Exam One: Introduction and Chromatographic Methods Flashcards
What is limit of detection or detection limit?
The minimum concentration or mass of an analyte that can be detected at a known confidence level
Figure(s) of merit for Detection Limit
Cm Cm=Sm-S(bar)m/m
What is k commonly when calculating Sm`
3
What is linearity lost at high analyte concentration
High analyte concentration can lead to interaction between the analyte and other molecules
LOD
Limit of Detection
LOQ
Limit of Quantification
LOL
Limit of Linear Response
What is selectivity?
Th degree at which a method is free of interference from other species int he sample matrix
Figure(s) of merit of selectivity
selectivity coefficient kAB=(slope of B)/(slope of A)
Selectivity= (Slope of A x [A])+(slope of B x [B])+ā¦.
If there is less interference, selectivity is ___
better
5 Important Sās of Method Selection
Sensitivity Selectivity Speed Skill \$\$
Questions to ask when defining a problem when selecting a method
What accuracy is required?
How much sample is available?
What is the concentration range of the analyte?
What components of the sample may cause interference?
What are the physical and chemical properties of the sample matrix?
How many samples are to be analyzed?
What kind of relationship generally exists between signal and analyte concentration?
Linear
Based on k, which is determined from standards
External Standard Calibration Methods
Single-Standard Calibration
Multiple-Standard Calibration
Standard Addition Methods
Single Standard Addition
Multiple Standard Addition
Three points when completing single standard calibration
- Good accuracy is achieved by having same conditions between the unknown and standard
- The standard concentration should be as close to the concentration of the unknown as possible
- The unknown signal should be near the middle
What conditions should be kept constant between standard and unknown sample
analysis time pH solvent temperature analyst
How can you get a standard concentration close to an unknown if the unknown concentration is obviously unknown?
- choose a standard concentration, do analysis
- Get concentration of unknown
- Prep new standard with concentration closer to unknown
What is single standard calibration method?
A single standard is used to determine the concentration of an unknown sample under the same conditions
What is multiple standard calibration method?
Multiple standard with difference concentrations are prepared and analyzed. Their signals are used to make a calibration graph, which is then used to determine the unknown sample concentration
When are standard addition methods used?
When the sample matrix of a sample is very complex
What is single standard addition?
Unknown solution is spiked with known standard and that is used to determine the unknown concentration
Can be done with just spiking or can be done in volumetric flasks
What is multiple standard addition?
Using spiked standards to create a calibration graph to determine the concentration of an unknown
Why are internal standards used?
To compensate for several types of errors caused by uncontrollable variablesā¦ coupled with other methods
What is an internal standard
substance that is similar in chemical and physical properties of the analyte that is added at a constant amount to help ācorrectā possible errors
Why is chromatography used
To separate compounds based on physio-chemical property differences
6 separation methods
Filtration Centrifuging Distillation Precipitation Solvent Extraction Chromatography
Filtration separates compounds based on
particle size
Centrifuging separates compounds based on
Density
Distillation separates compounds based on
Vapor pressure (boiling point)
Precipitation separates compounds based on
Solubility
Solvent Extraction separates compounds based on
Distribution coefficient between organic and aqueous phase
Chromatography separates compounds based on
Distribution coefficient between stationary and mobile phase
What is LLE?
Liquid-Liquid Extraction- the transfer of a solute from one liquid phase to another (usually aqueous to organic)
What indicates greater separation/ affinity to the organic phase
A larger K value
What is K
The Partition Coefficient
Solubility of A in organic phase / Solubility of A in aqueous phase
If an analyte is present in two forms in the aqueous and or organic phase separation is based on ___
D- distribution coefficient
What is extraction
Aqueous to Organic
What is back extraction
organic to aqueous
If an analyte is a BASE
BELOW the pKa, itās mainly BH+
ABOVE the pKa, itās mainly B
If the analyte is a ACID
BELOW the pKA itās mainly HA
ABOVE the pKA itās mainly A-
Types of solvent etxraction
single-step solvent extraction
multiple step solvent extraction
continuous solvent extraction
counter-current solvent extraction
Using __ volumes of ____ solvent multiple times makes for good extaction
Small volumes of Organic Solvent
What can be analyzed in continuous solvent extraction
solids
What canāt be used in continuous solvent extraction
Volatile compounds
What assumptions are made in countercurrent extraction
DA=1
Vaq=Vor
Extraction Efficiency then equals 50%
in chromatography, what does a greater K value indicate
the analyte has high affinity for the stationary phase
How is chromatography classified
On the physical means by which the stationary and mobile phases come into contact
Chromatography classification
Planner Chromatography
Column Chromatography
What is planner chromatography carried out on
A flat medium stationary phase
Types of planner chromatography
Paper Chromatography (PC) Thin Layer Chromatography (TLC)
Figure of merit for planner chromatography
Retardation factor (Rf)
How is qualitative analysis done on planner chromatography
Comparing Rf values between sample and standards
How is quantitative analysis done by planner chromatography
Spot can be scraped from the plate, then analyte can be extracted and measured
In column chromatography what can the mobile phase be
liquid or gas
In column chromatography what can the stationary phase be
solid or liquid
What is LSC
Liquid- Solid Chromatography (liquid mobile phase, solid stationary phase)
What is LLC
Liquid- Liquid Chromatography (liquid mobile phase, liquid stationary phase)
What is GSC
Gas- Solid Chromatography (gas mobile phase, solid stationary phase)
What is GLC
Gas- Liquid Chromatography (Gas mobile phase, liquid stationary phase)
What are chromatographic methods based on
Separation mechanisms
5 separation mechanisms
- Adsorption chromatography
- Partition chromatography
- Ion-Exchange Chromatography
- Molecular Exclusion Chromatography
- Affinity Chromatography
What is SFC
Supercritical Fluid Chromatography
Stationary phase in adsorption chromatography
Solid Particles
Mobile phase in adsorption chromatography
Gas or Liquid ( LSC or GSC)
Separation mechanism of adsorption chromatography
adsorption of the solute to the stationary phaseā¦. the more adsorped the solute the slower it will move through the column
Stationary phase in partition chromatography
Liquid
thin liquid coat on solid support
Mobile phase in partition chromatography
Liquid or Gas (LLC or GLC)
Separation mechanism of partition chromatography
partition of solute between the two phases
Stationary phase in Ion-Exchange Chromatography
Anion or cation exchnagers (resin)
Mobile phase in Ion- exchange chromatography
liquid
Separation mechanism of ion-exchange chromatography
ion exchange
Stationary phase in molecular exclusion chromatography
molecular sleeve or porous gel
Mobile phase in molecular exclusion chromatography
Liquid or Gas
Separation mechanism of molecular exclusion chromatography
exclusion of larger solute molecules but not small molecules
Stationary phase in affinity chromatography
molecule (antibody) immobilized on support
Mobile phase in affinity chromatography
Liquid or gas
Separation mechanism in affinity chromatography
specific interactions such as antibody-antigen and chiral separation
Theories of chromatography
plate theory
rate theory
What does plate theory say
The column is divided into a large number of imaginary ātheoretical plateā segments (N)
What is assumed in plate theory
the solute equilibrates infinitely fast between mobile phase and stationary phase
There is no kinetic limitation
What is HETP
Height Equivalent to a Theoretical Plate (H)
What does HETP depend on
column length and number of plaes
What distribution is plate theory assumed to be similar to?
counter current solvent extarction
What does rate theory say?
The equlibration of the solute between mobile and phase is not infinitely fast and resulting shape depends on elution rate
What does rate theory describe in elution bands and in what terms?
In quantitative terms, rate theory describes shapes and breadth of elution bands
What do we focus on?
What factors broaden elution bands and how we can qualitatively improve the column efficieny
How is column efficiency estimated
van Deemter equation
What is A in the van deemter equation
multiple path effects coefficent
What is B in van deemter equation
longitudinal diffusion coefficeint
What is Cs in van deemter equation
mass transfer coefficient in the stationary phase
What is Cm in van deemter equation
mass transfer coefficient in the mobile phase
what us u (mu) in van deemter equation
linear velocity of mobile phase in cm/s
When looking at A, what leads to higher chromatographic efficeincy
Smaller particles and more uniform packing leads to smaller A which leads to a smaller H and a higher N which leads to high chromatographic efficiency
What does the multiple paths term (A) come from
the multiple paths of different lengths traveled by the solute
What does longitudinal diffusion term (B/u) come frome
The diffusion parallel to the direction of flowā¦. diffusion of the solute from the center of the solute zone (high concentration) to the edges of the zone (low concentration)
If the flow rate is low the ( less or more) greater the solute is spead
the greater the solute is spread bc it spends more time on the column
In which chromatography is the longitudinal diffusion term (B/u) more effective and why
GC more than LC due to a higher DM in gas mobile phase
where does the mass transfer term (Cu) come from
The equilibration of the solute between the mobile and stationary phase requiring a finite amount of time
When looking at Cu, what leads to higher chromtographic efficiency
A thinner stationary phase film, narrow column, and smaller column packing materials leads to a smaller Cu which leads to higher chromatographic efficiency
In the van Deemter plot, at what point is the optimum flow rate located
in the lowest point of plotting H when Contribution to H is a function of flow rate
7 ways to reduce band broadening
- smaller particles in stationary phase
- Narrower column
- uniform packing
- thinner stationary phase coating
- optimized mobile phase flow rate
- Fast injection of a SMALL sample size
- Controlling temperature (Specific to GC)
How is chromatography used quantitatively
Peak area is related to analyte concentration
Calibration standards and addition methods
How is chromatography used qualitatively
Retention times to identify compounds between standards and unknowns
spectroscopic methods to ID compounds
How is preparative chromatography used?
Using bigger columns to separate and purify a significant quantify of a compound in a mixture
Which chromatography is GSC
Gas-Solid Chromatographyā¦ Adsorption chromatography
Which chromatography is GLC
Gas-Liquid chromatographyā¦ Partition chromatography
What is GSC application limited
The semi permanent retention of active or molecules
severe tailing of elution peaks
What are the components of a gas chromatograph?
carrier gas supply flow controller syringe injector port detector data processor
What is the purpose of the injector port
The injector port flash evaporates the sample to introduces it to the column
What is the temperature of the injection port normally
normally at least 50 degrees C above the boiling point of the least volatile component of the sample
What makes up the injection port
septum
carrier gas port
heat source
liner
What should the carrier gas in GC be
inert
pure
detector compatable
inexpensive
What carrier gasses are common in GC
Helium
Nitrogen
Hydrogen
What pressure range does the inlet fall between
10-50 psi
In liquid injection, what syringe is used
microsyringe
What is an average liquid sample size fr ordinary packed column
0.1-20 ĀµL
Which GC column has split injection?
Capillary columns
Which GC column has split-less injection?
Packed column
How hoes a split/split-less injection work?
- Initially he split/purge is off
- Sample travels in
- After a specific TIME, the column closes
- split is introduced
- remaining sample outside the column is evacuated
What does the āsplitā flow do in a split/split-less injection
it bypasses the liner to help maintain column head pressure
How does split injection work
sample splitter delivers a small specific known fraction of the sample to the column while the remainder is expelled as waste
What is split-less flow controlled by
time
what is split flow controlled by
volume
In gas injection, what syringe is used
gas-tight syringe
How does a gas-tight syringe work?
- uptake of sample.
- locking
- insertion of syringe
- unlocking
- injection of sample
Pro of gas- tight locking syringe
good for qualitative analyss
con of gas-tight locking syringe
reproducible sample size cannot be obtained due to temperature
What are used for gas sample injection
Gas sampling valves
What do gas sampling valves do
the lead to more reproducible sample sizes- help quantitatively
Why is head space injection prefered
its the fastest and cleanest method or analyzing volatile compounds in complex samples (such as biological samples)
Steps to head space injection
- heat vial containing samples for certain time and specific temperature
- Insert syringe through septum and intake space above liquid sample
Components of solid phase micro extraction (SPME) appratus
Needle
Stainless steel tubing
fused silica fiber
coated phase
Steps of SPME
- inject needle
- push plunger (fiber then exposed to liquid or gas sample)
- release plunger (fiber extracted upward with analyte on fiber)
- Inject into system
common modes of SPME
direct (liquid exposure)
headspace (gas exposure)
Advantages of SPME
- combination of extraction, pre-concentration, and sample introduction
- solvent free
- rapid
- relatively inexpensive
- Easily coupled to GC and LC
- Used both in the laboratory and in the field
- Used in-vitro and in-vivo
- Automated
Ways to analyze solid samples by GC
digestion pyrolysis Direct Headspace SPME Headspace Solid sample capsules
How is solid sample digestion compelted
Solid sample digested in solvent and then injected as a liquid sample
What is pyrolysis
Solid sampel is heated in special sample holder at extreme temperatures, thus causing sample so decompose into gaseous state which can be introduced to GC
How are solid sample capsules used
sample inside capsule, introduced to injection area, once decomposed capsule is broken
CON: glass must be cleaned out
Tubing materials in packed column
stainless steel glass aluminium copper teflon
General tubing length and diameter of packed column
Length: 1-5 m
diameter: 2-4 m
What is included in the ideal solid support material in GSC
- small, uniform, spherical particles
- Good mechanical strength
- specific surface of at least 1mg^2/g
- inert at elevated temperatures
- uniformly coated by the liquid phase
Common solid support packing materials
naturally occurring diatomaceous earth
Chromosorb P, W, G
How does particle size of support effect column efficiency
decreasing the diameter of the support particle increases column efficiency
At what pressure is it inconventent for GC system to operate
> 50 psi
As particle size ___, pressure ___
as particle size decreases, pressure increases
What tubular (capillary) column is used is GSC
Stationary solid phase particles (PLOT) porous layer
What tubular (capillary) columns are used is GLC
- Solid support coated with liquid phase (SCOT) support coated
- Stationary Liquid phase (WCOT) wall coated
What tubing materials are used in tubular (capillary) columns
stainless steel aluminium copper plastic fused silica
General tubing length and diameter of tubular (caillary) column
length: 10-100 m
diameter: 0.15-0.32 mm
what are the desired properties of the stationary phase in GC
- less volatility
- Thermal stability
- Chemically inert
Two factors in separation in GC
distribution coefficient
volatility
What does a greater distribution coefficient (K) mean
the analyte has a higher affinity for the stationary phase and moves more slwoly with the carrier gas
What happens as compound volatility increases
the quicker it will move with the carrier gas.
More Volatile= ____ K
small
less volatile= ___ K
larger
If compounds are polar you want a ____ column
more polar
If compounds are nonpolar you want a _____ column
less polar
What does isothermal mean
constant temperature (or temperature ratio)
What is temperature programmng
inserting ramps and gradients and alteration of temperature through the run
What are the properties of an ideal detector
- adequate sensitivity
- good stability and reproducible
- linear response to solute
- temp between room and 400 degree c
- short response time independent of flow rate
- high reliability and ease of use
- Similar response towards all solutes, and predictable
- nondestructive
Common GC detectors
flame ionization detector (FID)
Thermal Conductivity Detector (TCD)
Electron capture detector (ECD)
Mass Spectrometer Detector (MSD)
how does Flame Ionization Detector (FID) work
solutes eluted from column, burned in small flame so ions and electrons are produced and carried through the flame
current flowing between anode and cathode is measured and translated into a signal
What carrier gasses can be used with FID
N2 or Helium
What is used if capillary (tubular) is used
make-up gas to fill up
Advantages of FID
highly sensitive large linear response range low noise rugged and easy to use little effect on detector response from flow change general detector to organic species
disadvantages of FID
destructive
requires additional gas
How does Thermal Conductivity detector (TCD) work
Thermal conductivity of the gas is measured by electrical resistance by a heated element whetstone bridge
Which carrier gases are commonly used in TCD
He and H2
True or False: Large molecules have larger conductivity
FALSE! Large molecules have smaller conductivity
_____ in some elements is related to temperature
Electrical resistance
Advantages of TCD
Simple
Larger Linear Response (10^5)
General response to inorganic and organic species
Nondestructive
Disadvantages of TCD
Relatively low sensitivity
Not very compatible with capillary (tubular) column
When micro-TCD is used, the interal volume is small
How does electron- capture detector (ECD) work?
Ionization of the carrier gas causes emission of electrons which leads to a constant standing current. When there is an organic analyte with electronegative functional groups present the current decreases leading to the capture of electrons
What is ECD good at analyzing with HIGH sensitivity
halogens
peroxides
quinones
nitro groups
What is ECD insensitive to
amines
alcohols
hydrocarbons
Advantages of ECD
highly sensitive
non-destructive
Disadvantages of ECD
limited linear response (10^2)
What is one of the most powerful detectors for GC
Mass Spectrometry (MS)
When GC is paired with MS what is it considered
hypernated method
What does MS separate on
mass to charge ratio (m/z)
Detectors for qualitative analysis
MS
FTIR
NMR
Using ____ is common in GC analysis
one or more internal standards
What kind of detectors can be used in preparative GC
destructive detectors
What detector is commonly used in GC
TCD
What is more common: preparative LC or preparative GC
preparative LC
Advantages of GC analysis
high resolution High speed high sensitivity high accuracy simple relatively cheap
Limitations of GC
Only can be used on volatile compounds
More sample prep is needed for samples to be clean
another instrument is usually needed for identification
Common GC samples
Almost all gasses volatile liquids solids (usually after digestion) Organic compounds some inorganic compounds Chiral spearation
What leads to separation in Liquid chromatography?
The affinity of the analyte between the mobile and stationary phase
What used to drive separation in early LC
gravity flow rates
TO increase efficiency what was introduced to LC
smaller packed columns with pressurized flow
In HPLC what is the diameter range of particles
3-10 Āµm
in UPLC what is the diameter range of particles
< 2.5 Āµm
As column particle size ___, LC column efficiency ___
as particle size decreases, column efficiency increases
In UPLC what can changed and not effect efficieny
The flow rate can be increased
What causes extracolumn band broadening in LC
The difference in flow rates between layers of liquid adjacent to the tube wall and center of the tube when a solute is carried through open tubes in the injection system, detector region and the piping.
What are the velocity differences in tubing
velocity is less by the wall
in the middle of the tube the velocity is greater
How can broadening be minimized
The radius of tubing components should be reduced
the length of extra column tubing should be as small as possible
Flow rate is LC is ___ than GC
less
LC separation is ___ than GC
slower
LC HETPs is ___ than GC
smaller
LC columns are ____ than GC
shorter
Why must dust and dissolved gasses be removed from LC solvent
dissolved gases can lead to irreproducible flow rates and band broadening
dust can clog the column
Both interfere with the detector performance
How can an LC be degasses
purging or sparging with an inert gas
LC pump requirements
pressure generation up to 6000 psi pulse free output 0.1 mL/min- 10 mL/min flow rates 0.5% or better relative flow reproducabilty corrosion resistance
Two major LC pumps
displacement pumps (screw driven syringe pum)p Reciprocating pump
Displacement pump function
large, syringe like chamber equipped with a plunger activated by a screw-driven mechanism
Advantages of displacement pumps
pulse free
Disadvantages of displacement pumps
limited pressure
limited solvent capacity
inconvenience when solvents need to be changed
isocratic only
Reciprocating pump function
Solvent pumped in a back and forth motion from motor piston with two ball check valves controlling flow of solvent in and out of cylinder
Advantages of reciprocating pumps
small internal volume high output pressure adaptability to gradient elution large solvent capacities consistent flow rates
Disadvantages of reciprocating pumps
pulsing flow (noisy base line)
How can pulsing flow be minimized in reciprocating pumps
pulse damper
What does the solvent portioning valve do?
used to introduce solvents from two or more reservoirs at ratios that can be varied continuously
Two types of LC elutions
Isocratic
Gradient
What is isocratic elution
single solvent or solvent mixture at constant composition
What is gradient elution
two (or more) solvent systems that differing in polarity and are varied in composition during separation
what is the most widely used LC injection mehtod
Sampling loop
What are LC columns normally made of
smooth-bored stainless steel tubing
heavy walled glass tubing
polymer tubing such as polyetheretherketone (PEEK)
Types of LC columns
Guard columns
analytical columns
What do guard columns do
short column before the analytical column to protect the analytica column from particle matter and compounds that bind irreversibly to the stationary phase
An LC column is generally held at this temperature
room temperature
Constant column temperature leads to
better more reproducible chromatograms
Types of LC column packing
pellicular particles
porous particles
What is a pellicular particle
contains a porous layer and non porous support
Which is better pellicular or porous?
porous bc smaller
Two basic LC detectors
Bulk-property detector
solute- property detector
Bulk- property reads
mobile phase bulk property such as refractive index, dielectric constant, density
Solute-property read
solute properties such as UV abs, fluorescence, diffusion current
Florescence detector advantage
one of the most sensitive LC detectors
Because fluorescence detection is so sensitive what is it usually used for?
trace analysis
Fluorescence detection disadvantages
Not all substances naturally fluoresce so their derivatives must be synthesized
Limited concentration range
Electrochemical detectors are based on what
amperometry (current)
voltammetry (current by applying voltage)
coulometry (charge)
conductometry (conductivity)
Refractive index detectors evaluates
whena solvent passes from one solvent into another causing a bend in the incident beam
Advantages of refractive index detectors
general detector (basically can respond to nearly all solutes) reliable and uneffected by flow rate
disadvantages to refractive index detectors
high temperature dependent
not as sensitive as other detectors
Not compatible with gradient
Why canāt LC be directly connected to MS
MS need sample in gaseous state
What is usually used to get LC sample into gaseous state for MS
Vaporization electrospray
A powerful analytical technique for separation, identification and determination is
LC-MS
LLC is also
Partition Chromatography
LSC is also
Adsorption chromatography
Classification of LC
partition chromatography adsorption chromatography ion chromatography size-exclusion chromatography affinity chromatograpy chiral chromatography
Most widely used type of HPLC is
partition
How do liquid liquid columns work
liquid is held in place by phsyical adsoprtion
how do liquid bonded phase columns work?
the liquid is attached to the support particles by chemical bonding (usually silica gel)
What is normal phase chromatography
Polar stationary phase, nonpolar mobile phase
what is reverse phase chromatography
nonpolar stationary phase, polar mobile phase
Types of partion chromatography
normal phase
reverse phase
In reverse phase which analytes elute first
more polar
in normal phase which analytes elute first
more non polar
Unreacted SiOH groups in silica column lead to
tailing
how is tailing minimized when using SiOH column
capping unreacted SiOHās with OH
Bond phase packing is reverse when
bonded coat is nonpolar
bond phase packing is normal when
bond coat is poalr
nonpolar stationary phases lead to _____
rapid mass transfer
In partition chromatography have ____ chains that are ____ retentive which permits the use of ____ samples
longer chains
more retentive
larger samples
In column selection for partition chromatography the ______ forces between the solute mobile phase and stationary phase need to be balanced
intermolecular forces
Polarity of analytes
hydrocarbons
Nonpolar molecules have weak ______
london forces
polar molecules have _____
dipole dipole forces
The retention factor (k) in LC can be changed with
solvent strength
the selectivity factor (alpha) in LC can be changed with
the chemical nature of the mobile phase
What is solvent strength related to
Polarity of the solvent
What is the polarity index (Pā) based on
solubility of substance in dioxane, nitromethane, and ethyl alcohol
The larger the polarity index number (Pā) the ___ polar
more polar
What is normally used in the solvent of reverse phase separations
water and polar organic solvent
When two bands overlap ______ must be changed
selectivity factor (k)
What is widely used to optimize solvent strength
Solvent Selectivity Triangle
How is qualitative LC used
retention times or spectroscopic detectors for ID
how is quanitative LC used
peak height or area and calibration standards used to determine concentration
internal standards
How is preparitive LC used
to separate components of a mixture on a large scale
Prepatarive LC application
pharmaceticals Biochemical food products industrial chemicals pollutants forensic science clinical chemistry
