atomic emission Flashcards
(9 cards)
What is atomic emission spectroscopy (AES) used for in pharmaceuticals?
Quantifying alkali metals, detecting metallic impurities, and supporting drug development and quality control.
How does flame photometry work in AES?
· 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
o Use air/acetylene flame – 2500 oC
· Photometry
o Simple analytical technique derived from flame tests.
§ (Na yellow, Ca brick red, K lilac).
o Ground state atoms are thermally excited then emit light of characteristic wavelength as they return to ground state.
o Used to measure Na, K, Li, Ca
o Photometer
How does an ICP torch work in atomic emission?
o Fuel and oxidant of flame replaced by Ar
o Powerful radio-frequency (rf) electrical current (0.5-3.0 kW; 15-50 MHz) applied to Cu load coil (induction coil)
o The rf current has an associated magnetic field with lines of force passing along the axis of a quartz tube placed inside the coil
o A spark is applied which ionises a few Ar atoms
o The e- formed are rapidly accelerated to & fro in circular orbits by the rapidly alternating electric field
o Collisions cause gas to heat up to high T (10,000 K) forming a plasma
What are the advantages of ICP-OES?
o Offers simultaneous analysis capability
o High T of plasma gives large choice of emission lines for wide range of elements
o Good detection limits
o Long linear calibration ranges
o Good precision (1 or 2 %)
What is “spectral line width” in atomic spectroscopy?
· 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.
What are sources of interference in atomic emission spectroscopy?
· Ionisation
o At high temperatures atoms such as K can lose an electron and therefore reduce the observed emission from the sample
§ K K+
o Must add more readily ionised element to the sample to compensate
§ E.g. strontium chloride solution is added to suppress ionisation of K in the BP assay of effervescent KCl tablets
· Spectral
o Two overlapping spectral lines
§ Must select a different non-overlapping emission line
§ There are more than 50,000 ICP-OES spectral lines documented
· Chemical
o Presence of low volatility compounds not readily atomised
o Sulphate and phosphate form involatile salts with metals and reduce sample reading
o Remove by adding lanthanum chloride which precipitates them out and replaces them with chloride anions
· Calibration graph non-linear at high concentrations.
o Due to reabsorption of emitted light by ground state atoms / ions in flame.
· Also ‘matrix effects’ i.e. effects due to excipients (Physical effects)
o Density, surface tension and viscosity all affect rates of aspiration and nebulisation
o Alters the rate of aspiration relative to the standard solutions thereby producing different results
§ e.g. sugar in syrups increase viscosity, formation of less volatile salts
How is “standard addition” used to counter matrix effects?
· Used to eliminate ‘matrix effects’i.e. interference due to excipients in sample.
· 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
· Method
o If interference is present which reduces sample absorbance then low concentration will result if use a normal calibration graph
o If we add the standard in incremental amounts to our sample and plot the readings against concentration of added standard – standard addition graph
explain the principle of atomic emission spectroscopy
· Atomic emission uses energy from the flame to excite the electron to the excited state.
o This then relaxes back to the ground state giving off the photon of light.
· Atomic absorption uses a light source of a specific wavelength to cause the electron to be promoted to the excited state.
o The flame is only used to atomise the sample. The amount of this light absorbed is then measured.
· Transitions between similar states are not normally allowed:
o i.e. p-p, s-s, d-d
· Energies of more than 5.2 eV cause the electron to be removed altogether from the atom - ionisation
· More emission lines than absorption lines. Absorption always from the ground state. Emission does not have to go back to ground state
explain how to use detection in atomic emission
· Flame photometers are very simple, cheap instruments
· Use photosensitive detectors similar to PMT but not necessarily as sensitive
· Light emitted from sample is detected and converted into an electrical signal which is then displayed
· There are 2 main designs for detection commercially available
· Simultaneous
· Sequential
