BEAM INJECTION FLAME FURNACE ATOMIC ABSORPTION SPECTROMETRY A NEW FLAME AAS METHOD The sensitivity of flame atomic absorption spectrometry measurements can be improved significantly by increasing the efficiency of aerosol generation/transport and prolonging the residence time of the free analyte atoms in the absorption volume. Both can be achieved using the new Beam Injection Flame Furnace AAS (BIFF-AAS). For the first time a continuous liquid sample introduction into a flame-heated tube furnace has become possible by sample transportation via a very narrow liquid jet. With this technique it is possible to introduce the sample over a distance of 10 cm into a flame-heated metal or ceramic furnace (Fig. 1). The liquid jet is created by means of an HPLC pump (50 to 400 bar), a gas pressure pump (2 to 10 bar), a peristaltic pump (< 2 bar) or a novel 6 bar flow-system which feeds a smooth jet nozzle with a diameter of 50 :m or smaller. The liquid jet passes a small sample introduction hole (2 mm diameter) and is then impacted on the opposite inner wall of the tube furnace leading to an immediately vaporizing aerosol (jet impact vaporization, JIV). The arrangement containing a peristaltic pump for the beam generation is shown in Fig. 2. The complete introduction of the whole sample directly at the place of atomization and the extended residence time inside the absorption volume (flame furnace) result in a considerable improvement in the power of detection compared with the standard working mode of flame AAS (sample introduction via a pneumatic nebulizer).
The experiments showed that a beam is stable over a distance of at least 15 cm if its speed is higher than about 12 m/s. Using a nozzle of 50 :m inner diameter, a 1.42 mL/min flow rate is necessary to reach this speed. The typical shape of a smooth jet nozzle is a cylinder with a ratio length/diameter > 3. The pressure drop of such a nozzle is less than 0.5 bar which allows the use of low-pressure pumps, for instance a peristaltic pump, for the beam generation. The requirements for the material of the tube furnace include good stability and chemical resistance against the corrosive conditions of an acetylene/air flame and water and acid vapors at temperatures higher than 1000 /C. Suitable tube materials are pure nickel, super alloys or non-porous Al2O3 ceramic. To increase the temperature inside the furnace and to achieve a more reducing atmosphere, a tube with 4 or more additional holes located on the bottom side is used. Currently, 25 elements have been determined using BIFF-AAS, of which 17 elements (Ag, As, Au, Bi, Cd, Cu, Hg, In, K, Pb, Pd, Rb, Sb, Se, Te, Tl, Zn) exhibit an improved power of detection from 6 fold to 202 fold compared to conventional FAAS [1]. The standard deviation amounts to 1.7 - 4.0 % (N = 12; 50 :L samples). Fig. 3 shows the signals of Cd sample in the low :g/L range measured with BIFFAAS.
The sample is injected via a sample introduction valve into a carrier stream as in all flow injection techniques including HPLC. Because of this, even sample volumes as small as 10 :L can be easily applied. Fig. 4 shows the determination of copper with increasing sample volumes and using BIFF-AAS as well as conventional flame- AAS. With 10 :L samples a clear Cu signal appears in case of BIFF-AAS but not when using the normal flame AAS. Working with a peristaltic pump allows a continuous sample introduction to be maintained. Generating the liquid jet with the aid of a high-pressure pump, BIFF-AAS can be applied as a simple, efficient interface between HPLC separation techniques and elemental determinations by flame-AAS.
To fill the gap in the pressure range between HPLC and peristaltic pumps a novel pump system based on a 6 bar low cost diaphragm pump has been developed (Fig. 5). The unavoidable strong pulsation has been overcome by using 50 m (2 mm i.d.) of highly flexible silicone tubing as a pulse suppression coil. This results in a smooth pulse free continuous flow whereby the pump delivers 100 ml/min in circulation. For analytical flow injection a restrictor is connected to a splitting point in the cycle to tap off the flow needed, down to 0.2-5.0 ml/min. The higher pressure of the diaphragm pump compared to the peristaltic pump permits the use of common HPLC-columns or low-pressure chromatography columns for online-sample pretreatment like preconcentration of traces or matrix separation. This system was successfully applied for the beam generation in BIFF-AAS and for on-line sample pretreatment in conventional AAS and ICP-OES respectively.
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