Atomic Spectroscopy Flashcards
Write the electron configuration
How do electrons fill orbitals
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
The 4s is filled before the 3d
Electron configuration of calcium
20 electrons
1s2 2s2 2p6 3s2 3p6 4s2
Atomic Absorption Spectrometry
AAS
energy absorbed by atoms to allow it to enter and excited state
Atomic Emission Spectrometry
AES
Energy emitted by an excited electron when it falls back to the ground state
Principle quantum number :
The number of the period that electron is in
n = 1,2,3,4
Orbital quantum number
oqn
l
s = 0
p = 1
d = 2
Delta n rules
Δ n = 0, +/1- 1,2,34
Delta l rules
Δ l = +/- 1
Allowed transitions of electrons are possible
Are only permitted if the rules for Δ n and Δ l are satisfied
Is the transition from _ possible
2p to 3p
Ground state 2p
Excited state 3p
Can an electron make this transition
Δn = 2 - 3 = -1 this rule is satisfactory
Δl = 1 - 1 = 0 not allowed
This transition can’t happen
Is the transition from _ possible
2p - 4s
Δ n = 2 - 4 = -2 satisfied
Δ l = 1 - 3 = - 2 not allowed
Energy formula
E = hf
E = c / λ
Difference between atomic emission and absorbance spectroscopy and how are they both measured
AAS = energy absorbed by an atom to reach an excited state
AAS instrument
AES = energy released or emitted by an excited atom
AES instrument
Plasma
Spectral lines
The wavelengths absorbed by an element which corresponds to allowed transitions the outer electron can undergo
eg sodium absorbs are 3 wavelengths therefore sodium has 3 spectral lines which corresponds to 3 transition that the outer electron can u servo in sodium
Use an element to describe the events when an element is excited and light is omitted by atoms which has been inserted into a flame
Sodium has its outer electron in the 3s orbital
It can undergo these 3 transitions
These transitions correspond to the 3 different wavelengths we see in the spectral for sodium
3s-3p,4p,5p
Use a specific element, eg ____, to explain the atomic events occurring when the element is excited and light is emitted by atoms which have been inserted into a flame.
Find outer electron
See possible transitions that would be characteristic of ____
Would be able to measure the wavelength of them and identify them as aluminium
AAS v AAS
How are the measured and the instrument
Basic theory of atomic emission
Electronic excitation
Excited electrons give off an amount of energy when they fall back down and you can measure it and it equates to UV Vis radiation ie light
Standard addition approach when is it used
Very complex matrix / complex sample eg colour
Take the sample and add it to standards
X intercept is the concentration of your standard
Calibration method when there is lots of variation in your experiment due to the instrument
Use internal standard
concentration v ratio
Transition metals valence electrons
2
Why is it important to use the same acids, etc, in standards as in samples?
To match standards to sample and avoid interference
In atomic spectroscopy, distinguish clearly between atomic emission and atomic absorption methods. Briefly outline why one might choose to use emission methods in certain situations.
AAS measures energy absorbed to excite an electron
AES measures the energy released by an excited electron
In atomic absorbance spectroscopy, absorbance is associated with:
electronic transitions
The purpose of the flame in a flame AA spectrometer is to:
Desolvate and atomise the sample
The Principle Quantum Number describes:
The energy level of an electron.
The ground state electron configuration of Ga, Galium, with atomic number
31 is:
1s2 2s2 2p6 3s2 3p6 3d19 4s2 4p1
A hollow-cathode lamp is:
A narrow-band light source
Internal standard
Prepare a set of standard solutions for
analyte (A) but add a constant amount of a
second species (B) to each solution. Add
the same amount of the second species to
the sample. Prepare a plot of
SignalA/SignalB versus [A]. Use the ratio of
signals for the sample to find the unknown
Standard addition
Add one or more increments of a standard
solution to sample aliquots of the same
size. Each mixture is then diluted to the
same volume. Prepare a plot of Analvtical
Signal versus: volume of standard solution
added, or concentration of analyte added
External standard
Prepare a series of standard solutions
(analyte solutions with known
concentrations).Plot [analyte] vs.
Analytical Signal. Use signal for unknown to
V
find [analyte].
Atomic and Molecular Spectroscopy techniques are similar yet different,
select the true statement(s) below.
UV/Visible absorption spectra of atoms is a line spectra with multiple lines
while the UV/Visible Spectra of molecules consists generally of a few intense
broad absorption bands.
Outside energy can excite all electrons in an atom if identical to the
difference between ground state and excited state energy levels ?
False
The rules for allowed transitions in Atomic Spectroscopy are
Δn = 0, ‡1, ‡2, ‡3, ‡4: Δl = ‡1
Select the true statement(s) for Atomic spectroscopy:
Absorbance is proportional to concentration
Which one of the following is are part of an inductively-coupled plasma.
atomic emission spectrometer?
Plasma torch
Induction coil
Nebulizer
Spectroscopy
the interaction of atoms and molecules with Electromagnetic Radiation.
Effect of uv-visible radiation on double and triple bonds
The pi bonds electrons Easily affected – promoted from ground to excited state
The absorbance peak nm is due to an electron printed from ground to excites state
Chromophore
Structure or group which absorbs
Eg
ethene
C=C
Lambda max at 171nm
Chromagen
Molecule containing chromophore eg ethene
H3C=CH3
Spectrometer parts
Light source
Wavelength selection eg grating has good resolution
Sample in sample holder glass, plastic, quartz
detector photo diode array
Qualitative uses spectrometer
Identify substance from Spectrum:
Position of peaks
Position of troughs
Widths of peaks and troughs Peak ratios
Unambiguous identification is difficult, so UV- Visible Spectroscopy not very commonly used for identification – IR better
Quantitative Use spectrometer
Most important use – Beer-Lambert
Biochemical analyses – enzymes, proteins (and nucleic acids
Hospitals
Detection of drugs in horses, athletes
Monitor pollution, pesticide levels
Identify confiscated narcotic drugs
Deviations in B/L Law
High concentration
Changes to sample eg precipitation
Cloudy sample eg particles scatter light which raises absorbance when it settles absorbance changes
Could also be instrument factors coming into play
What happens in a nebuliser? Use a diagram.
The process of converting a solution into a fine spray of droplets
Sample in solution is mixed with the oxidant and the fuel and them combined are sprayed into a glass bead and this causes the liquid to break into five droplets, 10% of the droplets are small enough to pass through the baffles to reach the burner head and flame.
Write a note of flames in atomic spectroscopy
Why is the lamp cathode hollow?
The cathode is hollow for two reasons
To direct the light energy it is producing in one direction
To increase the chances of the metal being redepoisites on the cathode which means the lamp will last longer
What happens in the flame, step by step?
Explain why the monochromator in AAS is positioned where it is.
30. What kind of monochromator (and detector) is mostly used for AAS?
Positioned after the sample
Reduces interference from light of other wavelengths reaching the detector
Deflection grating is most commonly used splits light up into different wavelength s and the exit slit is positioned so a particular wavelength is selected
Detector most commonly used is the photo multiplier tube
What could you do if you had calcium phosphate in your sample?
To use a high temperature flame by switching to oxidant to nitrous oxides and acetylene instead of air and acetylene
Draw a series of labelled diagrams to show the construction, and main principles of operation, of the following: hollow cathode lamp, nebuliser, graphite tube burner
Hallow cathode lamp :
Hollow cathode
Anode
Glass tube that all this is positioned in
Gas like neon inside the lamp
What kind of radiation does a hollow cathode lamp produce?
The radiation produces by a hollow cathode lamp is very narrow and specific wavelength for example if you want to measure iron in a sample you need to use a hollow cathode lamp made of iron , that cathode will only produce energy that is enough to excite iron atoms in the sample
Suited only the metal we are interested in measuring
Compare flame and graphite furnace AAS.
Describe the structure of an AAS and explain in detail the functioning and usefulness of
such an instrument
Block diagram
Discuss the advantages and disadvantages of flameless AAS compared with flame AAS.
Compare graphite burner with flame AAS
Why is it important to use the same acids, etc, in standards as in samples?
Match sample to blank to avoid interference
We use acids to put metals into solutions
It’s important to use the same acid and quantity in your standard
Explain the circumstances under which it is appropriate to use the standard addition method
in atomic spectroscopy and advantages of this method.
Adding standards to your sample and measure absorwbance
Produces a standard addition curve
Line of best fit crosses x and y axis
Label x axis Concentration of added
Used for complicated matrix eg blood urine
Concentration of actual sample is where the line crosses the z axis and ignite the minus sign
Avoids interference from the matrix from interfering with results
Ensures results are true to real compositions
Write a note on interferences in AAS and how to overcome them
5 types of interference
Spectral: iron and platinum
Inflated absorbance
Use a different wavelength to overcome it
Chemical: refractory compounds don’t dissociate in flame no free atoms produced in the flame
Changed to nitrous oxide and acetylene use a chelating EDTA agent
Which will break down these compounds, common with calcium
Use a releasing agent : prevents formation of refractory complexes
Ionisation complexes: atoms absorbing energy become ions
Add a ion surpressoer or use a cooler flame
Physical / efficiency of nebuliser
Use standard addition
Molecular: molecules that remain despite the process of atomisation eg molecules not evaporating or decomposing
Use background correction will overcome this
What could you do if you had calcium phosphate in your sample?
This is chemical interference abs there’s theee things you can do to overcome this
Increase temperature is flame chelating agent use a releasing agent
What are the uses of chelating and releasing agents in AAS? Give some examples.
To prevent chemical interference break down the stable complexes that are formed
EDTA
Strontium chloride
Prevents formation abs breaks them down to complete atomisation
What would you do if your sample contained an element which absorbs at the same
wavelength as your analyte of interest?
Spectral interference
Use a different wavelength eg iron and platinum remover e wavelength
The problems associated with AAS can be listed as follows:
Background molecular absorption
Chemical interference
Ionisation
Physical interference
Spectral interference
Describe what these processes are and some of the ways of dealing with these problems
Causes
How to overcome
What is Plasma?
Very hot conducting gas usually argon that has a significant concentration of cations and electrons
Explain the flame photometer with reference to atomic spectroscopy.
Diagram of flame photo metre had no lamp first thing you include is the sample the monochromiter and the sample
Sample is sucked up into nebuliser where it is combined with an oxidant and a gas the gas is ignited and burned , atomisation happens in the flame emitted energy is filtered through a mocochronotet and detected on the detector
In atomic spectroscopy, distinguish clearly between atomic emission and atomic absorption methods. Briefly outline why one might choose to use emission methods in certain situations.
The energy amount is the same
Use energy diagram
ICP is a powerful instrument and can looked at a broader range of periods this is why emission might be usrd
Give the functions of the three streams of Argon in an ICP torch.
The argon in the outer tube helps to cool the source and the sample is introduced into the inner tube by the nebuliser
A spark ionises the argon in the intermediate tube ( the curl) the resulting kind and electrons and this gives the plasma
Why do we not need a lamp in Atomic Emission spectroscopy?
Measuring emission we don’t want to use a lamp that could give an energy source that could be absorbed
Flame photometer
Plasma icp
Heat energy excites electrons which give off uv abs visible radiation that radiation can. S measured abs us specific to each element
If you are measuring two elements with emission lines very close together, what might you be able to do?
Spectral interference
Switch the spectral ljne ie switch the wavelength
Draw a diagram of the arrangement of the various component parts of a simultaneous ICP spectrometer.
What are the advantages of both sequential and simultaneous ICP spectrometry systems?
Sequential
Flexible measure at any wavelength but time consuming uses a lot of sample
Simultaneously: measure many enn my enemy’s at the same time uses little sample but is expensive and come with a pre set wavelength
State the advantages and disadvantages of the ICP compared with conventional flame in atomic spectroscopy.
See icp v flame photometer
Give 8
Explain how an inductively coupled plasma torch works and why it is used.
In emission it provides very high energy for excitation of electrons that is measured as emission
Describe simultaneous and sequential atomic emission spectroscopy systems. Use diagrams.
Sequential
One monochromitee and one detector
Simultaneously: polyomyre multiple detectors can measure multiple samples at the one time
Draw a diagram of the arrangement of the various component parts of a simultaneous ICP spectrometer.
Absorbance
The light energy that is absorbed by a chromophore which has double or triple bonds
Transmittance
The light energy that reaches the detector
Not a very useful parameter to use as it does not give a straight line but this value can be converted to absorbance which can give a trend line with an equation of a line
UV vis sources
Deuterium (2H) discharge lamp
Tungsten in glass and vacuum
Tungsten Hallogen
The effective bandwidth
is defined as the width of the radiation at half of its maximum throughput
Sample holders in spectroscopy
Visible: plastic
UV vis: quartz
Detectors in spectroscopy
Photomultiplier
Photo diode
Photo diode array
Spectroscopy uses
Proteins and nucleic acid analysis
Drug detection in humans and horses
Pollution pesticide levels
Indemnify narcotics
Deviations in Beer LL
Instrument factors
Large bandwidth leads to non monochromatic radiation
Electronic problems
Or possible the sample under analysis doesn’t actually absorb light in this portion of the EM spectrum you could over come this by mixing the sample with a reagent to form a coloured complex
