Navigating Interferences in AAS and AES for Accurate Results

Getting accurate results from atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES) is not always straightforward. Various interferences – spectral, chemical, and physical – can distort your measurements and lead to incorrect data. Whether you are using flame AAS (FAAS), graphite furnace AAS (GFAAS), or inductively coupled plasma AES (ICP-AES), understanding these interferences and knowing how to manage them is essential for reliable environmental monitoring and trace metal analysis.

What are interferences in AAS and AES? 🔗

In atomic spectrometry, an interference is any phenomenon that causes the analytical signal to deviate from what it should be for a given analyte concentration. Interferences lead to systematic errors by either enhancing or suppressing the signal, and they fall into three broad categories: spectral, chemical, and physical. Each category operates through a different mechanism and requires its own set of correction strategies. Recognising which type of interference is at play is the first step toward producing trustworthy analytical data.

Types of interferences 🔗

Spectral interferences 🔗

Spectral interferences occur when radiation from a source other than the target analyte reaches the detector and is incorrectly counted as part of the analyte signal. This can happen when the absorption or emission line of a different element overlaps with that of the analyte. For instance, vanadium has an absorption line at 308.211 nm, which sits extremely close to an aluminium line at 308.215 nm. In practice, though, direct atomic line overlap is rare in AAS because the absorption lines are inherently very narrow.

A more common form of spectral interference is background absorption. This happens when molecular species or particulate matter in the flame or furnace scatter or absorb the source radiation across a broad wavelength range. Incomplete combustion of organic materials, formation of refractory oxides, and the presence of unvaporised solvent droplets all contribute to elevated background signals. Background absorption becomes particularly problematic at wavelengths below 300 nm, where light scattering intensifies.

Modern instruments address spectral interferences through several background correction methods. Deuterium (D₂) lamp correction uses a continuum light source alongside the hollow cathode lamp. Since the D₂ lamp emits broad-spectrum radiation, only the background absorbs its light, while both the analyte and background absorb from the hollow cathode lamp. Subtracting one from the other isolates the true analyte signal. Zeeman background correction applies a magnetic field to split the absorption line into polarised components, allowing precise separation of analyte and background absorbance. A third option, Smith-Hieftje correction, pulses the hollow cathode lamp at alternating high and low currents – at high current, self-absorption broadens the emission line so only background is measured, while at low current both analyte and background signals are captured.

Chemical interferences 🔗

Chemical interferences arise when matrix components react with the analyte and alter its atomisation behaviour. If the analyte forms stable, refractory compounds in the flame or furnace, fewer free atoms are produced, and the absorption signal drops. A classic example is the suppression of the calcium signal in the presence of phosphate or sulphate ions, which form thermally stable calcium phosphate or calcium sulphate compounds that resist decomposition at typical flame temperatures.

Several strategies counter chemical interferences. Releasing agents, such as lanthanum or strontium, are added to preferentially bind with the interfering anion, freeing the analyte for atomisation. Protective chelating agents like EDTA can wrap around the analyte atom, shielding it from reacting with matrix components. Another direct approach is simply increasing the atomisation temperature – switching from an air-acetylene flame (approximately 2300 °C) to a hotter nitrous oxide-acetylene flame (approximately 2700 °C) – to break apart refractory compounds more effectively.

Physical interferences 🔗

Physical interferences result from differences in the physical properties of the sample compared to the calibration standards. Variations in viscosity, surface tension, density, and dissolved solids content affect how efficiently the sample is nebulised and transported into the flame or plasma. A highly viscous sample, such as a digested soil extract with heavy dissolved salts, will nebulise less efficiently than a dilute aqueous standard, producing a weaker signal even if analyte concentrations are identical.

The most common remedies include matrix matching (preparing calibration standards in a matrix similar to the samples), dilution to reduce matrix effects, and the method of standard additions, which accounts for matrix-specific signal suppression or enhancement by spiking the analyte directly into the sample.

Ionisation interferences 🔗

Ionisation interference is a specific type of chemical interference that affects easily ionised elements. In a hot flame, alkali and alkaline earth metals like sodium, potassium, and barium can lose electrons, forming ions instead of neutral atoms. Since ions do not absorb at the same wavelengths as their neutral atoms, the measured signal decreases. This is especially common in nitrous oxide-acetylene flames, which are hot enough to ionise Group I and Group II elements significantly.

The fix is to add an ionisation suppressant – an element that ionises even more easily than the analyte, such as caesium or potassium chloride. The suppressant floods the flame with free electrons, pushing the equilibrium back toward neutral atom formation for the analyte.

Common interferences in FAAS and GFAAS 🔗

Flame AAS (FAAS) 🔗

FAAS is an open system where the sample is aspirated into a flame and atomised continuously. The main interference challenges in FAAS include ionisation effects (as described above), chemical interferences from refractory compound formation, and background absorption from molecular species in the flame.

Background absorption is particularly troublesome when analysing complex environmental matrices – water samples with high dissolved salt content, for instance. Molecular species such as NaCl can absorb radiation at specific wavelengths; a high concentration of sodium chloride absorbs at 213.9 nm, the primary resonance line for zinc, creating a direct interference in zinc determination. Switching to a hotter flame or selecting an alternative analytical line can help resolve this.

FAAS also faces transport interferences. Because only about 5% of the aspirated solution actually reaches the flame (the rest is drained from the spray chamber), any change in the aerosol characteristics caused by the sample matrix directly impacts signal intensity. Regular calibration and matrix matching are essential to maintain accuracy.

Graphite furnace AAS (GFAAS) 🔗

GFAAS operates in a confined, enclosed graphite tube, which gives it far superior sensitivity compared to FAAS – typically detecting analytes in the low micrograms-per-litre (ppb) range. However, this enclosed environment also amplifies matrix interferences. Matrix components cannot escape easily, leading to more severe chemical interactions during atomisation.

Matrix interferences in GFAAS can be quite challenging, often manifesting as poor recoveries and distorted peak shapes. Salt matrices, for example, can severely suppress absorption signals by shifting atomisation temperatures or forming thermally stable compounds that trap the analyte.

Memory effects present an additional challenge unique to GFAAS. Residues from previous samples can persist in the graphite tube and contaminate subsequent analyses, especially for elements like chromium and vanadium that form carbides with graphite.

The primary strategy for managing GFAAS interferences is chemical modification. Chemical modifiers alter the thermochemical behaviour of both the analyte and the matrix. Common modifiers include palladium nitrate, magnesium nitrate, and ammonium phosphate. Palladium-magnesium nitrate mixtures are considered nearly universal modifiers – they stabilise volatile analytes (like cadmium and lead) so that higher pyrolysis temperatures can be used during the ashing step, which removes more matrix components before atomisation occurs.

The stabilised temperature platform furnace (STPF) concept, developed by Walter Slavin based on Boris L’vov’s research, combines several strategies to make GFAAS nearly interference-free. This approach uses a L’vov platform inside the graphite tube to delay atomisation until the gas-phase temperature has stabilised, along with chemical modifiers and effective background correction. Additionally, the US EPA recommends specific modifier combinations – such as palladium with magnesium nitrate and a hydrogen-argon gas mixture – for regulated environmental methods involving GFAAS.

Minimising interference in ICP-AES 🔗

ICP-AES operates at plasma temperatures reaching 6000-10000 K – far hotter than any flame. This extreme heat eliminates most chemical interferences, since virtually all compounds are broken down into free atoms and ions. However, the high temperature also generates dense, complex emission spectra with numerous lines, making spectral interferences the dominant challenge in ICP-AES.

Spectral interference management 🔗

Because ICP-AES excites atoms to produce rich emission spectra, the risk of line overlap from different elements is much higher than in AAS. Spectral interferences include direct spectral overlap, wing overlap, and nearby emission lines that can complicate background correction. A direct overlap occurs when two emission lines are so close together that the spectrometer cannot resolve them separately.

The first line of defence is careful wavelength selection. Most elements have multiple emission lines, and choosing a line that is free from known interferences is often the simplest solution. Modern high-resolution ICP spectrometers can separate closely spaced lines, reducing spectral overlap significantly. When line overlap cannot be avoided, inter-element correction (IEC) factors are applied – a mathematical approach that subtracts the contribution of the interfering element from the analyte signal. This is widely accepted in regulated methods such as US EPA Method 200.7 and Method 6010D.

Background correction in ICP-AES 🔗

Background emission in ICP-AES comes from recombination continuum radiation, molecular band emission, and stray light within the spectrometer. Effective background correction is critical. Two-point background correction measures the background intensity on either side of the analytical emission line and interpolates to estimate the background at the analyte wavelength. This works well for smooth, slowly varying backgrounds.

For more complex backgrounds – common when analysing environmental samples with many matrix components – multi-point background modelling or full spectral fitting approaches provide better results. These methods use software algorithms to model the background structure and subtract it from the analyte signal more accurately. Modern advanced software algorithms can automatically detect and correct spectral interferences through spectral deconvolution and real-time correction during analysis.

Matrix matching and internal standards 🔗

Physical interferences in ICP-AES arise from differences in viscosity, density, or dissolved solids between samples and calibration standards. These differences affect nebulisation efficiency and aerosol transport, leading to signal suppression or enhancement.

Matrix matching is one of the most effective tools for minimising these effects. This involves preparing calibration standards in a solution that closely resembles the sample matrix. For example, when analysing seawater, standards should be prepared in synthetic seawater or with added NaCl at matching concentrations. While this requires extra effort during standard preparation, it substantially improves accuracy.

Internal standardisation is another powerful approach. An internal standard element – typically yttrium, scandium, or indium – is added at the same concentration to all samples and standards. Because the internal standard experiences the same matrix effects as the analyte, normalising the analyte signal against the internal standard signal compensates for variations in sample introduction and plasma conditions.

When the matrix is entirely unknown and cannot be replicated, the method of standard additions remains a reliable option. Known amounts of the analyte are spiked into aliquots of the sample, and the resulting calibration curve is used to determine the original analyte concentration while inherently accounting for matrix effects.

Optimising plasma conditions 🔗

Maintaining robust plasma conditions is fundamental to reducing interferences in ICP-AES. Proper optimisation of RF power, gas flow rates, and viewing configuration (axial vs. radial) all play a role. Radial viewing, for instance, is less susceptible to matrix effects than axial viewing, though it typically offers slightly lower sensitivity. A robust ICP-AES setup minimises matrix effects by ensuring that variations in sample composition do not significantly alter plasma energy transfer. Dual-view instruments, which can measure both axially and radially, offer flexibility to balance sensitivity and matrix tolerance depending on the analytical requirements.

Choosing the right correction strategy 🔗

No single correction method works for every situation. The best approach depends on the technique being used, the nature of the sample matrix, and the analyte of interest. For FAAS, ionisation suppressants and hotter flames solve many problems. For GFAAS, chemical modification combined with optimised temperature programming is the standard approach. For ICP-AES, high-resolution spectrometers, careful wavelength selection, background correction, and internal standardisation form the core toolkit.

In environmental monitoring – where samples range from clean rainwater to heavily contaminated industrial effluents – analysts often need to combine multiple strategies. Running interference check solutions, validating results with certified reference materials, and using the method of standard additions for particularly difficult matrices are all part of good analytical practice.

What do you think? When working with complex environmental samples, how do you decide which interference correction approach to prioritise – and have you ever encountered a matrix so challenging that standard correction methods fell short?

References

1. https://www.jove.com/science-education/v/14819/atomic-absorption-spectroscopy-interference
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