Gas Chromatography Flashcards
Definition of GC
A tool for the sepration of compounds in mixtures based on the polarity of compounds
What’s the preresiquite for GC
GC is for compounds that are volatile or that can be made volatile on derivationzation
On what does the separation of compounds depends on?
it depends on the partitioning behaviour difference between mobile and stationary phase
what happend in at the SP?
Sample gets adsorbed in the SP of the column, which is seperated by the carrier gas
What’s the property of the carrier gas?
iner gas either helium or nitrogen
What has to be done with liquid sample before injection?
There have to be vaporied
On what does the elution depends?
it depends on the polarity of the compounds
What does the response of the GC means?
it is proportional to the concentration of the analyte in the introduced sample
Which kind of detector can be used for Residual solvent Analysis?
Flame ionization (FID) thermal conductiveity detecor (TCD) Mass spectrometery (MS)
Where can GC be used in the pharmaceutical field?
Residual solven analysis, analysis of various functional groups
percentage of purity of pharmaceutcail compounds
identification of impurities
Which other detectors can be used for identification of specific functional groups
IR, UV, MS
Disadavantage of GC?
applicable ony for volatile compounds
How does the flam ionisation detector works?
it detects the amount of inos release from the combustion of a sample in ahydrogen falme which is proportinal to the amout or organic compound present in the ignited
what is the column velosity?
it describe how quick the chromatography will take place and how the column can resolve the peaks efficiently.
What does higher velocities means?
it means the chromatogrma will be produce faster compared to a low velocity
On what does the velocity value depends on?
it depends on the column charateristics, sample temperature, carrier gas
what is the carrier-gas flowrate?
it describe how well the colume suits the hardwars as its entrance and exit
What happend when the sample is not well delivered to the coulmn
it can lead to peak distortion, at too high or too low column
what affects the separation efficiency?
the chose of carrier gas and linear velocity
What does is mean, when the temperature is kept constant
one has a isocratic elution process
Learn how to draw the GC
Do you know how to do it?
Why it is important to know the boiling point of the samples?
Compound with higher boiling point will have higher affinity to the stationaty phase
Which interaction between the compounds and SP can occur
Hydrophobic interaction (van der waals forces
How does the increase of carbon chain affect the polarity of alcohols?
increase in the length of the hydrocarbon chane, decrease the hydrophilicity of the whole molecule.
How are the components sepearted?
the volatile analytes of the mixture are separated through differences in boiling points and retention time in terms of vapor pressures and distribution coefficient
example of mobile phase
inert carrier gas like helium,hydrogen or nitrogen
different between the inert gases
Hydrogen gas is used for short analysis time, higher flow rate can be achieved
explain the two types of cappilary columns
non-polar column, longer, higher diameter and film thickness
polar column, shorter length, narrower diameter and thinner film thickness
what affects the volatility?
vapor pressure (boiling point), solubility and interaction in the SP
the vapor pressure changes the
equilibrium betwen MP and the SP if specific interactions occur
what’s the fundamental paramter in the GC
retention time. it is the time measuremnet for solutes to pass through the colume from injection to detection
other various important parameters are
retention factor, peak area, peak height and peak form
on what does the seperation time is based
on the time spent in the mobile phase and in the SP
how does the molecular weight and the bond strength influence the RT
it increases the Rt in the stationary phase
how can the retenion time be reduced?
by increasing the capillary temperature
what’s the ad von a temp. gradient?
it can strengthen the seperation efficiency from a mixture of analytes with varying boiling points
whats the phase ratio
it is the ratio of the column inner diameter and the SP film thickness
low phase ratio–> high value of retention factor with broad peak
importance of the column length
have to be considered inorder to achieved high separation efficiency and short retention time
factors that influence the separation are
vapor pressure, boiling tempertaure, polarity, column temperature, carrier gas flow rate, amount of material injected and column length
what’s the benefit of FID?
it reacts to all hydrocarbon gases like alcohol and has a broad varity of measurement
Which gases can the FID not detect?
non burned gases like h20, c02, s02, nox and noble gas
what’s the optimal separation
peaks are away from each other and as narrow as possible
what’s the plate height?
it is the constant of proportionality between the variance of the band and the distance it has traveled
plate height can also be describe as?
the length of a coulmn required for one equilibration of solute between the MP and SP
how does the plate height affects the bandwidth
if the plate height is small then the bandwidth is narrow
what happend if the column has more theoretical plates?
the separation is better
which equation can be used to describe the separation efficacy
the van deemter
what happend to the PH if the flow rate is increased?
the plate height is decreases, because compounds have stay less time inside the column and their longitudinal diffusion will be less
What is important while injecting the samples?
it should be injected carefully and fast, leading to a narow bandwidth from the injection
what causes broad bands?
when one component would stick in the SP
What’s the relation between the thickness of the SP and the plate height?
Reduction of the thickness of the SP it leads to reduction of the plate height, because of faster diffusion of the compound from the SP, far fom the SP and inside the MP
What’s the relation between the radius and the plate height?
small radius, leads to decrease of the PH, due to the reduction of the travel distance of the component
How does an increase of the temp influnece the diffusion coefficient?
increase of the temp, leads to an increase of the diffusion coefficient
what’s the benefits of GC?
bozj qualitative (identification of individual components) and quantitative (concentration of particular components)
what’s important during a quantitative anaylsis
creation of a calibration curve
What’s the main different in the carrier gas used in GC
it does not undergo interactions with analyte itself
How is the componten divided during GC-process
the non-polar component ratain on the SP, while the polar are carrier through the column
Why is the column temperature mainted in a higher degree
inoder to maintain the compound in the gas phase and to reduced the retention time
what influences the RT?
interaction of the compound in SP and higher solubility of the compound (increases the RT)
increase of column length, also increase the RT as well as the resolution
increase of temp, decrease the RT, due increase of the kentic energy which decrease the solubility of the compound
increase of the Flow Rate, we increase the rate at which the mixture moves, thus decreasing the RT
What affects the retention factor
higher impact has the film thickness , compared to the inner diameter of the column
How does the film thickness affect the RF
extremely volatile compounds, a thick film colum should be used to increase the retention, whereas if one is analyzing high molecular weight compound, a thinner film should be used
Thicker and thinner film
Thinner film, reduction of the retention, compared to the thicker film
Inner diameter and the RF
smaller inner diamter produces higher retention compared to larger ID column
why do small ID leads to higher RF
it is due to the less available mobile phase volume in the column
what are the ad and disad von longer and shorter column in GC
Longer column provide more resolving power, but increase analysis time
Which factor is most influence by the column length?
the retention factor has the greates effect on resolution
what happend if the column length is doppelt?
it increases the resolution as well as the retention time
larger and smaller ID incomparison to their peaks?
smaller ID columns generate more plates per meter and sharper peaks, leading to better separation efficiencies
When should smaller ID be used?
When more complex samples need to be analyzed
when is the disad von smaller ID?
loading capacities are lower
what could be the cause of a poor resolution?
Non-selective SP
Poor efficiency
Sample overlaod
Incorrect analytical condititions
what could be the cause of a poor retention time
Resolution/integration issues
incorrect column/oven temperature
injection daly between pushing start and actual injection, if manual injected
Cause for an split peaks
Incomplete vaporization
Sample loading capacity exceeded
cause for unstable baseline
Carrier gas leak or contamination
injector or detector contatmination
cause for Broad peaks
low flow rate
column film is too thick
slow GC oven program
