Spectroscopy

Question 1

define spectroscopy

Answer 1

describes the quantitative study or electromagnetic spectra, dealing with radiation in the visible, ultraviolet, and infared regions of electromagnetic spectrum

Question 2

define electromagnetic spectrum

Answer 2

it is a radiant energy that can be described as a transverse wave.

the wave is in terms of wavelength, and frequency is one complete cycle on the wave. The wavenumber is the reciprocal of the wavelength

as wavelength increase, energy decrease

Question 3

define photons

Answer 3

they are discrete packets of energy, E = photons energy

the energy of a photon is transferred tot he electron of the atoms, and it can either be absorbed or emitted

Question 4

explain the difference between absorption and emission of a photons energy

Answer 4

absorption: increase the energy of the photon, promotes it to a higher energy level, the increase of energy is qual to the energy of the absorbed radiation, absorption spectroscopy

emission: an atom ejects a photon of energy, decrease the energy of the atom, it is denoted to a lower energy level, the decrease of energy is equal to emitted radiation, emission spectroscopy

Question 5

what are the 3 kinds of transitions a molecule can do?

Answer 5

transition: molecule is pormotes from one energy level to another
1) rotational: absorbing energy increase the rotation of the molecule around its axes, promotes to high energy level
2) vibrational: vibrate relative to each other, molecule absorbed small amount of energy to promote to higher energy level
3) electronic: electrons may be raised to higher electron energy corresponding to

electronic>vibrational>rotational (order of processes)

Question 6

what is the general schemata of a spectrophotometer?

Answer 6

1) have a lgiht source
2) goes though a monochromator (wavelength selector)
3) only that wavelength go though to the sample
4) that light is shown onto a light detector
5) creates a signal to be able to understand the absorption of the sample

Question 7

what are all the important parts of a spectrophotometer?

Answer 7

1) light source: readily detectable output of radiation over the wavelength region
2) wavelength selector: restrict the radiation being measured to a narrow band that is absorbed by the sample (should be a max wavelength for the sample)
3) sample cell: holding the sample, and must be clear so the light can pass through it, always in a corvette
4) detectors: detecting how much light has been absorbed or emitted by the sample

Question 8

what are 3 important parts of the wavelength selector?

Answer 8

1) lense/mirrors: use to focus the radiation
2) entrace and exit slits: used to restrict unwanted radiation
3) dispering element: used to separate the wavelength

Question 9

what are the different kinds of wavelength selectors?

Answer 9

-prisms: the way light will refract, light refracts because the index of reaction of a prism material is different from air
-diffraction gradient: has a lot of parallel lines to scatter the incident beam, create equal dispersion of all wavelengths
-optical filters: they absorb all the radiation except for the desired wavelength and pass radiation
-bands: output of continuous wavelengths, bandwidth is the width of the band of wavelength passed at one-half of the intensity of the nominal wavelength (half of highest wavelength = bandwidth)

Question 10

what are the 4 kinds to photon detectors:

Answer 10

1) phototubes
2) photomultiplier tubes: photo-emissive cathode and series of dynodes, ejected electrons are accelerated towards the additional electrodes (dynodes)
3) photodiodes
4) photovoltaic tubes:

Question 11

define Beer’s Law

Answer 11

amount of monochromatic radiation obsrobed by a sample
- transmittance: fraction of indicident light that passes through the sample, T = transmitted radiation, P/ incident radiation, Po
- absorbance: opposite of transmittance, not linear though, A= -log(T)

A = Ebc

Question 12

define spectrophotometric titrations

Answer 12

where one or more reactants/products absorb radaition or that an absorbing indicator is present. Spectrophotometer serves as an indicator that minotr the transmittance of the solution at a suitable wavelength

Concentration = (total/initial volume) x observed absorbance

as titrant in added to analyte, absorbance increases
- on the graph, once all the analyte is reacted, the absorbance will plateau

Question 13

define luminescences

Answer 13

there is an emission of light
- bio
- chemical: glow stick
- phosphorescent: highlights and glow in the dark stickers
- electric: alarm clock

Question 14

how is a molecule typically found?

Answer 14

at room temperature, it is found at ground state. when apired, they will have opposite spinds. when it absorbs a photon, it will get bumped up to a higher energy level.

Question 15

explain the nonradiation pathway to return to ground state?

Answer 15

vibrational relaxation: loses energy and goes down to a level with the same electronic state
internal conversion: moving from a higher level to a lower one, always happens with states in the same spin
external conversion: absorbed energy is transferred to solvent
intersystem crossing: moving from high to low level state that has a different spin

Question 16

explain the radiative pathways to return to ground state?

Answer 16

florescence: going from lowest vibrational to a lower state where they both has same spin. once the radiation is remove, it also stops (same spin, need light)
phosphorescence: going from lowest vibrational to a lower state where they both has different spin, can continue after radiation is removed (different spin, can go on after light)

can be show with a Jablonski diagram

Question 17

define florescence spectrophotometer

Answer 17

excited spectrum: constand emission wavelength is slected and intensity is measured (similar to absorption)

emission spectrum: excitating wavelength is held constant and the fluorescent intensity is measured as a function of the emission wavelength
LOOK AT PICTURE ON PG 22

Question 18

what is the Stokes shift

Answer 18

distance in nm between the max peak excitation wavelength and the maxi peak emission wavelenght

Question 19

define atomic spectroscopy

Answer 19

Using high temperautes to break the bonds of the moelcules to produce free atoms.
- sample is introduced to flame and concentration of an element is measured by atomic (AAS) or emission (no lamp required, electron going to excited states, AES) of radiation

Question 20

define atomization

Answer 20

when a molecule is broken up into atoms
- flame: replaces and acts as the sample cell, use a premix under, help stop big impurities from going into the fire to be analyzed
- furnaces: provide greater sensitivity than flame and require less sample (drying,charring, atomization, vaporization)
-plasmas: electrically neurtal, highly ionized gas, plasma goes through a torch, electrons are stripped and caught up in magnetic field that collide with other plasma, many more advantages but more expensive

Question 21

define inference

Answer 21

any effect that changes the signal when analyte concentration remain unchanged

Question 22

what are the 3 types of interference?

Answer 22

spectral: analyte signal overlap with other species in sample or with flame/furnace signals. can overcome by choosing different wavelength

chemical: any substance that decreases the extend of the atomization of the analyte, releasing agants are chemcails added to decrease this, high temperature of flame can overcome this.

ionization: ionization suppressor is added to sample to decrease ionization of analyte, use this to overcome