Exam 3 Flashcards
What are the two natures of electromagnetic radiation?
wave and particle
What is the equation for photon energy? (relationship between energy and frequency)
E = hv = hc/(wavelength)
h = Planck’s constant (6.626 x 10 ^ -34 Js)
c = the speed of light (2.998 x 10 ^ 8 m/s)
What is the unit for the particle nature of electromagnetic raidation?
photons
What are the equations for the wave nature of electromagnetic radiation?
Speed of propagation (in a vacuum) = (c) = 2.998 x 10^8 m/s
Frequency (v) = c / (wavelength)
Describe the electromagnetic spectrum. Which way does energy increase? What are the types of waves in order?
What are the effects of electromagnetic radiation on various waves? Radio waves? Microwaves? Infrared? Visible? Ultraviolet? X-rays? Gamma Rays?
What are the two steps always involved in luminescence spectroscopy?
- Excitation (photon absorption) : atom enters excited state
- Relaxation: atom returns from excited state to ground state
What are four types of molecular spectroscopy? What is the excitation process (same for all types)? and what is each type’s relaxation process?
Why is an object red?
A red object absorbs visible wavelengths except for red;
red is reflected (and transmitted, if the object is clear.)
What is lambda max?
the wavelength a sample absorbs most strongly is called (lambda symbol) max
What is spectrophotometry?
any technique that uses light to measure chemical concentrations
What is frequency? What are the units for frequency?
- the number of complete oscillations that the wave makes
- units = sec^-1
- one oscillation per second is called one hertz (Hz)
What are the fundamental molecular spectrophotometry equations?
Transmittance T = I / Io
Absorbance A = -log T
What is luminescence?
the emission of light from an excited state of a molecule
fluorescence and phosphorescence are both examples of this
What is the lowest energy state of a molecule?
the ground state
What is fluorescence?
the radiational transition S1 -> S0
What is phosphorescence?
the radiational transition T1 -> S0
Compare phosphorescence and fluorescence. (in terms of energy and wavelength)
The energy of phosphorescence is less than the energy of fluorescence so phosphorescence comes at longer wavelengths than fluorescence
What is a singlet state?
the state in which the spins are opposed
What is a triplet state?
the state in which the spins are parallel
Describe how you would sketch the visible spectrum of a substance knowing only is color and absorbance or transmittance.
The color of transmitted light is the complement of the color that is absorbed. If blue green light is absorbed, red light is transmitted
What are the two main types of luminescence experiments?
emission spectrum and excitation spectrum
How do you produce an emission spectrum?
- If we hold the excitation wavelength fixed and scan through the emitted radiation, an emission spectrum is produced.
- An emission spectrum is a graph of emission intensity versus emission wavelength (observed at lower energy than the absorption spectrum and tends to be the mirror image of the absorption spectrum)
How do you produce an excitation spectrum?
- An excitation spectrum is measured by varying the excitation wavelength and measuring emitted light at one particular wavelength
- An excitation spectrum is a graph of emission intensity vs excitation wavelength (similar to an absorption spectrum (absorption vs wavelength))
What is the main problem with fluorescence and phosphorescence as analytical tools?
Most analytes don’t fluoresce/phosphoresce
What are the advantages of fluorescence/phosphorescence as analytical tools?
- For molecules that luminesce, luminescence spectrophotometry is usually very sensitive and has low detection limits – even single molecules in favorable cases
- Since most compounds don’t fluoresce the method is usually free of background interference by the solvent
What do many analytes that fluoresce have in common?
contain aromatic rings
What distinguishes fluorescence from phosphorescence?
- Phosphorescence emits photons at a longer wavelength than fluorescence
- Phosphorescence emission occurs due to relaxation from the excited triplet state while fluorescence results from relaxation from the excited singlet state
- The lifetime of the excited state is orders of magnitude longer in phosphorescence (ns to ms but sometimes hours) than fluorescence (ps to ns)
What can be measured using light scattering?
suspended particles can be measured using light scattering
Analytical applications:
- Quantifying particulate matter in natural water
- Determining polymer size (MW) and shape
* Both synthetic and natural polymers (proteins)
* MALS (multi-angle light scattering)
What are two types of light scattering techniques?
- Turbidimetry - measuring how much light reaches the detector directly through the sample
- Nephelometry - measuring how much light is scattered below (at a right angle) by the sample
What is the beer-lambert law?
- A = abc, where a is absorptivity coefficient, b is the path cell length (in cm) and c is the concentration of the analyte
- A = ebc only if the concentration units are mol/L and the path length units are in cm
- e = molar absorptivity or extinction coefficient, units are M^-1cm^-1
What is the beer lambert law for mixtures?
A = a1b1c1 + a2b2c2 + a3b3c3…
What is the basic design for most spectrophotometers?
light source, monochromator, sample, detector
What are light sources for UV-Vis light spectrophotometers?
- Tungsten lamp for wavelengths >350 n
- Deuterium lamp for wavelengths <350 nm
What are gratings made of? What are two types of gratings in monochromators?
- Flat glass or metal with grooves, spacing of the grooves must be even
- May be transmission (inexpensive) or reflection gratings (expensive, high quality)
What are three types of detectors for spectrophotometers?
A phototube light detector, A photomultiplier tube light detector, A CCD or Charge Coupled Device (aka Diode Array)
How does a phototube light detector work?
photons produce current, more photons = more current, problem – not incredibly sensitive
How does a photomultiplier tube light detector work?
photons produce current, more photons = more current, amplifies the current produced by a photon
What is a CCD and how does it work?
- Charge Coupled Device (aka Diode Array)
- highly sensitive photon detector.
- divided up into a large number of light-sensitive small areas known as pixels, which can be used to assemble an image of the area of interest.
- a silicon-based multi-channel array detector of UV, visible and near-infra light.
Describe the function of a spectrophotometer with a monochromator.
- selects one wavelength at a time and sends it through the sample to the detector
- rotates grating to select each wavelength (moving parts)
- rotates the grating slowly and plots detector response vs time
Describe the function of a spectrophotometer with a polychromator.
- accepts all wavelengths from the sample, separates them with a grating and sends each wavelength to a unique array detector element
- no need to rotate grating
- measures light intensities at all wavelengths
What is stray light and what does it affect?
- Affects Beer-Lambert law calibration plots
- The absorbance that the spectrophotometer reports curves off due to stray light and the large range of wavelengths that our spectrophotometer can absorb
- Stray light is a constant problem with every instrument and causes lower absorbance values to be reported than are actually true
Describe atomic spectrophotometry. What is analyzed? Why is it useful?
- Analysis of the atoms (not molecules) in samples.
- Nearly always used for elemental analysis
- Crucial: the atoms of each element absorb and emit light at specific, unique wavelengths. That makes it possible to analyze a particular element in a sample even though the sample contains a mixture of elements.
What is spectral interference and where is it relevant?
an overlap (either partial or direct) of the analyte of interest by an interfering element, atomic spectrophotometry
What are the two types of atomic spectrophotometry?
emission and absorption
Describe the process of atomic emission spectrophotometry.
- Sample gets sucked up into a nebulizer, which atomizes the sample, extra drains to liquid waste, the flame then excites the atoms which release a wavelength of light which travels through the optical filter to the detector
- A yellow piece of glass will transmit yellow light, which is the light that sodium produces
- Could use a monochromator instead of an optical filter - But this is a more expensive strategy
- Using a very hot flame allows for the highest detection limits
What is the self-absorption/ inner filter effect? Where does it apply?
- Self-absorption/Inner filter effect causes the absorption spectrum to not be straight: We want a linear plot of intensity vs concentration, however it is not linear, it begins to curve. At low concentrations it behaves linearly, but at high concentration excited atoms are surrounded by ground state atoms. This means that at high concentrations excited atoms may interact with a ground state atom before hitting the detector. The higher the concentration the more likely an excited atom with hit a ground state atom before getting to the detector
- Atomic emission spectroscopy
What is the process of atomic absorption spectrophotometry?
- Atoms are nebulized into the flame, a hollow cathode lamp produces the exact wavelength that the atoms will absorb, each cathode lamp is specific to the element
- If there is no copper in the sample all of the light goes through to the detector
- If you have more copper in the sample some of the light is absorbed and doesn’t reach the detector
- The higher the concentration of the analyte the less light gets to the detector
- Absorption type experiment where the flame is the “cuvette”
Why do you want a hot flame in atomic emission spectroscopy?
- Using a very hot flame allows for the highest detection limits
- With a hotter flame, a greater fraction of atoms become excited and then relax to emit light
- An ICP or Inductively Coupled Plasma Torch produces better sensitivity and detection limits by producing temperatures up to 10,000 kelvin
Why do you want a wide flame is atomic absorption spectrophotometry?
- The large path length gives you bigger absorption, pathlength in beer lambert law
- The light has to go through a wider “cuvette” of analyte atoms if it is wider
- If you have too much signal you can rotate the flame for a shorter pathlength
What is an isotope?
Isotopes differ by the number of neutrons present
What is nominal mass?
- calculated by assuming the molecule contains only the most abundant isotopes, each having an integer atomic weight
What is exact mass?
- calculated from the exact atomic weights of the isotopes that are actually present in that specific molecule
What is average mass?
calculated from the molecular formula by using abundance-weighted atomic weights (i.e., those from the periodic table
What are the four essential components of a mass spectrometer?
- Sample inlet
- Ion source
- Mass analyzer
- Ion detector
What are 3 types of MS sample inlets?
direct, HPLC, GC
What are 3 types of MS ion sources?
electron, chemical, electrospray
What are 3 types of MS mass analyzers?
magnetic, quadrupole, time of flight
What are two ion detectors in MS?
electron multiplier, metal plates (Fourier transform)
Describe the process of electron ionization (MS)
- In a high vacuum, bombard sample molecules with a beam of electrons having enough energy to ionize them.
- If the beam knocks electrons off neutral molecules (the usual case), positive ions are formed. If molecules capture electrons, negative ions are formed. (This isn’t rare but is less common.)
- A current runs through a metal filament and an electron beam travels from the filament to the ground plate
- Most MS electron ion sources use an electron beam energy of 70eV to ensure ionization of most analytes
Describe the process of chemical ionization (MS).
- Mix a reagent gas with your analyte: (CH4 , C4H10 , NH3 , NO, O2 , SF6 , etc.)
- Reagent gas gets ionized and forms a reactive species
- The peak for the protonated molecule usually predominates
What is the problem with mass spec?
- Problem: To obtain a mass spectrum of a protein, one must (1) get the molecules into the gas phase, and (2) ionize them.
- Polymers are not volatile; heating just decomposes them.
- Most mass analyzers have m/z ranges below ~2000, so heavy ions must bear many + charges (i.e., high z) to produce m/z peaks in that range.
What is electrospray ionization?
- Process: Dissolve a sample in solvent (often aqueous and acidified). Use a charged electrode to inject positive charged into solution. Spray the solution into a vacuum to form fine droplets.
- Results: Solvent begins to evaporate, H+ concentrates in droplets, Droplets explode due to + charge repulsion, rest of solvent evaporates and is pumped away, analyte is now in the gas phase and highly charged
What is a magnetic sector mass analyzer?
- Charged ions travel in circular paths when placed in a magnetic field
- As you change the magnetic field you allow differently weighted ions to travel to the detector
What is a quadrupole mass analyzer?
- Four charged rods, oscillate the charges on the rods
- If the charges are oscillated quickly – allows you to see small ions
- If the charges are oscillated slowly – allows you to see large ions
How does time of flight MS work?
- Gas-phase ions are created
- The potential on grid 1 is lowered for 10 µs to accelerate ions by 20 kV between the back plate and grid 2.
- Ions pass through grid 2 into the high-vacuum drift region. Lighter ions travel faster than heavier ions with equal kinetic energy from 20 keV acceleration. Ions reach the detector in order of increasing mass-to-charge ratio (m/z).
- This method uses an electric field to accelerate ions to the same potential. The ions advance to the detector through a “flight” tube where the time to reach the detector is related to the mass of the ion.
- There is some spread in the kinetic energy of ions leaving the source region, so there is a spread in arrival time at the detector for ions of equal m/z.
What is a problem with time of flight and how is it overcome?
- There is some spread in the kinetic energy of ions leaving the source region, so there is a spread in arrival time at the detector for ions of equal m/z.
- Reflectron/electrostatic mirror overcomes this problem
What is a reflectron/electrostatic mirror and how does it work?
- Composed of positively charged ring electrodes that reflect the ions
- The advantage of reflecting ions through 180° with an electrostatic mirror is that ions with greater kinetic energy penetrate into the reflector deeper than ions with less kinetic energy. Faster ions travel a longer path, so all ions of the same m/z reach the detector located beside the ion source at the same time, enabling high-resolution mass spectra to be obtained.
What does mass spectrum display?
shows the mass/charge ratio (m/z) of various species, not just the mass
How do different ionization strategies produce different mass spectra?
- EI is the best to produce molecular fragmentation patterns
- CI is good for finding the molecular weight from [MH]+, which is nearly always the species produced by CI
Why is MS useful?
- high information content (types of isotopes present, fragmentation patterns, MW) and its high sensitivity and low detection limit
- It can be interfaced to GC and HPLC to obtain spectra as tiny amounts of analytes elute from the column
What is the natural abundance of carbon?
The natural abundance of carbon 12 is 98.94% and of carbon 13 is 1.07%
What is chromatography?
involved physical separation of an analyte mixture
What are four types of chromatographic methods? What are they classified by?
Classification by mobile and stationary phase
GLC, GSC, LLC, LSC
What are the five major types of chromatography?
Adsorption, Partition, Ion-exchange, Size exclusion, affinity
What is adsorption chromatography?
Solute equilibrates between the mobile phase and the surface of the stationary phase
What is partition chromatography?
Solute equilibrates between the mobile phase and the film of liquid attached to the stationary phase (via solubility in the liquid stationary phase)
What is ion-exchange chromatography?
Ions in the mobile phase are attracted to counterions covalently attached to the stationary phase (on solid ion exchange resin)
What is size-exclusion chromatography?
Different sized solutes penetrate pores in the stationary phase to different extents. Largest solutes are eluted first
What is affinity chromatography?
Solute in the mobile phase is attracted to specific groups covalently attached to the stationary phase. (make a column stationary phase for a particular analyte)
What can be run on GC? How can molecules be modified to be run?
- GC can only be used for the roughly 5% of all compounds, which is the group that is volatile enough to be vaporized. This is an important 5% (mostly small organic molecules).
- Some analytes having high boiling points can be made more volatile by derivatizing them to reduce hydrogen bonding, thus making them suitable for analysis by GC. – If GC can be used, use it
What are the GC instrument temperatures?
o Injector > highest BP
o Column is hot but usually < highest BP
o Detector > injector
High temperatures and high flow rates decrease the retention time, but also deteriorate the quality of the separation.
What are two types of GC columns?
Packed and open tubular
Describe packed columns.
- Glass or stainless steel
- Diameter 1/8-1/4 inch, length = 4-10 ft
- The packing is solid particles (for GSC) or “support” particles coated with liquid (for GLC).
- offer high sample capacity
Describe open tubular columns.
- Fused silica coated with polyimide
- Inner diameter = 0.1 to 0.53 mm, length = 5 to 100 meters (30 m typical)
- The liquid or solid stationary phase layer (usually 0.1 to 5 um thick) is coated on the interior surface of the tubing.
- Give better separation efficiency (smaller plate height), shorter analysis time, and increased sensitivity to small quantities of analyte
Why are peaks with long retention times broad in isothermal GC?
Long retention time, goes through column slowly, comes off of the column slowly
What are two types of open tubular columns?
- Wall-coated: liquid stationary phase bonded to the wall of the column
- Porous-layer: solid stationary phase particles on wall of the column
What are three types of GC detectors?
thermal conductivity, flame ionization, and ms
What is the principle of a thermal conductivity detector (GC)? A +D?
- measures how well flowing gas cools a hot metal filament, causing the current running through a coil to be affected
Advantages: - Universal. Detects any substance having a thermal conductivity that differs from the carrier gas.
- Can detect analytes that don’t ionize well (e.g., permanent gases, water vapor)
- Nondestructive. (Possible to collect the analytes.)
- Easy to use / low maintenance. (Just turn on the switch.)
- Needs no extra gases, etc.
Disadvantages:
~100-1000 times less sensitive than most other types.
What is the principle of a flame ionization detector (GC)? A + D?
- Combustion of the analyte produces ions that conduct current between the two charged electrodes.
Advantages: - Very sensitive for most analytes, especially organics.
Disadvantages: - Destroys the sample by burning it
- Requires extra gases (dust-free air and H2)
- Extremely insensitive to some especially stable or oxidized analytes (noble gases, N2, O2, H2. CO2, H2O, formaldehyde, formic acid)
What is the principle of a MS detector for GC? A +D?
- Use a quadrupole or time of flight mass analyzer
Advantages: - Very sensitive for most analytes (Even better than FIDs)
- Can determine the full mass spectrum for every peak in the chromatogram – lots of extra information
Disadvantages - Require more maintenance than most detector types
- expensive
What are zeolites?
- Can use zeolites (molecular sieves) as GC stationary phases in PLOT columns to separate gases
- Cage-type molecules with a cavity on the inside – effective filtration based on size
What are bonded phase columns (GC)?
the stationary phase is covalently bonded to the silica in the wall of a capillary column. This raises the upper temperature limit at which the column can be used without boiling stationary phase off the column.
What are the effects of boiling point on GC separation?
(often, but not always, the highest BP compound elutes last; lowest BP usually elutes first); the analyte boiling points are always an important factor in GC separations
What are the effects of polarity on GC separation?
(if using a polar stationary phase, polar analytes will have longer retention times than if a nonpolar stationary phase is used)
What is the basic strategy to run GC?
As a first approach, use a nonpolar column if the analytes are nonpolar with very different boiling points. Use a polar stationary phase if the analytes are polar or if the sample contains analytes having a range of polarities but similar boiling points
What is temperature programming in GC? Useful for? Disadvantages? Effects?
- the best way to run samples that contain analytes with a very wide range of boiling points;
- disadvantage is that the column must be cooled back to the initial temperature before starting the next run, which usually takes an extra 5-15 min;
- when using a temp. program, the baseline often shifts near the end of a chromatogram and the peaks with long retention times are narrower than they would be in an isothermal run
Why does a thermal conductivity detector detect all analytes except the carrier gas?
The thermal conductivity detector measures changes in the thermal conductivity of the gas stream exiting the column. Any substance other than the carrier gas will change the conductivity of the gas stream. Therefore the detector responds to all analytes.
What are three types of open-tubular columns?
a poly-dimethylsiloxane coated open tubular column, poly-ethylene glycol coated open tubular column, and porous layer open tubular column
What types of solutes are typically separated with a poly-dimethylsiloxane coated open tubular column, poly-ethylene glycol coated open tubular column, and porous layer open tubular column?
- Poly(dimethylsiloxane) – nonpolar, most suitable for nonpolar solutes such as nonpolar organic solvents or petroleum products
- Poly(ethylene glycol) – highly polar and suited for separation of amines and fatty acid methyl esters
- Porous layer open tubular columns are used for separation of compounds such as permanent gases like O2 N2 or CO that are too volatile to be adequately retained by a wall coated open tubular column
If you are extracting an anionic compound into a hydrophobic solvent do you need an ion pairing agent with a hydrophobic cation or a hydrophobic anion?
For example extracting EDTA complexed with Al into hexane. Need a hydrophobic cation.
What is a selected ion monitoring?
A selected ion chromatogram displays detector current for just one or a small number of m/z values. For a selected ion chromatogram, the detector is not measuring the signal for all values of m/z in each time interval. The detector is set at just the desired values of m/z and collects that information for the whole time.
What is selected reaction monitoring?
In selected reaction monitoring an ion of one m/z value is selected by the first mass separator. This ion is directed to a collision cell in which it undergoes collisional induced dissociation to produce fragment ions. One of those fragment ions is then selected by a second mass separator and passed through to the detector. The detector is responding to just one product ion from the selected precursor ion. This technique is called MS/MS because is involves two consecutive mass separation steps. The signal/noise ratio is improved because noise (extraneous signals) is very low. There are few sources of the precursor ion other than the desired analyte, and it is unlikely that other precursor ions of the selected m/z can decompose to give the same product ion being monitored.
Where is it most accurate to measure absorbances?
- In the range A = 0.3 – 2
- If absorbance is too high, too little light reaches the detector for accurate measurement. If absorbance is too low, there is too little difference between sample and reference for accurate measurement
