Elemental Analysis at the Atomic Level
Atomic spectroscopy encompasses techniques that measure the absorption, emission, or mass of individual atoms to determine the elemental composition of a sample. Unlike molecular spectroscopy, which probes bonds and functional groups, atomic spectroscopy focuses exclusively on the identity and concentration of elements. It is the primary tool for measuring trace metals in environmental samples, biological fluids, foods, and industrial materials, with detection limits that can reach parts per trillion.
The three pillars of atomic spectroscopy are atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), and mass spectrometry of atomized samples (ICP-MS). Each converts the sample into free atoms or ions, typically using a flame, furnace, or plasma, and then measures a property of those atoms to determine concentration.
Atomic Absorption Spectroscopy
AAS measures the absorption of light by ground-state atoms in the gas phase. A liquid sample is aspirated into a flame (air-acetylene at about 2300 degrees Celsius, or nitrous oxide-acetylene at about 2900 degrees Celsius), where the solvent evaporates and the analyte is atomized. A hollow cathode lamp, which contains the element of interest as the cathode material, emits narrow spectral lines characteristic of that element. Ground-state atoms in the flame absorb these specific wavelengths, and the decrease in light intensity at the detector is proportional to the concentration of the element in the sample, following the Beer-Lambert law.
Graphite furnace AAS (GFAAS), also called electrothermal AAS, replaces the flame with a small graphite tube that is electrically heated in three stages: drying (110 degrees Celsius), ashing (400-1400 degrees Celsius to destroy the matrix), and atomization (1800-2800 degrees Celsius). GFAAS achieves detection limits 100-1000 times lower than flame AAS because the atoms are confined in the small tube for a longer time, increasing the absorption signal. Detection limits for lead by GFAAS are typically around 0.05 micrograms per liter (50 parts per trillion).
AAS is inherently a single-element technique: the hollow cathode lamp emits lines of only one element, so separate lamps are needed for each element. While multi-element lamps exist, AAS is best suited for targeted analysis of a few specific elements rather than broad elemental surveys.
Atomic Emission Spectroscopy and ICP-OES
In atomic emission spectroscopy, atoms are excited to higher energy states by a high-temperature source, and the light emitted as they relax is measured. The inductively coupled plasma (ICP) is the most powerful excitation source, reaching temperatures of 6000-10,000 K. A stream of argon gas is ionized by a radiofrequency field (typically 27.12 MHz at 1-2 kW), creating a stable, annular plasma into which the sample aerosol is injected.
ICP optical emission spectroscopy (ICP-OES), also called ICP-AES, disperses the emitted light with a grating spectrometer and measures the intensities of characteristic emission lines for each element. Because the plasma excites virtually all elements simultaneously, ICP-OES is inherently a multi-element technique, capable of determining 20-70 elements in a single measurement. Detection limits are typically 1-100 micrograms per liter for most elements, with linear dynamic ranges spanning five to six orders of magnitude.
ICP-OES is the workhorse of environmental, agricultural, and industrial elemental analysis. Water treatment plants use it to monitor heavy metals (arsenic, lead, cadmium, chromium, mercury) against regulatory limits. Geochemists use it to determine major and trace elements in rocks and minerals. The food industry uses it to verify nutritional labeling (calcium, iron, zinc) and screen for contaminants.
ICP Mass Spectrometry
ICP-MS combines the plasma atomization and ionization source of ICP-OES with a mass spectrometer detector, achieving detection limits 100-1000 times lower. The plasma ionizes the sample atoms, and the resulting ions are extracted through a sampling cone into a mass analyzer (quadrupole, time-of-flight, or sector field). Because each isotope of each element has a unique mass-to-charge ratio, ICP-MS provides both elemental and isotopic information.
Detection limits for most elements by ICP-MS are in the range of 0.001-0.1 micrograms per liter (1-100 parts per trillion). For some elements, such as uranium and thorium, detection limits reach below one part per quadrillion. This extraordinary sensitivity makes ICP-MS the technique of choice for ultra-trace analysis in semiconductor manufacturing (where metallic contamination at parts-per-trillion levels can ruin microchips), nuclear forensics, and clinical toxicology.
Isotope ratio measurements by ICP-MS are used in geochemistry (uranium-lead dating), food authentication (geographic origin of wines and olive oils via strontium isotope ratios), and anti-doping analysis (detecting synthetic testosterone by carbon isotope ratios).
Sample Preparation
All atomic spectroscopy techniques require the sample to be in solution (or a gas). Solid samples must be dissolved, typically by acid digestion. Open-vessel digestion with nitric acid and hydrochloric acid (aqua regia) is suitable for many metals. Microwave-assisted digestion in closed vessels with HNO3, HCl, HF (for silicates), or H2O2 achieves complete dissolution more rapidly and with less risk of contamination or analyte loss.
Spectral and Matrix Interferences
Spectral interferences occur when emission lines of different elements overlap (in ICP-OES) or when polyatomic ions have the same nominal mass as the analyte (in ICP-MS). For example, ArO+ at m/z 56 interferes with the major isotope of iron (56Fe). Collision/reaction cells in modern ICP-MS instruments remove these interferences by reacting with the interfering polyatomic species using hydrogen or helium gas.
Matrix interferences arise when the physical properties of the sample solution (viscosity, surface tension, dissolved solids) differ from those of the calibration standards, affecting nebulization efficiency and transport to the plasma. Internal standardization (adding a known concentration of a non-analyte element such as yttrium or indium to all samples and standards) corrects for these effects.