UV-Vis

Question 1

what’s the used of UV-vis spectroscopy?

Answer 1

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

Question 2

What are the main two properties of an absorbance peak

Answer 2

Absorption wavelength and Absorption intensity

Question 3

How is the extinction coefficient defind?

Answer 3

the value of k for a 1 % sample concentration of 1 cm thickness

Question 4

Which electronic transition state are possible?

Answer 4

pi-pi, n-p, n-delta and delta-delta

Question 5

pi-pi*

Answer 5

have the hightes probability of transition, due to their overlapping

Question 6

n-pi*

Answer 6

the n-orbital do not overlap well with p orbital, so the probaility of this excitation is small

Question 7

different between the energy in pi and delta

Answer 7

delta-delta require higher engery than pi-pi*

Question 8

why are functional groups containing multiple bonds capable of absorbing radiation above 200 nm (UV)

Answer 8

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

Question 9

what have to happend for excitation to occur

Answer 9

the energy of the light photon must match that of the abnd gap

Question 10

relation between the amount of energy and the wavelength

Answer 10

higher energy–> short wavelength and low energy –> long wavlength
the amount of energy os proportional to the wavlength of the light

Question 11

set up for UV

Answer 11

Lamp (source), monochromator, sample area and detector

Question 12

ideal light source

Answer 12

yielding of a constant intensity over all wavelengths with low noise and long term stability

Question 13

example of light source

Answer 13

Deuterium arc lamp, tugszen-halogen lapm and xenon lamp

Question 14

on what dose the dispersion by prima depends?

Answer 14

onn refraction of light which is wavelength dependent

Question 15

What are the advantages and disadvantages of decreasing monochromator slit width?

Answer 15

wider slit width gives higher sensitivity becuase higher radiation
narrow slit width gives better resolution

Question 16

the monochromator exit slit?

Answer 16

it dtermines the width of radiation (bandwidth) emitted from the monochromator

Question 17

How can the problem of low sensitivty be overcome?

Answer 17

by increasing the sensitivty of the detector

Question 18

selection of wavlength

Answer 18

Absorbance measuremnets are always carried out at fixed wavlength

Question 19

Factors that have to be consider when chosen a wavelength

Answer 19

give the highest possible sensitivity

the lowest sample concentration can be measured with fair accuracy

Question 20

what happends when a molecule absorbs light?

Answer 20

two possible transition sate
single state-electron spins opposed
triplet state- electron spins are parallel

Question 21

single state and triplet state

Answer 21

triplet state has lower energy than singelt state

Question 22

dalte-dalte transition

Answer 22

in vaccum UV,

Question 23

n-dalte

Answer 23

saturated compunds with non-bonding electrons

Question 24

n-pi, pi-pi

Answer 24

requires unsaturated functional groups (UV-Vis)

Question 25

limitations of UV-Vis

Answer 25

worse LOD than fluori

overlapping absorption bands may intefere

Question 26

what is the specific absorption

Answer 26

it is the absortion of a 1% solution (m/V) (10 g -1) measured at a thickness of 1 cm

Question 27

what if the SA is unknown

Answer 27

a calibration line is needed for the determination

Question 28

equation of the A with SA

Answer 28

A= A%-1 cm -1 xCxb