3.1 introduction to fbg sensors -...

Fiber Bragg Grating Chemical Sensor

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3.1 Introduction to FBG Sensors

The need for technologically advanced sensors capable of providing rapid and

robust results are expected to lead to the increasing use of fiber optic sensors in

various fields. In the very beginning, the research on fiber optic sensors was taken up

merely as curiosity. Increased R&D activities brought many newer high performing

fiber optic devices which strengthened flying wings of fiber optic sensor field. FBG is

one such device that solved many problems of optical networking and fiber optic

sensors. Now research on FBG sensors is one of the hottest topics that may replace

the conventional sensors mainly because mass of the sensor is reduced many a times.

Fiber Bragg gratings are excellent fiber-optic sensing elements and have attracted

considerable interest in various fiber optic sensor implementations in the past years.

FBGs behave as filters by reflecting a narrow band of incoming spectrum [1, 2] and

have found applications in optical telecommunication networks and in sensing

parameters like temperature, pressure, displacement etc [3-6]. They are integrated into

the light guiding core of the fiber and are wavelength encoded, eliminating the

problems of amplitude or intensity variations suffered by many other types of fiber

sensors. In addition to the well-known advantages of fiber-optic sensors such as

electrically passive operation, immunity to RFI and EMI, high sensitivity, compact

size and potentially low cost; FBG based sensors have an inherent self-referencing

capability and can be easily multiplexed in a serial fashion along a single fiber

forming quasi distributed sensor [1, 2]. FBG sensors can be produced in glass or

plastic optical fibers to meet harsh environmental conditions and structural demands

for a variety of applications. Due to their narrow-band wavelength reflection, they are

also conveniently multiplexed in fiber optic networks [7]. Because of the wavelength-

encoded nature, FBG based sensors offer several advantages over competing


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conventional methods. They can perform the functions of virtually any conventional

sensor; often faster and with greater sensitivity and they can perform measurement

tasks that would be impractical with conventional sensors. They are real-time,

repeatable and reusable and have the ability to perform quasi-distributed (multipoint)

sensing through the use of multiplexing [8]. They also allow remote operation.

Compared to other variations of the fiber optic sensors, spectrally based sensors such

as FBGs are inherently resistant to errors induced by intensity variations. Potential

error sources include variable losses due to connectors and splices, microbending loss,

mechanical creep and misalignment of light sources and detectors [9, 10]. In the race

with conventional sensors, fiber Bragg grating sensors have created a special position.

Fiber gratings have been embedded in composite materials for smart structures

monitoring and tested with civil structures to monitor load levels. They have also been

successfully tested as acoustic sensing arrays. Fiber Bragg grating sensors have

gained wide acceptance for structural health monitoring applications for the

measurement of strain, stress, vibration, acoustics, acceleration, pressure and

temperature in various fields like defense, aviation, automobile, civil structures,

petrochemical industries, etc. It can be easily cast, embedded or surface mounted on a

structure or packaged in strong, rugged materials to withstand harsh environments.

Central Glass and Ceramic Research Institute (CGCRI), Kolkata has

demonstrated on-line monitoring of temperature of overhead high voltage power

conductors with fiber Bragg-grating based sensors at the Kolkata power grid [11].

Many commercial companies in Europe have developed sensors based on fiber

Bragg gratings that include temperature sensors, strain sensors and extensometers

ready to be embedded or retrofitted on the structure to be monitored. SCAIME is a

company that released FBG based temperature sensors in the range -30OC to +50OC


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and -30OC to +80

OC with precession 0.5

OC and 1

OC respectively. With AdvOptics,

SCAIME has developed and installed fiber optic extensometers on the new fleet of

the Monaco harbor, both under sea level and in the structure surface [12]. O/E Land

INC, UK is another company that has developed temperature sensor in the range

-50OC to 120OC with resolution of 1OC and they are commercially available in

market. These new fiber grating temperature sensors can be widely used in

construction, aerospace, electrical, petrochemical, biomedical, nuclear power station

and other industry applications. Depending on the packaging material they can be

used for different applications. The company has come up with FBG sensors having

stainless steel jacket for harsh environment; also with Teflon material and with

ceramic material suitable for non metal applications like high voltage transformer,

micro wave and strong electromagnetic field, as well as high temperature applications

[13].

The variables measured by a FBG sensor can be expanded using an additional

sensitive material or a transducer, which strains the Bragg grating under an external

influence. Several examples of chemical detection with FBGs such as hydrogen gas

sensing using a palladium-coated FBG, measuring salinity using hydrogels and

detecting hydrocarbon leaks using polymers [14, 15] have been demonstrated. A fiber

optic based humidity sensor has been fabricated using FBG coated with polymide to

assess moisture levels in 200 year old limestone walls around Worcester College,

Oxford [16]. This humidity sensor was based on the strain effect induced in the Bragg

grating through the swelling of the moisture sensitive polymer coating. A direct

indication of the humidity level is given by the shift of the Bragg wavelength caused

by the expansion of the sensing material. FBG chemical sensors are in developing

stage; even though some FBG chemical sensors are demonstrated, lot of research is


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still needed in aspect such as packaging, repeatability, reliability to commercialize

them.

3.1.1 FBG Chemical Sensors

Refractive index of a compound depends upon its chemical composition. Hence,

in biological and chemical sensing area refractive index (RI) is an important

parameter. Fiber gratings have provided good solutions in this area, because FBG

chemical sensors and biosensors are based on RI induced detection. Normal FBG is

intrinsically insensitive to external refractive index. The existing FBG RI sensors are

based on the interaction between the evanescent field of the core mode and

surrounding material. Therefore either cladding should be removed [17, 18] or

special kind of fibers like D-fiber [19] or side polished fiber [20] can be used to

design FBG chemical sensors. Wei Liang et al. proposed fiber Fabry-Perot

interferometer where, interferometer sensor is fabricated by removing cladding of the

fiber between two identical FBGs. The sensor is used to measure refractive index of

isopropyl alcohol of different concentrations [17, 21]. Although the interferometric

sensor is sensitive even for low concentration, it cannot act as point sensor. Fiber

Bragg Gratings have also proved promising candidates in the field of chemical

sensors as RI sensors and solution concentration sensors using many designs [22-24].

The principle of operation of FBG sensor relies on the dependence of the Bragg

resonance on effective refractive index and the grating pitch. In FBG chemical

sensors, pitch of the grating is unaltered as chemicals at room temperature cannot

change the pitch. Hence, effective refractive index of core mode has fundamental role

to play in FBG chemical sensors. Since the effective refractive index is not influenced

by the external medium for standard optical fibers, no sensitivity to external refractive

index is expected. However, if the fiber cladding diameter is reduced along the


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grating region, the effective refractive index of the core mode is significantly affected

by external refractive index. As a consequence, shifts in the Bragg wavelength

combined with a modulation of the reflected amplitude are expected [25]. We apply

the wet chemical etching process to reduce the cladding diameter over Bragg grating

region to design FBG based chemical sensor.

3.2. Theory of FBG Chemical Sensor

In optical fibers, the effective refractive index (neff) of the fundamental mode is

practically independent of the refractive index of the medium surrounding the

cladding. However, if the cladding diameter is reduced, neff shows dependence on the

external refractive index, leading to a shift in the reflected wavelength (λB). Unlike

the FBG sensors for temperature and strain measurements, in this case, only the

refractive index is affected by measurand, neff changes while the grating pitch is

practically unchanged. As external refractive index increases, higher sensitivity is

observed, since the fundamental mode is less confined in the core region leading to an

increased interaction with the external medium. In addition, such interaction increases

the sensor sensitivity as cladding diameter is reduced.

The FBG works on the principle of Bragg reflection by reflecting Bragg

wavelength (λB) given by [1, 2, 26]

λB = 2 neff Λ (3.1)

where neff is the effective refractive index of the core and Λ is grating period. The

reflected wavelength mainly depends on grating pitch and effective refractive index.

The change of Bragg wavelength due to the changes in measurand is used in FBG

sensing applications. As such FBG is unaffected by surrounding RI, since the

reflected spectrum is result of coupling of core bound mode with backward


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propagating modes. In FBG chemical sensors, the change of Bragg wavelength is due

to change in RI induced by change in the chemical composition around the sensor. In

order to measure small changes in composition, it is important for the optical mode to

penetrate evanescently into the surrounding solution. To design the FBG chemical

sensor, the cladding above the grating region should be etched so that core mode

interacts with external medium immediately surrounding the core. This can be done

by etching the fiber cladding above the grating region to a diameter such that core

mode interacts with the surrounding environment. With this configuration the value of

effective refractive index of the waveguide mode is directly affected by the RI of the

medium where the fiber is immersed. When the optical fiber is etched to a point

where the fundamental waveguide mode is affected, the modified propagation

constant of the mode can be written as [27-29]

(3.2)

where effn)/2(0 λπβ = is propagation constant of core mode under normal

conditions, surn is RI of surrounding medium, cln is RI of cladding, k is wave vector,

pη is the fraction of the total power of unperturbed fundamental mode that flows in

the etched region and therefore lost to the surrounding medium. If pA is area of the

cross section of the etched region, then pη is given by [29],

∫

∫

∞

=

A

A

pdA

dA

p

2

2

ψ

ψ

η (3.3)

where ∞A is area of cross section of unetched fiber and ψ is wave function of

fundamental mode in weakly guiding circular fiber with step index profile, defined

as[29]

)(0 clsurp nnk −+= ηββ


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,)(

)(

0

0

UJ

URJ=ψ 0 ≤ R ≤ 1 (3.4)

,

)(

)(

0

0

WK

WRK=ψ 0<R<∞ (3.5)

Here, R=r/ρ where ‘r’ is the fiber radius and ρ is the core radius, J0 is the Bessel

function of first kind and K0 is modified Bessel function of second kind. U and W are

the scalar mode parameters for the fiber core and cladding respectively.

Due to the power loss that occurs in the etched region of the fiber, the reflected

power of the FBG decreases when the fiber radius becomes smaller. After certain

stage, decrease of fiber radius due to etching will also have consequences on the value

of Bragg wavelength because the effective refractive index of the core mode begins to

change. If 1pη and 2pη are two values of pη at two different diameters of etched fiber,

from equation 3.2 the associated variation of the modified propagation constant of the

fundamental mode would be

(3.6)

From definition of propagation constant, this equation indicates that there is a

variation in the effective RI of the waveguide mode is given by

η∂∂=∂ nneff (3.7)

where clsur nnn −=∂ and 21 pp ηηη −=∂

Combining equations 3.1 and 3.7

ηλ ∂∂Λ=∂Λ=∂ nneffB 22 (3.8)

The above equation indicates that the etching process is associated with variation of

Bragg wavelength of the FBG given by

))(( 2112 clsurpp nnk −−=− ηηββ


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effB n∂Λ=∂ 2λ (3.9)

When etching process is stopped, pη becomes constant value and equation (3.9)

becomes

)(2 clsurpoB nn −Λ=∂ ηλ (3.10)

In this stage, the variation of the Bragg wavelength of FBG depends only on the

variation of the refractive index of the surrounding medium. Unlike in pressure,

temperature or strain sensor, in FBG chemical sensor only the effective RI is affected

while the gratings pitch is practically unchanged. It is considered that temperature of

the medium surrounding FBG is maintained at constant room temperature (25OC).

This constitutes the principle of chemical sensing by FBG.

3.3 FBG Chemical Sensor Fabrication

Fiber Bragg grating fabricated in boron-germanium co-doped single mode fiber

with Bragg wavelength (λB) centered at 1546.96nm was used to design chemical

sensor. As mentioned before, FBG as such is insensitive to the surrounding RI with

cladding above the grating region. To fabricate chemical sensor, cladding above the

FBG region was etched. Cladding was etched by dipping the segment of FBG in 40%

HF solution for 55minutes. In order to etch the cladding, a groove was made in candle

that holds the FBG. The groove was filled with 40% HF solution. In the beginning no

shift of the Bragg wavelength was observed, because there was no interaction of the

core mode with outer surrounding medium. After some time, when cladding diameter

is decreased to certain extent, the evanescent wave of core mode starts reacting with

the external medium leading to shift in the Bragg wavelength. At regular intervals

Bragg reflected spectrum was recorded. During etching, blue shift in Bragg

wavelength was observed. This is related to monotonic decrease in the effective


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refractive index due to the extent of etching process [22, 25]. The plot of Bragg

wavelength shift with respect to time is shown in the Fig. 3.1. The shift in Bragg

wavelength is not linear. The shift during etching traces a polynomial. During etching,

peak reflectance of FBG also diminishes with time. This may be due to the etching

induced multimodal propagation conditions along the etched region [28]. The fiber

was washed with plenty of water to remove HF solution action completely and dried.

Fig. 3.2 is schematic diagram of FBG where cladding is removed by etching

process. The diameter of etched FBG is 10.38µm (measured under high resolution

micrometer at University Center for Scientific Instruments, KUD). The image of

etched fiber grating region and its thickness comparison with un-etched fiber is shown

in Fig. 3.3.

10 20 30 40 50 60

1546.55

1546.60

1546.65

1546.70

1546.75

1546.80

1546.85

1546.90

1546.95

1547.00

Wav

elen

gth

in

mic

rom

eter

Time in minutes

Experimental data

Polynomial fit to the data

Figure 3.1: Shift of Bragg Wavelength during etching.


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Fig. 3.2: Schematic of cladding etched above the grating region.

Cladding etched FBG Comparison of cladding etched FBG with optical fiber

Figure 3.3: FBG etched in 40% HF solution.

3.4 Design of FBG Chemical Sensor

Now in cladding removed FBG, the evanescent field of core mode interacts with

surrounding medium. Hence, this cladding etched FBG is directly employed for

chemical sensing – alcohol sensor and manganese sensor.

3.4.1 Alcohol sensor

3.4.1.1 Alcohols

Alcohols are compounds in which one or more hydrogen atoms in an alkane have

been replaced by an -OH group. Alcohols with less number of carbon atoms (<5) are

used as common laboratory solvents and also solvents in paints, markers and

cosmetics such as perfume and deodorant. Most of the alcohols are used as additives

for petrol.

Cladding etched

grating region

Core Cladding


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Methanol as common laboratory solvent is especially useful for High

Performance Liquid chromatography (HPLC) and UV/VIS spectroscopy. Methanol is

used as starting compound in the synthesis of other chemicals such as formaldehyde

and from there into products like plastics, plywood, paints, explosives and permanent

press textiles. Other chemical derivatives of methanol include dimethyl ether, which

has replaced chlorofluorocarbons as an aerosol spray propellant; dimethyl ether that

can be mixed with liquefied petroleum gas for cooking and can be used as a diesel

replacement for transportation fuel. One of the potential drawbacks of using high

concentrations of methanol in fuel is the corrosive to some metals. It is poisonous and

causes death in humans if consumed in high quantity. Methanol is also used as a

solvent and as antifreeze in pipelines and windshield washer fluid. Methanol is mixed

with water and injected into high performance diesel and petrol engines for an

increase of power and a decrease in intake air temperature in a process known as

water methanol injection [30].

In many countries like Brazil and Australia ethanol is directly used as motor fuel.

Ethanol as fuel reduces emission of harmful products such as carbon monoxide,

oxides of nitrogen and other ozone forming pollutants emitted by petroleum products

as petrol and diesel. Major application of ethanol is in beverage industry. Ethanol is

also used as antiseptic as it kills bacteria, fungi and viruses by denaturing its proteins

and dissolving lipids. Ethanol has many industrial applications [31].

Propanol is used as a solvent for waxes, vegetable oils, resins, cellulose esters

and ethers. It is found in inks, brake fluids and polishing compounds those have been

used as degreasing agent, antiseptic and a chemical intermediate for synthesis of

organic compounds. More recently, it is being used as a hand disinfectant by health

care workers [32].


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Butanol is used as an ingredient in perfumes and as a solvent for the extraction of

antibiotics, hormones and vitamins during the manufacture; as solvent for paints,

coatings, natural resins, gums, synthetic resins, dyes, alkaloids and camphor. Other

miscellaneous applications of butanol are as a swelling agent in textiles, as a

component of hydraulic brake fluids, cleaning formulations, degreasers and

repellents; and as a component of ore floation agents and of wood-treating systems.

Butanol has been proposed as a substitute for diesel fuel and petrol. Used as starting

chemical, butanol is transformed into a variety of other chemicals, such as butyl

acrylate, butyl methacrylate, butyl glycol ethers and butyl acetate which have further

industrial applications [33].

Octanol is manufactured for the synthesis of esters those used in perfumes and

flavorings. It is used to model the partitioning of pharmaceutical products between

water and the cytosol. Other uses include experimental medical procedures for

controlling nervous disorder known as Essential Tremor and other types of

involuntary neurological tremors [34, 35].

3.4.1.2 Experiment

In this experiment we chose first members of homologous alcohol series -

methanol, ethanol, propanol, butanol and octanol. The refractive index of these

alcohols increases with increase in carbon atoms in the compound.

The experimental setup of FBG alcohol sensor is shown in Fig.3.4. The broad

band light from JDS Uniphase source is launched into FBG through 2X2 coupler

(3dB coupler). FBG sensor was inserted in test tube containing different alcohols

carefully one after the other. During experiment care was taken so that FBG sensor

head was freely and completely dipped in alcohol without touching the wall of the

container. FBG sensor was dipped in the alcohol for a minute to stabilize the spectrum


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before recording. Grating spectra measurement have been carried out by recording

reflected spectrum from the sensing grating for each alcohol sample. The reflected

spectrum of FBG sensor was observed and recorded using Anritsu OSA (MS90A) for

every alcohol solvent. Fig. 3.5 shows reflected spectra of FBG sensor for different

alcohols.

Figure 3.4: Experimental setup of FBG alcohol sensor.

Figure 3.5: FBG reflected spectra for different alcohols.

FBG sensor

immersed

in alcohol


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3.4.1.3 Results and Discussion

When we put FBG sensor in different alcohols i.e. methanol (n=1.328), ethanol

(n=1.361), propanol (n=1.385), butanol (n=1.398) and octanol (n=1.429), the reflected

spectra is shown in the Fig.3.5. The reflected spectra show the red shift as the ambient

refractive index increases. When ambient liquid changed from methanol to octanol,

ambient refractive index changed from 1.328 to 1.429 resulting into Bragg

wavelength shift from 1546.41nm to 1546.49nm. Refractive index of ethanol,

propanol and butanol vary between1.328 to 1.429. Bragg wavelength shift is linear

with refractive index in our experimental verification. Fig. 3.6 shows the Bragg

wavelength (λB) shift as function of refractive index of alcohol. λB is almost linear

with standard deviation of 0.0056 and correlation factor 0.987 for least square linear

fit to the experimental data. The slope is 0.8156. Hence, the sensitivity of the alcohol

sensor is 0.8156nm/RIU.

1.32 1.34 1.36 1.38 1.40 1.42 1.44

1546.40

1546.42

1546.44

1546.46

1546.48

1546.50

Octanol

Butanol

Propanol

Ethanol

Methanol

Bra

gg W

ave

leng

th (

nm

)

Refractive index

Alcohols

Linear Fit

Figure 3.6: Bragg wavelength shift in alcohol series.


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3.4.2 Manganese Sensor

3.4.2.1 Manganese in Water

Manganese is a component of several enzyme systems and is essential for normal

bone structure. Intake varies greatly depending mainly on the consumption of rich

sources, such as unrefined cereals, green leafy vegetables and tea. The usual intake of

this mineral is 2–5mg/day and absorption is 5–10%. Different techniques have been

proposed for the determination of manganese concentration.

Manganese is present in ground waters primarily as the divalent ion (Mn2+

), due to

the lack of subsurface oxygen. Surface waters may contain combinations of

manganese in various oxidation states as soluble complexes or as suspended particles.

The occurrence of manganese in public water supplies presents more of an aesthetic

problem than a potential health hazard. Mn is an element essential to humans, animals

and plants for proper functioning of some enzymes and hormones. Although most

amounts of Mn supplied to the body is through food, it also enters the body through

water. Even though ground has excellent mechanism of filtering out dissolved

chemicals and gases, they can still occur in large enough concentrations to cause

problems. In low concentrations it produces extremely objectionable stains due to

oxidation on everything with which it comes in contact. Deposits collect in pipelines

and tap water may contain black sediment and turbidity due to precipitated manganese

[36]. Higher concentration of Mn in water may bring about various health related

problems in both animals and plants. Adverse neurological effects (decreased

performance in school and in neurobehavioral examinations of the WHO core test

battery) were reported in 11 to 13year-old children who were exposed to excess

manganese through ingestion of contaminated water and from wheat fertilized with

sewage water [36-40]. Long term studies concluded that progressive increases in the


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manganese concentration in drinking-water are associated with a progressively higher

prevalence of neurological problems of chronic manganese poisoning [41]. Contrary

to the above study, another long-term drinking-water study found no neurological

effects were found in older people consuming well water containing at least 0.3 mg of

manganese per liter for 10–40 years [42]. It is found from research on animals that the

higher concentration of manganese input (via food or water) brings about many

complications such as neurotoxicity, reproductive problems. Manganese causes dark

stains in laundry and on plumbing fixtures, tends to deposit in water lines and imparts

an objectionable taste to beverages such as coffee and tea. Manganese levels in

natural waters rarely exceed 1ppm, but levels of 0.1ppm are sufficient to cause the

taste and staining problems. World Health Organization recommends a limit of

0.05ppm manganese in consumer usable water, although this may vary with local

circumstances.

3.4.2.2 Instruments used for Comparison

We designed chemical sensors to determine the concentration of dissolved

chemical species in water present at ppm level. Our results were compared with well

established sophisticated spectroscopic instruments - atomic absorption spectrometer

and inductively coupled plasma spectrometer.

Atomic Absorption Spectrometer

Atomic absorption spectroscopy (AAS) is the measurement of absorption of

optical radiation by atoms in the gaseous state. The original equipment was developed

by Walsh. Majority of free atoms in the commonly used flames are in ground state.

The flames also not have enough energy to excite these atoms (except for Group I

elements of the periodic table). A light source emitting a narrow spectral line of the

characteristic energy is used to excite the free atoms formed in the flame. The


97

decrease in energy (absorption) is then measured. The absorption is proportional to

concentration of free atoms in the flame. Atomic absorption spectroscopy is similar to

UV-VIS spectroscopy and Lambert-Beer’s law is applicable. Photograph of AAS in

our University Center for Scientific Instruments (USIC) is shown in Fig. 3.7.

There are three basic components for every AA spectrophotometer:

1. Light source - It is designed to emit the atomic spectrum of a particular element.

Specific lamps are selected according to the element to be determined. The hollow

cathode lamp (HCL) or electrodeless lamps (EDL) are widely used.

2. Sample cell - Where an atomic sample vapor is generated in the light beam from

the source. This is usually done by introducing the sample into a burner system

(flame) or electrically heated furnace or platform, aligned in the optical path of the

spectrophotometer.

3. Specific light measurement - Includes several components:

a) A monochromator to disperse several wavelengths of lights that are emitted

from the light source to isolate a particular line of interest.

b) A detector to produce an electrical current that is dependent on the light

intensity. This electrical current is amplified and processed by the

instrument electronics to produce a signal, which is a measure of the light

attenuation occurring in the sample cell.

c) The signal is further processed to generate instrument readout in

concentration units.


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Figure. 3.7: Atomic absorption spectrometer.

Inductively Coupled Plasma Spectrometer

Inductively coupled plasma spectrometry (ICPS) is a powerful tool for the

determination of metals in a variety of different sample matrices. With this technique,

liquid samples are injected into a radiofrequency (RF)-induced argon plasma using

one of a variety of nebulizers or sample introduction techniques. The sample mist

reaching the plasma is quickly dried vaporized and energized through collisional

excitation at high temperature. The atomic emission emanating from the plasma is

viewed in either a radial or axial configuration, collected with a lens or mirror and

imaged onto the entrance slit of a wavelength selection device. Single element

measurements can be performed cost effectively with a simple

monochromator/photomultiplier tube (PMT) combination and simultaneous

multielement determinations are performed for up to 70 elements with the

combination of a polychromator and an array detector. The analytical performance of

such systems is competitive with most other inorganic analysis techniques, especially

with regards to sample throughput and sensitivity. We made ICP measurements at

Met-Chem Laboratories, Bangalore. ICP spectrometer is shown in Fig. 7.8.


99

Inductively Coupled Plasma Spectrometer (ICPS) has components such as the

nebulizer, spray chamber, plasma torch, interface and detector.

Figure 3.8: Inductively coupled plasma spectrometer

3.4.2.3 Experiment

Standard solutions of manganese concentrations varying from zero to 0.09ppm

(0.02, 0.04, 0.06, 0.09ppm) were prepared by dissolving manganese chloride in

distilled water. To 8ml of each standard solution and test sample (collected ground

water) pinch of ascorbic acid was added and swirled vigorously until ascorbic acid

dissolves completely. Then 2ml of ammonium hydroxide was added and mixed

vigorously. Then to each solution, three drops of potassium cyanide (KCN) and N,N

dimethylformamide were added one after the other. Later 0.25ml of 1-(2-pyridylazo)-

2-napthol (PAN) indicator was added swiftly with pipette and mixed immediately and

left for 10minutes to complete the reaction. Light orange solution will form if Mn2+

is

present.

Ascorbic acid reduces all oxidized forms of manganese into Mn2+. Ammonium

hydroxide is added to maintain the alkaline pH of the solution. Iron, cadmium, zinc,

cobalt and nickel present in the test sample can be effectively masked with potassium


100

cyanide. N,N dimethylformamide is added to catalyze the reaction. PAN combines

with Mn2+

to form orange colour complex compound. If PAN indicator is added

slowly, turbidity will be formed. The chemical reaction of formation of complex is

given below [43].

Orange complex compound (soluble in water)

All standard and test samples along with reagents were allowed for

measurements. Optoelectronics setup for concentration measurement comprises of

17mW broad band source of wavelength range 1530-1600nm, 3dB coupler to collect

reflected signal form sensor head, OSA (Wistom with Proximion software). Fig. 3.9

shows the experimental setup. FBG sensor head connected to broad band source was

immersed in a test tube containing solution for the measurement and allowed for a

minute to stabilize the spectrum. Reflected spectrum was recorded on OSA for all

samples one after the other. Each time, the grating region was cleaned properly with

isopropyl alcohol before inserting in a manganese solution of different concentrations

to avoid contamination. When the FBG sensor is dipped in manganese solutions of

different concentrations, the medium surrounding sensor is head changed. Hence, the


101

effective refractive index of FBG core mode changes, resulting in shift in Bragg

wavelength (λB). Fig. 3.10 shows the combined reflected spectra of FBG for different

concentrations of manganese with reagents. The experiment was carried out at room

temperature.

Figure 3.9: Experimental setup for FBG manganese concentration sensor.

1545.50 1545.75 1546.00 1546.25 1546.50 1546.75 1547.00

0

2

4

6

8

10

12

14

16

18

Reflecte

d P

ow

er

(mic

row

att)

Wavelength (nm)

Water

0.02ppm

0.04ppm

0.06ppm

0.09ppm

Test sample

Figure 3.10: Reflected spectra overlap for different Manganese concentrations.


102

3.4.2.4 Results and Discussion

The spectral response of FBG sensor for manganese solutions of different

concentration in the range 0 to 0.09ppm is shown in Fig. 3.10 It is observed that as the

concentration of surrounding medium increases, the reflected spectrum shifts towards

longer wavelength. With increase in concentration of a solution, refractive index also

increases. Hence, as the concentration of the solution surrounding etched FBG

increases, the Bragg wavelength (λB) shifts towards longer wavelength side. The plot

of λB against concentration of manganese solutions is presented Fig. 3.11. The shift of

Bragg wavelength is linear. Experimental data fit very well with straight line.

Correlation factor of the least square linear fit for the data is 0.98, standard deviation

is 0.0072 and the slope is 0.88. There is shift of 76pm in λB when the concentration

of solution surrounding the FBG sensor is varied from zero to 0.09ppm. Hence, the

sensitivity of the sensor is 0.844nm/ppm. From the graph of least square linear fit to

the experimental data, we can find out the concentration of the manganese in our test

sample and it is found to be 0.029ppm.

0.00 0.02 0.04 0.06 0.08 0.10

1546.10

1546.11

1546.12

1546.13

1546.14

1546.15

1546.16

1546.17

1546.18

1546.19

Wa

vele

ngth

(n

m)

Concentration in ppm

Experimental data

Linear fit

Figure 3.11: Bragg wavelength as function of manganese solution concentration.


103

0.00 0.02 0.04 0.06 0.08 0.10

1.3

1.4

1.5

1.6

1.7

1.8

Pea

k P

ow

er

(mic

row

att)

Concentration of Mn Solution (ppm)

Experimental data Linear fit

Figure 3.12: Peak power of λB as function of manganese solution concentration.

On observing spectral response of FBG for different concentrations of manganese

solution (Fig. 3.10), the peak power of Bragg wavelength also shows linear behavior.

As concentration of solution increases, the RI of medium surrounding solution also

increases. Hence, more evanescent field of core mode penetrates into the surrounding

medium and reflected power decrease. With the increase in concentration of

manganese solution, the reflected power for FBG decreases. The graph of peak power

as function of solution concentration is plotted in Fig. 3.12. The peak power at

different concentrations can be fitted to least square linear fit with very high

correlation factor of 0.999 and minimum standard deviation – 0.0074.

3.5. Conclusions

A simple and low cost FBG chemical sensor is presented on the principle of

refractive index variation. We have combined FBG and wet chemical etching

technique of silica to demonstrate FBG as alcohol sensor and manganese

concentration sensor. A simple and low cost procedure involving wet chemical


104

etching of cladding in 40% HF solution is carried out for the sensor preparation. The

experimental characterization of the FBG sensor for refractive index change and

solution concentration change has been carried out.

In the investigated refractive index range of 1.328-1.427 for alcohols, the Bragg

wavelength shifts from 1546.41 to 1546.49nm. Thus the refractive index sensitivity of

fabricated FBG sensor is 0.8156nm/RIU.

We have determined the concentration of dissolved manganese in the ground

water (test sample) present at ppm level using our designed FBG chemical sensor

fabricated in boron co-doped single mode optical fiber. The manganese concentration

is found to be 0.029ppm. This result is compared with measurement done using

highly sophisticated and reliable atomic absorption spectrometer in our university and

also with inductively coupled plasma method. The comparison is given in the Table

3.1 and results are in agreement. The sensitivity of chemical sensor is found to be

0.844nm/ppm.

This highly flexible, robust, sensitive FBG chemical sensor is addition to the

already established list of FBG sensors. It can be applied to many fields, including

biochemical sensing and environmental monitoring.

@measurement was made in USIC (University Scientific Instrument Center, Karnatak University

Dharwad, India.)

**Measurement was made at Met – Chem Laboratories, Bangalore. (India Pvt. Ltd.).

Table 3.1: Manganese concentration measured with different methods

Chemical

Species

FBG Sensor

(ppm)

Atomic absorption

Spectrometer@

(ppm)

Inductive coupled

plasma**

(ppm)

WHO

Permissible

Limit

(ppm)

Manganese 0.029 0.048 <0.05 0.05


105

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