Chemiluminescence Notes

Instrument Technician - Chemiluminescence

Chemiluminescence

Rationale

Chemiluminescent analyzers are used in large industrial plants and government agencies to monitor nitrogen oxide pollutants. Government regulations mandate the measurement and reporting of these emissions. Understanding the operation of chemiluminescent analyzers is essential for their selection, installation, and maintenance.

Outcome

Upon completion of this module, you should be able to select, install, and maintain chemiluminescent analyzers.

Objectives

1. Describe the chemical reactions related to chemiluminescent analysis.

2. Describe the components of a chemiluminescent nitric oxide (NO) analyzer.

3. Describe the principles of analysis and application of chemiluminescent analyzers.

Introduction

This module explains the theory of chemiluminescence and the operating principles behind chemiluminescent nitric oxide analyzers. Specific terminology related to these analyzers will also be covered.

Objective One: Chemical Reactions Related to Chemiluminescent Analysis

Chemiluminescence

Luminescence is the emission of visible light from cold substances or processes, often called cold light. Two types of luminescence are important in environmental analyzer monitoring applications:

1. Fluorescence

2. Chemiluminescence

Fluorescence is the rapid emission of low-energy visible light when some substances are exposed to high-energy UV radiation. This principle is used in sulfur dioxide ($SO<em>2$) analyzers that measure parts per billion (ppb) levels of $SO</em>2$ in the atmosphere.

Chemiluminescence is the emission of light energy from certain chemical reactions. For example, the reaction between nitric oxide (NO) and ozone ($O<em>3$) produces radiation in the visible and infrared parts of the spectrum. NO is a pollutant by-product formed from nitrogen ($N</em>2$) and oxygen ($O_2$) during high-temperature fuel combustion. Chemiluminescence is used to measure ppb levels of nitrogen oxides in ambient air.

Phosphorescence is related to fluorescence. Phosphorescent materials emit low-energy light when exposed to high-energy radiation, but they do so slowly. These are the glow-in-the-dark materials used in toys and watch dials. Unlike fluorescent substances, phosphorescent materials continue to glow for hours after the energizing light source is removed.

Chemiluminescent Chemical Reactions and Equations

A chemiluminescent chemical reaction produces light energy, represented as:

reactants -> products + light

The chemiluminescent reaction between nitric oxide (NO) and ozone ($O<em>3$) produces nitrogen dioxide ($NO</em>2$) and oxygen ($O_2$), along with light:

NO + O3 -> NO2 + O_2 + light

Nitric oxide gains an oxygen atom to become $NO<em>2$, and $O</em>3$ loses an oxygen atom to become $O_2$.

Chemiluminescent reactions also produce excited molecules, similar to how substances like $SO_2$ can exist in an excited state before fluorescing. Excited molecules can release energy as light or transmit it to other molecules through collisions.

Chemiluminescence is a two-step process:

1. A molecule, such as $NO_2$, is formed with high energy electrons.

2. The excited molecule loses its excess energy as light to become a non-excited molecule in its normal state.

Initially, the electrons in $NO<em>2$ produced from the NO-$O</em>3$ chemical reaction are in an excited state. If these excited $NO_2$ molecules do not lose energy through collisions, they emit energy in a broad band of wavelengths ranging from the red end of the visible spectrum to the invisible infrared part of the EMR spectrum.

Only a percentage of the excited $NO_2$ molecules emit a photon of light to return to their normal energy state. Many excited molecules lose energy by colliding with other gas molecules, resulting in heat transfer.

The total number of photons emitted depends on the number of NO molecules involved in the reaction. The number of NO molecules is proportional to the concentration of NO. The intensity of the emitted light is proportional to the number of photons emitted from the reaction. Therefore, the intensity of the emitted light is also proportional to the concentration of NO.

Chemiluminescence Liquid Reactions

Glow sticks are an example of a chemiluminescent liquid reaction. A glow stick contains a chemical solution called Cyalume and a dye in a plastic tube. A smaller breakable glass tube holds hydrogen peroxide ($H<em>2O</em>2$) solution. When the plastic tube is bent, the inner glass tube breaks, and the hydrogen peroxide solution mixes with the Cyalume. The resulting chemiluminescent reaction produces visible light.

Objective Two: Components of a Chemiluminescent Nitric Oxide (NO) Analyzer

Chemiluminescent Nitric Oxide Analyzers

Nitric oxide and nitrogen dioxide emitted during high-temperature fuel combustion are collectively called oxides of nitrogen, represented as NOx. Nitric oxide quickly converts to nitrogen dioxide ($NO_2$) when it contacts oxidizing agents like ozone or oxygen in the air.

Chemiluminescent nitric oxide (NO) analyzers typically display three concentrations on ambient air and conditioned stack gas samples:

1. NO

2. $NO_2$

3. NOx (NO + $NO_2$)

Analyzers for ambient air samples detect and measure low ppb concentrations, while stack gas analyzers measure high ppm concentrations in flue gas. Chemiluminescent and UV stack gas analyzers measure ppm levels of NO, $NO_2$, and NOx.

The choice between UV and chemiluminescent analyzers for NO, $NO_2$, and NOx stack gas analysis depends on factors such as:

• Cost

• Installation issues

• Maintenance issues

• Hot/wet or cool/dry sampling

• In-situ or extractive sampling

• Interferences

Chemiluminescent analyzers can only handle cool/dry samples, restricting their sampling options to:

• Extractive cool/dry sampling: extracts a sample of stack gas and conditions it by filtering and drying.

• Dilution probe: filters and dilutes an extracted sample with pure dry air to reduce water vapor.

• A sample of cool/dry ambient air.

Analyzer Components and Operation

A typical chemiluminescent NO-$NO_2$-NOx analyzer includes:

• Reaction chamber: where sample NO reacts with excess ozone at a constant flow rate.

• Photomultiplier tube (PMT): measures the intensity of light emitted from the chemiluminescent reaction.

• Electronics: processes the PMT signal into a proportional concentration value.

The functions of these components are as follows:

• Ozonator: generates $O<em>3$ from $O</em>2$ or dry air using a UV lamp or high voltage electrical field. The ozonator supplies more ozone than required, ensuring complete reaction with NO.

• Red light filter: selects and passes the wavelengths of light emitted by the chemiluminescent reaction.

• Photomultiplier Tube (PMT): measures the intensity of the emitted light. In low-level (ppb) ambient air analyzers, the PMT is cooled to reduce electrical noise.

• Pump: exhausts the reaction chamber and draws a sample at atmospheric pressure through the analyzer. Excess ozone and $NO_2$ are removed by passing them through a scrubber before safe venting.

• Signal Processing: digital or analog electronics convert the PMT signal to a concentration reading in ppb or ppm. Microprocessor-controlled NO analyzers correct concentration readings for changes in sample gas pressure and temperature using pressure and temperature sensors.

Ozonator

An ozonator generates ozone. Oxygen molecules ($O<em>2$) convert to ozone ($O</em>3$) when flowing through a high voltage electrical field (approximately 10,000 V ac). A high voltage discharge breaks down $O<em>2$ molecules into O atoms, which then recombine into $O</em>3$ molecules.

Ozone is also generated using UV light. When an oxygen molecule is radiated with high-energy UV radiation (UV less than 240 nm), it splits into two single oxygen atoms (dissociates). Some of these single oxygen atoms react with oxygen molecules to form ozone.

Chemiluminescence Detector

The reaction between NO and $O<em>3$ produces excited $NO</em>2$, which emits light. At very low (ppb) concentrations of NO, very low amounts of $NO<em>2$ are produced. Not all excited $NO</em>2$ molecules release light; some lose their energy through collisions before emission.

Analyzer designs maximize chemiluminescence by:

• Using high concentrations of ozone from the ozonator to create as many excited $NO_2$ molecules as possible.

• Maintaining the reaction chamber at a reduced pressure to reduce gas molecule collisions with the excited $NO_2$ molecules.

A photomultiplier tube measures the intensity of the emitted light. An optical filter transparent only to the wavelengths of infrared (IR) and a small portion of red visible light makes the detector more selective. Cooling the PMT using a thermoelectric cooler reduces electronic noise.

Thermoelectric coolers are solid-state devices. When a dc current passes through them, they transfer heat from a heat-absorbing cool side of the semiconductor to a heat-releasing hot side.

Ozone Scrubber

Excess ozone, used to ensure all NO reacts, must be removed with an ozone scrubber to prevent personnel exposure. A scrubber contains a solid packing material that either retains the scrubbed chemical or reacts with it to produce harmless oxygen. Scrubbers must be maintained regularly by replacing their internal packing or installing a new device.

Objective Three: Principles of Analysis and Application of Chemiluminescent Analyzers

Nitric Oxide Analyzer Operation

A typical rack-mounted chemiluminescent analyzer has a front panel display with output information menus such as:

• Sample concentrations

• Calibration

• Instrument controls

• Help and error messages

The operator uses push buttons to provide input, using up and down arrow keys to select different menus and individual items in each category.

Chemiluminescent NO-$NO<em>2$-NOx analyzers may measure trace ppb levels in ambient air or ppm levels in conditioned stack gas samples. Both ambient air and stack gas normally contain NO and $NO</em>2$. The chemiluminescent reaction is between NO and $O<em>3$. Because $NO</em>2$ has no effect on the reaction, it must first be converted to NO to measure its concentration. After conversion, the analyzer reads the sum of the converted NO plus the original NO in the sample, called NOx.

Operating Modes

Two modes of operation are selected alternately by a solenoid-operated, three-way valve:

1. NO mode: connects the gas sample directly to the reaction chamber.

2. NOx mode: converts $NO<em>2$ present in the sample to NO. The NO from the $NO</em>2$ adds to the NO present in the original sample to give the NOx reading.

In the NO mode, an $NO<em>2$ to NO converter changes $NO</em>2$ to NO using a heated metal reactant. Constant flow rates of $O_3$ and NO enter a reaction chamber where the chemiluminescent reaction occurs. The instrument uses devices such as capillary restrictions and pressure regulators to maintain constant flow rates.

• NO Measurement Mode: sample containing both NO and $NO<em>2$ connects directly to the reaction chamber, bypassing the $NO</em>2$ to NO converter.

• NOx Measurement Mode: the three-valve switches and connects the sample $NO<em>2$ and NO to the converter. The $NO</em>2$ converts to NO which adds to the original NO in the sample. Only NO enters the reaction chamber. The analyzer reads this total concentration of NO as NOx.

The analyzer automatically switches the electrically operated three-way valve back and forth between the NO and NOx modes, providing the following data:

• Measured NO concentration ([NO]) when in the NO mode.

• Measured total nitrogen oxides concentration ([NOx]) when in the NOx mode.

• $NO<em>2$ concentration calculated as [$NO</em>2$] = [NOx] - [NO].

Example

A chemiluminescent analyzer detector measures the following NO concentrations on an ambient air sample containing NO and $NO_2$:

• 51.9 ppb after passing it through an $NO_2$ to NO converter.

• 1.9 ppb after bypassing the $NO_2$ to NO converter.

To calculate the analyzer readings:

• The concentration of NOx is 51.9 ppb: 51.9 ppb [NOx] = [NO] + [$NO_2$].

• 1.9 ppb is [NO], the NO present in the sample.

• The instrument calculates [$NO<em>2$] by subtracting [NO] from [NOx]: [$NO</em>2$] = [NOx] - [NO] = 51.9 ppb - 1.9 ppb = 50.0 ppb.

Flow Control

Accurate concentration readings require NO and $O<em>3$ flow controls. Flow controls ensure all NO is always converted to $NO</em>2$ in the reaction chamber, achieved by using more $O_3$ than NO.

The intensity of the light emitted depends on the number of NO molecules entering the chamber, which depends on NO concentration and flow rate. Maintaining a constant NO flow rate ensures light intensity depends only on NO concentration.

A simple flow control system uses capillaries to restrict flow and a vacuum pump. A capillary is a thick-walled glass tube with a very small bore of known diameter. Flow rates depend on the diameter of the capillaries and the pressure drops across them.

Sample gas and dry air are supplied at atmospheric pressure. The differential pressure across a capillary is the difference between atmospheric pressure and the reduced pressure at the vacuum pump. Constant atmospheric pressure results in a constant differential pressure. The diameters of the capillaries determine the relative flow rates of sample gas and ozone.

Typically, a particulate filter traps any solids in the sample at the inlet to the analyzer.

Common maintenance tasks for flow control systems:

• Replace blocked filters.

• Clean or replace blocked capillaries.

• Rebuild or replace the vacuum pump when the vacuum pressure is insufficient.

• Replace the charcoal scrubber which removes $O<em>3$ and $NO</em>2$ from the pump exhaust gas periodically.

Pressure and Temperature Corrections

Modern instruments use microprocessors to continuously correct concentration readings for changes in sample gas temperature and pressure. Atmospheric pressure may vary by approximately ±5% over several weeks, and analyzer temperature may also fluctuate. These changes affect gas density and the number of gas molecules in a given volume.

Instruments store temperature and pressure values measured at calibration time and continuously correct the concentration data. The formula for corrected concentration (C) is:

$corrected \space concentration \space (C) = C<em>m \times \frac{T</em>c}{T<em>m} \times \frac{P</em>m}{P_c}$

Where:

• $C_m$ = current measured concentrations

• $T_c$ = temperature at calibration

• $T_m$ = current temperature

• $P_c$ = pressure at calibration

• $P_m$ = current pressure

Gas calculations require absolute temperature and pressure units, such as Kelvin ($^{\circ}C$ + 273.15) and mm Hg absolute.

Calibration

Chemiluminescent NO-$NO_2$-NOx analyzers require an accurately known ppb level concentration and a source of pure zero air to check or adjust the analyzer zero.

Pure zero air should be dry and contain no contaminants that could react with NO, $NO<em>2$, or $O</em>3$. A series of chemical scrubbers and a filter dry, purify, and remove solids from an instrument air supply. Typical components include:

• Silica gel: removes water vapor.

• Purafil: removes acid gases such as H₂S, $SO<em>2$, $NO</em>2$, and NO.

• Activated charcoal: removes remaining traces of a wide range of gases.

• Five-micron particulate filter: protects downstream analytical equipment from solid particles.

Zero air is used to:

1. Check or adjust the trace chemiluminescence analyzer zero.

2. Prepare standard NO/air gas mixtures to check or adjust the analyzer NO span.

3. Prepare standard $NO<em>2$/air gas mixtures to check or adjust the analyzer $NO</em>2$ span.

Trace level calibration gas mixtures in the low ppm and ppb concentration range cannot be purchased accurately. Commercial procedures such as weighing are inaccurate, and storage in cylinders results in decreasing gas concentrations over time.

Chemiluminescent NO-$NO<em>2$-NOx analyzers are calibrated using a diluter or a dynamic gas calibrator to dilute a high concentration calibration gas with zero air. The calibrator adjusts the flow rates automatically to generate a particular concentration of gas ($C</em>{mix}$) according to the equation:

$C<em>{mix} = C</em>{cal} \times \frac{F<em>{cal}}{F</em>o + F_{cal}}$

Where:

• $C_{cal}$ = concentration of the calibration gas in the cylinder

• $F_{cal}$ = flow rate of the calibration gas from the cylinder

• $F_o$ = flow rate of zero air

• $C_{mix}$ = concentration of the diluted gas

Calibration