Atomic Spectroscopy

Question 1

What is atomic spec?

Answer 1

• Spectral emission occurs when an elctron transitions from a higher energy state to a lower energy state
• The energy of an emitted photon corresponds to the energy difference between the two states therefore the emission of each atom is going to be characteristic

Question 2

Four principles to remember about atomic spec

Answer 2

1. The energy of each state is fixed
2. The energy difference between them is fixed
3. The transition will always produce a photon with the same energy
4. Important to remember that the energy is limited by the levels

Question 3

What are the 3 main principles of atomic emission spectrophotometry? (What elements can be used)

Answer 3

1. Based on electron transitions
2. Atoms are thermally excited so they emit light when they move from a higher energy level to a lower energy level and the emitted radiation is measured
3. Only a limited number of elements are sufficiently excited by the thermal energy for AES measurements - limited primarily to the analysis of alkaline metals and some alkaline earth metals

Question 4

How does atomic emission spectrophotometry work? (3 points)

Answer 4

• e- in the outer shell gain energy and are elevated to a higher electronic state
• The excess energy is lost by transition to the stable electronic ground state with emission of energy
• Emission of energy is associated with different transitions

Question 5

What are the three main components of the instrumentation used for AES?

Answer 5

1. Flame or plasma source
2. Monochromator/Filter
3. Detector

Question 6

Instrumentation for AES: Flame detector or plasma source

Answer 6

• (aq) sample containing metal is volatilised (atomised) and excited using natural gas or compressed air at 2000°C
• Higher temps (2500°C) may be achieved using air/acetylene mixtures e.g. required for AES of Mg

Question 7

Instrumentation for AES: Monochromator/Filter

Answer 7

• Radiation emitted by excited atoms can be broad (due to interference by sample components and the flame)
• Set to monitor the wavelength of interest and filter out other wavelengths

Question 8

Instrumentation for AES: Detector

Answer 8

• Used to measure the intensity of emitted radiation (related to conc) - more atoms, the higher the intensity of the bands it’s going to be
• Photosensitive cell/photomultiplier

Question 9

What factors interfere with AES?

Answer 9

1. Ionisation
2. Sample viscosity
3. Anionic interference

Question 10

ABS Interfering factors: Ionisation

Answer 10

• Some atoms may lose an e- at high temps
• This reduces observed emission
• Other readily ionised elements in the sample may affect degree of ionisation
• Solution = add excess of readily ionised element e.g. K as an ionisation depressant

Question 11

Interfering factors: Sample viscosity

Answer 11

• Organic substances in a sample can increase or decrease the viscosity which subsequently affects the rate of sample transfer to the flame
• This affects the reading (if viscosity increases, false low readings occur)

Question 12

Interfering factors: Anionic interference

Answer 12

• Anions form in-volatile salts (-ve charged) with metal ions and can reduce the sample reading. Examples include = sulfate and phosphate ions
• Solution = add lanthanum chloride to precipitate anions

Question 13

Applications in pharmaceutical analysis: AES

Answer 13

1. Analysis of trace metals in sample matrices
2. Multi elemental analysis
3. Quantification of alkali metals in infusion and dialysis solutions
4. Determination of metallic impurities in raw materials and formulations

Common elements = Ca, Ba, Na, Li and Know

Question 14

Quantification of AES in pharmaceutical analysis

Answer 14

• Involves construction of calibration curve
• Flame photometry readings are proportional to the conc of metal at a particular wavelength
• The conc can be determined from emission reading using the equation for calibration line

Question 15

Principles of atomic absorption spectrophotometry

Answer 15

• Many heavier atoms e.g. Zn and Al cannot be thermally excited at ground state to excited state due to large amount of energy required.
• Instead of using thermal energy to excite atoms we use radiation at a defined wavelength from a hollow cathode tube which is coated with the metal being analysed - passed through flame
• The atoms become volaltised in the flame
• The volatile atoms absorb radiation with energy corresponding to the difference between he ground and excited states
• The amount absorbed relates directly to the conc of metal atoms in the flame

Question 16

What is the similarities and differences between AES and AAS in terms of principles?

Answer 16

Similar to principles of AES - based on electronic transitions

HOWEVER

There is a greater number of atoms in the ground state available for excitation than the number that can become excited and emit energy in AES. Therefore AAS is more sensitive than AES and a greater spectrum of metals may be analysed.

Question 17

What are the 4 main parts of instrumentation involved in AAS?

Answer 17

1. Flame
2. Light source
3. Monochromator
4. Detector

Question 18

AAS Instrumentation: Flame

Answer 18

• Used to volatise the sample
• Air/acetylene at 2500°C
• Nitrous oxide/acetylene up to 3000°C - required to volatilise salts of Al or Ca
  ATOMS ARE NOT EXCITED BY THE FLAME UNLIKE AES

Question 19

AAS Instrumentation: Light source

Answer 19

• A hollow cathode lamp used to excite volatilised sample
• Lamp is coated with the same metal as the element to be analysed in the sample
• Excitation of the atoms in the element in the coating produces a narrow band of radiation which can be efficiently absorbed by the volatilised atoms in the flame
• The volatilised atoms , which are mainly in their ground state, will absorb radiation with an energy corresponding to the difference between their ground and excited states
• Different lamp required for each element
• Only one element can be analysed at a time

Question 20

AAS Instrumentation: Monochromator

Answer 20

• Set to monitor the wavelength of interest
• Narrows the width of the band of radiation being examined (filters out other wavelengths)
• This cuts out interference by radiation from other sources e.g. flame or other elements in the sample

Question 21

AAS Instrumentation: Detector

Answer 21

• Unabsorbed radiation passes through the detector
• Photosensitive cell/Photomultiplier

Therefore, the more absorption , the more atoms that are in the sample, so higher conc —> the lower the radiation we recover

Question 22

What factors interfere with AAS?

Answer 22

1. Ionisation
2. Sample viscosity

Question 23

AAS Interfering factors: Ionisation

Answer 23

• More common in hot flames
• Excess energy in flames causes excitation and ionisation of atoms resulting in depletion of ground state atoms
• Solution = add excess of readily ionised element e.g. K as ionisation depressant

Question 24

AAS Interfering factors: Sample viscosity

Answer 24

viscosity
- Organic substances in the sample can increase or decrease the viscosity which subsequently affects the rate of sample transfer into the flame
- This affects the reading - if the viscosity increases = false low readings occur

Question 25

What does atomic spectrophotometry require careful verification of?

Answer 25

1. Interference possibilities 2. Calibration 3. Linear dynamic range Unfortunately, deviations from linearity usually occur.

Question 26

How many standards should be prepared when doing a calibration curve?

Answer 26

5 standards should be prepared

Question 27

If sample conc Is too high, to permit accurate analysis:

Answer 27

1. Dilute the sample 2. Using an alternative wavelength having a lower absorptivity 3. Reducing the path length by rotating the burner head

Question 28

Applications of AAS in pharmaceutical analysis

Answer 28

Quantification of metals in: - infusion and dialysis solutions - tissue samples including blood, bone marrow, urine, hair, and nails E.g. measurement of Na and K electrolytes in plasma Determination of metal impurities in pharmaceutical preparations