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31 Cards in this Set
- Front
- Back
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What is the difference between accuracy and precision?
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Accuracy refers to the "correctness" of a measurement, whereas precision refers to its reproducibility
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How is precision reflected in the shape of the Gaussian?
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Narrow Gaussian corresponds to high precision, whereas a broad Gaussian refers to low precision
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What degrees of confidence does plus or minus a single, two, and 3 standard deviations from the mean refer to?
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68%
95% >99%, respectively |
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Define: specificity
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The ability to distinguish the signal of interest from interferences
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How is the signal-to-noise ratio calculated for a DC measurement?
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S/N = mean/standard deviation = one inverse relative standard deviation
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How is the detection limit defined?
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Technically this is arbitrary, but it is generally set around 3-4, according to most manufacturers
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What are the major types of instrument noise?
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Thermal (Johnson) Noise
Shot Noise Flicker (1/f) Noise Environmental Noise |
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What is thermal (Johnson) noise?
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"White noise"
This comes from thermal electronic motion in the machine's conductive elements resulting in an artificial voltage |
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How can the effects of Johnson noise be reduced?
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-Reducing resistance in the machine
-Reducing temperature of the machine -Decreasing the measured bandwidth (note that this increases scan time) |
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What is shot noise?
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-Noise related to charged particles crossing a junction (evacuated space) between an anode and a cathode.
-Has to do with the randomness of the rate at which a charge will cross a junction, leading to fluctuations in current |
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Control over which variable limits shot noise?
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Bandwidth
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What is flicker noise?
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Flicker noise = 1/f noise is a ubiquitous and inexplicable noise source that is inversely proportional to frequency, and becomes relevant at measurement frequencies under 100 Hz
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What is environmental noise?
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-Disturbances in measurement that come from conductors in a machine picking up electromagnetic signals from the environment.
-This includes radio and television signals, ac power lines, lightning, ionospheric disturbances, etc. |
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What are some hardware mechanisms for noise reduction/signal-to-noise enhancement?
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Shielding via Faraday cages, coaxial cables
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What is the photoelectric effect?
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That incident photons on a photoemissive surface will release electrons with a kinetic energy equal to the energy of the photon less the work function corresponding to the energy required to free the electron
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What kind of noise is limiting for photomultiplier tubes?
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Shot noise, because this has to-do with charges passing across junctions. Therefore there will be fluctuations due to current in amplification.
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Arrange Flame AAS/AES, Furnace AAS, ICP-AES, and ICP-MS from low-to-high detection limits
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Furnace = low/sub-ppt
ICP-MS = ppt ICP-AES = low ppb-high ppt Flame-AAS = low ppb Flame-AES = ppb |
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What sort of noise is targeted by low-pass filters?
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Thermal noise, shot noise
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What sort of noise is eliminated by high-pass filters?
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Low-frequency flicker noise, drift
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What is the advantage of analog filters over digital?
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By the time the signal has been digitized, it's nearly impossible to eliminate this noise
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At about what frequency should analog data be sampled for digitizing?
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At about 10 times the Nyquist frequency (= twice the frequency of sampling). Certainly no less than the Nyquist frequency, nor anything so great that it would incorporate additional noise
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How does signal increase with sampling? Random noise? Shot noise?
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-Signal and shot noise increase linearly with repeated sampling
-Random noise increases with the square root of the number of repeated samples |
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What is the sign relationship between Gibb's free energy and the cell potential?
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They have opposite signs
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What is the activity of a solid?
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1
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List the absorption/emission techniques we've discussed in order of their detection limit (low to high)
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Furnace AAS
ICP-AES ICP-MS Flame-AAS Flame-AES |
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What are the assumptions of Beer's Law?
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-Low analyte concentration
-Monochromatic source -Refractive index of sample is concentration-independent |
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What are the requirements for a reference electrode?
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-Known electrode potential
-Potential independent of concentration of any ions in the solution under study |
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In order to bring an electrochemical reaction to equilibrium, the overpotential at the cathode must be of what sign? At the anode?
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Cathode reactions (reductions) require negative overpotential, whereas anodic reactions (oxidations) require a positive overpotential
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True or false: half-cell potentials are written as reductions
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True
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What information is given by the charge flow at application of an over-potential, and what by the potential?
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Charge flow gives amount and potential gives identity
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Why does is sampling done toward the end of the droplet lifetime in differential pulse polarography?
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Because at this time faradaic currents are significant (if decreased by mass transfer effects), but the non-faradaic charging current effects due to the double layer are largely obsolete
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