Quiz 4 Flashcards
What is the carrier gas?
is what transports gaseous analyte through the GC column, has a high purity and is less than 1 ppb O2.
What is the stationary phase in gas-liquid partition chromatography?
This is a nonvolatile liquid bonded to the inside of the column or to a fine solid support (such as silica or aluminum particles)
Where is the analyte adsorbed in gas-solid adsorption chromatography?
The analyte is adsorbed directly on the solid particles of the stationary phase
Describe how a gas chromatogram works?
volatile (liquid or gas) sample is injected through a septum (rubber disk) into a heated port to make it into a gas. The vapor is swept through the column located in the column oven by He, N2, or H2 carrier gas,
The column must be hot enough to provide sufficient vapor pressure for the analytes to be eluted.
The detector which is hotter than the column shows the response of the separated analytes
What are open tubular columns?
These are the most frequently used S phase in GC, have fused silica in them.
What are open tubular columns coated in?
They are coated with polyamide (can deal with temps upto 350 C) to protect it from atmopsheric moisture and breaking
What are the benefits of open tubular columns?
Give higher resolution (reduced A term)
Shorter analysis time (as analyte doesn’t have to go through packing material like in a packed column)
Greater sensitivity (reduced A term)
Whats a negative about open tubular columns?
They have a lower sample capacity (due to their small diameter)
What is the length of a open tubular column?
15-100m
How thick is the stationary phase in an OT column?
0.1-5 um
How big is the diameter in a OT column?
0.1-0.53 mm
Name three types of OT columns?
Wall coated open tubular column, support coated open tubular column (solid support coated with stationary liquid phase), porous layer open tubular column (stationary solid phase particles)
What does a narrower column in GC indicate?
This indicates a higher resolution/ seperation
What do narrow columns require on terms of pressure and what do they have in terms of sample capacity?
They require higher pressure and have less sample capacity.
At what diameter can u not use Mass spec as a detector and why?
If the column diameter is greater than 0.32 mm you cant use mass spec as it tends to overload the vaccum system of the MS
As column length increases, what also increases?
The number of theoretical plates increases (N), as resolution is prportional to sqr root N, than as column length increases resolution also increases a long with rentention time.
what increases as stationary phase thickness increases in an OT column?
The retention time and resolution of peaks with a retention factor (K) less than 5 also increases.
How do you decide what liquid s phase to go with?
By polarity, if analyte is non polar choose s phase that’s non polar and vice versa
As the GC column ages, what happens to the stationary phase?
the stationary phase gets lost as silanol groups are exposed which can increase tailing (distorts peaks shape due to analyte lingering on s phase) which increases bleeding (when s phase decomposes/evaporates in mobile phase) which then decreases performance.
In non polar analytes and s phase, what is retention time determined by?
Volatility, with more volatile ones (goes by BP) eluting the fastest
In polar analytes and s phase, what is retention time determined by?
Determiend by polar interactions ex: H bonding
How can we make our column selective?
Can use certain non polar or polar s phases to control which compounds elute fastest or not.
How do you reduce the tendency of the stationary phase to “bleed” from a column at high temp?
you chemically bond (covalently attach) it to a silica surface and have it covalently cross linked to itself
How do you monitor column performance?
You peridoically measure rention factor (K) or N
At high operating temperatures what happens to the stationary phase?
high operating temperatures → stationary phases will
decompose → slow bleed → elevated background in
detector → reduced signal-to-noise & detector contam-
ination
What is the newest type of stationary phase for GC?
ionic liquids
How do ionic liquids s phase work?
These melt below room temperature and have low volatility at high temps, they provide multiple types of solvation interactions and therefore off novel selectivity for polar analysis
What are the diff solid stationary phases used for open tubular columns?
polymers, alumina, and molecular sieves
What are packed columns?
These are filled with fine solid particles coated with non volatile stationary phase or the solid itself in the staionary phase
Compared to OT columns how do packed columns differ?
They have better sample capacity, but give broadder peaks, longer retention times, and less resolution
What type of analysis are packed columnns used for?
preparitive seperations- these require alot of s phase to isolate mg amounts of analyte
What is the solid support in packed columns?
often silica that has been silanized to reduce h bonding to polar solutes
What is temp and press programming?
in GC temp is often increased during separation which increases analyte vapor pressure and decreases retention times of late eluting compounds and gives sharper peaks.
What is the isothermal temp limit?
Is temp that GC column can be at for a long time w out damage
What is the programmed temp limit?
is temp that column can only be exposed to for few minutes.
Which carrier gas is most often used?
He as compatible with most detectors
Why is H2 better than He ass a carrier gas and why is it not used?
gives faster seperation, not used as it’s explosive of greater than 4%
What are the plate heights of all three carrier gases at different velocities?
they have all the same plate height
Going from N2, He, and H2 what happens to resolution and analysis time?
Resolution increases, analysis time decreases
How does a sample injection work?
liquid volumes are injected through a rubber septum into a heated glass port, however solvent isnt immediately evaporated as the solvent washes the needle and the airplug expels solvent fro, the needle.
What is the injection port in a sample injection lined with?
silanized glass liner, carrier gas sweeps vaporized sample from the port into the Chromatogorpahy column
How much analyte is used in a sample injection?
0.1-2 ul
When do we do a split injection?
if analyte of interest is greater than 0.1% of sample
How much sample is deposited into a column via split injection?
0.2-2%
What is the proportion of sample that does not reach column in split injection?
50:1-600:1
Why is quantitative analysis bad in split injection?
because split ratio is not reproducible
When do we do a splitless injection?
analytes of interest <0.01% of sample → splitless injection
most appropriate
(80% on column)
Is there a different port used for split vs splitless injections?
Np, only diff is that there’s no mixing chmaber for splitess
What is solvent trapping and why is it used in sample injection?
make the initial column temp 40 degrees below bp of solvent which condenses in beginning of column, then a ssolutes catch up with solvent they get trapped which leads to shapr peaks in respons egraph.
What is on column injection?
The solution is injected directly onto the column (no loss of solute)
Why use on column injection?
preferred for quantitative purposes & for samples that
decompose above their BP
What are two ways to do qualitative analysis in GC?
use MS and compare results to spectral library or use spiking
How can we confirm our indentification of analyte in GC?
By comparing it to multiple columns with diff staionary phases
Name two universal detectors?
Thermal conductivity and flame ionization
Name two selective detectors?
electron capture and flame photometric
What is quantitative analysis based on?
base don area of chromatographic peak
What is the linear response concentration range?
over which peak area ≈ to quantity of component
The more orders of mag the linear cocn range covers the less?
you have to dilute ur analyte
What is a thermal conductive detector?
is a universal detector, less sensitive than other detetcors and so useful for packed columns and operated with He.
How does a thermal conductive detector work?
You have a hot filament, when eluent from GC enters helium streams in and out and the heat from the hot filament gets dissipated by it, then when analyte enters filament it gets hotter and we then see a signal as it measures ability of analyte to conduct heat and increase electrical resistnce so voltage across the filament chnages
Why is TCD a universal detector?
Because every molecule will conduct heat one way or another in this detector, picks up all alaytes.
whats the linear response range for TCD?
10^5
What is a flame ionization detector?
Here you have flame made by hydrogen and air , and whatever comes off GC column enters the detector and is infused in the flame.
How does a flame ionization detector work?
Here carbon atoms form CH radicals, and only 1 in 10^5 carbon atoms produce an ion
What is the DL of the flame ionization detector compared to the thermal conductive detector?
is 100 fold lower
What is the linear response range for the flame detector?
10^7
if there is no analyte what is the current between the flame tip and collector?
10^-14, but if there is than 10^-12
What is an electron capture detector sensitive too?
particularly sensitive to
halogen-containing molecules,
conjugated carbonyls, nitriles,
nitro compounds and organo-
metallic compounds, but rela-
tively insensitive to hydro-
carbons, alcohols & ketones
In ECD what are the requirements for the carrier gas?
must either be N2 or 5% Ch4 in Argon and must be ionized by high energy electrons (beta rays)
How does ECD work?
formed electrons attracted to anode
→ produce a small current
when analyte with high electron
affinity enter → detector capture some
electrons & decrease conductivity
Is ECD sensitive?
Yes kinda like MS
whats an example of another detector?
flame photometric detector measures optical emission from sulfur or other selected elements.
How does sulfur chemoluminescence detecor work?
sulfur oxide formed through flame reacts with ozone to produce excited state, when it releases back to ground state it releases light that the detector captures as a signal.
What does the intensity of signals tell you?
Intensity of signals tell you how much of each specific compound there is
What is the benefit of using a GC-MS?
the MS can sweep through a variety of m/z very quickly, can tell MS to print what’s coming off chromatograph in order to get qualitative and quantitative information
Name a case in which you would use, a GC-MS?
When identifying spills
What is selected ion monitoring/selected reaction monitoring?
When you measure one component in a complex chromatogram of poorly seperated compounds, useful when we wanna see if a specific compound is present but we don’t have a standard for it
What does selected ion monitoring do to the DL?
It lowers the DL by a factor of 10^2-10^3 compared to total ion monitoring (m/z scanning)
In total ion monitoring how many spectra are seen over a span of 10 minutes?
1072
To quantify benzene if using a GC-MS technique what do you do?
We do standard addition to see how big peaks get
What is selected reaction monitoring?
Another mode of GC-MS
Describe how selected reaction monitoring works with an example?
If interested in whether there’s felsothion in an orange peel you extract orange peel w organic solvent and inject it into GC, find fulsothion in a collision cell 1, through quardruple collide prominent ion in second collison cell to fragment it and then in third cell observe the speciifc species w the specific mas to charge ratio you created.
What are three element specific plasma detectors?
FAA, ICP-AES, ICP-MS- can analyze all component sina mixture by element.
What is sample preperation?
The process of transforming the sample into a form suitable for analysis
What is solid phase microextraction?
extracts compounds from liquids, air, and sludge wout use of solvent
What is the key component in SPME?
the filament which has 10 -100 um thick film of s phase (is coated with hydrocarbons)
How does SPME work?
if you drop fiber into hydrophobic solution and stir it for a certain time it will pick up molecules as inke dissolves like, then you inject that solution into a GC
How much analyte is extracted by SPME?
only a fraction, the time you leave the needle in stirring it in the analyte must be optimized for the matrix of that solution
What is the concentration of analyte sitting on SPME needle propotional to?
Is proportional to analyte conc in solution.
