7 The flame side of the AA measures in parts per billion. Wh excitation for thes

Question

7 The flame side of the AA measures in parts per billion. Wh excitation for these two kinds of the spectroscopy? asures concentrations in parts per million range and the furnace side of the AA at is the major difference in terms of sample introduction and electron concentrati 8 What are the advantages of using the FAAS?* disadvantages? 9 What are the advantages of using the GFAAS?* disadvantages? 10 Why must all equipment in this analysis be acid washed? 11 What are the units of ppm in mass/mass ratio? In mass/volume ra 12 What the units of ppb in mass/mass ratio? In massvolume ratio?

Explanation / Answer

Flame atomic absorption spectrometry (FAAS)

Flame atomic absorption spectrometry (FAAS) is one of the most widespread traditional analytical techniques for trace element determination, but it often suffers from poor sensitivity due to the low nebulization efficiency and the short residence time of free atoms in the flame. On the basis of conventional FAAS, flame furnace atomic absorption spectrometry (FF-AAS) is developed with a tube (flame furnace) placed on top of the FAAS burner for the atomization. Sample is introduced via beam injection (BIFFAAS) or thermospray (TS-FF-AAS). Due to the total sample introduction and prolonged residence time of free atoms in the flame furnace, marked sensitivity improvement is obtained for volatile and semivolatile elements over conventional FAAS. TS-FF-AAS can be employed as an element-selective detector for GC, HPLC, or CE for studying of metal speciation analysis and metallomics.

The flame AAS technique was welcomed by analytical chemists as a powerful alternative to spectrophotometric methods, which can require tedious chemical pretreatment of samples, such as buffering, complexation, extraction, and other operational steps, whereas dissolution of the sample into an aqueous medium was usually sufficient for flame AAS.

FAAS often suffers from low or poor sensitivity, which greatly holds back or inhibits its further application in ultratrace determination. The low sensitivity of FAAS can be ascribed to two major origins: poor sampling efficiency and short residence time of free atoms, as compared with its analog, GF-AAS. Due to the employment of pneumatic nebulization, a very poor sampling efficiency in FAAS is obtained, typically lower than 10%.

FLAME FURNACE ATOMIC ABSORPTION SPECTROMETRY

Contrary to conventional FAAS, sample introduction efficiency was 100% as sample was all delivered into flame furnace via the sample introduction hole. From the operation of conventional FAAS, the typical sample aspirating rate is about 5–10 mL/min. Considering the efficiency of pneumatic nebulization (5– 10%), the actual sampling rate is about 5–15 µL/s, which is far lower than that 414 P. Wu et al. of flame furnace AAS (1.5–2.0 mL/min, total sample introduction). In other words, the exact sample amount introduced into the atomization cell by flame furnace AAS is larger than that by conventional FAAS. Further, because the gas expansion rate inside the flame furnace is expected to be lower than that in an unobstructed flame because the gas can only exit from the two open ends of the tube, this prolongs the residence time of free atoms inside the flame furnace.

The FF-AAS technique is mainly limited to those elements with relatively low appearance temperatures (9), such as Cd (11, 12), Pb (13, 14), and Zn (12, 15). Fortunately, these elements are the subjects of routine analysis in a variety of samples including food, water, botanic samples, etc., and this makes FF-AAS very promising for these applications. On the other hand, leaking acetylene into the flame furnace leads to a reducing environment inside the flame furnace, which is particularly useful for the atomization of several elements prone to form refractory oxides in conventional FAAS.

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