Review Basic Concepts in Spectroscopy

Question 1

Describe why electromagnetic radiation is called “wave-particle duality”.

Answer 1

It behaves as a particle with no mass traveling as a sinusoidal wave at the speed of light. It both has magnetic and electric field components that are orthogonal to each other. Therefore, ER travels as a 3D plane

Question 2

What is wavelength?

Answer 2

“space” that takes up by one cycle

Question 3

What is wavenumber?

Answer 3

The reciprocal of the wavelength: n=1/λ

Question 4

What is power, P?

Answer 4

The energy of the beam that reaches a given area per second. The flux of energy per unit time. Power is proportional to amplitude, A2.

Question 5

What is the index of refraction η?

Answer 5

Relates velocity of light in vacuum to medium (η=c/ν)

Question 6

What is the general expression of speed of light?

Answer 6

C=nλ=f(medium) λ

Question 7

What is Planck’s Quantum Theory?

Answer 7

Quantum is the smallest quantity of energy that can emitted (or absorbed) in the form of EM, where h is Planck’s constant of h=6.63x10-34 J*s.

Energy is always emitted (or absorbed) in whole number multiples of hn

Question 8

What is the photoelectric effect?

Answer 8

Albert Einstein proposed that light is composed of particles, called photons.

Each photons has an energy content of E=hv

Energy above a specific threshold can dislodge electrons from a metal surface. This phenomenon can be used to construct a photon detector.

Question 9

How can atoms and molecules store energy?

Answer 9

Atoms and molecules can store energy in very specific ways, which is defined by energy states. Since the energy photons is defined by its frequency, only photons with very specific energy can be absorbed by a given atom or molecule. If an atoms or molecule is in an excited state and then returns to a less energetic state, photons with specific frequencies will be emitted.

Question 10

What is spectrometer?

Answer 10

Measures the intensity of radiation emitted by the sample

Question 11

What is a spectrophotometer?

Answer 11

an instrument with a monochromator to select λ

Question 12

What is spectrograph?

Answer 12

An instrument with a photographic plate as a detector

Question 13

What is photometer?

Answer 13

An instrument with a filter to select wavelength range

Question 14

What is colorimeter?

Answer 14

Photometer using the human eye as a detector (visible λ).

Question 15

Spectroflurometer?

Answer 15

A Fluorometer with monochromator

Question 16

What are the major components in any spectroscopic instrument?

Answer 16

• Stable course of radiant energy
• Transparent container for folding the sample
• Device that isolates a restricted region for measurement
• Radiation detector
• Signal processor or readout

Question 17

What is the problem for using hollow cathode lamps?

Answer 17

Only a fraction of “source beam” can be absorbed by an atomic sample, so there is large background signal occurs.

Question 18

What is the electrodeless discharge lamp?

Answer 18

RF or microwave radiation used to excite metal or salt of material of interest. Intensities of these lamps are one to two orders of magnitude larger than hollow cathode lamps but performance is not as reliable.

Question 19

What are the two types of line sources for atomic absorption?

Answer 19

Hollow cathode lamps and electrodeless discharge

Question 20

What is λ band and bandwidth?

Answer 20

A narrow continuous group of λ’s. effective bandwidth is an indication of the quality of the wavelength selector. The Range of wavelengths or frequencies of an absorption or emission band, typically measured at a height equal to half of the peak height. Also , the width of radiation emerging from the exit slit of a monochromator.

Question 21

What are the types of wavelength selectors?

Answer 21

Filters: interference filters, interference wedges, and absorption filters

Monochromators: prisms, and gratings, such as echellette grating, concave, and holographic

Question 22

How does interference filter works?

Answer 22

It limits transference to narrow band. Wavelength passed depends on thickness of dielectric (t) and refractive index (n).

Question 23

What is absorption filters?

Answer 23

Generally less expensive. Normally used in visible region for inexpensive deveis. Colored glass or dye suspended in gelatin. With narrow bandwidth, there is low tranmission

Question 24

What is grating used for?

Answer 24

To achieve dispersion through constructive interference

Question 25

An Echellette Grating with 1450 blazes/mm was irradiated with a polyatomic beam at a incident angle of 48 degree to the grating normal. Calculate the wavelengths of rations that would appear at reflection angles of +20, +10, and 0 degree.

Answer 25

Question 26

Describe the function of a detector or electromagnetic radiation transducers.

Answer 26

A radiation transducer is s device that converts the radiation into a quantifiable value. Most modern transducers convert signals to an electrical signal

Question 27

What is the generic relationship between the radiant power of the radiation and the signal.

Answer 27

S=kP + kd. Note: kd is also called the dark current. Background current in absence of source. P is power

Question 28

Name the three types detectors

Answer 28

Thermal detectors
Photon detectors
Multichannel detectors

Question 29

Name the examples and functionality of thermal detectors

Answer 29

Thermoelectric detector, bolometer, and pyroelectric

Sense the change in temperature

Question 30

Name the examples of photon detector

Answer 30

Respond to incident photon arrival rates rather than to photon energies

Phototube, photomultiplier tube (PMT)

Question 31

Name the examples of multichannel detector

Answer 31

Photographic emulsions, arrays of thermal detectors

Question 32

What are the important requirements for detector

Answer 32

• High sensitivity with low noise level
• Short response time
• Long term stability to ensure quantitative response
• Produces and electron signal easily amplified

Question 33

Describe the characteristics of phototube

Answer 33

The photons strike photoemissive surface of the cathode and transfer energy to loosely bound surface electrons. The electrons escape from the surface and are collected at the anode causing current to flow.

Question 34

Describe the characteristics of photomultiplier tube (PMT)

Answer 34

An electron is emitted from a photoemissive surface. An ejected electron is accelerated by an electric field; strikes another electron active surface, causing additional emitted electrons.

Question 35

Describe the photovoltaic or barrier-layer cells. Also give range, advantages, and disadvantages.

Answer 35

• A simple rugged device containing a thin layer of semiconductor such as selenium coated with silver or gold. Photons are ejected from semiconductor and can flow to silver or gold collector electrode.
• Range: 350 to 750 nm
• Advantages: low cost and rugged, no power required
• Disadvantages: lack of sensitivity at low light levels, difficult to amplify signal, can fatigue with continuous illumination

Question 36

Describe silicon diode transducers. Include range and advantages.

Answer 36

• A silicon chip containing a reverse-biased pn junction. Sensitivity between vacuum phototube and photomultiplier.
• Range: 190 to 1100 nm
• Advantages: low cost and can be miniaturized.

Question 37

Describe multichannel photon detectors

Answer 37

• Arrays of detectors usually contained on a semiconductor chip
• Three types in common usage;
• Photodiode array (PDA)
• Charge-injection devices (CIDS)
• Charge-couple devices (CCDS)
• 2nd and 3rd point above are Charge-Transfer Devices (CTDs)

Question 38

Describe the photodiode arrays (PDAs). What are the disadvantages?

Answer 38

Disadvantage: not as sensitive as photomultiplier
Series of silicon diode transducers on a single integrated circuit.

Question 39

What is the relationship between emission power and concentration of sample?

Answer 39

S=k’C

Question 40

What is the relationship between transmittance and power and absorbance and power?

Answer 40

Question 41

What is the equation of beer’s law? Define each terms

Answer 41

Absorbance is directly proportional to the path length, b, the concentration, c, and the a proportional constant called the absorptivity, a

Question 42

Describe the function atomic absorption. Include application and instrument cost.

Answer 42

Sample solution is spayed into a flame where it is vaporized and converted to atomic atoms. Atomic atoms can absorb narrow bandwidth of light generated from a hollow cathode tube.
Application: trace analysis of metals (ppm-ppb level)
Instrument cost: $30L-$100K with accessories

Question 43

Explain what is this spectrum, function, and application

Answer 43

Some transitions are more probable than others
The nature of the allowed and favoured transitions is governed by a complex set of “selection rules”, in which some types of transitions are forbidden.
For example, singlet-singlet transitions are much more probable than singlet triplet transitions. As a result, the emission or absorption lines of these transitions are much more intense.

Question 44

Define singlet and triplet state. Which state is more favourable?

Answer 44

If the magnetic spin of electrons is the same, they are said to be in a triplet state (spins unpaired)

If spins are opposed, they are singlet state (spins pairs).

If paired, the effects of magnetic electrons and magnetic orbital spin splitting cancel and so the p-splitting effect is cancelled. However, in triplet states, all of the p, d, and f states are split

So singlet state is the most stable

Question 45

What is the p-splitting effect?

Answer 45

If the electron spin magnetic field and “orbital motion” magnetic field are the same, the energy of the state is higher than if they are different. Analogous to bringing 2 north magnetic fields together.

Question 46

What is atomic line width?

Answer 46

Narrow line width reduce the possibility of spectral overlap and thus interferences. The band width at half height is used to indicate width. The is also sometimes called the effective line width Δλ_{<span>l/2</span>}

Question 47

Name the four sources of line broadening and describe how they can affect it.

Answer 47

Uncertainty effect: broadening due to the uncertainty principle relating to the uncertainty in state lifetimes

Doppler effect: broadening due to the Doppler effect (see image). Common for sound waves
The magnitude of the Doppler shift increases with the velocity that the emitting species moving perpendicular to the detector.

Pressure effects: Broadening due to collisions of emitted or absorbing species causing small changes in the energy level of the ground state.
Electric and magnetic effects: broadening due to the Zeeman effect or in other words the splitting of states due to presence of electric (or mag. Fields).

Question 48

How does temperature affect Atomic Spectra?

Answer 48

In very hot flames only a small fraction of atoms are in excited states. Absorption and fluorescence are much less temperature dependant.
High temperature in fact increases atomization of the sample, while also enhancing Doppler Effect and increasing ionization. Although, adsorption involves more states, it also involves measuring a small difference (A=logP₀ – logP), thus trade off

Question 49

What are the complications and sources of continuum spectra (explain)

Answer 49

Molecular species produce broad absorption bands due to the close “spacing” of vibration and rotation states associated with excited states and ground states.
These broad continuum spectra can interfere (overlap) with a number of discrete atomic transitions.
Formation of molecular species can also reduce the effective gaseous atomic concentration of analyte, also resulting in reduced analyte signal. (undesirable)
Selecting appropriate characteristics signal, altering flame chemistry, and in some cases by background subtraction can adverse the effect.

Question 50

What is nebulization?

Answer 50

The formation of small droplets