Atomic Spectroscopy

Atomic Spectroscopy

***Atomic*** ***spectroscopy*** can be used to determine the elemental composition of a sample - which elements are in the sample and how much of each element is in the sample

In atomic spectroscopy:

• Heat is used to vaporize and decompose samples into atoms and ions.

• The atoms of each element absorb and emit light at ***characteristic wavelengths***

• The amount of light that is absorbed or emitted at the characteristic wavelength enables the analyst to determine the ***element concentration***

 **Types** of Atomic Spectroscopy

Atomic emission spectroscopy

Atomic absorption spectroscopy

Atomic fluorescence spectroscopy

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1\. **Atomic Emission Spectroscopy**

Most common form of atomic spectroscopy

 

Requires no lamp, collisions in flame promote some atoms to excited electronic states from which they can emit photons to return to lower energy states, emission intensity proportional to concentration

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2\. **Atomic Absorption Spectroscopy** 

The flame atomizes samples to produce atoms that are primarily in the ground electronic state, the atoms are illuminated by characteristic wavelengths from a hollow-cathode lamp from which atoms absorb a portion of the light

3\. **Atomic Fluorescence Spectroscopy**

The flame atomizes samples to produce atoms that are primarily in the ground electronic state, a line source irradiates atoms in the flame to promote them to an excited electronic state, fluorescence emission from excited state atoms is then measured (proportional to concentration)

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**Atomization**

**Atomization is** the process in which a compound is decomposed into its atoms at high temperature

In atomic spectroscopy, analyte is *atomized* in a flame, an electrically heated furnace, or a plasma. Flames were used for decades, but they have been largely replaced by the plasma and the graphite furnace

**The** ***analytical signal*** is the time-integrated absorbance (the peak area) measured during the *atomizing* step

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Free Atom Formation in Flames

Physical and chemical processes in the burner head and flame that convert an **aqueous analyte** into **gaseous atoms**

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**Graphite Furnace Advantages over Flames**

1\. Lower minimum sample volume:

2\. Longer sampling time in optical path:

3\. Easier preconcentration

4\. No nebulization step required in a graphite furnace

Disadvantages

Graphite furnaces require more operator skill to find proper conditions for each sample; have a limited lifetime

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**Heating Program for a Graphite Furnace**

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• **Drying—**dries the sample

• **Charring—**decomposes the sample

• **Atomizing—**atomizes the sample

• **Cleaning (not shown)—**cleans out remaining residue

 

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**Inductively Coupled Plasma**

•Is twice as hot as a combustion flame

• Has fewer interferences than a flame

• Is used for simultaneous, multi-element analysis

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**HOW TEMPERATURE AFFECTS ATOMIC SPECTROSCOPY**

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Temperature determines the degree to which a sample breaks down into atoms and the extent to which a given atom is found in its ground, excited, or ionized states

 

**The Hollow-Cathode Lamp** 

is filled with low-pressure Ne or Ar

The **cathode** is made of the element to be analyzed

 

A voltage is applied between the anode and cathode which ionizes the Ne, the atoms sputtered from the cathode are excited by collisions with high-energy electrons and relax back to the ground state by emitting light

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**Beer's law requirement**

Hollow-cathode lamp lines should be substantially narrower than the linewidth of analyte atoms in the flame or furnace

A **charge injection device (CID) array detector** 

offers superior detection characteristics, CID pixels can be read *individually* at any time and can measure a dim spot adjacent to a bright spot

**Background Signal** 

arises from absorption, emission, or scatter by everything in the sample besides analyte as well as from absorption, emission, or scatter by flame, plasma, or furnace

**Background correction** 

is necessary in atomic spectroscopy to isolate the analyte signal from the background signal

**Interference**

Interference is any effect that changes the signal while analyteconcentration remains unchanged

 

 

**Types of Interference**

 

**Spectral**, **Physical**, **Chemical**, **Ionization**

 

***1.Spectral interference****:*

Refers to the overlap of analyte signal with signals due to other elements or molecules in the sample or with signals due to the flame or furnace

To **solve the problem** of elemental spectral interference:

• Choose another wavelength for analysis  

• Use an instrument with higher resolution

***2. Physical interference:*** 

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caused by any component of the sample that alters sample transport or nebulization

 

***3. Chemical interference:***  

caused by any component of the sample that alters any of the chemical processes involved in atomization of analyte

***Releasing agents:*** chemicals that are added to a sample to decrease chemical interference

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4\. Ionization interference:

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can be a problem in the analysis of alkali metals because they have low ionization potentials

 

An **ionization suppressor** decreases the extent of ionization of the analyte

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**Correcting for Interference with Standard Addition**

The ***method of standard addition*** compensates for many types of interference by adding known quantities of standard to the unknown in its complex matrix

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**INDUCTIVELY COUPLED PLASMA–MASS SPECTROMETRY**

Ions generated in a plasma can be directed into a mass spectrometer, which:

Separates the ions by mass-to-charge ratio

Measures ion current at each mass-charge ratio

**Atomic Isobaric Interference -** occurs when the interfering species has the same mass-to-charge ratio as the analyte ion

**Laser-induced breakdown spectroscopy** 

records the atomic emission spectrum of the temporary plasma created by laser ablation

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**X-RAY FLUORESCENCE**

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**X-ray fluorescence** is the emission of X-rays following the absorption of X-rays by a material

Elements are identified by their peak energies and quantified by the number of photons in each peak