Quiz 2 Material Flashcards
Define spectroscopy.
The study of the interaction (absorption &/or emission) of electromagnetic radiation with matter
How do spectroscopy methods differ?
-
Region of spectrum
- UV
- visible
- infrared
- microwave
- radio
-
Type of radiation-matter interactions
- Absorption
- Emission
-
What is analyzed
- Molecule
- Atom
Describe light as a wave. [5]
- Wavelength, λ → the distance between successive maxima
- Amplitude → magnitude of the electric vector at the wave maxima
- Frequency, v → # of occurrences per unit time e.g., cycles per second
- Wavenumber, ṽ → wavelengths per unit length
- Wavelength and frequency are related by the speed of light, which we treat as a constant, c = 3x108m/s
Describe waves of light & interference.
- Maximum constructive interference (a) waves are in phase → amplitude = 2A
- Interference (b) 90° out of phase → amplitude = 1.4A
- Minimum destructive interference (c ) → waves 180° out of phase → amplitude = 0
Describe light as wavelike particles.
What is photon flux?
Describe the properties of light. [8]
Describe the properties of light. [8]
Energy is proportional to frequency (and wavenumber)
True or False?
True.
Energy is inversely related to wavelength.
True or False?
True.
Energy is inversely related to frequency (and wavenumber).
True or False?
False.
Energy is proportional to frequency (and wavenumber).
Energy is proportional to wavelength.
True or False?
False.
Energy is inversely related to wavelength.
Describe the relationship between energy, frequency, and wavelength.
- Energy is proportional to frequency (and wavenumber).
- Energy is inversely related to wavelength.
Discuss the energy of visible light.
- Mostly food analysis focuses on ultraviolet, visible, and infrared.
Describe transitions used for quantization of energy. [4]
- A given transition corresponds to a certain energy.
- Transitions:
- Translational
- Rotational
- Vibrational
- Electronic
- A photon of a particular energy causes a given transition.
Describe how the internal energy of a molecule or atom varies.
- atoms and molecules exist predominantly in their ground state
- a species struck by a photon may absorb the photon
- the species energy is increased by an amount equal to the photon energy, hv
- the internal energy of a molecule or atom varies in a series of discrete steps
- “The set of available energy levels for any given atom or molecule will be distinct for that species.”
When increasing energy, what are the transitions that occur?
- Only electronic transitions occur when considering the transitions of atoms.
- When considering molecules, all three types of transitions are important.
What wavelength of light is associated with electronic transitions?
UV-VIS light (180 - 750nm)
What wavelength of light is associated with vibrational energy?
IR light (0.78 - 300μm)
What wavelength of light is associated with rotational energy?
Microwave light (0.75 - 3.75mm)
Describe the wavelength regions, spectroscopic methods, and associated transitions. [7]
Describe absorption of energy in spectroscopy.
- Energy from a photon of electromagnetic radiation is transferred to the molecule or atom.
- Molecule/atom goes from the ground state to an excited state.
- An absorption spectrum characteristic for a particular molecule/atom
Why do we see continuous spectra, and not discrete bands in spectroscopy?
Smearing
What is the source of nonbonding valence electrons?
N, O, S, P
What is the source of pi(bonding)-electrons?
Double, triple bonds
What is a pigment?
Molecules that absorb VIS light
UV-VIS light has enough energy to cause […].
Outer shell electronic transitions
Conjugation lowers excitation energy (longer wavelength).
True or False?
True.
- electron delocalization lowers the required excitation energy
- absorption shifts +30nm for each additional conjugated double bonds
- Generally a system of greater than or equal to 7 conjugated double bonds will absorb VIS (e.g., flavonoids and carotenoids)
Conjugation increases excitation energy (shorter wavelength).
True or False?
False.
- electron delocalization lowers the required excitation energy
- absorption shifts +30nm for each additional conjugated double bonds
- Generally a system of greater than or equal to 7 conjugated double bonds will absorb VIS (e.g., flavonoids and carotenoids)
Describe the UV absorption of proteins.
- Below 200nm, the backbone of the protein is being excited
- Higher wavelengths are due to the side chains of the protein being excited.
Describe the absorption of chlorophyll (a pigment; highly conjugated system).
- Pigments absorb in the visible region.
What is fluorescence?
Absorption + Emission = fluorescence
Fluorescence = radiative decay (emitted photon = lower E than absorbed photon)
Describe the population of states.
Boltzmann Distribution → the population of a state decreases exponentially with the energy of the state.
Which phenomena associated with light are most readily explained by considering the wave nature of light? [3]
- Interference, diffraction, and refraction.
- The phenomena is associated with light propagation → includes refraction, diffraction, and interferences, all of which are important in spectroscopy.
Which phenomena associated with light are most readily explained by the particle nature of light? Explain these phenomena based on your understanding of the quantum nature of electromagnetic radiation.
- Interaction of light with matter, which is the basis of absorption and emission spectroscopy
- Potential energy spacing between allowed internal energy levels are characteristic of a species (= qualitative ‘fingerprint’)
- The phenomena associated with the interaction of light with matter, absorption being the most important for spectroscopy.
- Experiments related to the photoelectric effect illustrate that the light is quantized. Absorption is a simple capture of this bundle of energy, which must match the energy differences in the molecule/atom.
What does it mean to say that the energy content of matter is quantized?
- Potential or internal energy of an atom or molecule does not vary in a continuous manner but rather in a series of discrete steps.
- The quantum nature of atoms and molecules puts limitations on the energy levels that are available to them.
- The energy levels of matter are not continuous. For any given matter there are specific energy levels in which it can reside. If plotted, the energy levels would have a digital type readout rather than an analogue type readout.
Molecular absorption of radiation in the UV-VIS range results in transitions between what types of energy levels?
Electronic energy levels
Higher energy levels compared to IR range.
Molecular absorption of radiation in the IR range results in transitions between what types of energy levels?
Vibrational energy levels
Lower energy level compared to UV-VIS.
How do the allowed energy levels of molecules differ from those of atoms? Answer with respect to the diagram.
- Atoms → only electronic transitions
- Molecules → all three transitions are relevant
- Molecules will have many more energy levels due to the vibrational and rotational energy levels that occur in multi-atom structures.
- Atoms will have only electronic levels. Within each electronic energy level, molecules will have their vibrational and rotational energy levels.
In fluorescence spectroscopy, why is the length of the emitted radiation longer than the wavelength of radiation used for excitation of the analyte?
- In most cases, only a fraction of the energy difference between the excited and ground states is lost in the emission process.
- The other fraction of excess energy is dissipated as heat during vibrational relaxation.
- The excited species undergoes vibrational energy relaxation down to the lowest vibrational energy level within the excited electronic state, and then undergoes a transition to the ground electronic state through the emission of a photon.
- The photon emitted will have an energy that equals the energy difference between the lowest vibrational level of the excited electronic state and the ground electronic state level it descends to.
- The fluorescing molecule may descend to any of the vibrational levels within the ground electronic state.
- The wavelength of the emitted light is longer because typically some of the energy associated with the absorbed light is dissipated as heat (vibrational relaxation) and thus, the emitted light is of lower energy. The lower energy associated with the photons of the emitted light means that the corresponding light waves are of longer wavelength
- E = hv = hc/wavelength
What are the wavelength limits of Ultraviolet?
10 - 380 nm
What are the wavelength limits of visible light?
380 - 750 nm
What types of spectroscopy can be used with ultraviolet?
Absorption
Emission
Fluorescence
What types of spectroscopy can be used with visible light?
Absorption
Emission
Fluorescence
What types of transitions in chemical systems with similar energies are there in UV-VIS?
Outer-shell electrons in atoms, bonding electrons in molecules.
What is the principle of UV-VIS spectroscopy?
What is an inconvenience?
What is an advantage?
- pass a beam of photons through a sample
- measure the amount of light that is transmitted (or absorbed)
- relate the %T (or %Abs) to [analyte]
- Inconvenience → T is NOT linear with concentration
- Advantage → A is linear with concentration (within limits)
How is transmittance, T calculated?
How is absorbance, A calculated?
What are assumptions associated with UV-VIS spectroscopy?
What is a solution to this problem?
A = log(P0/P) = alc
- Assumption → Attenuation of beam solely due to absorption by sample = not completely valid! (i.e., light scattering, light reflection)
- Solution → measure the light exiting a reference sample (e.g., cuvette + solution, no analyte); use this as P0; ‘zeroing’ the instrument
In UV-VIS spectroscopy, discuss deviations from Beer-Lambert Law [3].
-
Analyte concentration > 10mM
- Intermolecular distances decrease
- Crowding
- Electrostatic interactions
-
Chemical processes
- Reversible association/dissociation of molecules
- Ionization (unbuffered system)
-
Polychromatic light
- Absorptivity is defined at a specific wavelength (monochromatic light)
- Multiple wavelengths = different absorption
Describe sample preparation for UV-VIS spectroscopy.
- Homogenization and clarification (e.g., centrifuge, filter)
- Chemical modification → absorb at desirable wavelength range
- Reference/blank sample = similar preparation/modifications w/o analyte
- Sample cell:
- Quartz cuvette (UV, VIS)
- Plastic (UV-VIS), 23-900nm
- Fused silica (UV)
- Silicate glass (VIS)
- Plastic (VIS)
- Match the application (wavelength range) with the correct cuvette material)
Describe why wavelength selection is important in UV-VIS spectroscopy. [2]
Maximize sensitivity
Greater adherence to Beer’s Law (less variation in absorptivity)
Describe calibration of UV-VIS spectroscopy instruments.
- 0% transmittance → block the detector
- 100% transmittance → measure the reference cell (set as P0)
Describe calibration curves in UV-VIS spectroscopy.
- Linear → obey Beer’s Law
- Non-linear → due to concentration-dependent changes in chemistry of system (change in absorbance per unit change in concentration is not constant → due to limitations in instrument
Discuss relative error in UV-VIS spectroscopy.
- Get lower relative errors with measurements at intermediate transmittance (%T = 15-65%; A = 0.2-0.8)
- Relative errors will be larger outside this range.
Describe the light source & features in UV-VIS spectroscopy instrumentation.
Describe the wavelength isolator/dispersion element of a UV-VIS spectroscopy instrument.
Describe the typical phototube design of a UV-VIS detector.
Describe double-beam spectrometer in UV-VIS spectroscopy.
- Measure reference and sample simultaneously
- Minimize errors due to drift/light fluctuations
- But, also lowers the intensity (Beam sharing)
Which compounds absorb 200-800nm?
What compounds give fluorescence?
Describe properties of fluorescent molecules. [5]
- Generally molecules that fluoresce are highly conjugated systems.
- Properties include:
- Maximum excitation and maximum emission wavelengths
- Stokes shift (λEMISSION − λEXCITATION)
- Extinction coefficient
- Quantum yield (conversion of absorption → emission)
- Lifetime (duration of excited state)
Describe fluorescence spectroscopy instrumentation and its differences from UV-VIS absorption spectrophotometer.
- Two wavelength isolators (for excitation and emitted light)
- Detector is 90° with regard to light source
Which is more sensitive?
Fluorescence or UV-VIS absorption spectroscopy?
Fluorescence is 10-1000x more sensitive.
Compare cuvettes - absorption vs. fluorescence.
Describe fluorescence spectroscopy applied to concentration.
Describe fluorescence applied to protein unfolding.
Describe fluorescent dye-binding experiments.
- Exposed non-polar surface area → increase in fluorescence means that there is binding to the protein because there are some hydrophobic surfaces exposed
- Amyloid fibrils → increase in fluorescence due to dye binding to beta-sheet structures of the amyloid fibrils
Describe how fluorescence can be used to determine egg freshness.
Describe how UV-VIS may be used for quantitative & qualitative measurements.
-
Qualitative
- Determine the presence of particular analytes
- Comparison of samples based on relative changes in amplitude
-
Quantitative
- Determine the amount of analyte in the sample
- Relate intensity to [analyte] using a known factor
Why is it common to use absorbance values rather than transmittance values when doing quantitative UV-VIS spectroscopy?
- T and %T are not directly proportional to the concentration of the absorbing analyte in the sample solution (i.e., nonlinear relationship). Under appropriate conditions, absorbance is directly proportional (i.e., linear relationship) to the concentration.
What criteria should be used to choose an appropriate wavelength at which to make absorbance measurements, and why is that choice so important?
- Choose the wavelength at which the analyte demonstrates maximum absorbance, and where absorbance does not change rapidly with changes in wavelength, if possible. This provides maximum sensitivity (i.e., absorbance change per unit change in analyte concentration), and greater adherence to Beer’s law.
Describe the similarities and differences between a phototube and a photomultiplier tube. What is the advantage of one over the other?
- Similarity → both detectors function by converting the energy associated with incoming photons into electrical current
- Advantage of photomultiplier tube is that signal is continuously amplified, so radiation can be less intense and still be detected (= more sensitive than phototube)
Your lab has been using an old single-beam spectrophotometer that must now be replaced. You obtain sales literature that describes single-beam and double-beam instruments.
What are the basic differences between a single-beam and double-beam spectrophotometer, and what are the advantages and disadvantages of each?
Single-beam → radiant beam follows only one path (i.e., from source through the sample to the detector)
Double beam → radiant beam is split so half the beam goes through one cell-holding compartment and the other half of the beam through the second, or beam is alternatively passed through the sample and reference cells by means of a rotating sector mirror
Advantages of double beam → can simultaneously measure and compare relative absorbance of a sample and a reference cell; can compensate for deviations or drifts in the radiant output of the source.
Disadvantage of double beam → radiant power of the incident beam is decreased because beam is split, causing inferior signal-to-noise ratios
Explain the similarities and differences between UV-VIS spectroscopy and fluorescent spectroscopy with regard to instrumentation and principles involved.
What is the advantage of using fluorescence spectroscopy?
Similarities → essentially the same instrumental components; both are based on absorption of radiant energy by the analyte, and the detection of radiant energy at the detector.
Differences → Fluorescence spectroscopy depends on measuring radiation emitted by the analyte as it relaxes from an excited electronic energy level to its corresponding ground state. The analyte is originally activated to the higher energy level by the absorption of radiation in UV or VIS range. Fluorometers have two wavelength selectors (i.e., excitation beam and emission beam versus 1 for spectrometer). Fluorometers have detector arranged so that emitted radiation is travelling at an angle of 90° relative to the axis of the excitation beam (to minimize signal interference due to transmitted source radiation and radiation scattered from the sample).
Advantage of fluorescence spectroscopy → 10-1000x more sensitive than UV-VIS absorption spectroscopy; has helpful applications regarding protein unfolding; amyloid fibrils; and egg freshness
What are the wavelength limits and types of transitions in Infrared Spectroscopy
0.075 - 1000um
Vibrational positions of atoms in molecular bonds
IR covers a broad region of light compared to UV and VIS.
Describe the three sections and identify the most useful.
When can a molecule absorb IR radiation?
If it vibrates such that its charge distribution (therefore its electric dipole moment) changes during vibration
What is harmonic approximation?
A model to approximate the energy of a vibrational transition (IR spectroscopy)
IR absorption is unique to each group of atoms.
Different bond strengths and different masses give rise to discrete energy levels
What is fundamental absorption?
Frequency of radiation that will make vibrating molecular functional group move from lowest vibrational state to first excited state.
What are overtones?
Absorption of radiation to move vibrating molecular functional group to higher excited states → 2-3x higher energy = less likely (less absorption at these frequencies)
Mid-IR generally involves overtones.
True or False?
False.
Mid-IR generally involves fundamental absorption.
Mid-IR generally involves fundamental absorption.
True or False?
True.
Near-IR generally involves overtones.
True or False?
True.
Near-IR generally involves fundamental absorption.
True or False?
False.
Near-IR generally involves overtones.
What are the two main Mid-IR instrument designs?
Dispersive IR
Fourier Transform IR
Describe dispersive IR.
Describe Fourier Transform IR.
- No means to select a single wavelength to shine on the sample
- The sample is exposed to all wavelengths at once
- Radiation is not dispersed; instead, uses an interferometer
- All wavelengths pass through the sample and arrive at the detector simultaneously.
- Mathematical treatment (FT) converts results into IR absorption spectrum
What are the advantages of Fourier Transform IR over Dispersive IR? [4]
- Faster
- More sensitive
- Better wavelength resolution
- Better wavelength accuracy
Describe FTIR instrument design.
- IR beam is split then recombined using mirrors
- Intensity of radiation reaching detector varies as function of optical path difference as one mirror is moved → causes interference
- Interferogram → intensity as a function of optical path difference
- Converted to intensity vs wavenumber using FFT
What are the light sources used in FTIR instrument design?
- Inert solids heated electrically to 1000-1800°C
- Three sources → Nernst glower, Globar, Nichrome coil
What are detectors in FTIR instrument design?
- Variations of thermocouples → output voltage varies with temperature changes caused by radiation striking detector
- Several types → Golay, pyroelectric, semiconductor
Describe MID-IR sample handling for measuring solids, pastes, and viscous liquids (e.g., peanut butter).
- Attenuated Total Reflectance IR (ATR IR)
- Measures total energy reflected from surface of sample in contact with IR transmitted crystal
Functional group spectroscopy = Near-IR spectra.
True or False?
False.
Functional group spectroscopy = Mid-IR spectra.
Functional group spectroscopy = Mid-IR spectra
True or False?
True
Describe Near-IR spectra.
- higher energy
- spectra consist mainly of overtones
- Lower intensity
- Broader absorption
- Arise primarily from functional groups that have an H atom attached to a C, N, or O
- e.g., common groups in water, organic compounds
Describe Near-IR instrumentation.
- Design
- Dispersive (common)
- FT-IR
- Mode
- Transmission (Liquids; quartz cuvette)
- Reflectance:
- Specular (light reflects back towards light source) → no information; not measured
- Diffusive (light reflects in random angles) → penetrates sample & partially absorbed; provides information (is measured)
- Used for solid foods
What is the equation for reflectance?
Describe qualitative analysis in Mid-IR & Near-IR applications in food.
- Compare spectra of sample and reference sample, for example:
- Distinguish between wheat types (hard red spring vs. hard red winter)
- Distinguish between orange juice and other juices
- Authentication of olive oil
Discuss specific applications of automated FTIR instruments.
Higher energy → lesser amplitude.
True or False?
False.
Higher energy → greater amplitude.
Describe FTIR to study secondary protein structure.
What is synchrotron radiation?
- Electron accelerator and storage ring
- Accelerating electrons gives off EM radiation
- Very intense light source
IR instrumentation is never portable.
True or False?
False.
IR instrumentation can be portable.
True or False?
True.
What is the primary method for measuring the mineral content (& toxicants like heavy metals) of food?
Atomic absorption & atomic emission spectroscopy
Note → minerals are present as minor constituents → need to quantify against a background of other components
When is atomic absorption spectroscopy used in mineral analysis rather than titration, colorimetric assays, or ion selective electrodes? [3]
- several minerals determined
- large sample number
- typically for nutrition labelling
How are samples prepared for atomic absorption analysis?
Most samples are wet ashed
What does AAS measure?
What does AES measure?
Compare atomic versus molecular absorption/emission.
Atomic only has electronic transitions!
Minerals do not have complex line spectra.
True or False?
False.
Each element has a unique set of energy levels = unique line spectra
Minerals have complex line spectra.
True or False?
True.
In line spectra there tend to be more emission spectra than absorption spectra.
True or False?
True.
In line spectra there tend to be less emission spectra than absorption spectra.
True or False?
False.
Emission and absorption spectra are often not unique for each element.
True or False?
False.
They are unique.
Describe the basic components [6] of atomic absorption spectroscopy instrumentation.
- Light source
- Beam chopper
- Atomizer (+ nebulizer)
- Monochromator
- Detector
- Readout device (CPU)
Describe the light source for AAS.
Hollow Cathode Lamp (HCL)
Cathode is made of the element to be measured.
What is the principle of atomization in AAS?
- Introduce sample as small droplets/fine mist, at high temperatures
- Solvent evaporates
- Sample vaporizes
- Molecules decompose to atoms
-
Flame atomizer
- Nebulizer: sample → fine mist
- mix with oxidant-fuel (air-acetylene)
- introduce to flame, 5-10cm long
-
Graphite furnace
- sample (~ul) carried through graphite tube
- electrically heated
What is considered ideal in atomization? [2]
Maximize atomization while minimizing ionization
Neutral atoms and ions have different line spectra
Describe the factors that affect the frequency of vibration of a molecular functional group and thus the frequencies of radiation that it absorbs.
Also, explain how the fundamental absorption and overtone absorptions of a molecule are related.
- Factors include strength of the bond and the mass of the molecular system.
- Fundamental absorption is the initial frequency of vibration of the bond.
- Overtone absorption are of a frequency 2-3 times that of the fundamental frequency; intensity of these absorptions is much lower than the fundamental absorption, since these transitions are less favoured.
Describe the essential components of an FT mid-IR spectrometer and their function, and compare the operation of the FT instrument to a dispersive instrument.
What advantages do FT instruments have over dispersive IR spectrophotometers?
FT Instrument: IR source: interferometer (splits then recombines a light beam); mirrors and mirror drive; sample cell ;detector; amplifier; computer
FT operation: radiation is not dispersed, but rather all wavelengths arrive at the detector simultaneously, a mathematical treatment (a fourier transfrom) is used to convert the results into a typical IR spectrum → uses an interferometer to split then recombine a light beam
Dispersive instrument → use a monochromator to disperse the individual frequencies of radiation and sequentially pass them through the sample so that the absorption of each frequency can be measured
Advantages of FT instruments (vs. dispersive) → can acquire spectra more rapidly, with greatly approved signal-to-noise ratio
Of the three antioxidants butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and propyl gallate, which would you expect to have a strong IR absorption band in the 1700-1750cm-1 spectral region?
A strong absorption band in the 1700-1750cm-1 region is indicative of the presence of a carboxyl group. Single propyl gallate is the only one of the three compounds to have a carboxyl group in its structure, only propyl gallate sould exhibit a strong band in this region.
Describe the two ways in which radiation is reflected from a solid or granular material.
Which type of reflected radiation is useful for making quantitative measurements on solid samples by near-IR spectroscopy?
How are near-IR reflectance instruments designed to select for the desired component of reflected radiation?
Radiation is reflected form a solid or granular material by:
Specular reflection → the mirror-like reflectance when radiation is reflected from a sample surface.
Diffuse reflectance → when radiation penetrates through the surface of the sample, and is reflected off several sample particles before it exits the sample; diffuse reflectance is useful for NIR reflectance spectroscopy
NIR reflectance instruments are designed with filters selected to transmit wavelengths that are known to be absorbed by the sample constituents.
AAS and AES instruments rely on energy transitions in atoms of elements being measured.
What is an ‘energy transition’ in this context and why can it be used to detect and quantify a given element in a sample containing multiple elements?
What is the source of energy that produces this energy transition in an AAS instrument? In an AES instrument?
Energy transition → is the energy change associated with a transition between two energy levels for an atom (e.g., ground and excited states). Atoms absorb or emit radiance of discrete wavelengths because the allowed energy levels of electrons in atoms are fixed and distinct. In other words, each element has a unique set of allowed electronic transitions and therefore a unique spectrum, enabling accurate identification and quantification even in the presence of other elements.
In AAS → the source of energy is radiation from a hollow cathode lamp (or an electrode-less discharge lamp)
In AES → the source of energy is flame (heat) or inductively coupled plasma torch (radiofrequency power).
Your boss wants to purchase an AAS instrument for your analytical lab because it is cheaper but you want an ICP-OES instrument because it is more versatile and will greatly increase your sample throughput. To convince your boss, you need to educate him.
Explain the underlying principles of operation for an ICP-OES instrument in language your boss can understand. Describe the instrument you want to purchase.
Principles → measures emission of energy (at specific wavelength) as atom excited by plasma returns to ground state → amount of radiant energy emitted is proportional to concentration of specific element.
Instrumentation → atomization-excitation source → ICP torch (plasma); Echelle optics (or monochromaters for older PMT-based instruments) → solid-state array detectors → computer/readout device
In the quantitation of Na by atomic absorption, KCl, or LiCl was not added to the sample.
Would you likely over- or under-estimate the true Na content? Explain why either KCl or LiCl is necessary to obtain accurate results.
- Underestimate, due to ionization interference, since Na is easily ionized
- Add ionization suppressants to increase the concentration of electrons in the flame, to shift equilibrium to the left
The detection limit for calcium is lower for ICP-OES than it is for flame AAS. How is the detection limit determined, and what does it mean?
- Detection limit (or LOD) is usually defined as the lowest concentration of an analyte that can be distinguished from the appropriate blank with a given level of confidence.
- To determine the LOD for a particular element using a particular instrument, one would simply prepare the appropriate blank, tune the instrument to measure the element, and analyze the blank several times. Then, calculate the mean and standard deviation of the reading and plug the values into the above equation. Variability in the blank signal is often referred to as ‘noise’. Therefore, the noise in the AAS signal is generally higher than the noise in an ICP-OES signal.
Discuss AAS calibration and use
Discuss AAS calibration and use
- Absorbance vs. concentration will deviate from linearity predicted by Beer’s law when concentration exceeds a certain level.
- Properly constructed calibration curves using pure standards are essential for accurate quantitative measurements.
- If values for linear ranges are not provided by manufacturer, the linear range of an element should be established by running a series of standards of increasing concentration and plotting absorbance versus concentration.
- The concentration of the unknown sample solution should be adjusted so that the measured absorbance always falls within the linear range of the calibration curve.
- If values are provided by the manufacturer, measured absorbances deviating from these values indicate appropriate adjustments are required (e.g., flame characteristics modified, lamp alignment changed, etc.)
Describe spectral interferences in AAS.
-
Spectral overlap = absorption of source radiation by other elements
- Use different wavelength, or narrow monochromator slit width
Describe the principle of AES.
- Sample solution is volatilized, then atomized, and excited by heat (flame), light (laser), electricity (ars or sparks), or radio waves (inductively coupled plasma, ICP)
- Atoms in an excited state emit energy (specific wavelengths for each element) when they return to a lower energy state/ground state
- Amount of radiation emitted is proportional to concentration.
Describe AES using inductively coupled plasma (ICP).
-
Flame AES
- Flame = atomization & excitation source
- Useful only for elements of relatively low excitation energy (Na, K)
-
Inductively coupled plasma-optical emission spectroscopy
- Plasma = atomization & excitation source
- Argon gas in a quartz tube is heated using rf power applied to copper coil
- Induces a magnetic field
- Ar, ions, and electrons are accelerated in a circular path → coupled to magnetic field
- Produces high energy electrons and argon ions (6000-7000K)
- Excites elements in the sample
Describe Echelle Optical System.
- Uses two dispersing components in series:
- Prism (x-axis) and diffraction grating (y-axis)
- Results in 2-D dispersion of light
- Allows high bandpass and resolution
- Detection using a solid-state array detector:
- Complementary metal oxide semiconductor
- Charge coupled device
- Charge injection device.
Describe spectral interferences in ICP-OES.
- Background shift interference
- Certain ions may increase background
- Perform background subtraction
- Certain ions may increase background
- Spectral overlap interference
- Overlap of emission lines of different elements
- Choose a different emission line, or use a correction factor
- Overlap of emission lines of different elements
