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Fichier Détails
| Cartes-fiches | 24 |
|---|---|
| Langue | English |
| Catégorie | Chimie |
| Niveau | Université |
| Crée / Actualisé | 06.01.2016 / 24.06.2016 |
| Lien de web |
https://card2brain.ch/box/intro_to_analysis_spectroscopy_and_chromatography
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| Intégrer |
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Define: analyte (1p)
analyte(s): compound(s) of interest in an analytical procedure
Define "analytical chemistry" and the two common categories of it (3p)
Analytical chemistry: measurement of propertie of a substance or system such that information about the chemical make-up of that system is obtained
1. Qualitative analysis: analysis by physical properties without quantitative data (some or all of the constituents of the system in question are identified)
2. Quantitative analysis: the concentration (or mass) of some or all constituents in a system are measured and identified
What are some factors to method selection? (5p)
1. Elemental or molecular in form
2. Organic or inorganic
3. Volatile or non-volatile
4. Low, medium, or high molecular weight
5. Ionic or non-ionic
What are the two most important considerations for method selection? Define them. (2p)
1. Selectivity: ability of the method to discriminate between different atoms/ions/compounds
2. Sensitivity: magnitude of the change in signal that results from an incremental change in concentration
Definte: limit of detection (LOD) (1p)
Limit of detection (LOD): lowest comcentration of a compound that may be statistically differentiated from the background noise
What two ways can you increase sensitivity? (2p)
1. Improve equipment used
2. Reduce the background noise
What are "descriptors" and what is the minimum number of them necessary to make a clear judgement? (2p)
Descriptors: element or term, qualities, that ha s the function of describing or identifying someone or something in particular
At least 2, completely independent descriptors are required to recognize someone from a distance
What does it mean when a presumptive colour test indicates that the object is positive for a certain substance? (1p)
The object is presumed to be positive for the substance, and fails to be excluded from the evidence
Define the two main types of spectroscopy. What subfields of these do we use in forensic sciences? (4p)
1. Electron spectroscopy: analysis of the interaction between matter and electronic radiation
2. Optical spectroscopy: analysis of the interaction between matter and light
Absorbance spectroscopy: analysis of the absorption of light by its chemical constituents in a mixture
Atomic spectroscopy: study of electromagnetic radiation absorbed and emitted by atoms
There are energy levels since electrons operate in waves. How is this seen visually, and how can it be explained on a molecular level? (2p)
We see these as colours and light, as the sum of photons that are leaving the article due to an absorption of light.
How are electrons promoted to higher energy levels? (1p)
They absorb incident photons in certain wavelengths so that they absorb the exact amount of energy required to ascent energy levels
What is the power spectrum based on? (1p)
Intensity
What are the 4 main components necessary for an absorption spectroscopy experiment in a lab? (4p)
1. Light source
2. Wavelength isolation (grating, prism)
3. Focusing optic (mirrors, lenses)
4. Detector
List 4 advantages and 3 disadvantages to absorption spectroscopy. (7p)
1. Spectral data is possible (absorption spectra are characteristic of the compound(s) absorbing
2. Technology widely available
3. May be used as detection scheme in chromatography, improving selectivity
4. Multiple species may absorb at a given wavelength, which is useful if ε(λ) is known for multiple λ fir each species in the particular matrix in question
5. ε is environmentall sensitive (ph, solvents, etc.) so the matrix effect may be problematic
6. Sensitivity is typically limited, so it isn't good for trace analysis
7. Linearity of Beer's law is limited (often to <0.01M)
Atomic spectrscopy requires knowledge of.... (1p)
The elemental composition (relative) of a sample aquired
How is the solution in question prepared for Flame Atomic Absorption Spectrometry? (2p)
1. The solutions are converted to aerosols by nebulizer and introduced into a flame
2. Molecular and ionic species may then be atomized in the flame
Name 5 types of optical spectroscopy common in forensic science. (5p)
1. UV-visible absorbance spectroscopy
2. Infrared absorbance spectroscopy
3. Atomic absorbance spectroscopy
4. Atomic emission spectroscopy
5. Atomic/molecular fluorescence spectroscopy
Define: fluorescence (1p)
Fluorescence: form of luminescence which occurs when an electron is excited through proton absorption, and the excited state energy is dissipated through re-emission of a photon
Define: stokes shift (1p)
Stokes shift: the difference (in wavelength or frequency units) between positions of the band maxima of absorption and emission spectra of the same electronic transitition
Simplified - difference between energy of excitation and energy of emission
Fluorescence intensity is proportional to... (2p)
Fluorophone concentration (over a limited C region)
Incident power
How is thermal relaxation affected by collisions, and thus affecting fluorescence? (1p)
More collisions = more thermal relaxation = less fluorescence
How is the fluorescence affected by the frequency of the wavelength? (1p)
The lower the wavelength, the more likely for the object to glow (fluoresce)
How does a larger stokes shift affect the background noise and thus, sensitivity? (2p)
The larger the stokes shift, the more background noise that gets filtered out.
This increases the sensitivity.
List 2 advantages and 2 limitations of fluorescence spectroscopy (4p)
1. Significantly more sensitive than absorptive methods
2. Selectivity can be enhanced by judcious choice of both the wavelengths of the excited state ot emissive state
3. May require labelling when used as detection scheme in chromatography or electrophoresis
4. Environmentall sensitive to quenchers, solvents, etc.
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