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Chemistry 254 Exam 1
| Term | Definition |
|---|---|
| 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 |
Created by:
Marlene the Chemist
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