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Atomic Emission Spectroscopy

Lecture 21

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Qualitative analysis is accomplished by comparison of the wavelengths of some emission lines to standards while the line blackness serves as the tool for semiquantitative analysis.

Polychromators are also available as multichannel arc and spark instruments. However, these have fixed slits at certain wavelengths in order to do certain elements and thus they are not versatile.

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Grating

DetectorsPotential Source

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Recently, arc and spark instruments based on charge injection and charge coupled devices became available. These have extraordinarily high efficiency and performance in terms of easier calibration, short analysis time, as well as superior quantitative results.

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Grating

CCD or CID DetectorPotential Source

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Characteristics of Arc Sources1. Typical temperatures between 4000-5000 oC are high

enough to cause atomization and excitation of sample and electrode materials.

2. Usually, cyanogens compounds are formed due to reaction of graphite electrodes with atmospheric nitrogen. Emission bands from cyanogens compounds occur in the region from 350-420 nm. Unfortunately, several elements have their most sensitive lines in this same region which limits the technique. However, use of controlled atmosphere around the arc (using CO2, Helium, or argon) very much decreases the effect of cyanogens emission.

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3. The emission signal should be integrated over a minute or so since volatilization and excitation of atoms of different species differ widely. While some species give maximum signal, others may still be in the molecular state.

4. Arc sources are very good for qualitative analysis of elements while only semiquantitative analysis is possible. It is mandatory to compare the emission spectrum of a sample with the emission spectrum of a standard. In some cases, a few milligrams of a standard is added to the sample in order to locate the emission lines of the standard and thus identify the emission wavelengths of the different elements in the sample. A comparator densitometer can be used to exactly locate the wavelengths of the standard and the sample components.

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Standard

Sample

The lines from the standard are projected on the lines of the combined sample/standard emission spectra in order to identify sample components. Only few lines are shown in the figure.

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Why use Carbon in Atomic Spectroscopy?

We have previously seen the use of graphite in electrothermal AAS as well as arc and spark AES, even though molecular spectra are real problems in both techniques due to cyanogens compounds absorption and emission. The reasons after graphite common use in atomic spectroscopy can be summarized below:

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1. It is conductive.2. It can be obtained in a very pure state.3. Easily available and cheap.4. Thermally stable and inert.5. Carbon has few emission lines.6. Easily shaped.

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Spark SourcesMost of the instruments in this category are arc

based instruments. Spark based instruments are of the same idea except for a spark source substituting an arc source. The spark source is constructed as in the figure below where an AC potential in the order of 10-50 KV is discharged through a capacitor which is charged and discharged through the graphite electrodes about 120 times/s; resulting in a discharge current of about 1000 A.

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This very high current will suffer a great deal of resistance, which increase the temperature to an estimated 40000 oC. Therefore, ionic spectra are more pronounced.

Transformer

Potential Source

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An introduction to Ultraviolet/Visible Absorption Spectroscopy

Chapter 13

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In this chapter, absorption by molecules, rather than atoms, is considered. Absorption in the ultraviolet and visible regions occurs due to electronic transitions from the ground state to excited state. Broad band spectra are obtained since molecules have vibrational and rotational energy levels associated with electronic energy levels. The signal is either absorbance or percent transmittance of the analyte solution where:

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Absorption measurements based upon ultraviolet and visible radiation find widespread application for the quantitative determination of a large variety species.

Beer’s Law:A = -logT = logP0/P = bc

A = absorbance = molar absorptivity [M-1 cm-1]c = concentration [M]P0 = incident powerP = transmitted power (after passing through sample)

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UV-Vis Absorption Spectroscopy

Lecture 22

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Measurement of Transmittance and Absorbance:The power of the beam transmitted by the analyte solution is usually compared with the power of the beam transmitted by an identical cell containing only solvent. An experimental transmittance and absorbance are then obtained with the equations.

P0 and P refers to the power of radiation after it has passed through the solvent and the analyte.

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Beer’s law and mixturesBeer’s law and mixtures• Each analyte present in the solution absorbs Each analyte present in the solution absorbs

light! light! • The magnitude of the absorption depends on The magnitude of the absorption depends on

its its • A A total total = A= A11+A+A22+…+A+…+An n

• A A total total = = 11bcbc11++22bcbc22+…++…+nnbcbcnn

• If If 1 1 = = 2 2 == n n then simultaneous determination is then simultaneous determination is impossibleimpossible

• Need to measure A at nNeed to measure A at n’s (get n’s (get n22 ’s’s) to solve ) to solve for the concentration of species in the mixturefor the concentration of species in the mixture

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Limitations to Beer’s Law

• Real limitations• Chemical deviations• Instrumental deviations

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1.      Real Limitations a.       Beer’s law is good for dilute analyte

solutions only. High concentrations (>0.01M) will cause a negative error since as the distance between molecules become smaller the charge distribution will be affected which alter the molecules ability to absorb a specific wavelength. The same phenomenon is also observed for solutions with high electrolyte concentration, even at low analyte concentration. The molar absorptivity is altered due to electrostatic interactions.

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b.      In the derivation of Beer’s law we have introduced a constant (). However, is dependent on the refractive index and the refractive index is a function of concentration. Therefore, will be concentration dependent. However, the refractive index changes very slightly for dilute solutions and thus we can practically assume that is constant.

c.       In rare cases, the molar absorptivity changes widely with concentration, even at dilute solutions. Therefore, Beer’s law is never a linear relation for such compounds, like methylene blue.

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2.      Chemical Deviations This factor is an important one which

largely affects linearity in Beer’s law. It originates when an analyte dissociates, associates, or reacts in the solvent, or one of matrix constituents. For example, an acid base indicator when dissolved in water will partially dissociate according to its acid dissociation constant:

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HIn H+ + In-

It can be easily appreciated that the amount of HIn present in solution is less than that originally dissolved where:

CHIn = [HIn] + [In-]

Assume an analytical concentration of 2x10-5 M indicator (ka = 1.42x10-5) was used, we may write:

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1.42x10-5 = x2/(2x10-5 – x)Solving the quadratic equation gives:X = 1.12x10-5 M which means:[In-] = 1.12x10-5 M[HIn] = 2x10-5 – 1.12x10-5 = 0.88x10-5 MTherefore, the absorbance measured will be the sum

of that for HIn and In-. If a 1.00 cm cell was used and the for both HIn and In- were 7.12x103 and 9.61x102 Lmol-1cm-1 at 570 nm, respectively, the absorbance of the solution can be calculated:

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A = AHIn + AIn

A = 7.12x103 * 1.00* 0.88x10-5 + 9.61x102 * 1.00 *1.12x10-5 = 0.073

However, if no dissociation takes place we may have:A = AHIn

A = 7.12x103 * 1.00 * 2x10-5 = 0.142 If the two results are compared we can calculate the

% decrease in anticipated signal as:% decrease in signal = {(0.142 – 0.073)/0.142}x100%

= 49%

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However, at 430 nm, the molar absorptivities of HIn and In- are 6.30*102 and 2.06*104, respectively.

A = AHIn + AIn

A = 6.30*102 * 1.00* 0.88x10-5 + 2.06*104 * 1.00 *1.12x10-5 = 0.236

Again, if no dissociation takes place we may have:A = AHIn

A = 6.30*102 * 1.00 * 2x10-5 = 0.013 If the two results are compared we can calculate the %

increase in anticipated signal as:% decrease in signal = {(0.236 – 0.013)/0.013}x100% = V.

large

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Comparison between results obtained at 570 nm and 430 nm show large dependence on the values of the molar absorptivities of HIn and In- at these wavelength.

At 570 nm:A = AHIn + AIn

A = 7.12x103 * 1.00* 0.88x10-5 + 9.61x102 * 1.00 *1.12x10-5 = 0.073

And at 430 nm:A = 6.30*102 * 1.00* 0.88x10-5 + 2.06*104 * 1.00

*1.12x10-5 = 0.236

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Chemical deviations from Beer’s law for unbuffered solutions of the indicator Hln.

Note that there are positive deviations at 430 nm and negative deviations at 570 nm. At 430 nm, the absorbance is primarily due to the ionized In- form of the indicator and is proportional to the fraction ionized, which varies nonlinearly with the total indicator concentration. At 570 nm, the absorbance is due principally to the undissociated acid Hln, which increases nonlinearly with the total concentration.

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Calculated Absorbance Data for Various Indicator Concentrations

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An example of association equilibria Association of chromate in acidic solution to form

the dichromate according to the equation below: 

2 CrO42- + 2 H+ Cr2O7

2- + H2O 

The absorbance of the chromate ions will change according to the mentioned equilibrium and will thus be nonlinearly related to concentration.

A = CrO4 *b*CCrO4 + Cr2O7 *b*CCr2O7

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3.      Instrumental Deviations a.       Beer’s law is good for monochromatic

light only since is wavelength dependent. It is enough to assume a dichromatic beam passing through a sample to appreciate the need for a monochromatic light. Assume that the radiant power of incident radiation is Po and Po’ while transmitted power is P and P’. The absorbance of solution can be written as:

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A = log (Po + Po’)/(P + P’)

P = Po10-bc, substituting in the above equation:

A = log (Po + Po’)/(Po10-bc Po’10-’bc)

Assume = ’ = A = log (Po + Po’)/(Po + Po’) 10-bc

A = bc

However, since ’ # , since is wavelength dependent, then A # bc

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The effect of polychromatic radiation on Beer’s law.

In the spectrum at the top, the molar absorptivity of the analyte is nearly constant over band A. Note that in Beer’s law plot at the bottom, using band A gives a linear relationship. In the spectrum, band B corresponds to a region where the absorptivity shows substantial changes. In the lower plot, note the dramatic deviation from Beer’s law that results.

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Therefore, the linearity between absorbance and concentration breaks down if incident radiation was polychromatic. In most cases with UV-Vis spectroscopy, the effect is small especially at the wavelength maximum. The small changes in signal is insignificant since differs only slightly.

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UV-Vis Absorption Spectroscopy

Lecture 23

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b.      Stray Radiation Stray radiation resulting from scattering or

various reflections in the instrument will reach the detector without passing through the sample. The problem can be severe in cases of high absorbance or when the wavelengths of stray radiation is in such a range where the detector is highly sensitive as well as at wavelengths extremes of an instrument. The absorbance recorded can be represented by the relation:

A = log (Po + Ps)/(P + Ps)Where; Ps is the radiant power of stray radiation.

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Instrumental Noise as a Function in Transmittance

The uncertainty in concentration as a function of the uncertainty in transmittance can be statistically represented as:

 sc

2 = (dc/dT)2 sT2

 A = -log T = bc = -0.434 ln Tc = -(1/b)*0.434 ln T (1)dc/dt = - 0.434/bT sc

2 = (-0.434/bT)2 sT2 (2)

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Dividing equation 2 by the square of equation 1 (sc

/c)2 = (-0.434/bT)2 sT2/{(-0.434 ln T)2/(b)2}

sc/c = (sT / T ln T)

 Therefore, it is clear that the uncertainty in

concentration of a sample is nonlinearly related to the magnitude of the transmittance. Substitution for different values of transmittance and assuming sT is constant, we get:

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Therefore, an absorbance between 0.2-0.7 may be advantageous in terms of a lower uncertainty in concentration measurements. At higher or lower absorbances, an increase in uncertainty is encountered. It is therefore advised that the test solution be in the concentration range which gives an absorbance value in the range from 0.2-0.7 for best precision.

However, it should also be remembered that we ended up with this conclusion provided that sT is constant. Unfortunately, sT is not always constant which complicates the conclusions above.

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Effect of bandwidth on spectral detail for a sample of benzene vapor. Note that as the spectral bandwidth increases, the fine structure in the spectrum is lost. At a bandwidth of 10 nm, only a broad absorption band is observed.

EFFECT OF bandwidth

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Effect of slit width (spectral bandwidth) on peak heights. Here, the sample was s solution of praseodymium chloride. Note that as the spectral bandwidth decreases by decreasing the slit width from 1.0 mm to 0.1 mm, the peak heights increase.

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Effect of Scattered Radiation at Wavelength Extremes of an Instrument

Wavelength extremes of an instrument are dependent on type of source, detector and optical components used in the manufacture of the instrument. Outside the working range of the instrument, it is not possible to use it for accurate determinations. However, the extremes of the instrument are very close to the region of invalid instrumental performance and would thus be not very accurate. An example may be a visible photometer which, in principle, can be used in the range from 340-780 nm. It may be obvious that glass windows, cells and prism will start to absorb significantly below 380 nm and thus a decrease in the incident radiant power is significant.

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What defines the instrumental wavelength extremes?

Three main Factors:1. Source2. Detector3. Optical components (lenses, windows, etc)Measurements at wavelength extremes should

be avoided since errors are very possible due to:

1. Source limitations2. Detector limitations3. Sample cell limitations4. Scattered radiation

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Spectrum of cerium (IV) obtained with a spectrophotometer having glass optics (A) and quartz optics (B). The false peak in A arises from transmission of stray radiation of longer wavelengths.

B: UV-VIS spectrophotometerA: VIS spectrophotometer EFFECT OF SCATTERED

RADIATION

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The output from the source at the low wavelength range is minimal. Also, the detector has best sensitivities around 550 nm which means that away up and down this value, the sensitivity significantly decrease. However, scattered radiation, and stray radiation in general, will reach the detector without passing through these surfaces as well as these radiation are constituted from wavelengths for which the detector is highly sensitive. In some cases, stray and scattered radiation reaching the detector can be far more intense than the monochromatic beam from the source. False peaks may appear in such cases and one should be aware of this cause of such peaks.

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Instrumentation• Light source - selector• Sample container • Detector• Signal processing• Light Sources (commercial

instruments)– D2 lamp (UV: 160 – 375 nm)– W lamp (vis: 350 – 2500 nm)

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SourcesDeuterium and hydrogen lamps (160 – 375 nm)

DD22 + E + Eee → D → D22* → D’ + D’’ + * → D’ + D’’ + hh

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(a) A deuterium lamp of the type used in spectrophotometers and (b) its spectrum. The plot is of irradiance Eλ (proportional to radiant power) versus wavelength. Note that the maximum intensity occurs at ~225 m.Typically, instruments switch from deuterium to tungsten at ~350 nm.

Deuterium lampUV region

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(a) A tungsten lamp of the type used in spectroscopy and its spectrum

(b). Intensity of the

tungsten source is usually quite low at wavelengths shorter than about 350 nm. Note that the intensity reaches a maximum in the near-IR region of the spectrum.

Visible and near-IR region

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The tungsten lamp is by far the most common source in the visible and near IR region with a continuum output wavelength in the range from 350-2500 nm. The lamp is formed from a tungsten filament heated to about 3000 oC housed in a glass envelope. The output of the lamp approaches a black body radiation where it is observed that the energy of a tungsten lamp varies as the fourth power of the operating voltage.

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Tungsten halogen lamps are currently more popular than just tungsten lamps since they have longer lifetime. Tungsten halogen lamps contain small quantities of iodine in a quartz envelope. The quartz envelope is necessary due to the higher temperature of the tungsten halogen lamps (3500 oC). The longer lifetime of tungsten halogen lamps stems from the fact that sublimed tungsten forms volatile WI2 which redeposits on the filament thus increasing its lifetime. The output of tungsten halogen lamps are more efficient and extend well into the UV.

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Tungsten lamps (350-2500 nm)

Why add IWhy add I22 in the lamps? in the lamps?W + IW + I22 → WI → WI22

Low limit: 330 nm

1)Low intensity2)Glass or quartz envelope

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3. Xenon Arc Lamps Passage of current through an atmosphere of

high pressured xenon excites xenon and produces a continuum in the range from 200-1000 nm with maximum output at about 500 nm. Although the output of the xenon arc lamp covers the whole UV and visible regions, it is seldom used as a conventional source in the UV-Vis. The radiant power of the lamp is very high as to preclude the use of the lamp in UV-Vis instruments. However, an important application of this source will be discussed in luminescence spectroscopy which will be discussed later.

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UV-Vis Absorption Spectroscopy

Lecture 24

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Instrumental Components• Source - selector (monochromators)• Sample holders • Cuvettes (b = 1 cm, typically)

1. Glass (Vis)2. Fused silica (UV+Vis)

• Detectors– Photodiodes– PMTs

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Sample ContainersSample containers are called cells or cuvettes and are

made of either glass or quartz depending on the region of the electromagnetic spectrum. The path length of the cell varies between 0.1 and 10 cm but the most common path length is 1.0 cm. Rectangular cells or cylindrical cells are routinely used. In addition, disposable polypropylene cells are used in the visible region. The quality of the absorbance signal is dependent on the quality of the cells used in terms of matching, cleaning as well as freedom from scratches.

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Instrumental designs for UV-visible photometers or spectrophotometers. In (a), a single-beam instrument

is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector.

Types of Instruments

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1. Single beam–Place cuvette with blank in place

in the instrument and take a reading 100% T

–Replace blank with sample and take reading % T for analyte (from which absorbance is calc’d)

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Most common spectrophotometer Spectronic 20

1. On/Off switch and zero transmission adjustment knob

2. Wavelength selector/Readout

3. Sample chamber

4. Blank adjustment knob

5. Absorbance/Transmittance scale

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End view of the exit slit of the Spectronic 20 spectrophotometer pictured earlier

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• Single-Beam Instruments for the Ultraviolet/Visible Region

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• Single-Beam Computerized Spectrophotometers Inside of a

single-beam spectrophotometer connected to a computer.

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2. Double beam (most commercial instruments)– Light is split and directed towards both reference

cell (blank) and sample cell– One or two detectors; electronics measure ratio

(i.e., measure/calculate absorbance)– Advantages:

• Compensates for fluctuations in source intensity and drift in detector

• Better design for continuous recording of spectra

• Faster

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General Instrument DesignsDouble Beam: In - Space

Needs two detectors

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General Instrument DesignsDouble Beam: In - Time

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Merits of Double Beam Instruments

1.Compensate for all but the most short term fluctuation in radiant output of the source

2.Compensate drift in transducer and amplifier

3.Compensate for wide variations in source intensity with wavelength

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3 .Dual Beam Instruments

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4. Multichannel Instruments• Photodiode array detectors used

(multichannel detector, can measure all wavelengths dispersed by grating simultaneously).

• Advantage: scan spectrum very quickly “snapshot” < 1 sec.

• Powerful tool for studies of transient intermediates in moderately fast reactions.

• Useful for kinetic studies.• Useful for qualitative and quantitative

determination of the components exiting from a liquid chromatographic column.

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A multichannel diode-array spectrophotometer

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Location of Sample cellIn all photometers and scanning spectrophotpmeters

described above, the cell has been positioned after the monochromators. This is important to decrease the possibility of sample photodecomposition due to prolonged exposure to all frequencies coming from the source. However, the sample is positioned before the monochromator in multichannel instruments like a photodiode array spectrophotometer. This can be done without fear of photodecomposition since the sample exposure time is usually less than 1 s. Therefore, it is now clear that in UV-Vis where photodecomposition of samples can take place, the sample is placed after the monochromators in scanning instruments while positioning of the sample before the monochromators is advised in multichannel instruments.

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5 .Probe Type Instruments

These are the same as conventional single beam instruments but the beam from the monochromators is guided through a bifurcated optical fiber to the sample container where absorption takes place. The attenuation in reflected beam at the specified wavelength is thus measured and related to concentration of analyte in the sample.

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A fiber optic cable can be referred to as a light pipe where light can be transmitted by the fiber without loss in intensity (when light hits the internal surface of the fiber at an angle larger than a critical angle). Therefore, fiber optics can be used to transmit light for very long distances without losses. A group of fibers can be combined together to form a fiber optic cable or bundle. A bifurcated fiber optic cable has three terminals where fibers from two separate cables are combined at one end to form the new configuration.

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Fiber optic probeinstrument

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6 .Double Dispersing Instruments

The instrument in this case has two gratings where the light beam leaving the first monochromators at a specified wavelength is directed to the second grating. This procedure results in better spectral resolution as well as decreased scattered radiation. However, double dispersing instruments are expensive and seem to offer limited advantages as compared to cost; especially in the UV-Vis region where exact wavelength may not be crucial.

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Optical diagram of the Varian Cary 300 double-dispersing spectrophotometer. A second monochromator is added immediately after the source.