Midterm/Exam one (E1-E5) Flashcards
ER
Sensitivity
- slope of a calibration curve
- shows significant response difference
ER
Limit of Detection
smallest concentration of analyte that can be detected with confidence by an instrument
- 3(st dev of the y-intercept)/slope
ER
Limit of Quantification
smallest concentration of analyte that can be quantified/ analyzed by a method
- 10(st. dev of the y-intercept)/ slope
ER
Limit of Linearity
the concentration range from limition of quantification until it is no longer linear
ER
What calculation measures precision?
Standard deviation/ relative standard deviation (RSD%)
ER
What calculation measures accuracy?
percent relative error
ER
Why is it better to use one 25 mL pipet to pipet 15 mL versus a 10 mL pipet and a 5 mL?
uncertaintly is propagated when using multiple methods of glassware
E1
Beers Law equation
A= EbC = -log (T)
e= molar absorptivity
b = path length
C = concentration
T = transmittance
E1
Three limitations to Beers Law
- only valid at small concentrations
- narrow range of wavelengths
- minimal stray light/ outside light interference
E1
How do high concentration of analyte affect Beers law?
At low concentrations the analyte particles are independent of each other, at high concentrations the particles may interact with each other and affect the refractive index or molar absorptivity
E1
How does stray light affect absorbance
Stray light decreases absorbance because the introduced/ stray light reaches the detector and indicates in increase in transmittance
E1
Why is it important to have a narrow wavelength of light introduced to the sample for beers law?
If a sample has multiple absorbing species but only one is of interest then you only want the wavelength to be absorbed by the species of interest, otherwise you would be recording the absorbance of multiple species
E1
In E1 the Lego colorimeter was better than the measurenet for bromophenol blue, why?
The LEDs of the measure net did not correspond to the dyes ideal wavelength, the yellow LED of the lego colorimeter did
E1
What is a photodiode array instrument? Why is it preffered?
A photodiode array features a difraction grating that separates wavelengths of light and directs them to a series of detectors that use signal averaging to create a spectrum of the sample.
- by using this intrumentation you can be sure the limitation of narrow wavelengths is true
E1
What is transmittance?
The amount of light that passes through a sample
- T= Pt/P0
- a high transmittance indicates a low absorbance and vice versa
E1
For an absorbance versus concentration plot what two variables from Beers Law form the slope?
molar absorptivity and path length (eb)
E1
An increase in molar absorptivity indicates an increase in what else?
Absorbance
e = A/bC
ppm =
parts per million
- mg/ L
ppb =
parts per billion
- microgram/ L
Dilution factor
vol of glassware/ vol of pipet
OR
final vol/ vol of added sample
True or false?
Multiple dilution factors for a sample are added together?
False, they are multiplied together
E1
What is the difference between stray light and light scattering?
- Stray light has a negative net error on absorbance measurements because transmittance is decreased
- Scattered light has a positive net error on absorbance measurements because less light reaches the detector
E2
Explain the process of fluorescence
When a molecule absorbs high energy light atoms within it are excited to a higher energy state, fluoresecnce occurs when those electrons return to the ground state by releasing a photon
E2
What molecular structures promote fluoresence?
- aromatic fused ring structure
- electron donating groups
- heteroatom containing ring structure
E2
Quantum yield
the fraction of excited state molecules that return to the ground state via fluorescence
E2
How does temperature affect quantum yield of a molecule?
increasing T decreases quantum yield because more molecules remain excited or lose energy via non radiative processes
E2
Internal conversion
a molecule in the ground state of an excited level transitions to a higher level of a lower energy excited state while conserving electron spin
opposite of intersystem crossing
E2
External conversion
The transfer of energy of excited molecules to the environment via collisions
E2
Why might processes like external and internal conversion be favored over flourescence?
A molecule will take the shortest pathway from the excited state to the ground state and these may be quicker
E2
Why do fluorescence instruments measure the sample at 90 degrees from the source light?
If the detector were to be placed head on from the source light it would measure the radiation of the source light and the sample resulting in a net positive error.
E2
How can orientation of the cuvette in a fluorescence instrument affect the intensity of the signal?
An increase in volume of the sample increases the sensitivity, if the cuvette is placed horizontally instead of vertically the volume is increased
E2
Why are emission wavelengths lower in energy than excitation wavelengths?
The change in energy from the ground state to the excited state is larger than that for the excited state to the ground state because of energy lost by vibrational relaxations
E2
Intersystem crossing
non radiative relaxation mechanism where electrons in the ground level of an excited state transtion to a higher level of a lower energy excited state without conserving electron spin
opposite of internal conversion
E2
Fluorescence quenching
often done by halides, reduces the intensity of fluorescence signal
E2
True or false? The extent to which a halide can quench a molecule decreases as size of the halide increases?
False, quenching abilites for halides increase as size increases
E2
Does basicity or acidity increase fluorescence?
Basicity
pH > 7
E2
Why do metal ligand complexes promote fluorescence?
They can hinder quick proton transfer mechanisms that occur faster than fluorescence
E2
Why is the Perkin-Elmer instrument more sensitive than the MeasureNet colorimeter?
The Perkin-Elmer instrument has a Xe bulb as opposed to an LED source that can excite molecules at a number of different wavlengths as oppsed to the one emission of the LED bulb
E2
Stern Volmer equation
Fo/F = 1 + Ksv[halide]
y= mx + b
E2
Fluorescence intensity equation
F = kC
- fluorescence is proportional to concentration
C= concentration k = product of constants
E3
Rayleigh scattering
- more intense than raman scattering
- emitted photons have the same energy as the absorbed photons
- elastic
E3
Raman scattering
- less intense signals than rayleigh scattering
- emitted photons have a different wavelength than absorbed photons
- inelastic
E3
Stokes scattering
- a kind of raman scattering
- emitted photons have less energy than the absorbed photons (loss of E)
- favored over anti stokes
E3
Anti stokes scattering
- kind of raman scattering
- emitted photons have more energy than absorbed photons (gain in E)
- less favored than stokes scattering
E3
Raman active
to be raman active there must be a change in the polarizability of the molecule
ex: induced dipole moment
E3
IR active
to be IR active the dipole moment of the molecule must change (change in charge distribution)
E3
Why is a high powered laser necessary for Raman spectroscopy?
Raman signals are very weak compared to Rayleigh signals, a sufficent noise to signal ratio must be seen and that is created by a high powered light source (eg laser)
E3
Calculation for shift in cm^-1
= (1/ x nm - 1/y nm) 1*10^7
x = excitation wavelength and y = wavelength of peak
E3
Why was the 532 nm Raman instrument not suitable for measuring fluorescein?
The wavelength was too close to the emission wavelength causing the solution to fluoresce and wash out any other signal
this is why the wavelength cannot be too small/ too high energy
E3
Why are IR peaks different from Raman peaks?
IR spectra often have an overlap of signals, Raman peaks are less cluttered and feature less overlap of signals
E3
What is the main advantage of Raman over IR?
aqueous samples can be used with Raman, for IR samples must be dried extensivley before analysis
E3
What is surface enhanced Raman spectroscopy? (SERS)
- sample is placed on a textured metal or in a collidal solution and plasmons produced by the laser enhance the Raman signal
- can be used for highly diluted samples because of signal enhancement
E4
Explain the difference between an electrothermal and flame source for AAS
- A flame source has better repetition of sample delivery (more precise) but effiency of atomization is low because of the temperature
- An electrothermal source has high effiency of atomization due to temperatures but poor repetition of sample delivery (less precise)
E4
Explation why atomization efficeny for flame AAS is low
if droplets of sample are too large they cannot be carried by the combustion gases to be atomized and concentration of solution is lower than the initial sample
E4
Explain the process a sample undergoes for atomization
aq. analyte to free atoms
- the solvent is removed and the sample is vaporized by a flame or electrothermal source
- the vaporized sample is exposed to increased temperatures to create free atoms and ions
E4
Boltzmann equation
Nstar/N = (gstar/g)e^(-delta E/kT)
E4
Do we want to maximize or minimize the boltzmann equation? (Nstar /N)
We want to maximize N star because that means there are more atoms in the excited state
E4
True or false? A large delta E and small T are favored in AAS?
False, a small delta E and large T are favored
- increases sensitivity
E4
How do organic solvents improve the aspiration process?
recall ethanol vs water
they have lower surface tensions and allow for smaller droplets to form, improves atomization efficency esp in flame
E4
Why is a hollow cathode lamp used for AAS instead of a continuous source?
the bandwidth is much smaller, allows for increased sensitivity and measurement
- also the hollow cathode lamp emits at the exact wavelength of the metal of interest
E4
How does a hollow cathode lamp work?
an electric current creates anions and cations within a gas, cations collide with the negative cathode that then emits metal atoms that emit light that serve as the source light for the instrument
M — M star + hv
E4
What is a releasing agent?
A releasing agent forms compounds with species that cause chemical interference and allow the atom of interest to be released for detection
Ex: La3+ for phosphate and aluminum
E4
What is the downside to using a hollow cathode lamp?
A new one must be used for each individual metal that needs to be studied
E4
Why should an external calibration curve be used for many unknown samples opposed to a standard addition?
A standard addition method would require far more samples
ex: six unknowns would require 36 samples in total to be made
E4
What is water hardness a measure of?
The concentration of CaCO3 in water
- we used tap water and assed its hardness by converting concentration of Ca to CaCO3 using molar masses
E5
Which sample had the highest average signal?
strontium had the highest average signal which is to be expected considering its low ionization energy
- signal increased as you moved down the column (ionization energy trend)
E5
Considering LOD why is an ICP-OES superior to a flame instrument?
The LOD in ppb for the flame instruments were thousands of times more than the ICP-OES
- low limit of detection notes increased sensitivity for dilute samples
E5
One-point ratio method
standard signal/ ppb of standard = unknown signal/ppb of species unknown
- use molar masses to convert
E5
What is the difference between atomization and excitation?
- atomization is the conversion of a sample into free atoms and ions by a thermal energy source
- excitation is the transition of an electron from a lower level ground state to a higher energy level excited state by energy input (whether Eth or light/ radiation)
E5
As you progress further into the excited states there are —- atoms to return to ground states
less, so transitions with more atoms returning to the ground state are more favorable
E5
What is self absorption?
Self absorption occurs most often in flame spectroscopy where excited atoms at the center of the flame emit radiation that is then absorbed by atoms in the cooler part of the flame so a lower emission signal than expected is recorded
E5
What is ICP-MS
this is coupling emission with mass spec, allowing for analysis of structure and qualitiative analysis concerning concentrations
- also has very low detection limits
E5
Which metals are best used with flame emission spectroscopy?
alkali metals (earth and non)
- low ionization energies
E5
Which metals are best suited for ICP-OES
all metals but transition metals must be used with ICP-OES bc of higher ionization energies
remember plasma has a higher T
E5
Why was potassium used on the medical nebulizer instrument?
this instrument had a potassium filter attatched
What is the effect of stray light on a curve of absorbance vs concentration?
Lower slope and flattening of the curve because of false decrease in absorbance
practice exam Q 1
E2
When colored soda was added to a solution of quinine why was the fluorescence decreased?
The colored particles in the added soda absorbed the radiation of quinine due to it being a complimentary color
practice exam Q 4
Why does a high powered laser benefit fluorescence but not absorbance?
One of the constants that is a part of the fluorescence equation is Po (the source light) which increases signal, but absorbance is part of a ratio so increased Po would have no effect
practice exam Q 5
E5
What does the coil on the ICP torch do?
Creates a magnetic field that creates the hot argon plasma as it moves in a circular motion
practice exam Q 12
How do you find the dilution factor for a ratio?
add the numbers
ex: 1:1 = a DF of 2
How do you find transmittance from voltage values?
V/Vo
practice exam Q 17
What are the possible energy transitions when a metal is introduced into a flame?
electronic only
