Spectroscopy

Question 1

Define spectroscopy

Answer 1

Study of interaction of light with matter. Use of absorption emission or scattering of electromagnetic radiation by atoms or molecules to quantitatively or qualitatively study the atoms or molecules or to study physical processes

Question 2

Define Infrared spectroscopy

Answer 2

Study of interaction of infrared light with matter

Question 3

Advantage to IR and Raman

Answer 3

Non-destructive, little sample preparation and they complement each other

Question 4

What is the main idea behind IR and Raman?

Answer 4

IR looks at light that is absorbed, transmitted and reflected whilst Raman looks at scattered light

Question 5

Define vibration

Answer 5

Negatively charged electrons in one atom are pulled toward the positively charged nucleus of another and vice versa. The electrons in one atom repel those in the other as well as the protons in the nuclei. Constant push and pull gives vibrations

Question 6

What are the types of molecular vibration?

Answer 6

Symmetric, Anti-symmetric, wagging, scissoring, rocking and twisting/torsion

Question 7

What is vibrational spectroscopy?

Answer 7

Measuring the energy required for bonds to vibrate which is region of interest

Question 8

What are the factors influencing vibrational frequency?

Answer 8

Mass of attached atom, strength of chemical bond, hybridisation and resonance

Question 9

What is the total IR range split into?

Answer 9

Near IR (700nm-2.5microm), mid IR (2.5-25 microm) and far IR (25microm-1mm)

Question 10

What is special about mid range IR?

Answer 10

Considered to be chemical fingerprint region

Question 11

How does IR work?

Answer 11

Molecule absorbs photon of IR radiation making it vibrate giving individual vibration frequency and different kinds of vibrations. When energy is identical to energy of vibration changing vibrational energy from ground state to excited state. 1 photon process. Show what molecules are present and at what concentrations. Can be measured in absorbance and % transmission

Question 12

What bonds can IR not measure?

Answer 12

Bonds that exist in pure metal and non-metal elements (lead, arsenic) or diatomic gases (chlorine, oxygen)

Question 13

What conditions are needed to be IR active?

Answer 13

Frequency of radiation must match frequency of bond vibration, bond undergoing vibration must be polar (dipole moment) and greater the polarity, the stronger the absorption.

Question 14

What is FTIR and how does it work?

Answer 14

Fourier Transform Infrared. Uses beam splitter to transmit some of light incident upon it and reflect some light incident. Light transmitted by beam splitter travels toward fixed mirror and light reflected travels toward moving mirror. They are then reflected by mirrors and travel back to beam splitter where recombined into single beam, leaves interferometer, interacts with sample and strikes detector

Question 15

What is the Michelson Interferometer?

Answer 15

Heart of FTIR. Splits single light into 2 and then rejoins. Allows to pass through sample and reference

Question 16

What are advantages of FTIR?

Answer 16

Measure spectra with high signal to noise ratio, mulitplex advantage, wavenumber precision

Question 17

What are advantages of IR and FTIR?

Answer 17

Almost universal, spectra information rich, fast and easy, inexpensive, sensitive

Question 18

What are disadvantages of IR and FTIR?

Answer 18

Cant detect some molecules, mixtures or water

Question 19

How does Raman work?

Answer 19

Measurement of wavelength and intensity of inelastically scattered light. 2 photon process. Occurs at wavelengths that are shifted from the incident light by energies of molecular vibrations

Question 20

What is the difference between incident and scattered light?

Answer 20

Different energies and amount of vibrations

Question 21

What does Raman need to work?

Answer 21

Polarisation (must be able to distort electron cloud), induced dipole moment and band intensity

Question 22

What is the application of IR and Raman?

Answer 22

Identification of drugs, small peptides, pollutants and food contaminants. Non destructive detection of bodily fluids at scene and trace evidence such as fibres, pain and plastic

Question 23

What gives a strong IR and weak Raman?

Answer 23

Dipole moment

Question 24

What gives a strong Raman and weak IR?

Answer 24

Polarisability