A Sensit ive Premixed 0xy a c ety Ie ne A t omiz e r- Bu rne r )e c t ro metry For Flame Emission and Absorption Spectrometry SIR: The observations (1-4) that atomic emission and absorption spectra of many elements are strikingly enhanced in fuel-rich oxyacetylene flames has greatly expanded the potential scope of application of these techniques. Although total consumption burners of the Beckman type (Beckman Instruments, Inc. #4030) have been used in our studies (f-S), several experimental difficulties are encountered when these burners are operated under fuel-rich conditions. First, the turbulent mixing principle employed in this burner gives rise to a high background noise which limits the ultimate sensitivities attainable. Second, a deposit tends to form at the capillary-oxygen orifice during prolonged operation of the Beckman burner under fuel-rich conditions (1, 9). The formation of this deposit affects the atomization efficiency and greatly increases the spectral background. By modifying the Beckman burner into a premixed type, we have been able to circumvent the difficulties associated with deposit formation as well as improve the sensitivities of
detection by an order of magnitude or more.
As shown in Figure 1, this modification, in essence, provides a premixing channel for the oxygen and acetylene after the sample atomization has occurred. A copper or brass tube is attached t o the outside of a standard Beckman burner by means of three sets of aligning screws placed at 120’ intervals about the base of the tube. A graphite tube, which provides the premixing channel, is inserted into the copper tube. I t is important that the shoulder on the graphite tube be placed so that there is a 0.2- to 1.0-mm. space between the top of the Beckman burner and the bottom of the graphite tube. Three equally-spaced air holes (-2 mm. in diameter), located as shown on the drawing, allow a small amount of air t o mix with the gas stream and also provide venting should the burner “strike back.” A picture of an assembled burner is shown in Figure 2. The solution atomized at the tip of the capillary must flow axially throngh the channel in the graphite rod. The alignment of the atomizer with respect to the premixing channel is therefore
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flow of gases causing the flame to flicker, and in a tendency for the the flame t o “strike hack.” To align the burner conveniently and precisely, the entire assembly, without the graphite rod, is held in an inverted position by clamping onto a ring stand. A distilled-water line is connected t o the oxygen inlet of the burner. When the water flow rate through the oxygen inlet is sufficient, a fine jet of water will issue from the oxygen port. The copper tube is then centered by means of the aligning screws so that the water jet travels axially through the copper tube. The grapite rod is then inserted into the copper tube and the alignment screws are adjusted until the water jet travels axially through the graphite tube. A properly aligned burner will not “strike back” when the acetylene flow is turned off first and the graphite rod will be cool to the touch even after continued operation. The burner will normally remain aligned for an extended period of time provided the alignment screws are tightened. I n operation, the burner is used in much the same manner as a standard Beckman burner. Relatively high oxygen and acetylene flow rates increase
GRAPHITE ROD 12.0m m dig.
4 Figure 1.
Diagram
of premixed burner
assembly
>ALIGNING
S C R W S 12-56)
Figure 2. Photograph of premixed burner assembly 910
ANALYTICAL CHEMISTRY
Table 1. Estimated Practical Detection Limits for Several Elements"
Detection limit (P.PJn. )* ;Standard Beckman Preburner mixed Line (-4.) (3) burner 4379.2 3 4408.2, 0.3
Element
v
4408.5
Nb 4058.9 12 1.0 Ti 3998.6 5 0.5 3798.3 0.5 0.03 Mo W 4008.8 90 4.0 Re 3460.5 3 0.3 a Using ethanol-con
