Atomic Spectrometry Notes

Atomic Spectrometry

Learning Objectives

• Define and explain the principles behind Atomic Absorption Spectroscopy (AAS), Atomic Emission Spectroscopy (AES), and Flame Emission Spectroscopy (FES) in the context of clinical laboratory analysis.
• Identify and describe the key components of AAS, AES, and FES instrumentation.
• Calculate the concentration of analytes (e.g., calcium, sodium, potassium) in biological fluids.
• Compare and contrast AAS, AES, and Flame Emission Spectroscopy in terms of sensitivity, specificity, clinical applications, and limitations in the analysis of elements.

Introduction

• Atomic spectroscopic methods are used for qualitative and quantitative determination of more than 70 elements.
• Two major types:
  • Optical atomic spectroscopy
  • Atomic mass spectroscopy
• Atomization, in which a sample is volatilized and decomposed to produce gas-phase atoms and ions, is required as the first step in all atomic spectroscopic procedures.
• Most common atomization methods:
  • Inductively coupled plasmas (ICPs)
  • Flames
  • Electrothermal atomizers
• Atomic absorption (AA) spectroscopy uses flames and electrothermal atomizers.
• Optical emission and atomic mass spectrometry use inductively coupled plasma.

Optical Atomic Spectroscopy

• Optical atomic spectroscopy encompasses several techniques used to analyze the elemental composition of a sample by studying the light emitted or absorbed by atoms.
• Main types:
  • Atomic Emission Spectroscopy (AES)
  • Atomic Absorption Spectroscopy (AAS)
  • Atomic Fluorescence Spectroscopy (AFS)

Emission Spectra

• Atomic Emission Spectroscopy (AES) is used to analyze the elemental composition of a sample by examining the light emitted when atoms are excited to higher energy levels.
• Analyte atoms are excited by heat or electrical energy.
• Energy is typically supplied by a:
  • Plasma
  • Flame
  • Low-powered discharge
  • High-powered laser
• Before the external energy source is applied, the atoms are usually in their ground state (lowest-energy).
• Applied energy causes atoms to momentarily enter an excited state (higher-energy).
• These excited electrons are unstable and eventually return to lower energy levels, releasing the absorbed energy in the form of photons (light particles).
• A resonance transition is a transition to or from the ground state.
• A resonance line is the spectral line produced as a result.

Absorption Spectra

• Atomic Absorption Spectroscopy (AAS) is used to measure the concentration of elements by detecting the absorption of light by free atoms in a gaseous state
• An external source of radiation interact on the analyte vapor.
• If the source radiation is of the appropriate frequency (wavelength), it can be absorbed by the analyte atoms and promote them to excited states.

Sample Introduction Systems for Atomic Spectroscopy

• Two classes of atomization devices:
  • Continuous atomizers such as plasmas and flames that introduce samples as a continuous stream.
  • Discrete atomizers such as the electrothermal atomizer that are used to inject individual samples using a syringe or autosampler.
• Processes that must occur to produce free atoms or elementary ions are complex and involve multiple steps.

Flame Atomizers

• A flame atomizer, the most common used atomization technique, consists of a pneumatic nebulizer that converts the sample solution into a mist or aerosol that is introduced into a burner.
• The high-pressure gas is usually the oxidant with the aerosol-containing oxidant being mixed subsequently with the fuel.
• The burners are generally premixed, laminar flow burners.
• The aerosol flows into a spray chamber where it encounters a series of baffles that remove all but the finest droplets.
• The sample spray is mixed with fuel and oxidant gas in the spray chamber before these components are burned in a slotted burner.
• Regions of a Flame:
  • When a nebulized sample is carried into a flame, the droplets are desolvated in the primary combustion zone.
  • The resulting finely divided solid particles are carried to a central inner core. This is the hottest part of the flame. Particles are vaporized and converted to gaseous atoms, elementary ions, and molecular species and excitation of atomic spectra takes place.
  • The atoms, molecules, and ions are carried to the outer edge (outer cone) where oxidation may occur. This is called the secondary combustion zone.
• The temperature of the flames produced is relatively low, so the technique is only suitable for the elements that are easily excited such as alkali and alkali earth elements.

INSTRUMENT USED IN ATOMIC SPECTROSCOPY: Flame Emission Spectroscopy (FES)

• Also known as flame photometry, it is a specific type of AES where a flame is used to excite the atoms in the sample.
• It is used to determine the concentration of certain elements by measuring the light emitted when they are introduced into a flame.
• Simple and inexpensive method for analyzing certain elements, particularly alkali and alkaline earth metals
• Often used in clinical and environmental laboratories for measuring the concentration of specific ions, such as sodium and potassium.
• A sample containing the analyte of interest is introduced into a flame.
• The heat of the flame excites the atoms of the analyte, causing their electrons to move to higher energy levels.
• When these excited electrons return to their ground state, they emit light at specific wavelengths.
• The intensity of the emitted light is directly proportional to the concentration of the analyte in the sample.
• Components:
  • Sample Introduction System
    • Nebulizer: Converts the liquid sample into a fine mist (aerosol).
    • Atomizer: Carries the aerosol into the flame.
  • Flame (Excitation Source)
    • Commonly a mixture of air-acetylene or nitrous oxide-acetylene.
    • Provides the thermal energy to excite the electrons of metal atoms
  • Monochromator (Wavelength Selector)
    • Disperses the emitted light into its component wavelengths.
    • Allows selection of a specific wavelength for detection.
  • Detector
    • Photomultiplier tube (PMT) or photodiode detects light intensity at the selected wavelength.
    • Converts light into an electrical signal.
  • Readout Device/Data System
    • Displays or records the intensity data, often in terms of emission intensity vs. concentration.

INSTRUMENT USED IN ATOMIC SPECTROSCOPY: Atomic Absorption Spectroscopy (AAS)

• Used to measure the concentration of elements by detecting the absorption of light by free atoms in a gaseous state.
• When light of a particular wavelength is passed through a sample containing these atoms, the atoms will absorb the light, and the amount of light absorbed is proportional to the concentration of the element in the sample.
• Relatively low equipment and operating costs. Only a small amount of sample is needed.
• Can only analyze one element at a time. Not suitable for ultra-trace levels (typically detects ppm to low ppb).
• The sample is first converted into free atoms in the gaseous phase. This is typically done by introducing the sample into a flame or a graphite furnace.
• A light beam from a hollow cathode lamp (HCL) specific to the measured element is passed through the atomized sample.
• The atoms of the element in the sample absorb light at their specific wavelength from the HCL beam.
• A detector measures the amount of light that passes through the sample, and the amount of light absorbed is calculated.
• The amount of light absorbed is directly proportional to the concentration of the analyte.
• Components:
  • Light Source: hollow cathode lamp (HCL), which emits light at a specific wavelength that can be absorbed by the measured element.
  • Atomizer: This is where the sample is converted into free atoms. The sample is introduced into either a flame or a graphite furnace.
    • A flame uses a mixture of fuel and oxidant gases to create heat, atomizing the sample.
    • A graphite furnace uses an electrically heated graphite cylinder to vaporize the sample.
  • Monochromator
  • Detector
  • Readout Device

Assignment

A Medical Laboratory Scientist working at the Biochemistry Section of a tertiary hospital. The attending physician requests serum calcium analysis for five patients with varying clinical symptoms, including muscle cramps, cardiac arrhythmias, and confusion. You proceed to analyze the samples using Atomic Absorption Spectroscopy (AAS) and obtain the following data:

Calibration Standards:

Patient Sample Results:

Questions:

Show calculation and report answer in four significant figures.

1. Construct the calibration curve equation from the standard data.
2. Find the value of the slope and intercept.
3. Find the value of r-squared.
4. Calculate the calcium concentration (mg/L) for each patient.
5. Convert the calcium concentrations to mg/dL.

REFERENCES:

• Fundamentals of Analytical Chemistry 10th Edition: Skoog, et al., Cengage Learning (2021)
• Quantitative Chemical Analysis: Harris, D.C. (2020)