Spectroscopy Flashcards
A sample in a 1.0-cm cell transmits 80% light at a certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, what is its concentration?
c = 0.050 g/L
absorption FORMULA
A = -log T = log 1/T = log P0/P =abc
Transmittance Formula
T = P0/P
PERCENTAGE TRANSMITTANCE
%T = P0/P x 100
amount of monochromatic radiation absorbed by a sample
Bouguer–Lambert-Beer’s law/
Beer’s Law
study of the interaction of electromagnetic radiation in
all its forms with matter
spectroscopy
Latin “spectron”
ghost/ spirit
σκοπειν
to see
Method involves in the interaction of light
excitation and detection
TRUE OR FALSE
Electromagnetic radiation moves through a medium other than a vacuum with a velocity, v, less than that of the speed of light in a vacuum
TRUE
Change in intensity I of incident light
ABSORPTION
Excitation induces emission of light from the sample
EMISSION
emission from excited electronic singlet states
FLUORESCENCE
emission from excited electronic triplet states
PHOSPHORESCENCE
light scattering involving vibrational transition
RAMAN SCATTERING
Two types of spectrometers
DISPERSIVE
FOURIER TRANSFORM
TRUE OR FALSE
A source of electromagnetic radiation must provide an output that is both intense and stable in the desired region of the electromagnetic spectrum.
TRUE
A source that emits radiation over a wide range of wavelengths,
with a relatively smooth variation in intensity as a function of
wavelength
continuum
emit radiation at a few selected, narrow wavelength ranges
LINE SOURCES
TRUE OR FALSE
The ideal wavelength selector has a high throughput of radiation and a narrow effective bandwidth
TRUE
Why is high throughput desirable?
because more photons pass through the wavelength selector, giving a stronger signal with less background noise
provides a higher resolution, with spectral features separated by more than twice the effective bandwidth being resolved
narrow effective bandwidth
The simplest method for isolating a narrow band of radiation
absorption or inference filter
work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum
ABSORPTION FILTER
piece of colored glass
use constructive and destructive interference to isolate a narrow range of wavelengths
INTERFERENCE FILTERS
more expensive, but have narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at least 40%.
INTERFERENCE FILTERS
LIMITATIONS OF USING FILTERS
- Do not allow for a continuous selection of wavelength
- Available for only selected nominal ranges of wavelengths
have a diffraction grating to disperse the radiation into its component wavelengths
MONOCHROMATORS
simultaneously allows source radiation of all
wavelengths to reach the detector
INTERFEROMETERS
The signal at the detector shows intensity as a function of the moving mirror’s position, expressed in units of distance or time
interferogram
time domain spectrum is also called
interferogram
convert a signal consisting of photons into an easily measured electrical signal
Modern detectors
used for optical
spectroscopy
Photon Transducers
contain a photosensitive surface that absorbs radiation in the ultraviolet, visible, and near infrared (IR), producing an electric current proportional to the number of photons reaching the transducer
. Phototubes and photomultipliers
used to calibrate the detector’s response, to amplify the signal from the detector, to remove noise by filtering, or to mathematically transform the signal
Signal Processors
These have very high excitation energies and do not contribute to absorption in the visible or UV regions
Closed shell electrons that are not involved in bonding
These also possess too high an excitation energy to contribute to absorption of visible or UV radiation
Covalent singlebond electrons
These are less tightly held than σ electrons
and can be excited by visible orUVradiation
Paired nonbonding outershell electrons
These are the most readily excited and are responsible for the majority of visible and UV light absorption.
Electrons in π (pi) orbitals
formed by head to head overlap of atomic orbitals
sigma bond
strong and have a high bond energies
sigma bond
can exist independently
sigma bond
found in single, double and triple bonds
sigma bond
the overlapping orbitals can be pure or hybrid
sigma bond
formed by the side-to-side overlap of atomic orbitals
pi bond
relatively weak
pi bond
must exist along the sigma bond
pi bond
found in double and triple bonds
pi bond
the overlapping bonds must be unhybridized
pi bond
The absorbing groups in a molecule
chromophores
A molecule
containing a chromophore
chromogen
does not itself absorb radiation, but, if present in a molecule, it can enhance the absorption by a chromophore and/or shift the wavelength of absorption when attached to the chromophore
auxochrome
hydroxyl groups, amino groups, and halogens
absorption maximum shifted to longer wavelength
bathochromic shift
absorption maximum shifted to shorter wavelength
hypsochromic shift
an increase in molar absorptivity
hyperchromism
a decrease in molar
absorptivity
hypochromism
The amount of monochromatic radiation absorbed by a sample is described by
BEER’S LAW
The relationship between the incident radiation and the transmitted radiation
TRANSMITTANCE
The product of the absorptivity and the molecular weight of the absorbing species is called
molar absorptivity
A = εbc
