ICP forward power Frequency of rf power supply Number of simultaneous channels Observation height Plasma Ar flow rate Auxiliary Ar flow rate Carrier Ar flow rate Supplementary buffer flow rate Emission lines 

Mostly, mass spectrometers have been used widely for nanodetection in NLC and NCE due to its low detection limits and ease of hyphenation with microfluidic devices. However, attempts have been made to couple other detectors with NLC and NCE. The state of the art of hyphenation of detectors in NLC and NCE is still in its development stage. More advances are expected in the near future for detection at extremely low concentrations for a wide range of molecules. 

Feustel, J. Muller, V. Relling, In Proceedings of Micro Total Analysis Systems 1994, Dordrecht, The Netherlands Kluwer Academic Publishers (1994). 

International Conference on Solid State Sensors and Actuators (Transducers-97), Chicago, IL, June 16-19, p. 927 (1977). 

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Applications Atomic emission spectrometry has been used for polymer/additive analysis in various forms, such as flame emission spectrometry (Section 8.3.2.1), spark source spectrometry (Section 8.3.2.2), GD-AES (Section 8.3.2.3), ICP-AES (Section 8.3.2.4), MIP-AES (Section 8.3.2.6) and LIBS. Only ICP-AES applications are significant. In hyphenated form, the use of element-specific detectors in GC-AED (Section 4.2) and PyGC-AED deserves mentioning. 

Atomic absorption spectrometry used either by direct aspiration (to determine total mercury) or as an element-specific detector for gas chromatography (to determine organically bound mercury) are now discussed. 

For the more volatile components of water samples, i.e. those with boiling points up to about 250°C, gas chromatography has been a favoured technique for several decades. However, with the realization that retention time measurements alone are insufficient to identify organics there has been an increasing move in recent years to connect a gas chromatograph to a mass spectrometer in order to provide unequivocal identifications. Element-specific detectors are another recent development. 

The application of atomic spectroscopic instruments as element-specific detectors in chromatography has been reviewed by van Loon More recently, Krull has extensively reviewed their use in high pressure liquid chromatography (HPLC). Atomic spectrometry has found wide acceptance in the field of liquid chromatography because, in most cases, the fractions can be directly analysed after elution from the column. However, it is possible to use the technique for the analysis of solid samples without first dissolving the matrix. This is particularly useful after electrophoresis, where the fractions are fixed either in a gel or on paper. Kamel et al. have shown that it is possible to cut the appropriate sections and insert them into the carbon furnace for analysis. The disadvantage of this approach is that the precision is usually poorer (about 10%) and it is difficult to calibrate the instrument. Nevertheless, this approach is very useful if it is used for qualitative speciation. 

The compounds MMA, DMA, and TMAO are reduced in acidic aqueous media by borohydride solutions to methylarsine (MeAsH2, bp 2°C), dimethylarsine (Me2AsH, bp 35°C), and trimethylarsine (Me3As, bp 55°C), respectively. These products are useful derivatives for speciation analysis of arsenic because they are readily separated from complex sample matrices and may be further separated from each other by distillation (41) or by gas chromatography (42) prior to their determination by element-specific detectors. Consequently, arsine generation techniques are the most commonly used methods for determining MMA, DMA, and TMAO in marine samples. 

Transient signals are typically obtained in atomic spectrometry when samples are introduced by flow injection techniques or when the spectrometer is used as an element-specific detector in hyphenated techniques. Inductively coupled plasma mass spectrometry has nowadays become the detection technique of choice for multielement-specific detection in speciation as it allows multielemental... 

Taylor, H. E., Garbarino, J. R., Murphy, D. M., and Beckett, R. (1992). Inductively coupled-plasma-mass spectrometry as an element-specific detector for field-flow fractionation particle separation. Anal. Chem. 64, 2036-2041. 

The atomic emission detector is a tunable, element-specific detector that uses microwave-induced helium plasma to generate temperatures high enough to break molecular bonds. The generated free atomic species undergo electron excitation to higher energy states, followed by relaxation and photon emission at characteristic frequencies... 

If the species of interest happens to be volatile, it can be collected in the head space or atmosphere of a closed system, and subsequently determined by gas chromatography (GC), MS, or a combination of these (GC-MS). In some circumstances it is possible to convert non-volatile compounds into a volatile form by appropriate derivativisation (e.g. by alkylation, or formation of a metal chelate). Separation and analysis can then be based on GC. Species containing elements such as Sn, Pb, Hg, As, Sb, Bi, Se or Te can be separated from the matrix by conversion into a chemically stable and volatile hydride (e.g. by treatment in acid solution, with sodium borohydride, NaBH4). Mixtures of hydrides can be separated by GC and detected by an electron capture unit, or if only one element is of interest, the volatile hydride(s) can be fed to an element-specific detector such as an AAS unit (fitted with a heated quartz tube cell). 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

In a hybrid technique the separation process and elemental detection occurs on-line. The two separate techniques must therefore be coupled by an interface. In designing the most effective coupled technique the detection system must be compatible with the separation process. The separation process in these systems is usually some form of chromatography, but the detectors normally used for chromatography lack the selectivity and sensitivity required for speciation studies. The favoured detectors for hybrid systems are the very sensitive and selective element-specific detectors. Selectivity is very useful in speciation studies because it means that only species of the element of interest will be detected, thus simplifying the sample preparation step. 

Kollotzek, D., Oechsle, D., Kaiser, G., Tschopel, R and Tolg, G. (1984) Application of a mixed gas microwave induced plasma to an on-line element-specific detector for high performance liquid chromatography. Fresenius Z. Anal. Chem., 318, 485-489. 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

Improvement in sensitivity can be obtained by increasing the temperature of the sample or by the salting-out effect, which is particularly useful for compounds such as phenols and fatty acids which form strong hydrogen bounds in aqueous solutions. With some compounds, the use of a more sensitive detector such as an electron-capture detector or an element-specific detector will enhance sensitivity. 

Most OP insecticides may be determined directly by GC using the phosphorus-selective flame photometric detector (FPD) which helps to minimize clean-up. However, it must be emphasised that the FPD is only a "selective" detector for phosphorus (at 540 nm) or sulphur (at 394 nm) and not an "element specific detector". 

Gas chromatography (GC) analysis with element-specific detectors, for example, a nitrogen-phosphorus detector (NPD), a flame photometric detector (FPD, in sulfur or phosporus mode) and/or an atomic emission detector (AED) ... 

G. Raber, K. A. Francesconi, K. J. Irgolic, W. Goessler, Determination of arsenosugars in algae with anion-exchange chromatography and inductively coupled plasma mass spectrometry as element specific detector, Fresenius J. Anal. Chem., 367 (2000), 181-188. 

The second major environmental application of FFF has been the use of an element-specific detector, usually in series with a UV detector, to provide elemental composition data along with the PSD. Graphite-furnace atomic absorption spectrometry has been used off-line on fractions collected from the FFF run. However, the multi-element detection, low detection limits and capability to function as an online detector have made inductively coupled plasma mass spectrometry (ICP-MS) the ideal detector for FFE85-86 The sample introduction system of the ICP-MS is able to efficiently transport micron-sized particles into the high-temperature plasma,... 

Q. Tu, T. B. Wang, X. Jia, and X. Bn, Speciation analysis of halogenides and oxyhalogens by ion chromatography with inductively coupled plasma mass spectrometer as element-specific detector, Merck Research Laboratories internal communication. 

Methods involving reduction to produce hydrides followed by separation and detection by an emission-type detector have been used for organoarsenic compounds . The design of a glow discharge tube proposed earlier as an element-specific detector for gas... 

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