Burner heads

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. 

Enclosed flares are composed of multiple gas burner heads placed at ground level in a staeklike enclosure that is usually refractory or ceramic lined. Many flares are equipped with automatic damper controls that regulate the supply of combustion air depending on temperature which is monitored upstream of the mixing, but inside the staek. This class of flare is becoming the standard in the industry due to its ability to more effectively eontrol emissions. Requirements on emissions includes carbon monoxide limits and minimal residence time and temperature. Exhaust gas temperatures may vary from 1,000 to 2,000 F. 

The flueless appliance has gas supplied to the burner head, and air passes outside the baffles at a designed velocity. 

Generally, this is a compromise in monobloc burners incorporating a fan and burner head within one casting. 

FIGURE 9.9 A drawing of the top of a burner head (looking down). The slot is either 10 cm long (for air-acetylene flames) or 5 cm long (for nitrous oxide-acetylene flames). 

The burner head itself is interesting. The flame must be wide (for a long pathlength), but does not have to be deep. In other words, the burner has a long slot cut in it (5 or 10 cm long) to shape the flame in a wide but shallow contour. Typically, the flame is 10 cm wide for air-acetylene and 5 cm wide for N20-acetylene. See Figure 9.9. 

The burner head can be adjusted vertically, horizontally (toward and away from the operator), and rotationally. Initially, it should be adjusted vertically so that the light beam passes approximately 1 cm above the center slot and horizontally so that the light beam passes directly through the center of the flame, end to end. All three positions can be optimized by monitoring the absorbance of an analyte standard while making the adjustments. A maximum absorbance would indicate the optimum position. 

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. 

A number of instrument variables need to be set prior to making measurements. These include slit, wavelength, lamp current, lamp alignment, amplifier gain, aspiration rate, burner head position, acetylene pressure, air pressure, acetylene flow rate, and air flow rate. Some instruments are rather automated in the setup process, while others are not. Your instructor will provide detailed instructions for the particular instrument you are using. Be sure to turn on the fume hood above the flame. 

Fuel flow rate. Your instructor will show you how to vary the fuel flow rate to the burner head and also how to read the flow rate on the flow meter. Measure absorbance values at ten different flow rates, selecting a flow rate range that will maintain a flame while bracketing what the manufacturer s literature or your instructor may suggest as the optimum. 

Controls that need to be optimized are wavelength, slit width, lamp current, lamp alignment, aspiration rate, burner head position, and fuel and oxidant flow rates. See Section 9.3.5 for details. 

The design of the burner head and the method of atomization of the sample both influence the sensitivity which can be achieved. The burner head is designed to give a long narrow flame so that as many atoms as possible are presented in the light path. It needs to be kept spotlessly clean to minimize... 

Q. How is it possible to redesign a slot burner to take up to 40% dissolved solids (cf. the normally acceptable 4-6%) when using a highly conductive material for the burner head ... 

Atomic absorption spectrometer with air-acetylene burner head. Pressurized acetylene cylinder. Air compressor. 

Ensure that an air-acetylene burner head is fitted. 

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. 

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