Spectroscopic Analyzers Notes

Instrument Technician: Spectroscopic Analyzers

Spectroscopy

Spectroscopy is the science of how matter interacts with electromagnetic radiation.

Spectroscopic analyzers perform:

• Qualitative analysis: Identifies unknown substances (common in labs).

• Quantitative analysis: Determines substance concentrations in mixtures (labs or process applications).

Electromagnetic Radiation and Spectrum

Described by:

1. Wave theory of matter

2. Photon (particle) theory of light

Wave Theory of Matter

Radiation travels as sine waves (electric and magnetic vibrations at right angles, peaking simultaneously).

• Wavelength: Distance between equivalent points on successive waves.

• Frequency (f): Wave cycles per second (Hertz [Hz]).

$Frequency (f) = \frac{Speed \, of \, light \, (c)}{Wavelength \, (\lambda)} = \frac{3.00 \times 10^8 m/s}{\lambda \, m}$

• Speed of light in air: $3.00 \times 10^8$ m/s

Example:

Calculate the frequency of electromagnetic radiation with wavelengths:
a) $1 \times 10^{-5}$ m
b) $1 \times 10^{-7}$ m

Solution:
a) $f = \frac{3.00 \times 10^8}{1 \times 10^{-5}} = 3.00 \times 10^{13} Hz$
b) $f = \frac{3.00 \times 10^8}{1 \times 10^{-7}} = 3.00 \times 10^{15} Hz$

• Wavelength decreases, frequency increases.

Photon (Particle) Theory of Light

Radiation consists of particles (photons) with energy but no mass, moving at the speed of light.

Photon energy (E):

$Energy (E) = Constant (h) \times frequency (f) = \frac{h \times Speed \, of \, light \, (c)}{Wavelength \, (\lambda)}$

High energy EMR has high frequency and short wavelength.

Electromagnetic Spectrum

Arranges EM radiation by energy, wavelength, and frequency into groups (regions):

• Gamma ray (shortest wavelength, highest energy)

• X-ray

• Ultraviolet

• Visible

• Infrared

• Microwave

• Radio wave (longest wavelength, lowest energy)

Spectroscopic analyzers use:

• Infrared (IR)

• Visible

• Ultraviolet (UV)

Visible light is a narrow region. Infrared and ultraviolet radiations occur at the two ends of the visible part of the spectrum.

Absorption and Emission Spectrums

Atoms: Basic components of matter with a positive nucleus and negative electrons.

Energy levels: Electron energy amounts. Outermost electrons absorb EMR energy, raising them to an excited state. Excited electrons emit EMR energy, returning to their normal state.

• EMR energy absorbed or emitted falls in the visible and ultraviolet regions.

White light: Contains all visible EMR wavelengths (red to violet). Objects change colour after absorbing visible EMR energy.

• Objects absorbing all wavelengths appear black.

• Objects absorbing none appear white.

• Transmitted light passes through transparent materials.

Electromagnetic Radiation Emission

Devices (filament and vapour lamps) emit EMR.

Two types:

1. Continuous

2. Line

Continuous Emission

Heated solid objects emit a range of EMR wavelengths (UV, visible, IR depending on temperature).

Prism: Separates EMR wavelengths by refraction angle (depends on wavelength).

Emission spectrum: Plot of wavelength versus intensity (photons emitted per wavelength).

• Peaks show maximum intensity at a particular wavelength (depends on object temperature).

• Hotter objects have higher energy (shorter wavelength) peaks.

Heated filament lamps are used in laboratory spectroscopic analyzers where wavelength must continuously vary over a range of values.

Line Emission

Vapour lamps: High voltage arc through low-pressure gas/vapour excites electrons, emitting EMR.

• Energy/wavelength depends on atoms present.

• Sodium vapour lamps emit yellow-orange light.

• Mercury vapour lamps emit blue light and some UV.

Vapour lamps emit only a few wavelengths (vertical emission lines).

Line emission spectrum: "Fingerprint" of gas/vapour type. Chemists use it to identify elements in unknown substances.

• Process analyzers using UV radiation make use of mercury vapour lamps that produce several UV emission lines.

Electromagnetic Radiation Absorption

Removal of energy from radiation beam by substances (e.g., process gas samples).

Absorption spectrum: Plot of EMR transmitted, showing dark lines/bands due to absorption of specific wavelengths.

• Gases, vapours, or liquids allow bands of EMR wavelengths to pass.

Gas atoms absorb specific wavelengths. Comparing the original emission spectrum of the lamp against the absorption spectrum shows drops in radiation intensity for each wavelength that the gas absorbs.

Spectroscopic Analyzers

Quantitative and qualitative analysis based on light absorption/emission.

Three types:

1. Photometric analyzers

2. Spectrophotometers

3. Ambient air pollutant analyzers

• Ambient air pollutant analyzers measure parts per billion (ppb) levels of sulfur dioxide and nitrogen monoxide in the air using EMR emission.

Photometric Analyzers

Uses:

• Concentration measurements (quantitative)

• Light absorption

• Infrared (IR), Visible Light, Ultraviolet (UV)

Parts:

• EMR Source

• Wavelength Selector

• Sample Cell

• EMR Detector

• Concentration Readout Device

Electromagnetic Radiation (EMR) Source

Provides radiation for sample absorption. Different types for IR, visible, and UV photometers.

Hot wires and lamp filaments are continuous emission sources, while vapour lamps are line emission sources.

• Hot wire emits IR, little visible light.

• Incandescent lamps produce visible light with a heated filament inside a glass bulb.

• Vapour lamps (e.g., mercury) are high-intensity UV sources.

Wavelength Selector

Monochromator: Selects desired wavelengths, blocks others. Can be before OR after sample cell.

Interference filter: Optical device passing desired wavelength, blocking others via interference. Interaction between the EMR of wavelengths, causing them to either cancel out or reinforce each other.

• Internal reflection creates out-of-phase radiation, causing destructive interference.

• Only in-phase reflected rays pass through.

• Spacing between glass discs selects a single wavelength.

Using interference filters with a line emission source selects specific wavelengths for accurate concentration measurements.

Sample Cell

Places conditioned samples in UV, IR, or visible radiation path. Analyzes continuously via sample flow.

• Cell body: Resists chemical attack and temperature/pressure effects (Teflon, stainless steel, metal alloys).

• Cell windows: Transparent to radiation, unaffected by sample (see table below).

Cell path length: Sample length within the cell. Liquids have more mass & molecules than the gases = Shorter cell path length. Optimum length depends on sample absorption strength.

• Liquid cells: < 1 cm.

• Gas cells: up to several meters.

Open path in-situ applications: No sample cell. Radiation travels directly through the gas.

Electromagnetic Radiation (EMR) Detector

Receives radiation passing through the wavelength selector and the sample. Converts light intensity into an electrical signal.

IR detectors require different tech than UV and Visible EMR. Three types of devices that you can use to detect UV and visible radiation are: Photodiodes, Phototubes, and photomultiplier tubes (PMT).

Ultraviolet and Visible Electromagnetic Radiation Detectors

• Photodiodes: Solid-state diode with a transparent window over the junction. Photons create current proportional to radiation intensity.

• Phototubes: Evacuated glass tube with a negative photocathode (emits electrons when photons hit) and a positive anode (collects electrons).

• Photomultiplier Tubes (PMT): Is similar to the phototube but with Dynodes (more sensitive). Similar to phototubes but with dynodes, making it more sensitive and allowing it to accurately measure low light levels.

Infrared (IR) Detectors
IR detectors:

1. Thermal (Thermocouples and Thermopiles)

2. Photo (Conductive and Voltaic)

• Photo Conductive detectors change electrical conductivity in proportion to IR intensity

• Photo Voltaic generate an electrical current proportional to IR intensity

Readout Device

Converts the electrical signal from the detector into a concentration reading.

• Increasing concentration increases absorption, reducing intensity.

• Concentration is proportional to log $\frac{I<em>{IN}}{I</em>{OUT}}$

• Modern analyzers use microprocessors to calculate the logarithm ratio values and display proportional concentration readings.

Spectrophotometers

Laboratory analyzers that plot absorption spectrums. Are used for qualitative and quantitative analysis

• Qualitative analysis: Identify unknown substances by comparing absorption spectrums with known compounds.

• Quantitative analysis measures the drop of intensity when a sample absorbs a single wavelength of radiation.

Spectrophotometers scan ultraviolet, visible and infrared regions.

• Absorption Spectrums have Inverted Peaks

• Emission Spectrums have regular peaks

• Continuous emission (EMR) source: Provides a range of wavelengths.

• Dispersive monochromator (variable wavelength selector): Separates each wavelength rather than using filters.

Dispersive Monochromators

Selects a single wavelength from a group by dispersing them at different angles.

1. Prisms

2. Diffraction gratings

Prisms

Transparent triangular devices refracting radiation.

• White light disperses into rainbow colours.

Diffraction Gratings

Parallel grooves on a reflective/transparent surface. Radiation creates interfering waves.

• Diffraction is another name for the destructive and reinforcing (constructive) interaction of radiation waves.

• Different wavelengths interfere at different angles (angles of diffraction).

Beer-Lambert Absorption Laws

Applies to Infrared (IR) and Ultraviolet (UV) absorption analyzers.

• $I_{IN}$ = Intensity of EMR entering the sample

• $I_{OUT}$ = Intensity after passing through the sample

• c = Concentration of the absorbing substance

• b = Path length of the radiation through the sample

Two ways to look at the effect of the sample:

1. Sample transmittance (EMR that passes through)

2. Sample absorbance (EMR that is absorbed)

Sample Transmittance

Fractional intensity of EMR that transmits through the sample:

$Fractional \, transmittance = T = \frac{I<em>{OUT}}{I</em>{IN}}$

Expressed as percentage intensity (%T):

$\%T = T \times 100\%$

Example

UV analyzer measures sulfur dioxide in stack gas. The gas transmits one-third of the UV intensity. Find the %T.

$T = \frac{1}{3} = 0.333$

$\%T = 0.333 \times 100\% = 33.3\%$

Sample Absorbance

Base 10 logarithm of the ratio of entering EMR to exiting EMR:

$Absorbance \, (A) = log \frac{I<em>{IN}}{I</em>{OUT}}$

Two differences between transmittance (T) and absorbance (A) exist.

1. Absorbance is logarithmic, while transmittance is linear.

2. Absorbance is the fraction of EMR intensity that the sample absorbs, while transmittance is the fraction of light the sample transmits.

Must convert %T to fractional T

$T = \frac{\%T}{100\%}$

Example

An IR spectrophotometer measures a %T value of 30.0% on a sample that contains carbon monoxide. Calculate the absorbance value of the sample.

$T = \frac{30.0\%}{100\%} = 0.300$

$A = log(\frac{1}{0.300}) = log(3.33) = 0.522$

Sample Concentration

$A = abc$

Absorbance (A) = absorptivity (a) x path length (b) x concentration (c)

Absorptivity (a) is equal to absorbance (A) divided by concentration and path length.

$a = \frac{A}{bc}$

Example 1

Sulfur dioxide has an absorbance of 0.22 when the sample concentration is 30.0 ppm, using a 12 cm path length cell. Calculate the absorptivity value in this example.

$A = \frac{0.22}{12 \times 30.0} = 6.1 \times 10^{-4} \frac{1}{cm \cdot ppm}$

Example 2

A sample of sulfur dioxide (SO2) gas that passes through an 18 cm cell in a UV photometric analyzer transmits 42% of the UV radiation. Calculate the concentration of SO2 in the gas sample if its absorptivity (a) is $5.0 \times 10^{-4} \frac{1}{cm \cdot ppm}$.

$T = \frac{\%T}{100\%} = \frac{42\%}{100\%} = 0.42$

$A = log(\frac{1}{0.42}) = log(42) = log(2.38) = 0.38$

$c = \frac{A}{ab} = \frac{0.38}{(5.0 \times 10^{-4}) \times 18} = 42 ppm \, SO_2$

New path length equation:

$New \, path \, length = Old \, path \, length \times \frac{Old \, upper \, range \, value}{New \, upper \, range \, value}$

Example 3

A sulfur dioxide photometric analyzer uses a cell path length of 12 cm to measure the absorbance of samples in a concentration range of 0 ppm to 1000 ppm. What should this path length be if the range changes from 0 ppm to 100 ppm?

$New \, path \, length = 12 \, cm \times \frac{1000 \, ppm}{100 \, ppm} = 120 \, cm$

Photometric Absorption analyzers are most accurate with maximum sample absorbance and absorbance value around 0.5

Fluorescence and Chemiluminescence

Fluorescence: Emission of lower energy EMR from a substance absorbing higher energy radiation.

Sulfur Dioxide (SO2) is a fluorescent gas that emits visible light under UV exposure (Ideal for detection)

UV --> Visible Light

Chemiluminescence: Light emission from a chemical reaction.

$NO + O<em>3 \rightarrow NO</em>2 + O_2 + Visible \, EMR$