Forensic Analysis and Analytical Chemistry: Spectroscopy Lecture Notes

Course Information

• Course Title: Forensic Analysis (CHEM20451)

• Course Title: Analytical Chemistry (CHEM-20551)

• Lecturer: Dr. Muhammad Zaheer

• Contact Details:

  • Email: muhammad.zaheer03@ntu.ac.uk

  • Room: Erasmus Darwin Building (ERD) 230

  • Office Hours: Monday 10 - 11 am

  • Response Time: Email responses within 24 hours, in-person meetings by prior arrangement if outside office hours

Required Text

• Chapter 18: Quantitative Methods of Analysis

  • Authors: Daniel C. Harris

  • Publisher: W. H. Freeman, New York

  • Editions Available:

  • 10th Ed. eBook available through NOW

  • 9th Ed. Available in Clifton Library (543.1 HAR)

  • 8th Ed. Available in Clifton Library (543.1 HAR)

• Directed Reading

Key Components of Optical Instruments

• 5 Key Components:

  • Source: Provides radiation

  • Sample Holder: Holds the sample for analysis

  • Wavelength Selector: Selects particular wavelengths of light

  • Detector: Measures the intensity of light

  • Output: Produces a signal based on the detected light

Radiation Sources

• Types of Radiation Sources:

  • Continuum Sources:

  • Tungsten: Covers visible to infrared (IR), resembling a filament bulb.

  • Deuterium: Operates in the ultraviolet (UV) region (e.g., like UV sterilization lamps).

  • Line Sources:

  • Hollow Cathode Lamps: Used in Atomic Absorption Spectroscopy (AAS)

  • Lasers: Employed in Raman and fluorescence spectroscopy.

Learning Objectives

By the end of this lecture, students should be able to:

• Identify main components of a spectroscopic instrument.

• Explain functionality of radiation sources, filters, monochromators, and detectors.

• Describe effects of spectral bandwidth and stray light on measurement quality.

• Differentiate between single- and double-beam designs.

• Relate spectroscopic components to practical applications in forensic and analytical fields.

Properties of Light Sources

Laser Operation (Energy Levels)

1. Pumping: Energy supplied to raise electrons to higher energy levels (E₃).

2. Relaxation: Electrons drop quickly to an intermediate energy level (E₂).

3. Population Inversion: More electrons are in E₂ than in E₁ (a non-equilibrium situation).

4. Stimulated Emission: Incoming photons trigger other electrons in E₂ to drop to E₁, emitting identical photons.

5. Amplification: A cascade effect builds a strong, coherent beam.

Sample Holders

• Materials: Cuvettes must be made from materials nearly transparent in the spectral region of interest:

  • UV Range (below 350 nm): Quartz or fused silica

  • Visible Range (above 350 nm): Silicate glass and some plastics

• Standard Cell Length: 1 cm is commonly used for measuring absorbance in UV and visible regions.

• Caution: Cuvettes are delicate and require careful handling.

Wavelength Selection (Monochromators)

• Purpose: Required to either input light from a source or analyze emitted light from a sample.

• Types of Monochromators:

  • Refracting Prisms

  • Diffraction Gratings

Effective Bandwidth

• Definition: The effective bandwidth is the range of wavelengths that can pass through the monochromator and depends on:

  • Quality of the dispersing element (prism or grating)

  • Width of the slit (narrower width is preferable)

  • Focal length of the optics

• Typical Ranges:

  • High-quality instruments: < 1 nm (UV–Vis)

  • Low-cost instruments: > 20 nm

• Quantitative Work Range: Commonly between 1 and 20 nm.

Spectral Bandwidth

• Wide Slit Widths:

  • Increases light intensity reaching the detector

  • Improves signal-to-noise ratio leading to better precision

  • However, may reduce wavelength resolution and lead to peak distortion when the bandwidth is equal to or greater than the peak width.

Monochromator Designs

• Refracting Prism:

  • Light enters through entrance slit, passes through a collimating mirror to a prism where it is dispersed by refraction.

  • The angle of rotation selects the desired wavelength.

• Diffraction Grating:

  • Uses closely spaced grooves to split light into its component wavelengths via interference.

  • The selected angle determines which wavelength is directed toward the exit slit.

• Resolution of a Grating Formula: $R = \frac{\lambda}{\Delta\lambda} = nN$

  • Where:

  • $\lambda$ is the wavelength

  • $n$ is the diffraction order

  • $N$ is the number of grooves of the grating illuminated.

Detector Types

• Phototube:

  • Comprised of a semicylindrical photocathode that emits electrons when light strikes it.

  • Generates a photocurrent that can be amplified by a series of dynodes.

• Photomultiplier Tube (PMT):

  • Involves multiple dynodes to amplify the signal from emitted photoelectrons, converting low light intensities into measurable electric signals.

• Silicon Photodiode:

  • Used to measure radiant power, forming a “hole” in the depletion layer in response to incident light.

Photodiode Array Detectors

• Functionality:

  • Simultaneously detect several wavelengths across the UV-Vis spectrum using a diffraction grating to disperse light onto multiple photodiodes.

  • Each diode records the intensity of light at a specific wavelength, allowing for real-time spectrum creation.

Spectrophotometer Types

• Single-Beam Spectrophotometer:

  • Measures sample and reference alternately but can be sensitive to light intensity drift.

• Double-Beam Spectrophotometer:

  • Splits light between the sample and reference simultaneously for real-time analysis, improving accuracy and stability.

Instrumentation Summary for Spectroscopy

1. Light Sources:

   • Deuterium lamp for UV region (200-400 nm)

   • Tungsten lamp for visible region (400-700 nm)

2. Monochromators:

   • Use diffraction gratings or prisms to select specific wavelengths.

3. Sample Holders:

   • Use quartz cuvettes for UV; glass or plastic for visible light.

4. Detectors:

   • PMTs for high sensitivity, Photodiode Arrays for simultaneous multiple wavelength detection.

5. Measurement Types:

   • Single-beam: Sequential measurements of sample and reference.

   • Double-beam: Real-time dual measurements for improved accuracy.