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3rd Year Fall > C3110 Final > Flashcards

C3110 Final Flashcards

1
Q

Mobile Phase

A

The phase in which the solute moves through the column

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2
Q

Eluent

A

Fluid entering column

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3
Q

Eluate

A

Fluid leaving the column

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4
Q

Stationary phase

A

Determines the amount of time the solute spends in the column

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5
Q

2 Main types of columns

A
  1. Packed
  2. Open Tubular
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6
Q

What are the 5 mechanisms of separation?

A
  1. Adsorption
  2. Partition
  3. Ion-exchange
  4. Size exclusion
  5. Affinity
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7
Q

Definition of retention time

A

Time between injection onto the column and when the component reaches the detector

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8
Q

Definition of retention volume

A

Volume of mobile phase reuired to elute a particular solute from the column

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9
Q

Dead time

A

Time required for an unretained solute to travel through the column

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10
Q

Volume flow rate

A

How many mL of solvent per minute travel through the column

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11
Q

Linear Flow rate

A

how many cm are travelled in 1 min by the solvent

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12
Q

What do we keep constant in terms of flow rate when changing column size?

A

Linear flow rate

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13
Q

Adjusted retention time

A

Time required for a solute to elute beyond the time for the solvent

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14
Q

Relative retention equation

A

a= tr2’ / tr1’

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15
Q

What value of alpha gives better separation?

A

Higher

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16
Q

Retention factor equation

A

k= (tr-tm) / tm

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17
Q

2 factors that contribute to how well compounds are separated

A
  1. Difference in elution time between peaks (farther apart = better separation)
  2. Peak broadness (wider peaks = poorer separation)
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18
Q

Column Resolution

A

how far apart two bands are relative to their widths (ability of the column to separate 2 solutes)

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19
Q

Plate Height

A

proportionality constant between variance of a band and distance travelled; length of column required for one equlibration

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20
Q

What is the plate height/number relation?

A

N=L/H

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21
Q

Does smaller or larger plate height give better resolution?

A

Smaller

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22
Q

Is a band narrow or wider for lower number of theoretical plates?

A

Wider

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23
Q

In the Purnell resolution equation, how are resolution and plate number related?

A

Resolution is proprtional to the square root of N

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24
Q

If the column length is increased by a factor of 4, how much does the resolution increase?

A

Factor of 2

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25
Q

What are the 2 main sources of extra column broadening?

A
  1. Variation due to injection or detection
  2. Variance due to connective tubing
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26
Q

What equation describes on column broadening?

A

Van Deemter Eqn

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27
Q

Describe what vD eqn terms are present in packed, OT, and capillary columns

A

Packed: A, B, C
OT: B and C
Capillary: B

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28
Q

Describe longitudinal diffusion

A

Solute molecules diffuse away from the concentrated center of a band in both directions due to a concentration gradient. Inversely proportional to flow rate. Greater for gases than liquids.

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29
Q

Describe finite equilibration time

A

The solute must diffuse from the mobile phase to the surface of the sp for equlibration to occur. The time required depends on the distance the solute must travel to reach sp (and inversely on how fast it diffuses)

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30
Q

Describe the multiple flow paths term

A

Some flow paths through a packed column are longer than others.

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31
Q

What are the advantages of OT columns? (3)

A
  1. Higher resolution
  2. Shorter analysis time
  3. Increased sensitivity
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32
Q

Disadvantages of OT columns (1)

A

Lower sample capacity

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33
Q

Fronting

A

Too much solute has been applited (or too concentrated) and the solute becomes more soluble in sp, eventually resembling sp.

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34
Q

Tailing

A

Small quantities of solute are retained more strongly than large quantities. Leads to a long tail of gradually decreasing concentration

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35
Q

Separation Process in GC

A

Gaseous analyte transported through a column by inert mp (carrier gas)

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36
Q

2 Main types of GC Separation

A
  1. Partition
  2. Adsorption
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37
Q

Common carrier gases

A

He, N2, or H2

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38
Q

Column Oven

A

Provides temperature control for the column

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39
Q

Importance of Temp Control in GC

A

Column must be hot enough to provide sufficient vapour P for analytes to be eluted in a reasonable time

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40
Q

In what type of chromatography are long, narrow OT columns made of fused silica coated with polyimide used?

A

GC

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41
Q

Wall Coated OT

A

Liquid sp on inner wall of the column

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42
Q

Support coated OT

A

Solid particles coated with liquid sp attached to inner wall of column

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43
Q

Kovats Retention Index for a linear alkane

A

100 times the number of carbon atoms

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44
Q

Temperature Programming

A

Temperature of the column is raised during separation to increase analyte vapor pressure and decrease RT of late-eluting compounds

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45
Q

Lower Temp Limits

A

Isothermal T limit at which column can be kept for a long time

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46
Q

Upper Temp Limit

A

Column should only be exposed for a few minutes at the end of a T run

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47
Q

What is a consequence of using really high temperature in GC?

A

Sp bleeding

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48
Q

3 Types of Injection into OT columns

A
  1. Split Injections
  2. Splitless Injections
  3. On-column Injections
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49
Q

Split Injection

A

Used for high concentrations of analyte. High T. Mixing chamber to promote vaporization and mixing of analytes. A large portion of the sample is vented (only 0.2-2% of sample goes to column). Can be inaccurate because split ratio is not reproducible.

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50
Q

Splitless Injection

A

Trace analysis. Same port used as split injection. No mixing chamber. A large volume of dilute solution is injected slowly. Around 80% of the sample is applied to the column.

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51
Q

Solvent Trapping

A

Initial column T is 40C below boiling point of solvent, which condenses. As solutes become trapped in a narrow band at the head of the column, detection occurs.

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52
Q

Cold Trapping

A

Column T is 150C lower than the boiling points of analytes. Solvent and low boiling solutes are quickly eluted. High boiling solutes trapped in a thin band at the head of the column. T is increased to initiate chromatography.

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53
Q

On Column Injection

A

Sensitive compounds that decompose above their boiling points. Solution injected directly to the column without entering a hot port. Initial column T is low enough to condense solutes in a narrow zone. Warming initiates chromatography.

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54
Q

List a couple common GC detectors (8)

A
  1. Thermal Conductivity
  2. Flame Ionization
  3. Electron capture
  4. Flame photometric
  5. Nitrogen-phosphorus
  6. Sulfur chemiluminescence
  7. Photoionization
  8. Mass Spec
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55
Q

Briefly describe thermal conductivity detection in GC

A

Common in the past, simple and universal. Not very sensitive.

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56
Q

Describe thermal conductivity detection (the process).

A

Eluate from column flows over a hot filament (tungsten-rhenium). H2 and He have thermal conductivities about 6-10 times that of organics. A drop in eluates thermal conductivity causes the filament to heat and electrical resistance to increase, changing the voltage across the filament. This is measured by the detector.

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57
Q

Advantages of Thermal Conductivity

A

Simple
Large linear range
Responds to organics and inorganics
Nondestructive to sample
Responds to everything but carrier gas

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58
Q

Disadvantages of Thermal Conductivity

A

Low sensitivity
Won’t work well for complex mixtures

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59
Q

Describe flame ionization detection (process)

A

Eluate is burned in a mixture of H2 and air. Carbon atoms produce CH radicals, which further produce CHO+ ions and electrons in the flame. Ion production is proportional to number of carbons entering the flame. Current changes in presence of analyte.

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60
Q

Advantages of FID (4)

A
  1. Insensitive toward noncombustible gases
  2. Good for samples contaminated with water/oxides
  3. Good detection limits (100 times better than thermal conductivity)
  4. High linear response
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61
Q

Disadvantages of FID (2)

A
  1. Destructive to sample
  2. Requires additional gases and controllers
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62
Q

Describe electron capture detection

A

Sample eluate from column is passed over a radioactive beta emitter, usually 63Ni. An electron from the emitter causes ionization of the carrier gas which produces a cloud of electrons. With no analyte, there is a constant current between electrode. Current decreases when analyte with EN FGs enter and capture electrons.

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63
Q

What is electron capture detection used for typically?

A

Environmental samples, anything with halogens

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64
Q

What detector is also called an alkali flame detector?

A

Nitrogen-phosphorus

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65
Q

Which detector measures optical emission from selected elemetns?

A

Flame photometric

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66
Q

Which detector uses a vacuum UV source to ionize aromatic and unsaturated compounds?

A

Photoionization

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67
Q

Sample Prep Processes (4)

A
  1. Extracting analyte from complex matrix
  2. Preconcentrating dilute analytes
  3. Removing or masking interfering species
  4. Derivatizing analytes into a more convenient or easily detectable form
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68
Q

Solid Phase Extraction

A

Sorbent is loaded into a cartridge and sample is loaded onto the phase, retaining analytes that strongly interact with the phase.

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69
Q

Solid-phase mixroextraction

A

Extracts compounds from liquids, air, or sludge without solvent.
Fused silica fiber coated with sp is attached to a syringe. A large volume of solution is extracted; mass extracted is proportional to concentration of analyte in solution.

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70
Q

Which sample prep method uses a magnetic stir bar to absorb hydrophobic analytes?

A

Stir-bar sorptive extraction

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71
Q

Purge and Trap

A

Method for removing volatile analytes from liquids or solids, concentrating the analytes, and introducing them into GC. Goal is to remove 100% analyte from sample.

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72
Q

Thermal Desorption

A

Method for relasing volatile compounds from solid samples.

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73
Q

Order of decisions in method development (GC), (5)

A
  1. Goal of analysis
  2. Sample Prep
  3. Detector
  4. Column
  5. Injection
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74
Q

Typical length and inner diameter of fused-silica capillary tubes

A

50cm
25-75um

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75
Q

Principle of CE

A

Different ions have different mobilities and move through the column at different speeds.

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76
Q

What is the only source of band broadening in CE?

A

Longitudinal diffusion

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77
Q

Describe electrophoresis

A

When an ion with charge q is placed in an electric field E, the force on the ion is qE. The ion reaches a steady speed when the accelerating force equals the frictional force.

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78
Q

Electrophoretic mobility

A

Proportionality constant between the speed of the ion and the electric field strength

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79
Q

Describe electroosmosis (EOF)

A

Inside wall of a fused-silica capillary is covered with silanol, which have negative charge above pH 3. A tightly adsorbed layer of cations partially neutralizes the negative layer. Remaining charge is neutralized by mobile cations in the diffuse part of the double layer in the solution near the wall. Excess cations in the diffuse part are attracted to the cathode and impart net momentum, called EOF

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80
Q

What is the contrast between EOF and hydrodynamic flow?

A

In hydrodynamic flow, the velocity flow is fastest at the center and slower at the walls due to friction. This is not the case in EOF.

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81
Q

Apparent mobility

A

Sum of electrophoretic mobility and electroosmotic mobility

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82
Q

True or false: smaller ions have smaller electrophoretic mobility

A

False

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83
Q

True or false: At neutral or high pH, electroosmosis transports anions to cathode because electroosmosis is faster than electrophoresis.

A

True

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84
Q

What is the number of theoretical plates proportional to in CE?

A

Apparent mobility, applied electric field, and distance to the detector.

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85
Q

At a fixed voltage, EMF is strongest in shorter or longer capillary?

A

Shorter

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86
Q

2 Methods of Sample Injection in CE

A
  1. Hydrodynamic injection (P)
  2. Electrokinetic Inject (electric field)
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87
Q

What is BGE and how does it change during CE runs?

A

Background electrolyte
pH will be altered during electrophoresis, potentially forming a gradient and effecting migration times. BGE must be replaced after every run to avoid this.

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88
Q

How do we reverse the direction of EOF?

A

Add a cationic surfactant

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89
Q

What is Micellar electrokinetic chromatography?

A

Used to separate neutral species. Uses a pseudostationary phase in the form of a micellar phase.

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90
Q

Describe the process of MEKC

A

In the absence of micelles, neutral analytes reach detector at the same time. Negative micelles, injected into sample, reach detector at a specific time. If a neutral molecule equilibrates between solution and micelle, the migration time is increased, since the micelles migrate slower.

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91
Q

What term does MEKC contain that CE does not in the vD eqn?

A

Mass transfer, C

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92
Q

How can we use MEKC to separate 2 chiral molecules?

A

ADD BGE modifiers, such as cyclodextrins

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93
Q

Name 5 common CE detectors (briefly describe each)

A
  1. UV; poor sensitivty and background must have low absorption
  2. Fluorescence; high dynamic range
  3. Contactless conductivity detection; electrodes outside capillary that measures conductivity differences
  4. Amperometric detection; sensitive to analytes that oxidize or reduce
  5. Electrospray MS; low detection and gives structural info
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94
Q

Advantages of CE compares to chromatography (4)

A
  1. High resolution
  2. Low waste (Green)
  3. Simple equipment
  4. Low sample consumption
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95
Q

Disadvantages of CE (4)

A
  1. Higher detection limits (insensitive)
  2. Irreproducibility
  3. Insolubility of analyes in BGE
  4. Challenging scale up
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96
Q

What is a real world advance of CE?

A

Lab on a chip devices
Zipchip

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97
Q

What are the 6 components of HPLC?

A
  1. Autosampler
  2. Solvent delivery system
  3. Sample injection valve
  4. High-P chromatography column
  5. Detector
  6. Computer
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98
Q

Why do smaller particles give better resolution in HPLC? (2)

A
  1. More uniform flow reduces A term
  2. Less distance the solute must diffuse reduces C term
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99
Q

What do smaller particles give in HPLC? (4)

A
  1. Higher plate number
  2. Shorter run time
  3. Lower detection limits
  4. Higher P
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100
Q

Describe HPLC column sp

A

Filled with sp of highly pure, spherical, microporous particles of silica.

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101
Q

Guard Column

A

Protects the main chromatographic phase in the column

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102
Q

Most common bonded phase

A

C18

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103
Q

RPLC

A

sp is nonpolar/weakly polar, solvent is polar. Less polar solvent is a stronger mp.

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104
Q

NPLC

A

Polar sp and less polar solvent. More polar solvent has higher strength.

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105
Q

Eluent strength

A

Measure of the solvent adsorption E on bare silica; value of pentane defined as 0

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106
Q

What is HILIC and what is it used for?

A

HILIC is hydrophilic interaction chromatography. Used to separate molecules too polar to be retained by RPLC.

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107
Q

Sparging System

A

Removes dissolved gases by sweeping them out of solution with fine bubbles of an inert gas such as He.

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108
Q

Why is sparging necessary in LC?

A

Dissolved oxygen can absorb UV and interfere with UV detection. Other gases can cause issues for the column, pumps, and detectors.

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109
Q

What is the process to clean a column before storage (LC)?

A
  1. Replace aqueous buffer with water and wash with 5-10 mobile phase volumes.
  2. Wash with 10-20 volumes of strong eluent.
  3. Store column with solvent to inhibit bacterial growth
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110
Q

Why would a material with a high UV cutoff not be used very often as a solvent with UV detection?

A

A high UV cutoff results in very many materials absorbing UV and thus is not very sensitive.

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111
Q

Describe the general elution problem and what is used as a solution.

A

The general elution problem is that one set of conditions is not suitable to separate a complex mixture of analytes in a reasonable amount of time. Gradient elution is a solution.

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112
Q

Gradient Elution

A

Uses a continuous change of solvent composition to increase the mobile phase strength.

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113
Q

How do spectrophotometric detectors work?

A

A photodiode array records the spectrum of each solute as it is eluted. Spectra can be matched with library spectra to identify the analyte.

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114
Q

Fluorescence Detection

A

Eluate is excited with a laser and fluorescence at longer wavelengths is measured. Up to 100 times more sensitive than UV detectors, but few analytes fluoresce.

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115
Q

Electrochemical Detector

A

Responds to analytes that can be reduced or oxidized. Potential is controlled with respect to a reference electrode, while current is measured between working electrode and auxillary electrode.

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116
Q

Refractive Index Detection

A

Responds to almost every solute. Detection limits 1000 times poorer than UV. When a solute with a different refractive index than the mobile phase enters the cell, a beam of light is deflected. Useless for trace analysis and gradient elution.

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117
Q

Common methods of LC-MS interface

A
  1. ESI
  2. APCI
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118
Q

What does a performance qualification sample (internal standard) tell us?

A

If there is a problem with the run, the internal standard indicates whether or not the issue is with the chromatography of the analyte or with the instrument.

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119
Q

Anion exchangers

A

Contain positively charged groups bound to the sp, that attract solute anions

120
Q

Cation exchangers

A

Contain negatively charged groups bound to the sp, that attract solute cations

121
Q

Resin

A

Amorphous particles of organic material

122
Q

Which resins are commonly used in ion-exchange?

A

Polystyrene resins

123
Q

When would it be ineffective to use a weakly acidic resin?

A

Above pH where they become protonated and lose their ion-exchange capacity

124
Q

Selectivity coefficient

A

Equilibrium constant for ion-exchange reactions

125
Q

What factors contribute to favoring of ions in ion-exchangers?

A

Higher charge
Decreased hydrated radius
Increased Polarizability

126
Q

The binding of one divalent metal ion releases how many hydronium ions?

A

2

127
Q

Equivalents

A

The amount of charge exchanged; amount of cation that will exchange with one mole of a monovalent ion

128
Q

Ion-exchange capacity

A

The number of ionic sites on the resin that can participate in the exchange process

129
Q

List the 3 classes of ion exchangers and when they are used.

A
  1. Resins, small molecules
  2. Gels, large molecules
  3. Inorganic, harsh chemical conditions
130
Q

How is gradient elution used in ion-exchange (2 methods)

A
  1. Ionic strength
  2. Changing pH
131
Q

List 3 applications of ion-exchange chromatography

A
  1. Convert one salt into another
  2. Preconcentration of trace components to obtain enough for analysis
  3. Purify water
132
Q

What are 3 desired attributes of a new chromatographic method?

A
  1. Adequate resolution
  2. Short run time
  3. Rugged
133
Q

What should k be for a good separation?

A

Between 0.5 and 20.
Too small = first peak distorted by solvent.
Too large = too long.

134
Q

What are the 4 initial steps of method development?

A
  1. Determine goal
  2. Select method of sample prep
  3. Choose detector
  4. Test initial system
135
Q

What are two methods to optimize a separation?

A
  1. Adjust mobile phase composition
  2. Add gradient elution
136
Q

What starting conditions need to be considered for a method (6)?

A
  1. Sp
  2. Column
  3. Flow rate
  4. Mobile phase
  5. Temperature
  6. Sample size
137
Q

What needs to be checked for efficiency during method development?

A

N, number of theoretical plates

138
Q

What are 4 ways the separation factor can be adjusted in method development?

A
  1. Fine-tune the solvent composition
  2. Vary column T
  3. Change solvent polarity
  4. Change stationary phase
139
Q

How are the column dimensions (and what dimensions) optimized in method development?

A

Length, particle size, flow rate.
Changing column length may increase separation time.

140
Q

How does pH affect retention?

A

Depending on at what pH different forms of a compound are protonated/deprotonated, they may be similar in retention.

141
Q

What is a supercritical fluid?

A

Formed above the critical T/P of a substance where a distinct liquid/gas phase cannot exist.
Densities, viscosities, and other properties are intermediate between gas and liquid.

142
Q

What is an important property of SCFs?

A

Ability to dissolve large non-volatile molecules

143
Q

What is one condition to use SCFs for LC?

A

Their critical T must lie within the operating conditions of the instrument.

144
Q

Supercritical Fluid Chromatography (SFC)

A

Provides a system intermediate between GC and HPLC. Usefule for compounds that are non-volatile or thermally labile

145
Q

Why is temperature control important in SFC?

A

Small changes in T have large changes in density of a SCF.

146
Q

What is the effect of Pressure on SFC?

A

Density is also easily impacted by small pressure changes. Increasing P increases the solvent effectivity and shortens elution times.

147
Q

Which term is better for SFC than HPLC in the van Deemter eqn?

A

C term because lower density and lower IM forces of SFCs leads to faster partitioning

148
Q

What sp is used for SFC?

A

Packed columns, similar to LC and capillary, similar to GC

149
Q

Which detectors can be used for SFC?

A

Any previous GC/LC detector

150
Q

Comparison of SFC to GC and LC (5)

A
  1. SFC usually faster than LC
  2. Less band broadening than in GC
  3. Plate height lower at higher flow rates for SFC compared to LC
  4. Faster separations with no loss in selectivity or resolution
  5. Variation in mobile phase can be used to change selectivity factors
151
Q

What are the advantages of using supercritical solvents? (3)

A
  1. Cheap
  2. Easily recovered or vented
  3. Green
152
Q

Advantages of SFC (sample prep) (4)

A
  1. Fast extractions
  2. Solvent strength is altered by changing P
  3. Liquid easily removed
  4. Liquid is cheap, non-toxic, and inert
153
Q

Explain liquid-liquid extraction

A

Usually used in prep scale work-ups to isolate and/or remove interferences. Used to extract a nonvolatile analyte from aqueous solution into an organic solvent.

154
Q

Continuous liquid-liquid extraction

A

Solvent boils from flask and condenses into extraction vessel. Dense droplets of solvent falling through the liquid column extract the analyte. When the liquid level is high enough, extraction solvent is pushed through the return tube to the solvent reservoir. Analyte is slowly transferred from the light liquid at the left into the dense liquid in the reservoir.

155
Q

Steps in dispersive liquid-liquid microextraction (4)

A
  1. Extraction solvent plus dispenser solvent in a syringe is transfer to aqueous sample containing analyte in centrifuge tube
  2. Liquid injected rapidly to create emulsion and mixed with vortex. Fine droplets of extration solvent form
  3. Centrifuge to separate extraction solvent and water. Analyte dissolved in dense extraction solvent
  4. Remove some solvent and inject to a column
156
Q

Solid Phase Extraction

A

Used to concentrate and purify samples for analysis. An aqueous solution containing analyte passed through a small volume of sp. Analyte adheres to sp. Concentrated analyte is eluted using a small amount of eluent such as MeOH.

157
Q

What does QuEChERS stand for

A

Quick, Easy, Cheap, Effective, Rugged, Safe

158
Q

What is QuEChERS used for

A

To remove matrix components from extract prior to chromatography

159
Q

Describe the QuEChERS process. (4)

A
  1. Analytes extracted from solid samples with acetonitrile and centrifugation.
  2. Aliquot of extract deposited into QuEChERS tube with MgSO4, ion exchangers and other sorbents
  3. Agitate and centrifuge
  4. Analyze aliquot by LC
160
Q

Advantages of QuEChERS (2)

A
  1. Little organic solvent required
  2. Many samples can be handled at once
161
Q

Mass Spectrometry

A

Technique for studying masses of atoms/molecules/fragments using behavior of their ions in the gas phase and under the influence of electric or magnetic fields

162
Q

3 Essential Components of any Mass Spectrometer

A
  1. Ion source
  2. Mass Analyzer
  3. Detector
163
Q

Most common ionization sources (gas) (2)

A
  1. EI
  2. CI
164
Q

Most common ionization sources for liquids (2)

A
  1. ESI
  2. APCI
165
Q

Most common ionization sources for solids (2)

A
  1. LDI
  2. MALDI
166
Q

Explain how MS occurs

A

Ions are created or volatilized in the ion source through reactions with electrons (EI and ESI) or chemical reactions (proton transfer or adduct formation, CI and APCI). Ions are accelerated by an electric field, and then ions are separated by their mass-to-charge ratio in the mass analyzer. Electron multiplier detector converts each arriving ion into electrons that reach anode where current is measured.

167
Q

Mass-to-charge ratio

A

Particles with the same m/z move down the same path in a vacuum when subjected to the same magnetic and electric fields

168
Q

Molecular ion

A

m/z particle represents the intact molecule with only one electron removed

169
Q

Protonated molecule

A

[M+H]+

170
Q

Base Peak

A

Highest intensity m/z ion in the spectrum

171
Q

Nominal Mass

A

Integer mass of the most abundant naturally occuring stable isotope

172
Q

Exact Mass

A

Exact mass of the most abundant naturally occurring stable isotope

173
Q

Isotope Ratio

A

Mass spectrum give peaks corresponding to the same molecule with different isotopes

174
Q

Nitrogen Rule

A

For compounds with an odd nominal mass, the number of N atoms is odd. An even nominal mass indicates an even number of N atoms or 0.

175
Q

Resolution in MS

A

Smallest difference in m/z values that can be detected as separate peaks

176
Q

Resolving power

A

Ability of a mass spectrometer to separate 2 peaks with similar mass

177
Q

For which compound is the base peak also the molecular ion?

A

Benzene

178
Q

What are the 3 main types of fragmentation processes (note that 1 is specific to ketones)

A
  1. Homolytic cleavage
  2. Heterolytic cleavage
  3. McLafferty rearrangement
179
Q

How can Cl and Br be specifically identified using MS?

A

The M+2 peak of Br is in a 1:1 ratio, whereas for Cl it is in a 1:3 ratio.

180
Q

Types of Mass Spectrometers (6)

A
  1. Magnetic sector
  2. Transmission quadrupole
  3. Time of flight
  4. Quadrupole Ion Trap
  5. Linear Ion Trap
  6. Orbitrap
181
Q

Magnetic Sector MS

A

separates gas ions by accelerating them in an electric field and deflecting ions of different m/z ratio through different arcs in a magnetic field. Detected by an electron multiplier.

182
Q

Conversion dynode

A

Ensures all ions produce a similar electrical response at detector for magnetic sector MS

183
Q

What is an electric sector used for in magnetic sector MS?

A

Reduce the spread of KE

184
Q

Double-focusing MS

A

Attains high resolution by employing an electric sector with the magnetic sector to select ions with a narrow range of KE.

185
Q

Go back and clarify differences between TOF and all other MS, write it out explicitly, might have to use textbook or another source cuz its not clear in notes

A

Fo

186
Q

Most common TOF (2)

A
  1. Quadrupole
  2. Ion Trap
187
Q

Why does MS require high vacuum?

A

Prevent molecular collisions during ion separation

188
Q

Electron Ionization

A

Most common ionization source. Used with GC. Electrons emitted from a hot filament are accelerated through 70V before interacting with incoming molecules M. Resulting molecular ion can have so much extra E that it breaks into fragments. These fragments and the ion are accelerated to mass analyzers. The molecular ion may have a small or absent peak.

189
Q

Chemical Ionization

A

Less fragmentation than EI. Ionization source is filled with a reagent gas and energetic electrons convert methane into a variety of reactive products, including CH5+ which reacts with analyte M to give MH+, the most abundant ion in the MS.

190
Q

Describe APCI

A

APCI uses a corona discharge to create a variety of gas ions from aerosol droplets. Both APCI and ESI produce unfragmented ions.

191
Q

Describe ESI

A

Employs high voltage at the exit of the column, with coaxial N2 flow, to create a fine aerosol of charged droplets containing ions. Analyte often associated with other ions to give new species. Controlling pH helps ensure selected analytes are in anionic or cationic form

192
Q

Reconstructed Total Ion Chromatogram

A

Shows the signal from all ions above a chosen m/z emerging from chromatography as a function of time

193
Q

Extracted Ion Chromatogram

A

Shows the signal for one ion taken from the complete MS (scans all, shows one)

194
Q

Selected ion monitoring

A

One or a few values of m/z; improves signal-to-noise because all time spent measuring just that one or a few ions

195
Q

Selected reaction monitoring (MS/MS)

A

A precursor ion is isolated by one mass falter and passes into a collision cell in which it breaks into products. One or more product ion is then selected by a second mass filter for passage to the detector. Extremely selective for one analye

196
Q

What are 3 types of scans for MS/MS and briefly describe

A
  1. Product ion scan: precursor ion is focused in Q1 and transferred to collision cell where it interacts and fragments, and then the fragments are measured.
  2. Precursor ion scan: the fragments are selected and the original Q is measured to see what produces those fragments
  3. Neutral Loss Scan: everything is passed through, and the ions that are lost are measured
197
Q

Atomic Spectroscopy (Overview)

A

Samples are vaporized and decompose into atoms and ions. Concentrations are measured based on their absorbance or emission of light. Gives the ability to distinguish one element from another in a complex mixture. Analyte can be measured at ppm or lower.

198
Q

Atomic Absorption

A

A hollow-cathode lamp passes light through the sample in the flame and the intensity of the transmitted radiation is measured by a photon transducer. Bandwidths of 10-100nm. Spectra consist of sharp lines (so sharp that there is little overlap)

199
Q

Atomic Fluorescence spectroscopy

A

Atoms in the flame are irradiated by a laser to promote them to an excited state from which they can fluoresce to return to the ground state.

200
Q

Atomic Emission Spectroscopy

A

Radiation from hot atoms whose electrons have been promoted to an excited state in the flame or plasma is emitted

201
Q

Flame atomizer

A

A liquid sample is aspirated through a plastic tube into the flame, where liquid evaporates and the remaining solid is atomized in the flame

202
Q

Premix Burner

A

Fuel, oxidant, and sample are mixed prior to introduction into the flame.

203
Q

Pneumatic Nebulizer

A

Sample solution is drawn in by rapid flow of oxidant past the tip of the sample capillary

204
Q

How does a premix burner work to prepare sample for a flame?

A

The liquid breaks into a fine mist as it leaves the capillary. Spray is directed against a glass bead, upon which droplets break into smaller particles. Mist, oxidant, and fuel flow past baffles that promote mixing and block large droplets.

205
Q

Nebulization

A

Formation of small droplets

206
Q

Aerosol

A

Fine suspension of liquid (or solid) particles in a gas

207
Q

In flame atomization, about how much of the initial sample is contained in the aerosol reaching the flame?

A

5%

208
Q

List several of the processes that can occur in a flame (most important one first)

A
  1. Excitation/Emission
  2. Ionization/suppression
  3. Vaporization
  4. Oxidation
  5. Decomposition
209
Q

What is the most common fuel used for flame atomization?

A

Air/acetylene, giving temps of 2400-2700K

210
Q

Rich flame

A

Excess fuel; increases sensitivity because excess carbon reduces metal oxides/hydroxides

211
Q

Lean flame

A

Excess oxidant; gives a hotter flame

212
Q

In cyanogen/oxygen fuel, which component is the fuel and which is the oxidant?

A

Cyanogen - fuel
Oxygen - oxidant

213
Q

What must be subtracted in flame atomization to get the analyte signal?

A

The light emitted by the flame must be subtracted from the total signal

214
Q

Graphite furnace

A

Atomization source; more sensitive than a flame and requires less ample.

215
Q

How does a graphite furnace work for atomization?

A

1-100uL of sample is injected into the furnace, through a hole at the center. Light from a HCL travels through the window at each end of the graphite tube. Sample injected onto platform, which is heated by radiant E from wall; sample vaporized when wall reaches a constant T.

216
Q

Disadvantages of graphite furnace (3)

A
  1. Requires more operator skill
  2. Limited lifetime
  3. Memory effect causes interference from previous runs
217
Q

Advantages of Furnace over flames (6)

A
  1. Less sample volume
  2. Furnace confines sample in light path for several seconds
  3. Pre-concentration possible; multiple aliquots can be deposited onto furnace
  4. Nebulization can dilute sample; no nebulization in graphite furnace
  5. Solids can be analyzed
  6. No combustible gases required
218
Q

Matrix Modifier

A

Substance added to sample to reduce loss of analyte during charring by making matrix more volatile or analyte less volatile

219
Q

Inductively Coupled Plasma

A

Twice as hot as flame; stable; inert Ar environment; simultaneous multi-elemental analysis by atomic emission or MS; costs more to purchase and operate.

220
Q

How does ICP for atomization work?

A

High-purity argon is fed into the plasma gas inlet at 14-18L per minute. A spark from a tesla coil initiates ionization of Ar, forming Ar+ and electrons, accelerated by the radio frequence field. Electrons collide with Ar atoms and transfer E to the entire gas (T 6000-10000K). Quartz torch protected from overheating by coolant gas.

221
Q

What instrument in ICP is used to reduce the concentration of analyte needed for an adequate signal?

A

Ultrasonic nebulizer

222
Q

Piezoelectric crystal

A

One whose dimensions changed in an applied electric field. Sinusodial voltage applied btwn two faces of the crystal cause it to oscillate

223
Q

How does Temperature affect Atomic Spectroscopy? (2)

A
  1. Deterines the degree to which a sample breaks down into atoms.
  2. Determines the extent to which an atom is found in its ground, excited, or ionized state. (Boltzmann distribution)
224
Q

Due to the Boltzmann distribution, how does varying the temperature by small amounts impact the ground and excited state populations?

A

Ground state: negligible influence
Excited state: significant increase (10K could have up to 4% increase in population)

225
Q

If the excited state population increases by 4%, how does the emission intensity change?

A

Increase by 4%

226
Q

By what principle are linewidths determined?

A

Heisenberg uncertainty principle

227
Q

Dopple Broadening in Atomic Spectroscopy

A

Atomic motion occurs in every direction. An atom moving toward radiation source sees a higher frequency light than one moving away. There is no shift for atoms moving perpendicular to the source.

228
Q

Pressure broadening in atomic spectroscopy

A

Caused by collisions of the emitting or absorbing species with other atoms or ions in a hot environment. Collisions between atoms shorten the lifetime of the excited state. Collision frequency is proportional to P. This is similar in magnitude to Doppler broadening.

229
Q

What do we use to produce narrow lines of correct frequency in atomic spec?

A

Hollow-cathode lamps

230
Q

Hollow-Cathode Lamp

A

Consist of a tungsten anode and a cylindrical cathode, and a glass tube filled with neon or argon. Cathode is made of metal whose spectrum is desired. Potential difference is applied across the electrodes, generating a current and ionizing inert gas. Gas cations can acquire enough KE to dislodge some metal atoms from cathode producing an electron cloud (sputtering). A portion of sputtered atoms are in excited state and emit radiation as they return to ground state.

231
Q

Background correction

A

Distinguishes analyte signal from absorption, emission, and optical scattering by/from a sample matrix (flame, plasma, or furnace).

232
Q

Why is background correction critical for graphite furnaces?

A

Residual smoke from charring

233
Q

List the 3 main background correction methods

A
  1. Beam chopping
  2. Deuterium lamp
  3. Zeeman
234
Q

Describe beam chopping as a correction method

A

Signal reaching detector while beam is blocked must be from flame emission. Difference between two is analytical signal. Does not correct for scattering

235
Q

Describe deuterium lamp background correction

A

Broad emission from a D2 lamp is passed through the flame in alternation with that from the hollow cathode. Light from hollow cathode is absorbed by analyte and background; but, light from D2 lamp is only absorbed and scattered by the background. Difference between the 2 is the absorbance of the analyte

236
Q

Describe the Zeeman effect background correction for graphite

A

A magnetic field applied parallel to light path through the furnace is pulsed on and off, splitting analyte signal into 3 parts: Two shifted and one not. Unshifted signal does not absorb light. Sample and background observed when the field is off, thus, only background is observed when the field is on. Difference between them is the corrected signal.

237
Q

Compare the sensitivities for flames, furnaces, and ICP

A

Detection limits for furnaces is 2 orders of magnitude lower than for flames, because the sample is confined in the small volume of the furnace for a long time.
ICP has intermediate sensitivity; close to the sensitivity of the graphite furnace

238
Q

Spectral Interference

A

Overlap of the analyte signal with signals due to other elements or molecules in the sample or with signals due to the flame or furnace

239
Q

Chemical Interferences

A

Caused by a component of the sample that decreases the extent of atomization of the analyte. Elements that form very stable diatomic oxides are not completely atomized. Molecular spectra are much broader than atomic spectra. This can be removed by extraction or Chromatography.

240
Q

Releasing Agents

A

Chemicals added to a sample to decrease chemical interference

241
Q

Ionization Interferences

A

Can be a problem in the analysis of alkali metals with low ionization potentials. Ionized atoms have diff E levels than neutral atoms, so desired signal is decreased.

242
Q

Ionization Suppressor

A

Decreases the extent of ionization of the analyte

243
Q

Standard Addition

A

Compensates for many types of interferences by adding known quantities of analyte to the unknown.

244
Q

Describe laser ablation

A

A pulsed laser beam is focused onto a solid sample. An explosion occurs of particles, atoms, electrons, and ions in the gas phase

245
Q

Which laser is commonly used for laser ablation?

A

Nd:YAG (neodymium-doped yttrium aluminum garnet crystal)

246
Q

Laser-Induced Breakdown Spectroscopy

A

Analyzes the ablation product in a plasma by observing atomic emission. Suitable for any element that emits a photon with a measurable UV or visible wavelenth

247
Q

What is the emission due to in laser ablation?

A

Emission is due to formation of a short-lived plasma under sample irradiation with a highly energetic laser pulse. Can be used for depth profiling

248
Q

ICP-MS

A

When ICP plasma is directed into an MS, separating and measuring ions according to m/z ratios.

249
Q

What is a downside of ICP-MS

A

Requires extremely pure water and trace-metal grade HNO3, very expensive

250
Q

What is the ICP source rich in?

A

Ar+
Secondary ions ArH+, ArC+, ArN+, etc.

251
Q

Isobaric interference in ICP-MS

A

They interfere with the measurement of analyte ions of the same m/z charge ratio

252
Q

Dynamic Rxn Cell in ICP-MS

A

Uses thermodynamically favorable reactions to reduce isobaric interference

253
Q

X-Ray Fluorescence

A

Emission of X-Rays following absorption of X-Rays by a material. Elements identified by peak energies and quantified by number of photons in each peak

254
Q

What is the hump in XRF that makes it not good for quantification in this region?

A

Bremsstrahlung

255
Q

Why is XRF semi-quantitative? (3)

A
  1. Higher uncertainty (RSD ~30%)
  2. Matrix effects (self-absorbance)
  3. Matrix components (absorbing/scattering X-rays)
256
Q

Describe handheld XRFs

A

Can detect levels as low as 1ppm for As, Pb, and Hg. Should be mounted in a lead-lined test stand to prevent user exposure.

257
Q

What is electric charge measured in?

A

Coulombs (C)

258
Q

What is the charge of an electron?

A

1.602*10^-19C

259
Q

What do electrons appear as in electrochemical reactions?

A

Reactants or products

260
Q

Electric current

A

Measured in ampere (A); represents rate of electron flow. Provides a measure of rate of an electrochemical reaction

261
Q

Electric potential

A

Measured in volts (V). The difference between two points exerts a force on electrons and causes a current to flow.

262
Q

Potentiometry

A

A difference in concentration (chemical potential) generates a potential difference

263
Q

Potential vs. Current

A

-Potential controls/measures E of electrons in a material. Electrons move from low potential to high potential.
-Potential is related to thermodynamics
-Current is related to kinetics

264
Q

What are the 3 main electroanalytical techniques?

A
  1. Coulometry
  2. Potentiometry
  3. Amperometry
265
Q

Potentiometry definition

A

Measurement of voltage at zero current using 2 electrodes to provide chemical information

266
Q

Applications of potentiometry (2)

A
  1. Measuring activities of specific ions
  2. Detecting end-point for titrations
267
Q

Indicator electrode

A

An electrode that responds to an analyte

268
Q

Reference electrode

A

An electrode with known constant potential

269
Q

Junction Potential

A

Develops at the interface btwn 2 dissimilar electrolyte solution. Typically small and are found at each end of the salt bridge connecting 2 half-cells.

270
Q

How are junction potentials minimized?

A

By choosing ions with similar mobilities

271
Q

Ion Selective Electrodes

A

Do not involve a redox process. Responds selectively to one ion through a thin membrane that bindings only a specific ion.

272
Q

Components of Ion-selective electrodes (4)

A
  1. Hydrophobic organic polymer membrane
  2. Viscous organic solution
  3. Ion-exchanging ligand that binds the analyte cation
  4. A hydrophobic anion
273
Q

How do ISE work?

A

The inner filling solution contains a constant concentration plus counter-ion. Outside analyte solution contains constant and counteranion. C+ can diffuse across the membrane but not R-, B-, or A-. Difference in potential across membrane is measured by 2 reference electrodes on the inside and outside. When the concentration of C+ in analyte solution changes, voltage changes

274
Q

K+ ISE

A

Uses valinomycin as the ligand. Chosen because it has a high affinity for K+ and low affinity for other ions. Membrane is PVC and a liquid plasticizer

275
Q

What is the most common ISE?

A

Glass electrode

276
Q

Describe Glass Electrode

A

pH sensitive part is the thin glass bulb at the bottom of the electrode. 2 Reference electrodes measure potential across the membrane. Salt bridge is the porous plug

277
Q

How do Glass Electrodes work?

A

Each side of the glass membrane is hydrated and separated by a dry glass layer. Metal cations can diffuse out of the glass in these regions and into the solution. H+ can diffuse into the membrane and replace the metal cations, in an ion-exchange equilibrium.

278
Q

What are the 8 sources of errors in pH Measurements?

A
  1. Standards
  2. Junction potential
  3. Junction potential drift
  4. Sodium error
  5. Acid error
  6. Equilibration time
  7. Temperature
  8. Cleaning
279
Q

What are the 4 classes of ISE?

A
  1. Glass Membrane
  2. Solid-state electrodes
  3. Liquid-based electrodes
  4. Compound electrodes
280
Q

What are solid-state ISE based on?

A

An inorganic crystal

281
Q

Describe the fluoride electrode

A

Used to monitor the fluoridation of municipal water supplies. Employs a crystal of LaF3 doped with Eu2+, creating anion vacancies, with a filling solution. F- migrates across the crystal. Adjacent F- can jump into the vacancy, leaving a new vacancy behind. F- diffuses from one side to the other, jumpint through the vacancies. The response is recorded

282
Q

What is measured in coulometry

A

The number of moles of electrons

283
Q

What is constant during coulometry?

A

Current

284
Q

Advantages of Coulometry (3)

A
  1. Precision
  2. Sensitivity
  3. Generation of unstable reagents in situ
285
Q

Amperometry

A

Current at the working electrode is proportional to analyte concentration

286
Q

2 Common Amperometric techniques

A
  1. Clark electrode
  2. Glucose biosensor
287
Q

How does the clark electrode in amperometry work?

A

Measures dissolved O2

288
Q

How does the glucose biosensor work in amperometry?

A

Generates H2O2 by oxidation of glucose, which is measured. A mediator is employed to rapidly shuttle electrons between electrode and analyte.

289
Q

Coulometric glucose monitor

A

Counts electrons released by oxidation of all glucose in a small blood sample

290
Q

What is electrical wiring in amperometry?

A

Enzyme and mediator; increases signal from the desired reaction and decreases background current from mediator diffusing to the auxillary electrode

291
Q

What is voltammetry?

A

A collection of methods in which the dependence of current on the applied potential of the working electrode is observed

292
Q

How does voltammetry work?

A

Current from an oxidation or reduction is measured as the potential of the working electrode is changes

293
Q

Residual current

A

Due to oxidation or reduction of species in solution other than analyte. Should be subtracted

294
Q

Faradaic current

A

What is measured in voltammetry; due to reduction or oxidation of analyte at working electrode

295
Q

Charging current

A

Comes from the flow of ions toward the electrode surface and interferes with detection of the reduction reaction

296
Q

What happens to limiting current for quantitative analysis? (3)

A
  1. Is controlled by analyte diffusion rate
  2. Convection is minimized by using an unstirred solution
  3. A high concentration of supporting electrolyte is used to minimize migration
297
Q

Microelectrodes

A

Low capacitance increases sensitivity by reducing charging current, permits rapid voltage scanning (short-lived species can be studied).

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