chrom final Flashcards
(24 cards)
What are the key characteristics of a carrier gas?
very pure, non-reactive, usually N2, H2, He or Ar
What changes are seen in a chromatogram when we move to different carrier gases? what changes?
different optimum linear velocity so faster run times
Why does the injector need to be heated 50C above the bp of the least volatile component?
What is the role of the septa and how do we know when it is failing?
septa = barrier between GC and outside world
failing = loss of analyte, contamination, noise, RT shift, smaller peaks
What is the difference between split and splitless injections?
1) split: a portion of the injected sample is discarded to avoid overloading the column.
* allows larger injection samples
* split valve opens
* vapourized sample splits between column and split vent
2) splitless: best for trace analyte of high boiling point analytes in low boiling solvents
* low concentration samples
* traps sample at head of column
* split valve is closed and sample is injected
* sample is trapped in column
* split valve is opened and sample is swept out
What are the differences between packed and capillary columns?
- Packed columns: larger ids, are shorter and are packed with material coated in stationary phase.
- Capillary columns: stationary phase coated on the inside, they are smaller Ida, longer, and more efficient
What is column bleed and what are the symptoms?
high temperatures can lead to column bleeding
*elevated background signals, reduces signal to noise, contaminates detector
What is temperature programming and when do we use it?
start at low temperature and increase over time (temp gradient)
Name and describe the mechanism for detection in the four GC detectors discussed in class
When are they most useful?
What are their advantages and disadvantages?
1) Thermal Conductivity: measures the ability to trasnsport heat from cold to hot regions
*Advantages: simple, large range, organic and inorganic, non-destructive to sample
*Disadvantages: low sensitivity,
2) Flame Ionization: responds to organic compounds
*Advantages: cost effective, large range, 100 x higher sensitivity , responds to all C-H bond samples
*Disadvantages: destroys the sample, requires additional gasses and controllers, doesnt provide molecular structure info.
3) Electron Capture: most popular. used with environmental testing
*Advantages: high sensitivity,
*Disadvantages:
4) Mass spectrometer
*Advantages:
*Disadvantages:
Describe the advantages and disadvantages of liquid liquid
Describe the advantages and disadvantages of solid phase extraction
less solvents used
less time and labour
less cost
Describe the four main applications of solid phase extraction: trace enrichment, pass through, catch and release and quechers
1) quechers: quick, easy, cheap, effective, rugged, safe. uses sorbent instead of cartridge. sample shaken to separate layers
2) Catch and release: analyte caught on top, wash to get rid of interference, strong solvent to release analyte.
3) trace enrichment: low concentration analyte retained, then concentrated with strong solvent
4) pass through: analyte passes through filter
How can columns be degraded?
by chemicals. usually pH
What are the pH limits for a silica column
pH 2-8
What is hydrophobic collapse?
complete loss of retention….
What are the symptoms of column clogging?
How can we repair it, without opening the column?
increases backpressure
Why is a column heater important from a reproducibility perspective?
temperature impacts reproducibility. reduced variability
what is a flow cell?
minimizes turbulence
facilitate clearing of air bubbles
what are the two main types of detectors
destructive (uv) and non destructivr (mass spec)
What are the two types of UV/Vis detectors and how do they differ?
variable wavelength detector: can look at one or more wl at a time. sensitive and specific. cant go back after to look at other wl
diode array detector : can look at entire spectral range, extract at any point
What is meant by the term peak purity?
constant ratio for a pure compound
What are the advantages of fluorescence detectors?
you can look at only one compound if youd like
more sensitive than UV
What is UV Cut off and how does it impact our mobile phase selection and chromatography?
below this wl should not be used
What is being measured when using refractive index?
bending of light as it passes from one medium to another
