Spectroscopy

Question 1

What is a reference solution?

Answer 1

This is a solution with no solutes in it and is purely made out of solvent.

Question 2

What must be done to a sample solution before measuring absorbance and cocnentration?

Answer 2

The sample must be homogenized so that the composition will be uniform throughout and clarified so that the dust and debris located in it will not absorb UV light and interference absorbance readings.

Question 3

What characteristics is a sample holder or cuvette required to have?

Answer 3

A cuvette is required to not be susceptible to absorbing light at the same wavelength range as the sample. It should have path length of about 1-100mm and width of 4mm.

Question 4

What are two reasons why you should choose the wavelength range of highest absorption for spectroscopy measurement?

Answer 4

This range of light is what solutes are more most capable of absorbing or most sensitive and will adhere to beer’s law the most. This means that the absorbance will increase consistantly as the number solutes in the solution increase.

Question 5

Compare and contrast linear calibration curves. Which do you want and why?

Answer 5

Calibrated: Linear curve between absorption and concentration
Non Calibrated: Logarithmic curve, as concentration increases, chemical changes affected by concentration prevent any change in absorbance from occuring.

Question 6

Components of a UV spectrometer:

Answer 6

1. Light source
2. Monochromoter: Has polychromatic light enter from an entrance source, hit a convave mirror, reach a reflective grading unit and split into multiple waves which reflect onto a second concave mirror and then reflect into an exit source which selects the desired wavelength out of the many based on the size of it’s slit.
3. Radiation detector- can come in three forms, phototube, photomultiplier and photodiode detector. It works by absorbing the radiation coming from photons and converting it into an electrical signal. As the Radiant power increases the signal does as well
4. Sample Cuvette
5. Readout device: Converts electical signal into readable data analysis

Question 7

E=hv

Answer 7

Energy of a photon is directly proportional to frequency. As the frequency of a photons light wave increases the energy it contains increases.

Question 8

Brightness, radiant power or power of light

Answer 8

This is dependent upon:

1) the number of photons released per unit of time
2) energy of the wave

Question 9

Interference

Answer 9

When two waves come into contact with one another and their amplitudes add up to produce one large wave. There are two types of interferences, constructive interferences and destructive interferences. Constructive interferences are when waves in phase combine to produce one larger wave and destructive interferences are when two waves out of phase cancel each other out.

Question 10

Beer’s Law

Answer 10

A=abc,
A= the amount of light a solute has absorbed
b=pathlength
c=Concentration of solute
Absorbance is directly proportional to concentration, and in a properly calibrated linear curve, the absorbance will increase as concentration of solutes increase.

Question 11

Absorbtion

Answer 11

Once an atom or molecule absorbs energy from a photon when light is shined on it, the atom or molecule jumps from a ground state to higher energy state. Atoms increase to higher electronic energy states, while molecules increase higher electronic states and rotational and vibrational energy states for the bonds between atoms.

Question 12

Emission

Answer 12

Once atoms and molecules move from a higher energy state to a lower energy state they will release energy one part in the form of radiation and another part in the form of heat phosphorescence or flourescence (do all molecules release flourescent light when relaxed?).

Question 13

What is the concentration of compound Y in an unknown solution if the solution has an absorbance of 0.846 in a glass cuvette with a pathlength of 0.2 cm? The absorptivity of compound Y is 54.2 cm−1 (mg/mL)−1 under the conditions used for the absorption measurement.

Answer 13

0.078 mg/mL
This problem illustrates (1) that concentration need not be expressed in units of molarity and (2) that the pathlength of the cuvette must be considered when applying Beer’s law. In the present problem the analyte concentration is given in mg/mL: thus, the absorptivity must be in analogous units:
Apply:𝑐=𝐴/𝜀𝑏
A
p
p
l
y
\:
c
=
A
/
ε
b
where:
𝐴=0.846𝜀=54.2 cm−1(mg/mL)−1𝑏=0.2cmAnswer:0.078 mg/mL