Quiz 2 Flashcards
What is the purpose of a radiation detector?
convert energy to electrical signal for detection and analysis
ideal detector properties
high sensitivity, high signal to noise ratio, constant response, good wavelength range, fast response, 0 output
can any detector combine high sensitivity, high signal to noise, constant wavelength response, and linear energy response?
no because of dark current
spectral response
slope vs wavelength
sensitivity
slope of response versus power
detection limit
smallest significant measure
Why can there never be an infinitely narrow spectroscopic line?
transition state lifetime
types of photon detectors
photon detectors, phototubes, photomultipliers, photoconductivity detectors, silicon diodes
types of thermal detectors
pneumatic cells, thermocouples, bolometers, pyroelectric detectors
types of multichannel detectors
photodiode arrays, vidicons, charge transfer devices
QM definition of spectral line broadening
atomic and molecular energy transitions occur at discrete, well defined energy
What is the classical approach to explain natural line broadening?
damped oscillating dipole function
How does energy decay result in a damped oscillating dipole function
decays from excited to ground
how is excited state lifetime related to damping constant
by lifetime of decay
How is the excited state lifetime related to the half width of a lorentzian distribution
longer ESL = sharper line
What is the most common form of the heisenberg uncertainty principle?
one cannot measure exactly both the momentum and position of the electron
How are experimentally measured line widths related to natural line widths
experimental»_space; natural 10^3
How is the wavelength and frequency of molecules moving toward the detector affected by their motion?
towards detector shortens wavelength increasing frequency
How is the wavelength and frequency of molecules moving away from the detector affected by their motion?
moving away from detector lengthens wavelength decreasing frequency
How is the half width of a gaussian distribution related to temperature
increasing temp increases bandwidth
What would be one way to reduce the doppler line broadening mechanism?
cooling
How does molecular collisional frequency change with sample pressure or temperature
increasing pressure or temp increases frequency
what is the effect of collisions on spectroscopic excited states
decrease excited state, broaden bands
How is the half width of the pressure broadened line related to excited state lifetime?
decrease excited state lifetime broadens band
What would be one way to reduce the half width of a pressure broadened line shape?
lowering temp slows collisional broadening
What is johnson noise
thermal
what is shot noise
particles crossing junction
what is chopping
synchronous demodulation; signal in phase passes positive, blocks negative
When should a lock in amplifier be used
when signals are obscurred
What is transmittance
power after divided by power before
What is the absorption coefficient (a)?
absorptivity, proportionality constant
What is the molar absorption coefficient ε
used instead of (a) when mol/L and cm
How do instrumental sources of deviation depend on concentration?
it does not
How can the chemical and instrumental sources be distinguished when concentration is constant?
concentration only effects chemical; instrumental depends on b when c is constant
How does polychromatic radiation affect beer’s law linearity?
increases deviations, increases ε’ + ε’’
How does stray light affect beer’s law linearity?
negative
How does slit width affect beer’s law linearity?
decreases absorbance
How do high or low transmittance readings affect beer’s law linearity?
absorbance greater than 1 AU decreases precision
How do chemical reactions and equilibria affect linearity?
What type of photochemical effects will influence linearity?
What is the effect of fluorescence?
increases T, negative deviation
What is the effect of light scattering?
decreases T, positive deviation
What is spiking?
add increments of standard to identical volume samples
what is the method of standard addition?
spiking, dilute, measure; all constituents but reagent should be identical
What types of samples are most used with the method of standard addition?
complex samples with high potential matrix effects
assumptions for method of least squares
assume linear relationship and measurement error (not x)
what is ordinary or simple linear regression?
area peak versus concentration
what is the independent variable in linear calibration curve
x
what is the dependent variable in linear calibration curve
y
how is external calibration performed
series of known [analyte] standards; obtain response signal vs concentration then plot and predict
external calibation
calibrate when no matrix interference in analyte
method of single addition
matrix almost identical for successive additions in standard
For a standard addition method, what is the difference between the method of continuous variation and the method of single addition
single assumes linearity, uses only 2 increments
What are the advantages of the method of single addition compared to the method of continuous variation?
saves time and sample
Internal standard method
substance added in constant amount to all standards in analysis
How is an internal standard used in a calibration
plot ratio of signals to concentration
advantages of internal standard
compensate for errors
disadvantages of internal standard
hard to find and introduce reproducibly
How does a response factor influence the internal standard method
used when unknown and standard do not respond identically
How is precision measured?
standard deviation, average, variation
How is bias measured?
population mean minus the true value
How is calibration sensitivity related to the slope of the calibration curve?
slope limits sensitivity
How is the minimum detectable signal related to the blank signal and standard deviation of the blank?
LOD is the concentration that gives a signal equal to the blank plus 3 standard deviations of the blank
How is dynamic range related to LOQ and LOL?
dynamic range is the distance between LOQ and LOL
