MAAC: Analytical Spectrophometry 4 Flashcards
What does atomic spectroscopy give information on?
What are uses?
Only get information on atoms in atomic spectroscopy not molecules
- Quantification of alkali metals in salts, infusion and dialysis solutions
- Determination of metallic impurities for preparing these solutions
Drug research, development and production is dependent on elemental analysis:
- Testing of individual ingredients
- Continuing through production to final quality control
- Impurities can affect drug efficacy and metabolism
What are the 2 techniques available?
Atomic absrobance spectroscopy (AA)
Flame photometry, (FP) and ICP-OES
Describe the basic mechanism of Atomic emission spectroscopy
Atomic Emission:
- Electron is promotes from ground state to excitated state due to excitation caused by thermal energy (e.g. heat/ flame)
- After cooling, the electron falls back down to ground state releasing the excess energy in the form of a photon of light.
- This emission is measured
What is the basic mechanism that occurs in atomic absorption?
- Electron is promoted from ground state to excited state using a specific wavelength of light
- The energy absorbed onn excitation is measured
What electrons are of importance?
What is different about E levels in this type of spectroscopy and fluorecence spectroscopy?
- OUTER ELECTRONS
- No vibrational E levels
What are the equations equal to Delta Energy.
What is delta E also called?
Does the electron always have to move up to the next E level?
What transitions are electrons not able to do?
When does ionisation occur?
Transisitions between similar states are generally not allowed i.e s-s, p-p, d-d
Energies of over 5.2 eV causes the electron to be removed all together from the atom - ionisation
Describe the spectral line widths
i.e compare this to molecular spectrosopcy. What is this advantages of atomic vs molecular?
Measure peak width at half max intensity.
- Describes the narrow spread of wavelengths over which absorption and emission is observed for a given electronic transition
- Less than 0.01 nm
- Contrast to electron transitions in molecules which are many tens of nm wide
- This narrowness is a distinct advantage as it reduces spectral overlap and makes atomic spectroscopy very specific
Describe how a flame spectroscometry set up works
The flame causes thermal excitation of the sample and the ground state electon is promoted to excited energy level. A photon of light of specific wavelength is emitted as electoron returns to ground state
- Sample needs to be aqueous and is placed in beaker connected by a capillary tube to nebuliser
- Nebuliser and mixing chamber convert sample to fine spray. Any large sample leaves the chamber as waste
- The sample travels to the flame where and causes excitation of the sample and atomic emission given off
- The light passes through a filter then lens which directs the light onto a photodetector which measures and displays the reading
What are the characterisitics of the flame? and use?
- •Flame used in atomic emission is used to volatilise the solution containing the metals
- •Typically use natural gas/compressed air for flame at 2000 oC
- •Higher temperatures required for other metals such as Mg
- Use air/acetylene flame – 2500 oC
Describe wavelength detection in terms of how it is done and why
- In atomic emission there are many different emission lines produced by each metal atom
- Must select one wavelength to select the metals for analysis
- Removes any interfering emission from the flame or other components
- Selected using either a filter or a monochromator
- High end instruments use monochromators
- Require very high specifications due to narrow bands in atomic emission
- Cheaper instruments use filters set for each alkali metal
- High end instruments use monochromators
Describe the use of a detector in flame spectrocopy
•Flame photometers are very simple, cheap instruments
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•Use photosensitive detectors similar to PMT but not necessarily as sensitive
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•Light emitted from sample is detected and converted into an electrical signal which is then displayed
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
- •An ICP torch is a device that produces a plasma
- A fireball of atoms, ions and electrons interacting at very high energies with temperatures up to 10,000 K
- •Therefore it makes for a very efficient atomisation source
- •Exposing a sample to high temperature plasma converts a very large proportion of its constituent atoms to an excited state
- •ICP-OES can identify and determine simultaneously up to 40 elements with detection limits of parts per billion
Describe the basic process of the ICP
A – Ar gas swirls through
B – RF energy applied to induction coil
C – Spark causes some Ar to ionise and form free e-
D – e- accelerated by RF fields, cause collisions and further ionisation so plasma forms
E – flow from the nebuliser, carrying the sample, drives a channel through the plasma
Difference between flame and ICP
- Lower temp, simpler optics and detector techniques, less excitation therefore emission. Build in filters means less element can be analysed and needs to be done one at time
What desgins of spectorphotomers is there?
•There are 2 main designs for detection commercially available
–Simultaneous
–Sequential
Simultaneous - multiple photodetecrots at precise angles/positinos to detect different wavelengths of light
Sequential - monochromator used to split light into compondent wavelengths and wavelength to analyse chosen by moving position of difraction grating
Advantages of ICP-OES?
•Advantages of ICP-OES
–Offers simultaneous analysis capability
–High T of plasma gives large choice of emission lines for wide range of elements
–Good detection limits
–Long linear calibration ranges
–Good precision (1 or 2 %)
What are sources of interference examples?
1. Ionisation:
- At high temperatures atoms such as K can lose an electron and therefore reduce the observed emission from the sample
- Must add more readily ionised element to the sample to compensate. This will produce electrons and drive the equilibrium to the left, supressing ion formation of metal
- E.g. strontium chloride solution is added to suppress ionisation of K in the BP assay of effervescent KCl tablets
2. Spectral:
- Two overlapping spectral lines
- •Must select a different non-overlapping emission line
- •There are more than 50,000 ICP-OES spectral lines documented
3. Chemical:
- Presence of low volatility compounds not readily atomised
- Sulphate and phosphate form involatile salts with metals and reduce sample reading
- Remove by adding lanthanum chloride which precipitates them out and replaces them with chloride anions
- press ionisation of K in the BP assay of effervescent KCl tablets
4. Calibration graph non-linear at high concentrations.
- Due to reabsorption of emitted light by ground state atoms / ions in flame.
- Excited electrons of falling realse photon of light. This can be reasborbed by ground electrons meaning emission never reaches detector and is not measured.
- To avoid this use non-resonance light - this means not using transisition involving ground state. Not all electrons fall to ground state. These electrons do not self-absorb therefore emission is detected
5. Also ‘matrix effects’ i.e. effects due to excipients (Physical effects)
- Density, surface tension and viscosity all affect rates of aspiration and nebulisation
- Alters the rate of aspiration relative to the standard solutions thereby producing different results
- e.g. sugar in syrups
How can the matrix effects be resolved?
Standard addition
What is standard addition and how is it carried out?
- Increasing volumes of standard solution added to sample (spiked).
- Amount of drug in sample found by extrapolation of graph to x axis.
- Increases accuracy and precision of assay.
- If interference is present which reduces sample absorbance then low concentration will result if use a normal calibration graph
- If we add the standard in incremental amounts to our sample and plot the readings against concentration of added standard – standard addition graph
- Increases accuracy and precision of assay.
How do you determine if interference is present in standard additions?
Gradient of graph should be same as normal calibarion
