Spektrofotometer inframerah pada umumnya digunakan untuk : Menentukan gugus fungsi suatu senyawa...

Spektrofotometer inframerah pada umumnya digunakan untuk :Menentukan gugus fungsi suatu senyawa organik Mengetahui informasi struktur suatu senyawa organik dengan membandingkan daerah sidik jarinya

SPEKTROFOTOMETER INFRAMERAH

Pengukuran pada spektrum inframerah dilakukan pada daerah cahaya inframerah tengah (mid-infrared) yaitu pada panjang gelombang 2.5 - 50 µm atau bilangan gelombang 4000 - 200 cm-1. Energi yang dihasilkan oleh radiasi ini akan menyebabkan vibrasi atau getaran pada molekul. Pita absorbsi inframerah sangat khas dan spesifik untuk setiap tipe ikatan kimia atau gugus fungsi. Metoda ini sangat berguna untuk mengidentifikasi senyawa organik dan organometalik.

SpektroskopiSpektroskopiSPEKTROFOTOMETER INFRAMERAH

Spektrum yang dihasilkan berupa grafik yang menunjukkan persentase transmitan yang bervariasi pada setiap frekuensi radiasi inframerah.

SpektroskopiSpektroskopi

A Simple AnalogyA Simple AnalogyIf you have a weight on a spring, you can set it into If you have a weight on a spring, you can set it into

motion by tapping it at its natural resonance motion by tapping it at its natural resonance frequency. The best way to input energy to it is to frequency. The best way to input energy to it is to tap it 90 degrees out of phase. We can diagram the tap it 90 degrees out of phase. We can diagram the experiment as follows: experiment as follows:

Interaksi dengan cahayaInteraksi dengan cahaya

When atoms or molecules absorb light, the incoming When atoms or molecules absorb light, the incoming energy excites a quantized structure to a higher energy excites a quantized structure to a higher energy level. The type of excitation depends on the energy level. The type of excitation depends on the wavelengthwavelength of the light. of the light.

Electrons are Electrons are promoted to higher orbitalspromoted to higher orbitals by ultraviolet by ultraviolet or visible light, or visible light, vibrationsvibrations are excited by infrared are excited by infrared light, and rotations are excited by microwaves. light, and rotations are excited by microwaves.

AbsorptionAbsorption

An absorption spectrum is the absorption of An absorption spectrum is the absorption of light as a function of wavelength. The light as a function of wavelength. The spectrum of an atom or molecule spectrum of an atom or molecule depends on depends on its energy level structureits energy level structure, and absorption , and absorption spectra are useful for identifying of spectra are useful for identifying of compounds. compounds.

Measuring the concentration of an absorbing Measuring the concentration of an absorbing species in a sample is accomplished by species in a sample is accomplished by applying the applying the Beer-Lambert LawBeer-Lambert Law. .

AbsorptionAbsorption

Atoms or molecules that are excited to high Atoms or molecules that are excited to high energy levels can decay to lower levels by energy levels can decay to lower levels by emitting radiation (emission or luminescence). emitting radiation (emission or luminescence).

For atoms excited by a high-temperature energy For atoms excited by a high-temperature energy source this light emission is commonly called source this light emission is commonly called atomic or optical emission (see atomic or optical emission (see atomic-atomic-emission spectroscopyemission spectroscopy), ),

For atoms excited with light it is called atomic For atoms excited with light it is called atomic fluorescence (see fluorescence (see atomic-fluorescence atomic-fluorescence spectroscopyspectroscopy). ).

EmissionEmission

For molecules it is called For molecules it is called fluorescencefluorescence if the if the transition is between states of the same spin transition is between states of the same spin

and and phosphorescencephosphorescence if the transition occurs if the transition occurs between states of different spin. between states of different spin.

The emission intensity of an emitting substance is The emission intensity of an emitting substance is linearly proportional to analyte concentration linearly proportional to analyte concentration at low concentrations, and is useful for at low concentrations, and is useful for quantitating emitting speciesquantitating emitting species. .

EmissionEmission

When electromagnetic radiation passes through When electromagnetic radiation passes through matter, most of the radiation continues in its matter, most of the radiation continues in its original direction but a small fraction is original direction but a small fraction is scattered in other directionsscattered in other directions..

Light that is scattered at the same wavelength as Light that is scattered at the same wavelength as the incoming light is called the incoming light is called Rayleigh scattering. Rayleigh scattering.

ScatteringScattering

Light that is scattered in transparent solids due Light that is scattered in transparent solids due to vibrations (phonons) is called to vibrations (phonons) is called Brillouin Brillouin scatteringscattering. .

Brillouin scattering is typically shifted by 0.1 to 1 Brillouin scattering is typically shifted by 0.1 to 1 cmcm-1-1 from the incident light. Light that is from the incident light. Light that is scattered due to vibrations in molecules or scattered due to vibrations in molecules or optical phonons in solids is called Raman optical phonons in solids is called Raman scattering. Raman scattered light is shifted by scattering. Raman scattered light is shifted by as much as 4000 cmas much as 4000 cm-1-1 from the incident light. from the incident light.

ScatteringScattering

IntroductionIntroductionAtomic-absorption (AA) spectroscopyAtomic-absorption (AA) spectroscopy uses the uses the

absorption of light to measure the concentration of absorption of light to measure the concentration of gas-phase atoms. gas-phase atoms.

Since samples are usually liquids or solids, the Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame analyte atoms or ions must be vaporized in a flame or graphite furnace. or graphite furnace.

Atomic-Absorption Spectroscopy (AA)Atomic-Absorption Spectroscopy (AA)

The atoms absorb ultraviolet or visible light and The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy make transitions to higher electronic energy levels. levels. The analyte concentration is The analyte concentration is determined from the amount of absorptiondetermined from the amount of absorption. .

Applying the Beer-Lambert Law directly in AA Applying the Beer-Lambert Law directly in AA spectroscopy is difficult due to variations in spectroscopy is difficult due to variations in the atomization efficiency from the sample the atomization efficiency from the sample matrix, and nonuniformity of concentration matrix, and nonuniformity of concentration and path length of analyte atoms (in graphite and path length of analyte atoms (in graphite furnace AA). furnace AA).


Concentration measurements are Concentration measurements are usually determined from a usually determined from a working working curvecurve after calibrating the instrument after calibrating the instrument with standards of known with standards of known concentration. concentration.


Atomic-absorption spectroscopy (AAS)Atomic-absorption spectroscopy (AAS) and and atomic-emission spectroscopy (AES)atomic-emission spectroscopy (AES) both rely both rely on the analyte existing as free atoms in the gas on the analyte existing as free atoms in the gas phase. There are two common types of phase. There are two common types of interferences that reduce the concentration of interferences that reduce the concentration of free gas-phase atoms: ionization and the free gas-phase atoms: ionization and the formation of molecular species. formation of molecular species.


Note that the distribution of gas-phase atoms between the Note that the distribution of gas-phase atoms between the ground and excited states is a physical property that depends ground and excited states is a physical property that depends on the temperature of the environment. This distribution will on the temperature of the environment. This distribution will affect the analytical signal, but it is not a chemical affect the analytical signal, but it is not a chemical interference. interference.

Concentration measurements are usually determined from a Concentration measurements are usually determined from a working curveworking curve after calibrating the instrument with standards after calibrating the instrument with standards of known concentration. To prevent any bias due to of known concentration. To prevent any bias due to differences between the standards and the samples, any differences between the standards and the samples, any reagents that are added to reduce chemical interferences reagents that are added to reduce chemical interferences should be added to the standards as well as the sample should be added to the standards as well as the sample solution. solution.


Preventing IonizationPreventing IonizationSince samples are usually liquids or solids, the Since samples are usually liquids or solids, the

sample must be vaporized and atomized in a high-sample must be vaporized and atomized in a high-temperature source such as a flame, graphite temperature source such as a flame, graphite furnace, or plasma. This high-temperature furnace, or plasma. This high-temperature environment can also lead to ionization of the environment can also lead to ionization of the analyte atoms.analyte atoms.


Analyte ionization can be Analyte ionization can be suppressedsuppressed by adding a by adding a source of electrons, which shifts the equilibrium of source of electrons, which shifts the equilibrium of the analyte from the ionic to the atomic form: the analyte from the ionic to the atomic form:

Analyte <--> AnalyteAnalyte <--> Analyte++ + e + e--

Cesium and potassiumCesium and potassium are common ionization are common ionization suppressors that are added to analyte solutions. suppressors that are added to analyte solutions. These atoms are easily ionized and produce a high These atoms are easily ionized and produce a high concentration of free electrons in the flame or concentration of free electrons in the flame or plasma. plasma.


Preventing Refractory FormationPreventing Refractory FormationSome elements can form refractory compounds that Some elements can form refractory compounds that

are not atomized in flames or plasmas. An example are not atomized in flames or plasmas. An example is the presence of phosphates, which interferes is the presence of phosphates, which interferes with calcium measurements due to formation of with calcium measurements due to formation of refractory calcium phosphate: refractory calcium phosphate:

3 CaCl3 CaCl22 (aq) + 2 PO (aq) + 2 PO

443-3- (aq) --> Ca (aq) --> Ca

33(PO(PO44))22 (s) + 6 Cl (s) + 6 Cl-- (aq) (aq)


Formation of refractory compounds can be Formation of refractory compounds can be prevented or reduced by adding a prevented or reduced by adding a releasing releasing agentagent. For calcium measurements, adding . For calcium measurements, adding lanthanium to the sample (and standard) lanthanium to the sample (and standard) solutions binds the phosphate as LaPOsolutions binds the phosphate as LaPO

44. LaPO. LaPO44

has a very high formation constant Khas a very high formation constant Kff and and

effectively ties up the phosphate interferent. effectively ties up the phosphate interferent.


Light sourceLight sourceThe light source is usually a The light source is usually a hollow-cathode hollow-cathode

lamplamp of the element that is being measured. of the element that is being measured. LasersLasers are also used in research instruments. are also used in research instruments. Since lasers are intense enough to excite Since lasers are intense enough to excite atoms to higher energy levels, they allow AA atoms to higher energy levels, they allow AA and atomic fluorescence measurements in a and atomic fluorescence measurements in a single instrument. single instrument.

The disadvantage of these The disadvantage of these narrow-band lightnarrow-band light sources is that only one element is sources is that only one element is measurable at a time. measurable at a time.

Instrumentation of AASInstrumentation of AAS

Light separation and detection – Light separation and detection – AAS use AAS use monochromatorsmonochromators and and detectors detectors for uv for uv

and visible light. The main purpose of the and visible light. The main purpose of the monochromator is to monochromator is to isolateisolate the absorption the absorption line from background light due to line from background light due to interferences. interferences.

Simple dedicated AAS instruments often replace Simple dedicated AAS instruments often replace the monochromator with a bandpass the monochromator with a bandpass interference filter. Photomultiplier tubes are interference filter. Photomultiplier tubes are the most common detectors for AAS.the most common detectors for AAS.


AtomizerAtomizerAA spectroscopy requires that the analyte atoms be in AA spectroscopy requires that the analyte atoms be in

the the gas phasegas phase. Ions or atoms in a sample must . Ions or atoms in a sample must undergo desolvation and vaporization in a high-undergo desolvation and vaporization in a high-temperature source such as a temperature source such as a flame or graphite flame or graphite furnace. furnace.

Flame AA can only analyze solutions, while graphite Flame AA can only analyze solutions, while graphite furnace AA can furnace AA can acceptaccept solutions, slurries, or solid solutions, slurries, or solid samples. samples.

Sample solutions are usually aspirated with the gas Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small flow into a nebulizing/mixing chamber to form small droplets before entering the flame. droplets before entering the flame.


Excitation Excitation -- A flame provides a high-temperature source -- A flame provides a high-temperature source

for for desolvating desolvating and and vaporizingvaporizing a sample to a sample to obtain free atoms for spectroscopic analysis. obtain free atoms for spectroscopic analysis.

In atomic absorption spectroscopy In atomic absorption spectroscopy ground stateground state atoms are desired. atoms are desired.

For atomic emission spectroscopy the flame For atomic emission spectroscopy the flame must also excitemust also excite the atoms to higher energy the atoms to higher energy levels. levels.


The graphite furnace has The graphite furnace has several advantages over a several advantages over a flameflame. .

1.1. It is a much more efficient atomizer than a flame and It is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities it can directly accept very small absolute quantities of sample. of sample.

2. It also provides a reducing environment for easily 2. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms. organic matter, and vaporize the analyte atoms.


Temperatures of Some Common FlamesTemperatures of Some Common Flames


FuelFuel OxidantOxidant Temperature (K)Temperature (K)

HydrogenHydrogen AirAir 2000-21002000-2100

AcetyleneAcetylene AirAir 2100-24002100-2400

HydrogenHydrogen OxygenOxygen 2600-27002600-2700

AcetlyleneAcetlylene Nitrous OxideNitrous Oxide 2600-28002600-2800

A flame atomic-absorption spectrometerA flame atomic-absorption spectrometer a g a graphite-furnace atomic-raphite-furnace atomic- absorption spectrometerabsorption spectrometer


IntroductionIntroductionAtomic fluorescence is the optical emission from gas-Atomic fluorescence is the optical emission from gas-

phase atoms that have been excited to higher phase atoms that have been excited to higher energy levels by absorption of electromagnetic energy levels by absorption of electromagnetic radiation. radiation.

The main advantageThe main advantage of fluorescence detection of fluorescence detection compared to absorption measurements is the compared to absorption measurements is the greater sensitivity achievable because the greater sensitivity achievable because the fluorescence signal has a very low background. The fluorescence signal has a very low background. The resonant excitation provides selective excitation of resonant excitation provides selective excitation of the analyte to avoid interferences. the analyte to avoid interferences.

Atomic-Fluorescence Spectroscopy (AFS)Atomic-Fluorescence Spectroscopy (AFS)

AFSAFS is useful to study the electronic structure of is useful to study the electronic structure of atoms and to make quantitative measurements. atoms and to make quantitative measurements. Analytical applications include flames and Analytical applications include flames and plasmas diagnostics, and enhanced sensitivity in plasmas diagnostics, and enhanced sensitivity in atomic analysis. atomic analysis.

Atomic-Fluorescence Spectroscopy (AFS)Atomic-Fluorescence Spectroscopy (AFS)

Analysis of solutions or solids requires that the analyte atoms be desolvated, vaporized, and atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace.

A hollow-cathode lamp or laser provides the resonant excitation to promote the atoms to higher energy levels. The atomic fluorescence is dispersed and detected by monochromators and photomultiplier tubes, similar to atomic-emission spectroscopy instrumentation.

Instrumentation of AFSInstrumentation of AFS

IntroductionLight emission from atoms or molecules can be used to

quantitate the amount of the emitting substance in a sample. The relationship between fluorescence intensity and analyte concentration is:

F = k * QE * Po * (1-10[-epsilon*b*c])

where F is the measured fluorescence intensity, k is a geometric instrumental factor, QE is the quantum efficiency (photons emitted/photons absorbed), Po is the radiant

power of the excitation source, epsilon is the wavelength-dependent molar absorptivity coefficient, b is the path length, and c is the analyte concentration (epsilon, b, and c are the same as used in the Beer-Lambert law).

Quantitative FluorimetryQuantitative Fluorimetry

Expanding the above equation in a series and dropping higher terms gives:

F = k * QE * Po * (2.303 * epsilon * b * c)

This relationship is valid at low concentrations (<10-5 M) and shows that fluorescence intensity is linearly proportional to analyte concentration.

Determining unknown concentrations from the amount of fluorescence that a sample emits requires calibration of a fluorimeter with a standard (to determine K and QE) or by using a working curve.


LimitationsMany of the limitations of the Beer-Lambert law also

affect quantitative fluorimetry. Fluorescence measurements are also susceptible to inner-filter effects. These effects include excessive absorption of the excitation radiation (pre-filter effect) and self-absorption of atomic resonance fluorescence (post-filter effect).

Specific fluorescence techniquesAtomic fluorescence spectroscopy (AFS) Molecular laser-induced fluorescence (LIF)


IntroductionAtomic emission spectroscopy (AES or OES) uses quantitative

measurement of the optical emission from excited atoms to determine analyte concentration. Analyte atoms in solution are aspirated into the excitation region where they are desolvated, vaporized, and atomized by a flame, discharge, or plasma. These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels. The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. The spectra of multi-elemental samples can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer. Since all atoms in a sample are excited simultaneously, they can be detected simultaneously, and is the major advantage of AES compared to atomic-absorption (AA) spectroscopy.

Atomic Emission Spectroscopy (AES, OES)Atomic Emission Spectroscopy (AES, OES)

InstrumentationAs in AA spectroscopy, the sample must be converted to free atoms,

usually in a high-temperature excitation source. Liquid samples are nebulized and carried into the excitation source by a flowing gas. Solid samples can be introduced into the source by a slurry or by laser ablation of the solid sample in a gas stream. Solids can also be directly vaporized and excited by a spark between electrodes or by a laser pulse. The excitation source must desolvate, atomize, and excite the analyte atoms. Since the atomic emission lines are very narrow, a high-resolution polychromatic is needed to selectively monitor each emission line. :

Instrumentation of Atomic Emission Spectroscopy (AES, OES)Instrumentation of Atomic Emission Spectroscopy (AES, OES)

The Nature of Electronic TransitionsThe Ultra-Violet(UV) and Visible regions of the electromagnetic spectrum are

associated with a large enough Kinetic Energy that the energy that is absorbed will affect the energy states of electrons occupying the molecular orbitals within the molecule. If the energy of the radiation is equal to or greater than the the energy of transition for an electron to be promoted to the next available molecular orbital then energy will be absorbed by that electron and be promoted to the higher energy molecular orbital. This absorption of energy will occur when energy from the UV or Visible regions are supplied. Infrared radiation is not energetic enough to cause electronic transitions within molecules.

UV and Visible SpectrophotometryUV and Visible Spectrophotometry

to to ** Transitions TransitionsFor molecules that possess bonding as in alkenes, alkynes, aromatics, acyl compounds or nitriles, energy that is available can promote electrons from a

Bonding molecular orbital to a Antibonding molecular orbital. This is called a ---> * transition. The energy difference for such a transition to occur will

depend upon the atoms bonded to each other, other atoms attached as well as the relationship between two or more bonds within the molecule. bonds

between two carbon atoms will have a different a ---> * transition compared to bonds between a carbon and an Oxygen atom (a carbonyl) or a bond

between a carbon atom and a nitrogen atom (a nitrile). This is because there will be a different energy gap between the Bonding and Antibonding molecular

orbital energy states. Other atoms such as Hydrogen( as in an aldehyde) or another SP3 carbon( as in a ketone) that would be bonded to one of the bonded

atoms in the molecule would also cause the energy of transition to vary. The greater the energy of transition the shorter the wavelength of UV or visible

radiation will have to be for electrons to be promoted from the bonding to the antibonding state.


Every group of atoms with bonding will have a different wavelength where maximum abosrption will take place. This is called the “Max", the wavelength

where maximum absorption takes place, and the group of atoms with the bonding is called a "chromaphore". Each chromaphore will have a different energy of transition between the bonding and antibonding molecular orbitals for which the electron transition takes place. For example, alkenes and non-conjugated polyenes will have lamda max absorbances that are below 200

nanometers(nm). Such a short wavelength which indicates a larger energy of transition is because such chromaphore molecules have only ---> *

transitions.


n to * TransitionsEven lone pairs that exist on Oxygen atoms and Nitrogen atoms may be promoted

from their non-bonding molecular orbital to a antibonding molecular orbital within the molecule. This is called an n---> * transition and requires less

energy(longer wavelength) compared to a ---> * transition within the same chromaphore .


Acyl compounds containing the carbonyl C=O will have a lamda max at longer wavelengths above 200 nm compared to non-conjugated

alkenes and alkynes. For example, ethene has a lamda max of 171 nm whereas acetone, CH3-CO-CH3 having a C=O has a lamda max of 280

nm, 109 nm longer. A longer wavelength indicates a shorter energy gap between molecular orbitals for the electron to be propelled to.


The Conjugation Effect on Lamda MaxConjugated polyenes will have lamda max that are higher than 200 nm. This

would indicate that the Pi--->Pi* transition involves a smaller amount of energy. If we compare the molecular orbital levels in a non-conjugated alkene

with the molecular orbitals of a conjugated diene, we find that for a conjugated diene there are two Pi bonding and two Pi antibonding molecular orbitals in

the diene compared to one each in the alkene. (See Fig 2 below)


The Conjugation Effect and Color Changes During Chemical ReactionsDuring a chemical reaction it is possible to go from a colorless starting material

where the lamda max is well within the colorless UV region to a colored product where the lamda max has shifted into the Visible region of the

spectrum. For example in the Benzoin Condensation of Benzaldehyde the colorless liquid, Benzaldehyde is converted to light yellow Benzoin. If we

compared the extent of conjugation between the reactant and the product we would see that the degree of conjugation roughly doubled. This doubling of the conjugation shifted the lamda max of absorption from the colorless UV region into the Blue end of the Visible region. Since Benzoin absorbs nearer the blue end of the visible spectrum it reflects radiation nearer the red end hence the reason for it appearing yellow. The conjugation effect can explain the color change in most cases where chemical change results in the change in color.


Spektrofotometer inframerah pada umumnya digunakan untuk : Menentukan gugus fungsi suatu senyawa...

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