Atomic spectroscopy: emission and absorption

Sprinkle a pinch of salt into a gas flame and it flares bright yellow — the 589 nm doublet of sodium atoms. The yellow is so bright you might guess that most sodium atoms in the flame are glowing. The opposite is true. Clinical labs measuring serum sodium and potassium by flame photometry, and environmental labs measuring lead in drinking water by atomic absorption, all start from the fact that almost every atom in a flame is sitting in its ground state.

Two consequences decide which method to use:

• Emission is proportional to $N_1$ and extremely sensitive to temperature, so the flame must be very steady. It suits elements that are easy to excite: flame photometry of the alkali metals (Na 589 nm, K 766/770 nm, Li 671 nm) is still in routine use.
• Absorption is proportional to $N_0 \approx N_\text{total}$ and almost independent of temperature. Zinc’s resonance line is at 213.9 nm, an excitation energy of 5.80 eV: at 2500 K only 6 × 10⁻¹² of the zinc atoms are excited, so flame emission is useless — yet atomic absorption is still sensitive.
• Inductively coupled plasmas (ICP) run at 6000–10 000 K: at 6000 K the excited fraction of zinc rises to about 4 × 10⁻⁵, nearly every element can be measured by emission, and dozens at once (ICP-OES).

Atoms have no vibrational or rotational levels, so their lines start out extremely narrow. Three effects broaden them:

• natural width: the excited state lives only briefly (16 ns for sodium 3p), giving $\Delta\lambda \approx 1\times10^{-5}$ nm;
• Doppler broadening: atoms move toward or away from the observer; at 2500 K the sodium line’s Doppler width is about 0.0044 nm (4.4 pm);
• collisional (pressure) broadening: in an atmospheric-pressure flame it is comparable, about 0.003 nm.

An ordinary monochromator passes about 0.2 nm — fifty times wider than the absorption line. With a continuum source the line removes only a sliver of the bandpass and the signal is almost invisible. In 1955 Alan Walsh’s solution was a hollow-cathode lamp whose cathode is made of the element itself: it emits exactly that element’s lines, narrower still than the absorption line in the flame. The monochromator only has to separate that line from the lamp’s others. Other elements’ lines almost never fall at exactly the same place (0.01 nm apart is enough to avoid each other), which is where atomic absorption’s element specificity comes from.

Interferences and remedies. Chemical: phosphate forms involatile compounds with calcium and lowers its signal — add a “releasing agent” such as lanthanum or strontium, or use the hotter nitrous oxide–acetylene flame. Ionisation: potassium and sodium partly ionise in hot flames, leaving fewer ground-state atoms — add plenty of easily ionised caesium or potassium as an “ionisation buffer”. Background absorption by salt particles and molecules is subtracted with a deuterium lamp or the Zeeman effect. Self-absorption: in flame emission the line centre becomes opaque at high concentrations, and cooler atoms reabsorb the emitted light, bending the calibration; an ICP is optically thin and stays linear over 4–6 orders of magnitude.