analytical instrumentation 1

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ANALYTICAL INSTRUMENTATION

ANALYZERS

SILICA ANALYZER:

In thermal power plants, silica content is measured in steam before turbine.silica analyzers arealso used for anion exchanger effluent monitoring of mixed-bed exchangers.

A silica analyzer manufactured by Electronic Instruments Ltd(EIL), U.K. for continuousautomatic stream monitoring works on the calorimetric analysis principle which is based upon thewell known molybdenum blue method.

Ammonium molybdate solution (pH7), sulphuricacid and a reducing solution are added to ametered volume of sample via separate measuring cylinders (to eliminate the precipitation ofmolybdic acid).


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The flow diagram of the analyzer. The inlet is at the top to prevent contamination by any solutionfrom the previous analysis, and the cuvette is drained completely after every analysis cycle to

prevent the accumulation of gas or air bubbles in the measuring cuvette.

The analysing cycle takes twelve minutes and consists of two overlapping sequences.The first measures a chemical blank.

The second is the actual quantitative determination. An associated sequence timer controls the programme of operations.In the first sequence,

ammonium molybdate solution, sulphuric acid and reduction solution are simultaneously added tothe mixing vessel. This solution is diluted with sample to a suitable volume and is then emptied

into the measuring cuvette where it is measured and then drained away.

In the second sequence, the reagents are added in the normal order; sample first then ammonium

molybdate and sulphuric acid. The reduction solution is added five minutes later.

All materials coming in contact with analysis liquor are either made of plastic or of metalcomponents coated with plastics.

The reason for the use of blank on each cycle is to give the analyzer long term stability bycompensating for the effects of variables such as coloration of the sample or reagents,

temperature, or ageing of the lamp of photo cells.

The silicon photo-voltiac cells are conventionally illuminated by a common light source. They are

connected in parallel opposition and the differential signal is fed into a very low input impedance

current amplifier. For the measurement of the blank solution, the Auto-Compensation Uni(ACU) driven by the current amplifier is connected to its internal motor potentiometer which

corrects the zero of the current amplifier.

For the measurement of silica concentration, this correction remains fixed and the output from theACU is connected to the read-out unit. The ACU now controls a motor driven potentiometer in

the read-out unit, producing a current output to feed a meter or recorder. A scale length control

adjusts the gain of the amplifier.

Temperature compensation of the amplifier output, for changes in sample temperature, is providedby a thermistor located in the analyzer constant head unit.

The analyzer is normally supplied in a cubicle, complete with analyzer, measuring electronics, andmanual facilities for testing and calibration. Reagents are contained in the rear of the cabinet andthese require regular replenishment. To assist in providing reliable and repeatable measurements,

the complete cabinet is temperature controlled by the use of the thermostas with electrical heaters.


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SODIUM ANALYZER:

Sodium analyzers find applications in thermal power plants for determining sodium ionconcentration in boiler waters, monitoring carryover detection of condenser leaks and theexhaustion of water treatment plant cation exchange units.

The sodium analyzer manufactured by Electronic Instruments Ltd(EIL), U.K. is based upon theuse of a specific ion electrode.

In fact, ion selective electrode(ISE) is today considered to be one of the most powerful tools forspecific analysis, i.e. to determine the specific constituents like cyanide fluoride, NH3 etc. in

sample; and is thus widely used for industrial water pollution monitoring.

ISE is an electrochemical sensor that develops an electric potential which can be related to theconcentration of a specific ion in solution. The potential developed is actually proportional to the

logarithmic of the ion activity, which can differ greatly from ionic concentration in non-dilute

solutions. Where activity is a measure of the number of free (dissociated) ions in solution,

concentration includes both free and bound (undissociated) ions.

As the interest in this case is for concentration and not activity, the bound ions must first beliberated so that they can contribute to the measurement. As certain ions in the sample interfere

with the specific measurement either by forming bonds with the ions to be measured or by being

measured themselves it is essential that the sample be prepared properly before making anymeasurement.

Sample is preferred by adding a reagent which forms bonds with the interfering ions, therebyeither freeing the ions of interest, or preventing the interfering ions from entering into the

measurement.

If the concentration of interfering ions(like H and OH ions)be too high, then the adjuster solutionwill not be able to provide this over-riding ionic strength, and for such case sample pH must be

adjusted to within desired range.


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The pH value of the sample is maintained in the flow cell by adding ammonia gas to the sample.A calomel reference electrode is used to complete the electrode pair across which a potential is

developed dependent on the sodium ion activity in the sample.

The potential is proportional to the logarithm of the sodium ion concentration, thus enabling lowconcentrations to be measured accurately and also provide high range capacity. The sequence of

operation is as follows. Sample water flows to a constant head tank to ensure a fresh sample andit is pumped anaerobically. At a constant rate, into the flow cell, where it is equilibrated with

ammonia gas.

The ammonia gas is derived by pumping air through a 25% ammonia solution and passing theammonia saturated air to the flow cell. The sample then flows past the measuring electrodes to adrain outlet. The analyzer also includes a facility for automatic standardization.

The standardization sequences commences by activating a valve to stop the sample flow and toallow the standard sodium ion solution to be pumped into the flow cell. When the electrodes

have stabilized in the new solution the amplifier output is compared with a pre-set standard value

in the auto compensation unit, and any error is used to drive a servo potentiometer circuit, to

adjust the output to the correct value.


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Cleaning of electrode at specified interval is essential for electrode longevity and accuracy. Somemanufactures have automated the procedure of cleaning using solid state timers and achieving

electrode cleaning by mechanical (brush), spray nozzle, or acoustical means. In mechanicalsystem the brush strokes the probe 2-4 times in minute; in chemical method, clean fluid is

sprayed on membrane and in acoustical method, ultrasonic waves vibrate deposits off the spray

ISE based monitors can be used for measuring the following effluent parametersAmmonia

ChlorideCopperCyanide

Fluoride

Nitrate

NitriteSulphide

Water hardness etc.


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PH meterA pH meter is an electronic instrument measuring the pH (acidity or alkalinity) of a

liquid .

A typical pH meter consists of a special measuring probe (a glass electrode) connected toan electronic meter that measures and displays the pH reading.

The pH probe measures pH as the activity of hydrogen ions surrounding a thin-walledglass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that

is measured and displayed as pH units by the meter.

Calibration and use:-

For very precise work the pH meter should be calibratedbefore each measurement. For

normal use calibration should be performed at the beginning of each day. The reason for

this is that the glass electrode does not give a reproducible e.m.f. over longer periods of

time.Calibration should be performed with at least two standard buffer solutions that span the

range of pH values to be measured.

For general purposes buffers at pH 4 and pH 10 are acceptable.

The calibration process correlates the voltage produced by the probe with the pH scale.

After each single measurement, the probe is rinsed with distilled water to remove anytraces of the solution being measured, blotted with a clean tissue to absorb any remaining

water which could dilute the sample and thus alter the reading, and then quickly

immersed in another solution.

Types of pH meters

1. Null Detection Type PH meter

2. Chopper Amplifier Type PH meter.
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ll-Detector Type pH Meter:


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Chopper Amplifier Type pH Meter:


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Electrodes for pH measurement:

There are different types of electrodes for the measurement of pH. Such as

1. Hydrogen electrode2. Glass electrode

3. Calomel electrode or Reference electrode4. Silver/Silver chloride Reference electrode


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Hydrogen electrode:

Glass electrode:


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Calomel Electrode or Reference Electrode:


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Silver/Silver Chloride Electrode:


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CONDUCTIVITY METER

An electrical conductivity meter (EC meter) measures the electrical conductivity in a

solution. Commonly used in hydroponics, aquaculture and freshwater systems to monitorthe amount of nutrients, salts or impurities in the water.

An electrical conductivity meter

Principle of operation:

The common laboratory conductivity meters employ a potentiometric method and four

electrodes. Often, the electrodes are cylindrical and arranged concentrically. The

electrodes are usually made of platinum metal. An alternating current is applied to theouter pair of the electrodes. The potential between the inner pair is measured.

Conductivity could in principle be determined using the distance between the electrodesand their surface area using the Ohm's law but generally, for accuracy, a calibration isemployed using electrolytes of well-known conductivity.

Industrial conductivity probes often employ an inductive method, which has theadvantage that the fluid does not wet the electrical parts of the sensor. Here, two

inductively-coupled coils are used. One is the driving coil producing a magnetic field and

it is supplied with accurately-known voltage. The other forms a secondary coil of a

transformer. The liquid passing through a channel in the sensor forms one turn in thesecondary winding of the transformer. The induced current is the output of the sensor.

Temperature dependence:Electrical conductivity

The conductivity of a solution is highly temperature dependent, therefore it is importantto either use a temperature compensated instrument, or calibrate the instrument at the

same temperature as the solution being measured. Unlike metals, the conductivity of

common electrolytes typically increases with increasing temperature.

Over a limited temperature range, the way temperature affect conductivity of a solution

can be modeled linearly using the following formula:
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where

Tis the temperature of the sample,Tcalis the calibration temperature,

Tis the electrical conductivity at the temperature T,

Tcal is the electrical conductivity at the calibration temperature Tcal,

is the temperature compensation slope of the solution.

The temperature compensation slope for most naturally occurring waters is about 2 %/C,however it can range between 1 to 3 %/C. The compensation slope for some common

water solutions are listed in the table below.

Aqueous solution at 25 C Concentration (mass percentage) (%/C)

HCl 10 1.56

KCl 10 1.88

H2SO4 50 1.93

NaCl 10 2.14

HF 1.5 7.20

HNO3 31 31

Conductivity factor:

conductivity factor (CF) of dissolvedsalts in a given solution is a measurement of

conductivity. Using the electrical conductivity between two electrodes in a water

solution, the level of dissolved solids in that solution can be measured. Measurements canthen be used to dose the solution with the necessary nutrients in the case of hydroponics.

Conductivity measurements are also used in ecology and environmental sciences toassess the level of nutrients in lakes and rivers
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Ultraviolet-visible spectroscopy orultraviolet-visible

spectrophotometers (UV-Vis orUV/Vis):

It refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible

spectral region. This means it uses light in the visible and adjacent (near-UV and near-

infrared (NIR)) ranges. The absorption or reflectance in the visible range directly affectsthe perceived color of the chemicals involved. In this region of the electromagnetic

spectrum, molecules undergo electronic transitions. This technique is complementary to

fluorescence spectroscopy, in that fluorescence deals with transitions from the excited

state to the ground state, while absorption measures transitions from the ground state to

the excited state.

UV/Vis spectroscopy is routinely used in the quantitative determination of solutions of

transition metal ions highly conjugatedorganic compounds, and biological

macromolecules.

Solutions of transition metal ions can be colored (i.e., absorb visible light)

because d electrons within the metal atoms can be excited from one electronic

state to another. The colour of metal ion solutions is strongly affected by the

presence of other species, such as certain anions orligands. For instance, the

colour of a dilute solution ofcopper sulfate is a very light blue; adding ammonia

intensifies the colour and changes the wavelength of maximum absorption (max).

Organic compounds, especially those with a high degree ofconjugation, also

absorb light in the UV or visible regions of the electromagnetic spectrum. The

solvents for these determinations are often water for water soluble compounds, or

ethanol for organic-soluble compounds. (Organic solvents may have significant

UV absorption; not all solvents are suitable for use in UV spectroscopy. Ethanol

absorbs very weakly at most wavelengths.) Solvent polarity and pH can affect the

absorption spectrum of an organic compound. Tyrosine, for example, increases in

absorption maxima and molar extinction coefficient when pH increases from 6 to

13 or when solvent polarity decreases.

While charge transfer complexes also give rise to colours, the colours are often

too intense to be used for quantitative measurement.

The Beer-Lambert law states that the absorbance of a solution is directly proportional to

the concentration of the absorbing species in the solution and the path length. Thus, for a
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fixed path length, UV/Vis spectroscopy can be used to determine the concentration of the

absorber in a solution. It is necessary to know how quickly the absorbance changes with

concentration. This can be taken from references (tables ofmolar extinction coefficients),

or more accurately, determined from a calibration curve.

A UV/Vis spectrophotometer may be used as a detector forHPLC. The presence of an

analyte gives a response assumed to be proportional to the concentration. For accurate

results, the instrument's response to the analyte in the unknown should be compared with

the response to a standard; this is very similar to the use of calibration curves. The

response (e.g., peak height) for a particular concentration is known as the response factor.

The wavelengths of absorption peaks can be correlated with the types of bonds in a given

molecule and are valuable in determining the functional groups within a molecule. The

Woodward-Fieser rules, for instance, are a set of empirical observations used to predict

max, the wavelength of the most intense UV/Vis absorption, for conjugated organic

compounds such as dienes and ketones. The spectrum alone is not, however, a specific

test for any given sample. The nature of the solvent, the pH of the solution, temperature,

high electrolyte concentrations, and the presence of interfering substances can influence

the absorption spectrum. Experimental variations such as the slit width (effective

bandwidth) of the spectrophotometer will also alter the spectrum. To apply UV/Vis

spectroscopy to analysis, these variables must be controlled or accounted for in order to

identify the substances present.
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Beer-Lambert law

The method is most often used in a quantitative way to determine concentrations of an

absorbing species in solution, using the Beer-Lambert law:

,

WhereA is the measured absorbance,I0 is the intensity of the incident light at a given

wavelength,Iis the transmitted intensity,L the path length through the sample, and c the

concentration of the absorbing species. For each species and wavelength, is a constant

known as the molar absorptivity or extinction coefficient. This constant is a fundamental

molecular property in a given solvent, at a particular temperature and pressure, and has

units of 1 /M* cm or oftenAU/M* cm.

The absorbance and extinction are sometimes defined in terms of the natural logarithm

instead of the base-10 logarithm.

The Beer-Lambert Law is useful for characterizing many compounds but does not hold as

a universal relationship for the concentration and absorption of all substances. A 2nd

order polynomial relationship between absorption and concentration is sometimes

encountered for very large, complex molecules such as organic dyes (Xylenol Orange or

Neutral Red, for example).

Ultraviolet-visible spectrophotometer

The instrument used in ultraviolet-visible spectroscopy is called a UV/Vis

spectrophotometer. It measures the intensity of light passing through a sample (I), and

compares it to the intensity of light before it passes through the sample (Io). The ratioI/

Iois called the transmittance, and is usually expressed as a percentage (%T). The

absorbance,A, is based on the transmittance:

A= log(%T/ 100%)

The UV-visible spectrophotometer can also be configured to measure reflectance. In this

case, the spectrophotometer measures the intensity of light reflected from a sample (I),

and compares it to the intensity of light reflected from a reference material (Io)(such as a
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white tile). The ratioI/Iois called the reflectance, and is usually expressed as a

percentage (%R).

The basic parts of a spectrophotometer are a light source, a holder for the sample, a

diffraction grating in a monochromatoror a prism to separate the different wavelengths

of light, and a detector. The radiation source is often a Tungsten filament (300-2500 nm),

a deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm),

Xenon arc lamps, which is continuous from 160-2,000 nm; or more recently, light

emitting diodes (LED)[5]

for the visible wavelengths. The detector is typically a

photomultiplier tube, a photodiode, a photodiode array or a charge-coupled device(CCD).

Single photodiode detectors and photomultiplier tubes are used with scanning

monochromators, which filter the light so that only light of a single wavelength reaches

the detector at one time. The scanning monochromator moves the diffraction grating to

"step-through" each wavelength so that it's intensity may be measured as a function of

wavelength. Fixed monochromators are used with CCDs and photodiode arrays. As both

of these devices consist of many detectors grouped into one or two dimensional arrays,

they are able to collect light of different wavelengths on different pixels or groups of

pixels simultaneously.

Fig: Spectrophotometer

A spectrophotometer can be eithersingle beam ordouble beam. In a single beam

instrumentall of the light passes through the sample cell.Io must be measured by
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removing the sample. This was the earliest design, but is still in common use in both

teaching and industrial labs.

Diagram of

a single-beam UV/Vis spectrophotometer.

.

In a double-beam instrument, the light is split into two beams before it reaches the

sample. One beam is used as the reference; the other beam passes through the sample.

The reference beam intensity is taken as 100% Transmission (or 0 Absorbance), and the

measurement displayed is the ratio of the two beam intensities. Some double-beam

instruments have two detectors (photodiodes), and the sample and reference beam are

measured at the same time. In other instruments, the two beams pass through a beam

chopper, which blocks one beam at a time. The detector alternates between measuring the

sample beam and the reference beam in synchronism with the chopper. There may also

be one or more dark intervals in the chopper cycle. In this case the measured beam

intensities may be corrected by subtracting the intensity measured in the dark intervalbefore the ratio is taken.
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Diagram of double beam UV/VIS Spectrophotometer.

Samples for UV/Vis spectrophotometry are most often liquids, although the absorbance

of gases and even of solids can also be measured. Samples are typically placed in a

transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape,

commonly with an internal width of 1 cm. (This width becomes the path length,L, in the

Beer-Lambert law.) Test tubes can also be used as cuvettes in some instruments. The type

of sample container used must allow radiation to pass over the spectral region of interest.

The most widely applicable cuvettes are made of high quality fused silica orquartz glass

because these are transparent throughout the UV, visible and near infrared regions. Glass

and plastic cuvettes are also common, although glass and most plastics absorb in the UV,

which limits their usefulness to visible wavelengths.

Specialized instruments have also been made. These include attaching

spectrophotometers to telescopes to measure the spectra of astronomical features. UV-

visible microspectrophotometers consist of a UV-visible microscope integrated with a

UV-visible spectrophotometer. These are commonly used for measuring thin film

thickness in semiconductor manufacturing, materials science research, measuring the

energy content of coal and petroleum source rock, and in forensic laboratories for the

analysis of microscopic amounts of trace evidence as well as questioned documents.

A complete spectrum of the absorption at all wavelengths of interest can often be

produced directly by a more sophisticated spectrophotometer. In simpler instruments the

absorption is determined one wavelength at a time and then compiled into a spectrum by
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the operator. By removing the concentration dependence, the extinction coefficient ()

can be determined as a function of wavelength.

IR spectrophotometry

Spectrophotometers designed for the main infrared region are quite different because of

the technical requirements of measurement in that region. One major factor is the type of

photo sensors that are available for different spectral regions, but infrared measurement is

also challenging because virtually everything emits IR light as thermal radiation,

especially at wavelengths beyond about 5 m.

Another complication is that quite a few materials such as glass and plastic absorb

infrared light, making it incompatible as an optical medium. Ideal optical materials are

salts, which do not absorb strongly. Samples for IR spectrophotometry may be smeared

between two discs ofpotassium bromide or ground with potassium bromide and pressed

into a pellet. Where aqueous solutions are to be measured, insoluble silver chloride is

used to construct the cell.

Sources

An inert solid is electrically heated to a temperature in the range 1500-2000 K. The

heated material will then emit infra red radiation.

The Nernst gloweris a cylinder (1-2 mm diameter, approximately 20 mm long) of rare

earth oxides. Platinum wires are sealed to the ends, and a current passed through the

cylinder. The Nernst glower can reach temperatures of 2200 K.

The Globar source is a silicon carbide rod (5mm diameter, 50mm long) which is

electrically heated to about 1500 K. Water cooling of the electrical contacts is needed to

prevent arcing. The spectral output is comparable with the Nernst glower, execept at

The incandescent wire source is a tightly wound coil of nichrome wire, electrically

heated to 1100 K. It produces a lower intensity of radiation than the Nernst or Globar

sources, but has a longer working life.
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Detectors

There are three categories of detector;

Thermal

Pyroelectric

Photo conducting

Thermocouples consist of a pair of junctions of different metals; for example, two pieces

of bismuth fused to either end of a piece of antimony. The potential difference (voltage)

between the junctions changes according to the difference in temperature between the

junctions

Pyroelectric detectors are made from a single crystalline wafer of a pyroelectric material,

such as triglycerine sulphate. The properties of a pyroelectric material are such that when

an electric field is applied across it, electric polarization occurs (this happens in any

dielectric material). In a pyroelectric material, when the field is removed, the polarization

persists. The degree of polarization is temperature dependant. So, by sandwiching the

pyroelectric material between two electrodes, a temperature dependant capacitor is made.

The heating effect of incident IR radiation causes a change in the capacitance of the

material. Pyroelectric detectors have a fast response time. They are used in most Fourier

transform IR instruments.

Photoelectric detectors such as the mercury cadmium telluride detector comprise a film

of semiconducting material deposited on a glass surface, sealed in an evacuated envelope.

Absorption of IR promotes nonconducting valence electrons to a higher, conducting,

state. The electrical resistance of the semiconductor decreases. These detectors have

better response characteristics than pyroelectric detectors and are used in FT-IR

instruments - particularly in GC - FT-IR.

Pneumatic detector

A pressure-sensitive detectorbased on the pressure increase of a gas. A special type is the

Golay cell where the pressure change is detected by observing the deflection off one of

the chamber walls.


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Types of instrument

Dispersive infra red spectrophotometers

These are often double-beam recording instruments, employing diffraction gratings for

dispersion of radiation.

Radiation from the source is flicked between the reference and sample paths. Often, anoptical nullsystem is used. This is when the detector only responds if the intensity of the

two beams is unequal. If the intensities are unequal, a light attenuator restores equality by

moving in or out of the reference beam. The recording pen is attached to this attenuator.

Fourier-transform spectrometers

Any waveform can be shown in one of two ways; either infrequency domain ortime

domain.

Dispersive IR instruments operate in the frequency domain. There are, however,

advantages to be gained from measurement in the time domain followed by computer

transformation into the frequency domain.

If we wished to record a trace in the time domain, it could be possible to do so by

allowing radiation to fall on a detector and recording its response over time. In practice,

no detector can respond quickly enough (the radiation has a frequency greater than 1014

Hz). This problem can be solved by using interference to modulate the i.r. signal at a

detectable frequency. TheMichelson interferometeris used to produce a new signal of a

much lower frequency which contains the same information as the original IR signal. The

output from the interferometer is an interferogram.


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The Michelson interferometer

Radiation leaves the source and is split. Half is reflected to a stationary mirror and then

back to the splitter. This radiation has travelled a fixed distance. The other half of the

radiation from the source passes through the splitter and is reflected back by a movable

mirror. Therefore, the path length of this beam is variable. The two reflected beams

recombine at the splitter, and they interfere (e.g. for any one wavelength, interference

will be constructive if the difference in path lengths is an exact multiple of the

wavelength. If the difference in path lengths is half the wavelength then destructive

interference will result). If the movable mirror moves away from the beam splitter at a

constant speed, radiation reaching the detector goes through a steady sequence of maxima

and minima as the interference alternates between constructive and destructive phases.

If monochromatic IR radiation of frequency,f ( ir ) enters the interferometer, then the

output frequency,fm can be found by;

Wherev is the speed of mirror travel in mm/s

Because all wavelengths emitted by the source are present, the interferogram is extremely

complicated.

The moving mirror must travel smoothly; a frictionless bearing is used with

electromagnetic drive. The position of the mirror is measured by a laser shining on a


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corner of the mirror. A simple sine wave interference pattern is produced. Each peak

indicates mirror travel of one half the wavelength of the laser. The accuracy of this

measurement system means that the IR frequency scale is accurate and precise.

In the FT-IR instrument, the sample is placed between the output of the interferometer

and the detector. The sample absorbs radiation of particular wavelengths. Therefore, the

interferogram contains the spectrum of the source minus the spectrum of the sample. An

interferogram of a reference (sample cell and solvent) is needed to obtain the spectrum of

the sample.

After an interferogram has been collected, a computer performs a Fast Fourier Transform,

which results in a frequency domain trace (i.e. intensity vs. wave number) that we all

know and love.

The detector used in an FT-IR instrument must respond quickly because intensity

changes are rapid (the moving mirror moves quickly). Pyroelectric detectors or liquid

nitrogen cooled photon detectors must be used. Thermal detectors are too slow.

To achieve a good signal to noise ratio, many interferograms are obtained and then

averaged. This can be done in less time than it would take a dispersive instrument to

record one scan.

Advantages of Fourier transform IR over dispersive IR;

Improved frequency resolution

Improved frequency reproducibility (older dispersive instruments must be

recalibrated for each session of use)

Higher energy throughput

Faster operation

Computer based (allowing storage of spectra and facilities for processing spectra)

Easily adapted for remote use (such as diverting the beam to pass through an

external cell and detector, as in GC - FT-IR)


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GAS CHROMATOGRAPHY

Introduction

Gas chromatography - specifically gas-liquid chromatography - involves a sample being

vaporized and injected onto the head of the chromatographic column. The sample istransported through the column by the flow of inert, gaseous mobile phase. The column

itself contains a liquid stationary phase which is adsorbed onto the surface of an inertsolid.

Have a look at this schematic diagram of a gas


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Gas chromatography (GC), is a common type ofchromatography used in analytic

chemistry forseparating and analyzing compounds that can be vaporized without

decomposition. Typical uses of GC include testing the purity of a particular substance, or

separating the different components of a mixture (the relative amounts of such

components can also be determined). In some situations; GC may help in identifying a

compound. In preparative chromatography, GC can be used to prepare pure compounds

from a mixture.

In gas chromatography, the moving phase (or "mobile phase") is a carrier gas, usually

an inert gas such as helium or an unreactive gas such as nitrogen. The stationary phase is

a microscopic layer ofliquid orpolymeron an inert solid support, inside a piece

ofglass ormetal tubing called a column (a homage to the column used in distillation).
http://en.wikipedia.org/wiki/Chromatographyhttp://en.wikipedia.org/wiki/Analytic_chemistryhttp://en.wikipedia.org/wiki/Analytic_chemistryhttp://en.wikipedia.org/wiki/Separation_processhttp://en.wikipedia.org/wiki/Vaporisedhttp://en.wikipedia.org/wiki/Preparative_chromatographyhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Inerthttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Reactivity_(chemistry)http://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Glasshttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Glasshttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Reactivity_(chemistry)http://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Inerthttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Preparative_chromatographyhttp://en.wikipedia.org/wiki/Vaporisedhttp://en.wikipedia.org/wiki/Separation_processhttp://en.wikipedia.org/wiki/Analytic_chemistryhttp://en.wikipedia.org/wiki/Analytic_chemistryhttp://en.wikipedia.org/wiki/Chromatography


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The instrument used to perform gas chromatography is called a gas chromatograph (or

"aerograph", "gas separator").

The gaseous compounds being analyzed interact with the walls of the column, which is

coated with different stationary phases. This causes each compound to elute at a different

time, known as the retention time of the compound. The comparison of retention times is

what gives GC its analytical usefulness.

Gas chromatography is in principle similar to column chromatography (as well as other

forms of chromatography, such as HPLC, TLC), but has several notable differences.

Firstly, the process of separating the compounds in a mixture is carried out between a

liquid stationary phase and a gas moving phase, whereas in column chromatography the

stationary phase is a solid and the moving phase is a liquid. (Hence the full name of the

procedure is "Gas-liquid chromatography", referring to the mobile and stationary phases,

respectively.) Secondly, the column through which the gas phase passes is located in

an oven where the temperature of the gas can be controlled, whereas column

chromatography (typically) has no such temperature control. Thirdly, the concentration of

a compound in the gas phase is solely a function of the vapor pressure of the gas.[1]

Gas chromatography is also similar to fractional distillation, since both processes separate

the components of a mixture primarily based on boiling point (or vapor pressure)

differences. However, fractional distillation is typically used to separate components of a

mixture on a large scale, whereas GC can be used on a much smaller scale (i.e. micro

scale). Gas chromatography is also sometimes known as vapor-phase

chromatography (VPC), or gas-liquid partition chromatography (GLPC). These

alternative names, as well as their respective abbreviations, are frequently found

in scientific literature. Strictly speaking,

The Modern Gas Chromatograph

The modern gas chromatograph is a fairly complex instrument mostly computer

controlled. The samples are mechanically injected, the analytical results are

automatically calculated and the results printed out, together with the pertinentoperating conditions in a standard format. However, the instrument has evolvedover many years although the majority of the added devices and techniques

were suggested or describe in the first three international symposia on gaschromatography held in 1956, 1958 and 1960. These symposia, initially

organized by the 'British Institute of Petroleum' have been held every two yearsever since 1956 and the meetings have remained the major stimulus for

developing the technique and extending its capabilities. However, the majority

of the techniques and devices that have been incorporated in the modernchromatograph, were described, reported, or discussed in the first triad of

symposia.
http://en.wikipedia.org/wiki/Elutionhttp://en.wikipedia.org/wiki/Column_chromatographyhttp://en.wikipedia.org/wiki/High-performance_liquid_chromatographyhttp://en.wikipedia.org/wiki/Thin_layer_chromatographyhttp://en.wikipedia.org/wiki/Ovenhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Vapor_pressurehttp://en.wikipedia.org/wiki/Gas_chromatography#cite_note-Pavia-0http://en.wikipedia.org/wiki/Gas_chromatography#cite_note-Pavia-0http://en.wikipedia.org/wiki/Gas_chromatography#cite_note-Pavia-0http://en.wikipedia.org/wiki/Fractional_distillationhttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Scientific_literaturehttp://en.wikipedia.org/wiki/Scientific_literaturehttp://en.wikipedia.org/wiki/Boiling_pointhttp://en.wikipedia.org/wiki/Fractional_distillationhttp://en.wikipedia.org/wiki/Gas_chromatography#cite_note-Pavia-0http://en.wikipedia.org/wiki/Vapor_pressurehttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Ovenhttp://en.wikipedia.org/wiki/Thin_layer_chromatographyhttp://en.wikipedia.org/wiki/High-performance_liquid_chromatographyhttp://en.wikipedia.org/wiki/Column_chromatographyhttp://en.wikipedia.org/wiki/Elution


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The layout of the modern gas chromatograph is shown as a block diagram infigure 1.

Instrumental componentsCarrier gas

The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium,argon, and carbon dioxide. The choice of carrier gas is often dependent upon the type of

detector which is used. The carrier gas system also contains a molecular sieve to remove

water and other impurities.

Injection Devices

The basic injection devices that are used in chromatography, such as the

external loop valve, have been discussed in book 1. In gas chromatography twobasic types of sampling system are used, those suitable for packed columns and

those designed for open tubular columns. In addition, different sample injectors

are necessary that will be appropriate for alternative column configurations. It


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must be stressed, however, that irrespective of the design of the associated

equipment, the precision and accuracy of a GC analysis will only be as good asthat provided by the sample injector. The sample injector is a very critical part

of the chromatographic equipment and needs to be well designed and well

maintained.

Sample injection port

For optimum column efficiency, the sample should not be too large, and should be

introduced onto the column as a "plug" of vapor - slow injection of large samples causesband broadening and loss of resolution. The most common injection method is where a

micro syringe is used to inject sample through a rubber septum into a flash vaporizer port

higher than the boiling point of the least volatile component of the sample. For packed

columns, sample size ranges from tenths of a micro liter up to 20 micro liters. Capillary

columns, on the other hand, need much less sample, typically around 10-3

capillary GC, split/split less injection is used. Have a look at this diagram of a split/split

less injector;

The injector can be used in one of two modes; split or split less. The injector contains aheated chamber containing a glass liner into which the sample is injected through the

septum. The carrier gas enters the chamber and can leave by three routes (when the

injector is in split mode). The sample vaporizes to form a mixture of carrier gas,vaporized solvent and vaporized solutes. A proportion of this mixture passes onto the

column, but most exits through the split outlet. The septum purge outlet prevents septum

bleed components from enter the column.


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Open Tubular Column Injection Systems

Due to the very small sample size that must be placed on narrow bore capillarycolumns, a split injection system is necessary.

The basic difference between the two types of injection systems is that the

capillary column now projects into the glass liner and a portion of the carriergas sweeps past the column inlet to waste. As the sample passes the column

opening, a small fraction is split off and flows directly into the capillarycolumn, ipso facto this device is called a split injector. The split ratio is changed

by regulating the portion of the carrier gas that flows to waste which is achieved

by an adjustable flow resistance in the waste flow line. This device is only usedfor small diameter capillary columns where the charge size is critical.

Columns

There are two general types of column, packed and capillary (also known as open

tubular). Packed columns contain a finely divided, inert, solid support material

(commonly based on diatomaceous earth) coated with liquid stationary phase. Mostpacked columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Capillary columns have an internal diameter of a few tenths of a millimeter. They can beone of two types; wall-coated open tubular (WCOT) or support-coated open

tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated

with liquid stationary phase. In support-coated columns, the inner wall of the capillary is

lined with a thin layer of support material such as diatomaceous earth, onto which the

stationary phase has been adsorbed. SCOT columns are generally less efficient thanWCOT columns. Both types of capillary column are more efficient than packed columns.

In 1979, a new type of WCOT column was devised - the Fused Silica Open

Tubular (FSOT) column;


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These have much thinner walls than the glass capillary columns, and are given strength

by the polyimide coating. These columns are flexible and can be wound into coils. Theyhave the advantages of physical strength, flexibility and low reactivity.

The Packed GC Column

Packed columns are usually constructed from stainless steel or Pyrex glass.

Pyrex glass is favored when thermally labile materials are being separated such

as essential oils and flavor components. However, glass has pressure limitationsand for long packed columns, stainless steel columns are used as they can easily

tolerate the necessary elevated pressures. The sample must, of course, beamenable to contact with hot metal surfaces. Short columns can be straight, andinstalled vertically in the chromatograph. Longer columns can be U-shaped but

columns more than a meter long are usually coiled. Such columns can be

constructed of any practical length and relatively easily installed. Pyrex glasscolumns are formed to the desired shape by coiling at about 700C and metal

columns by bending at room temperature. Glass columns are sometimes treated

with an appropriate silanizing reagent to eliminate the surface hydroxyl groups

which can be catalytically active or produce asymmetric peaks. Stainless steelcolumns are usually washed with dilute hydrochloric acid, then extensively with

water followed by methanol, acetone, methylene dichloride and n-hexane. This

washing procedure removes any corrosion products and traces of lubricatingagents used in the tube drawing process. The columns are then ready forpacking.

Column temperature

For precise work, column temperature must be controlled to within tenths of a degree.

The optimum column temperature is dependent upon the boiling point of the sample. As

a rule of thumb, a temperature slightly above the average boiling point of the sample

results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution,but increase elution times. If a sample has a wide boiling range, then temperature

programming can be useful. The column temperature is increased (either continuously orin steps) as separation proceeds.

Detectors

There are many detectors which can be used in gas chromatography. Different detectors

will give different types of selectivity. A non-selective detector responds to all


36/75

compounds except the carrier gas, a selective detector responds to a range of compounds

with a common physical or chemical property and a specific detector responds to a singlechemical compound. Detectors can also be grouped into concentration dependant

detectors and mass flow dependant detectors. The signal from a concentration dependant

detector is related to the concentration of solute in the detector, and does not usually

destroy the sample Dilution of with make-up gas will lower the detectors response. Massflow dependant detectors usually destroy the sample, and the signal is related to the rate

at which solute molecules enter the detector. The response of a mass flow dependant

detector is unaffected by make-up gas. Have a look at this tabular summary of commonGC detectors:

Detector TypeSupport

gasesSelectivity Detectability

Dynamic

range

Flame

ionization

(FID)

Mass flowHydrogen

and airMost organic cpds. 100 pg 10

7

Thermal

conductivity

(TCD)

Concentration Reference Universal 1 ng 107

Electron

capture

(ECD)

Concentration Make-up

Halides, nitrates,nitriles, peroxides,

anhydrides,

organometallics

50 fg 105

Nitrogen-

phosphorusMass flow

Hydrogen

and airNitrogen, phosphorus 10 pg 10

6

Flame

photometric(FPD)

Mass flow

Hydrogen

and air

possibly

oxygen

Sulphur, phosphorus,

tin, boron, arsenic,

germanium, selenium,

chromium

100 pg 103

Photo-

ionization(PID)

Concentration Make-up

Aliphatics, aromatics,

ketones, esters,aldehydes, amines,

heterocyclics,

organosulphurs, someorganometallics

2 pg 107

Hallelectrolytic

conductivity

Mass flowHydrogen,

oxygen

Halide, nitrogen,

nitrosamine, sulphur


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The effluent from the column is mixed with hydrogen and air, and ignited. Organiccompounds burning in the flame produce ions and electrons which can conduct electricity

through the flame. A large electrical potential is applied at the burner tip, and a collector

electrode is located above the flame. The current resulting from the pyrolysis of anyorganic compounds is measured. FIDs are mass sensitive rather than concentration

sensitive; this gives the advantage that changes in mobile phase flow rate do not affect

the detector's response. The FID is a useful general detector for the analysis of organiccompounds; it has high sensitivity, a large linear response range, and low noise. It is also

robust and easy to use, but unfortunately, it destroys the sample

The Nitrogen Phosphorus Detector (NPD)

The nitrogen phosphorus detector (NPD), is a highly sensitive but specific

detector and evolved directly from the FID. It gives a strongresponse to organic compounds containing nitrogen and/or phosphorus. Although

it appears to function in a very similar manner to the FID, in fact, it operates on

an entirely different principle. A diagram of an NP detector is shown in figure

24.


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The Electron Capture Detector

The electron capture detector contains a low energy source which is used to

produce electrons for capturing by appropriate atoms. Although tritiumadsorbed into a silver foil has been used as the particle source, it is relatively

unstable at high temperatures, the Ni63

source was found to be preferable. The

detector can be used in two modes, either with a constant potential appliedacross the cell (the DC mode) or with a pulsed potential across the cell (the

pulsed mode). In the DC mode, hydrogen or nitrogen can be used as the carriergas and a small potential (usually only a few volts) is applied across the cell that

is just sufficient to collect all the electrons available and provide a smallstanding current. If an electron capturing molecule (for example a molecule

containing an halogen atom which has only seven electrons in its outer shell)

enters the cell, the electrons are captured by the molecule and the moleculesbecome charged. The mobility of the captured electrons is much smaller than

the free electrons and the electrode current falls dramatically. The DC mode of

detection, however, has some distinct disadvantages. The most serious objectionis that the electron energy varies with the applied potential. The electron

capturing properties of a molecule varies with the electron energy, so the

specific response of the detector will depend on the applied potentialOperating in the pulsed mode, a mixture of 10% methane in argon is employedwhich changes the nature of the electron capturing environment. The electrons

generated by the radioactive source rapidly assume only thermal energy and, in

the absence of a collecting potential, exist at the source surface in an annularregion about 2 mm deep at room temperature and about 4 mm deep at 400C. A

short period square wave pulse is applied to the electrode collecting the

electrons and producing a base current. The standing current, using 10%

methane in argon is about 10-8

amp with a noise level of about 5 x 10-12

amp.The pulse wave form is shown in figure


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Wave form of Electron Capture Detector Pulses

In the inactive period of the wave form, electrons having thermal energy only

will attached themselves readily to any electron capturing molecules present in

the cell with the consequent production of negatively charged ions. The

negative ions quickly recombine with the positive ions and thusbecome unavailable for collection. Consequently the standing current measured

during the potential pulse will be reduced.

The period of the pulsed potential is adjusted such that relatively few of theslow negatively charged molecules (molecules having captured electrons and

not neutralized by collision with positive ions) have time to reach the anode, but

the faster moving electrons are all collected. During the "off period" the

electrons re-establish equilibrium with the gas. The three operating variables arethe pulse duration, pulse frequency and pulse amplitude. By appropriate

adjustment of these parameters the current can be made to reflect the relative

mobilities of the different charged species in the cell and thus exercise somediscrimination between different electron capturing materials. A diagram of an

electron capture detector is shown in fig.


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Electron Capture Detector

Gas SuppliesGases for use with the gas chromatograph were originally all obtained from gas

tanks or gas cylinders. However, over the past decade the use of gas generatorshave become more popular as it avoids having gases at high pressure in thelaboratory which is perceived by some as potentially dangerous. In addition, the

use of a hydrogen generator avoids the use of a cylinder of hydrogen at high

pressure which is also perceived by some as a serious fire hazard despite thefact that they have been used in laboratories, quite safely for nearly a century.

Supplies from Gas Tanks

Gasses are stored in large cylindrical tanks fitted with reducing valves that areset to supply the gas to the instrument at the recommended pressure defined by

the manufacturers. The cylinders are often situated outside and away from the

chromatograph for safety purposes and the gasses are passed to thechromatograph through copper or stainless steel conduits at relatively low

pressure. The main disadvantage of gas tanks is their size and weight which

makes them difficult to move and replace.

Pure Air Generators.

Air generators require an air supply from air tanks or directly from the

laboratory compressed air supply. The Packard Zero Air Generator passes thegas through a 0.5filter to remove oil and water and finally over a catalyst to

remove hydrocarbons. The hydrocarbon free air is then passed through a

0.01 cellulose fiber filter to remove any residual particulate matter that may bepresent. The manufacturers claim the resulting air supply contains less than 0.1

ppm total hydrocarbons and delivers air at 125 psi at flow rates up to 2,500 cc

per min.

Preparative Gas Chromatography

Gas chromatography has not been used extensively for preparative work

although its counterpart, liquid chromatography, has been broadly used in thepharmaceutical industry for the isolation and purification of physiologically

active substances. There are a number of unique problems associated with

preparative gas chromatography. Firstly, it is difficult to recycle the mobilephase and thus large volume of gas are necessary. Secondly, the sample must befully vaporized onto the column to ensure radial distribution of the sample

across the column. Thirdly, the materials of interest are eluted largely in a very

dilute form from the column and therefore must be extracted or condensed from

the gas stream which is also difficult to achieve efficiently. Finally, the efficientpacking of large GC columns is difficult. Nevertheless, preparative GC has been

successfully used in a number of rather special applications; for example theisolation of significant quantities of the trace components of essential oils fororganoleptic assessment.

The layout of a preparative gas chromatograph is shown in figure38


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Figure 38 Layout of Preparative Gas Chromatograph

Air cannot normally be used as the mobile phase due to likely oxidation and so

either a gas tank or a gas (e.g., nitrogen) generator must be used. As the flow

rates can be large, more than one generator operating in parallel will often benecessary. The sample is usually placed on to the column with a syringe pump

and rapidly vaporized in a suitable heater. Passing the gas in vapor form ontothe column helps evenly distribute the sample radially across the column. Thedetector that is used must have specifications that are almost opposite to those

of an analytical detector. It should function well at high concentrations of

solute, have a generally low sensitivity, if in-line it must be non-destructive and

have minimum flow impedance. It need not have a particularly linear response.The katharometer is one of the more popular detectors for preparative GC. The

column outlet is passed to a selection valve that diverts the eluent to its

appropriate collecting vessel. The collecting vessel may be cooled in ice, solidcarbon dioxide or if necessary liquid nitrogen (liquid nitrogen can only be used

if a low boiling gas such as helium is employed as the carrier gas). In some

cases the solutes contained in the eluent can be extracted into an appropriateliquid or onto the surface of a suitable adsorbent. the desired fractions are thenrecovered by distillation or desorption.

The Moving Bed Continuous Chromatography SystemThe concept of the moving bed extraction process was originally introduced for

hydrocarbon gas adsorption by Freund et al. (13) and was first applied to gas

liquid chromatography by Scott (14). A diagram of the moving bed systemsuitable for GC was proposed by Scott and is shown in figure 39.


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The feasibility of this process was established for a gas chromatographic

system, subsequently, its viability was also confirmed for liquidchromatography which will be discussed in Book 19. The moving bed system

takes a continuous sample feed and operates in the following way. The

stationary phase, coated on a suitable support, is allowed to fall down a column

against and upward stream of carrier gas. In the original device of Scott, thepacking (dinonyl phthalate coated on brick dust) was contained in a hopper at

the top of the column and was taken off from the bottom the column by a

rotating disc feed table and returned to the hopper by a simple air-lift device.

Courtesy of Butterworths Scientific Publications Ltd. [Ref. 10]

ApplicationsGas chromatography has a very wide field of application but its first and main

area of use is in the separation and analysis of multi component mixtures such

as essential oils, hydrocarbons and solvents. Intrinsically, with the use of theflame ionization detector and the electron capture detector (which have very

high sensitivities) gas chromatography can quantitatively determine materials

present at very low concentrations. It follows, that the second most importantapplication area is in pollution studies, forensic work and general trace analysis.

GasolineGasoline is a multicomponent mixture containing a large number ofhydrocarbons, many of which have very similar molecular weights and all are

almost exclusively dispersive in interactive character. The structures of many of

the hydrocarbons are also very similar and there are many isomers present. As aconsequence, due to their interactive similarity the separation factors between

individual components are very small.


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It follows that columns of very high efficiency will be mandatory to achieve an

effective separation. It is clear that open tubular columns are ideal for this typeof separation problem. In fact, it would be impossible to separate the

components of gasoline efficiently with a packed column, even one that is 50 ft

long, and even if the inherent long analysis times could be tolerated. In addition

this type of separation demands the maximum number of theoretical plates andtherefore not only must open tubes be used but tubes of relatively small

diameter to produce the maximum number of theoretical plates. In fact, several

hundred thousand theoretical plates will be necessary and so the column mustalso be very long. As has already been discussed, it is necessary to use small

radius open tubular columns with a split injection system. Furthermore, as a

result of the wide range of molecular weight of the components present,

gasoline has a relatively wide boiling range and so will require a temperatureprogram that will heat the column to 200 C or more. A thermally stable

stationary phase must be employed.The individual gasoline components are

present over a wide concentration range and thus, for accurate quantitative

results, the linear dynamic range of the detector must also be large. These latterdemands mandate that the detector must be the FID.

Food and Beverage Products

Due to the likely contamination of food and beverage products with pesticides,herbicides and many other materials that are considered a health risk, all such

products on sale today must be carefully assayed. There is extensive legislation

controlling the quality of all human foods and drinks, and offensives carry very

serious penalties. In addition, the condition of the food is also of great concernto the food chemist, who will look for those trace materials that have been

established to indicate the onset of bacterial action, aging, rancidity ordecomposition. In addition, tests that identify the area or country in which thefood was processed or grown may also be needed. The source of many plants

(herbs and spices) can often be identified from the peak pattern of the

chromatograms obtained directly from headspace analysis. Similarly, unique

qualitative and quantitative patterns from a GC analysis will often help identifythe source of many alcoholic beverages.

Unfortunately, food analysis involves the separation and identification of very

complex mixtures and the difficulties are compounded by the fact that thecomponents are present at very low concentrations. Thus, gas chromatography

is the ideal (if not only) technique that can be used successfully in food and

beverage assays and tests.The potential carcinogenity of the aromatic hydrocarbons make their separationand analysis extremely important in environmental testing. However, thearomatics can pose some serious separation problems (for example, the m-

andp-xylenes) due to the closely similar chemical structure and characteristics.

The xylene isomers differ in structure (although not optically active) havesimilar spatial differences to pairs of enantiomers. It follows, chiral stationary

phases that separate enantiomers can also be used for separating spatial isomers

that are not necessarily optically active. Nevertheless, the separation ratios ofsuch isomeric pairs (even on cyclodextrin stationary phases) is still very small,

often in the 1.021.03 range. As a consequence, the use of high efficiency


44/75

capillary columns is essential, if reasonable analysis times are also to be

maintained.

Introduction

Liquid chromatography (LC) was the first type of chromatography to be discovered and, in the

form of liquid-solid chromatography (LSC), was originally used in the late 1890s by the Russian

botanist, Tswett (1), to separate and isolate various plant pigments. The colored bands he produced

on the adsorbent bed evoked the term chromatography (color writing) for this type of separation.

Initially the work of Tswett was not generally accepted, partly due to the original paper being in

Russian and thus, at that time, was not readily available to the majority of western chemists and

partly due to the condemnation of the method by Willstatter and Stoll (2) in 1913. Willstatter and

Stoll repeated Tswett's experiments without heeding his warning not to use too "aggressive

adsorbents as these would cause the chlorophylls to decompose. As a consequence, the

experiments of Willstatter et al. failed and their published results, rejecting the work of Tswett,

impeded the recognition of chromatography as a useful separation technique for nearly 20 years. In

the late 1930s and early 1940s Martin and Synge introduced a form of liquid-liquid

chromatography by supporting the stationary phase, in this case water, on silica gel in the form of a

packed bed and used it to separate some acetyl amino acids. They published their work in 1941 (3)and in their paper recommended the replacement of the liquid mobile phase with a suitable gas,

which would accelerate the transfer between the two phases and provide more efficient

separations. Thus, the concept of gas chromatography was born. In the same paper in 1941, Martin

and Synge suggested the use of small particles and high pressures in LC to improve the separation

that proved to the critical factors that initiated the development of high performance liquid

chromatography.


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The statement made by Martin in 1941 contains all the necessary conditions to realize

both the high efficiencies and the high resolution achieved by modern LC columns.

Despite his recommendations, however, it took nearly fifty years to bring his concepts to

fruition. Activity in the field of liquid chromatography was eclipsed in the 1950s by the

introduction of gas chromatography and serious attempts were not made to improve LC

techniques until the development of GC neared completion in the mid 1960s. The major

impediment to the development of LC was the lack of a high sensitive detector and it was

not until the refractive index detector was developed by A. Tiselius and D. Claesson (4)in 1942 could the technique be effectively developed. Tswett's original LC consisted of a

vertical glass tube, a few centimetres in diameter and about 30 cm high, packed with the

adsorbent (calcium carbonate). The plant extract pigments were placed on the top of the

packing and the mobile phase carefully added to fill the tube. The solvent percolated

through the packing under gravity, developing the separation, which could be seen as

different colored bands at the wall of the tube. The simple apparatus of Tswett contained

all the essentials to achieve a chromatographic separation. The contemporary

chromatograph, however, is a very complex instrument operating at pressures up to

10,000 p.s.i providing flow rates ranging from a few microliters per minute to 10 or 20

ml/minute depending on the type of LC that is carried out. Modern detectors can detect

solutes at concentration levels of 1x10-9 g/ml and an analysis can be completed in a few

minutes with just a few micrograms of sample.

CO Monitoring:

Co monitoring consisting of two methods:1. Chromatographic technique

2. Infrared method

Chromatographic Technique:--It's range is 0-200ppm

--Sensitivity 0.1ppm

Diagram:


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Theory:

--Any series of copound which has high or regionalblevapour pressure can be supperated

by gas chromatograph.

--This is useful for supperation of Hydrogen,Carbon,Nytrogen and other organiccompounds.

--In this sample and air contains CO is passed through stripped column or pre-column

then heavy hydro carbon are retainetheir,other than CH4,CO.

--These are passed into the chromatographic column, and then toacatelic chamber andhere the CO is reduced to methene(CH4).Which is detected by flame ionization detector

from which peaks are obtain in which the 1st peak corresponds to CH4 and 2ndcorresponds to CO.

Advantages:

--Its an accurate.--Highly Sensitive

--It can also measure CH4(Methene).

Disadvantages:--It is expensive and complex.

Infrared Method:--This is uasally range of 0-25,50,100 ppm levels.

Diagram:


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Advantages:

--Highly accurate and stable.

Disadvantages:

--It is Sensitive.

NOx Analyzers:If detection of nitric oxide is important to your research, have you thought of

detecting nitrite, which can bring you more insight? The detection of nitric oxide (NO) in

biological liquid samples is extremely difficult due to the transient half-life of NO

molecules. Some report the biological lifetime to be in the order of milliseconds. Onceyou remove the sample from the animal or other source, it is almost impossible to directly

detect nitric oxide. Thus, detection of nitrite and nitrate, considered to be the major

metabolites of NO, can be used to determine the NO levels. Moreover, if you evaluate the

nitric oxide level with an in-vivo sensing procedure, it shows only one aspect of the nitricoxide profile. Monitoring nitrite level has become increasingly important in recent years.

Studies have shown nitrite to be indirectly related to physiological activity and it is also a

signaling molecule.The ENO-20s high sensitivity and specificity are accomplished with the

combination of a dizao coupling method and chromatography. Nitrite and nitrate are

separated from other substances on a unique separation column and mobile phase. Nitrite

then reacts with a compound called Griess reagent and generates diazo compounds whichhave a red color. The level of nitrite can be monitored with peak height or area with a

retention time of 4.5 min from the injection of the sample. Nitrate is reduced to nitrite on

a reduction column which reacts to the Griess reagent as well. The nitrate peak has an 8


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min retention time. The level of diazo compound is measured by absorbance at 540 nm

using a visible detector. The separation column is robust as well as the entire ENO-20.Normal lifetime of the column is at least 3 months with regular use. All separation and

detection technologies are provided by Eicom for the ENO-20 including mobile phase

ingredients, Greiss reagent, separation columns, and reduction columns. You can relax

and detect nitrite and nitrate with 10 nM sensitivity (0.1 pmol)

Nox analyzers are classified into three types:They are

1. Laser opto acoustic spectroscopy2. Chemiluminescence method

1. Laser opto acoustic spectroscopy:Radiation source can be output from a laser, a monochromator furnishing

radiations in UV, IR, or a FT-IR spectrometer. All radiation must be pulsed at an

acoustical frequency 50-1200Hz. PA cell is filled with transparent gas often air or helium

and cell volume is kept small, less than 1cm3

in order to preserve the strength of the

acoustical signal.

Fig:1

One main advantage of opto acoustic spectroscopy is the ability to get informationabout the depth in the sample of the absorption. The amount of the sample contributing to

the PA signal is proportional to the thermal diffusion depth. This thermal diffusion depth, is inversely proportional to the modulation frequency f. Figure 3 shows a model

sample that has a thermally thin surface layer (thickness


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relative to the light absorption and non- radiative decay, an absorption in the bulk will

have a phase lag between the time of absorption and the thermal signal. However, asurface absorption should not have a phase lag since the heat doesn't have far to travel to

generate the detected pressure change in the transfer gas.

Fig 2

Advantages:

The sample does not have to be dissolved in some solvent or embedded in a solid

matrix, it is to be used as its.

Conventional absorption spectroscopy is based on excitation by electromagnetic

radiation with intensity I and the measurement of reflected or transmitted light

intensity I. Thus, the absorbencies derived indirectly from transmittance or

reflectance, whereas in PAS pressure waves are detected which are generateddirectly by the absorbed energy.

PA signal is not influenced by the scattering particles.

PAS allows the determination of absorption coefficients over several orders ofmagnitude. This analytical technique can be applied to the measurement of weak

absorption using PA cells with relatively small path lengths, allowing compact

and mobile set-ups

PA signal depends on the incident radiation power hence the sensitivity can betuned to desired range by choosing an appropriate radiation source (for example, a

lamp versus a laser).

PAS is useful for sample that are powered, amorphous or otherwise notconductive to reflective or transmission form of optical spectroscopies.

2.Chemiluminescence method and Catalytic thermal decomposition:

Sample is reacted with oxygen using oxidation catalyst to transform the nitrogen

compounds into nitric monoxide (nitrogen and nitrogen dioxide do not transform to nitricmonoxide). When nitric monoxide is reacted with ozone, nitrogen dioxide in semi-stable

status is produced. When the nitrogen dioxide in semi-stable status changes into stable

nitrogen dioxide, it emits lights and the strength of the light is in proportional to theconcentration of nitric monoxide. Thus the total nitrogen concentration in the sample is

identified by measuring the strength of the light. An example of total nitrogen analyzer

using catalytic thermal decomposition and chemiluminescences method is illustratedbelow. Sample is weighed and then put into combustion tube, where the sample is heated

to a high temperature and oxidized with oxidation catalyst into nitric monoxide. After


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oxidation, the sample is dehumidified with a dryer and fed into the detector, where the

sample is reacted with ozone as below.

NO + O3 --> NO2 + O2+ h

The strength of light of wavelength of 590 to 2500nm is measured.

Fig 3

(a).120C decomposition and UV absorptiometery:

Add alkali solution of potassium peroxodisulfate to the sample and heat themixture to approx. 120C to transform all the nitrogen compounds to nitric acid ions.

Control the pH of this solution to 2 to 3 using hydrochloric acid, and measure theabsorption of nitric acid ions in UV zone to obtain the total nitrogen concentration. An

example of total nitrogen analyzer using 120C decomposition and UV absorptiometer isillustrated below. Sample is weighed and put into the sample water container. Specifiedamount of potassium peroxodisulfate and sodium hydroxide solutions are added to the

sample, and the mixture is put into the thermal decomposition chamber. In the chamber,

the mixture is heated at 120C for 30 minutes to oxidize the nitrogen compounds intonitric acid ions. The solution after the decomposition is put into reaction-cooling chamber

to cool off. The cooled solution is then adjusted to pH 2-3 by adding hydrochloric acid.

The absorbency of nitric acid ions in the solution is measured using UV absorptiometer ata wavelength of 220nm.


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Example of total nitrogen analyzer of 120C decomposition- UV

absorptiometery

Fig.4

(b) Photo oxidation decomposition and UV absorptiometery:

Add alkali solution of potassium peroxodisulfate to the sample and radiateultraviolet ray to transform all the nitrogen compounds to nitric acid ions. Control the pH

of this solution to 2 to 3 using hydrochloric acid, and measure the absorption of nitric

acid ions in UV zone to obtain the total nitrogen concentration. An example of total

nitrogen analyzer using photooxidation and UV absorptiometer is illustrated below.Sample is weighed and put into the mixing chamber. Specified amount of potassium

peroxodisulfate and sodium hydroxide solutions are added to the sample, and the mixture

is put into the UV oxidation apparatus. In the chamber, the mixture is heated at 90C andexposed to ultraviolet ray for 15 minutes to oxidize the nitrogen compounds into nitric


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acid ions. The solution after the decomposition is weighed and then adjusted to pH 2-3 by

adding hydrochloric acid. The absorbency of nitric acid ions in the solution is measuredusing UV absorptiometer at a wavelength of 220nm.

Example of total nitrogen analyzer of photooxidation decomposition- UVabsorptiometery:

Fig.5

Advantage of Chemiluminescences method:

Nitric oxide and ozone react in the gaseous phase to produce chemicalluminescence. It is possible to detect nitrite in the liquid phase using this procedure but itis not as simple as using the ENO-20. What types of samples require the nitrite assay?

Usually the sample matrix is liquid; the ENO-20 is the perfect device for this. In the

process of reducing nitrite/nitrate, the system is heated which results in condensation on

the inner surface of the glassware after cooling down. The condensation dissolves thenitrite/nitrate and will have an effect on carry-over when the condensation

evaporates during the next process. This is a common reason for variation in the detected

values of nitrite using the chemiluminescense procedure. As explained above, the ENO-20 installed with an autosampler yields hands free analysis. You do not need to clean the

glassware of the detection system following the analysis of several samples or following

the sample conversion from liquid to gas.


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H2S Analyzer Sample System

THE NEED

A critical measurement of natural gas quality is theconcentration of hydrogen sulfide(H2S). Remarkably,thecommon technique for making this measurement, leadacetate tape,

is at least 70 years old. Users of this archaictechnology report general dissatisfaction with

it due to high maintenances, the cost and shelf life of tape cassettes, the need for reagents,difficulty handling H2S overload conditions, the sensitivity of the technology to

ambientextremes, and, not least, used cassette disposal.

AMETEK Western Research ha

analytical instrumentation 1

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