Inexpensive Instrumental Analysis II: Introductory Spectroscopy Louis ~amaley,'Kathleen Young, Robert D. Guy, and Roger Stephens Dalhousie university, Halifax, ~ ~ , ~ a nB3H a d4J3 a In order to introduce more instrumental technioues into the first laboratory in analytical chenustry and td provide students with a basic understandine ofboth the ~rincioles hehind those techniques and theinstrumeniation involved, we are developing a set of inexpensive, easily constructed, and open instruments that provide quality results ( I ) . In this way enough instruments can be made available for each student to work individually. The various forms of spectroscopy are the techniques most freouentlv used in chemical analvsis. Thuq. students in aualykcal ciasses should be introiuced to such techniques as early as possible, both in lectures and in the laboratory. In the laboratory this is perhaps most easily accomplished through the use of flame emission or flame absorption measurements for elemental analysis and ordinary UV-vis absorption spectmphotometry for molecular analysis. These techniques are simple, are used routinely in analysis, and are covered in all introductory texts on instrumental analysis. We describe below both a simple atomic flame spectrometer and a solution spectrophotometer that are easily constructed, yet provide accurate, reliable results. Atomic Flame Spectrometer Commercial flame atomic absorption instruments are expensive and usually require the use of acetylene as fuel. Coleman (2), who described an experiment on the determination of lead in gasoline, has mentioned the precautions that must be used with acetylene. Melucci (3) recommended natural gas rather than acetylene. He converted a commercial instrument for use with natural gas in the analysis of sodium, noting that this resulted in a loss in sensitivity by a factor of 3. The calibration curve he obtained covered a range of 5-100 ppm of sodium. Other atomic absorption experiments have been described recently for the analysis of lead in brass (4) and calcium in fruit juices (3. All of the above have used commercial instrumentation. To implement the policy described above, we have designed and built a spectrometer for either flame emission or absorption measurements using a propanelair (or natural gaslair)flameas atom source an interference filter as monochromator a phototransistor as detector glass optics The use of these components keeps the instrument simple, compact, and inexpensive without compromising the teaching effectiveness or the analytical capability of the apparatus for elements that emit or absorb visible light. With the substitution of quartz optics and a photomultiplier detector the instrument can be converted for flame measurements in the ultraviolet or for cold vapor measurements on elements such as mercury. 'Author to whom correspondence should be addressed.
HCL POWER SUPPLY
AMPLIFIER AND METER
Figure 1. Flame spectrometer block diagram, showing the hollow cathode lamp (HCL), lens (L), chopper (C),chopper motor (M), burner (B), flame (F), nebulizer (N), light beam (LB), filterldetector (FID),optical rail (R), waste (W), and sample (S).
Adjustable Components Figure 1is a block diagram of the instrument showing a front view of the optical components. These components mount on an optical rail made from a square, hollow aluminum beam and can be adjusted to any position along the rail. In addition, the heights of the lenses and the detector can also be varied. The overall dimensions of the optical components are 60 cm long x 30 cm high x 30 cm deep (24 in. x 12 in. x 12 in.). Displaying the Pulsed Beam A conventional, hollow cathode lamp is used as the light source in the absomtion mode. In order to make the instrument insensitive background light, from both the flame and other sources in the laboratow. the lieht from the hollow cathode lamp is pulsed and detected &h an amplifier system tuned to the pulse frequency. This can be done in two ways: Use a power supply that pulses the current through the lamp, or place a mechanical chopper in the light beam right after the lamp. The use of pulsed current requires more complex circuitry, but this method is entirely electronic and provides better rejection of background light. This is more important for an instrument in which all components are open to the laboratory. However, this arrangement does not readily show the student that the beam is pulsed, so we use a mechanical chopper. This allows the hollow cathode lamp to be powered by a simple dc supply See-Through Nebulizer The burner, constructed of brass, uses a slot covered by a perforated screen (10 cm x 0.5 cm) to prevent flashback. The nebulizer is machined from clear, acrylic plastic. AlVolume 71 Number 4 April 1994
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thoueh this urevents the use of most nonaaueous solvents. it algws ththe' student to observe the nebugzation process: Laboratory compressed air at about 40 psi serves as the nebulization gas and oxidant, whereas propane or natural gas serves as the fuel. The monochromator (a 25-mm-diameter interference filter) is mounted in the same housing as the detector. a simple ~hototransistor.Inexuensive glass lenses be- focusing. Inexpensive Parts and Easy Construction The electronics are based on simole ouerational amolifier circuits assembled on ~ector6oariusing standard breadboardim techniaues and mounted in a small aluminum cabinet (36 x 18 'x 10 cm). The analytical result-intensity in the emission mode or percent transmittance in the absorption mode-
