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

As medições de AAS podem encontrar interferências espectrais, químicas ou físicas que obscurecem ou alteram a magnitude do sinal do analito detectado.
A interferência espectral ocorre quando um sinal de um interferente ou a chama de atomização se sobrepõe ao sinal do analito, elevando ou mascarando falsamente a absorbância do analito.
Isso pode ser resolvido usando Zeeman, Smith-Hieftje ou técnicas de correção de fundo de deutério. Além disso, os espectrômetros de alta resolução podem minimizar a interferência espectral de linhas espectrais sobrepostas. 
As fontes de interferência química incluem interações de analitos com constituintes da matriz, produzindo sais não voláteis que reduzem a eficiência da atomização.
Tal interferência pode ser evitada usando altas temperaturas de atomização ou adicionando agentes complexantes ou liberantes.
Além disso, alguns analitos atômicos podem ionizar durante a análise, resultando em absorção reduzida. No entanto, a adição de um elemento ionizado mais facilmente suprime a ionização do analito.
As interferências físicas decorrentes de fatores não químicos, como variações nas taxas de fluxo de gás ou temperatura da chama, são minimizadas por meio de calibração frequente, padrões internos ou adição de quantidades conhecidas de analito na amostra.

espectroscopia de absorção atômica: interferência

Atomic Absorption Spectroscopy: Interference

A interferência leva a erro sistemático em medições de absorção atômica (AA) ao aumentar ou diminuir o sinal analítico ou o fundo. Essas interferências podem ser agrupadas em três categorias principais: interferência espectral, interferência química e interferência física.

A interferência espectral ocorre quando sinais de outros elementos ou moléculas se sobrepõem ao sinal do analito, elevando ou mascarando falsamente a absorbância do analito. Essa interferência pode ser corrigida usando métodos de correção de fundo Zeeman, Smith–Hieftje ou deutério.

O método de correção Zeeman usa um campo magnético para dividir a linha de absorção em três componentes polarizados: dois σ (deslocados) e um π (não deslocado). As absorbâncias do analito e do fundo são medidas separadamente alternando o campo magnético, aumentando a precisão em matrizes complexas.

O método de correção Smith-Hieftje envolve pulsar uma lâmpada de cátodo oco (HCL) em altas correntes, fazendo com que a linha de emissão se amplie e sofra autorreversão, onde a linha analítica central diminui. Isso leva a uma forte emissão em ambos os lados da linha, absorvida pelo fundo. A absorbância é medida em condições normais e de alta corrente, permitindo a diferenciação entre sinais de analito e de fundo. Embora exija apenas uma única fonte de luz, a sensibilidade do método diminui, especialmente quando a autorreversão é insuficiente ou a recuperação é muito lenta.

O método de correção de fundo deutério (D_2) usa uma lâmpada D_2 como uma fonte de luz de amplo espectro para corrigir a absorção de fundo na espectroscopia de absorção atômica (AAS). Um espelho giratório alterna entre a lâmpada de cátodo oco de banda estreita (HCL) e a lâmpada D_2 de banda larga. A lâmpada D_2 mede a absorbância de fundo em uma ampla faixa de comprimento de onda, enquanto a HCL mede a absorbância de analito e de fundo em um comprimento de onda específico. A diferença entre os dois sinais isola a absorbância do analito. Embora seja de baixo custo, ele não tem precisão em medições de alta exatidão.

Além disso, espectrômetros de alta resolução podem minimizar a interferência espectral de linhas espectrais sobrepostas. Às vezes, o analito pode ser repetidamente extraído com um solvente antes da análise.

Interferências químicas ocorrem quando os componentes indesejados da matriz interagem com o analito, reduzindo a eficiência da atomização. Um modificador químico, como um agente de liberação ou agente complexante, pode ser adicionado à amostra para aumentar a atomização ou evitar a formação de compostos interferentes. Interferências químicas comuns incluem interferências devido à ionização e formação de compostos refratários.

Elementos ou compostos que ionizam na mesma temperatura que o analito podem alterar sua ionização. A ionização pode ser suprimida adicionando um excesso de uma solução contendo um elemento que ioniza mais facilmente, o que reprime a ionização do analito.

Além disso, reações químicas entre o analito e outras espécies na matriz da amostra podem formar compostos não voláteis que não atomizam prontamente. Isso dificulta a formação de átomos livres para absorção. Essa interferência é evitada pela adição de um concorrente químico ou pelo uso de temperaturas muito altas.

Os padrões de calibração podem ser preparados com uma matriz de amostra semelhante às amostras reais, o que ajuda a compensar as interferências químicas decorrentes da matriz.

As interferências físicas decorrentes de fatores não químicos, como variações na taxa de fluxo de gás ou mudanças na temperatura da chama, afetam o processo de nebulização ou atomização. Essas interferências podem ser resolvidas usando padrões internos ou diluindo a amostra. Modificar matrizes de amostra e preparar padrões de calibração com uma matriz semelhante pode reduzir ainda mais as interferências físicas.

Espectroscopia de Absorção Atômica: Interferência

A interferência leva a erros sistemáticos nas medições de absorção atômica (AA), aumentando ou diminuindo o sinal analítico ou o fundo. Estes...

atomic-absorption-spectroscopy-interference

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Atomic Spectroscopy

Espectroscopia Atômica

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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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Element Analysis with Atomic Light Spectra

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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How Heat Changes Atomic Spectra

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

AAS Sample Prep and Calibration

Atomic absorption spectroscopy (AAS) depends on careful sample preparation and calibration. Samples must be turned into clear solutions before measurement. That often means extra treatment to dissolve materials such as soils, animal tissues, and minerals. Common sample prep methods include hot mineral acids, wet ashing, combustion in closed containers, high-temperature ashing, and fusion with reagents. These steps help make the sample suitable for AAS analysis. Solutions that contain organic...

atomic emission spectroscopy: overview

Atomic Emission Spectroscopy for Element Analysis

Atomic emission spectroscopy is used to identify the elements in a sample by measuring the light they emit. Atoms are first excited to higher energy levels by thermal energy from hot sources such as plasma, arcs, or sparks. When they fall back to lower energy levels, they give off light at specific wavelengths. Each element produces its own set of lines, so the emission spectrum can show both what is present and how much is there. AES instruments are similar to atomic absorption spectrometers,...

atomic emission spectroscopy: instrumentation

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AES Sample Handling and Detection

Atomic emission spectrometry (AES) uses sample handling, atomization, and detection parts to measure emitted light from atoms and ions. The sample must first be changed into a gas-phase form so it can produce a measurable signal. AES uses two main atomizer types. Continuous atomizers, such as flames and plasmas, feed the sample in a steady stream. Discrete atomizers add one sample at a time with syringes or autosamplers. The electrothermal atomizer is the most common discrete type. Many...

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

Inductively coupled plasma atomic emission spectroscopy uses a plasma torch to analyze elements in a sample. This method is also called inductively coupled plasma optical emission spectroscopy, or ICP-OES. It is widely used because the plasma creates a strong environment for elemental analysis. The ICP source, or torch, contains three concentric quartz tubes. Argon gas flows through the tubes. A spark from a Tesla coil starts the ionization of argon and creates a high-temperature plasma. The...

inductively coupled plasma atomic emission spectroscopy: instrumentation

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ICP-AES Instrument Types and Detection

Inductively coupled plasma atomic emission spectroscopy, or ICP-AES, uses an inductively coupled plasma to detect and analyze elements in a sample. The method measures the light given off by atoms, which helps identify and compare different elements. It is a common approach in atomic emission spectroscopy. ICP-AES instruments are built in three main forms: sequential, simultaneous multichannel, and Fourier transform instruments. Fourier transform designs are less commonly used, but they are...

atomic emission spectroscopy: lab

Plasma Sources in Atomic Emission Spectroscopy

Atomic emission spectroscopy uses plasma sources to identify elements by their light. The method creates abundant spectra with characteristic emission lines. Inductively coupled plasma, or ICP, is especially useful because it gives strong quantitative data. It works well under stable conditions, with low noise, low background, and few interferences. Newer air-operated microwave sources are also being studied. These sources may be more cost-effective than conventional ICP systems. Atomic...

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Atomic fluorescence spectroscopy measures elements by detecting the light atoms give off after they are excited by electromagnetic radiation. The atoms are first placed in a flame, furnace, or plasma. They absorb energy, become excited, and then release that energy as they return to their original state. The emitted light is called fluorescence. The fluorescence is observed at a right angle to the incoming beam. This setup helps separate the emitted light from the light used to excite the...

flame photometry: overview

Flame Emission Spectrometry Basics

Flame emission spectrometry uses a flame to excite elements in a sample so their light can be measured. It is also called flame photometry. The method can be used for both qualitative and quantitative analysis of elements. The idea became established in the early 1860s when Kirchhoff and Bunsen showed that specific elements give off characteristic radiation in a flame. The first instrument for this purpose measured sodium in plant ash with a Bunsen flame. A major problem was how to add the...

flame photometry: lab

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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