Analytical Chemistry 2210 OSU
Beers law
A = Ebc
Colorimetry 1
A chemical analysis method that uses color intensity and change to determine solution concentration
Spectrophotometry 1
A colorimetric method used to determine the amount of analyte in a sample by the samples ability to absorb light at a given wavelength
corrected absorbance equation
CA=(total volume/initial volume)*(observed absorbance)
What will be the effect of a white light?
A white light will produce a complex spectra impossible to interpret and its necessary to use only defined light regions for spectrochemical analysis
Absorbance formula
A=-logT
Relaxation Processes from Excited State
There are multiple possible relaxation pathways Vibrational, Rotational relaxation occurs through collision with solvent or other molecules
Furnace Atomization
Usually made of graphite, Graphite furnace employs temperature programming to produce elemental vapor
solving for concentration of abs. vs wavelength graph
a) Write two equations b) Find all ε with standards c) measure A for mixture at two λ
grating equation
an equation that can be used to determine accurately the wavelength of monochromatic light sent through a diffraction grating, dsinθ = nλ.
spectrophotometer
an instrument used to make absorbance or transmittance measurements
Spectrophotometry 2
any technique that uses light to measure the chemical concentrations
Fluorescence Resonance Energy Transfer (FRET)
applicable to bio-sensing and determining distance between acceptor and donor
transverse heating
heating by induction and not conduction
Amplitude
height of a waves electric vector
The energy of the incident light must be greater than the work function of the cathode
important?? idk
change this to allow specific wavelength through the exit slit
incidence angle
Isosbestic Points
indicates that only 2 interconverting species are present in solution
energy gap for emission
is exactly the same as for absorption
Excitation Spectra
measure fluorescence or phosphorescence at a fixed wavelength while varying the excitation wavelength
Emission Spectra
measure fluorescence or phosphorescence over a range of wavelengths using a fixed excitation wavelength
light detector
measures the amount of light being transmitted through the sample photomultiplier usually works by converting light signal into an electrical signal
atomic spectroscopy
specific absorption or emission patterns are used to identify the elemental composition of the substance
Transmittance
the relative amount of light making it through the sample (P/Po)
transverse heating process
the sample is injected onto the platform, which is heated by radiant energy from the wall, The sample is vaporized when the wall reaches a constant temperature
why is a correction needed on a spectrophotometric titration curve
the volume becomes different after each addition of titrant
intersystem crossing
transition between a singlet electronic state to a triplet electronic state by overlapping vibrational states
internal conversion
transition between singlet electronic states through overlapping vibrational states
energy comparison of triplet state to singlet state
triplet state has lower energy in general than singlet state
deuterium lamp
ultraviolet light (160-375 nm)
Wavelength selector (monochromator)
used to select a given wavelength of light from the light source
process of exciting molecules of IR wavelengths
vibrational excitation
tungsten lamp
visible light (320-2500 nm), based on black body radiation
Stray light, also called optical noise
wavelength outside of the bandwidth of the monochromator that reaches the detector
ground to excited state
when a chemical absorbs light, it goes form a low energy state (ground) to a higher energy state (excited)
Inductively Coupled Plasma
• Twice as hot as the flame • Stable • Inert Ar environment • Simultaneous multi-element analysis • Costs more to purchase and operate than the flame
Furnace Atomization features
• Usually made of graphite • Heated electrically via temperature programming • Light from hollow cathode tube enters furnace • Only 1-100 μL of sample required • More sensitive than flame because more time spent in the furnace (signal averages longer)
The atomic absorption and emission spectra consist of sharp lines while the molecular spectra exhibit broad absorption bands. This is because: a) the atomic absorption and emission are faster than the corresponding molecular processes. b) high temperature increases the population of the atomic excited states. c) atomic spectra are recorded in the gaseous phase only. d) atomic energy levels do not have vibrational and rotational sublevels. e) molecular peaks are broadened by the solvent.
d
Which of the following statements is not true about an excitation spectrum? a) It is nearly the same as the absorption spectrum. b) It is a mirror image of the emission spectrum. c) It is collected at variable excitation and fixed emission wavelength. d) It has a higher energy than the emission spectrum. e) It is collected at constant excitation and variable emission wavelength.
e
How much energy in kJ/mol is released when nitrogen emits Ka radiation at 0.392 keV
3.78x10^4 kJ/mol
wavelength range of visible light
400-750 nm
Explain what is meant by the Doppler Effect. Rationalize why Doppler broadening increases with increasing temperature and decreasing mass
- doppler effect occurs because an atom moving toward the radioation source sees a higher frequency than an atom moving away from the source - increasing temperature gives an increased speed, hence more broadening -increasing mass gives a reduced speed, less broadening
Explain why X-Ray fluorescence is observed when a matter absorbs X-rays of sufficient energy. Why does each element have a unique X-ray signature?
-absorption of x-rays ionizes electrons from the atom. when an electron is removed from the inner shell, an electron from an outer shell of the atom falls into the vacancy. The excess energy of the electron making the transition is emitted as x-rays. -electronic energy levels are different for each element
the figure on the right shows a temperature profile for a furnace atomic absorption experiment. Explain the purpose of each different part of the heating profile
-drying: removes water from sample -char/ashing: intended to remove as much matrix as possible without evaporating the analyte -atomization: vaporizes analyte into free gas-phase atoms
What is the purpose of a monochromator in atomic spectroscopy? Why is the use of a chopper essential in atomic spectroscopy?
-monochromators are used after atomization and absorption/emission. therefore, the purpose is to disperse the light to enable different wavelengths to scanned, where they reach the detector one at a time. -Chopper is essential to different signal from the flame. The chopper imposes frequently on the atomic line, which can be filtered from the frequency independent light from the flame
Beer's Law- relative amount of a certain wavelength of light absorbed (A) that passes through the sample is dependent on
1. Distance the light must pass through the sample (cell path length-b) 2. amount of absorbing chemicals in the sample (analyte concentration-c) 3. ability of the sample to absorb light (molar absorptivity - E)
Ways to Atomize a Sample
1. Flames 2. Furnace 3. Inductively coupled plasma (ICP)
materials for a sample cell and their light range use
1. glass- visible region 2. Quartz- ultraviolet 3. NaCl, KBr- infrared region
what happens when a molecule absorbs light?
1. molecule promoted to a more energetic excited state 2. infrared and microwave radiation 3. combined electronic, vibrational & rotational transitions 4. relaxation processes from excited state 5. fluorescence and phosphorescence
one photon produces how many electrons?
10^6 to 10^7
combined electronic, vibrational and rotational transitions
Absorption of photon with sufficient energy to excite an electron will also cause vibrational and rotational transitions
Fluorescence and Phosphorescence Intensity
At low concentration, emission intensity is proportional to analyte concentration. I=k*Po*c
Double-Beam Instrument
Continuously compares sample and blank Automatically corrects for changes in intensity of light source
Energy formula
E=hc/λ
homogeneous
Even if all atoms or molecules in a sample were identical, there would still be broadening
energy comparison of vibrational and rotation states to electronic state
Excited vibrational and rotational states are lower energy than electronic state
True or False: The purpose of a monochromator in the atomic spectroscopy is to isolate narrow monochromatic lines from source.
False
Spectrophotometric titrations
For total iron-binding capacity or transferrin saturation
energy of phosphorescence (P) compared to energy of fluorescence (F)
P lower in energy than F
Absorbance
The amount of light absorbed by a sample
Wavelength
The distance between two corresponding parts of a wave
Atomization by Inductively Coupled Plasma
The induced current, composed of ions and electrons flowing in a closed circular path, heats the support gas to a temperature on the order of 9000 to 10000 K and sustains the ionization necessary for achieving a stable plasma
Flame Atomization
The process of reducing a molecular sample into a population of its constituent atoms by burning that sample in a flame
True or False: Sensitivity with an inductively coupled plasma is enhanced by using piezoelectric crystal as an ultrasonic nebulizer
a
diffraction grating
a device made of thousands of closely spaced slits through which light is passed in order to produce a spectrum
The first excited state of Ca is reached by absorption of 422.7 nm light. a. find the energy difference (kJ/mol) between ground & excited states b. the degeneracies are g*/g=3 for Ca. Find N*/N at 2500 K c. by what % will N*/N change with 15 K rise in temp d. Find N*/N at 6000 K
a. 283.0 kJ/mol b. 3.67x10^-6 c. 8.2% increase d. 1.03x10^-2
two possible transitions in excited state of molecules
a. single state- electron spins opposed b. triplet state- electron spins are parallel
Comparison of Atomization Sources a. Flame b. Furnace c. Plasma
a. • Inexpensive, readily available • Limited T range • Requires large amounts of sample • Instability in flame leads to lack of precision b. • High temperature and low volume • Highest sensitivity for absorption • Prone to interference effects • Requires specialized skill to operate c. • High temperature and highly stable • Inert Ar plasma is free of most interference effects • Simultaneous multi-element analysis by atomic emission (to be discussed) • Most expensive to purchase
Why is specificity and selectivity inherent in atomic spectroscopy ?
because each atom produces a specific set of line spectra
process of exciting molecules of X-ray wavelengths
bond breaking/ionization
Doppler Broadening
broadens the bandwidth to 10-3 - 10-2 nm, caused by randomness in the velocity of the atoms
The Doppler effect is one of the reasons for line broadening in the atomic absorption and emission. Which of the following factors increases magnitude of Doppler broadening? a) higher atomic mass b) decreasing temperature c) higher velocity of the absorbing or emitting species d) higher number of atoms moving perpendicular to the transducer's path e) higher concentration of atoms in the plasma
c
The UV/Vis spectrum of acetone has two absorption maxima at 187 nm and 273 nm. The lowest energy absorption is due to: a) s - s* transition. b) p - p* transition. c) n - p* transition. d) s - p* transition. e) n - s* transition.
c
Colorimetry 2
changes the solutions ability to absorb light
Reagent blanks are primarily used to
compensate for any absorbance due to reagents or contamination
color of light absorbed and observed are
complementary
The most common source in atomic absorption is the hollowcathode lamp. Which of the following statements is not true about the hollow-cathode lamp? a) Doppler broadening and pressure broadening are reduced. b) The anode and the cathode are sealed in a glass tube. c) The ionized inert gas is sputtering off atoms from the cathode. d) It produces single-wavelength monochromatic radiation. e) The performance of the lamp depends on the ionization potential of the carrier gas.
d
Two solutions, A and B, of a certain compound, have absorbances AA = 1 and AB = 2. Which statement about the relation between their transmittances, TA and TB , is correct? a) TA = 2 × TB b) TA = 0.1 × TB c) TB = 100 × TA d) TA = 10 × TB e) TB = 2 × TA
d
Which of the following statements is not true about the isosbestic points? a) This is the wavelength where the absorption spectra of two species with a constant total concentration cross each other. b) The total absorbance does not change during a chemical reaction, or a physical change of the sample. c) The molar absorptivity of the species is the same. d) More than two species exist in equilibrium. e) The total analytical concentration remains the same.
d
Which of the following statements is true about fluorescence? a) It is faster than absorption. b) The fluorescence efficiency decreases when the temperature decreases. c) It is a T1 - S0 transition. d) It occurs from the lowest vibrational level of the excited state. e) Fluorescence comes at longer wavelengths than phosphorescence.
d
relative rates of relaxation
depend on the molecule, the solvent, temperature, pressure, etc.
Jablonski Diagram
diagram that illustrates the electronic states of a molecule and the transitions between them
the consequence of interference
diffraction
Intersystem crossing is: a) a nonradiative transition between states with the same spin. b) a radiative transition between states with different spin. c) a radiative transition between states with the same spin. d) a nonradiative transition between the vibrational levels of the same electronic state. e) a nonradiative transition between states with different spin.
e
process of exciting molecules of UV wavelengths
electronic excitation
luminescence
emission of light from an excited state molecule
Fluorescense
emitting a photon by relaxing from an excited singlet electronic state to a ground singlet state
Phosphorescense
emitting a photon by relaxing from an excited triplet electronic state to a ground singlet state
Energy relationship to wavelength
energy increases as wavelength decreases
radiationless transitions
energy is converted to heat
Atoms moving toward the light source
experiences more oscillation than the atom moving away from the source
Kjeldahl nitrogen analysis
method for determining nitrogen concentration in organic substances
infrared and microwave radiation
not energetic enough to induce electronic transition change vibrational, translational and rotational motion of molecule
Frequency
number of oscillations per unit time
single-beam instrument
sample and blank are alternatively measured in same sample chamber
inhomogeneous
occurs in both atoms and molecules • simply means that not all atoms or molecules in your sample are the same
electronic relaxation
occurs through the release of a photon (light)
absorbance sometimes called
optical density (OD)
when correct wavelength is absorbed
oscillations of the atom vibration is increased in amplitude molecule rotates or moves faster
Light waves consist of
perpendicular oscillating electric and magnetic fields
the only photons to be absorbed
photons with energies exactly equal to the energy different between two electron states
absorption spectrum
plot of absorbance vs wavelength for a compound
Automization
production of free gas-phase atoms
Energy relationship to frequency
proportional
process of exciting molecules of microwave wavelengths
rotational excitation
types of vibrational states
symmetric, asymmetric, in-plane scissoring, out-of-plane twisting , in-plane rocking, out-of-plane wagging
Po
the amount of light passing through the system with no sample present (i.e., blank)
Conduction
the center of furnace is hotter than the end, which causes condensation into the end creating memory effects
