Gaseous hydride analysis

The EPA has approved special methods for arsenic (EPA Method 7061) and selenium (EPA Method 7741) analysis by gaseous hydride generation technique, which offer a unique combination of selectivity and sensitivity. This technique has an advantage of being able to isolate these elements from complex matrices that may cause interferences in analysis with other techniques. 

Figure 5.8. The analyte is converted into a gaseous hydride (e.g., As — ASH3), which is purged into the heated furnace. There it decomposes into free As atoms for analysis.

In connection with the FIA system, the amalgamation attachment allows fully automated on-line concentration of the analyte. This technique improves the detection limit for mercury by an order of magnitude or even more. Gaseous hydrides do not interfere due to the low temperature of the gauze, thus, the analysis with NaBH4 is also interference-free. 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

A number of elements form volatile hydrides, as shown in the table. Some elements form very unstable hydrides, and these have too transient an existence to exist long enough for analysis. Many elements do not form stable hydrides or do not form them at all. Some elements, such as sodium or calcium, form stable but very nonvolatile solid hydrides. The volatile hydrides listed in the table are gaseous and sufficiently stable to allow analysis, particularly as the hydrides are swept into the plasma flame within a few seconds of being produced. In the flame, the hydrides are decomposed into ions of their constituent elements. 

Laser-induced breakdown spectrometry was used for the analysis of gaseous samples containing elements such as F, Cl, S, P, As and Hg in air, and hydrides of column 111 and V elements (e.g. B,H, PH,) [184-189]. The aim was to measure trace amounts of analytes in hostile environments and gas impurities for hydride work. Mercury was detected at the parts-per-billion level in air using a photodiode array detector that recorded single-shot spectra over a range of 20 nm [186]. Cremers el al. [189] reported limits of detection of 8 and 38 pg/ml for chlorine and fluorine, respectively, the source of both... 

There are sample introduction systems that can handle slurries of particles suspended in hquids. Powders can be injected directly into the plasma for analysis. Lasers, sparks, and graphite furnaces (exactly the same as AAS graphite furnaces) are used to generate gaseous samples from sohds for introduction into the plasma. Hydride generation for As and Se and cold-vapor Hg introduction are used for ICP as for AAS these two techniques were discussed in Chapter 6. 

Gaseous samples can originate from sources as simple as process gas streams in industrial settings to more sophisticated systems such as gas chromatographic effluents or hydride gas generation apparatus. An example of the direct sampling of a gas stream from an industrial process is illustrated by the analysis of impurities in silane (SiH ) gas used in the fabrication of silica films for the manufacture of photovoltaic cells. Arsenic and iodine are measured at the subnanogram concentration level, with detection limits of approximately 0.5 parts per billion (v/v). 

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