modern methods in techniques of analytical chemistry: spectroscopic techniques
TRANSCRIPT
Modern methods in techniques of analytical
chemistry: Spectroscopic techniques.
1) Describe the absorption of radiation by molecules and its relationship to molecular structure. 2)Make quantitative calculations, relating the amount of radiation absorbed to the concentration of an absorbing analyte.
3)Describe the instrumentation required for making measurements.
Scope:
The Electromagnetic Radiation• Light is a form of electromagnetic radiation.
Electromagnetic radiation can be considered as a form of radiant energy that is propagated as a transverse wave.
It vibrates perpendicular to the direction of propagation, imparts a wave motion to the radiation
Wavelength, λ = c/ v
Wavenumber, ṽ = v/c =1/λ
Wavelength ,λ is the distance between the neighboring peaks of two wave.
Frequency, v (nu) the number of cycles passing a fixed point per unit time.
Frequency is measure in Hertz, 1 Hz= 1 s-1.
Wavenumber, ṽ (nu tilde) the number of complete wavelengths in a given length.
e.g.: A wave number of 5 cm-1 indicates there are 5 complete wavelength in 1 cm.
Where c = velocity of light (3 x 1010 cm/s)
Electromagnetic radiation possesses certain amount of energy
E = hν = hc/λ
E = energy of a photonh = Planck’s Constant (6.62 x 10-34 joule second (J-s))
It is apparent that the shorter the wavelength or the greater frequency, the greater energy.
In spectrometric methods, the sample solution absorbs electromagnetic radiation from appropriate source, and the amount absorbed is related to the concentration of analyte in the solution.
109 1
07 1
05 1
03 1
01 1
0-1
10-3
10-5
10-7
10-9
10-1
1
gamma
X-rays
Ultra Violet
Infra red
microwave
Radio waves
500
600
700
Violet, indigo, blue
Green, yellow
Orange, red
Electromagnetic Spectrum
How does matter absorb radiation?
There are 3 basic process:1)Rotational transition- molecules rotate, absorb radiation and raised to higher
rotational energy level
2) Vibrational transition- Atoms or group of atoms within molecule vibrate
relative to each other, absorb radiation and raised to higher vibrational energy level.
3) Electronic transition- Electrons of molecule raised to higher electron energy.
Wavelenghts and Color
Wavelength of Maximum
absorption (nm)
Color absorption Color Observed
380-420 Violet Green-yellow
420-440 Violet-blue Yellow
440-470 Blue Orange
470-500 Blue-green Red
500-520 Green Purple
520-550 Yellow-green Violet
550-580 Yellow Violet-blue
580-620 Orange Blue
620-680 Red Blue-green
680-780 Purple Green
Irradiance or radiant power, P (Wm-2), is the energy per second per unit area of the beam of light.
Schematic of a simple spectrometer.
Derivation of Beer’s LawDerivation of Beer’s Law
Irradiance In, P0Irradiance In, P0
Irradiance Out, P
Irradiance Out, P
PathlenghtPathlenghtReadout
Transmittance is T=Po/P =10-kbTransmittance is T=Po/P =10-kb
Transmittance, T, is the fraction of original light not absorbed by the sample.
Po = power of incident lightP= power of transmitted light
Putting transmittance in logarithmic form:
log T = log P/Po = -kb
Similar law holds for the dependence of T on concentration, c
T=Po/P =10-k’c and log T = log P/Po = -k’c
Combining these two laws,
T = P/P0 =10-abc (Beer’s Law)
Where ε is the analyte’s molar absorptivity. ε = a x molecular weight
log T = log P/P0 = -abc
A = -log T = log 1/T = log P0/P = abc = log 100 – log %T
Continue…..
Where
A = absorbance, a = analyte’s absoptivity b = pathlenght through the material; c= concentration,
OR A = εbc
Spectroscopy Nomenclature Recommended Name Unit
Absorbance (A) -
Absorptivity (a) cm-1 g-1 L
Pathlength (b) cm
Transmittance (T) -
Wavelength (λ) nm
Concentration (c) moles per liter (moles L-1)
Example 1:
A sample in a 1.0-cm cell is determine with a spectrometer to transmit 80% light at certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, What is the concentration of the substance?
Solution:
The percent of the transmittance is 80%, and so T = 0.80.
log 1/T =abc
log 1/0.80 = 2.0 cm-1g-1L x 1.0 cm x c
log 1.25 = 2.0 g-1L x c
c = 0.10/2.0 = 0.050 gL-1
Example 2:
A solution containing 1.00 mg ion (as thiocyanate complex) in 100 mL was observed to transmit 70.0% of the incident light compared to an appropriate blank. (a)What is the absorbance of the solution at this wavelength?(b) What fraction of light would be transmitted by a solution of iron four times as concentrated.
Solution:
(a)T = 0.700 A = log 1/0.700 = log 1.43 = 0. 155
(b) 0.155 = ab (0.0100 g/L) ab = 15.5 L/g From A= abc A = 15.5 L/g x (4x0.1000 g/L) = 0.620
log 1/T = 0.620 T = 0.240
The absorbance of the new solution could have been calculated more directly:
Beer’s Law Assumption/Limitation
• The light being shined on the sample must be monochromatic (one color or wavelenght)
• The analyte must not be participate in a concentration dependent equilibrium
This isn’t good technique for many weak acid systems, as dilution increases dissociation and HA and A- probably don’t have the same absorbance.
Beer’s Law Assumption/LimitationProblems: Calibration curves are found to be nonlinear because occuring of deviation. Deviation from linearity are divided into three categories:
1)Fundamental : Law is valid for low concentration analyte. At higher concentration, there will be interaction between particle of analyte that may change the value of ε.
2) Chemical: when the absorbing species is involved in an equilibrium reaction.
HA ↔ H+ + A-
HA will absorbs the wavelength and contribute to ε and C value. However, if the equilibrium shifts to right, less HA available for absorption and will result in non linearity of the curve.
3) Instrumentation: 2 principal limitation: i) Stricly valid for purely monochromatic instrumentation. ii) The effect of ‘leakage’ light from imperfections within wavelength selector. This phenomena is called stray radiation.
Deviation from beer’s law:continue…
From the beer’s law, the absorbance against the conc. A straight line passing through origin is obtained (linear graph)However, deviation might occurs. Deviation is due to the
following factors:- A foreign substance having colour particle may affect the
absorption & extinction coefficient. Deviation also occur if colored solute ionized or dissociates in
the solution; e.g.- benzyl alcohol in chloroform Due to the presence of impurities that fluoresce or absorb at
the absorption wave length. If monochromatic light is not used deviation may occurs. If width of the slit is not proper. If the solution species undergoes polymerization
Block of Diagram - Spectrometer
Source Monochromator Sample DetectorReadout (meter or recorder)
Sources : Tungsten lamp (visible), hydrogen or deuterium discharge tube (ultraviolet), hot wires, light bulb or glowing seramic (IR), laser Monochromator: prism, difraction grating, optical filters.Sampel cell: cuvets, KBr, UV/IR quartz.Detector: phototube, photomultiplier tube, spectrophotometer (UV) thermocouples, bolometers (IR)
Two types of Monochromator
Grating: • A fundamental property of gratings is that the angle of deviation of all but one of the diffracted beams depends on the wavelength of the incident light. • Therefore, a grating separates an incident polychromatic beam into its constituent wavelength components, i.e., it is dispersive. Because of their dispersive properties, gratings are commonly used in monochromators and spectrometers.
Two types of Monochromator
Prism: •Electromagnetic radiation is refracted because index of refraction of prism material is different from air.•Shorter wavelengths are refracted more than longer wavelength•The effects of refraction is to ‘spread’ the wavelength apart into different wavelength•By rotating the prism, different wavelength can be made to pass through an exit slid and to the sample
Types of Instruments
•Single–beam : instrument has one sample holder, you must swap blank and sample.• less expensive, sophisticated instrument and excellent result can be obtained.• Modern single beam instrument are smaller, faster, more sensitive and more economical then the older versions.
Spectroscopic – Single Beam
Spectroscopic – Double Beam
•Double-beam: instrument splits light output between two holders so you can measure blank and sample.•Normally use as a recording instrument
26
Introduction to UV-Visible Absorption Spectroscopy
Applications
Protein
Amino Acids (aromatic)
Pantothenic Acid
Glucose Determination
Enzyme Activity (Hexokinase)
Pyridoxine Vitamin B12 Niacin Metal Determination (Fe) Fat-quality Determination (TBA) Enzyme Activity (glucose oxidase)
UV-Vis Spectrophotometer
1) The spectrophotometer should be turned on at least 30 minutes before calibration. Adjust the wavelength to the required setting.
2) With the 'Press for Zero Set' button depressed adjust the 'Zero Adjustment' dial until the meter needle is aligned with 0 % transmission (lower scale).
3) Then place an appropriate blank in a cell in the cell holder and adjust the '100 % Transmission' dial until the meter needle is aligned with 100 % transmission (lower scale).
4) The blank can be replaced with the sample(s) of interest and the absorbance read from the upper scale.
5) This procedure should be made for each wavelength of interest. The spectrophotometer should remain on until all the required readings are made.
Calibration of the Spectrophotometer
Example :
ppm Zn Absorbance
0.00 0.000
1.00 0.193
2.00 0.344
3.00 0.565
4.00 0.727
unknown 0.270
1) Plot a graph of absorbance versus the concentration of standard Zn on a graph paper.2) Recognize a linear regression of the plot.3) Calculate the concentration of Zn in the unknown.
Regression line and equation
Regression line and equation
Calculate the concentration of Zn in the unknown.
Solution: y = 0.1816 x + 0.0006 x = 0.1816 (0.270) + 0.0006 = 0.0496 ≈ 0.05
Concentration Vs. Absorbance
absorbanceRegression line (absorbance)
Fourier Transform Infra Red-FTIR
FTIR
The IR source will split into 2 path
The IR source will split into 2 path
Instead of grating monochromators, FTIR uses interferometer to obtain a spectrum
Functional Groups Vibration mode Frequency (cm-1)Alkane
C-H strechching 3000-2850 -CH2
CH2 (long chain)bending 1465
700
-CH3 bending 1450 dan 1375 Alkene
C=C bending 1660-1600 =C-H bending
1000-650
Carboxylic AcidC-OH strechching 3400-2400 C=O strechching 1730-1700C-O strechching 1320-1210
AlcoholO-H
C-O-Hstrechching
bending3400-33001440-1220
C-O strechching 1260-1000Ester
strechching
strechching
1750-1735
1740-1715
EpoxideC-O strechching 1300-1000
Ring strechching
Between 880 and 750 Beyween 950 and 815
1280-1230
R OR
O
C CH
O
ORH
H
O
Correlation Table- FTIR
Example FTIR Spectrum
OH
3456 cm-1
CH
2925 cm-
1
CH
2854 cm-
1
CH2
1465 cm-
1
CH3
1374 cm-
1
CH2
722 cm-1
=C-H
1035 cm-
1
C=C
1641 cm-
1
C-O
1246 cm-
1
C=O
1736 cm-
1
Atomic Spectrometric Methods
Atomic absorption spectrophotometry (AAS)
-To measure metal content (refer to periodic table) e.g. copper, iron, natrium, magnesium, manganese, zinc, etc.- cannot detect non-metal compound
Hollow cathode lamp- Source of the analytical light line for the element of interest - Give a constant and intense beam of that analytical line
Nebulizer - Suck up liquid sample at a controlled rate - Create a fine aerosol spray for introduction into the flame - Separate large and fine aerosol- Mix the aerosol, fuel and oxidant thoroughly before combusted in slotted burner which provide a flame
• Flame - Create atoms of the element of interest, Feo, Cuo, Zno, etc.
• Monochromator - Isolate the analytical line photons passing through the flame - Remove scattered light of other wavelengths from the flame- In doing this, only a narrow spectral line impinges on the PMT.
Photomultiplier tube (PMT)- This is the detector. The PMT determines the intensity of photons of the analytical line exiting the monochromator. -The PMT is the most commonly used detector for atomic absorption spectroscopy. -However, solid state detectors are now replacing conventional vacuum-type photomultipliers. -High tech electronics amplify, filter, and process the electrical signal, using a series of chips and microprocessors, transmitting the result to an internal or external computer which handle all data-handling and display
Graphite Furnace AA-The graphite furnace is an electrothermal atomiser system that can produce temperatures as high as 3.000°C. -The heated graphite furnace provides the thermal energy to break chemical bonds within the sample held in a graphite tube, and produce free ground state atoms.- Ground-state atoms then are capable of absorbing energy, in the form of light, and are elevated to an excited state. -The amount of light energy absorbed increases as the concentration of the selected element increases. -Flame AA can only analyse solutions, but graphite furnace can accept very small absolute quantities of solution, slurry or solid samples.
How to run your sample?
Chapter 8&9 - 41
ANALYTICAL TECHNIQUES
• Beer's law, A = k×C, not always true making a calibration curve necessary.
• Effect from matrix:e.g. high viscosity, chemical reaction with analyte
• Standard addition method is used to minimize the effects from the matrix
Example of standard addition plot
Concentration of analyte in sample = intercept at y-axis
slope
AAS - ApplicationQualitative analysis
- to identify an element. - However, due to interferences, atomic emission is far more convenient to qualitative emission.
Quantitative analysis
-Quantitative analysis with spectroscopic methods is based on the Beer-Lambert Law.