Final/ Exam 2 (E6-E10) Flashcards
(87 cards)
1
Q
E6
What is flow injection analysis (FIA)
A
FIA is a non robotic automated system that handles sample preparation and injection into an instrument
2
Q
E6
What is one advantage and disadvantage of FIA?
A
advantage: reduces error from manual methods or degredation of sample due to time
disadvantage: dispersion/ dilution of the sample from injection to detection
3
Q
E6
Dispersion
A
= concetrantion of sample at detector/concentration of og sample
ideally the value should be one
4
Q
E6
What is High-performance liquid chromatography/ HPLC
A
HPLC is a form of liquid chromatography to separate samples
for temperature senstive and biochemical species (species not for GC)
5
Q
E6
Adsorption
A
occur when the sample is more attracted to the stationary phase
6
Q
E6
Partition
A
when the sample moves between stationary and mobile phase, does not have a preference for the stationary phase (adsorption)
7
Q
E6
Reverse Phase chromatography
A
non polar stationary phase + polar mobile phase
- partition separation
- non polar compounds retained longer
8
Q
E6
Normal Phase chromatography
A
polar stationary phase + non polar mobile phase
- adsorption separation
- polar compounds retained longer
9
Q
E6
Hildebrand solubility parameter
A
measures how much energy is required to pull two solvent molecules apart
- the higher the value the more polar the molecule
- intermolecular forces
10
Q
E6
In experiment E6 was the red or blue dye more polar? Why?
A
The red dye was more polar
- eluted first
- had less fractions
- structure has less non polar groups present
11
Q
E6
Ion pair chromatography
A
- acidic, basic, or amphoteric compounds to be separated
- counter ion is added to mobile phase to pair with ion in solution (can increase or decrease interactions depending on polarity of stationary phase)
12
Q
E6
the more — the ion pairing agent, the greater the retention time of the ion pairing compound
A
hydrophobic/non polar
13
Q
E6
When a solute passes through a column for chromatography it establishes – — with the mobile phase and stationary phase
A
an equilibrium
- by considering it an equilbrium we are able to establish analytical equations
- mass transfer
14
Q
E6
What are theoretical plates and why do we want to minimize their height?
A
theoretical plates are where an equilibrium between a solute and stationary phase and mobile phase is established
- the height must be minimized so the number of theoretical plates can be maximized and column efficency improves
- N= L/H
15
Q
E6
Size exclusion chromatography
A
- separates a sample on its ability to move between the pores of a stationary phase
- large species elute first
- useful for assesing native vs denatured protein
- polymer vs monomer samples
16
Q
E6
in E6 (FIA pt. 1) how was the ferric ion (Iron III) determined?
A
By formation of a red/ orange complex FeSCN2+ by reaction with thiocyanate (SCN-)
17
Q
E6
What is the difference between hydrophilic interaction chromatography and reverse phase LC?
A
- Retention in HILIC is based on the hydrophilicity of the compound (aka more polar analyte retained longer)
- Retention in reverse phase LC is based on hydrophobicity of the compound (aka less polar analyte retained longer)
18
Q
E6
What is the Van Deemter equation?
A
H= A +B/u +u[ Cm + Cs]
- the relationship between H (height of the theoretical plate) and u ( velocity) tells us that at optimal velocity the height is minimized
- A B and C terms are constants
18
Q
E6
What is the A term in the Van deemter equation?
H= A +B/u + u[Cm + Cs]
A
A is the eddy diffusion term
- independent of velocity
- depends on size of stationary particles (small and uniform minimizes A)
19
Q
E6
What is the B term in the Van deemter equation?
H= A + B/u u[Cm+ Cs]
A
B is the longitudional diffusion term
- moves randomly from high concentration to lower concentration
- inversely related to velocity
20
Q
E6
What is the C term in the Van deemter equation?
H= A + B/v + v[Cm + Cs]
A
The C term is mass transfer
- movement of a solute between the MP and SP
21
Q
E7
Gas Chromatography
A
similar to LC but samples must be introduced in gaseous form and the mobile phase is gaseous
22
Q
E7
Do compounds with low boiling points elute first or last in GC?
A
- low bp = nonpolar
- they will elute first, they have minimal interaction with the stationary phase
23
Q
E7
Do compounds with high boiling points elute first or last in GC?
A
- high bp = polar
- elute last, more interaction with the stationary phase
24
# E7
How do compounds that have similar structure elute on a GC column?
in order of boiling point, from lowest to highest
25
# E7
Explain the homebuilt GC instrument/s used in experiment E7
- syringe based injection port with a silica column
- convection oven to maintain stable temperature
- GC1 had a polar column
- GC2 had a nonpolar column
26
# E7
True or false? branched carbons can efficently be determined by the homebuilt GC instrument?
False, the calibration curve was poor in comparison to the non branched ester curve
27
# E7
What was the purpose of using the internal standard method for determination of methyl salicylate?
to overcome variability in injection of sample, detector response and change in column performance
28
# E7
What does the slope of the log(tr) versus number of carbons represent?
the free energy change
- slope = -deltaG/RT
29
# E7
What two compounds were switched in predicted retention order during the temperature study?
Toluene and Octane
- toluene eluted last instead of second likely due to aromatic interactions with the aromatic stationary phase
30
# E7
Was the temperature program or isothermal program preffered during the temperature studies?
the temperature program was preferred because an isothermal run takes too long and has poor separation or does not feature peaks at all
31
# E7
What does the value of Kd (equilibrium partition coefficent) tell you?
Kd= [Ss]/[Sm]
- large Kd= favors the stationary phase, increased retention time
- small Kd= favors the mobile phase, decreased retention time
32
# E7
What is a flame ionization detector?
- a flame is generated and an electric potential is applied to the flame
- the applied potential reacts to the introduction of a sample
- low detection limits and no interference by water or CO2
- on its own cannot perform identification of unknown compounds
- only functions if a C-H bond is present in analyte
33
# E7
Explain the resolution equation: Rs = [(N)^1/2/4][(a-1)/a][kB/(kB+1)]
- Resolution (Rs), which improves with separation of peaks (a value of 1.5 is optimal)
- N^1/2/4: efficency
- a: selectivity factor (magnitude does not change greatly)
- kB/kB+1: retention (can approximate one)
| Rs1/Rs2= sqrt N1/ sqrt N2
34
# E7
How is retention time related to alkyl chain carbon number?
a longer carbon chain = a longer retention time
- proportional relationship
35
# E7
How is temperature related to retention time?
An increase in temperature (standardly) decreases retention time
- inverse relationship
36
# E8
Formula for molar solubility at low ionic strength (Ksp)
A +B ---- C +D
Ksp = [C][D]=s^2
37
# E8
Formula for molar solubility at high ionic strength
A + B --- C + D
Ksp = (C activity coefficent)(D activity coeffiecent) *[C][D]= activity coefficents *s^2
38
# E8
nernst equation
Emeas= E* +0.0591log(Ag)
- where E* = E. - Eref + Ejnc = y-int of potential vs log[Ag+]
39
# E8
How do inert ions increase solubility of slightly soluble salts?
| specifically sodium nitrate solution
they create ionic atmospheres around the salt ions that decrease charge interactions that form a precipitate/ crystal lattice structure
- Nitrate ions collect around the cation
- sodium ions collect around the anion
40
# E8
True or false a large Ksp indicates a more soluble species?
True, an increase in Ksp indicates an increase in solubility
41
# E8
Explain how the fluoride ion selective electrode works
- single crystal (LaF3) for ionic charge transport
- gaps in the crystal lattice allow for fluoride ions to pass through the membrane, creating a concentration difference between the inner and outer portions of the electrode
- movement of the F- ion creates a potential difference that is proportional to the concentration of F- (-log [F-])
42
# E8
Reduction occurs at the ---
cathode
| indicator electrode
43
# E8
Oxidation occurs at the ---
anode
| reference electrode
44
# E8
E(degree sign)
standard state reduction potential
E* = E. - Eref + Ejnc
45
# E8
Emeas
measured potential
Emeas= E* +0.059log(activity Ag)
46
# E8
Eref
potential of the reference electrode
Ecell= Eind - E ref +Ejnc
47
# E8
Eind
potential of the indicator electrode
Ecell = Eind - Eref + Ejnc
48
# E8
How is the concentration of an unknown analyte calculated by a potentiometric titration?
Solve using trendline of Emeas vs -log[X-]
49
# E8
How do you determine the concentration of different analytes in a mixture useing potentiometric titration values?
- solve for moles using concentration of titrant (was Ag+ for example)
- use moles to solve for concentration using the volume of the mixture (mol/L)
- Ksp can help determine the order and what endpoint volume to subtract from
50
# E8
How do ion selective electrodes work?
- function based on a membrane and concentration differences
- measure potential as a function of concentration
- interior solution is in contact with a reference solution
- exterior solution is in contact with the analyte solution
51
# E8
Examples of ion selective electrodes
- gas sensing electrode (concentration of dissolved gas)
- Fluoride ISE
- Glass pH electrode
52
# E9
Voltammetry
apply a time dependent potential to an electrochemical cell to measure the resulting current as a function of the applied potential
- the current is often proportional to the concentration of the analyte
53
# E9
Voltammogram
Current (A) vs Potential (V)
54
# E9
Cyclic voltammetry
Scans in both the positive and negative directions to get a characteristic 'duck' shape voltammogram
- anodic peak (negative)
- cathodic peak (positive)
55
# E9
True or false? The potential for the cathodic peak is always greater than that for the anodic peak?
False, the anodic potential is always greater than the cathodic potential
| think of the shape of CV, the Epa is shifted to the left
56
# E8/E9
What is the difference between measurements/ quantitative analysis of potentiometric measurements versus voltammetric or coulometric measurements?
- Potentiometry is measuring the signal as voltage (dependent variable is voltage)
- Voltammetry and coulometry are measuring the signal as current (dependent variable is current)
57
# E9
How do you know if a process is diffusion controlled?
If the plot of current versus SQRT(scan rate) is linear
58
# E9
How do you know if no side reactions or adsorption is occuring during a voltammetric measurement?
If the absolute value of the ipa/ipc ratio is about 1 the process is well behaved
| this also means it is reversible
59
# E9
How can you estimate the number of electrons in the redox process during voltammetric measurements?
Epa -E pc = 0.059/n
60
# E9
How can you measure the diffusion coefficent using voltammetric measurements?
Use the Randles-Sevcik equation to solve for SQRT(D)
- K''=(2.69*10^5)(n^3/2)(A)(D^1/2)(C)
- the slope of the ipc vs SQRT(scan rate) is = K''
61
# E9
Why does a stirred CV look different than an unstirred CV?
Because diffusion is no longer the only form of mass transport occuring, stirring introduces convection mass transport which inhibits the formation of a diffusion layer
- product is constantly introduced to sensor = higher current signal
62
# E9
Differential pulse voltammetry (DPV)
measures in pulses and takes the signal once at the start of the pulse and once at the end where the difference between the two signals is proportional to sample concentration
63
# E9
Square wave volammetry (SWV)
measures in pulses, current is measured at positive (cathodic pulse) and negative (anodic pulse) pulses where the difference is proportional to the concentration
64
# E9
Why are DPV and SWV more sensitive than CV?
because of the loss of the interfering charging current
- CV does not scan in pulses so there is no loss in charging current
- SV also scans at high frequency which can promote sensitivity
65
# E9
Why could a coulometric titration be more beneficial than a buret titration?
1. you can utilize unstable reagents (like I2) that are generated during the titration
2. Small concentrations can be used because it is easy to measure a small change in charge
66
# E9
What three electrodes are used in cyclic voltammetry? Which ones generate the current?
- working, counter, and reference electrodes
- working and counter electrode produce the current
67
# E9
How did we calculate the grams of ascorbic acid present in the sample for the coulometric titration?
W= (current A)(time s)(molar mass)/(number of e-)(Faraday constant)
- time was calculated from average midpoint (vertical point on titration curve)
68
# E9
What is the Karl Fischer titration?
H2O + SO2 + I2 --- SO3 + 2HI
- titration used to calculate the amount of water present in a sample
- sulfur dioxide and organic base to titrate water
- organic base stabilizes SO3 formation
- Ethanol reacts with SO3 to drive the reaction forward
69
# E9
In a coulometric titration where is the unstable reagent formed?
- Using I2 as an example, I2 is generated at the anode by oxidation to act as a reducing agent for the analyte
| group questions, Q5
70
# E9
True or false? the formation of hydrogen gas in a coulometric titration is not of interest because it is not electrochemically active?
True
71
# E9
Faradaic Current
The current of interest that is generated at the working electrode by a oxidation or reduction reaction
| can be obscured by charging current
72
# E10
What are the three major steps in MS?
1. convert neutral atoms or molecules into a beam of ions
2. ions are then separated based on their mass to charge ratio
3. the separated ions are then measured for their abundance
| A MS measures abundance versus m/z
73
# E10
How is the molecular ion peak formed? (M+)
by removal of a single electron from a species
74
# E10
Which elements are most likely to have an M+2 peak?
| use this to identify if they're in the structure
Br and Cl
75
# E10
Nitrogen rule
if the m/z of the molecular ion peak is even 0 or an even number of of nitrogens are present
- if the m/z of the molecular ion peak is odd there is an add number of nitrogens present
76
# E10
The --- stable carbocation tends to form during fragmentation of a molecule
most
| (most) allylic>benzylic>tertiary>secondary>primary (least)
77
# E10
How do you calculate probability of a species?
probability= (# of combinations)(product of isotopic combinations)
| combinations = (# of isotopes)^number of atoms
78
# E10
What kinds of molecules is MS /not/ suited to detect?
isomers, these have the same molecular weight and often result in similar fragmentation which means simialr m/z and similar spectrums in general
79
# E10
Tandem mass spectroscopy
tandem mass spectroscopy uses multiple quadrupoles in which a precursor ion is chosen to undergo further fragmentation into product ions.
- this gives us extensive structural information on a molecule and helps to positvley identify a molecule
80
# E10
How does a quadrupole work?
An electric field is directed through the apparatus, ions with the m/z that is not of interest go off course and hit the sides of the quadrupole to be neutralized
81
# E10
What is hard ionization?
Hard ionization results in a smaller (or lack of) molecular ion peak
- it can also be called electron impact ionization
- produces ions with lots of excess energy, which fragment easily
82
# E10
What is soft ionization?
Soft ionization is more likely to produce a molecular ion peak
- can also be called chemical ionization
- ions do not have as much excess energy and are less likely to fragment
83
# E10
What is a coulombic explosion and what does it have to do with MS?
A coulombic explosion occurs before a molecule enters the MS portion of an MS-LC instrument **during electron spray ionization**
- ionized droplets of the analyte in solvent are forced apart into smaller and smaller charged droplets that reach the MS
- a coulombic explosion occurs and releases the ionized compounds from the solvent drops
84
# E10
Would you choose a GC- or LC- MS instrument to analyze a protein?
You would chose a LC-MS for the electron spray ionization
- proteins have very large molecular masses
- large mw can form small m/z ratios by **electron spray ionization**
85
# E10
What is MALDI? What compounds is it best suited for?
matrix-assisted laser desportion ionization
- the analyte is embedded onto a matrix and rapidly heated by a laser without fragmentation
- the heating removes the adsorbed analyte by movement into the gas phase
- it is best suited for peptides, lipids, saccharides, and high mw biological molecules
86
# E10
What is DART?
Direct analysis in real time mass spectrometry
- uses open air sampling, which has not been used extensively before
- does not require a vaccum or high temperature and pressure environment for analysis
