Cold mercury vapour generation

The techniques discussed in this chapter vary in automatability and frequency of use. Thus, while automatic hydride and cold mercury vapour generation are implemented in laboratory-constructed or commercially available dynamic equipment that is straightforward, easy to operate and inexpensive, automating laboratory headspace modes and solid-phase microextraction is rather complicated and commercially available automated equipment for their implementation is sophisticated and expensive. Because of its fairly recent inception, analytical pervaporation lacks commercially available equipment for any type of sample however, its high potential and the interest it has aroused among manufacturers is bound to result in fast development of instrumentation for both solid and liquid samples. This technique, which is always applied under dynamic conditions, has invariably been implemented in a semi-automatic manner to date also, its complete automatization is very simple. 

The techniques dealt with in this chapter are discussed in terms of similarities. For this reason, hydride generation and cold mercury vapour generation are addressed first, notwithstanding the limited scope of this technique as regards analytes — scope that can... 

The characteristics of the separator and its associated equipment used for hydride and cold mercury vapour generation are dictated by the state of the sample. 

Features of methods based on hydride or cold mercury vapour generation... 

Table 4.1 [37-43] shows the figures of merit of typical hydride generation and cold mercury vapour formation methods using atomic absorption or fluorescence detection. 

Typical cold vapour generation AAS system used for mercury determination. The same system can be used with a flame in place of the Pyrex tube to allow the determination of hydride -forming elements. 

Gaseous and volatilised analytes can also be easily determined by FAAS and ETAAS. For example, the determination of several elements by the formation of covalent volatile hydrides e.g. arsenic, selenium) and cold vapour generation (mercury and cadmium) is feasible with good analytical sensitivity (see Section 1.4.1.1). 

To implement an easy and automated means for chemical vapour generation procedures (hydride generation for arsenic, selenium, etc., and cold vapour mercury), which allows for a reduction on the interferences caused by first-row transition metals (such as copper and nickel). FI methods may be readily coupled with almost all the atomic-based spectroscopic techniques (including graphite furnace atomisers). 

G. Tao, S. N. Willie, R. E. Sturgeon, Determination of total mercury in biological tissues by Bow injection cold vapour generation atomic absorption spectrometry following tetramethylammonium hydroxide digestion, Analyst, 123 (1998), 1215D1218. 

F. Ubillus, A. Algeria, R. Barbera, R. Farre, M. J. Lagarda, Methylmercury and inorganic mercury determination in fish by cold vapour generation atomic absorption spectrometry, Food Chem., 71 (2000), 529-533. 

Ultrasonic slurry formation has been frequently used prior to cold-vapour and hydride generation. Both procedures usually involve a drastic treatment of the slurry to ensure complete transfer of the target species to the liquid phase for subsequent formation of the gas phase — after a normally long standing time — which is the only phase reaching the atomizer in the case of hydride generation and the detection point in the case of mercury vapour formation. The gaseous analytes or their hydrides are most often obtained in a commercial or laboratory-made dynamic flow injection manifold. 

Cold vapour generation is the term exclusively reserved for mercury. Mercury in a sample is reduced to elemental mercury by tin (II) chloride (eqn 27.2) ... 

Lee SH, Jung KH, Lee DH. 1989. Determination of mercury in environmental samples by cold-vapour generation and atomic-absorption spectrometry with a gold-coated graphite furnace. Talanta 36(10) 999-1003. 

Hutton, R.C. and Preston, B. (1980) A simple non-dispersive atomic-fluorescence spectrometer for mercury determination, using cold-vapour generation. Analyst, 105, 981-984. 

Hydride generation AAS (HGAAS) and cold vapour AAS (CVAAS) are special combinations of chemical separation and enrichment with AAS. In HGAAS the analyte is transformed to a volatile hydride, stripped off by an inert gas and atomized in a quartz tube, flame-in tube etc. About ten elements (As, Se, Bi, Sb etc.) can be determined by this technique. The accuracy and detection limits depend on the proper isolation of the hydride. CVAAS is the universally acknowledged most sensitive method for determination of Hg. The generation of elemental mercury vapour is similar to the hydride generation however the quartz cell may not be heated and this gives the name of the method. 

Mercury can be determined in plasma AES by reducing it first to elemental mercury and then transporting the mercury vapour into the plasma. The same reduction methods may be used as for AAS. Commercial hydride generation systems can be adopted to the cold vapour method. The detection limit is about 0.02 mgP ... 

For mercury, the borolydride technique is used to generate elemental mercury vapour from a mercury soluhoa This is then passed irrto the merenry hght beam without any need for a flame to decorrrpose mercury corrrporrrrds (so it is cahed cold vapour AAS). 

In analogy to sample introduction by hydride generation, mercury trace analysis is possible by reducing Hg compounds to the metal using the cold vapour technique or the determination of iodine at the ultratrace level (after oxidation with 70 % perchloric acid of iodide to iodine) via the gas phase. 

Because MIPs are formed at low temperatures, liquid samples cannot be introduced because they extinguish the plasma, even small amounts of organic vapour. However, the on-line coupling of HPEC to MIP-OES has been described for the speciation of mercury and arsenic compounds. Continuous cold vapour (CV) or hydride generation (HG) techniques were used as interfaces between the exit of the HPEC column and the MIP, held in a surfatron at reduced pressure [24]. 

A flame AAS (FAAS) detector can monitor the GC effluent continuously to provide on-line analysis. However, as the gas flow rates for the flame are quite high, the residence time in the flame is short, and this can adversely affect the detection limits. Detection limits in the microgram range are usually achieved. Improved detection limits can be obtained if the additional techniques of hydride generation or cold vapour mercury detection are used as described in Section 4.6. 

It should be pointed out that few elements are present in most natural waters at concentrations where flame spectroscopic techniques are directly applicable. Those that are include calcium, magnesium, sodium, potassium, and, in some samples and if conditions are very carefully optimized, manganese, iron, and aluminium. Zinc, and sometimes cadmium, may be determined directly by AFS. Mercury and hydride-forming elements may be determined if cold vapour and hydride generation sample introduction techniques are employed, as discussed in... 

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