Anh văn chuyên ngành hóa 3

LOGO
SPECTROS
COPY
LECTURE : Võ Uyên Vy
GROUP : 4

Introduction.
 Before the beginning of the 20th century most
quantitative chemical analyses used titrimetry as
the analytical method analysts achieved
highly accurate result.
 But limited Other methods developed
during this period extended quantitative analysis
to include trace level analytes Colorimetry.
 One example of an early colorimetric analysis is
Nessler’s method.
SPECTROSCOPY
Group 4 – DHHC6B

Nessler’s method.
 The Nessler’s for ammonia.
 It was first proposed in 1856.
 Nessler’s found that adding an alkaline solution
of HgI2 and KI to a dilute solution of ammonia
produced a yellow to reddish brown colloid with
the color determined by the concentration of
ammonia.
A comparison of the sample’s color to that
for a series of standards was used to determine the
concentration of ammonia.
SPECTROSCOPY
Group 4 – DHHC6B

Introduction
 At the end 19th century, spectroscopy was limited to:
 The absorption.
 Emission.
 Scattering of UV/VIS.
 Infrared electromagnetic radiation.
 During the 20th , spectroscopy has been extended to include
other form of electromagnetic radiation (photon spectroscopy)
• X-rays
• Microwaves
• Radio waves
• Energetic particles such as: electrons and ions
SPECTROSCOPY
Group 4 – DHHC6B

Introductions.
SPECTROSCOPY
Group 4 – DHHC6B

Introduction
 Spectroscopy is used to qualitatively or
quantitatively study the atoms or molecules, or to
study physical processes.
 The interaction of radiation with matter can
cause redirection of the radiation and/or
transitions between the energy levels of the
atoms or molecule.
SPECTROSCOPY
Group 4 – DHHC6B

Introduction
 A transition from a lower level to a higher level
absorption ( transfer energy)
 A transition from a higher level to a lower level
 emission (transfer energy)
 Redirection of light due to its interaction with
matter scattering (may or may not occur
with transfer of energy)
SPECTROSCOPY
Group 4 – DHHC6B

Absorption
SPECTROSCOPY
Group 4 – DHHC6B
Type of excitation depend on the wavelength of the light
UV/Visible promoted electrons to higher orbital
Infared excited vibrations
Atoms or molecules absorb light  a higher energy level
Microwaves excited rolations
Measuring the concentration of absorbing species
in a sample is accomplished by Beer-Lambert Law

Absorption
Useful for
indentifying
of compounds
Depend on its
energy level
structure
A function
of wavelength
The absorption
of light
SPECTROSCOPY
Group 4 – DHHC6B

Emission
SPECTROSCOPY
Group 4 – DHHC6B
1.How is the emitting radiation?
2. Atomic – emission spectroscopy and
Atomic – fluorescence spectroscopy
3. How is the flourescence of molecules
and the phosphorescence of molecules?

Emission
SPECTROSCOPY
Group 4 – DHHC6B
 Atoms or molecules at higher energy
level low levels by emitting
radiation (emission or luminescence)
 Atoms by a high-temperature
energy source, this light emission
atomic or optical emission.
 Atoms excited with light atomic
fluorescence.
decay
Excited
called
called

Emission Group 4 – DHHC6B
 For molecules it is called :
• Fluorescence if the transition is between states of
the same spin.
• Phosphorescence if the transition occur between
states of different spin.
The emission intensity of an emitting substance is
linearly proportional to analytes concentration at low
concentration, and is useful for quantitating emitting
species.
SPECTROSCOPY

UV/VIS and infrared
spectrophotometry
1
Colorimetric:
visible light was
absorbed by
sample, was the
earliest
application of
molecular
absorption
spectroscopy
2
Concentration of
analyte was
determined by:
• Using Nessler
tubes.
• Using an
instrument called
a colorimeter.
3
IR was discovered
in 1800, their
uses in optical
molecular
absorption
spectroscopy
Group 4 – DHHC6B
SPECTROSCOPY
UV radiation was
discovered in
1801, was limited
by the lack
convenient for
detecting the
radiation.
4

Introduction UV/VIS
spectrophotometer
 The UV/VIS spectrophotometer uses two light
sources:
 A deuterium (D2) lamp for ultraviolet light.
 A tungsten (W) lamp for visible light.
SPECTROSCOPY
Group 4 – DHHC6B

Introduction UV/VIS
spectrophotometer
 Principles of machine operation:
SPECTROSCOPY
Group 4 – DHHC6B

Single-Beam UV/VIS
Spectrophotometer
 Single-Beam spectrophotometer are often
sufficient for making quantitative absorption
measurements in the UV/VIS spectral region.
 The concentration of analyte in solution can be
determined by:
- Measuring the absorbance at a single
wavelength.
- Applying the Beer-Lambert Law.
SPECTROSCOPY
Group 4 – DHHC6B

Single-Beam UV/VIS
Spectrophotometer
A light-
emitting diode
(LED)
Instrumentation
A photodiode
dectector
A sample
container.
The simplest instruments use a
single-wavelength light source.
SPECTROSCOPY
Group 4 – DHHC6B

Single-Beam UV/VIS
Spectrophotometer
SPECTROSCOPY
Group 4 – DHHC6B

Dual-Beam UV/VIS
Spectrophotometer
 In UV absorption spectroscopy, obtaining a
spectrum requires manually measuring the
transmittance of the sample and solvent at each
wavelenght.
 The double-beam design greatly simplifies this
process by measuring the transmittance of the
sample and solvent simultaneously.
SPECTROSCOPY
Group 4 – DHHC6B

Dual-Beam UV/VIS
Spectrophotometer
 Instrumentation.
reference
sample
detector
detector
ratio
Mono-
chromator
LAMP
SPECTROSCOPY
Group 4 – DHHC6B

Applications.
 Absorption measurements based upon
ultraviolet or visible radiation find
widespread application for the qualitative
and quantitative determination of molecular
species
SPECTROSCOPY
Group 4 – DHHC6B

Applications.
Quantitative analysis by
absorption measurements
Applications to
absorbing species
Applications to
nonabsorbing species
Application of absorption
measurement to qualitative
SPECTROSCOPY
Group 4 – DHHC6B

 UV/VIS spectrophotometry have somewhat
limited application for qualitative analysis.
Unambiguous identification is impossible.
 Confirmation of the presence of an aromatic
amine or a phenolic structure may be obtained
by comparing the effects of pH on the spectra of
solutions containing the sample with those.
Application of absorption
measurement to qualitative
analysis
SPECTROSCOPY
Group 4 – DHHC6B

 Absorption spectroscopy is one of the most
useful and widely used tools available to the
chemist for quantitative analysis.
 Important characteristics of spectrophotometric
and photometric methods include:
SPECTROSCOPY
Group 4 – DHHC6B

• Wide applicability to both organic and
inorganic systems1
• Typical sensitivities of 10^-4 to 10^-5 M2
• Moderate to high selectivity3
4
5 • Ease and convenience of data acquisition
• Good accuracy
SPECTROSCOPY
Group 4 – DHHC6B

Applications to absorbing
species.
 Spectrophotometric analysis for any organic compound
containing one or more of these groups is potentially
feasible.
 A number of inorganic species also absorb and are thus
susceptible to direct determination; we have already
mentioned the various transition metals. In addition, a
number of other species also show characteristic
absorption.
 Examples include nitrite, nitrate, and chromate ions;
osmium and ruthenium tetroxides; molecular iodine; and
ozone
SPECTROSCOPY
Group 4 – DHHC6B

Applications to
 Numerous reagents react selectively with
nonabsorbing species to yield products that
absorb strongly in the ultraviolet or visible
regions.
 The successful application of such:
 Reagents to quantitative analysis usually requires that
the color
 Forming reaction be forced to near completion
SPECTROSCOPY
Group 4 – DHHC6B

Applications to
• Forming reagents are also frequently employed for the
determination of absorbing species such as transition-
metal ions
• The molar absorptivity of the product will frequently be
orders of manitude greater than that of the uncombined
psecies
Note
SPECTROSCOPY
Group 4 – DHHC6B

Applications to
nonasorbing species.
 A host of complexing agents find appilication in the
determination of inorganic species.
 Typical inorganic reagents include:
 Of even more importance are organic chelating
agents that form stable, colored complexes with
cations.
The
thiocyanate
ion for Fe, Co,
Mo
The anion of
H2O2 for Ti,
Va, Cr
Iodide ion for
Bi, Pb, Te
SPECTROSCOPY
Group 4 – DHHC6B

Procedure
Cleaning and
handing of cells
Selection of wavelength
Standard addition
method
Variables that
influence absorbance
Determination of the
relationship between
absorbance and concentration
SPECTROSCOPY
Group 4 – DHHC6B

Procedural details
The pH of the solution
The temperature
High electrolyte concentration
Variables that
influence
absorbance
The nature of the solvent
The presence of interfering subtances
SPECTROSCOPY
Group 4 – DHHC6B

Procedural details Group 4 – DHHC6B
 Spectrophotometric absorbance measurements are
ordinarily made at a wavelength corresponding to an
absorption peak, because the change in absorbance
per unit of concentration is greatest at this point; the
maximum sensitivity is thus realized.
 In addition, the absorption curve is often flat in the
region; under these circumstances, good adherence
to Beer’s law can be expected. Finally, the
measurements are less sensitive to uncertainties
arising from failure to reproduce precisely the
wavelength setting of the instrument.
SPECTROSCOPY

 After deciding upon the conditions for the analysis,
it is necessary to prepare a calibration curve from a
series of standard solutions. These standards should
approximate the overall composition of the actual
samples and should cover a reasonable
concentration range of the analyte.
 Seldom, if ever, is it safe to assume adherence to
Beer’s law and use only a single standard to
determine the molar absorptivity. The results of an
analysis should never be based on a literature value
for the molar absorptivity.
SPECTROSCOPY

 It is apparent that accurate spectrophotometric
analysis requires the use of good – quality, matched
cells. These should be regularly calibrated against
one another to detect differences that can arise from
scratches, etching, and wear.
 Equally important is the use of proper cell cleaning
and drying techniques.
 Erickson and Suries recommend the following
cleaning sequence for the outside windows of cell.
SPECTROSCOPY

 Prior to measurement, the cell surfaces are cleaned
with a lens paper soaked in spectrograde methanol.
 The paper is held with a hemostal; after wiping, the
methanol is allowed to evaporate, leaving the cell
surfaces free of contaminants.
 The authors showed that this method was far
superior to the usual procedure of wiping the cell
surfaces with a dry lens paper, which apparently
leaves lint and films on the surface.
SPECTROSCOPY

 Ideally, calibration standards should approximate
the composition of the sample to be analyzed not
only with respect to the analyte concentrations of the
other species in the sample matrix, in order to
minimize the effect of various components of the
sample on the measured absorbance.
 For example, the absorbance of many colored
complexes of metal ions is decreased to a varying
degree in the presence of sulfate and phosphate ions
as a consequence of the tendency of these anions to
form colorless complexes with metal ions.
SPECTROSCOPY

 The color – formation reaction is often less complete
as a consequence, and lowered absorbances are the
results. The matrix effect of sulfate and phosphate can
often be counteracted by introducing into the
standards amounts of the two species that
approximate the amounts found in the samples.
 When complex materials as solid, minerals, plant
ash are being analyzed, preparation of standards that
match the samples is often impossible. When this is
the case, the standard addition method is often helpful
in counteracting matrix effects.
SPECTROSCOPY

 The standard addition method can take several
forms. The one most often chosen for photometric or
spectrophotometric analyses, and the one that will be
discussed here, involves adding one or more
increments of standard solution to the sample
aliquots of the same size.
SPECTROSCOPY

 Each solution is then diluted to a fixed volume
before measuring its absorbance. It should be noted
that when the amount of sample is limited, standard
additions can be carried out by successive
introductions of increments of the standard to a
single measured aliquot of the unknown.
Measurements are made on the original and after
each addition. This procedure is often more
convenient for voltammetric and potentiometric
measurements and will be discussed in later sections
of the text.
SPECTROSCOPY

Anh văn chuyên ngành hóa 3

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (16)

Destaque

Destaque (6)

Semelhante a Anh văn chuyên ngành hóa 3

Semelhante a Anh văn chuyên ngành hóa 3 (20)

Último

Último (20)

Anh văn chuyên ngành hóa 3