Spectroscopy Flashcards
study of the interaction of electromagnetic radiation with matter. It explains how materials absorb, transmit or emit radiation
spectroscopy
give qualitative and quantitative information about the system studied
spectroscopic methods
form of energy that travels through space at an extremely high velocity
electromagnetic radiation
in electromagnetic fields, the waves consist of ________ electric and magnetic fields
perpendicularly oscillating
fails to account for phenomena associated with the absorption and emission of radiant energy. For these processes, electromagnetic radiation can be treated as discrete packets of energy or particles called photons or quanta
the wave model
linear distance between sucessive maxima or minima of a wave
wavelength
is a vector quantity that provides a measure of the electric or magnetic field strength at a maximum in the wave
amplitude (of an electromagnetic wave)
the number of oscillations that occur in one second
frequency
unit of frequency of an EMR
Hertz
the frequency of a beam of EMR does not ____ as it passes through different media
change
the number of waves per centimeter and is equal to 1/wavelength
wavenumber
In a vacuum, light travels at its ____ velocity and this velocity is giving the special symbol c
maximum
Speed of light in a vacuum
2.99792 x 10^8
Speed of light formula
Frequency x wavelength
in a medium containing matter, light travels at a veocity ________ C because of interaction between the electromagnetic field and electrons in the atoms or molecules of the medium. Radiation velocity and wavelength both decrease as the radiation passes from a vacuum or from air to a denser medium. Frequency remains constant
less than
a particle of electromagnetic radiation having zero mass and an energy of hV.
photon
photon energy equation
Planck’s constant x velocity of light in a vacuum x wavenumber
E = hcν
Planck’s constant
6.63x10^-34 Js
types of interaction of electromagnetic radiation with matter
- absorption
- emission
- reflection
- scattering
- diffraction
- interference
process where a molecule absorbs EMR that excites electrons. This absorption of energy causes an electronic transition from a groung state (non-excited) to an excited state
absorption
transition from a higher to lower state which releases EMR from the atom or molecule
emission
electronic transitions involving pi, sigma and non bonded electrons
organic molecules
electronic transitions involving d and f electrons
inorganic compounfs
electronic transitions involving change transfer electrons
inorganic compounds
Uses ultraviolet and visible radiation.
Absorbed EMR induces electronic transitions (absorption peaks)
Identifies functional groups
Access to molecular structure and oxidation state
UV-visible spectroscopy
Pros of UV visible spectroscopy
- economic
- non-invasive (fibre optics allowed)
- versatile (solid, liquid gas)
- extremely sensitive (concentration
Cons of UV-visible spectroscopy
- No atomic resolution
- Broad signals (resolution)
- Time resolution (S/N)
An electron in a bonding s orbital is excited to the corresponding antibonding orbital. The energy required is large. For example, methane shows an absorbance maximum at 125nm (which has only C-H bonds and can only undergo transitions of this type). Absorption maxima due to these transmitions not being seen in typical UV-VIS spectra (200-700 nm)
σ → σ*
Electronc transitions in organic molecules
Absorption of radiation by ________ in the wavelength region between 180 and 780 results from interactions between photons and electrons that either participate directly in bond formation or that are localised about such atoms as oxygen, sulphur, nitrogen and the halogens
organic molecules
consists of an electron donor group bonded to an electron acceptor
charge-transfer complex
when a ____ absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor. The excited state is thus the product of a kind of internal oxidation/reduction process
charge-transfer complex
radiant power in watts incident on sample
incident radiant power P0 (incident intensity)
radiant power transmitted by sample
transmitted radiant power P
Log (incident radiant power/transmitted radiant power)
Log(P0/P) or Log T
Absorbance A
transmitted radiant poiwer/incident radiant power
P/P0
Transmittance T
Length over which attenuation occurs
path length of sample, b
As light traverses a medium containing an absorbing analyte, the intensity ____ as the analyte becomes excited
decreases
As light traverses a medium containing an absorbing analyte, the intensity ____ as the analyte becomes excited
decreases
Absorbance is directly proportional to the concentration of the absorbing species c, to the path length b of the absorbing medium and a is a proportionality constant called absorptivity
A = Log (P0/P) =abc
Beer’s Law
Absorbance is directly proportional to the concentration of the absorbing species c, to the path length b of the absorbing medium and a is a proportionality constant called absorptivity
A = Log (P0/P) =abc
Beer’s Law
equal to the sum of the absorbances of the individual components in the solution
total absorbances
Limitations of Beer-Lambert’s Law
- Real deviations
- Chemical deviations
- Instrumental deviations
These are fundamental deviations due to the limitations of the law itself
Beer-Lambert’s Law
Real deviations
Beer-Lambert’s Law
These are deviations observed due to specific chemical species of the sample which is being analysed
Beer-Lambert’s Law
Chemical deviations
Beer-Lambert’s Law
These are deviations which occur due to how the absorbance measurements are made
Beer-Lambert’s Law
Instrumental deviations
Beer-Lambert’s Law
Concentration effects of Beer-Lambert’s Law
A higher concentrations, the individual particles of the analyte no longer behave independently of one another. The resulting interaction between particles of analyte may change the value of Ɛ
Beer’s law strictly applies only when measurements are made with ___________
monochromatic source radiation
If the cells holding the analyte and blank solutions are not of equal path length and equivalent in optical characteristics, an intercept will occur in the calibration curve and the equation will be
A = Ɛbc +k
why we need a reference or standard measurement
To compensate for reflection and scattering which causes attenuation of the beam. The analyte solution is compared with an identical cell containing only the solvent or a reagent blank
the attenuation of the course radiation at the selected wavelength is measured
absorption measurements
the source excites the analyte and causes the emission of characteristic radiation, which is usually measured perpendicular to the incident source beam
fluorescence measurements
To be suitable for spectroscopic studies, a source must generate a beam of radiation that is ________ for easy detection and measurement
sufficiently powerful
To be suitable for spectroscopic studies, a source must generate a beam of radiation that is ____ for a reasonable period of time
stable
used to isolate the desired wave so that only the band of interest is detected and measured
monochromator/ filter
operate by blocking or absorbing all but a restricted band of radiation
filters
optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest.
Wikipedia
Interference filter
https://en.wikipedia.org/wiki/Interference_filter
This type of silter usually consists of a coloured glass plate that absorbs part of the incident ration and transmits the desired band of wavelengths
absorption filter
device that identifies, records, or indicates a change in one of the variables in its environment such as pressure, temperature or electromagnetic radiation
Detector
Converts non-electrical quantities such as light intensity, pH, mass and temperature into electrical signals that can be subsequently amplified, manipulated, and finally converted into numbers proprtional to the magnitute of the original quantity
Transducer
detector
The electrical response of the detector =
proportionality constant * radiant power of the beam
G = KP
Measures the sensitivity of the detector
proportionality constant
Photon detector which is based on the interaction of radiation with a reactive surface to produce electrons
photoemission
Photon detector which is based on the interaction of radiation with a reactive surface to promote electrions to energy states in which they can conduct electricity
photoconduction
consists of a tiny blackened surface that absorbs infrared radiation and increases in temperature as a result. The temperature rise is converted to an electrical signal that is amplified and measured
thermal detector
the number of photoelectrons ejected from the photocathode per unit time is ________ proportional to the radiant power of the beam striking the surface
directly
In a detector, when a voltage is applied across the electrodes, the emitted ________ are attracted to the positively charged wire anode. These electrons produce a photocurrent in the circuit.
photoelectrons
In a detector, when a voltage is applied across the electrodes, the emitted photoelectrons are attracted to the positively charged wire anode. These electrons produce a ________ in the circuit.
photocurrent
voltage =
current * resistance
sample containers which must have windows that are transparent in the spectral region of interest
cells/curvettes
absorption in the infrared region arises from _______
molecular vibrational transitions
provide more specific qualitative information and are called fingerprints because no other chemical species will have an indentical one
IR spectra
IR spectroscopy gives ____ data
qualitative
atoms of a molecule change their relative positions without changing the position of the molecular centre of mass
vibration
cannot vibrate or rotate but can move in x,y and z (translation)
single atom
DOF of the vibration of a single atom
3
DOF in a molecule
number of atoms * 3 DOF
number of vibrations in a linear molecule
(3 x number of atoms) - 5
number of vibrations in a nonlinear molecule
(3 x number of atoms) - 6
number of vibrations in a solid
(3 x number of atoms) - 3
Most possible vibrational modes
- symmetric stretch
- asymmetric stretch
- wagging
- twisting
- scissoring
- rocking
Bonds containing heavier atoms vibrate more ____ than lighter ones
slowly
Bonds containing ____ atoms vibrate more slowly than lighter ones
heavier
stronger bonds generally vibrate ________ than weaker bonds
faster
vibrational frequency =
1/2 pi c * √force constant/reduced mass
reduced mass =
mass 1 * mass 2/ mass 1 + mass 2
In IR spectroscopy, the ____ the atoms involved in the bond, the lower the absorption frequency, assuming a constant bond strength
heavier
In IR spectroscopy, the heavier the atoms involved in the bond, the ____ the absorption frequency, assuming a constant bond strength
lower
sensitive to homo-nuclear molecular bonds such as C-C and C=C
raman spectroscopy
vibrational mode is associated with a change in dipole movement
IR active
vibrational mode is associated with a change in polarisability and totally symmetric vibrational mode
Raman active
dipole moment
Raman spectroscopy
changes at a vertain rate per second. If this changing dipole frequency matches a frequency in the IR electromagnetic region then absoprtion of that particular IR frequency takes place
used for the qualitative and quantitative determination of more than 70 elements. Typically, these methods can detect parts per million to parts per billion amounts
atomic spectroscopic methods
spectroscopic determination of atomic species can only be performed on a _ medium
gaseous
first step in all atomic procedures, a process in which a sample is volatilised and decomposed in such a wat as to produce gas phase atoms andions
atomisation
requires a light source, an atom course a monochromator to isolate the specific wavelength of light to be measured, a detector to measure the light accurately
atomic absorption spectroscopy
before external energy is applied, atoms are in their _______
ground state
applied energy causes atoms to be momentarily in a higher energy _______
excited state
transition to or from the ground state is called a ______
resonance transition
resulting spectral line from a resonance transition
resonance line
In atomic absorption spectroscopy, ____ tells the element present in the sample
wavelength
In atomic absorption spectroscopy, ____ gives the concentration of the element
intensity
the oldest and most commonly used atomisers in atomic absorption spectroscopy
flame atomisers
thermal equilibrium is ordinarily not reached in this region, and it is therefore, seldon used for flame spectroscopy
Flame structure
primary combustion zone
This area is relatively narrow in stoichiometric hydrocarbon flames, is often rich in free atoms and is the most widely used part of the flame for spectroscopy
Flame structure
interzonal region
In the secondary reaction zone, the products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings
secondary combustion zone
it provides enhanced sensitivity because the entire sample is atomised in a short period and the average residencec time of the atoms in the optical path is a second or more
electrothermal atomisation
a few microlitres of sample are deposited in the furnance by syringe or autosampler. Next, a programmed series of heating events occur: drying, ashing and atomisation
electrothermal atomisation
spectroscopy which detects chemical elements which uses inductively coupled plasma to produce excited atoms and ions which emit electromagnetic radiation at wavelengths characteristic of a particular element
inductively coupled plasma atomic emission spectroscopy
conducting gaseous mixture containing a significant concentration of ions and electrons
plasma
type of plasma source which offers the greatest advantage in terms of sensitivity and freedom from interference
inductively coupled plasma
photoluminescense process in which atoms or molecules are excited by absorption of electromagnetic radiation. The excited species then relax to the ground state, giving up their excess energy and photons
fluorescence
luminescence which is longer lasting than fluorescence
phosphorescence
produced when a chemical reaction yields an electronically excited molecule which emits light as it returns to the ground state
chemiluminescence
chemiluminescence in a biological system
bioluminescence
electromagnetic spectroscopy method which analyses fluorescence from a sample
molecular fluorescence spectroscopy
diagam which illustrates the electronic states and often the vibrational levels of the molecule
Jablonski diagram
produced by a transition from an excited singlet state to the ground singlet state
molecular fluorescence
produced by a transition from an excited triplet state to the ground singlet state
molecular phosphorescence
