Vapour generation

ICP-SFMS (Thermo Finnigan, Flement) with cold vapour generation was developed with a guard electrode and a gold amalgamation device using an Au-sorbent for sample pre-concentration to improve the sensitivity. Instrumental parameters of ICP-SFMS such as take-up time, heating temperature of Au-sorbent, additional gas flow, and sample gas flow were optimized. Detection limit calculated as 3 times the standard deviation of 10 blanks was 0,05 ng/1, RSD = 7-9 %. 

Atomic Fluorescence System - Millennium Excalibur PSA 10.055 -was used in our work. This system consists of the autosampler, the integrated continuous flow vapour generator and the atomic fluorescence spectrometer with the boosted dischar ge hollow cathode lamp and a control computer. 

Mercury generally is found in low and trace concentrations. So there is need to determine Hg in ranges corresponding to various types of water samples. Detection levels of Hg can be improved by the use of vapour generation technique. This technique allows to sepai ate the analyte from the sample matrix and so to overcome the matrix interference. The fluorescence technique, with its high sensitivity and linearity, in combination with vapour generation, provides for a possibility to detect Hg in parts per trillion per liter regions. 

To obtain the calibration standards, take aliquots ranging from 50 /xL to 300 juL As, from the working standard solution, using an Eppendorf micropipette. Add the appropriate microlitre quantities to the reaction vessel of the vapour generation system, together with 10 mL of hydrochloric acid (AM), delivered from a calibrated dispenser. 

Start the vapour generator cycle so that the absorption cell is flushed with argon gas and the pre-set volume of NaBH4 (1 mL) is pumped into the sample vessel. After the pre-selected reaction time (0.5 minute), AsH3 vapour is flushed into the absorption tube. Record the value of each arsenic signal as a peak height measurement. Read off the arsenic concentration of the sample, which is displayed on the instrument video screen. 

The ions formed are then directed though a sampling cone at 90° to the direction of vapour flow - to minimize the chances of blocking of the entrance to the mass spectrometer - into the source of the mass spectrometer, while the vast majority of the vapour generated by the mobile phase is removed by a pump directly opposite the capillary. 

During air-pressurised discharge of a hot 53% aqueous solution of the nitrate salt from a reaction vessel via a filter press, a violent explosion occurred. The nitrate salt begins to decompose below 100°C, and at the likely internal temperature of 142°C, decomposition would be expected to be very rapid, involving much gas/vapour generation according to the equation below. 

Table 5.3 sets out the advantages and disadvantages of the batch and continuous flow techniques. The introduction of continuous-flow hydride/vapour-generation has substantially advanced the value and acceptance of the technique for trace elemental analysis. Appfied Research Laboratories (now part of Fisons Elemental), P.S. Analytical and Varian have all introduced continuous-flow hydride/vapour-generation systems, whilst Perkin Ehner has used the flow injection modification to automate the techniques with their instrumentation. 

Figure S.2 shows a schematic diagram of the automatic hydride/vapour-generator system designed by P.S. Analytical. This has been widely used to determine hydrideforming elements, notably arsenic, selenium, bismuth, tellurium and antimony, in a wide range of sample types. To provide a wide range of analyses on a number of matrices the chemistry must be very well defined and consistent. Goulden and Brooksbank s automated continuous-flow system for the determination of selenium in waste water was improved by Dennis and Porter to lower the detection levels and increase relative precision [10, 11]. The system described by Stockwell [9] has been specifically developed in a commercial environment using the experience outlined by Dennis and Porter.

Fig. 5.2 Schematic diagram of a continuous flow vapour generator.

Fig. 5.4 Calibration graph for vapour generation linked to atomic absorption spectroscopy.

Figure S.4 shows a calibration graph of arsenic concentrations obtained by using a Perkin Elmer 2100 atomic-absorption system bnked to a P.S. Analytical hydride/vapour generator (PSA 10.003). An electrically heated tube has been used in this work and the spectral source was an electrodeless discharge lamp. Alternatively, a flame-heated tube can be used.

Fig. S. S Membrane separator designed to improve performance of ICP/MS for arsenic and selenium by vapour generation techniques.

Atomic absorption coupled to cold vapour generation. 

Atomic absorption following trapping on gold from cold vapour generation. 

Hydride/vapour generation techniques provide extremely good sensitivity. When coupled to continuous flow methodologies for use in routine analysis, simple and reliable analytical techniques are provided. TTie extension of chemistries and sample transfer systems to provide analytical protocols to cope with a wider range of elemental analyses should be pursued in the search for lower detection levels. While multi-element techniques offer very low levels of detection, the use of specific single element analytical instruments with detection capabihties similar to those described above may be the best route for routine laboratories with high sample throughput. 

SEC. S.2] HYDRIDE/VAPOUR-GENERATION TECHNIQUES 163 288,145 counts (0.1 second)... 

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