Slot nebulizer

The cone-spray nebulizer is a more efficient design of the previously described slot nebulizer. A diagram of this type of nebulizer is shown in Figure 5.16. Instead of a slot, where sample approaches the orifice from only one direction, the cone spray consists of a three-dimension funnel-shaped depression made in an inert material, such as sapphire, with an orifice positioned in the bottom of the funnel. Sample is introduced into the funnel from above and is concentrated on the orifice as it flows to the bottom. A commercially available version of this nebulizer has an orifice with a 216-p,m diameter and operates at about 32 psi.The long-term precision of this nebulizer has been shown to be about 1% over an 8-h period of time. 

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

Flame atomization assembly equipped with spray chamber and slot burner. The inset shows the nebulizer assembly. 

Flame Sources Atomization and excitation in flame atomic emission is accomplished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and injected directly into the flame. 

Finally, periodic cleaning of the burner head and nebulizer is needed to ensure minimal noise level due to impurities in the flame. Scraping the slot in the burner head with a sharp knife or razor blade to remove carbon deposits and removing the burner head for the purpose of cleaning it in an ultrasonic cleaner bath are two commonplace maintenance chores. The nebulizer should be dismantled, inspected, and cleaned periodically to remove impurities that may be collected there. 

The flow-shear nebulizer consists of a spherical surface with a fine slot through which the argon gas passes horizontally, and creates an aerosol stream flowing normal to the tangent at the slot. The Babington flow-shear nebulizer has been used for FAA, by Fry and Denton(36). The Fry and Denton version requires a flow rate of nine to twelve liters per minute of nebulizing gas. It may be possible to select a proper orifice size to obtain adequate aerosol production with a nebulizing gas flow rate of one liter per minute which is more suited to most ICP systems. A peristaltic pump transfers the solution to the nebulizer. 

Figure 21-5 (a) Premix burner. (fc>) End view of flame. The slot in the burner head is about 0.5 mm wide, (c) Distribution of droplet sizes produced by a particular nebulizer. 

The development of new, highly efficient nebulizers, described in detail in the section on ICP-OES (Section 12.2.4.4.1), has meant that a more concentrated aerosol and a more sensitive FAAS determination is achievable. Similarly, the use of slotted tube atom traps (STATs) and water-cooled atom traps (WCAT)10 11—the latter have undergone modification in recent years12—enhances sensitivity with regard to volatile elements like Cd and Pb because of the long residence time of these atoms in the tube. 

The burners used in flame spectroscopy are most often premixed, laminar flow burners. Figure 28-11 is a diagram of a typical commercial laminar-flow burner for atomic absorption spectroscopy that employs a concentric tube nebulizer. The aerosol flows into a spray chamber, where it encounters a series of baffles that remove all but the finest droplets. As a result, most of the sample collects in the bottom of the spray chamber, where it is drained to a waste container. Typical solution flow rates are 2 to 5 mL/min. The sample spray is also mixed with fuel and oxidant gas in the spray chamber. The aerosol, oxidant, and fuel are then burned in a slotted burner, which provides a flame that is usually 5 or 10 cm in length. 

Nebulizer-bumer system. The purpose of the system is to produce rmiformly fine fog of droplets from the test solution. The burner has a long and narrow slot at the top so that the flame provides a long absorption path for the incident radiation. The fuel-oxidant system used may be acetylene air, acetylene-nitrous oxide, hydrogen-air etc. 

An FIA system is especially useful in the analysis of solutions containing high levels of solids such as saturated salt solutions or dissolved fusion mixtures. The burner slot or nebulizer will not become blocked since the system is continuously and thoroughly rinsed with the carrier stream after each sample measurement. The FIA system requires less than 400 /zl of sample solution, which is much less than with continuous aspiration. Thus, FIA-FAAS is the preferred technique when only small sample amounts are available. Low sample consumption is also beneficial for routine analysis, since with fully automated sequential multi-element analysis more determinations can be performed with a given sample volume. 

A Perkin-Elmer (PE) Model 603 spectrophotometer equipped with a manual gas control system, a stainless steel nebulizer, a burner mixing chamber, a flow spoiler and a 10 cm. (one-slot) burner head was used in the experimental validation of the flame AAS analytical technique. A PE cadmium hollow cathode lamp, operated at the manufacturer s recommended current setting for continuous operation (4 mA), was used as the source lamp. Instrument parameters are listed in Attachment 1. 

Aside from the HCL, an AA spectrophotometer contains familiar components as shown in Figure 5.41. A peristaltic pump draws sample into the nebulizer, where it is converted to a fine mist Fuel and oxidant gases such as acetylene and air are added, and the flow is directed into the flame, which is long and thin (called a slot-type flame) to provide an optimum path length for absorbance measurements. Light from the HCL enters the flame, interacts with it, and passes to the detector. A monochromator is placed between the flame and the detector to filter out flame emission signals. 

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