UV-Vis Flashcards
what’s the used of UV-vis spectroscopy?
it is used to detect compounds which can absorbed light at a specific wavelength in the ultraviolte (180-400 nm) and visible (400-800 nm) range
What are the main two properties of an absorbance peak
Absorption wavelength and Absorption intensity
How is the extinction coefficient defind?
the value of k for a 1 % sample concentration of 1 cm thickness
Which electronic transition state are possible?
pi-pi, n-p, n-delta and delta-delta
pi-pi*
have the hightes probability of transition, due to their overlapping
n-pi*
the n-orbital do not overlap well with p orbital, so the probaility of this excitation is small
different between the energy in pi and delta
delta-delta require higher engery than pi-pi*
why are functional groups containing multiple bonds capable of absorbing radiation above 200 nm (UV)
it is due to their n-pi* and pi-pi*, thus decrease the HOMO and LOMO gap, less energy is needed for the excitation, longer wavlength
what have to happend for excitation to occur
the energy of the light photon must match that of the abnd gap
relation between the amount of energy and the wavelength
higher energy–> short wavelength and low energy –> long wavlength
the amount of energy os proportional to the wavlength of the light
set up for UV
Lamp (source), monochromator, sample area and detector
ideal light source
yielding of a constant intensity over all wavelengths with low noise and long term stability
example of light source
Deuterium arc lamp, tugszen-halogen lapm and xenon lamp
on what dose the dispersion by prima depends?
onn refraction of light which is wavelength dependent
What are the advantages and disadvantages of decreasing monochromator slit width?
wider slit width gives higher sensitivity becuase higher radiation
narrow slit width gives better resolution
the monochromator exit slit?
it dtermines the width of radiation (bandwidth) emitted from the monochromator
How can the problem of low sensitivty be overcome?
by increasing the sensitivty of the detector
selection of wavlength
Absorbance measuremnets are always carried out at fixed wavlength
Factors that have to be consider when chosen a wavelength
give the highest possible sensitivity
the lowest sample concentration can be measured with fair accuracy
what happends when a molecule absorbs light?
two possible transition sate
single state-electron spins opposed
triplet state- electron spins are parallel
single state and triplet state
triplet state has lower energy than singelt state
dalte-dalte transition
in vaccum UV,
n-dalte
saturated compunds with non-bonding electrons
n-pi, pi-pi
requires unsaturated functional groups (UV-Vis)
limitations of UV-Vis
worse LOD than fluori
overlapping absorption bands may intefere
what is the specific absorption
it is the absortion of a 1% solution (m/V) (10 g -1) measured at a thickness of 1 cm
what if the SA is unknown
a calibration line is needed for the determination
equation of the A with SA
A= A%-1 cm -1 xCxb
