Instrument Technician: Ultraviolet Analyzers

Ultraviolet Analyzers

Rationale

Ultraviolet analyzers are vital for:

• Identifying unknown compounds in laboratories.

• Continuously measuring chemical compound concentrations in liquids and gases in process applications.

• Process control.

• Environmental monitoring.

• Worker safety.

• Correct selection, installation, and maintenance is crucial.

Outcome

Upon completion of this module, you will be able to:

• Select, install, and maintain ultraviolet analyzers.

Objectives

• Describe the principles of analysis and application of ultraviolet (UV) analyzers.

• Describe the components of UV analyzers.

• Describe UV precautions and hazards.

• Explain the differences between UV absorption and UV emission (fluorescence) analysis.

Introduction

This module covers:

• Theory and operation of ultraviolet analyzers.

• Operating principles of various types of UV analyzers for different process applications.

• Differences between laboratory and process ultraviolet analyzers.

• Specific related terminology.

Objective One

Describe the principles of analysis and application of ultraviolet (UV) analyzers.

Ultraviolet Analyzers

• Spectroscopic analyzer type.

• Performs qualitative and quantitative analysis.

• Sends UV radiation through a gas or liquid sample.

• Measures the degree to which the radiation is absorbed by the sample.

• Laboratories use UV absorption spectrums for qualitative analysis.

UV Absorption Spectrum:

• Plot of absorbance or transmittance against UV radiation wavelengths.

• Process UV analyzers are often single-component analyzers.

• Designed to measure one type of substance.

• Measure the absorbance of a fixed wavelength of UV radiation.

Beer-Lambert Law

According to the Beer-Lambert Law, concentration is proportional to absorbance.

Ultraviolet Analysis Principles and Applications

• Ultraviolet radiation is electromagnetic radiation (EMR).

• Wavelength range: approximately 100 nm to 400 nm.

• One nanometre is 10^{-9} of a metre.

• The UV region begins at the violet end (400 nm) of the visible region of the EMR spectrum.

• As wavelength decreases, UV radiation increases in frequency and energy.

Absorption

• Ultraviolet radiation is absorbed by the electrons in molecules.

• This raises the energy level of the electrons.

• Some compounds absorb UV radiation and can be analyzed by measuring the amount of UV radiation they absorb.

Common Compounds

Many common compounds such as air, carbon dioxide (CO2) and water vapour (H2O) do not absorb UV radiation.

Important Difference:
Compared to the IR region where heteronuclear molecules such as CO2 and H2O strongly absorb IR radiation.

Heteronuclear molecules: molecules composed of more than one type of element.

Stack Gas Example

Large amounts of water vapour and carbon dioxide present in stack gas do not absorb UV radiation, so UV analyzers can measure SO2 and NO2 in stack gas without addressing interferences from these background gases.

UV Absorption Spectrum

A UV absorption spectrum generally shows one or two broad peaks, unlike IR spectrums which often contain many sharp absorption peaks.

Example: Sulfur Dioxide
Sulfur dioxide has a broad peak from about 250 nm to 330 nm, with maximum absorbance at about 280 nm.

UV Spectrophotometer

• Dispersive analyzer: UV radiation is separated into wavelengths.

• Most laboratory analyzers are UV spectrophotometers.

• Plots UV absorption against wavelength (absorption spectrum).

• Used for both qualitative and quantitative analysis.

Qualitative Analysis

Performed by comparing the wavelengths for maximum absorbance of unknown compounds with those of known compounds to identify the unknown substance.

Limitation:
UV absorption spectrums have few absorption bands, and bands for different compounds overlap, so it is not always possible to distinguish between compounds.

UV Photometer

• Non-dispersive analyzer: UV radiation is not separated into wavelengths.

• Most process UV analyzers are UV photometers.

• Designed to continuously measure the concentrations of specific compounds in process samples.

Example: Sulfur Dioxide in Stack Gas
A UV photometer for SO2 in stack gas is designed specifically for this application. The manufacturer needs to know which UV wavelengths are strongly absorbed and those not absorbed by SO2 to make the analyzer respond to changes in SO_2 concentration.

Laboratory Ultraviolet Spectrophotometers

• Determine the absorption spectrum of a gas or liquid.

• Uses components that provide a wide range of UV wavelengths.

• Uses a dispersive device (e.g., grating or prism) to separate the wavelengths individually for detection.

Main Parts:

• UV radiation source or lamp: generates a broad range of UV wavelengths.

• Sample cell with windows: allows UV radiation to pass through a fixed length of sample gas or liquid.

• Wavelength selector (e.g., grating): separates the UV into different wavelengths.

• UV detector: sends a signal proportional to the intensity of the UV wavelength.

• Detector signals are converted to either transmittance (T) or absorbance (A) values.

The instrument automatically plots the absorbance spectrum by synchronizing the position of the grating and the corresponding wavelength values to the recording time on its horizontal axis. Transmittance or absorbance values are plotted on the vertical axis.

Process Ultraviolet Photometers

Analyze process samples for specific compounds by measuring the UV absorbance value of a strongly absorbed wavelength and relating it to concentration.

Key Features:

• Do not require dispersive devices.

• Use filters to determine absorbance values at specific UV wavelengths.

Main Parts:

• UV source: emits specific wavelengths (one strongly absorbed, one not absorbed for reference).

• UV filters: select the absorbed and reference wavelengths.

• Sample cell with windows: allows UV radiation to pass through a fixed length of sample gas or liquid.

• UV detector: converts the intensities of the absorbed and reference wavelengths into an electrical signal.

• Concentration readout device: calculates concentration from absorbance values determined by the absorbed and reference intensity signals.

Note:
UV photometers designed for single-component analysis monitor two wavelengths:

• A wavelength that is strongly absorbed by the sample component.

• A wavelength that is not absorbed.

The intensity of the non-absorbed wavelength (I{Reference}) is a reference measurement. The intensity of the absorbed wavelength (I{Measure}) is compared against the non-absorbed reference intensity to determine the absorbance of the sample.

The instrument readout device calculates absorbance from the two intensity measurements as follows:

Absorbance A = log \frac{I{Reference}}{I{Measure}}

Example 1: Phenol in Waste Water

Scenario: An instrument manufacturer wants to set up a UV photometer to analyze phenol in waste water solutions. A laboratory spectrophotometer plots the UV absorption spectrum of a sample of phenol in water. Water does not absorb UV light, so it does not interfere with the phenol absorbance measurements.

Task: Find the values that should be chosen for the absorbed (measure) and unabsorbed (reference) wavelengths.

Solution:
The ideal measure wavelength is the one that is most strongly absorbed by the phenol so that small concentrations give large absorbance values for accurate measurements. From the figure, the ideal measure wavelength is approximately 260 nm.

The ideal reference wavelength should be one that is not absorbed by phenol so that it gives an absorbance value of zero. From the figure, the ideal reference wavelength could be any wavelength from approximately 300 to 400 nm.

Important: Ultraviolet photometers normally use line emission UV source lamps that produce high intensity UV radiation, but have a limited number of emitted wavelengths. A mercury vapour lamp is often used as a source of UV radiation. The UV photometer manufacturer can use only UV wavelengths that are actually produced by the UV lamp.

Example 2: Phenol in Wastewater with Mercury Vapor Lamp

Scenario: An instrument manufacturer is setting up a new UV photometer to analyze phenol in wastewater. The analyzer uses a mercury vapor lamp UV source. The manufacturer must choose measuring and reference filter wavelengths given the following mercury lamp line emission wavelengths:

• 405 nm

• 365 nm

• 313 nm

• 289 nm

• 265 nm

• 254 nm

Task: Find the wavelength that the measure UV filter should select and the wavelength that the reference UV filter should select.

Solution:
Choose the measure and reference wavelengths only from those emitted by the mercury lamp. The 265 nm mercury emission line has the highest absorbance value, so choose it as the measure wavelength and use a measure UV filter to select it.

Either the 405 nm, 365 nm or the 313 nm mercury emission lines have the lowest absorbance value, so one of these should be chosen as the reference wavelength. However an interfering compound commonly found with phenol absorbs UV at 365 nm and 313 nm. As a result, choose the 405 nm mercury emission line as the reference wavelength and use a reference UV filter to select it.

Example 3: Calculating Phenol Concentration

Scenario: A UV photometer measures the following phenol absorbance (A) values, the first when a calibration solution flows through the sample cell and the second when the process waste water is analyzed:

1. A=0.357 for 0.5 µg/L calibration solution and

2. A=0.298 for the process wastewater.

Task: Calculate the concentration of phenol in the wastewater.

Solution:
According to the Beer-Lambert law, absorbance (A) is proportional to sample component concentration (c) according to the following equation.

A = (ab) \times c = constant \times c

Since absorptivity (a) is a constant for the sample component at a specific wavelength and the path length (b) is fixed by the length of the sample cell, a times b (ab) has a constant value (c).

To calculate the concentration, follow these steps.

1. Absorbance (A) is proportional to concentration (c) according to the Beer-Lambert law, so use this formula.
   A = (ab) \times c = constant \times c

2. The constant (ab) is calculated from the calibration gas data as follows.

   constant = \frac{A}{c} = \frac{0.357}{0.5 \mu g/L} = 0.714 \text{ per } (\mu g/L)

3. The unknown concentration of phenol in the waste water (c in μg/L) can now be calculated from its absorbance (A) value as follows.

   c = \frac{A}{constant} = \frac{0.298}{0.714 (\mu g/L)^{-1}} = 0.417 \mu g/L

The unknown concentration is 0.417 μg/L.

Process Ultraviolet Analyzer Applications

Three important process UV analyzer applications are:

1. Environmental stack gas monitoring

2. Sulfur compound gas monitoring in a sulfur plant

3. Bleach monitoring in pulp mills

Environmental Stack Gas Monitoring

As a result of burning fuel to generate heat and produce steam, large industrial plants emit sulfur dioxide (SO2) and mixtures of nitrogen oxides known as NOx. These gases are the principle causes of acid rain, so their emission rates into the atmosphere are controlled by government regulations. Companies must monitor the emission rates of SO2, nitrogen monoxide (NO) and nitrogen dioxide (NO2) gases leaving their stacks to ensure they are below the permitted values. These gases all strongly absorb UV radiation, so UV stack gas analyzers have been used since the 1970s to monitor high concentrations (often hundreds of ppm) of these stack gases. Stack gas pollution monitoring is also known as source monitoring, since the gases in the stack are the source of the pollutants.

After the gases leave the stack and become diluted by the surrounding air, their concentrations drop to very low levels that cannot be monitored by UV absorption. Specially designed ambient air environmental analyzers measure concentrations as low as a few parts per billion (ppb). They use other techniques such as UV fluorescence for SO_2 and chemiluminescence for NO.

Stack gas analyzers use UV absorption to measure samples taken in several ways:

1. In-situ using a gas measurement cell placed directly in the stack gas

2. Extractive hot, wet sampling

3. Extractive cool, dry sampling

For in-situ measurements, the sample cell is located inside the stack. The filtered stack gas flows through the cell and absorbs UV radiation sent from a module located outside the stack. A mirror at the end of the sample cell reflects the UV beam back to the external module which selects specific wavelengths and measures their intensities. A separate electronics module processes the detector signals and calculates gas concentrations. The gas concentration is measured without removing any stack gas water vapour, so it is determined on a wet basis.

In hot, wet extractive sampling, a sample of the stack gas is drawn through a probe and kept hot, above its dew-point temperature, throughout sample conditioning and analysis. This ensures that the contained water does not condense. A heated sample cell inside the UV analyzer maintains the hot conditions so that the analyzed stack gas has the same water content as the gas inside the stack. Gas concentration measured with this technique is determined on a wet basis.

In cool, dry extractive sampling, a sample of the stack gas is drawn through a probe and kept hot, above its dew-point temperature, until it leaves the dryer and filter of the sample conditioning unit. The dried, filtered gas travels to a cool, dry UV analyzer in an unheated sample line since its dew-point temperature is now below the outside temperature, so water cannot condense. Gas concentration measured with this technique is determined on a dry basis.

Dry stack gas has a different composition than wet stack gas. This difference affects the SO2 and NOx concentrations. Measurements made on a dry basis are higher than wet basis concentrations. Stack gas concentration reporting is often required on a dry basis.

Monitoring Sulfur Compounds in Tail Gas Units

Sulfur recovery units (SRUS) convert hydrogen sulfide (H_2S), a by-product of refining sour oil and gas, into elemental sulfur. Conversion to sulfur is not 100% efficient, so the tail gas contains some of the following sulfur compounds:

• unconverted H_2S

• SO_2

• CS_2 (carbon disulfide)

• COS (carbonyl sulfide)

The tail gas sulfide compounds are highly toxic and foul smelling. They cannot be released to the atmosphere, so they are converted to H2S in a tail gas unit. The H2S is recycled back through the SRU, which converts it to sulfur.

Ultraviolet analyzers designed to measure H2S, SO2, CS_2 and COS monitor the tail gas to control the SRU so that it produces the maximum amount of sulfur and the minimum amount of sulfur compounds.

Sulfur vapour must be removed from the sample before it goes to the analyzer to ensure that the vapour does not condense to a liquid or a solid when it cools in the sample lines or sample cell. Cooled condensers remove liquid sulfur from the sample gas and reduce its vapour concentration before it goes to the analyzers. Sulfur vapour also absorbs UV radiation which UV analyzers must compensate for when analyzing sulfur compounds.

Pulp Bleach Monitoring

Chlorine (Cl2), chlorine dioxide (ClO2) and ozone (O_3) are three bleaching agents that pulp mills use to whiten wood pulp fibres to make white paper products. All of these gases absorb UV radiation, and UV photometric analyzers are available to measure their concentration in either the gas phase or when dissolved in water.

For example, CLO2 is an explosive gas which is generated as required and dissolved in water. The steps to prepare solutions of CIO2 in water to whiten wood pulp in pulp mill bleaching plants are:

1. Chemical reactions occur in a ClO_2 generator which produces the gas. A UV gas analyzer may monitor and control the gas concentration as it leaves the generator.

2. The ClO2 gas absorbs into water in an absorption tower. A ClO2 solution UV analyzer may monitor and control the concentration of the solution as it leaves the absorption tower.

3. The ClO2 bleaching solution is stored in a tank. A CIO2 solution UV analyzer may monitor its ClO_2 concentration as it leaves the storage tank to go to the bleaching plant.

4. The storage tank vents to the atmosphere through a scrubber that uses cold water or other solutions to absorb ClO2 and prevent it from being released to the atmosphere. A CIO2 UV gas analyzer monitors the vent gas to control ClO_2 emissions to the atmosphere.

Objective Two

Describe the components of UV analyzers.

Analyzer Components

All UV analyzers contain the following components:

• UV source

• UV sample cell

• UV wavelength selector

• UV detector

• Readout device to convert detector signals to concentration readings

There are two general types of UV process analyzers.

1. Non-dispersive UV process analyzers, also known as UV photometers, typically analyze a few specific compounds in a process sample.

2. Dispersive UV process analyzers are similar to laboratory spectrophotometers, but can scan a wide range of wavelengths instantaneously. This enables them to measure all of the UV absorbing compounds in a process sample.

Component Differences

Radiation Sources in Non-Dispersive Analyzers

Non-dispersive UV process analyzers require specific wavelengths:

• A measure wavelength that is strongly absorbed by the compound that is analyzed

• A reference wavelength that is not absorbed by any sample compounds

Two types of line emission UV lamps can provide specific, intense measure and reference wavelengths:

1. Metal vapour lamps

2. Hollow cathode lamps

Metal Vapor Lamps

Metal vapour lamps create specific UV emission wavelengths called lines by passing a high voltage electrical discharge through metal vapours. Mercury vapour lamps are the most common UV lamps because of the high intensities of their UV lines. Cadmium and zinc vapor lamps may be used to provide different measure and reference wavelengths. Ultraviolet lamps use quartz glass because ordinary glass absorbs UV radiation.

Hollow Cathode Lamps

Hollow cathode lamps also use high voltage electrical discharge, but the discharge strikes a hollow metal electrode, called a cathode, which produces metal vapour. The metal vapour emits UV lines which pass through the circular quartz window at the end of the lamp.

Many different types of hollow cathode lamps have the same design, but use different metal cathodes. The different types of metals give different sets of UV lines depending on the characteristics of the specific metal. Two common hollow cathode lamps are magnesium and cadmium.

Radiation Sources in Dispersive Analyzers

Dispersive UV analyzers require UV lamps that emit a broad range of wavelengths. Lamps relying on UV emissions from metal atoms in metal vapour and hollow cathode lamps do not provide a continuous source of UV wavelengths, but emit specific lines depending on the type of metal. However, when an electric arc passes through hydrogen gas (H2) or its isotope deuterium (D2), they emit a continuous spectrum of UV wavelengths.

Deuterium arc lamps provide a wide UV wavelength range in laboratory UV spectrophotometers and process dispersive UV analyzers. An electric arc strikes between two metal plates which excites the deuterium gas inside the lamp to produce UV radiation.

Sample Cells

Ultraviolet sample cells typically have quartz windows that allow UV radiation to pass through a fixed path length of the sample gas or liquid. The sample must not corrode the cell windows or the metal tubing, which is typically stainless steel. Other materials that prevent chemical attack by the sample are often available from instrument manufacturers.

The sensitivity of the UV analyzer depends on the cell path length. Liquids are denser than gases with more UV absorbing molecules per unit length, so gas sample measurements require a long path length (up to 1 m) to measure the same absorbance compared to a 1 mm liquid sample.

Note: For the most accurate measurements, the cell path length should be chosen so that the normal process sample concentration gives absorbance values of approximately 0.5.

Example - Optimizing Path Length

A UV gas analyzer has been optimized to measure 0 ppm to 100 ppm of gas, using a 30 cm path length cell. Determine whether the manufacture needs to increase or decrease the sample cell path length and calculate the new path length to maintain optimum absorbance measurements for a range of 0 ppm to 10 ppm.

The range has decreased from 100 ppm to 10 ppm, which is a factor of 10 (100 ppm divided by 10 ppm). If the path length is not changed, the decreased concentrations result in decreased absorbance (A) values since the Beer-Lambert Law states that absorbance (absorptivity path length) × concentration.

If concentration decreases by a factor of 10, the path length should be increased by a factor of 10 so that the measured absorbance stays the same.

New path length = Old path length x 10 = 30 cm x 10 = 300 cm.

New path lengths may be calculated using the following formula.

new \ path \ length = old \ path \ length \times \frac{old \ upper \ range \ concentration}{new \ upper \ range \ concentration}

Substituting the values given in this example, this is the calculation.

new \ path \ length = 30 cm \times \frac{100 ppm}{10 ppm} = 30 cm \times 10 = 300 cm

Manufacturers design sample cells so that they can be disassembled to enable the tubing and cell windows to be cleaned or replaced as necessary.

Wavelength Selectors in Non-Dispersive Analyzers

Single compound non-dispersive UV analyzers, such as SO_2 analyzers, require only two UV wavelengths for concentration measurements:

1. A strongly absorbed measure wavelength and

2. A non-absorbed reference wavelength

These two wavelengths are typically selected by sending the UV beam through an interference filter. Interference filters have a very thin internal reflective layer which creates multiple reflected waves. Interference filters only pass a very narrow band of wavelengths; the rest are cancelled out when they mix with the internally reflected waves. Source lamps typically do not provide equal intensities for the measure and reference UV wavelengths. Each filter is screened by light-blocking mesh to create two wavelengths of equal intensity.

Wavelength Selectors in Dispersive Analyzers

Dispersive UV analyzers must make measurements on a full spectrum of UV wavelengths. They typically use a reflection diffraction grating to separate the UV radiation from a deuterium lamp into a continuous set of separate wavelengths. A reflection diffraction grating also referred to as a reflection grating consists of a reflective surface marked with thousands of identical grooves per mm.

The UV radiation strikes the grooves of the reflection grating at the same angle, but reflections travel away from the grating at different angles depending on their wavelength. The reflected wavelengths from one groove interfere with the reflected wavelengths from other grooves. Constructive interference strengthens the wavelength; destructive interference cancels the wavelength. This phenomena of constructive and destructive interference is called diffraction.

The result is that the reflected UV radiation is separated into wavelengths. The direction of the strengthened waves moving away from the grating depends on their wavelength.

Ultraviolet Detectors

Three types of UV detectors are commonly used to measure UV radiation intensity:

1. UV photodiodes,

2. UV photomultiplier tubes (PMTs) and

3. UV photodiode arrays (PDAs).

Ultraviolet photodiodes and photomultiplier tubes (PMTs) measure the intensity of a single selected UV wavelength in non-dispersive UV analyzers. The UV PMTs are the most sensitive devices and are suitable for measuring low levels of UV intensity.

Photodiode arrays (PDAs) consist of thousands of individual photodiodes arranged in a rectangular pattern, known as an array. These PDAs simultaneously measure the intensities of hundreds of separate wavelengths of UV radiation coming from a reflection grating in a dispersive UV analyzer.

Photodiodes

A diode consists of two types of semiconductors, a p type and an n type, which meet at a junction. A reverse-biased diode has very high resistance, and circuit current flow is very small.

In a UV photodiode, a quartz window placed over the junction allows the junction to be exposed to UV radiation. The UV radiation passes through the quartz window, and the junction becomes electrically conductive, which reduces the diode's resistance and increases the current flow in the circuit. The conductivity of the reverse-biased photodiode is proportional to UV intensity, so the photodiode current is proportional to UV intensity.

Photomultiplier Tubes

A photomultiplier tube (PMT) generates electrons when UV radiation strikes a specially coated electrode called a photocathode. Electrons released by the photocathode impact plates called dynodes that multiply the number of electrons striking each successive dynode.

Each of the dynode plates are held at progressively higher positive potentials relative to the photocathode. The positive potentials between each dynode attract and accelerate the emitted electrons so that every time an emitted electron strikes a dynode, it knocks multiple electrons off the dynodes. In this way, the electrons emitted by the photocathode are multiplied.

The flow of electrons that finally collect at a positive electrode called an anode generates an electrical current flow through the PMT. Current flow measured between the anode and the power supply is proportional to the intensity of UV radiation entering the PMT.

Photodiode Arrays

Photodiode array UV detectors contain thousands of photodiodes to measure the intensities of a wide range of UV wavelengths at one time. They measure the intensities of all the wavelengths dispersed by a grating in a dispersive UV analyzer.

The diodes in a PDA are arranged in a rectangular pattern of rows and columns beneath a quartz window which transmits UV radiation. The device connects to a circuit board as an integrated circuit (IC) through a series of pins on each side of the package.

Readout Devices

Modern UV analyzers are typically microprocessor operated. Microprocessor based instruments typically have a front panel output display and a keypad for operator input. They calculate sample absorbance (A) values for each UV wavelength absorbed by a compound in the sample. UV detector measures the intensities of two wavelengths:

1. The measure wavelength

2. The reference wavelength

Microprocessor based UV analyzers first digitize the reference and measure detector output voltages. Then they calculate absorbance as the log of the ratio of reference wavelength intensity to measure wavelength intensity.

Instruments that do not use microprocessors to process digitized detector intensity signals use analog electronic devices to process the detector output voltages. The reference and measure detector signals pass through logarithmic amplifiers that generate the logs of their input voltages. A differential amplifier then subtracts the log of the measure signal from the log of the reference detector signal. This processing results in an absorbance signal.

Absorbance is proportional to concentration according to the Beer-Lambert Law.

Absorbance = (absorptivity \times path \ length) \times concentration

A = (ab) \times c = constant \times c

Ultraviolet analyzers usually have several output options such as 4 mA to 20 mA analog and digital outputs, plus a front panel display that shows concentration.

Analyzer Designs

Many different UV analyzer designs are available. Each has a different arrangement of component parts. Three common designs are:

1. The split-beam UV photometric analyzer

2. The filter-wheel or flicker UV photometric analyzer

3. The photodiode array dispersive UV analyzer

The split-beam and filter wheel are non-dispersive analyzers that use filters to select measure and reference wavelengths. The photodiode array analyzer is a dispersive analyzer since it uses a grating to disperse the UV radiation and measures the intensities of all the UV wavelengths.

Split-Beam Photometric Analyzers

The spit-beam UV photometric analyzer is a non-dispersive analyzer. The key component in a split-beam UV analyzer is a beam splitter, a semitransparent mirror consisting of a quartz plate with a reflective coating so thin that part of the UV radiation reflects and the other part travels straight through the device.

The components of the split-beam analyzer perform the following functions.

1. A metallic vapour lamp provides high intensity UV emission lines including the measure and reference wavelengths.

2. Both UV measure and reference wavelengths pass through the sample contained in a sample cell. Only the measure wavelength is absorbed by the sample.

3. The beam splitter splits the UV beam containing the measure and reference wavelengths into two beams.

4. A measure filter allows only the measure wavelength to pass on to the measure detector.

5. A reference filter allows only the reference wavelength to pass on to the reference detector.

6. The detectors generate electrical signals proportional to the intensity of the UV radiation falling on them.

7. A signal processor module converts the detector signals to absorbance and concentration values for output by a readout device.

Split-beam analyzers typically use interference filters to select and pass precise measure and reference wavelengths. Depending on the instrument design, they may use either photodiode or photomultiplier detectors.

The signal processor converts the reference (I{Reference}) and the measure (I{Measure}) detector signals to absorbance values. For UV absorbing compounds that follow the Beer-Lambert Law, their concentration (c) is proportional to their measured absorbance (A).

A = (ab) \times c = (constant) \times c

Filter-Wheel Photometric Analyzers

Like the split-beam photometer, a filter-wheel or flicker photometer also has two interference filters to select measure and reference wavelengths. However, the filters are part of a rotating wheel placed between the sample cell and a UV detector. As the wheel rotates it performs two functions.

1. It chops or interrupts the UV beam before it reaches the detector.

2. It alternates between sending the reference wavelength and the measure wavelength of UV radiation to the detector.

A UV line emission lamp provides UV wavelengths to analyze a UV absorbing compound in the sample. The reference, measure and other UV lamp wavelengths travel through the sample contained in the sample cell.

1. When the reference filter is in the path of the UV beam, only the reference wavelength passes through the filter to the PMT detector.

2. When the measure filter is in the path of the beam, it allows only the measure wavelength to pass on to the PMT detector.

3. As the filter wheel rotates, it blocks all UV radiation from reaching the detector until either the reference filter or measure filter is in the path of the UV beam.

The PMT detector signal is a series of electrical pulses which correspond in height to the intensities of the reference and measure UV intensities (I{Reference} and I{Measure}).

Readout electronics process the detector pulse train. A signal processor synchronizes to the rotation of the filter wheel to identify the reference and to measure intensity signals. They are then converted by either analog or digital means to absorbance values by performing the following calculation using the sizes of the two signals.

Absorbance A = \log \frac{I{Reference}}{I{Measure}} = \log(I{Reference}) - \log (I{Measure})

The greater the difference between the intensities of the reference and measurement beams is, the greater the absorbance and the greater the concentration of the measured component are. Concentration (c) is proportional to absorbance (A) according to the Beer-Lambert Law.

A = (ab) \times c = (constant) \times c

Calibration gases are required to calibrate the spit-beam UV photometric analyzer or the flicker UV photometric. Connecting a zero gas which has zero concentration of the detected component results in an electrical pulse train. The reference signal and measured signal are almost identical; when subtracted this results in a very low absorbance value. This value is the zero concentration signal.

Connecting a span gas which has a known concentration of the detected component results in an electrical pulse train. The reference signal is much larger than the measured signal; when subtracted this results in a large absorbance value. This value is the span concentration signal.

The flicker UV photometric analyzer uses the zero and span reference points when measuring the process sample to determine the concentration of the detected component measured.

Comparing Analyzer Designs

Split-beam and filter-wheel UV analyzers are different in two main ways.

1. The split-beam detector signals are not chopped; the filter wheel acts as a chopper, producing a detector signal pulse train in the filter-wheel analyzer.

2. The split-beam analyzer uses two detectors; the filter-wheel analyzer uses a single UV detector.

Both types of analyzer use a reference wavelength to compensate for changes in instrument components or operating conditions. When the signal processor determines absorbance, anything that equally affects both the reference and measure wavelengths cancels out.

Reference wavelength measurements compensate for the following factors in split-beam and filter-wheel UV photometers:

• Reduced UV lamp intensity as the lamps age

• Reduced UV intensity due to absorption and scattering by sample particles

• Reduced UV intensity due to dirty cell windows

• Line voltage variations that affect the detector and signal processor outputs

The filter-wheel UV photometer has three advantages over the split-beam instrument.

1. It can compensate for changes in detector sensitivity.

2. Pulse train signal processing reduces electronic noise.

3. The filter wheel may contain up to six filters, which allows it to analyze multiple compounds.

The filter-wheel photometer uses a single UV detector for both the reference and measure signals, so changes in detector sensitivity affect the reference and measure signals equally. These effects cancel out when the signal processor determines absorbance values from the two detector signals.

The split-beam analyzer uses two different detectors, so unequal changes in the sensitivity of the two detectors affects the absorbance. Unequal changes in the sensitivity results in errors until the analyzer is recalibrated.

The detector output from a split-beam analyzer is a pulse train of alternating reference and measure signals. The pulse train produced by a filter-wheel UV analyzer has the following advantages.

• The signal processing electronics can be optimized to reject all noise frequencies except that of the pulse train. For example, 60Hz noise from power lines occurs at different frequencies than the pulse train frequency, so the noise is rejected.

• A pulse train is not subject to electronic drift noise, called flicker noise, because the measurement signal is subtracted from the reference signal.

• Changes in the electronics because of aging equally affect both signals, so they cancel each other out.

An analyzer that calculates its output from a pulse train typically does not need to be calibrated as often as an analyzer that does not.

A split-beam analyzer can only analyze one UV absorbing compound in a sample because it has only a single measure filter to select the wavelength.

A filter wheel may accommodate up to six filters. Five of the filters may be used for measure wavelengths and one for reference. A filter-wheel analyzer can measure up to five different UV absorbing compounds in a process sample, using a different measure filter for each substance.