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Chemistry 254 Exam 1

TermDefinition
Standard A solution whose composition by virtue of the way that it was made from a reagent of known purity, or by virtue of its reaction with a known quantity of a std. reagent
Corrected Data Data that has been avg. of has had the blank absorbance subtracted
Direct Calibration Absolute std. response measured separately from samples
Standard Addition known quantities of analyte are added to the unknown. From the increase in signal, we deduce how much analyte was in the original unknown. This method requires a linear response to analyte
Internal Standard Known amount of a compound, DIFFERENT FROM ANALYTE, that is added to the unknown. Signal from analyte is compared with the signal from the internal standard to find out how much analyte is present
Calibration function quantitatively relate signal to chemical property
Figures of Merit dhow me judge/analyze our calibrations. What we have to show for them
Range concentration range over which linearity, accuracy, and precision meet specifications for analytical method
Linear Range concentration range over which calibration curve is linear
Dynamic Range concentration range over which there is a measurable response
Sensitivity the ability of an analytical method to distinguish analyte from everything else that might be in the sample
Detection Limit also called the lower limit of detection, is the smallest quantity of analyte that is "significantly different" from the blank
Quantitation Limit smallest amount that can be measured with reasonable accuracy
Reagent Blank has all the same chemicals except the analyte
Method Blank is a sample containing all components except analyte, and it is taken through all steps of the analytical procedure
Explain how the different calibration methods work and when/why each is used - standard addition more appropriate when sample composition unknown or complex; affects analytical signal - internal standards more useful for analyses in which the quantity of sample analyzed or the instrument response varies slightly from run to run
Explain the proper use of different types of blanks blanks account for interference by other species in the sample and for traces of analyte found in reagents used for sample preservation, preparation and analysis
Wavelength distance between consecutive crests of a wave
Frequency the number of cycles per unit of time for a repetitive event
Singlet State an electronic state in which all electrons are paired
Triplet State an electronic state in which there are two unpaired electrons
Absorption occurs when a substance is taken up inside of another. For the purposes of this course it is usually light (photons)
Vibrational Relaxation vibrational energy is transferred to other molecules through collisions, not emission of photons, to the lowest level of S1 (excited electronic state)
Internal Conversion going from S1 to a highly excited vibrational level of S0 (ground electronic state) that has the same energy as S1
Intersystem Crossing when a molecule crosses from S1 to T1
Fluorescence relaxing from S1-->S0 by emitting a photon
Phosphorescence relaxing from T1-->S0 by emitting a photon
Chromophore the part of a molecule responsible for light absorption
Transmittance fraction of original light that passes through the sample T = P/P0
Absorbance -log T
Absorption Coefficient light absorbed by the sample is attenued at the rate P2/P1 = e^-(alpha)b where P1 = initial radient power, P2 = power after transversing a pathlength b (alpha) = the absorption coefficient
Molar Absorptivity the characteristic of a substance that tells how much light is absorbed at a particular wavelength
Boer's Law states absorbance is directly proportional to the concentration of the light absorbing species A = (epsilon)bc
Instrumental Deviation when Boer's Law isn't followed because of the instrument. Like stray light, polychromatic light, and absorbances greater than 2
Chemical Deviation when Boer's Law isn't followed because of the reagent. Like the molecule can react with itself at high concentrations or reacts with contaminant or buffer
Colorometric Analysis procedure based on absorption of visible colors
Mirror Image Rule the emission spectrum is roughly the mirror image of the absorption spectrum
Excitation Spectrum measured by the varying the excitation wavelength and measuring emitted light at one particular wavelength. Graphs emission intensity versus excitation wavelength. Looks a lot like absorption spectrum. Constant (lambda)em, variable (lambda)ex
Emission Spectrum Graph of emission intensity versus emission wavelength. Constant (lambda)ex, variable (lambda)em
Explain how the speed of light changes in different media while its energy is constant Speed and wavelength change through different media to accommodate frequency. Frequency is directly proportional to energy by E = hv
Convert wavelength, frequency, and energy for EM radiation. You should be able to do this with energy units of J or kJ mol^-1 c = speed of light x = wavelength v = frequency h = Plank's constant Eph = e of a photon c = (lambda)v E = hv = h(c/(lambda))
Properly order rotational, vibrational, and electronic energy rot < vib < elec
Explain what is meant by a singlet or triplet electronic state Singlet is when all electrons are spin paired. Triplet is when electrons are not paired and with different spins
Correctly order photophysical processes from fastest to slowest Absorption > vib. relax > internal con > Fluorescence > Intersys. con > Phosphorescence
Write a chemical equation for Absorption S0 + hv ====> S1
Write a chemical equation for Vibrational Relaxation S1^Vn ==(-(delta)H)==> S1^V0
Write a chemical equation for Internal Conversion S1^V0 ====> S0^Vn
Write a chemical equation for Fluorescence S1^V0 ====> S0 + hv'
Write a chemical equation for Intersystem Crossing S1^V0 <====> T1^Vn
Write a chemical equation for Phosphorescence T1 ====> S0 + hv"
Broadband Source basically lamps that can be made out of different materials with different applications
Laser provide isolated lines of single wavelength for many applications
Stimulated Emission a photon can stimulate an excited molecule to emit a photon and return to its lower state
Population Inversion necessary for "lasing". Higher energy state has a greater population (n) than lower energy state in the lasing medium
Monochromator takes white light (all wavelengths) and only allows bands of a certain wavelength to pass
Diffraction the bending of light rays by a grating
Bandwidth range or width of wavelengths
Resolution the ability to separate two slosely spaced peaks. Function of slits and grating. R = (lambda)avg / (delta)(lambda)
Photoelectric Effect when energy from a photon causes an electron to leave
PMT (Photomultiplier Tube) electrons emitted from a photosensitive surface strike a second surface, called a dynode. The dynode is positive WRT the emitter, and electrons accelerate towards it. Multiple dynodes accelerate the electrons faster. Thus weak light signals can be seen.
N-Type Semiconductor has an excess of electrons. Negative.
P-Type Semiconductor has an absence of electrons. Provides holes for electrons to fit in.
N-P Junction N- and P-Type semiconductors together
Photodiode an N-P Junction with a wire to conduct electricity
Photodiode Array often used to detect many wavelengths at once instead of using a monochromator
Interferometer the machine that makes an interferogram
Retardation extra distance light travels because of the mirror movement
Interferogram plot of detector signal versus delta --> retardation
Fourier Transform magic math that takes an interferogram and turns it into an absorbance spectrum
Describe the function of the five major components in a spectrometer Sample holder holds the sample in place. Wavelength selector selects a single wavelength. Radiation source is a lamp or a laser. Detector is the instrument that receives the signal. Processor processes.
Know the basic lamp types used in spectrometers and their applications Lamp Typical Power Applications D2 low (<150w) UV W low Vis/NIR Xe high (>400w) UV-A&B&Vis, fluor. excitation Hg high (>400w) line source, lambda, chromato
Explain how three- and four-level lasers work 3 4 inversion nEy > nE0 nEy > nEx ~T req. ~TEy > ~TEn ~TEn<~TEx<~TEy ~T = squiggly T (average lifetime in excited state)
Explain the function of the grating and slits in a monochromator to select a specific wavelength with high resolution
Explain the effect of monochromator bandwidth on spectral quality/resolution A good monochromator separates light such that the angular dispersion between the bands of light is high. The bandwidth is determined by the slit size and grating. High resolution means a single wavelength and thus better data with less interference.
List pros and cons of PMTs vs Photodiodes PMT Pros: sensitive and fast Cons: narrow lambda range, sequential only Photodiode Pros: wide lambda range, simultaneous measurements of multiple lambda values (diode array) Cons: poor sensitivity
Explain what the Fourier Transform does the interferometer first takes a spectrum of the blank, and then one of the sample. Then the sample spectrum divided by the blank spectrum is the transmission spectrum of the sample
Explain the advantages of FT-IR over dispersive IR Faster Noise is averaged to all lambda, giving a good S/N ratio No slits --> larger signal