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Atomic Emission SpectroscopyMolecular Absorption Spectroscopy

Lecture 21

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SpectrographBeginning 1930s

• photographic film detector–Cheap–Long integration times–Difficult to develop/analyze–Non-linearity of line "darkness“

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Potential Source

Photographic Film

Graphite Electrodes

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The blackness of the lines on the photographic film is an indication of the intensity of the atomic line and thus the concentration of the analyte. The location of emission lines as compared to standard lines on a film serves to identify the wavelengths of emission lines of analyte and thus its identity. The use of spectrographs is not very convenient since a lot of time and precautions must be spent on processing and calibrating the photographic film.

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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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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.

TPP

PP

so lu tion

so lven t

0A

PP

PP

so lven t

so lu tion lo g lo g

0