Atomic spectroscopy 3.7.8



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3.7.8 Atomic spectroscopy

Atomic spectroscopy, i.e. emission, absorption and fluorescence, involves the input of energy (e.g. electromagnetic, thermal, chemical or electrical) into an atomic population, which is then converted into light energy by various atomic and electronic processes before the final measurement. The light energy is manifest in the form of a spectrum consisting of radiation at a number of discrete wavelengths. The quantized energy levels involved may be expressed as:

ΔE = E₁ − E₂ = hv

where E₁ and E₂ are the energies in the initial and final states respectively, h is Planck’s constant (6.626 076 × 10⁻³⁴ Js) and v is the frequency of the radiation.

An atom is said to be in the ground state when its electrons are at their lowest energy level. When energy is transferred to such atoms, as in emission spectroscopy, the energy transfer to individual atoms will vary and hence the resulting radiation will be at a number of different frequencies and will give rise to a complex emission spectrum.

The proportion of excited to ground state atoms in a population at a given temperature is expressed by the Boltzmann relationship:

	N_m	=	g_m	exp		(E_m − E_n)
	N_n		g_n			kT

where N is the number of atoms in a state n or m, g is the statistical weight for a particular state and k is the Boltzmann constant (1.380 658 × 10⁻²³ JK⁻¹).

Atomic absorption spectrometry is the measurement of the absorption of optical radiation by atoms in the gaseous state. Usually only absorptions involving the ground state, known as resonance lines, are observed. The absorption coefficient is determined by the product of the total number of atoms present per unit volume and the oscillator strength of the resonance line. The excitation energy is provided by a radiation quantum and no temperature factor is involved.

In quantitative spectroscopy, the absorbance A is often defined by:

A = log(I_o/I)

where I_o is the intensity of the incident beam and I is the intensity of the transmitted light. Thus we obtain a linear relationship where:

A = k_v log e

= 0.4343 k_vl

In this case l is the path length and so in practical terms k_v, the absorption coefficient at frequency v, is proportional to the number of atoms per cubic centimetre in the flame and A is therefore proportional to analyte concentration. The atoms excited by absorption of resonance radiation may also re-emit the energy giving rise to atomic fluorescence. The re-emitted energy may be of the same wavelength as the absorbed energy, or as is more often the case at a longer wavelength indicating that an intermediate state is involved with the partial loss of energy in some other form.

References

S. J. Hill (1999) Inductively Coupled Plasma Spectroscopy and its Applications, Sheffield Academic Press.
L. Ebdon, E. H. Evans, A. Fisher and S. J. Hill (1998) An Introduction to Atomic Absorption Spectroscopy, Wiley.
W. J. Price (1979) Spectrochemical Analysis by Atomic Absorption, Heyden.
B. Welz (1985) Atomic Absorption Spectrometry, 2 edn, VCH.

Values of the ratios of atoms in the excited state (N_j) to the number of atoms in the ground state (N₀) for typical elemental resonance lines

Line, nm	g_j /g₀	N_j/N₀
		2000 K	3000 K	4000 K	5000 K
Cs 852.1	2	4.44 × 10⁻⁴	7.24 × 10⁻³	2.98 × 10⁻²	6.82 × 10⁻²
Na 589.1	2	9.86 × 10⁻⁶	5.88 × 10⁻⁴	4.44 × 10⁻³	1.51 × 10⁻²
Ca 422.7	3	1.21 × 10⁻⁷	3.69 × 10⁻⁵	6.03 × 10⁻⁴	3.33 × 10⁻³
Zn 213.9	3	7.29 × 10⁻¹⁵	5.58 × 10⁻¹⁰	1.48 × 10⁻⁷	4.32 × 10⁻⁶
Note: g_j/g₀ are the statistical weights of the excited and ground state.

Detection limits for flame atomic absorption spectrometry

Element	Wavelength (nm)	Characteristic concentration (μg ml⁻¹)^a	Detection limit (μg m1⁻¹)^a	Normal range (μg ml⁻¹)	Flame Type
Ag	328.1	0.03	0.002	0.02–10	Air–C₂H₂
Al	309.3	0.8	0.03	0.3–200	N₂O–C₂H₂
As	193.7	0.5	0.3	3–150	N₂O–C₂H₂
Au	242.8	0.1	0.01	0.1–30	Air–C₂H₂
B	249.7	8.0	0.5	5–2000	N₂O–C₂H₂
Ba	553.5	0.2	0.02	0.2–50	N₂O–C₂H₂
Be	234.9	0.015	0.001	0.01–4	N₂O–C₂H₂
Bi	223.1	0.2	0.05	0.5–50	Air–C₂H₂
Ca	422.7	0.01	0.001	0.01–3	N₂O–C₂H₂
Cd	228.8	0.01	0.0015	0.02–3	Air–C₂H₂
Co	240.7	0.05	0.005	0.05–15	Air–C₂H₂
Cr	357.9	0.05	0.006	0.06–15	Air–C₂H₂
Cs	852.1	0.02	0.004	0.04–5	Air–C₂H₂
Cu	324.7	0.03	0.003	0.03–10	Air–C₂H₂
Dy	421.2	0.6	0.03	0.3–150	N₂O–C₂H₂
Er	400.8	0.5	0.03	0.5–150	N₂O–C₂H₂
Eu	459.4	0.3	0.02	0.2–100	N₂O–C₂H₂
Fe	248.3	0.05	0.006	0.06–15	Air–C₂H₂
Ga	287.4	0.08	0.08	1–200	Air–C₂H₂
Gd	368.4	20	2.0	20–6000	N₂O–C₂H₂
Ge	265.1	1.0	0.2	2–300	N₂O–C₂H₂
Hf	307.3	10	2.0	20–300	N₂O–C₂H₂
Hg	253.7	1.5	0.15	2–400	Air–C₂H₂
Ho	410.4	0.7	0.04	0.4–200	N₂O–C₂H₂
In	303.9	0.15	0.04	0.4–40	Air–C₂H₂
Ir	208.8	0.8	0.5	5–200	Air–C₂H₂
K	766.5	0.007	0.003	0.03–2	Air–C₂H₂
La	550.1	40	2.0	20–10 000	N₂O–C₂H₂
Li	670.8	0.02	0.002	0.02–5	Air–C₂H₂
Lu	336.0	7.0	0.3	3–2000	N₂O–C₂H₂
Mg	285.2	0.003	0.0003	0.003–1	Air–C₂H₂
Mn	279.5	0.02	0.002	0.02–5	Air–C₂H₂
Mo	313.3	0.3	0.02	0.2–100	N₂O–C₂H₂
Na	589.0	0.003	0.0002	0.002–1	Air–C₂H₂
Nb	334.9	20	2.0	20–6000	N₂O–C₂H₂
Nd	492.5	6.0	1.0	10–1500	Air–C₂H₂
Ni	232.0	0.07	0.01	0.1–20	Air–C₂H₂
Os	290	1.0	0.1	1–300	N₂O–C₂H₂
P	213.6	120	40	400–30 000	N₂O–C₂H₂
Pb	217.0	0.1	0.01	0.1–30	Air–C₂H₂
Pd	247.6	0.05	0.01	0.1–15	Air–C₂H₂
Pr	495.1	20	8.0	100–5000	N₂O–C₂H₂
Pt	265.9	1.0	0.1	1–300	Air–C₂H₂
Rb	780.0	0.05	0.009	0.1–15	Air–C₂H₂
Re	346.0	8.0	0.8	10–2000	N₂O–C₂H₂
Rh	343.5	0.1	0.005	0.05–30	Air–C₂H₂
Ru	349.9	0.4	0.08	1–150	Air–C₂H₂
Sb	215.6	0.3	0.04	0.4–100	Air–C₂H₂
Sc	391.2	0.3	0.05	0.5–80	N₂O–C₂H₂
Se	196.0	1.0	0.5	5–250	N₂O–C₂H₂
Si	251.6	1.5	0.25	3–400	N₂O–C₂H₂
Sm	429.7	6.0	1.0	10–1500	N₂O–C₂H₂
Sn	235.5	0.7	0.1	1–200	N₂O–C₂H₂
Sr	460.7	0.04	0.002	0.02–190	N₂O–C₂H₂
Ta	271.5	10	2.0	20–3000	N₂O–C₂H₂
Tb	432.7	7.0	0.7	7–2000	N₂O–C₂H₂
Te	214.3	0.2	0.03	0.3–60	Air–C₂H₂
Ti	364.3	1.0	0.08	1–300	N₂O–C₂H₂
Tl	276.8	0.2	0.02	0.2–50	Air–C₂H₂
Tm	371.8	0.3	0.02	0.2–100	N₂O–C₂H₂
U	358.5	100	40	400–30 000	N₂O–C₂H₂
V	318.5	0.7	0.07	1–200	N₂O–C₂H₂
W	255.1	5.0	1.0	10–1500	N₂O–C₂H₂
Y	410.2	2.0	0.2	2–500	N₂O–C₂H₂
Yb	398.8	0.06	0.004	0.04–15	N₂O–C₂H₂
Zn	213.9	0.008	0.008	0.01–2	N₂O–C₂H₂
Zr	360.1	9.0	1.0	10–2000	Air–C₂H₂

^a Both the characteristic concentration and detection limit are quoted for the most sensitive line.
Data supplied by Varian Ltd, for the Spectra AA series of spectrometers.
Note: The characteristic concentration is the concentration corresponding to an absorbance of 0.004 4.

Typical detection limits (in μg/L) attainable with various techniques of atomic emission spectrometry

Element	Flame AES^a	Graphite Furnace AES^a		ICP-AES^b
Ag	20	0	.45	3
Al	10	1		1	.5
As	50 000			12
Au	500	160		20
B	30 000	200		1	.5
Ba	1	4		0	.07
Be	40 000	460		0	.2
Bi	40 000	30		12
Ca	0.1			0	.03
Cd	2 000	50		1	.5
Co	50	10		5
Cr	5	1		4
Cs	8	18		3200
Cu	10	2		2
Fe	50	7		1	.5
In	5	0	.65	18
Ir	1 00 000	860		3	.5
K	3	0	.0015	10
Li	0.03	0	.07	0	.6
Mg	5	1		0	.1
Mn	5	1	.5	0	.3
Mo	100	16		4
Na	0.1	0	.0025	1
Ni	30	15		5	.5
P				18
Pb	200	27		14
Pd	50	60		7
Rb	0.3	0	.1	3
Si	5 000	90		5
Sn	300	15		15
Sr	0.1	1		7
Ta	18 000			9
Ti	200	17		0	.6
Tl	20	1		27
U	10 000	2500		18
V	10	9		2
W	500			17
Zn	50 000	1500		0	.9
Zr	3 000			1	.5

^a Reproduced with permission from B. Welz (1985) Atomic Absorption Spectrometry, VCH. ^b Reproduced with permission from Varian Ltd. Data for Liberty series of spectrometers.

Some detection limits in simultaneous multi-element fluorescence

Element	nm	Detection limit ppm		Flame

Aluminium	396.1		0.3	N₂O–C₂H₂
Silver	328.0		0.07	Air–C₂H₂
Calcium	422.7		0.003	Air–C₂H₂
Cadmium	228.8		0.03	Air–C₂H₂
Cobalt	240.7		0.06	Air–C₂H₂
Chromium	357.6		0.02	Air–C₂H₂
Copper	324.8		0.04	Air–C₂H₂
Iron	248.3		0.03	Air–C₂H₂
Magnesium	285.2		0.005	Air–C₂H₂
Manganese	279.5		0.003	Air–C₂H₂
Molybdenum	312.6		0.2	Air–C₂H₂
			1.0	N₂O–C₂H₂
			0.1	Argon-separated N₂O–C₂H₂
Nickel	232.0		0.08	Air–C₂H₂
Lead	405.8		0.07	Argon-separated N₂O–C₂H₂
Antimony	217.6		0.1	Air–C₂H₂
		(in MIBK)
Selenium	204.0		1.5	Air–C₂H₂
Zinc	213.9		0.07	Air–C₂H₂

Ref: W J Price (1979) Spectrochemical analysis by Atomic Absorption, Wiley.

S.J.Hill

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