Clarkson Spectroscopy
A liquid sample absorbs light of wavelength 250 nm. What lamp would you need to perform a spectroscopic analysis of this sample?
deuterium-- Deuterium arc lamp emits continuous radiation spectrum from 180 nm to 370 nm. Tungsten lamp have a range from 320 nm to 1100 nm.
relative symbols and meaning in *10
giga (9)(G), mega (6)(M), kilo (3)(k), hecto (2)(h), deca (1)(da), base unit (0), deci (-1) (d), centi (-2) (c), milli (-3)(m), micro (-6) (mu), nano (-9)(n)
chemiluminescence
is a process by which radiation is produced as a result of a chemical reaction
hyperchromism
shift of peaks to greater absorbance
What technological advance was the key to making Raman a feasible instrumental technique?
Development of the laser
Vibrational energy levels (n)
Different fixed energies that molecules can have as a result of bending and stretching of bonds.
What is the importance of conjugation in UV-VIS? What is the effect of extending conjugation?
Conjugation- rearrangement or electrons- during light absorption and excitation - photon absorption, and the electrons are promoted to higher energy levels. (b) A compound must have conjugation in order to be analyzed by UV-VIS spectroscopy -conjugated systems of atoms have unique properties that give rise to strong color -extending conjugation results in bathochromic and hyperchromic shifts in absorption
Discuss the effect of the stray light on the adherence to Beer's law.
Consequences: deviations may appear in the calibration/standard addition curve with smaller or larger than theoretical absorbance errors and this causes deviation from Beer's Law.
main types of molecular vibrations
asymmetric, scissoring, rocking, wagging, twisting, symmetric
auxochrome
functional group that does not itself absorb in UV region, but shifts the peak of a chromophore to longer wavelength while increasing their intensities
accuracy
indicates how close a measurement is to the accepted/true value ; also defined as a bias in a measurement system
chromophore
light absorbing functional group
Compared to the fluorescence emisssion, phosphorescence will have
longer wavelength
fluorescence
the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity. Note that this often involves singlet->singlet transitions in neutral molecules, since typically all electrons are paired in the ground state.
singlet state
the spins of the electrons of an atom or molecule are all paired so there is no net spin angular momentum or, in other words, a many-electron state in which all electron spins are paired. Total spin angular momentum S=0. This is commonly the multiplicity of neutral molecules, both in the ground state and in excited electronic states reached by spin-allowed optical transitions
The term masking generally refers to
the transformation of an interfering species into a form that is not detected.
Mechanism action of lasers
1) Pumping 2) Spontaneous emission (fluorescence) 3) Stimulated emission 4) Absorption
Angstrom to nm
A*.1
Formula for energy
E=hv=hc/lambda
What are the two most common nuclei used in NMR?
1H and 13C
Describe two advantages that Raman spectroscopy holds over IR spectroscopy
Can analyze smaller samples, doesn't degrade sample, can use water as a solvent.
What is chemical shift?
Chemical shift correlates to the frequency emitted by the protons in a molecule. Higher frequencies are emitted when there is less shielding. Chemical shift is defined as the observed shift from the standard (TMS) in Hz times 106 divided by the spectrometer frequency in Hz; the difference in parts per million (ppm) between the resonance frequency of the observed proton and that of the tetramethylsilane (TMS) hydrogens
In NMR what are the advantages of using a magnet with as great field strength as possible?
NMR uses measurements of absorption of electromagnetic radiation. Sensitivity and resoltuion increase with strength. Ratio of Nj/N0 increases-- easier to interpret by Boltzmann's rule.
Describe briefly the basic theory and principle of RAMAN spectroscopy.
Obtained by irradiating a sample with a powerful source of visible monochromatic radiation (e.g. laser). During irradiation, the spectrum of the scattered radiation is measured at an angle (usually 90°) with a spectrometer. The intensities of Raman are ~0.001% of the intensity of the source - more difficult to detect than IR bonds. But, the Raman scattered radiation is in the VIS and near IR region where there are more sensitive detectors available When light interacts with molecules the photons are dispersed or scattered. A small number scatters at a different frequency than the incident beam. This inelastic scattering can be measured to analyze the vibrational signal of a molecule
Should SO2 absorb strongly in the infrared? How many vibrational modes are expected? Draw the normal vibrational modes of SO2 and comment on whether they are expected to be IR active
SO2 is a symmetric nonlinear molecule with three atoms. There would be three modes of vibration: bend, symmetric stretch, and antisymmetric stretch. All of these are IR active because they would be expressed in the fundamental frequencies. There should be several absorption bands from the effects of the electronegative atoms on the protons in the molecule
List the types of materials used as sample holders in FTIR? Why only those can be used? What solvents should you use in used; what is the requirement for an "ideal" solvent?
Sample holders: Methylamine (40%) on KBr plates, NaCl pellets... Potassium bromide is transparent in IR (from 4000-400cm-1). NaCl pellets are also transparent to infrared radiation. Solvents used include CS2 CHCl3 & CCl4-- transparent in the absorption region of the spectrum. There are no absorptions in the area of those solutions
Why are stokes scattering much more likely to occur than anti-stokes. Explain.
Stokes lines occur from excitation of atoms grom ground state to excited. Intensity of stokes likes is proportional to the number of atoms in ground state whereas the antistokes lines are proportional to the number of atoms in the excited state. There are more atoms in ground state than excited state, so it is more likely for stokes scattering to occur
Why does the ratio of anti-stokes and stokes intensities increase with sample temperature?
Stokes scattering is from molecules staring at ground state and the radiations have lower energies and higher wavelengths than the excitation source-- they are being excited and a measurement of that scattered radiation. Anti-stokes is from moelcules starting at esxcited state, a state of higher energy and lower wavelength than that of the excitation source, and falling to ground state. More molecules get excited when the temperature is increased. The number of molecules in excited vibrational levels increases with temperature. The number of molecules in ground vibrational levels decreases with temperature. The ratio of anti stokes to stokes will increase with temperature.
What nuclear property is needed to see a NMR signal?
Strong magnetic fields are necessary-- there must be an applied magnetic field and one present in the molecule. The nuclear spin is effected and can be detected
What is SERS? Discuss the application/interest of using SERS in chemical analysis.
Surface-enhanced raman spectroscopy (SERS) is a technique used in detection of biological samples for its increased sensitivity. SERS can detect various diseases such as cancers. Using nano substrates to enhance the signals while fluorescence is suppressed, the technique increases the sensitivity of Raman spectroscopy lowering the limit of detection. It increases the probability of a Raman transition and is a promising new form of analysis for food and biological samples.
What is a virtual state in Raman?
The virtual state is the intermediate state resulting from the Raman process. It is short lived and between the electronic ground and excited state
What is the purpose of the deuterated solvent?
There is more solvent than sample in an NMR tube, the deuterated solvent has the deuterium atoms which absorb at a different frequency than the protons. It allows a standard to be defined at 0ppm to prevent TMR to be required-- the spectrometers can use the deuterium signal
ppm to ppb
parts per million * 1000
Hypochromism
shift of peaks to lower absorbance
Hypochromic shift
'blue shift' - shift of peaks to shorter wavelengths
Bathochromic shift
'red shift' - shift of peaks to longer wavelengths
In a hydrogen-oxygen flame, the atomic absorption signal from iron was found to decrease in the presence of large concentrations of sulfate ion. (a) Suggest an explanantion for this observation (b) Suggest three possible methods for overcoming the potential interference of sulfate in a quantitative determination of iron.
(a) Sulfate ion forms complexes with Fe(III) that are not readily atomized. Thus, the concentration of iron atoms in the flame is less in the presence of sulfate ions. (b) Sulfate interference could be overcome by (1) adding a releasing agent that forms more stable complexes with sulfate than does iron, (2) adding a protective agent, such as EDTA, that forms highly stable but volatile complexes with Fe(III), and (3) using a higher flame temperature (oxyacetylene or nitrous oxide-acetylene)
Give examples of factors that can affect the robustness of an analytical method
(a) changes in instrument operational conditions (ex: pH, t0), (b) different operators, (C) effect of ageing of reagents, (d) different laboratories
With specific reference to the analysis of an aqueous MgCl2 solution, describe the processes that are likely to occur during atomization/ionization.
Nebulization: Aqueous solution containg MgCl2 is converted to an aqueous aerosol. Desolvation. The solvent is evaporated leaving solid particles. Volatilization. The remaining water is evaporated and the particles are vaporized. Atomization. Mg atoms are produced Ionization of Mg to Mg+ Possible reaction of Mg to form MgOH and MgO
Discuss the major reasons why molecular phosphorescence spectrometry has not been widely used as molecular fluorescence spectrometry
Not as many molecules give good phosphorescence signals as fluorescence signals. As a result, the experimental requirements to measure phosphorescence are more difficult than those to measure fluorescence and the applications are not as large. For example, the triplet state has a long lifetime and is very susceptible to collisional deactivation. Thus, most phosphorescence measurements are made at low temperature in a rigid matrix or in solutions containing micelles or cyclodextrin molecules. Also, electronic methods must be used to discriminate phosphorescence from fluorescence.
Why is an electrothermal atomizer more sensitive than a flame atomizer?
Electrothermal atomizers rapidly atomize the sample in a short burst, producing a locally higher concentration of free analyte atoms per unit of time in the optical path than is possible with a rapidly flowing flame atomization source. This higher "effective concentration" allows for greater sensitivity than with a flame. Essentially, more of the analyte atoms have a chance to stay in the optical path long enough to contribute to the signal than is the case for a flame atomizer.
What are the most known emission techniques?
Fluorescence and phosphorescence.
Give an example of a fluorescence quenching reagent. What is fluorescence quenching? What is the role of fluorescence quenching in quantitative chemical analysis. Explain.
Fluorescence quenching - decrease in fluorescence when excited molecules relax to ground states via non-radiative pathways avoiding fluorescence emission. Example: oxygen Can be used as an indirect way to quantify analytes (e.g. quenching analyte) Types of quenching include: • concentration quenching (a decrease in the fluorescence-per-unit-concentration as the concentration is increased). Concentration quenching results from excessive absorption of either primary or fluorescent radiation by the solution. • chemical quenching. Chemical quenching is due to actual changes in the chemical nature of the fluorescent substance (conversion of a weak acid to its anion with increasing pH) • collisional quenching may be caused by nonradiative loss of energy from the excited molecules, and the quenching agent (such as oxygen) may facilitate conversion of the molecules from the excited singlet to a triplet level.
Why do quantitative and qualitative analyses often require different monochromator slit widths?
For qualitative studies the details of the spectra are important in identifying species, so a narrow monochromator slit width is desirable to reveal details in the spectrum. For quantitative studies a wider slit is preferred so that only the necessary details are visible.
In what state (gaseous or solution) you get greater detail in a UV-VIS spectra and why?
Gases allow greater detail - in gaseous state the molecules are sufficiently separated to vibrate and rotate freely (in solution phase where they have little freedom to rotate and the vibrational levels can be affected by the solvent).
What is the effect of bandwidth on spectral detail?
Greater detail is obtained with the narrowest slit
What is the source in atomic absorption. Describe the specific details of this source and how it works.
Hallow cathode lamp is used to produce narrow lines of the same frequency as the analyte of interest. HCL contain vapors of the same analyte as that being analyzed and is filled with neon or argon at a pressure of ~130-700 Pa. ❑ When an electric discharge is applied, this ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. ❑ Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light (atoms or molecules that are excited to high energy levels can decay to lower levels by emitting radiation (emission or luminescence)). • Cathode made of metal of interest (Na, Ca, K, Fe...) • Different lamp for each element which restricts multielement detection
How does the general design for fluorescence compare with the one for absorption and chemiluminescence measurements?
Here two wavelength are needed to select the excitation and emission wavelengths Emission and chemiluminescence differ from the other types in that NO external radiation source is required - the sample itself is the emitter. The radiation produced by the analyte vapor is isolated by the wavelength selector and converted to an electrical signal by the detector
Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum. Which one more closely resembles an excitation spectrum?
In a fluorescence emission spectrum, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength. In an excitation spectrum, the emission is measured at one wavelength while the excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum since the emission intensity is usually proportional to the absorbance of the molecule
What is the difference between a fluorescence excitation spectrum and a fluorescence emission spectrum? Which one resembles an absorption spectrum?
In a fluorescence emission spectrum, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength. In an excitation spectrum, the emission is measured at one wavelength while the excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum since the emission intensity is usually proportional to the absorbance of the molecule.
What do you measure in a kinetic study? What is the wavelength region at which you are measuring and how do you determine it.
In a kinetic study the absorbance of the solution is measured over time at a particular wavelength. The wavelength can be determined experimentally for either A or P (decrease in A; appearance of P) by first scanning a UV-VIS spectrum for either A or P.
What is the optimum absorbance range in order to ensure linearity of Bear's law and where the measurements are the most precise and accurate
Optimum absorbance range: 0.1-1
List the main causes for deviation in the Beer's law. Which are the ones that you can control/avoid in your analysis?
Real deviations- fundamental, real limitations of the law 1. Beer's law applies for low analyte concentrations - can be avoided by diluting the sample to be in the linear range 2. Refractive index - can be avoided by using a correction term εn/(n2+2)2for ε Instrumental deviations- occur when absorbance measurements are made 1. polychromatic radiation - Beer's law applies when measurements are made with monochromatic radiation - to avoid this effect, the absorbance is measured at the wavelength band near the wavelength at maximum absorbance 2. Instrumental noise 3. Slit width - use variable slit widths - greater detail and higher resolution are obtained with narrow slit. 4. Scattered radiation due to absorption of light on glass optics, prisms, etc. 5. Mismatched cells: use identical cells (for double beam instruments) or one cell with single beam instruments and keep it in the same position for both blank and analyte reading Chemical deviations - a result of chemical changes that occur when the concentration changes (e.g. degradation, pH dependent processes)
Quinine is one of the best-known fluorescence molecules and the sensitivities of fluorometers are often specified in terms of the detection limit for this molecule. Predict the part of the molecule that is most likely to behave as the chromophore and fluorescent center.
The fused aromatic ring portion is most likely to act as a chromophore (large conjugated pi-electron systems tend to show absorption in the UV/vis) and fluorophore (rigid ring structures hinder vibrational relaxation pathways for internal conversion, which competes with fluorescence)
how can you vary the temperature of the flame?
The graphite furnace consists of a small carbon tube supported between two graphite electrodes. Typically, 1 to 50 μl of a liquid sample is placed within the tube and the entire assembly is heated by passage of a low current through it. At this current, the temperature of the tube is just sufficient to remove the solvent from the sample. Then, a higher current is passed through the assembly to ash the sample if it contains a volatile matrix. Finally, the sample itself is vaporized by passage of a current of several hundred amperes through the tube. At this current, the tube can reach temperatures greater than 3000°K, sufficient for vaporization and atomization of most elements. The atomization time ranges between few milliseconds and seconds
Why is spectrofluorometry potentially more sensitive than spectrophotometry?
The key difference is that spectrofluorometry measures the intensity of fluorescence, while spectrophotometry (absorption spectroscopy) measures the ratio of the intensities of two beams (incident and passed beams). The fluorescence signal is directly proportional to the intensity of the excitation source. We can increase the intensity of the excitation source (use a laser!) and gain a subsequent increase in the fluorescence signal—potentially leading to greater sensitivity. Spectrophotometry, on the other hand, is an absorption technique. Absorption depends on the ratio of incident to passed light: so simply increasing P0 also increases P.
What are the main components of a laser?
The main component of lasers is Lasing medium - crystal (ruby), semiconductor (gallium arsenite), solution of organic dye, inert gas (Ar, Kr) Lasing medium - is activated / pumped by radiation from external source (except gas lasers) Formation of Photons with the same energy LASERS (Light Amplification by Stimulated Emission of Radiation) - start with material that will exhibit stimulated emission and populate upper states typically using another light source
Absorption spectra for molecules are more complex than atomic spectra because:
The number of energy states of molecules is enormous when compared with the number of energy states for isolated atoms.
What is the region of the spectrum in which you mostly characterize compounds in UV VIS spectroscopy?
Typically 200-700 nm; you could also run up to 900 or 1100 nm
Rotation energy (l)
consist of at least two atoms, do not remain stationary in space but are in constant state of motion. As the molecule rotates, its rotational energy continually changes and if the change is great enough, the electron in the molecule can gain that energy and transition to a higher quantum state
Beer's law
explains the relationship between absorbance, at a given wavelength and concentration, A = εlc-- ASSUME l IS ONE CM
A molecule which has absorbed a photon of energy in the visible range could move from the excited electronic state S1 to a triplet excited state T1 and then to a highly excited vibrational level of the ground state (S0). This process is
intersystem crossing and phosphorescence
Describe the theory of atomic absorption spectroscopy (AAS) -describe the electronic levels.
AAS: The elements of sample are converted to gaseous atoms or elementary ions; atoms in gaseous state absorb light & undergo electronic transitions (ex 3s-4s levels) . At room temperature, essentially all atoms of a sample are in ground state. Excitation of this e- to higher orbitals can be achieved by heat of a flame, plasma or an electric arc or spark. The lifetime of the excited atom is brief. Ex: In a hot gaseous medium, Na atoms absorb radiation of wavelength characteristics of electronic transitions from the 3s state to higher excited states
robustness/ruggedness
Ability of the method to withstand small, deliberate changes in operating parameters: method, reagents, equipment, operator, etc.
What is IR spectroscopy (basis of the method) and what is its main utility?
Applying light through a solution without the compound of interest is used to create a standard. The light excites the molecules and the emissions from frequencies not absorbed are a fingerprint of the organic compounds present. Different effects of shielding from the relative proximity of protons to other atoms changes the spectra and allows the identification. The compound of interest is then added to an identical solution and the spectra are compared to determine the functional groups in the compound. The IR spectrum allows the identification of functional groups in a compound. When analyzed it can be used to identify the compound
What are the advantages of decreasing monochromator slit width?
As the spectral bandwidth decreases, the structure in the spectrum is enhanced and more detail is revealed, allowing determination of more complex structures/spectra with higher resolution.
What is the shape of a spectra associated with atomic species. How is this as compared to molecular species?
Atomic spectra are simple line spectra because there are no bonds to vibrate or rotate around, just electrons to promote
Describe the flame as sample ionization method
Atomization process with a continuous sample introduction into plasma or flame. 1. The solution is converted into a spray by the nebulizer 2. The high t0 causes the solvent to evaporate, leaving dry aerosol particles 3. Further heating volatilizes the particles producing atomic, molecular and ionic species The flame has three regions: these are variable in size, depending on fuel / oxidant ratio, and fuel and oxidant types: - primary combustion zone: the thermal equilibrium is not achieved in this zone. - interzonal area: the highest temperature achieved, few cm in height with the highest population of free atoms. It is the most widely used part of the flame. - secondary reaction zone: the products from interzonal area are converted to stable molecular oxides
Why do some absorbing compounds fluoresce but others do not?
Compounds that fluoresce have structures that slow the rate of nonradiative relaxation to the point where there is time for fluorescence to occur. Compounds that do not fluoresce have structures that permit rapid relaxation by nonradiative processes.
Electronic Energy Levels
Definite fixed energies that electrons in an atom or molecule can take. Similarly, electronic transitions tend to accompany both rotational and vibrational transitions
In what situations you would use derivative spectroscopy. What are the advantages of derivative spectroscopy?
Derivative spectroscopy can be used for quantitative and qualitative purposes; it is especially useful for (1) qualitative identification of species with overlapping spectra (through feature enhancement) and (2) for simultaneous determination of two or more components in the mixtures. Derivative spectroscopy can reveal spectral details that are lost in a usual spectra and measure the analyte more accurately.
selectivity/ specificity
Describes the extent to which the method is free from interferences; Coefficient of selectivity is used to define selectivity: K(AB)=mB/mA-- selectivity coefficient for A with respect to B
What are the main types of UV-visible sources and what is the wavelength region in which they work?
Deuterium (or Hydrogen) Lamp for the UV range - 160-400 nm; usually switch to tungsten at 350 nm ▪ Tungsten Filament Lamps- this is usuallyW-Iodide 350-2500 nm ▪ Xenon Arc Lamps- 200-1000 nm ▪ Light-Emitting Diodes- 375-1000 nm
Would you use a tungsten or a deuterium lamp as a source of 300 nm radiation? Why is iodide introduced into a tungstein lamp?
Deuterium lamp (b) Iodine reacts with gaseous tungsten forming volatile WI2; when molecules of this compound strike the filament, it is decomposed, redepositing W, which increase its lifetime; the presence of iodine doubles the lifetime of the lamp
Briefly describe the functioning of ELISA. Where is the UV-VIS measurement used in an ELISA set-up?
ELISA- biochemical technique used to detect the presence of an antibody or antigen in a sample -(1) unknown amount of antigen is affixed to a surface; (2) specific antibody (linked to an enzyme) is washed over the surface to bind to the antigen; (3) add a substance that enzyme can convert to some detectable signal (colored or fluorescent product) -UV-VIS is used at the end to measure the absorbance of the solution; if there is more of the analyte (antibody/antigen) then more enzymes will be bound and unable to function to convert the substance to a detectable signal. This means that a substance with more analyte will have a lower absorbance
Describe the possible energy states of chemical species
Electronic / Vibration / Rotation
Would you use UV-VIS for qualitative purposes or not? Explain.
UV-VIS could be used for qualitative purposes. However, by itself an electronic spectrum is not sufficient to identify a compound. This could be a good method for mechanistic qualitative studies in pure solution especially aqueous (e.g. determination of analytical constants, effect of pH, ect); IR and NMR are far better methods for qualitative purposes.
Give example of properties of an "ideal" solvent to be used in a UV-VIS analysis.
UV-VIS transparent; do not have overplaying spectra with the species being analyzed; Examples of good solvents: water, ethanol, hexane, cyclohexane.
Explain the working principle of a photoelectric transducer.
Vacuum Phototube - The radiation causes emission of electrons from a photosensitive solid surface The number of electrons ejected is direct proportional to the radiant power of the beam that strikes the surface
When does stray light occur and what are the consequences of having stray light in a spectrophotometric analysis?
Wavelength of stray radiation may differ from that chosen for determination, and the radiation may not have passed through the sample
Is it possible to use UV-VIS spectroscopy to quantify a non-absorber species? Provide a specific example.
Yes, (1) by converting a non-absorber to an absorbing species or (2) by using an absorber that will change the concentration of your analyte (indirect detection) in a concentration dependent manner. Ex: (1) Organic chelating agents may be used to form stable, colored complexes with non-absorbing cations. For example, dimethylglyoxime may be used for the determination of nickel. The absorbance of the resulting bright red complex can be used as a measure of the concentration of nickel. Ex (2) Glucose with Cu - see experiment from the Spec lab
What are the possible electronic transitions occurring in the UV VIS region?
a) σ → σ* b) n → σ* x c) n → π*and π → π* The absorption peaks for these transitions fall in an experimentally convenient region of the spectrum and require less energy than σ → σ* and n → σ*
internal conversion
the intermolecular process in which a molecule crosses to a lower electronic state without emitting radiation or in other words, intramolecular processes which allow an excited state to relax to another electronic state or the ground state without emission of a photon and without a change in multiplicity.
vibrational relaxation
the process by which a molecule loses its excess vibrational energy without emitting radiation, or, dissipation of excess energy in a molecular excited state among the vibrational degrees of freedom.
intersystem crossing
the process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g., singlet → triplet).
phosphorescence
the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited triplet state to a lower state of a different spin multiplicity (e.g., a triplet → singlet transition)
triplet state
the spins of the electrons of an atom or molecule are unpaired so that their spin angular moments add to give a net non-zero moment or, in other words, a many-electron state in which two electron spins are parallel. Total spin angular momentum S=1. Typically not accessible by *allowed* optical transitions from the ground state for neutral molecules
Electromagnetic radiation in the IR range causes
vibrational and rotational excitation.
interference occurs
when a species other than the analyte causes an analysis to be inaccurate
precision
the degree of reproducibility of multiple measurements under a given set of conditions (variation in the result when the analytical process is repeated - coefficient of variation, standard deviation, standard error, or confidence interval)-repeatable, reliable, getting the same measurement each time
What are the principal components of a regular spectroscopic Instruments.
1. Stable source of energy. 2. Transparent sample holder. 3. A device that isolates a restricted region of the spectrum for measurement. 4. A radiation detection: converts radiant energy to a useable electrical signal. 5. A signal processor and readout + recorder (Computer)
What are some of the advantages of plasma compared with flame in atomic emission spectrometry?
:Advantages of plasma sources include: 1. Lower interferences 2. Multidetection: Emission spectra for many elements can be obtained with one set of excitation conditions. 3. Plasma sources usually have a linearity range that covers several decades in concentration. Flame atomic absorption requires a separate lamp for each element, which is not convenient when multiple elements are to be determined
What is an isosbestic point? When are isosbestic points observed and why?
Isosbestic point- the point at which one absorbing species is converted to another absorbing species during the course of the chemical reaction
Describe the absorption profiles in the following figure: (figure shows absorbance vs height)
Mg: maximum absorbance at ~ middle of the flame by combining two opposite effects. First - increase in absorbance as an increase of Mg atoms, and second a decrease in absorbance as consequence of Mg oxidation. Ag: Very resistant to oxidation (silver oxides are not readily formed), Its absorbance increases as a continuous increase in the number of atoms formed. Cr: forms very stable oxides, which generate a continuous decrease in absorbance.
Explain or define the mirror image
Mirror Image: the fluorescence spectra is roughly a mirror image of the absorption (excitation) spectra - fluorescence at a longer wavelength. This is due to the fact that the vibrational levels in the ground and excited states have nearly the same spacing and the molecular and orbital symmetries do not change.
What is the main requirement for a molecule to be measured by IR spectroscopy? What are the exceptions?
Molecules must have a bond which can exhibit a dipole moment. Any frequencies emitted on the electromagnetic spectrum from 14000-4000cm-1 can be detected by IR spectroscopy. Molecules absorb specific frequencies and the level of transmittance is detected. Infrared spectrum of tri-t-butyl carbinol is affected by steric interactions. In cyclopropane the H atoms have more s characters which make the bond frequencies greater than 3000cm-1. Substitution of halogen can affect C-H stretching. Nearly all molecules absorb infrared radiation, the exceptions being homonuclear diatomics such as O2 , N2 , and H2
Do molecules with single bond absorb light? What region of the spectrum you would expect absorption to occur in this case and what are the possible transitions in this region?
Molecules with single bonds do absorb light at wavelengths but at a wavelength less than 185 nm and some from 150-250 nm (for unsaturated compounds with lone pairs of electrons). The possible transitions include in this case σ → σ*, n → σ*, which require higher energy.
If the value of the molar absorptivity is very high, would you expect your method to be more sensitive or not?
More sensitive; you can determine lower concentrations of analyte
Describe the atomization process in AAS.
Nebulization, Desolvation, Vaporization, Atomization
Compare the operation of a single beam and a double-beam spectrophotometer. How are these different from a diode array spectrometer
Single beam and double beam spectrophotometers are both using a monochromator. - so they are both scanning spectrophotometers. For the single-beam, the monochromatic radiation from the filter or monochromator passes through the cell before the detector. For a double-beam monochromatic radiation from the filter is split into two beams that simultaneously pass through the reference and sample cells before reaching the detector Diode array spectrometers- photodiode array uses is a dispersion device (a diode array) placed after the reference or sample cell; here, the spectrum is obtained by electronic (and not mechanical scanning as for the monochromatros based). In this way all wavelengths are measured at once.
What are the procedures for measuring absorbance with a single-beam and a double beam spectrophotometer? Briefly describe the procedure that you would use to measure UV-VIS spectra with a single beam spectrophotometer.
Single-beam- take separate measurements of the reference (solvent) and sample at each wavelength; Fill a cuvette with the reference solution (solvent), and 'blank' the instrument. Next, fill cuvettes with each sample separately, place them individually into the spectrophotometer, and take separate measurements of each sample. The background was in this case stored by the instrument; the computer keeps in memory the last background run. Double beam- take measurements of blank and samples at the same time
Briefly describe the absorption process by a molecule M. Is this a destructive or non destructive process?
Step 1 M + hν M* electronically excited species having a short lifetime of ~10-8 -10-9 s its concentration is almost negligible Step 2 M* M + heat relaxation involving conversion of the excitation energy to heat (it could also involve fluorescence or phosphorescence) the amount of thermal energy resulted is quite small The method can be considered as non-destructive because the system is minimally disturbed (unless photochemical decomposition occurs)
What is stray light?
Stray (or scattered) light results from scattering or reflection of light on the surfaces, grating device, lenses, mirrors, filters, or windows.
Which of the following is proportional to concentration?
THIS ONE absorbance<-- percent transmittance transmittance
For the same concentration of Ni, the absorbance at 352.4 nm was found to be about 30% greater for a solution that contained 50% ethanol than from an aqueous solution. Explain.
The following factors can lead to a great number of Ni atoms in the viewing region of the flame: ▪ Alcohol may add its heat of combustion to the flame leading to a slightly higher temperature compared to water which cools the flame. ▪ Alcohol changes the viscosity of the solution which may increase the nebulizer uptake rate -more efficient atomization.
Discuss the effect of temperature in AAS.
The temperature affects the ratio between the number of excited and unexcited atomic particles. An increase in t produces an increase in the efficiency of the atomization process and thus an increase in the total nb of atoms in the vapor gas
What is the x axis in IR; what is the x axis in RAMAN?
The x axis in IR is the wavenumber and ranges from 400-4,000 (right to left). The x-axis provides the absorption number. The x axis in RAMAN is the Raman shift and is the measure of the frequency of the light emitted during scattering compared to the incident beam-- it is the energy difference at each frequency between the laser and the detected light expressed as a wavenumber.
How does the flame method compare to electrothermal atomization?
This type of atomization requires a graphite furnace, where after thermal pre-treatment the sample is rapidly atomized. Entire sample is atomized which generates enhanced sensitivity: Since the dilution and expansion effects of flame cells are avoided, and the atoms have a longer residence time in the optical path, a higher peak concentration of atoms is obtained: a higher sensitivity
Explain the phenomenon of absorption / emission of light by a molecule and describe the processes in terms of energy levels. How this information can be used for obtaining analytical information on an analyte of interest?
measuring emitted/absorbed radiation gives information When a molecule absorbs a photon, the light energy is transferred to the electrons in the molecules so that the energy of the molecule increases: the molecule is promoted to a higher energy, excited state. The molecules will usually remain in the higher energy, excited state for a very short period of time before they return to the lower energy, ground state. As the molecules return to the lower energy state from the higher energy state, they must lose energy. When a molecule emits a photon, the energy of the molecule is lowered. The lowest energy state is called the ground state
Transition between states
nebulization: sample + gaseous oxidant + gaseous fuel purged into flame spectra. ❑ desolvation: solid aerosol finely divided ❑ volatilization: formation of gaseous molecules ❑ dissociation: formation of atomic gas ❑ Ionization: formation of cations and electrons
ppm to g/L
parts per million *.001
What sample holders are accurate for... UV? VIS? IR? RAMAN?
quartz or silica; quartz, plastic or silica; KBr, NaCl, AgCl; simple glass capillaries can be used
external conversion
radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute
Which of the following types of transition would require the lowest energy of radiation?
rotational transitions