Atomic Absorption and Flame Emission Spectroscopy: Principles and Differences

Introduction to Optical Atomic Spectroscopy

In molecular spectroscopy, electronic transitions are accompanied by simultaneous vibrational and rotational transitions, resulting in broad absorption or emission bands. In optical atomic spectroscopy, however, the absorbing or emitting species are free, isolated gaseous atoms or elementary ions. Because isolated atoms possess no vibrational or rotational degrees of freedom, their electronic energy manifolds consist solely of discrete quantum states. Consequently, atomic spectra exhibit sharp, discrete line spectra rather than broad molecular envelopes.

The fundamental relationship governing any electronic transition between a lower state of energy E₀ and an upper state of energy E_j is Planck's postulate:

$$\Delta E = E_j - E_0 = h\nu = \frac{hc}{\lambda}$$

Where h is Planck's constant, c is the speed of light in vacuum, and λ is the resonance wavelength. In Atomic Absorption Spectroscopy (AAS), free ground-state atoms absorb resonant photons emitted by an external element-specific source. In Flame Emission Spectroscopy (FES), also referred to historically as flame photometry, the flame itself supplies thermal energy to excite atoms, and the radiant power emitted during spontaneous de-excitation back to the ground level is monitored. A solid foundation in fundamental electronic configurations and atomic energy levels is essential for interpreting the spectral terms involved in these transitions.

The Atomisation Pathway in Flames

Before an analyte element dissolved in an aqueous or organic solvent can absorb or emit optical radiation, it must be converted from solvated ionic or molecular complexes into free, gas-phase ground-state neutral atoms. This transformation is known as atomisation [परमाणुकरण]. The flame serves as the atomisation cell in both conventional flame AAS and FES.

The sample is introduced into the burner assembly through a pneumatic nebuliser as a liquid stream. The high-velocity oxidant gas shears the liquid into a fine mist or aerosol [फुहार]. Inside the spray chamber, flow spoiler baffles remove larger droplets, ensuring that only the finest aerosol droplets (typically less than 5 to 10 percent of the aspirated volume) enter the flame base. Once inside the flame, the analyte undergoes a rapid sequence of physical and chemical equilibria:

1. Desolvation: The high thermal energy of the flame rapidly evaporates the solvent, leaving behind a dry microcrystalline aerosol of solid analyte salts:

   $$M_{(aq)}^{z+} + z\,A_{(aq)}^{-} \xrightarrow{\text{evaporation}} MA_{z\,(s)}$$

2. Volatilisation: The solid micro-particles undergo vaporisation into gaseous molecules:

   $$MA_{z\,(s)} \xrightarrow{\text{heat}} MA_{z\,(g)}$$

3. Dissociation (Atomisation): At elevated flame temperatures, the gaseous molecules thermally decompose into neutral free gaseous atoms:

   $$MA_{z\,(g)} \xrightarrow{\Delta} M_{(g)}^{0} + z\,A_{(g)}^{0}$$

4. Excitation / Ionisation Equilibria: Depending on the element's first ionisation energy and the thermal regime of the flame, secondary reactions occur:

   $$M_{(g)}^{0} \xrightleftharpoons{\text{thermal } \Delta E} M_{(g)}^{*} \quad (\text{Thermal excitation for FES})$$

   $$M_{(g)}^{0} \xrightleftharpoons{\text{heat}} M_{(g)}^{+} + e^{-} \quad (\text{Unwanted ionisation loss})$$

   $$M_{(g)}^{0} + O_{(g)} \xrightleftharpoons{\text{equilibrium}} MO_{(g)} \quad (\text{Refractory oxide formation})$$

The chemical composition and temperature of the flame dictate atomisation efficiency. Common flame configurations include:

• Air – Acetylene (C₂H₂ / air): Operating at temperatures between 2100 °C and 2400 °C. This is the standard flame for easily atomised elements such as copper, lead, zinc, iron, calcium, and magnesium. By altering the fuel-to-oxidant ratio, an analyst can generate an oxidising (fuel-lean, blue) flame or a reducing (fuel-rich, luminous yellow) flame.
• Nitrous Oxide – Acetylene (C₂H₂ / N₂O): Operating at significantly higher temperatures between 2700 °C and 3000 °C. The high temperature and strongly reducing cyano- and carbonaceous radical environment are mandatory for dissociating refractory oxide-forming elements such as aluminium, silicon, titanium, vanadium, and molybdenum, which form metal-oxygen bonds that resist thermal cleavage in standard air-acetylene flames.
• Air – Propane or Air – Natural Gas: Lower-temperature flames (1700 °C to 1900 °C) utilized predominantly in simple flame photometers for determining alkali metals (sodium, potassium, lithium), where higher temperatures would cause extensive, undesirable ionisation.

The Hollow Cathode Source: Why Continuum Sources Fail in AAS

A fundamental milestone in the development of modern AAS was the realization by Alan Walsh in 1955 that conventional continuum sources (such as deuterium or xenon arc lamps) paired with standard prism or grating monochromators cannot provide sufficient sensitivity for atomic absorption.

As noted earlier, atomic absorption lines in a flame have a half-width of only 0.002 to 0.005 nm. High-quality grating monochromators generally provide a spectral bandpass between 0.1 and 0.5 nm—several hundred times wider than the atomic absorption profile. If a continuum source were employed, the atomic absorption would remove only an infinitesimal slice of the radiant energy passing through the exit slit. The detector would be flooded with unabsorbed radiation from adjacent wavelengths, resulting in an imperceptible drop in transmitted power, catastrophic signal-to-noise degradation, and severe non-linearity.

To overcome this limitation, AAS uses an element-specific Hollow Cathode Lamp (HCL) as a line source. An HCL emits atomic resonance lines that are narrower than the absorption profile of the analyte atoms in the flame, ensuring that the radiation passing through the flame matches the absorption frequency exactly.

Construction and Sputtering Mechanism of the HCL

An HCL consists of a sealed cylindrical glass tube fitted with a quartz front window (transparent to ultraviolet radiation). It houses two electrodes:

• An anode constructed from tungsten or nickel.
• A hollow cylindrical cathode whose interior cavity is machined from or lined with the ultra-pure target element being determined.
• An inert filler gas, typically neon or argon, filled at a low pressure of 1 to 5 torr.

When an operating potential of 300 to 500 V is applied across the electrodes, a glow discharge initiates. The step-by-step mechanism proceeds as follows:

1. Gas Ionisation: Electrons accelerate toward the anode, colliding with filler gas atoms to create positive inert gas ions:

   $$\text{Ne} + e^{-} \rightarrow \text{Ne}^{+} + 2e^{-}$$

2. Sputtering: The positive neon ions are accelerated through the potential gradient toward the negative potential of the hollow cathode. Striking the inner cathode surface with high kinetic energy, they physically dislodge neutral analyte metal atoms into the gas phase inside the cavity:

   $$M_{(\text{cathode surface})} + \text{Ne}_{(\text{fast})}^{+} \rightarrow M_{(g)}^{0} + \text{Ne}_{(\text{reflected})}$$

3. Excitation: Sputtered gaseous metal atoms undergo inelastic collisions with excited filler gas ions or energetic electrons, elevating the metal atoms to excited electronic states:

   $$M_{(g)}^{0} + \text{Ne}^{*} \rightarrow M_{(g)}^{*} + \text{Ne}$$

4. Line Emission: The excited metal atoms spontaneously relax back to the ground state, emitting narrow resonance radiation characteristic of the cathode metal:

   $$M_{(g)}^{*} \rightarrow M_{(g)}^{0} + h\nu_{\text{resonance}}$$

Because the internal pressure inside the sealed lamp is extremely low and the operating temperature inside the hollow cavity is kept moderate, pressure broadening and Doppler broadening inside the HCL are significantly smaller than in the hot atmospheric flame. Consequently, the emission line profile from an HCL is substantially narrower than the absorption line profile in the flame, satisfying the criteria for strict monochromaticity.

Beer-Lambert Law in AAS and Causes of Non-Linearity

When monochromatic radiation of incident radiant power P₀ passes through an atomic vapour of path length b containing a concentration c of absorbing ground-state atoms, the transmitted power P obeys the classical Beer-Lambert relationship:

$$T = \frac{P}{P_0} = 10^{-\varepsilon b c}$$

$$A = -\log_{10}(T) = \log_{10}\left(\frac{P_0}{P}\right) = \varepsilon b c = k \cdot c$$

Where A is absorbance, T is transmittance, ε is the atomic absorption coefficient at the resonance wavelength, and k is an empirical calibration sensitivity factor. In ideal conditions, a plot of absorbance versus analyte concentration yields a straight line passing through the origin. However, calibration curves in AAS frequently exhibit downward curvature (negative deviations) at elevated concentrations. The primary physicochemical causes include:

• Stray Light: Unwanted radiation reaching the photodetector without passing through the specific absorption path of the atomic cloud. Even a 1% stray light leak imposes an upper ceiling on measurable absorbance, bending the curve downward.
• Non-Monochromaticity: If the monochromator slit width transmits unresolved non-absorbing spectral lines emitted by the filler gas (Ne or Ar) or secondary emission lines of the cathode that lie in close proximity to the resonance wavelength, the effective absorbance deviates from linearity.
• Self-Absorption and Self-Reversal in the HCL: Operating the hollow cathode lamp at excessively high currents increases the population of cold, unexcited sputtered atoms within the lamp cavity. These cold atoms absorb the exact centre of the emitted line before it escapes the lamp envelope, splitting the emission line into a doublet (self-reversal) and drastically reducing slope sensitivity.
• Flame Equilibrium Perturbations: At high analyte concentrations, the atomisation efficiency may decrease due to incomplete droplet volatilisation, or the fraction of un-ionised atoms may shift if the analyte significantly contributes to the flame's free electron density.

Rigorous analytical calibration demands working within the linear dynamic range determined experimentally using standard reference solutions. Aspirants preparing for quantitative sections can review fundamental analytical concepts in our guide on classical wet chemical methods like titrimetry.

Interferences in Atomic Spectroscopy and Chemical Remedies

Interferences [व्यतिकरण अथवा बाधा] in atomic absorption and emission are categorized into four distinct classes: chemical, spectral, ionisation, and physical. Understanding their molecular mechanisms and corrective procedures is a core syllabus component for competitive examinations.

1. Chemical Interferences

Chemical interference occurs when an anion or co-existing cation in the sample matrix forms a thermally stable, refractory compound with the analyte, preventing quantitative dissociation into free atoms in the flame.

The classic textbook example is the determination of calcium in the presence of orthophosphate (PO₄³⁻) or sulphate (SO₄²⁻). In an air-acetylene flame, calcium and phosphate react during droplet desolvation to yield refractory calcium pyrophosphate (Ca₂P₂O₇), which does not atomise efficiently at 2300 °C. This results in a severe depression of the ground-state Ca⁰ population and a corresponding drop in absorbance.

Chemical interferences are eliminated through three specific strategies:

• Releasing Agents: A releasing agent is a cation that forms a thermodynamically more stable compound with the interfering anion than the analyte does. Adding an excess of Lanthanum chloride (LaCl₃) or Strontium chloride (SrCl₂) releases calcium entirely. The La³⁺ ions competitively sequester all phosphate ions to form thermally stable lanthanum phosphate (LaPO₄), leaving calcium free to atomise completely:

  $$\text{Ca}^{2+} + \text{PO}_4^{3-} + \text{La}^{3+}_{(\text{excess})} \rightarrow \text{LaPO}_{4\,(s)} + \text{Ca}^{2+}_{(\text{free})}$$

• Protective (Chelating) Agents: Introducing a strong organic complexing agent such as EDTA (ethylenediaminetetraacetic acid) or 8-hydroxyquinoline. EDTA binds preferentially with Ca²⁺ in the liquid phase, forming a stable chelate that prevents calcium from interacting with phosphate ions. In the flame, the organic EDTA ligand burns away cleanly, releasing unhindered neutral calcium atoms.
• Higher Temperature Flames: Switching from an air-acetylene flame to a hotter nitrous oxide-acetylene flame (2900 °C) provides sufficient thermal enthalpy to dissociate refractory pyrophosphates directly.

2. Ionisation Interferences

Ionisation interference is particularly prominent for elements with low first ionisation energies, specifically group 1 alkali metals (K, Rb, Cs, Na) and group 2 alkaline earths (Ba, Sr). In high-temperature flames, thermal energy is sufficient to strip valence electrons:

$$M_{(g)}^{0} \xrightleftharpoons{\Delta} M_{(g)}^{+} + e^{-}$$

Because the atomic resonance line of an uncharged atom M⁰ is completely different from the spectral lines of its singly ionised ion M⁺, ionisation depletes the ground-state atomic population, lowering both absorption in AAS and atomic emission in FES.

To eliminate ionisation interference, an ionisation suppressor (or ionisation buffer) is added in large excess to both samples and standards. An ionisation suppressor is a salt of an element with an even lower ionisation potential than the analyte—typically potassium chloride (KCl) or cesium chloride (CsCl) when analysing sodium or calcium. The suppressor ionises readily in the flame, flooding the plasma with free electrons:

$$\text{Cs}_{(g)}^{0} \rightarrow \text{Cs}_{(g)}^{+} + e_{(\text{high concentration})}^{-}$$

By Le Chatelier's principle, the vast excess of free electrons drives the analyte's dissociation equilibrium backward, suppressing analyte ionisation and restoring the neutral ground-state population M⁰ to near 100%.

3. Spectral Interferences and Background Correction

Spectral interference arises when radiation absorbed, emitted, or scattered by matrix constituents coincides with or overlaps the spectral bandpass of the monochromator. While direct overlap of atomic resonance lines between two different elements is exceptionally rare, non-specific background absorption is widespread:

• Light Scattering: Solid micro-particles of unvaporised refractory salts (such as high NaCl concentrations in seawater) scatter incident radiation, creating an artificial increase in measured absorbance.
• Molecular Absorption: Undissociated matrix molecules (such as CaOH or CaSO₄) exhibit broad band absorption spanning several nanometers, overlapping the narrow analyte resonance line.

Modern AAS instruments correct for background attenuation using advanced instrumental systems:

1. Deuterium Continuum Background Correction: A deuterium arc lamp (D₂) emitting a continuous spectrum in the ultraviolet region (190 to 350 nm) is aligned along the optical axis alongside the HCL. The instrument alternates light pulses from the HCL and the D₂ lamp through the flame. The narrow atomic absorption line of the analyte attenuates the HCL beam significantly, but absorbs an imperceptible fraction of the broad D₂ continuum. Conversely, broad molecular absorption and particulate scattering attenuate both the HCL and the D₂ beams equally. The electronics subtract the D₂ absorbance (background only) from the HCL absorbance (atomic + background), yielding pure atomic absorbance.
2. Zeeman-Effect Background Correction: An intense magnetic field (typically 0.5 to 1.0 Tesla) is applied across either the flame atomiser or the hollow cathode lamp. Under the magnetic field, the electronic energy levels of the analyte atom undergo splitting into σ (shifted in wavelength) and π (unshifted) components according to the anomalous Zeeman effect. Using a rotating linear polariser, the instrument measures total absorption (atomic + background) and background absorption alone at the exact identical wavelength, providing superior baseline accuracy even in complex biological or metallurgical matrices.

4. Physical (Matrix) Interferences

Physical interferences originate from changes in the bulk physical properties of the solution—such as viscosity, surface tension, density, or vapour pressure. For instance, high concentrations of mineral acids or dissolved salts reduce the liquid aspiration rate through the capillary and produce larger aerosol droplets, reducing atomisation efficiency. Physical interferences are resolved by matrix matching (preparing calibration standards in an identical solvent/acid matrix), significant dilution, or utilizing the method of standard additions.

Flame Emission Spectroscopy and the Boltzmann Distribution

Flame Emission Spectroscopy (FES) operates on the principle of thermal excitation. Analyte atoms absorb kinetic energy from flame collisions, exciting ground-state valence electrons to upper orbitals. As these excited atoms relax back to lower electronic levels or the ground state, they emit characteristic photons whose radiant power is monitored by a photomultiplier tube or solid-state detector:

$$I_{\text{emission}} \propto N_j$$

Where N_j is the population density of atoms residing in the excited state j. The thermal distribution of atomic populations across energy states at absolute temperature T is quantitatively governed by the Maxwell-Boltzmann distribution law:

$$\frac{N_j}{N_0} = \left(\frac{g_j}{g_0}\right) \exp\left(-\frac{\Delta E}{k T}\right) = \left(\frac{g_j}{g_0}\right) \exp\left(-\frac{h c}{\lambda k T}\right)$$

Where:

• N₀ and N_j are the number of atoms in the ground state and excited state, respectively.
• g₀ and g_j are the statistical weights (degeneracy, $g = 2J + 1$) of the ground and excited quantum states.
• ΔE is the excitation energy difference between the states ($E_j - E_0$).
• k is the Boltzmann constant ($1.3806 \times 10^{-23}\text{ J K}^{-1}$).
• T is the absolute flame temperature in Kelvin.
• λ is the transition wavelength.

Mathematical Significance of the Boltzmann Ratio

Evaluating this exponential relationship reveals the core operational contrast between AAS and FES. Consider the analytical determination of two contrasting elements at a representative flame temperature of 2500 K:

1. Sodium (Na) at λ = 589.0 nm (D-line):

   The ground state is 3s (²S_1/2, $g_0 = 2$) and the excited state is 3p (²P_3/2, $g_j = 4$). The excitation energy ΔE is approximately $2.11\text{ eV} \approx 3.37 \times 10^{-19}\text{ J}$.

   At $T = 2500\text{ K}$, $kT = (1.3806 \times 10^{-23})(2500) \approx 3.45 \times 10^{-20}\text{ J}$.

   $$\frac{N_j}{N_0} = \left(\frac{4}{2}\right) \exp\left(-\frac{3.37 \times 10^{-19}}{3.45 \times 10^{-20}}\right) = 2 \times \exp(-9.77) \approx 2 \times 5.7 \times 10^{-5} \approx 1.1 \times 10^{-4}$$

   Even for an easily excited alkali metal like sodium with a transition in the visible spectrum, only about 0.01% of the atoms reside in the excited state at 2500 K. More than 99.98% remain unexcited in the ground state!

2. Zinc (Zn) at λ = 213.9 nm (Ultraviolet transition):

   Here, the excitation energy ΔE is much larger, approximately $5.8\text{ eV} \approx 9.29 \times 10^{-19}\text{ J}$.

   $$\frac{N_j}{N_0} \approx \exp\left(-\frac{9.29 \times 10^{-19}}{3.45 \times 10^{-20}}\right) = \exp(-26.9) \approx 2 \times 10^{-12}$$

   For zinc at 2500 K, only 2 out of every one trillion atoms are excited. Thermal emission from zinc in a standard flame is virtually undetectable.

Why AAS is Less Sensitive to Temperature Fluctuation

The derivative of the Boltzmann equation with respect to temperature highlights a crucial stability factor:

$$\frac{d(N_j / N_0)}{dT} = \left(\frac{N_j}{N_0}\right) \left(\frac{\Delta E}{k T^2}\right)$$

Because N_j depends exponentially on temperature, a minor temperature fluctuation of just 10 K (less than 0.5% in flame temperature) can alter N_j by 4% to 10%, causing substantial drift in emission intensity in FES. Flame emission instruments therefore require exceptionally rigid gas flow controllers to maintain thermal stability.

In stark contrast, AAS measures the ground-state population N₀. Because N₀ constitutes over 99.9% of the total atom population ($N_{\text{total}} \approx N_0$), a 10 K increase that doubles N_j from 0.01% to 0.02% reduces N₀ from 99.99% to 99.98%—a negligible relative change of 0.01%. Thus, atomic absorption spectroscopy is inherently insensitive to modest temperature variations in the atomiser, providing remarkable signal stability.

Direct Comparison: AAS versus FES

The fundamental, instrumental, and operational differences between Atomic Absorption Spectroscopy and Flame Emission Spectroscopy are contrasted below:

Syllabus Relevance for Competitive Examinations

For aspirants preparing for national-level examinations such as IIT-JAM, CSIR-NET, GATE, and CUET-PG, atomic spectroscopy is a recurring theme within the analytical and physical chemistry sections. While individual examination weightages vary across years and branches, questions consistently evaluate core conceptual principles rather than surface-level memorisation.

Candidates are advised not to rely on anecdotal weightage claims or fixed question tallies. Instead, refer directly to the official syllabus notifications published by the respective organising institutes. Systematically build your own topic tally by solving authentic previous year test questions. In particular, students should concentrate on mastering:

• The physical derivation and mathematical application of the Boltzmann ratio, including computing state populations given degeneracies and wavelength transitions.
• The specific functions of hollow cathode lamp components (sputtering dynamics, filler gas roles, and Doppler broadening suppression).
• The chemical rationale behind releasing agents (e.g., La³⁺ for Ca²⁺ in phosphate matrices) and ionisation suppressors (e.g., Cs⁺ or K⁺).
• The operational principles of Deuterium continuum and Zeeman-effect background correction methods.

To strengthen your overall examination readiness, integrate systematic revisions with comprehensive curated analytical chemistry study material and test your problem-solving speed using authentic official previous year question papers. Dedicated students seeking rigorous, concept-first classroom or online coaching can explore structured modules through our academic enquiry portal or direct telephone guidance at 9212142427.

Frequently Asked Questions

Why can a single hollow cathode lamp generally determine only one element?

The hollow cathode lamp achieves the exceptional monochromaticity required for AAS because its cathode is constructed from the exact element under analysis. When filler gas ions sputter the cathode, the emitted spectrum consists strictly of the narrow resonance lines of that specific elemental lattice. A copper lamp emits copper lines, which cannot be absorbed by lead or zinc atoms in the flame. While some multi-element lamps exist (using alloys), cathode fabrication limits and spectral line overlaps restrict their general use.

Why does AAS demonstrate higher analytical sensitivity than FES for zinc and cadmium?

Zinc and cadmium have high electronic excitation energies, with primary resonance transitions occurring deep in the ultraviolet region (213.9 nm for Zn and 228.8 nm for Cd). According to the Boltzmann distribution, the exponential factor $\exp(-\Delta E / kT)$ at standard flame temperatures (2300 to 2800 K) yields an excited-state population ratio ($N_j / N_0$) on the order of 10⁻¹⁰ to 10⁻¹². Because flame emission depends strictly on this minute excited fraction, emission signals are virtually indistinguishable from baseline noise. Conversely, AAS measures the ground-state population ($N_0$), which comprises more than 99.999% of all vaporised zinc atoms, delivering outstanding signal sensitivity.

What is the exact chemical mechanism by which lanthanum chloride removes phosphate interference in calcium analysis?

When calcium solutions containing orthophosphate are atomised in an air-acetylene flame, they combine during solvent evaporation to form thermally refractory calcium pyrophosphate (Ca₂P₂O₇), which does not dissociate efficiently at flame temperatures. Lanthanum acts as a releasing agent because the enthalpy of formation of lanthanum phosphate (LaPO₄) is substantially more negative than that of calcium phosphate. When LaCl₃ is added in large excess, La³⁺ ions competitively bind all available phosphate ions to precipitate LaPO₄, releasing the Ca²⁺ ions to form simple chlorides that decompose effortlessly into free ground-state calcium atoms.

Why is a nitrous oxide-acetylene flame mandatory for determining aluminium by AAS?

Aluminium forms an exceptionally strong metal-oxygen bond in the flame, producing gaseous aluminium monoxide (AlO) and refractory Al₂O₃ species with dissociation energies exceeding 500 kJ/mol. An air-acetylene flame reaches a maximum temperature of approximately 2300 °C, which is insufficient to break this bond, resulting in near-zero free ground-state Al⁰ atoms. A nitrous oxide-acetylene flame attains temperatures near 2950 °C and generates a fuel-rich chemical environment rich in carbon- and nitrogen-containing radicals (such as CN and CH) that actively scavenge oxygen, effectively reducing aluminium oxide species and freeing elemental aluminium atoms.

How does source modulation distinguish analyte absorption from flame background emission in AAS?

In AAS, the flame atomiser is hot enough to excite a small fraction of analyte atoms thermally, causing the flame itself to emit light continuously at the exact resonance wavelength being measured. If uncorrected, this flame emission would enter the monochromator and strike the photodetector, artificially reducing the apparent absorbance. To prevent this, the hollow cathode lamp's radiant power is modulated (either mechanically using a rotating slotted chopper wheel or electronically by pulsing the lamp power supply at a designated frequency, such as 500 Hz). The detector amplifier is tuned strictly to this modulation frequency, amplifying only the alternating current (AC) signal originating from the lamp while completely discarding the unmodulated direct current (DC) emission from the flame.

What is the difference between Doppler broadening and pressure broadening in atomic flames?

Both phenomena broaden atomic absorption and emission profiles beyond their natural line width. Doppler broadening arises from the thermal Maxwellian velocity distribution of the absorbing or emitting atoms relative to the optical detector. Atoms moving toward the light source perceive radiation at a slightly higher frequency, while atoms moving away perceive a lower frequency, broadening the line profile symmetrically by approximately 0.001 to 0.005 nm. Pressure (or Lorentz) broadening is caused by electrostatic collisions between the analyte atoms and other gaseous molecules or ions in the high-pressure (1 atmosphere) flame environment during the finite lifetime of the excited state. These collisions perturb the ground and excited electronic energy levels, further widening the line profile by approximately 0.001 to 0.004 nm.