Spectroscopy - Test 1
Quantum transition 10m to 100cm
Nuclear magnetic resonance - Change to nuclei spin
Frequency (𝜈)
Number of cycles passing a point during a given time, or cycles per second. The unit is 1/s or just s^-1 also known as hertz(hz)
Diffraction
Occurs when an object causes a wave to change direction and bend around it
Applications of spectroscopy
Elemental analysis Molecular structure elucidation Functional group recognition Quantitation Forensics - Identify explosives Environmental - Detect ozone hole Biomedical - Breath ratio of oxygen to carbon dioxide
Fluorescence
Emission by a substance of electromagnetic radiation, usually visible, as the immediate result of (and only during) absorption of energy from another source.
Excitation
Energy absorption that forces an electron into a higher energy state in the form of the increased rotation, vibration or electronic excitation
Reflection
Energy waves bouncing off the surface of an object (mirrors or echoes return energy back to their source)
Photon emission
Excited species return to ground state
Amplitude
For a wave or vibration, the maximun displacement on either side of the equalibrium (midpoint) position.
Quantum transition at 100 pm to 10 pm
Gamma ray emission and absorption - Change of nuclear configuration
Qualitative
Having to do with the quality or qualities of something (as opposed to the quantity) Characteristics/descriptions What is it?
Interferometer
High-resolution spectrometer
Molar Absorptivity (ε)
How much light is absorbed at a particular wavelength by the absorbing species
For alkyl halides the wavelength required for the n->σ*transition increases with increasing atomic number of the halide (ie: R-Cl < R-Br < R-I) Why?
If wavelength increases then energy decreases. Moving down the periodic table the elements increase in size meaning the valence electrons are further from the nucleus and thus the attraction is less, because of this less energy is needed to alter the orbital positions of these electrons
Photon
Packets of quantised energy, which leads EMR to being thought of as a stream of discrete particles. The energy of these particles is directly proportional to the frequency of the radiation
Rotational spectroscopy
Periodic fluctuation of the dipole moment interacts with the sinusoidal electric field of radiation
Stray Radiant Energy
Radiation of any wavelength that contaminates the system
Atomic absorption
Simple spectra Small number of possible energy states Promotion of valence electrons to a higher level Can only have electronic transitions
Transmittance (T)
The fraction of the incident light which passes through the sample Ranges from zero to one Dimensionless
Real limitations of Beer's law
The law only holds for analytes that have been diluted to less than 0.01 M, this is because higher concentrations diminish the mean free path. Molecules effect the charge distribution of their neighbours. Extent of interaction is dependent on concentration Affected by the refractive index of the medium (this is rarely significant when the concentration is less than 0.01 M
Spectrometry
The measurement and interpretation of spectra. The utilization of spectra.
Absorbance (A)
The negative logarithm of transmittance. Dimensionless Absorbance is inversely proportional to transmittance so if %T is 100% then absorbance is zero Absorbance is directly proportional to the concentration of the absorbing species Above absorbance of 1.0 90% of light is gone, hence most systems don't operate above an absorbance of 1.0
Why is NO2 a brownish colour at room temperature and colourless when the temperature is reduced to 77K
The nitrogen oxide molecules combine to form the dimer N2O4. At warmer temperatures the NO2 is favoured however as the temperature decreases the reaction begins to favour N2O4 which is a colourless liquid
Chromophore
The part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others.
Atomisation in atomic absorption spectroscopy
The process of converting an analyte to a free gaseous atom, it requires that we strip away the solvent, volatilize the analytes, and, if necessary, dissociate the analyte into free atoms. There are two common atomization methods: flame atomization and electrothermal atomization
Luminescence
The re-emission of radiation. Rare
Briefly describe the chemistry involved in either hydride or Mercury generation sample introduction for AAS and outline the major advantages of your chosen protocol.
The reaction of many metalloid oxyanions with sodium borohydride and HCl produces a volatile hydride: H2Te, H2Se, H3As, H3Sb, etc. After being mixed together the liquid mixture flows through a tube of a specific length and is ultimately flowed into a gas/liquid separator where the hydride and some gaseous hydrogen bubble out and are purged into the optical cell via a gas transfer line. Most of the reagents introduced into the system flow to a waste container. Can be used to analyse metaloids like antimony, arsenic, selenium, and tellurium, instead of AAS, which may be plagued by interferences, poor reproducibility, and poor detection limits when analysing metaloids. Many of the main parts of the HGAAS system are identical to that of AAS: a hollow cathode lamp, air/acetylene flame, and optical system but include (in most systems) an optical cell and the relatively complex hydride generation system. The nebulizer required in AAS is not used in HGAAS.
Discovery of UV and Infrared light
UV - Photographic plate placed just beyond the violet of the visible light spectrum, when developed there was signs of EMR. Infrared - Thermometer was placed just beyond the red of the visible light spectrum and when EMR was directed through the prism the temperature rose.
Electromagnetic spectrum
Includes all forms of electromagnetic radiation; the types of radiation differ in their frequencies and wavelengths Electromagnetic waves are energy waves produced by the vibration of charged particles. They have both electrical and magnetic properties and travel through space at the constant speed of light. The wavelength of an electromagnetic wave is inversely proportional to its frequency.
Frequency equation
c = λ𝜈 where, 𝜈 = frequency in s^-1 c= speed of light (2.998x10^8 ms^-1) λ = wavelength in m c is the velocity of light in a vacuum only, to calculate the velocity through other media it is c/n, where n is the refractive index of the substance
Spectrum
A band of colours, as seen in a rainbow, produced by separation of the components of light by their different degrees of refraction according to wavelength.
Concentric (Gouy) nebuliser
A central capillary with the liquid and an outer capillary with the gas. The gas draws the liquid into the gas stream through induction, and the liquid is broken into a fine mist as it moves into the gas stream.
Atomic absorption spectrometer
A double-beam instrument that measures the ratio of P0/P. The hollow-cathode lamp emits the resonance lines of the test element. These are only absorbed by the test element atomic vapor. The hollow cathode is constructed of the test element (it is low pressure and low current as that minimises the collisions of the test species). Atomic vapor is produced which is excited and emits the lines of the element. The hollow cathode is a sealed glass tube with a quartz window, quartz because glass would absorb some wavelengths.
Snell's law
A formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.
Calibration curve of absorbance
A graph showing the value of absorbance versus concentration of analyte. When the corresponding property of absorbance is measured, its concentration can be determined from the graph. The curve plateus at an absorbance of 1
How do you use a continuum source for Atomic absorption spectroscopy?
A radiation source that is usually lamps or heated solid materials that emit a wide range of wavelengths that must be narrowed greatly using a wavelength selection element to isolate the wavelength of interest. Must be used with a high resolution spectrometer to isolate a narrow wavelength span.
Wave-particle duality
A single particle travelling through space has oscilating magnetic and electric fields that run perpindicular (90°) to each other.
Phosphorescence
A type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation, which provides an afterglow. The delay is caused by atoms being excited to energy states that do not decay rapidly. The afterglow may last from fractions of a second to hours or even days, depending on the type of material, temperature, and other factors.
Velocity
A vector quantity that includes the speed and direction of an object.
Bouguer-Lambert-Beer Approximation
AKA Beer's law Describes the amount of monochromatic radiation absorbed by a sample A = Absorbance ε = Molar absorptivity (cm^-1 mol^-1 L) b = path length (cm) c = concentration (mol L^-1)
What is the mechanism of interaction between incident EMR and the atom/molecule?
Absorption Reflection Transmission (Refraction, travelling through the medium)
Dipole
An unequal distribution of electrons in the molecule that causes separation of charge.
Why is the Ne resonance transition vastly more energetic than the Na resonance transition?
As the Neon has a full outer shell it takes a substantial amount of energy to excite one of the valence electrons to the 3s orbital, while Sodium's one valence electron is not held very strongly to the nucleus so it is easily moved to the 3p orbital.
Possible transitions of atoms and molecules
Atoms can only have electronic transitions. Molecules however can have electronic, vibrational and rotational transitions due to their bonds. The type of transition is dependent upon the wavelength of the incident EMR, and each of the levels are quantised.
Why are there different coloured emissions?
Because of the quantised energy levels. As the element/molecule comes down energy levels it emits light of particular frequencies. Because the energy levels are quantised it can only emit at these same frequencies.
What material would be suitable for the windows of a sample cell used to contain gaseous hydrogen fluoride for the measurement of its microwave rotational spectrum?
CaF2 Calcium Fluoride It is transparent of a broad range, UV - IR frequencies, and as it is the precursor to HF it shouldn't react with its gaseous form.
Apparent limitations (instrumental)
Cannot get monochromatic radiation, even a laser has a wavelength band, it is actually a symmetric band of polychromatic radiation centered around the desired wavelength. Molar absorptivity should not vary significantly across a portion of the band. Stray Radiant Energy
Refraction
Change in direction of radiation passing through the interface of different refractive indices
Apparent limitations (chemical)
Chemical reactions produce a species with a different absorption spectra to the analyte Dissociation, Association, Complex formation, Solvent reactions and Concentration dependent equilibrium
Quantitative
Definite and usually numeric Quantifies How much is there?
Electromagnetic energy and wavelength equation
E=h x c/λ where, E = Energy of a photon h = Planck's constant (6.626×10-34Js) c= speed of light (2.998x10^8 ms^-1) λ = wavelength in m Wavelength and energy are inversely proportional. Wavelength and frequency are also inversely proportional Frequency and energy are directly proportional
Electromagnetic energy equation
E=h𝜈 where, E = Energy of a photon h = Planck's constant (6.626x10^-34 J s) 𝜈 = frequency in s^-1 The shorter the wavelength the higher the energy but the lower the frequency.
Rayleigh scattering
Elastic scattering of light. No non kinetic transfer of energy between the molecules and the photons. A rayleigh shift occurs when there is no overall net transfer of energy.
Photon absorption
Electromagnetic radiation energy is transferred to atoms, molecules, etc. promoting them from a ground to an excited state. The energy of the exciting photon must exactly match the difference between the ground and one of the excited states of the absorbing species. These differences in energy are different for every species.
Quantum transition 100 cm to 1 cm
Electron paramagnetic resonance - Change to electron spin
Quantum transition at 1000 nm to 10 nm
Ultraviolet/visible absorption, emission and fluorescence - Change to outer electron distribution
Triplet excited states
Unpaired electrons with the same spin Spins parallel, unlikely configuration - low probability
Singlet excited states
Usually the ground state with all electrons paired Spins opposed, most likely configuration
Simple modes of vibration
Vibration can only exist between two or more atoms and the vibration changes the difference between the nuclei Stretching -Symmetric stretching -Asymmetric stretching Bending -Scissoring -Rocking -Wagging -Twisting
Wave/particle duality
Waves exhibit both wave and particle characteristics. A single particle travelling through space moves with oscillating electric and magnetic fields that are always 90 degrees to each other.
Quantum transition at 10 nm to 100 pm
X-ray absorption/emission, fluorescence or diffraction - Change to core electron distribution
%T
where, P = power (intensity)of transmitted radiation P0 = Power (intensity) of incident radiation
Speed of light
2.998 x10^8 cm s^-1
Visible spectrum
280 - 780 nm The only portion of the electromagnetic spectrum that we can see without instrumentation
Infrared absorption
3×10^12 to 3×10^14 Hz (100μm to 1μm) Vibrational spectroscopy Measures the vibrations of atoms, and determines the functional groups. Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wavenumber) The vibration can be in the form of a bend or a stretch for each bond and always results in a change in dipole moment. Energy transition 10^3 to 10^5 Jmol^-1
Visible/UV absorption
3×10^14 to 3×10^16 Hz (1000 nm to 10nm) Electronic spectroscopy Movement of valence electrons from the ground to the excited state Transition energy 10^5 to 10^7 Jmol^-1 Much less certain than IR for identification Overlapping bands can result in misleading qualitative analysis. Organic substances measured in the UV must usually be dissolved in organic solvents. The solvent may affect the spectrum due to solute-solvent interactions. A polar solvent may cause loss of fine structure. However it is desirable for quantitative analysis Aromatic compounds are good absorbers of UV radiation
X-ray absorption
3×10^16 to 3×10^18 Hz (10nm to 100pm) X-ray spectroscopy Movement of core electrons - can move from inner shell to outer shells by exciting them to a higher energy state Transition energy 10^7 to 10^9 Jmol^-1
Radio frequency absorption
3×10^6 to 3×10^8 Hz (10 m to 100 cm) NMR spectroscopy The magnetic dipoles (spin) of the nuclei of an atom allign in a magnetic field. Radio waves are able to cause a reversal in this spin causing a change in energy (10^-3 to 10^-1 J mol^-1)
Microwave absorption
3×10^8 to 3×10^10 Hz (100 cm to 1 cm) Electron Paramagnetic (Spin) Resonance (EPR/ESR) Uses radicals - unpaired electrons - with similar techniques as NMR but instead of reversing the spin of the nuclei, it reverses the spin of the electrons. An energy change of between 10^-1 to 10 Jmol^-1 in highly magnetic fields 3×10^10 to 3×10^12 Hz (1cm to 100μm) Rotational spectroscopy Microwave radiation used to measure the energy of rotational transitions of molecules in the gas phase. Only molecules with a permanent dipole can be investigated using this method. The periodic fluctuation of the dipole moment interacts with the sine wave of the radiation's electric field. Gives off sharp line spectra Energy of transitions between 10 to 10^3 Jmol^-1
Planck's constant
6.626x10^-34 J s
Grotrian Diagram
A Grotrian diagram shows the allowed electronic transitions between the energy levels of atoms. They can be used for one electron and multi electron atoms.
Raman scattering
Inelastic scattering of light in which the wavelength of scattered light is changed from that of incident light by an energy corresponding to the vibrational energy of the molecule responsible for scattering. Transfer of energy between molecule and photon, the energy change is referred to as raman shift. When the scatterred light has lost energy it is called a stoke shift, when it has gained electricty it is an antistoke shift. For a molecule to be raman active there must be able to be a change in its polarisability (change in size, shape or orientation of the electron cloud - only happens with symmetric stretching)
Quantum transition 100 μm to 1000 nm
Infrared absorption and raman scattering - Configuration change
Spectroscope
Instrument for performing spectroscopy
Atomic absorption lamp modulation
Intensity pulsed at a constant frequency Chopper or electronically Tuned detector Eliminates interferences from flame emission
Infrared light
Just below the visible light spectrum 0.78 μm - 300 μm
Ultra violet light
Just beyond the visible light spectrum. 200-380 nm
Self absorption
Lower concentration on excited atoms in the cool outer part of the flame than in the inner, hot part of the flame. The cool atoms (ground state) can absorb emissions from the hot ones and thereby decrease the observed signal. Only observed at high lamp currents Increased number of sputtered atoms
Classical Mechanics
Macroscopic bodies move in continuous range of energies. Microscopic bodies such as atoms and molecules can only absorb or emit discrete energies. Atoms can have only electronic transitions. Molecules can have electronic transitions as well as vibrational and rotational transitions (this is why atomic spectra are much simpler than molecular spectra). Rotational are the lowest energy transitions (long wavelength - microwave and far infrared), followed by vibrational (infrared to near infrared) and electronic transitions require the highest energy (visible to UV) Total energy of the transition is equal to the sum of all three transition types.
Raman spectroscopy
Method of vibrational spectroscopy that measures the vibrational energy levels of chemical bonds in a given sample. Demonstrates how an input of energy into a molecule will cause it to vibrate. Sample is irradiated with visible light which the molecules absorb and re-emit, however some of the energy is absorbed by the molecular vibrations causing a small portion to re-emit at a frequency different to that of the incident light. Part of the scattered light experiences quantised frequency changes.
Quantum transition 1 cm to 100 μm
Microwave absorption - Orientation change
Scattering
Small fraction of radiation is scattered at all angles from the original path. Two types of scattering Rayleigh = the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation Raman = Inelastic scattering of radiation. Part of the scattered light experiences quantised frequency changes
Common colours of elements in fireworks
Sodium - Yellow Barium - Green Strontium - Red Copper - Blue Titanium - White
Spectroscopy
Spectroscopy is the study of the way light (electromagnetic radiation) and matter interact. There are a number of different types of spectroscopic techniques and the basic principle shared by all is to shine a beam of a particular electromagnetic radiation on to a sample and observe how it responds to such a stimulus; allowing scientists to obtain information about the structure and properties of matter.
Maxwell-Boltzman Distribution
Statistical distribution of possible energy gas molecules within a sample may have. Minimum of 99.3% in ground state - Below a particular point on the energy axis that symbolises the activation energy.
Mean free path
The average distance traveled by a gas molecule between collisions.
Doppler broadening
The broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different Doppler shifts, the cumulative effect of which is the line broadening.
Why do objects appear coloured?
The color of an object we see is due to the wavelengths transmitted or reflected. Other wavelengths are absorbed. The more absorbed, the darker the color (the more concentrated the solution).
Wavelength (λ)
The distance of one cycle of a wave, such as two consecutive crest peaks, expressed as λ.
Absorption spectra
The distribution of wavelengths of light absorbed by a species.
Emission spectra
The distribution of wavelengths of light given off by a species in an excited state.
Excitation spectrum
The excitation spectrum corresponds to the absorption spectrum. In larger molecules, the vibrational spacings of excited states are similar to those in the ground state. So the emission spectrum may be a mirror image of the excitation spectrum.
Nebulisation in atomic absorption spectroscopy
The sample is aspirated through a capillary by the Venturi effect, using the support gas (fine mist - aerosol). A fine spray if produced, with larger droplets condensing and draining away, about 2 % gets through to the flame the rest is waste
Spectrometer
The spectrometer resolves polychromatic radiation into different wavelengths. There is: •a source of continuum radiation •a monochromator for selecting a narrow band of wavelengths (prism or diffraction grating) • a sample cell (must be transparent in the wavelength being measured) •a detector to convert radiant energy to electrical energy. Choice of detector is dependent on the wavelength of interest. These components vary for UV, visible, and IR regions.A type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation, which provides an afterglow. The delay is caused by atoms being excited to energy states that do not decay rapidly. The afterglow may last from fractions of a second to hours or even days, depending on the type of material, temperature, and other factors
As microwaves are relatively low energy electromagnetic radiation, how can they heat food so effectively?
The waves at a specifically set frequency agitate water molecules in food. As these water molecules get increasingly agitated they begin to vibrate at the atomic level and generate heat. This heat is what actually cooks food
Why are there two wavelengths for the one transition for both Neon and Sodium's resonance transitions?
There is more than one wavelength for the transition because P orbitals are split into doublets with slightly different energy levels as a result of spin-orbital coupling. The decision of which wavelength to use in spectroscopy depends on the level of analytical sensitivity needed.
Resonance transitions
Transitions between an excited electronic state and the ground state. Appear as resonance lines on an atomic emission spectra.
