Optical Atomic Absorption/Atomic Emission Spectrometry

Portions of Chapter 8-10.

In atomic spectroscopy, samples are vaporized at high temperatures (2000 – 8000K) and decomposed into atoms. The vapor phase atomic concentration is measure by absorption (Chapter 9) or emission (Chapter 10) of characteristic wavelengths of UV or visible radiation. These techniques exhibit high sensitivity and selectivity.

Atoms are produced by aspirating a sample with a nebulizer into a burner which desolvates, volatilizes, and breaks chemical bonds causing atomization.

The atoms in the flame (or plasma) are then detected by absorption of external radiant energy, or emission from the atoms themselves.

Atomic fluorescence will not be discussed, first will be atomic absorption, then atomic emission.

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Atomic line widths – narrow but not infinitely so as one would surmise from an atomic energy level diagram. (Important in atomic absorption instrumentation.)

Sources of atomic line broadening:Uncertainty Principle – “natural” line widths ~10-5 nm.Doppler Broadening – most relevant for this discussion.

Δλ/λo = ν/cVelocity v increases with Temperature (K) to the 0.5 power. The hotter the medium, the broader the greater the Doppler broadening.

Pressure Broadening – important at high pressures (i.e. not important in this context)

Atomic Absorption (Ch 9)Atomic absorption (AA) instruments routinely but not exclusively use flame atomization. A few things about flames which provide the ground state atoms used for AA.

Flashback when gas flow rate < burning velocity

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Higher temperatures required for harder to decompose samples. Interzonal region is free atom rich

Where in the interzonal region to perform AA, as well as the preferred type of flame (fuel “rich” or fuel “lean”) depends on the element. Below is the flame absorption profile for Ag (hard to form oxides), Cr (forms very stable oxides), and Mg (intermediate).

The position of the flame is very important for optimization.

Atomic absorption is almost exclusively used for quantitative analysis of elements in a sample using Beer’s Law.

Remember:1. Beer’s Law requires monochromatic radiation to irradiate the

sample. The spectral slit width must be < 10% of the absorption bandwidth.

2. Atomic absorption bands are extremely narrow.

Since the monochromator bandwidth >> absorption line width, a continuous source cannot be used for quantitative analysis for AA.

A line source narrower than the absorption line width is required.

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The most common AA source is a hollow cathode lamp. The cathode iscoated with the element to be analyzed. Ionization of the inert gas from a potential (~300 V) between cathode and anode causes the ionized gas to slam into the

cathode which sputters the cathode coating (same element as analyte) into the vapor phase. The excited state atoms in the source irradiate light of the same energy as the analyte. The bandwidth is narrower than the natural line width because the source is at room temperature and the atoms are at 2000+K in the flame, so Doppler broadening is much less in the source.

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AA/quantitative analysisIn theory, a calibration curve will follow Beer’s Law. There are many cases in which, and many reasons for, non-linearity. Section 9C discusses many, the data below shows various Sr calibration lines in different flames and in different matrices.

The details are unimportant to us here, but linearity cannot be assumed.

Standard addition calibration and other methods are used to defeat matrix effects common in AA.

AA’s place in analytical chemistry: Quantitative analysis of ~70 elements (1 at a time, metals and

metalloids only) with sub ppm detection limits Sample preparation is the most difficult and time consuming. Most

samples must be converted to homogeneous solutions (digestion procedures) prior to analysis

Linear range: 2-3 orders of magnitude No information on chemical form of element

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For atomic absorption read chapters 8 & 9 with particular attention to: Flame atomization in 9A Radiation sources and hollow cathode lamps in 9B AA Analytical techniques in 9D

For topics which are important for both AA and atomic emission (next thing)

Optical atomic spectra 8A Nebulizers (without the details) 8C-1

A related technique with the same information content as AA and used for the same purposes, is atomic emission. This is discussed in Chapter 10.

The most common method is ICP-AES. (Inductively Coupled Plasma Atomic Emission Spectrometry). With this method it is possible to perform simultaneous quantitative analysis of >70 elements with lower detection limits than AA.

For atomic emission to work, excited state atoms are required to emit light at characteristic wavelengths of that element.

This discussion requires use of the Boltzmann distribution discussed in Chapter 8 – the effect of temperature on atomic spectra.

Using our standby Na atom

The energy difference between the 3p and 3s atomic orbitals is about 2.1 eV. To do emission spectroscopy enough electrons must exist in the

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3p excited state atomic orbital of Na to emit light that can be detected (~589 nm, equivalent to 2.1 eV).

With, for example, an air/acetylene flame temperature of 2500K, using the Boltzmann function the fraction of excited state Na atoms can be calculated at this temperature.

First calculate ΔE, then put that into the Boltzmann equation.

What if you wanted to do flame emission of Mg?

In this case the 3p 3s transition corresponds to light emitted at 285.2 nm.

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Doing this the same way as before, this light has an energy of 6.94 x 10-

19 J.

Plugging this into the Boltzmann equation, as before..

Need a hotter flame. Can’t be done with fire (combustion), need a plasma.

The physics of the ICP is not important to us, what is important is its usefulness for AES.

In a 6000K plasma, the fraction of Mg atoms with electrons in a 3p atomic orbital excited state can be calculated with the Boltzmann equation.

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So a very hot “flame” or plasma is needed for atomic excitation. A second absolute requirement is a stable flame temperature.

Remember: both AA and AE are used primarily for quantitative analysis. But the signal generated and detected from these 2 techniques, absorption versus emission, is fundamentally different.

You have seen this already:

The signal intensity is directly proportional to the concentration of excited state analyte.

Earlier the %Na* in a 2500 K flame was found to be 0.0172%.Now calculate the %Na* in a 2510 K flame.

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A 10K change in flame temperature results in a 4% change in Na* concentration, which will result in a 4% change in the analytical signal output for an AE measurement.

Whether it be 2500 or 2510 K however, there is still about 99.98% of Na in the ground state. Thus the AA measurement is unaffected.

This brings up a second fundamental difference between any absorption versus emission measurement. All else being equal, an emission measurement will exhibit much lower (i.e. better) detection limits than an absorption measurement.

Consider the following scenario analogous to an absorption measurement where T = P/Po; Abs = -logT50,000 people in a stadium each with a flashlight on. Po = 50,000500 people turn off the flashlight. P = 49,500

Consider the competing scenario analogous to an emission measurement50,000 people in a stadium all with flashlights off.500 people turn on their flashlight.

A general rule: It is easier to detect a small signal in the absence of background (emission), than a small change in signal in the presence of a large background signal (absorption). This is why emission techniques are more sensitive than absorption techniques.

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The 2nd advantage of ICP-AES over AA is the ability to perform simultaneous quantitative analysis of many elements using a multichannel spectrometer since the source is the analyte.

In general, ICP-AES and AA provide the same information. ICP-AES is superior with respect to detection limits (lower), linear range (larger), and matrix effects (fewer). ICP-AES also affords the possibility of simultaneous multi-element analysis. Both methods suffer from sample

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preparation. Not surprisingly, an ICP-AES spectrometer is much more expensive to buy, but also to maintain.

Questions/problemsCh. 8: 1,4,6,9Ch. 9: 1d,f,g,l, 2,9,12,13a,14,20Ch.10: 1,2,9

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