Instrumental analysis exam 1 – Flashcards
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| Environmental Noise |
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| •Caused by interference from surroundings •Power lines at 60 hz; Radio, TV, elevators •Quiet region between 10^2 and 10^6 |
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| Grounding and Shielding |
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| •Surround instrument in a conducting material that is grounded and absorbs radiaton |
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| Difference amplifier |
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| •removes noise by subtracting the difference between a reference input and the signal input. •Good for removing drift noise, power line noise |
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| Analog Filters |
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| •Filter out noise at a different frequency from signals • Low pass filter: allows only low frequencies to pass •High Pass filter: allows only high frequency signals to pass •Band Pass Filter: Allows only a particular range of frequencies to pass |
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| Modulation |
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| •Shift the signal to a frequency region with less noise •Source modulation: modulate at radiation source ->Source chopper •Sample modulation:Sample alternatively presented to the system •Wavelength modulation:Wavelength is repeatedly varied over a wavelength range •Also can alter some property of carrier wave signal so that the carrier wave encodes signal information |
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| Lock-In amplifier |
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| •Used to extract signal that is engulfed by noise •Uses a reference signal to lock in to a signal at a specific frequency •Reference signal must have same frequency and fixed phase relationship |
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| Ensemble Averaging |
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| •Numerous data sets of the same sample are collected and averaged •In order for this to be effective, data must be sampled at a rate that is at least twice the frequency of the highest frequency component of the waveform->Nyquist frequency • S/N= Sqrt[n] (Si/Ni) |
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| Boxcar Averaging |
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| •Averages a small number of adjacent points |
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| Fourier Transform |
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| Convert time domain to frequency, filter out noise convert back to time domain |
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| Slit Diffraction |
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| m lambda=d Sin[Theta] |
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| REfractive index |
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| Ni= C/Vi |
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| Refraction |
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| Change in direction of light passing through a medium Sin[Theta1]/Sin[Theta2]=n2/n1 |
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| Scattering |
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| Light going through matter results in momentary absorbtion of photons by atoms followed by reemission •Rayleigh Scattering: Scattering by by molecules smaller than the wavelength of radiation-> no net energy change •Tyndall Effect: Scattering by large molecules •Raman Scattering:Results in a change of frequency due to vibrational energy level transitions |
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| Electronic States: |
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| •Singlet: no unpaired electrons •Triplet: Two unpaired |
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| Components of optical instrumentation |
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| •Source of radiant energy •Sample holder •Wavelength Selector •Radiation detector •Processor/readout |
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| Incandescent lamps |
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| •Continuum source •Spectral distribution described by blackbody equations • Tungsten lamps: Visible range, near IR |
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| Arc lamp |
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| •Continuum source •Xenon : Uv visible range • Deuterium or hydrogen lamp: true continuum from 160 nm out to visible |
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| Nernst glower |
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| •Continuum source • Visible to far IR range •Semiconductor material that is heated |
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| Globar |
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| •Silicon carbide rod • Range: IR to far IR |
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| Low pressure arc lamp |
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| •Line source •Hollow cathode tube with metal anode, emits radiation characteristic of the metal (AA) |
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| Lasers |
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| Light amplification by stimulated emission of radiation •Lasers work by: •Pumping: light or electronic source excites the active medium •Spontaneous emission: Active medium emits radiation randomly •Stimulated emission: active medium gives off radiation of the same wavelength that is colliding with it and in the same direction •Absorption: active medium absorbs radiation and becomes excited •Population inversion: when the number of species undergoing stimulated emission is larger than the number undergoing absorption-> when there are more species in the excited state than in the lower energy state •Line source |
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| Interference filter |
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| •Filters light from interference of radiation •Filtered wavelength can be selected based on thickness of dielectric material between two mirrors • lambda=2 d Ni/n Ni:refractive index of dielectric •Useful for Uv vis, and IR regions •effective bandwidth is 1.5% of peak transmitted wavelength |
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| Free Spectral range |
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| Range of wavelengths around chosen wavelength about which no other wavelength of radiation overlaps |
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| Absorption filters |
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| •Absorb certain wavelengths with colored glass or dye • Effective bandwidth of 30 nm- 250 nm |
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| Monochromator: Makeup |
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| Composed of: Entrance Slit Collimating lens dispersion element focus lens exit slit |
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| Monochromator: Gratings:types |
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| •Echellette n lambda=d(Sin[i]+Sin[r]) d: distance between gratings i:angle of inference r: angle of diffraction •Echelle Grating: n lambda=2 d Sin[i] |
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| Monochrometer: Gratings: performance |
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| •Resolving power R: limit of a gratings ability to seperate adjacent images R=mN m: order N: total number of grooves •Dispersion: ability of a monochrometer to separate different wavelengths-> reciprocal linear dispersion D^-1= d Cos[r]/n f f: focal length, d:distance between gratings r:angle of diffraction •Light gather power F number: F= f/d f: focal length d: distance between gratings |
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| Monochrometer: Gratings: slit width |
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| •Effective bandwidth: range of wavelengths that exit the monochrometer at a given wavelength Lamdaeff= 1/2 Wavelength difference Lamdaeff= w D^-1 w:exit slit width |
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| Detectors |
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| Should have: • High sensitivity • High signal to noise ratio •Constant response over a range of wavelength •Fast response times •No dark current •Signal proportional to radiant power |
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| Photovoltaic Cell |
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| •Radiant energy creates electron/hole pairs generating a potential difference max absorbance 550 nm |
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| Photodiode |
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| Reverse biased pn junction. Photons cause promotion of electrons from valence band to conduction band |
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| Phototubes |
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| Photoemissive material in cathode tube emits electrons when struck by radiation. •Can be useful from ultraviolet through visible regions |
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| Photomultiplier tube |
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| Much more sensitive to emitted photons •Fast response time •Can be damaged by bright light |
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| Charge coupled devices |
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| •Formed from p type silicon, electrons formed from absorption of radiation collect in a potential well below the electrode. •Charge under electrode accumulates during scan and can be measured |
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| Thermocouple |
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| A pair of junctions between two dissimilar metals. A voltage is generated by the difference in temperature between the junction metals |
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| Bolometer (Thermistor) |
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| As temperature increases valence band electrons are promoted to the conduction band which decreases resistance and increases conductivity |
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| Pyroelectric transducer |
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| •Special dielectric material that is polarized by an external magnetic field but remains polarized in a temperature dependent manner after the field is removed. •Heating the material results in a change in charge distribution and a measurable current in a circuit •Most common detector in FTIR |
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| IR |
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| 1 micron |
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| Visible light |
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| 400nm-650 nm |
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| Photoconductor |
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| •Semiconductor: acts as light dependent resistor |
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