Anh văn chuyên ngành hóa 3

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SPECTROSCOPY

LECTURE : Võ Uyên VyGROUP : 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.

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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.

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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

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Introductions. SPECTROSCOPY

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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.

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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)

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Absorption SPECTROSCOPY

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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 speciesin a sample is accomplished by Beer-Lambert Law

Absorption

An absorption spectrum

Useful for indentifying

of compounds

Depend on its energy level

structure

A function of wavelength

The absorption of light

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Emission SPECTROSCOPY

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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?

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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

EmissionGroup 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.

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UV/VIS and infrared spectrophotometry

1Colorimetric:

visible light was absorbed by

sample, was the earliest

application of molecular absorption

spectroscopy

2Concentration 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

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UV radiation was discovered in 1801, was limited by the lack convenient for detecting the radiation.

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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.

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Introduction UV/VIS spectrophotometer Principles of machine operation:

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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.

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A light- emitting diode (LED)

Instrumentation

A photodiode dectector

A sample container.

The simplest instruments use a single-wavelength light source.

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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.

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Dual-Beam UV/VIS Spectrophotometer

Instrumentation.

reference

sample

detector

detector

ratioMono-

chromator

LAMP

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Applications.

Absorption measurements based upon

ultraviolet or visible radiation find

widespread application for the qualitative

and quantitative determination of molecular

species

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Applications.

Quantitative analysis by absorption measurements

Applications to absorbing species

Application

Applications to nonabsorbing species

Application of absorption measurement to qualitative

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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

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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:

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• 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

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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

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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

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• 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

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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

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Procedure

Cleaning and handing of cells

Selection of wavelength

Standard addition method

Variables that influence absorbance

Determination of the relationship between absorbance and concentration

Procedural details

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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

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Procedural detailsGroup 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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Anh văn chuyên ngành hóa 3

Engineering

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