Torches, problems with

Several coating techniques are now available to overcome the oxidation problems with molybdenum above 300°C. One of these, based on molybdenum disilicide, is finding increased usage in flame breakout shields for aero-engines where tests have shown (unpublished work) that the coated material can withstand a high pressure torching type flame attack at temperatures in excess of 2(X)0°C. 

An additional problem is that known elements in the periodic table, e.g. boron (B), tungsten (W) and molybdenum (Mo) tend to stick in the transport fine, nebuliser, spray chamber and torch causing memory effects in atomic spectroscopy. Elements that stick cause problems with quantification, and detection Emits and it is important that methods of reducing these are rigorously applied in analysis of these elements. However, memory effects in ICP-OES are not as pronounced as they are with graphite furnaces for refractory elements but they are present to some extent, and must be reduced or removed. 

Improper operation of a process may result in the vessel s exceeding design temperature. Proper control is the only solution to this problem. Maintenance procedures can also cause excessive temperatures. Sometimes the contents of a vessel may be burned out with torches. If the flame impinges on the vessel sheh, overheating and damage may occur. 

The capacitatively coupled microwave plasma is formed by coupling a 2450 MHz magnetron, via a coaxial waveguide, to metal plates or a torch where the plasma is formed. Considerable problems have been encountered with this low-cost plasma, particularly from easily ionizable elements which cause dramatic changes in the excitation temperature in the plasma. 

Trace levels (10 to 10 g/g of sample) of silver can be accurately determined in biological samples by several different analytical techniques, provided that the analyst is well acquainted with the specific problems associated with the chosen method. These methods include high frequency plasma torch-atomic emission spectroscopy (HFP-AES), neutron activation analysis (NAA), graphite furnace (flameless) atomic absorption spectroscopy (GFAAS), flame atomic absorption spectroscopy (FAAS), and micro-cup atomic absorption spectroscopy (MCAAS). 

GC-MIP systems have been investigated in considerable detail. Because of the low power of the plasma, it is easily quenched if the normal, atomic spectrometric sample introduction techniques, such as nebulisation, are used. Capillary columns overcome this problem as they require only low flow rates and small sample sizes more compatible with stable plasma operation. The capillary columns can be passed out of the oven, down a heated line, and the end of the column placed in the plasma torch just before the plasma, thus preventing any sample loss. A makeup gas is usually introduced via a side arm in the torch to sustain the plasma (Fig. 4.1, Greenway and Barnett, 1989). Other dopant gases can also be added in this way to prolong the lifetime of the torch and improve the plasma characteristics. 

The use of correction equations should be avoided, especially if the Se mass counts are small compared with the interference present. Correction equations suffer from inherent problems in that the masses used to correct for interferences may themselves suffer from interferences [113, 114]. Examples of potential polyatomic interferences for Se are given in Table 20.2. These interferences cannot be separated from analytes using a Q detector. Many papers in the literature report different interferences when measuring Se in similar matrices. This is because the extent and magnitude of interferences depend on the Q-ICP-MS instrument used, the type of nebulizer, the plasma torch conditions, the final dilution volume of the sample, and the isotope concentration being measured [88]. 

The transfer line consists of an electrically heated stainless steel tube, through which an uncoated, yet deactivated fused silica transfer capillary is passed until the end of the plasma injector. All parts of the stainless steel transfer tube are heated, including the part inside the torch box. The ICP-MS instrumentation is prone to signal suppressions and/or instrumental drift. These problems can be compensated by the use of internal standards. In the case of GC-ICP-MS the internal standard can be added to the carrier gas of the GC apparatus. A suitable internal standard is Xenon (Xe) [41]. The 126Xe signal is monitored simultaneously with the other isotopes of interest. In this way instrumental drift and signal suppression can be corrected. 

As an example of problems involving long time scales consider the reaction of hydrogen and oxygen in a balloon at room temperature. The fact that there seems to be no detectable change in the concentration of either constituent over many months does not mean that the system is equilibrated insertion of platinum black as a catalyst leads to a measurable rate of formation of water, and heating the balloon with a torch leads to a violent reaction. 

There are other problems in the wet oxidation method. For one, the precision of the method is far from good. Customarily, three replicates are taken from each water sample. After acidification, bubbling, and the addition of persulfate (not necessarily in diat order), the sample ampoules are sealed with a torch. They are then held until it is convenient to heat them, usually for 1 hr at 140 °C, break the ampoule, and measure the carbon dioxide evolved. Since there is often a considerable delay between sampling and analysis, sometimes as much as a month, there is no opportunity to retrieve mistakes or accidents. Our own experience has been that wild values, perhaps caused by contamination, occur in about 20% of the samples. The use of either subjective judgment or some empirical rule-of-thumb not far removed from subjective judgment, for the elimination of these wild values is widespread. 

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