Advanced Methods for Trace Element and Heavy Metal Analysis
Cold Vapor Atomic Absorption Spectroscopy (CV-AAS)
Cold Vapor AAS (CV-AAS) is a specialized atomic absorption technique used only for mercury, because elemental mercury possesses a relatively high vapor pressure at room temperature. First, the mercury present in the sample is converted to Hg2+ using an oxidizing agent such as nitric or sulfuric acid. Then, Hg2+ is reduced to elemental mercury, Hg0, using SnCl2:
Hg2+ → Hg0
Since Hg0 is volatile, it passes into the gas phase and is swept by an inert gas into a quartz absorption cell. No flame is required for this process. Atomic absorption is measured at approximately 253.7 nm. The main advantage of this method is the low detection limit, typically in the parts per billion (ppb or μg/L) range.
Hydride Generation AAS (HG-AAS)
Hydride Generation AAS (HG-AAS) is utilized for elements capable of forming volatile hydrides, such as As, Bi, Sb, Se, Pb, and Sn. The sample is acidified, and sodium borohydride (NaBH4) is added to produce a volatile gaseous hydride. The hydride is then transported by an inert carrier gas, such as N2, to an atomization source—for example, a flame or a heated quartz tube. There, the hydride is decomposed into free atoms, and atomic absorption is measured. An important advantage is that the analyte is separated from the sample matrix before atomization, strongly reducing matrix interferences and improving detection limits from the μg/L to the ng/L range.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
The main difference between ICP-MS and techniques like CV-AAS or HG-AAS is that the latter are element-specific, whereas ICP-MS is a multi-element technique. In ICP-MS, the sample is introduced into an argon ICP, where the analytes are atomized and ionized. The positive ions are then transferred through the sampling and skimmer cones into the mass spectrometer, separated according to their mass-to-charge ratio (m/z), usually by a quadrupole, and finally detected. ICP-MS can analyze more than 90% of the elements in the periodic table, features a very large dynamic range, and offers very low detection limits. Furthermore, it can provide isotopic information, which AAS cannot. While its mass spectra are generally simpler than ICP-OES emission spectra, ICP-MS is much more expensive and complex. It can also suffer from isobaric and polyatomic spectral interferences; therefore, collision/reaction cells may be required.
Flame Atomic Absorption Spectroscopy (FAAS)
FAAS is an analytical technique used for the quantitative determination of elements based on the absorption of characteristic electromagnetic radiation by free ground-state atoms. The liquid sample is first aspirated into a nebulizer, where it is converted into a fine aerosol. Large droplets are removed in the spray chamber and sent to the drain, while the smaller droplets are transported to the flame. In the flame, the sample undergoes several steps: desolvation (solvent removal), vaporization (conversion to gaseous species), and atomization (dissociation into free atoms).
A hollow cathode lamp (HCL) specific to the element of interest emits narrow characteristic wavelengths through the atomic vapor in the flame. The ground-state atoms of the analyte absorb part of this radiation at their characteristic wavelength. The more analyte atoms present in the flame, the greater the absorption. After passing through the flame, the radiation enters a monochromator, which selects the analytical wavelength and helps remove interfering spectral lines. The selected radiation then reaches the detector, which measures the decrease in intensity. The absorbance is calculated from the incident and transmitted radiation using the formula:
A = log(P0 / P)
Within the linear range, the absorbance is proportional to the analyte concentration according to the Beer-Lambert relationship. FAAS is fast, relatively simple, and inexpensive; however, it requires relatively large sample volumes and has lower sensitivity than graphite furnace AAS because only a small fraction of the sample reaches the flame and the atoms have a short residence time in the optical path.
Inductively Coupled Plasma Optical Emission Spectrometry
ICP-OES, also called ICP-AES, is a multi-element analytical technique based on the emission of characteristic radiation by excited atoms and ions. A liquid sample is first converted into a fine aerosol by a nebulizer and transported by argon into the ICP torch. The ICP consists of three concentric quartz tubes surrounded by a radio-frequency (RF) induction coil. Argon is used as the plasma gas because it is chemically inert. The plasma is initially created by a Tesla spark, which produces Ar ions and electrons. An alternating magnetic field generated by the RF coil causes these charged species to move. Their resistance to this movement produces Ohmic heating, resulting in very high temperatures (typically 6000–8000 K), and further ionization sustains the plasma.
When the sample enters the plasma, it undergoes desolvation, vaporization, atomization, ionization, and excitation. The excited atoms and ions subsequently return to lower electronic energy levels and emit characteristic radiation:
M* → M + hν
Each element emits light at characteristic wavelengths used for qualitative identification, while the intensity of the emitted radiation is proportional to the concentration of the element. A spectrometer separates the different emission wavelengths, allowing sequential or simultaneous determination of many elements. ICP-OES can simultaneously measure a large number of elements and has a wide linear concentration range. It is particularly suitable for heavy-metal analysis because the high plasma temperature provides efficient atomization and excitation, even for difficult samples, and strongly reduces chemical interferences. As a multi-element technique, several heavy metals such as Pb, Cd, Cu, Cr, Ni, or Zn can be measured in the same analysis. It also offers good accuracy, reproducibility, and detection limits typically in the µg/L range. Its main disadvantages are spectral interferences due to the large number of emission lines and high operating costs, primarily due to argon consumption.
Comparing ICP-OES with Other Analytical Techniques
ICP-OES vs. ICP-MS
Compared with ICP-MS, ICP-MS provides much lower detection limits (typically around 0.1–10 ppb), higher selectivity, and can provide isotopic information because ions are separated according to their m/z ratio. However, ICP-MS is considerably more expensive and complex, and it can suffer from isobaric and polyatomic spectral interferences, sometimes requiring a collision/reaction cell. Therefore, if ultra-trace concentrations or isotope measurements are not required, ICP-OES can be a more practical and cost-effective choice for routine multi-element heavy-metal analysis.
ICP-OES vs. FAAS
Compared with FAAS, FAAS is simpler, faster, and cheaper, but it has a higher detection limit and is mainly a single-element technique because a specific hollow-cathode lamp is normally required for each element. In addition, only a small fraction of the introduced sample reaches the flame and the atoms have a short residence time, which reduces sensitivity. Therefore, FAAS is useful for relatively high concentrations or the analysis of only one or a few elements, whereas ICP-OES is much more efficient when many heavy metals must be determined simultaneously.
ICP-OES vs. GFAAS
Compared with GFAAS, graphite furnace AAS has much better sensitivity than FAAS and can reach very low detection limits using only a very small amount of sample. However, it is slow (approximately 100 seconds per sample), suffers from more matrix interferences, and is normally used for one element at a time. Therefore, although GFAAS may be preferable for the ultra-trace determination of a single heavy metal, ICP-OES is more suitable for routine analysis when several heavy metals must be measured simultaneously over a wide concentration range.