4103FSBMOL - Lecture 7 - Heavy Metal Detection. Flashcards
What are the properties of Heavy Metals?
- They have high densities.
- They are persistent in the environement.
- They can’t degrade or be destroyed.
How many heavy metals are there? Can you name a few examples?
There are 23 heavy metals: antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc.
How can Heavy metals get into the body?
- Ingestion (food and drink).
- Inhalation.
Finish the quote: “Some heavy metals are essential to the body. But at a high concentration they lead to ____________”.
Give examples.
Poisoning.
Zinc and Selenium.
What are the 5 heavy metal elements which are toxic to the body?
Mercury, Cadmium, Chromium, Lead and Arsenic.
How is Mercury toxic to the body?
It is absorbed quickly and can cause damage to the nerves, kidney, stomach, intestines and DNA.
How is Cadmium toxic to the body?
It can cause damage to the skeletal, cardiovascular, nervous and respiratory systems.
How is Chromium toxic to the body?
It can cause Respiratory issues, lug cancer, kidney and liver damage, nose bleeds and upset stomach’s.
How is Lead toxic to the body?
It can cause damage to the Liver, kidney, brain and bones.
How is Arsenic toxic to the body?
It can cause Gastrointestinal issues, problems with the nervous system and issues with DNA.
What does AAS stand for?
Atomic Absorption Spectroscopy.
What is Atomic Absorption Spectroscopy used for?
The detection of metals and metalloids. It is reliable and sensitive and can be used quantitatively - can quantify a concentration of an element.
Metalloid - a chemical element which has properties that are a mixture of, those of metals and nonmetals
What is the Sensitivity of AAS?
It works in ppb (parts per billion).
How many different elements can AAS analyse?
Over 62 different elements.
What does AAS measure?
The amount of emission given back after absorption.
In AAS, is Energy proportional or inversely proportional to the Wavelength?
Inversely Proportional.
Are AAS systems more stable at higher or lower energy?
They are more stable at a lower energy (ground state). Even in the flame, most of the atoms will be in their lowest energy state.
Is AAS a single or multi element technique?
Single-element. Separate measurements must be made if more than one metal is to be determined.
What are the principles of AAS?
- Free atoms (prepared in a solution) that are gas generated in an atomiser can absorb radiation at a specific frequency (An atomiser converts a liquid into a gas).
- AAS quantifies absorption of ground state atoms in the gaseous state.
- The atoms absorb UV or visible light and make transition to higher electronic energy levels.
- The analyte concentration is determined from the amount of absorption (amount absorbed is directly proportional to the concentration - beer lambert’s law).
- Instrument is calibrated with standards of known concentration (can’t go from zero).
What is AAS based off?
It is based on the properties of specific metal atoms to absorb specific wavelength of light. The electronics will measure light attenuation and convert those readings to the actual metal concentration of the sample.
What are the steps for Atomisation to turn your solution of analyte into atomic ions?
- Nebulization.
- Desolvation.
- Volatilisation.
- Dissociation (reversible).
- Ionisation (reversible).
- Excitation.
How does a Nebulizer work in AAS?
A nebulizer uses a flame oxidant gas to draw a solution into the nebulizer through a capillary tube. The solution is then accelerated through a narrow tube, which creates a fine spray of droplets called an aerosol.
How does Desolvation work in AAS?
The solvent is evaporated, resulting in dry nanoparticles of the sample remaining.
How does Volatilisation work in AAS?
The particles are converted to the gaseous phase by being volatised/ vapourised.
How does Dissociation/ Atomization work in AAS?
The key stage at which the population of ground state, freely dissociated atoms is created. Ground state atoms are the target for AAS analysis.
How does Ionisation work in AAS?
Some, but not all, free atoms will be converted to ions. This will depend on the flame conditions (gas mix) and the ionisation potential of the analytes on solution.
What are the different Flame mixes for AAS and what are the temperatures they operate at?
- Acetylene/ Air - 2100°C - 2400°C (most common).
- Acetylene/ N2O - 2600°C - 2800°C.
- Acetylene/ O2 - 3050°C - 3150°C.
How do you know what flame type to use in AAS?
It depends on the volatilisation temperature of the atom of interest.
What are the different parts of a Flame Structure?
- Interzonal region - inner section of the flame.
- Primary Combustion Zone - small inner section at the bottom.
- Secondary Combustion Zone - outer section of the flame.
- Fuel-oxidant mixture (gas supply).
Which is the best part of the Flame Structure?
The Interzonal Region as it is thehottest part of the flame and best for atomic absorption.
Fuel ________ flames are best for atoms because the likelihood of oxidation of the atoms is __________.
(1. rich/ weak, 2. increased/ reduced).
- Rich.
- Reduced.
Which section of the Flame Structure does oxidation of the atoms occur?
The secondary combustion zone where the atoms will form molecular oxides and are dispersed into the surroundings.
What are the 5 basic components of an Atomic Absorption Spectrometer?
What does each thing do?
- Light source: emits the spectrum of the element of interest.
- Absorption cell: where atoms of the sample are produced (flame, graphite, furnace).
- Monochromator: for light dispersion.
- Detector: that measures the light intensity.
- Display: that shows the reading after it has been processed by the instrument electronics.
What is the Light Source made up of?
Hollow cathode lamps. They emit a spectrum specific to the element of which it is made. This is focused through the sample cell into the monochromator. They contain a tungsten anode and a hollow cylinder cathode made of the element to be determined. They are sealed in a glass tube filled with inert gas (neon or argon). Each element has a unique lamp. The lamp holder has positions for 8 source lamps which allows having 8 lamps simultaneously that are prepared for work and can pass from one element to another.
What is the Nebuliser made up of?
It aspires the liquid sample at a controlled rate. It creates fine aerosol particles to be introduced to the flame. You mix the aerosol, fuel and oxidant thoroughly for introduction into the flame.
What are the 2 types of AAS Atomiser?
- Flame Atomiser.
- Graphite tube Atomiser.
What does a Flame Atomiser do?
A mixture of oxidant gas and a fuel (e.g. air-acetylene) reacted with a flame or nitrous oxide acetylene flame.
What does a Graphite tube atomiser do?
Uses graphite coated furnace to vaporise the sample. Samples are deposited in a small graphite coated tube then heated to vaporise and atomise the analyte.
What 2 different things can Free Atoms be turned into and back from?
Molecules and Ions.
What does a Monochromator do?
They select specific wavelengths of light that is absorbed by the sample and excludes other wavelengths not needed. The main purpose is to remove interference by isolating the absorption line from background light.
What is the most common detector which is used with AAS?
Photomultiplier tubes Detector.
How does a Photomultiplier tubes Detector work?
They convert a light signal to an electric signal that is proportional to the intensity of light. A signal amplifier processes the electrical signal.
What do we need to use to prepare all glassware and equipment for analysis?
We MUST clean everything with Nitric Acid and Deionised Water!
Why do we use Nitric Acid to clean glassware?
Because it removes all elements present.
It also makes sure the sample is suitable and to minimise interferences.
What is Sample Digestion?
Some samples must be digested before analysis in order to ensure accurate measurement. A Microwave pressure digestion system is used and the digestion duration is 15-20 mins.
What can you use to determine the concentration/ absorbance of Iron (Fe)?
A calibration curve.
What does ICP-MS stand for?
Inductively Coupled Plasma - Mass Spectrometry
How many elementd can ICP-MS run simultaneously?
40 elements.
ICP-MS is a Mass Spectroscopy method which detects what?
Ions of metals distinguishable by their m/z value (mass-to-charge ratio).
It detecs at parts-per-billion level.
What is ICP-MS based on the ions moving under?
The influence of electrical or magentic fields.
What are the components of the ICP-MS machine?
- Plasma Ion Source.
- Ion Lenses.
- Collision/ Reaction Cell.
- Quadrupole MS.
- Detector.
What is the role of the Plasma Ion Source in the ICP-MS instrument?
It decomposes the sample matrix and forms ions. It is in the shape of a torch with an induction coil which power is supplied to by a radio-frequency generator that makes an oscillating Electromagentic Field at the end of the torch. This enables formation of ions from atoms that had been introduced through the torch. This conversion is made by stripping electrons from the atoms.
What temperature does Plasma exist?
6000-10,000°K.
What is the role of the Ion Lense in the ICP-MS instrument?
To focus Ions and remove photons and neutrals. - Argon ions formed are caught in the oscillating fields and collide with other argon atoms forming argon discharge or plasma (ICP). Ion lense is a section that connects ionising section at ambient pressure to the MS at high vacuum. Composed of two metallic cones with small orifices. Exports ions produced in argon plasma and transports them to the MS.
What is the role of the Quadrupole Mass Analyser in the ICP-MS instrument?
It is made up of four parallel metal rods 15-20 cm in length and 1 cm diameter with DC and AC voltage (alternating with radiofrequency). Works as mass filter: allows passage of particular m/z ions only. Separates ions based on m/z. It can scan over m/z range spectrum.
What is the role of the Quadrupole Cross Section in the ICP-MS instrument?
Positive or negative bias on the rods keep the ions of interest in the middle of the four rods to the end, so they produce an electrical pulse in the detector. Then ions at a different m/z ratio are measured and the process is repeated until all ions in a multi-element determination have been measured.
What is the role of the Multi-collector Mass Analyser in the ICP-MS instrument?
Simultaneous detection with array of collectors (Faraday cups). Best for determination of isotope ratios.
It has applications in geochemistry and biomedical research.
What is the role of the Detector in the ICP-MS instrument?
High sensitivity and low background noise, it is an electron multiplier device that can generate a measurable signal pulse from the impact of a single ion.
What are the different type of Spectral Interferences which you can get with ICP-MS?
- Plasma Gas - Most abundant isotope of Ar is at mass 40 which dramatically interferes with the most abundant isotope of Ca at 40.
- Solvent Based - In HCl acid medium, Ar combines with Cl isotope at 35 amu, that interferes with the only isotope of As at mass 75.
- Isobaric - Direct overlap from a different element with an isotope of the same nominal mass.
What does XRF stand for?
X-Ray Fluorescence.
Are X-Rays a source of Monochromatic or Polychromatic light?
Polychromatic.
What can happen with a sample that is exposed to X-Rays?
An electron could be displaced (ionisation could occur).
What is the energy from X-Rays able to do to electrons in atoms?
It is sufficient enough to displace electrons from inside orbitals (K, L or M) of the atoms.
What does the ‘K shell’ look like?
It is the closest inner shell (1st shell) surrounding the nucleus. It contains a maximum of 2 electrons.
What does the ‘L shell’ look like?
It is the 2nd closest shell to the nucleus and contains a maxmum of 8 electrons.
What does the ‘M shell’ look like?
It is the 3rd closest shell to the nucleus and contains a maxmum of 18 electrons.
What does the ‘N shell’ look like?
It is the furthest shell from the nucleus (4th shell) and contains a maxmum of 32 electrons.
The energy of __________ photons is equal to differences in energy levels involved in electron transmission.
(Absorbed/ emitted).
Emitted.
What can you characterise the Transition between the L and K shell with?
(2nd shell to 1st shell).
Kα.
What can you characterise the Transition between the M and K shell with?
Kβ.
What can you characterise the Transition between the M and L shell with?
Lα.
What can you characterise the Transition between the N and L shell with?
Lβ.
The emission intensity is proportional to the _______________ of the atom in the sample.
Concentration.
What different ways can X-Rays interact with matter?
Give examples.
- Absorption - Medical X-Rays to see inside materials.
- Diffraction/ Scattering - X-Ray diffraction to study crystal structures.
- Causing X-Rays of different colours - X-Ray Fluorescence detmines elemental composition.
Are inner or outer electrons easier to remove from an atom?
Outer. Inner shell electrons are bound more tightly and are harder to remove from the atom.
Which electrons are X-Rays typically only able to affect?
The inner shells (K and L) electrons.
Which shell has the highest Binding Energy and which has the lowest?
- Highest = ‘K’ shell.
- Lowest = ‘N’ shell.
Which shell has the highest Potential Energy and which has the lowest?
- Highest = ‘N’ shell.
- Lowest = ‘K’ shell.
What is X-Ray Attenuation?
The decrease in intensity of an X-ray beam as it passes through matter.
What are the 2 types of X-Ray Scattering?
- Coherent (Rayleigh).
- Incoherent (Compton).
When does Coherent (Rayleigh) scattering happen?
When the X-ray collides with an atom and deviates without a loss in energy. An electron in an alternating electromagnetic field (e.g. X-ray photon), will oscillate at the same frequency (in all directions). It essentially becomes a small radiating dipole, scattering the incident energy in all directions at the same frequency of the X-ray. This is elastic scattering – no loss of in energy. One impact in XRF is that the X-ray line of the X-ray source may be “backscattered” off of the sample onto the detector and show up as something present in the sample.
When does Incoherent (Compton) scattering happen?
It happens where the incident X-ray loses some of its energy to the scattering electron. It is inelastic scatter. As total momentum is preserved, the wavelength of the scattered photon increases.
What are the components of the XRF instrument?
- X-Ray Tube Source.
- Silicon Drift Detector (SDD).
- Digital Pulse Processor.
- XRF Spectrum - by cps or keV software.
What does the ‘X-Ray tube source’ do?
High energy electrons are fired at an anode (made of Ag or Rh). It can vary the excitation energy from 15-50kV and the current from 10-200mA. Filters can be used for lower detection limits.
What does the ‘Silicon Drift Detector’ and Digital Pulse Processor’ do?
It uses an Energy-dispersive multi-channel analyser, which means no monochromator is needed. Pettier-cooled solid state detector monitors both the energy and number of photons over a preset measurement time.
What does the energy of a photon in keV software relate to?
The type of element.
What does the emission rate (cps) relate to?
The concentration of the element.
What is the concentration of an element determined from using the analyser software?
Factory callibration data, sample thickness as estimated from source backscatter and other parameters.
What are the 3 types of XRF instruments?
- Handheld.
- Portable.
- Benchtop/ Lab model.
Explain about the Handheld XRF instrument.
It is used in the field (in crime scenes). It is very easy to use. It can give accurate results.
Explain about the Portable XRF instrument.
It is used in adverse conditions in the field. It is a bit bigger than the handheld device. It is easy to use and can be used for screening. It can give accurate results when used by a knowledgeable operator.
Explain about the Benchtop XRF instrument.
It has complex software which is designed to give accurate results when used correctly by a trained professional. It is used in lab analysis and generates a report. It uses an autosampler - can run multiple samples automatically and uses optimised excitation.
What Brand is used by the Handheld XRF instruments?
Bruker (Tracer V).
What Brand is used by the Portable XRF instruments?
Innov-X (X-50).
What Brand is used by the Benchtop XRF instruments?
Thermo (ARL Quant’X).
Do Organic Elements give XRF peaks? Explain why?
Give examples of some elements that fall into this category.
They don’t give XRF Peaks. Fluorescence photons from these elements are too low in energy to be transmitted through air and are not efficiently detected using conventional Si-based detectors.
(i.e., H, C, N, O).
Do Low Z (atomic number) Elements give XRF peaks? Explain why?
Give examples of some elements that fall into this category.
They only give ‘K’ (1st Shell) Peaks. ‘L’ peaks (2nd shell). from these elements are too low in energy (these photons are not transmitted through air and not detected with conventional Si-based detectors).
(i.e., Cl, Ar, K, Ca).
Do Middle Z (atomic number) Elements give XRF peaks? Explain why?
Give examples of some elements that fall into this category.
They MAY give both K and L peaks.
(i.e., Rh through I).
Do High Z (atomic number) Elements give XRF peaks? Explain why?
Give examples of some elements that fall into this category.
They only give ‘L’ Peaks. K peaks from these elements are too high in energy (these electrons have high binding energies and cannot be removed with the limited voltage available in field portable analyzers).
(i.e., Ba, Hg, Pb, U).
What other things can you Analyse using XRF?
- Analysis of Coins (to determine counterfeit or authentic).
- XRF of Inks.
