Atomic Absorption Flame and Precautions

Air-acetylene is the preferred flame for the determination of approximately 35 elements by atomic absorption. The temperature of the air-acetylene flame is about 2300 °C. An air-acetylene flame can be used with all Perkin-Elmer burner heads. The step-by-step procedure is given for the operation of the acetylene/air flame.

For the air-acetylene flame, the acetylene flow is about 4 liters/minute, or 8.5 cubic feet/hour, using a heat combustion value of 1450 BTU per cubic foot. The heat given off would be approximately 12,300 BTU per hour. Usually, ordinary welding grade acetylene is adequate. However, welding grade acetylene is supplied dissolved in acetone. As tank pressure falls, the concentration of acetone entering the flame increases. This increase may produce erratic results when determining elements that require a fuel-rich flame or elements with analytical wavelengths in the lower UV range. Also, acetone passing into the gas control box may damage valves and tubing. For these reasons, acetylene tanks should be replaced when the tank pressure falls below 520 kPa (75 psig). Acetylene tanks should be stored and operated in a vertical position to prevent liquid acetone from reaching the cylinder valve. If the welding grade acetylene is not adequate for atomic absorption, atomic absorption grade acetylene may be available.

The nitrous oxide-acetylene flame has a maximum temperature of about 2900 °C and is used for the determination of elements which form refractory oxides. It is also used to overcome chemical interferences that may be present in flames of lower temperature. For the nitrous oxide-acetylene flame, the acetylene flow is about 14 liters per minute or 30 cubic feet per hour. Using a heat of combustion value of 1,450 BTU per cubic foot, the heat given off would be approximately 43,000 BTU per hour. Light emission from the nitrous oxide-acetylene flame is very strong at certain wavelengths. This may cause fluctuations in the analytical results for determinations performed at these wavelengths, particularly if the lamp emission for the element of interest is weak. Only the nitrous oxide burner head can be used with the nitrous oxide-acetylene flame. Ignition procedures and operating conditions for the nitrous oxide-acetylene flame are given in the appropriate instrument manual.

Gas Controls

Initial operating conditions, including pressure and flow settings for fuel and oxidant, are listed in the appropriate instrument manual. For flame ignition and adjustment procedures, consult the instrument operating manual. Adjustment of the burner head relative to the light path of the instrument is necessary to obtain maximum sensitivity.

The burner height may be adjusted by first lowering the burner (with the flame off) until the burner head is well below the light beam and then zeroing the readout. Slowly raise the burner until the head just intersects the beam (indicated by a positive reading on the readout). The burner should then be slowly lowered until the readout returns to zero (indicating the burner is just below the light beam). With elements that require a fuel-rich flame, the burner height may have to be optimized by aspirating a standard solution with the flame on and slowly lowering the burner until a maximum signal is obtained (recheck zero after lowering the burner and readjust if necessary). Lateral and rotational adjustment of the burner must be made with the flame ignited and while aspirating a standard solution of the element of interest. The burner is adjusted in both cases to obtain maximum absorption.

For many elements, the fuel/oxidant ratio must be adjusted for maximum sensitivity. Aspirate a standard solution and adjust the Fuel, checking zero after each flow change and readjusting as necessary. Set up the instrument for the determination of copper, magnesium, or other elements which have an absorbing wavelength above 250 nm and whose sensitivity is not dependent on the fuel/oxidant ratio. Do not select an element requiring a nitrous oxide-acetylene flame. Aspirate a standard solution of the element being determined and turn the knurled end cap of the nebulizer counterclockwise until air bubbles into the solution. If the flame becomes leaner during this operation, slight adjustment of the fuel flow may be necessary. While observing the absorbance reading, slowly turn the knurled end cap of the nebulizer clockwise. The absorbance reading may go through several peaks as the end cap is turned. Return the end cap to the peak which gives maximum absorbance (normally the first peak). After adjustment turn, the locking ring counterclockwise until it rests against the nebulizer end cap to prevent accidental misadjustment. Nebulizers do not normally require readjustment unless a different solvent is used.

Many times it is desirable or necessary to use solvents other than water. In selecting an organic solvent, the solubility of the sample, the miscibility of the solvent with water (for extractions), and the burning characteristics of the solvent must be considered.

Operation of the Air/Acetylene Flame

5. For a single-slot, 10-cm burner head turn the oxidant selector valve to AIR. Set the auxiliary Oxidant Flow valve for a flow meter reading of 50.

6. Adjust the FUEL Flow valve to bring the flat ball to about 20 on the flow meter.

7. Ignite the flame by pressing the IGNITE button and holding it until the flame is lit.

8. After ignition, minor adjustments in the flame conditions may be necessary. Allow several minutes warmup of the burner system. If the flame is uneven, shut it off and clean the burner head slot. Check the burner alignment.

9. A fuel-lean (oxidizing) air-acetylene mixtures gives a blue, hot flame. A fuel-rich (reducing) air/acetylene mixtures gives a yellow, cool flame. Adjust the flame to recommended conditions.