Excersises chapter 24, Gas chromatography Flashcards
What are the advantages and disadvantages of packed and open tubular columns in gas chromatography?
. The advantages of open tubular columns as compared to packed columns is that they have higher resolution, faster analysis, and more sensitivity. However, open tubular columns can accommodate less sample size than packed columns.
Meanwhile, packed columns can provide greater sample capacity than open tubular columns but they deliver broader peaks, less resolution, and longer retention times.
Describe the difference between wall coated and porous layer open tubular columns
b. In a wall-coated open tubular column, the inner wall is coated with a 0.1 to 5 um thick film of high-molecular-weight stationary liquid phase. On the other hand, a porous-layer open tubular column contains solid stationary phase particles on the inner wall of the column.
What are the advantages of covantly attaching the stationary phase to the column wall or cross-linking the stationary phase to itself?
c. The advantage of bonding (covalently attaching) the stationary phase to the column wall or cross-linking the stationary phase to itself is that the tendency of the stationary phase to bleed from column at high temperature is decreased. This method is necessary because thru time and column usage, the stationary can be lost, surface silanol groups are exposed and tailing increases.
Why do open tubular columns provide greater resolution than packed columns in gas chromatography
Open tubular columns eliminate multiple pathways, decreases plate height H and plate number N, and longer columns L can be used with the same elution time thanks to the lower resistance to gas flow
What are the advantages and disadvantages of using a narrower open tubular column
It increases the rate of mass transfer between the mobile and stationary phase by decreasing the time needed for solute in the mobile phase to diffuse to the stationary phase. The increased rate of mass transfer lowers plate height H, which increases plate number N and increases resolution. With lower plate hieght a shorter column can be used to obtain the same resolution in shorter time. The disadvatanges are that is has lower sample capacity and it requires high sensitivity detector.
What are the advantages and disadvantages of using longer open tubular columns
Larger N and greater resolutiom, disadvantages longer analysis time
What are advantages and disadvantages of using a thicker film of stationary phase
greater retention and sample capacity, disadvatangaes lower N since the rate of mass transfer in the stationary phase decreases, long retention time for high boiling compounds and increased bleeding
What are the advantages and disadvantages of temperature programming in gas chromatography
Compounds with a wide range of retention chrasteristics (such as variety in boiling points with similar polarities) can all have sufficient retention for resolution and yet all be eluted in a reasonable time. Peaks are sharper which improves sensitivty and a low intitial temperature allows for splitless or on column injection.
Disadvantages: There may be a sloping baseline if there is stationary phase bleed and time is needed to cool the columns between seperations.
What are the charasteristics of an ideal carrier gas?
It should be inert (not react with samples), low viscosity and compatible with a detector
Why do H2 and He allow more rapid linear velocities in gas chromatography than N2 does withous loss of column efficancy?
Diffusion of solute is more rapid for H2 and He then N2 therefore mass transfer of solute between mobile phase and stationary phase is faster.
When would you use a split, splitless, or on ccolumn injection in gas chromatography?
split is the ordinary mode and is best suited for high analyte concentrations and gas analysis. Splitless is useful for trace analysis and they are both comapatible with dirty samples. On column is best for quantitation and thermally sensitive solutes that might decompose during high termpature injection but this requires clean samples.
Explain how solvent trapping and cold trapping work in splitless injection
In solvent trapping the intial temperature is low enough to condense solvent at the beginning of the column. The solute is very soluble in the solvent and it will be trapped at a narrow band in the beginning. At cold trapping the initial temperature of the column is >1000 below the boiling point of the solute and a narrow band forms in the beginning of the column.
To which kinds of analytes does each gas chromatography detector respond? Thermal conductivity, flame ionization, electron capture, flame, , mass spectrometer?
Thermal conductivity reacts to all analytes, flame ionization carbon atoms bearing hydrogen atoms and electron capture molecules with halogens, CN, NO2 or conjugated C=O, flame with P and N and mass spectrometer to all analytes
State the order of decisions in method developmeant for gas chromatography
(1) Goal of the analysis, (2) sample preperation method, (3) detector (4) column, (5) injection method
