Q1(c). Sources of radiation for infrared spectrometry

Answer in brief

Electrically heated inert solids that emit a continuous, black-body-like spectrum: the Nernst glower, the Globar (silicon carbide) and the incandescent nichrome wire, covering the mid IR, 4000 to 400 cm⁻¹.

Key points

• Need: continuous radiation of steady intensity over the mid IR (4000–400 cm⁻¹); an inert solid heated to about 800–2000 °C radiates like a black body.
• Nernst glower: rod of zirconium oxide with yttrium and erbium oxides, 1500–2000 °C; non-conducting when cold, so it is preheated.
• Globar: rod of sintered silicon carbide, 1300–1700 °C; self-starting, sturdier, better beyond about 15 µm; the usual source of FTIR instruments.
• Incandescent wire: coil of nichrome (or Kanthal, rhodium) at about 1100 K; lower intensity, long life, no cooling.
• Outside the mid IR: tungsten filament lamp (near IR) and high-pressure mercury arc (far IR); CO₂ and diode lasers for single compounds.