Name: correcting errors in atomic absorption spectroscopy
Uploaded: 2024-12-05T18:32:14.000Z
Duration: 1 min 25 s
Channel: JoVE

Les mesures AAS peuvent rencontrer des interférences spectrales, chimiques ou physiques qui obscurcissent ou modifient l’amplitude du signal de l’analyte détecté.
L'interférence spectrale se produit lorsqu'un signal provenant d'un interférent ou de la flamme d'atomisation se chevauche avec le signal de l'analyte, augmentant ou masquant faussement l'absorbance de l'analyte.
Ce problème peut être résolu à l’aide de techniques de correction de l’arrière-plan Zeeman, Smith-Hieftje ou du deutérium. De plus, les spectromètres à haute résolution peuvent minimiser les interférences spectrales dues au chevauchement des raies spectrales. 
Les sources d’interférence chimique comprennent les interactions de l’analyte avec les constituants de la matrice, produisant des sels non volatils qui réduisent l’efficacité de l’atomisation.
De telles interférences peuvent être évitées en utilisant des températures d’atomisation élevées ou en ajoutant des agents complexants ou de démoulage.
De plus, certains analytes atomiques peuvent s’ioniser pendant l’analyse, ce qui entraîne une absorption réduite. Cependant, l’ajout d’un élément plus facilement ionisé supprime l’ionisation de l’analyte.
Les interférences physiques résultant de facteurs non chimiques, tels que les variations des débits de gaz ou de la température de la flamme, sont minimisées grâce à des étalonnages fréquents, des étalons internes ou l’ajout de quantités connues d’analyte dans l’échantillon.

spectroscopie d'absorption atomique: interférence

Atomic Absorption Spectroscopy: Interference

L'interférence entraîne une erreur systématique dans les mesures d'absorption atomique (AA) en renforçant ou en diminuant le signal analytique ou le bruit de fond. Ces interférences peuvent être regroupées en trois catégories principales: interférence spectrale, interférence chimique et interférence physique.

L'interférence spectrale se produit lorsque les signaux d'autres éléments ou molécules se chevauchent avec le signal de l'analyte, augmentant ou masquant de manière erronée l'absorbance de l'analyte. Cette interférence peut être corrigée à l'aide des méthodes de correction de bruit de fond Zeeman, Smith-Hieftje ou au deutérium.

La méthode de correction Zeeman utilise un champ magnétique pour diviser la ligne d'absorption en trois composantes polarisées : deux σ (décalées) et une π (non décalée). Les absorbances de l'analyte et du bruit de fond sont mesurées séparément en alternant le champ magnétique, ce qui améliore la précision dans les matrices complexes.

La méthode de correction Smith-Hieftje consiste à pulser une lampe à cathode creuse (HCL) à des courants élevés, ce qui provoque l'élargissement de la ligne d'émission et une auto-inversion, où la ligne analytique centrale diminue. Cela conduit à une forte émission des deux côtés de la ligne, absorbée par le bruit de fond. L'absorbance est mesurée dans des conditions de courant normal et élevé, ce qui permet de différencier les signaux d'analyte et de bruit de fond. Bien qu'elle ne nécessite qu'une seule source lumineuse, la sensibilité de la méthode diminue, en particulier lorsque l'auto-inversion est insuffisante ou que la récupération est trop lente.

La méthode de correction de bruit de fond au deutérium (D_2) utilise une lampe D_2 comme source lumineuse à large spectre pour corriger l'absorption de bruit de fond dans la spectroscopie d'absorption atomique (AAS). Un miroir rotatif alterne entre la lampe à cathode creuse à bande étroite (HCL) et la lampe D_2 à large bande. La lampe D_2 mesure l'absorbance de bruit de fond sur une large plage de longueurs d'onde, tandis que la HCL mesure l'absorbance de l'analyte et du bruit de fond à une longueur d'onde spécifique. La différence entre les deux signaux isole l'absorbance de l'analyte. Bien que peu coûteux, il manque de précision dans les mesures de haute précision.

De plus, les spectromètres à haute résolution peuvent minimiser les interférences spectrales dues au chevauchement des lignes spectrales. Parfois, l'analyte peut être extrait à plusieurs reprises avec un solvant avant l'analyse.

Les interférences chimiques se produisent lorsque les composants de la matrice indésirables interagissent avec l'analyte, réduisant l'efficacité de l'atomisation. Un modificateur chimique, tel qu'un agent de libération ou un agent complexant, peut être ajouté à l'échantillon pour améliorer l'atomisation ou empêcher la formation de composés interférents. Les interférences chimiques courantes comprennent les interférences dues à l'ionisation et à la formation de composés réfractaires.

Les éléments ou composés qui s'ionisent à la même température que l'analyte peuvent altérer son ionisation. L'ionisation peut être supprimée en ajoutant un excès d'une solution contenant un élément qui s'ionise plus facilement, ce qui réprime l'ionisation de l'analyte.

De plus, les réactions chimiques entre l'analyte et d'autres espèces dans la matrice de l'échantillon peuvent former des composés non volatils qui ne s'atomisent pas facilement. Cela empêche la formation d'atomes libres pour l'absorption. De telles interférences sont évitées par l'ajout d'un concurrent chimique ou l'utilisation de températures très élevées.

Les étalons d'étalonnage peuvent être préparés avec une matrice d'échantillons similaire aux échantillons réels, ce qui permet de compenser les interférences chimiques provenant de la matrice.

Les interférences physiques provenant de facteurs non chimiques, tels que les variations du débit de gaz ou les changements de température de la flamme, affectent le processus de nébulisation ou d'atomisation. Ces interférences peuvent être résolues en utilisant des étalons internes ou en diluant l'échantillon. La modification des matrices d'échantillons et la préparation d'étalons d'étalonnage avec une matrice similaire peuvent réduire davantage les interférences physiques.

Spectroscopie d'absorption atomique: interférence

Le brouillage conduit à des erreurs systématiques dans les mesures d’absorption atomique (AA) en augmentant ou en diminuant le signal analytique ou le bruit de fond. Ceux-ci...

atomic-absorption-spectroscopy-interference

Education

Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Atomic Spectroscopy

Spectroscopie atomique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

correcting errors in atomic absorption spectroscopy

Atomic absorption spectroscopy interference can create systematic error by making the analytical signal or the background look stronger or weaker than it should be. In atomic absorption measurements, these errors are usually grouped as spectral interference, chemical interference, and physical interference.

Spectral interference happens when signals from other elements or molecules overlap with the analyte signal, the substance being measured. This overlap can falsely raise or hide the analyte absorbance. It can be reduced with Zeeman, Smith-Hieftje, or deuterium background correction methods. High-resolution spectrometers can also help minimize overlap from nearby spectral lines, and in some cases the analyte can be repeatedly extracted with a solvent before analysis.

The Zeeman correction method uses a magnetic field to split the absorption line into three polarized components. Two are shifted sigma components, and one is an unshifted pi component. The analyte and the background are measured separately by alternating the magnetic field, which improves accuracy in complex samples. The Smith-Hieftje method uses a hollow cathode lamp at high current. This broadens the emission line and causes self-reversal, where the central analytical line decreases. Strong emission remains on both sides of the line and is absorbed by the background. Absorbance is measured under normal and high-current conditions to separate analyte and background signals, but sensitivity can drop if self-reversal is weak or recovery is slow. The deuterium background correction method uses a D2 lamp as a broad-spectrum light source. A rotating mirror switches between the narrow-band hollow cathode lamp and the broad-band D2 lamp. The D2 lamp measures background absorbance over a wide wavelength range, while the hollow cathode lamp measures analyte and background absorbance at one wavelength. The difference between the two signals isolates the analyte absorbance, although this method is less precise in high-accuracy work.

Chemical interferences occur when matrix components in the sample react with the analyte and reduce atomization efficiency. A chemical modifier, such as a releasing agent or complexing agent, can be added to improve atomization or prevent interfering compounds from forming. Common chemical problems include ionization interference and refractory compound formation. If another element or compound ionizes at the same temperature as the analyte, it can change the analyte ionization. This can be suppressed by adding an excess of an element that ionizes more easily. Chemical reactions can also form nonvolatile compounds that do not atomize easily, which limits free atom formation for absorption. Adding a chemical competitor or using very high temperatures can help prevent this. Matching the calibration standards to the sample matrix also helps correct for matrix effects.

Physical interferences come from non-chemical changes, such as gas flow rate changes or flame temperature changes. These factors can affect nebulization or atomization. They can be reduced with internal standards, sample dilution, and matrix matching. Preparing calibration standards with a similar matrix can also help reduce physical interference and improve measurement reliability.

Correcting Errors in Atomic Absorption Spectroscopy

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atomic spectroscopy: absorption, emission, and fluorescence

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Atomic spectroscopy is used to identify and measure elements by their light signals. It supports both qualitative analysis, which tells what is present, and quantitative analysis, which tells how much is present. The optical methods in this group are atomic absorption spectroscopy, atomic emission spectroscopy, and atomic fluorescence spectroscopy. These methods begin with atomization. In this step, a solid, liquid, or solution sample is changed into gas-phase atoms and ions. Once the sample...

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Heat changes atomic spectra by affecting atomization, excitation, ionization, and line shape. In atomic spectroscopy, atomization means converting a sample into gas-phase atoms and ions. The flame temperature needed for this step changes how efficiently the method works and shifts the balance between excited and ground-state atoms. At thermal equilibrium, the relative numbers of excited and ground-state atoms can be estimated with the Maxwell-Boltzmann distribution. A small temperature rise...

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Atomic absorption spectroscopy (AAS) measures trace metals by tracking how atoms absorb electromagnetic radiation (EMR). The absorbed light makes atoms move to a higher-energy orbit. In AAS, the key step is atomization, which converts the sample into gas-phase atoms. A flame or a furnace is typically used for this step. Inside the flame, some atoms become thermally excited. Most stay in the ground state, which is the lowest-energy state. These ground-state gas-phase atoms absorb EMR only when...

atomic absorption spectroscopy: instrumentation

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Atomic Absorption Spectroscopy: Key Parts

Atomic absorption spectrophotometry uses a set of key parts to measure light absorbed by atoms. The main components are a radiation source, an atomizer, a monochromator, and a detector. Each part has a specific job in the instrument. The radiation source is usually a hollow-cathode lamp, or HCL, or an electrodeless-discharge lamp, or EDL. Both produce a narrow emission line at the needed wavelength. Some instruments instead use continuum sources with high-resolution monochromators to narrow...

atomic absorption spectroscopy: radiation and light sources

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Choosing Light Sources for AAS Measurements

Atomic absorption spectroscopy (AAS) uses a light source that emits a very narrow range of wavelengths. This is needed because the source must match the absorption behavior of the analyte atom. The choice of source also depends on giving a precise, intense emission at the right wavelength for accurate detection. Two common line sources in AAS are hollow-cathode lamps (HCLs) and electrodeless-discharge lamps (EDLs). These lamps are selected based on the element being analyzed and the level of...

atomic absorption spectroscopy: atomization methods

atomic-absorption-spectroscopy-atomization-methods

Atomic Absorption Spectroscopy: Sample Atomization

Atomic absorption spectroscopy uses sample atomization to turn a liquid specimen into free atoms for analysis. The two main approaches are flame atomization and electrothermal atomization. Both methods prepare the sample so its atoms can be measured by the instrument. Flame atomization uses a nebulizer and spray chamber to mix the sample with a fuel-oxidant mixture. This creates a fine aerosol mist that enters the burner. The fuel and oxidant are usually close to a stoichiometric ratio, but a...

atomic absorption spectroscopy: interference

Atomic absorption spectroscopy interference can create systematic error by making the analytical signal or the background look stronger or weaker than it should be. In atomic absorption measurements, these errors are usually grouped as spectral interference, chemical interference, and physical interference. Spectral interference happens when signals from other elements or molecules overlap with the analyte signal, the substance being measured. This overlap can falsely raise or hide the analyte...

atomic absorption spectroscopy: lab

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atomic emission spectroscopy: overview

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atomic emission spectroscopy: instrumentation

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atomic emission spectroscopy: interference

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Reducing Interference in AES Signals

Atomic emission spectroscopy (AES) can produce interfering signals when atoms and molecules in a flame or plasma emit overlapping light. High-temperature atomizers excite many species at once, including oxides, hydroxides, and flame combustion products. These extra emissions can make the analyte signal harder to read. Spectral interference happens when emission lines or bands overlap. One way to reduce it is to improve instrument resolution. Another is to choose a different emission line for...

inductively coupled plasma atomic emission spectroscopy: principle

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ICP-AES Torch and Sample Analysis

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inductively coupled plasma atomic emission spectroscopy: instrumentation

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flame photometry: overview

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key terms and concepts

Learn how spectral, chemical, and physical interference affect atomic absorption spectroscopy and how background correction helps improve accuracy.

Correcting Errors in Atomic Absorption Spectroscopy | JoVE K12

analytical chemistry

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Correcting Errors in Atomic Absorption Spectroscopy

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