Lecture 7: Atomic Spectroscopy

Transcription

1 Lecture 7: Atomic Spectroscopy 1

2 Atomic spectroscopy The wavelengths of absorbance and emission from atoms in the gas phase are characteristic of atomic orbitals. 2

3 In the lowest energy transition, the spin of the electron is the same and it takes nm light to make the transition. It takes slightly more energy (λ = nm) to flip the spin in the excited state. 3

4 Atomic spectroscopy Atomic spectrometry is important in: Metallurgy Environmental science (Hg, Pb, Cu, Ag ) Clinical chemistry Atomic spectroscopy has excellent detection limit! Detection limits tend to be in the ppb range for most metals. What does this really mean?? What is the corresponding molar concentration? 4

5 How can we quantify atomic samples in the gas phase??? Gas sample???? 5

6 Atomic absorbance spectroscopy 6

7 Atomic absorbance Beer s law holds for atomic absorption measurements. The cuvette is usually a flame or oven. Light source: hollow-cathode lamp. Stray light: which must be dealt with. 7

8 Light source: hollow cathode lamp In liquids, spectral bandwidth is huge (~100 nm) because the electronic, vibrational, and rotational states combine. In atoms in the gas phase, the bandwidth is much narrower, nm, which no monochromator can resolve. Instead, we use a hollow cathode lamp, which produces light with a bandwidth of approaching nm. 8

9 Atomization: Nebulization The sample is converted to gas-phase atoms or ions for analysis. This process can occur in: inductively coupled plasma (ICP) furnace flame In ICP and flame atomic absorbance, a liquid sample is aspirated as a fine mist (aerosol) into the hot gas. Argon to torch capillary to waste 9

10 Atomization To convert the nebulized liquid to gas-phase atoms: 1. Desolvation: 2. Sublimation: 3. Atomization 4. Excitation 5. Ionization 6. Oxide formation NaCl (Aq) NaCl(s) NaCl (s) NaCl (g) NaCl (g) Na(g) + Cl(g) Na(g) Na*(g) Na(g) Na+ (g) Na(g) + O(g) NaO(g) 10

11 Atomization: the flame The flame is the cuvette in atomic absorbance experiments. The flame must convert the solution phase analyte into gaseous atoms. Temperature should be high but not too high! o Air-acetylene o Nitrous oxide (N 2 O)-acetylene o Oxygen-acetylene 2600 K 3000 K 3300 K 11

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13 A typical temperature program: Atomization: furnace 1. A low temperature is applied to the sample to remove solvent (~150 o C). 2. A higher temperature (~1000 o C) is applied to burn off organic matter that otherwise could produce smoke, interfering with the absorbance determination. 3. The temperature is ramped to 3000 o C to atomize the sample. Advantage - much more sensitive for some elements. Disadvantage - slow, limited linear dynamic range. 13

14 Atomic Emission In atomic emission spectroscopy, we measure the emission of light from atoms. The intensity is proportional to the concentration of atoms in the particular excited state. sdmiramar 14

15 Excitation The population in an excited level is given by the Boltzmann distribution. Example: What fraction of sodium atoms are excited at T = K? (λ = 589 nm; ΔE = J) N N 0 E kt 1.379x10 J 2e 2 e 1 19 J What fraction of sodium atoms are excited at T = K? N N 0 E kt e 2 e J J

16 Excitation source: inductively coupled plasma (ICP) The temperature of the plasma is ~10,000 K. 16

17 ICP-MS In 1980, Fassel and Houk developed ICP mass spectrometry as an analytical tool. It has four important attributes: It is extremely sensitive (parts-per-trillion). It has a huge dynamic range (6 orders of magnitude). It is able to analyze most of the periodic table, and it provides isotopic information. From Perkin Elmer web site 17

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19 ICP-MS ~1-mm a few torr 10-5 torr 19

20 Isobaric & Polyatomic Interferences Different elements can have isotopes with very similar masses. 113 In + and 113 Cd + have essentially the same masses, as do 115 In + and 115 Sn +. As a result, analysis of indium in the presence of cadmium and tin is difficult. More problematic are elements with major isotopes near argon. The major calcium isotope is 40 Ca, which accounts for 97% of the isotopic abundance. It suffers serious interference from 40 Ar, which is used to form the plasma. Instead, the less abundant 44 Ca + ion (2%) is used to analyze calcium. Although the plasma is at K, it cools upon entering the first vacuum chamber, and polyatomic ions can be formed, including: 40 ArO +, which overlaps with 56 Fe + ; 16 O 2+, which overlaps with 32 S + ; and 40 Ar 2+, which overlaps with 80 Se +. 20