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

Le misure AAS possono riscontrare interferenze spettrali, chimiche o fisiche che oscurano o modificano l'entità del segnale dell'analita rilevato.
L'interferenza spettrale si verifica quando un segnale proveniente da un interferente o dalla fiamma di atomizzazione si sovrappone al segnale dell'analita, elevando o mascherando falsamente l'assorbanza dell'analita.
Questo problema può essere risolto utilizzando le tecniche di correzione dello sfondo Zeeman, Smith-Hieftfje o deuterio. Inoltre, gli spettrometri ad alta risoluzione possono ridurre al minimo l'interferenza spettrale dovuta alla sovrapposizione di linee spettrali. 
Le fonti di interferenza chimica includono le interazioni dell'analita con i costituenti della matrice, producendo sali non volatili che riducono l'efficienza di atomizzazione.
Tali interferenze possono essere evitate utilizzando alte temperature di atomizzazione o aggiungendo agenti complessanti o distaccanti.
Inoltre, alcuni analiti atomici possono ionizzarsi durante l'analisi, con conseguente riduzione dell'assorbimento. Tuttavia, l'aggiunta di un elemento più facilmente ionizzabile sopprime la ionizzazione dell'analita.
Le interferenze fisiche derivanti da fattori non chimici, come le variazioni delle portate di gas o della temperatura della fiamma, sono ridotte al minimo grazie a frequenti calibrazioni, standard interni o l'aggiunta di quantità note di analita nel campione.

spettroscopia di assorbimento atomico: interferenza

Atomic Absorption Spectroscopy: Interference

L'interferenza porta a un errore sistematico nelle misurazioni dell'assorbimento atomico (AA) aumentando o diminuendo il segnale analitico o lo sfondo. Queste interferenze possono essere raggruppate in tre categorie principali: l’interferenza spettrale, l’interferenza chimica e l’interferenza fisica.

L'interferenza spettrale si verifica quando i segnali provenienti da altri elementi o molecole si sovrappongono al segnale dell'analita, elevando o mascherando falsamente l'assorbanza dell'analita. Questa interferenza può essere corretta usando i metodi di correzione dello sfondo Zeeman, Smith-Hieftje o deuterio.

Il metodo di correzione Zeeman usa un campo magnetico per dividere la linea di assorbimento in tre componenti polarizzate: due σ (spostate) e un π (non spostato). Le assorbanze dell'analita e dello sfondo vengono misurate separatamente alternando il campo magnetico, e migliorando la precisione nelle matrici complesse. Il metodo di correzione Smith-Hieftje prevede l'impulso di una lampada a catodo cavo (HCL) ad alte correnti, causando l'allargamento della linea di emissione e l'auto-inversione, dove la linea analitica centrale diminuisce. Questo determina una forte emissione su entrambi i lati della linea, assorbita dallo sfondo. L'assorbanza viene misurata in condizioni normali e ad alta corrente, consentendo la differenziazione tra i segnali dell’analita e di sfondo. Sebbene richieda solo una singola sorgente luminosa, la sensibilità del metodo diminuisce, soprattutto quando l'auto-inversione è insufficiente o il recupero è troppo lento. Il metodo di correzione dello sfondo al deuterio (D_2) utilizza una lampada D_2 come una sorgente luminosa ad ampio spettro, per correggere l'assorbimento dello sfondo nella spettroscopia di assorbimento atomico (AAS). Uno specchio rotante si alterna tra la lampada a catodo cavo a banda stretta (HCL) e la lampada D_2 a banda larga. La lampada D_2 misura l'assorbanza dello sfondo su un'ampia gamma di lunghezze d'onda, mentre l'HCL misura l'assorbanza dell'analita e dello sfondo a una lunghezza d'onda specifica. La differenza tra i due segnali isola l'assorbanza dell'analita. Sebbene sia poco costosa, manca di precisione nelle misurazioni ad alta accuratezza.

Inoltre, gli spettrometri ad alta risoluzione possono ridurre al minimo l'interferenza spettrale da linee spettrali sovrapposte. A volte, l'analita può essere estratto ripetutamente con un solvente prima dell'analisi.

Le interferenze chimiche si verificano quando i componenti indesiderati della matrice interagiscono con l'analita, riducendo l'efficienza di atomizzazione. Un modificatore chimico, come un agente di rilascio o un agente complessante, può essere aggiunto al campione per migliorare l'atomizzazione, o prevenire la formazione dei composti interferenti. Le interferenze chimiche comuni includono delle interferenze dovute alla ionizzazione e alla formazione di composti refrattari.

Gli elementi o i composti che si ionizzano alla stessa temperatura dell'analita possono alterarne la ionizzazione. La ionizzazione può essere soppressa aggiungendo un eccesso di una soluzione contenente un elemento che si ionizza più facilmente, questo reprime la ionizzazione dell'analita.

Inoltre, le reazioni chimiche tra l'analita e le altre specie nella matrice del campione possono formare dei composti non volatili che non si atomizzano facilmente. Questo ostacola la formazione di atomi liberi per l'assorbimento. Tale interferenza viene evitata aggiungendo un competitor chimico, o utilizzando delle temperature molto elevate.

Gli standard di calibrazione possono essere preparati con una matrice campione simile ai campioni reali, il che aiuta a compensare le interferenze chimiche derivanti dalla matrice.

Le interferenze fisiche derivanti dai fattori non chimici, come le variazioni della portata del gas o i cambiamenti della temperatura della fiamma, influenzano il processo di nebulizzazione o atomizzazione. Queste interferenze possono essere risolte utilizzando gli standard interni o diluendo il campione. La modifica delle matrici campione e la preparazione di standard di calibrazione con una matrice simile possono ridurre ulteriormente le interferenze fisiche.

Spettroscopia di assorbimento atomico: interferenza

L'interferenza porta a un errore sistematico nelle misure di assorbimento atomico (AA) aumentando o diminuendo il segnale analitico o il fondo. Questi...

atomic-absorption-spectroscopy-interference

Education

Didattica

JoVE Core

Analytical Chemistry

Chimica Analitica

Unit 1

Unità 1

Atomic Spectroscopy

Spettroscopia atomica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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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: 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

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Atomic Absorption Spectroscopy: Sample Atomization

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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: 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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ICP-AES Instrument Types and Detection

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flame photometry: overview

Flame Emission Spectrometry Basics

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Flame Photometry and Metal Detection

Flame photometry is a method for detecting metals by measuring the light they emit in a flame. A solution such as potassium chloride is aspirated into the flame. The solvent evaporates first. The remaining salt becomes dehydrated and then dissociates into free gaseous atoms in the ground state. Some of these atoms absorb energy from the flame and move to an excited state. When they return to the ground state, they emit photons at characteristic wavelengths. These emissions are very narrow...

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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