11 Atomic Spectroscopy

The chemical information obtained from atomic spectroscopy is elemental composition. Instrument designs for atomic spectroscopy include an atomizer, wavelength selector, photodetector, and, for absorption spectroscopy, a light source.

Atomization

An atomizer is required to produce an atomic population from a molecular sample. Classically, the energy (i.e. heat) source has been a flame into which a liquid sample is introduced as an aerosol via a component called a nebulizer; however, high-temperature ovens called electrothermal atomizers (ETAs; useful with both liquid and solid samples) and plasmas are now also used to create atomic populations.

Atomic Absorption Spectroscopy (AAS)

Flames and ETAs are conventionally used for atomization in AAS. Flame temperature is optimized to minimize the formation of relatively stable small molecules (e.g. oxides) and favour ground-state atoms.

A technical challenge for AAS is the narrow width of atomic absorption lines (< 0.01 nm). With broadband light sources, typical monochromator bandwidths are > 0.1 nm and thus have output that is too spectrally broad for good signal-to-noise ratios in AAS (only a small fraction of the total light is resonant with the atomic transition). Instead, the light for AAS is produced using hollow cathode lamps (HCLs) or electrodeless discharge lamps (EDLs) that are based on the element of interest. For example, a sodium-based HCL or EDL is used to measure atomic sodium in a sample. HCLs and EDLs use different mechanisms to create excited-state atoms, but both use the light from emissive transitions within the lamp to achieve the resonance and narrow spectral bandwidth required for absorption measurements. The measurement of different elements thus requires lamps based on those elements (e.g. Fe-based HCL to measure Fe, Zn-based EDL to measure Zn). AAS instruments often have turrets that house multiple lamps and facilitate switching between lamps.

Beyond the atomizer and HCL or EDL lamps, a conventional AAS instrument resembles a UV-visible spectrophotometer. The light transmitted through an atomized sample is directed to a monochromator (to select the desired line from the HCL/EDL) and onto a photodetector such as a PMT. If a flame atomizer is used, the lamp is modulated on/off to distinguish between light from the flame and light from the lamp. Various technical approaches are also used to correct for scattering by small carbonaceous particles (e.g. soot) in the flame. Chemistry-based methods are used to help reduce the formation of elemental oxides and reduce ionization in the flame.

Recent advances in monochromator and detector design have enabled the use of broadband (a.k.a. continuum) light sources in AAS with either flame or electrothermal atomization, but these instruments are not yet the norm.

Atomic Emission Spectroscopy (AES)

AES, which is also called optical emission spectroscopy (OES), requires an excited-state atomic population (without significant ionization, as ions have different energy levels and resonances than atoms). Although flames reach high enough temperatures to induce atomic emission from some elements, inductively-coupled plasma (ICP; a.k.a. RF plasma) torches and microwave plasma (MP) torches are much hotter and induce atomic emission from a much wider array of elements.

Plasma torches may be coupled with a liquid sample nebulizer (where the plasma both atomizes and excites the sample) or an ETA. In some AES instrument designs, the desired atomic emission line is selected by a monochromator and the intensity is measured via a photodetector. Spectra are measured by scanning the monochromator. In other instrument designs, combinations of gratings, prisms, and array detectors (e.g. CCD) enable the simultaneous detection of multiple emission lines or the rapid acquisition of full atomic emission spectra.

Laser-Induced Breakdown Spectroscopy (LIBS)

LIBS is a form of AES that uses pulses from a high-power laser to ablate a sample, generating a plume of picograms to nanograms of sample material and a transient micro-plasma at the ablation site, yielding an excited-state atomic population. An optical fiber system collects the atomic emission from the ablation site and is coupled to a spectrometer to acquire emission spectra. A key advantage of LIBS is that no sample preparation is required.

AAS or AES?

Overall, the choice between AAS and AES is determined by the requirements of the application. AAS is lower cost to purchase and operate, and, for some elements, can offer parts-per-billion (flame) and parts-per-trillion (electrothermal) detection limits. Flame AAS also offers the best precision. ICP-AES is more expensive, but can detect a larger number of elements and offers parts-per-billion detection limits for many elements with faster measurement times, larger dynamic range, and lower sample consumption. The nature of the samples (e.g. physical state, potential interferences, quantity) may also be a factor in choosing between AAS and AES.

Connections

• Instruments for AAS, AES, and LIBS have designs that are similar to those used for molecular spectroscopy (Ch. 7, Ch. 9), including some of the same components (e.g. slits, gratings, PMTs), where the atomizer for atomic spectroscopy replaces the sample cell for molecular spectroscopy.
• As with molecular spectroscopies (Ch. 7, Ch. 9), atomic spectroscopy measurements can be made at discrete wavelengths for known analytes or spectra acquired for surveying what analytes might be present.
• Calibration curves are used for quantitation, similar to molecular spectroscopy (Ch. 6, Ch. 9).

Post-Reading Questions

1. Why is a flame suitable for AAS but not preferred for AES?
2. A certain monochromator has a minimum bandwidth of 0.2 nm. An absorption line of interest has a width of 0.004 nm. To a first approximation, what is the percentage of output light from the monochromator that would be absorbed?
3. Do you expect that light from a flame would cause an atomic absorbance reading to be incorrectly high or incorrectly low? What about the effect of soot?
4. Speculate as to why the terminology is “atomic emission spectroscopy” instead of “atomic fluorescence spectroscopy.” (Hint: Consider the origin of the excitation energy.)
5. A company requires elemental analyses of samples on a routine basis, but only ever for the same three metal elements. Trace detection is not very important and the sample sizes are large. Would you recommend purchase of an AAS or an AES instrument? Why?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Draw and label block diagrams for AAS and AES instruments.
• Understand the design and operational principles of atomizers.
• Name the approaches for optimizing flame atomization and explain the associated chemistry.
• Given an analysis scenario, propose a rationale for selecting AAS, AES, or LIBS as the recommended method.
• Match an atomizer to an analysis scenario.
• Summarize chemical approaches to combating interferences in atomic spectroscopy.