A-chem test 3
(Types of chromatography) molecular exclusion
A type of chromatography in which solute molecules penetrate voids in the stationary phase. In this type of chromatography, the largest solute molecules elute from the column first.
How should a random heterogeneous material be sampled in order to construct a representative sample?
Divide the material into segments and use a table of random numbers to then collect random segments of the material.
For an open tubular column (not capillary electrophoresis), which term(s) in the van Deemter equation does(do) not affect the plate height?
Eddy diffusion
Definition of retention time (chromatography)
Elapsed time between sample injection and detection
What HPLC detectors are NOT compatible with separations performed using gradient elution?
Electrochemical detector Refractive index detector
How should a segregated heterogeneous material be sampled in order to construct a representative sample?
Take a representative amount from each region of the material that has a distinctly different composition.
Definition of linear flow rate (chromatography)
The distance traveled by the solvent per unit time.
In chromatography, what does the partition coefficient "K" describe?
The distribution of the solute between the mobile and stationary phases.
define flux
The medium used for a fusion.
(Sources of band broadening) extra-column band broadening
produced in the tubing before or after the chromatographic column
A detector with virtually no selectivity that is not compatible with gradient elution. It uses filtered visible light as the irradiation source. The main appeal of this detector is its universal response to all solutes, even those with little absorption in the ultraviolet region of the spectrum.
refractive index detector
A young researcher set out to develop a method for quantifying the polycyclic aromatic hydrocarbon (PAH) content in a contaminated soil sample by gas chromatography (GC). With an analysis goal already chosen, in what order should the young researcher proceed to complete the analysis? Arrange the possibilities from first step to final step.
select a sample prep method select a detector select a column choose a sample injection method
Arrange the ion monitoring modes that are used to generate a chromatogram in gas chromatography-mass spectrometry (GC-MS) from the most selective (top) to the least selective (bottom). Most selective to least
selective reaction monitoring selective ion monitoring reconstructed total ion chromatogram
What parameters cause band separation in gas-liquid chromatography?
small packing particles thin stationary phase layer rapid sample injection
Is MALDI considered a soft or a hard ionization technique?
soft
Which of the following trapping types in splitless injection removes solvent by setting the initial column temperature 40°C below the solvent boiling point to condense solvent at the front of the column?
solvent trapping
What detector(s) can be used in gas chromatography for the detection of the following list of analytes: Compounds ionized by UV radiation
thermal conductivity mass spectrometer photoionization
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? S
thermal conductivity mass spectrometer sulfur chemiluminescence
What detector(s) can be used in gas chromatography for the detection of the following list of analytes: Nitrogen & phosphorous containing compounds
thermal conductivity mass spectrometer thermionic
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? Aromatic and unsaturated compounds
thermal conductivity photoionization mass spectrometer
What parameters cause band broadening in gas-liquid chromatography?
very low flow rates low temperature large packing particles thick stationary phase layer
define fusion
Procedure that uses a molten salt to dissolve an otherwise insoluble substance.
Consider a reversed phase chromatographic separation of solutes A and B, where tm is the mobile phase retention time, tA and tB are the retention times of the solutes A and B, wA and wB are the baseline widths of peak A and peak B, wavg is the average of the two baseline widths, and kA and kB are the retention factors for solutes A and B. (a) Which of the following is a measure of kA? (b) Which of the following is a measure of the resolution of A and B? (c) How might tm be determined? (Check all that apply).
(a) (tA-tm)/tm (b) (tB-tA)/wavg (c) from column volume and flow rate from first disturbance of baseline
Reversed-phase chromatography (a) Polarity of phases: (b) Eluent strength of solvent: (c) Nature of solutes: (d) Nature of solute interaction:
(a) Polarity of phases: Stationary phase is non-polar. (b) Eluent strength of solvent: Increases as solvent becomes less polar. (c) Nature of solutes: Non-polar (d) Nature of solute interaction: More soluble in mobile phase as the polarity of the mobile phase decreases.
Normal-phase chromatography (a) Polarity of phases: (b) Eluent strength of solvent: (c) Nature of solutes: (d) Nature of solute interaction:
(a) Polarity of phases: Stationary phase is polar. (b) Eluent strength of solvent: Increases as solvent becomes more polar. (c) Nature of solutes: Polar (d) Nature of solute interaction: More soluble in mobile phase as the polarity of the mobile phase increases.
`Define the sample prep method: -derivitization -purge and trap -solid phase microextraction -preconcentration
-derivitization- Uses a chemical reaction to convert an analyte to a form more easily separated and detected. -purge and trap- An extraction method with 100% efficiency used for removing volatile analytes from solid or liquid samples. -solid phase microextraction- Extracting a portion of available analyte from a sample without the use of solvent. -preconcentration- Increasing the concentration of solute in a sample by the removal of solvent.
Describe the components of the high-performance liquid chromatography (HPLC) schematic: -pumps -injector -sample loop -detector -column -mixer
-pumps- Used to move the mobile phase through the system. -injector- Used to introduce the sample to the system. -sample loop- Holds a fixed amount of sample prior to injection. -detector- Used to monitor and measure the analytes as they move through the system. -column- Contains the support and stationary phase and is used to separate the analytes. -mixer- Combines the solvents in the appropriate ratio required for the mobile phase.
How does magnetic sector mass spec work?
1. Gaseous molecules enter ion source where they're ionized through collisions with electrons. 2. Ions accelerated out of ion source into analyzer tube (same KE) by voltage applied b/w ion accelerator plates. 3. Ions encounter mag. field applied perpendicular to their direction of travel. 4. Mag. field detects ions on circular path w/ radius dependent on m/z of ions, towards detector at the end of the analyzer tube. 5. Ions deflected on circular path w/ radius equal to radius of curvature will travel through slit leading to detector. Heavier ions not deflected enough and lighter ions deflected too much to reach detector. 6. Mass spec created by varying mag. field strength altering which ions reach detector.
Which of the following correctly describes what a user would observe if they collected a chromatogram generated in selected ion monitoring mode?
A chromatogram of ion intensity from one or more selected m/z values versus elution time.
Wall-coated open tubular column (gas chrom)
A column with a liquid stationary phase bonded to its wall.
Porous-layer open tubular column (gas chrom)
A column with a solid stationary phase coating the inside wall.
definition and usage of flux.
A flux is an alkali metal salt that is added in high concentration to sample that cannot be dissolved using traditional methods, such as acid digestion. The sample-salt mixture is heated to several hundred degrees and the sample is dissolved.
(chromatography terms) open tubular column
A hollow capillary tube with inner walls that are coated with stationary phase material.
(chromatography terms) packed column
A hollow tube that is filled with particles coated with stationary phase material.
Which of the following correctly describes solid-phase microextraction?
A method for isolating compounds for analysis by gas chromatography that uses a stationary phase coated fiber to extract compounds from gas or liquid samples
solid-phase microextraction (defn)
A method for isolating compounds for analysis by gas chromatography that uses a stationary phase-coated fiber to extract compounds from air or liquid samples.
Cold trapping (defn)
A method of analyte condensation at the begining of the column to produce a narrow injection band.
How are neutral species in solution converted to gaseous ions in the other interface?
A needle in the path of the aerosol has a high voltage applied to it. An electric corona forms around the charged needle which injects electrons into the aerosol and creates ions.
Whats an example of a sampling error?
A non-representative sample of an inhomogeneous material is taken for the analysis. A sampling error is an error in obtaining a representative sample
define solid-phase extraction
A procedure that uses a small amount of chromatographic stationary phase to isolate an analyte from a solution prior to analysis.
Describe chemical ionization.
A reagent gas is converted to reactive species by energetic electrons. These new reactive species then donate a proton to the analyte molecules to produce the protonated molecule, MH .
stir-bar sorptive extraction (defn)
A sensitive extraction technique for isolating compounds from liquid samples by a magentic rod coated with a layer of stationary phase material.
define wet ashing
Destruction of organic matter in a sample using a liquid reagent prior to analysis of an inorganic component.
Describe how ions are created via electron ionization.
Analyte molecules are bombarded with high energy electrons (usually 70 eV), allowing the molecules to absorb enough energy to ionize.
Shown below are three van Deemter curves for columns containing a microporous stationary phase with particle diameters of 5 μm, 3 μm, and 1.8 μm. Why does the van Deemter curve shift downward as the particle size decreases?
As the particle size decreases, the flow through the column becomes more uniform which reduces the contribution of the A term to the plate height.
Which of the following choices correctly describes the effect of increasing the particle size of the column packing in an HPLC column has on the terms of the van Deemter equation?
B is unchanged; A and C are increased.
Largest peak in mass spec
Base peak
Consider the van Deemter equation. Which of the following methods can be used to minimize the effects of the resistance to mass transfer term on plate height? Choose all that apply.
Decreasing stationary phase thickness Decreasing column radius
Definition of retention factor (chromatography)
Describes the amount of time that a sample spends in the stationary phase relative to the mobile phase. It is sometimes also called the capacity factor or capacity ratio.
Packed column (gas chrom)
Can be made of stainless steel or glass with a packed solid support of silanized silica.
Which of the following are required properties of a pump used in high-performance liquid chromatography?
Capable of producing flow rates from 0.1 to 10 mL/min Reproducible flows Resistance to corrosion from a variety of solvents
What methods can be used to improve the resolution between two compounds for a liquid separation using a packed chromatography column?
Change the composition of the mobile phase. Reduce the particle size of the packing material in the column. Increase the column length.
define derivatization
Chemical alteration of a molecule to make the molecule more easily detected.
define preconcentration
Concentrating trace analytes in a mixture prior to analysis.
What does a larger partition coefficient correlate to and why?
Correlates to a longer elution time because the fraction of solute in the stationary phase is larger
Which method is used to reduce stationary phase degredation (bleeding) in gas chromatography?
Covalently bonding the stationary phase to the support material.
A mixture of nonpolar compounds is separated by HPLC using an octyl (-(CH2)7CH3) stationary phase covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 55 vol% acetone and 45 vol% water to 85 vol% acetone and 15 vol% water?
Decrease
A mixture of polar compounds is separated by HPLC using bare silica as the stationary phase. How would the retention times of the compounds change if the solvent composition was changed from 40 vol% acetonitrile and 60 vol% tetrahydrofuran to 80 vol% acetonitrile and 20 vol% tetrahydrofuran?
Decrease
(Types of chromatography) partition
Equilibration of solute between the mobile phase and a liquid film stationary phase occurs in this type of chromatography.
Detectors used only for gas chromatography
Flame ionization detector thermal conductivity conductor electron capture detector
Choose the statements that are true regarding open tubular and packed columns in gas chromatography. (Hint: there is more than one correct answer).
For similar analysis times, open tubular columns provide better resolution than packed columns. Packed columns give broader peaks with longer retention times. Open tubular columns provide better separation efficiency with shorter analysis times.
Which of the following carrier gases is used to decrease the loss of column efficiency at high flow rates, because it allows the solutes to diffuse faster from the center of the column to the wall?
H2
Thermal conductivity detectors in gas chromatography respond to changes in the thermal conductivity of the eluate. Due to this, ideally the difference in conductivities of the carrier gas and the solute should be as great as possible. The two carriers of choice for use in thermal conductivity detection are
H2 He
Support-coated open tubular column (gas chrom)
Has a solid support that is coated with a liquid stationary phase.
(Types of chromatography) ion exchange
Mobile-phase ions are attracted to stationary-phase ions in this type of chromatography.
Put the following steps of a sampling operation in order from first to last.
Identify the population from which the sample will be drawn Collect a gross sample Reduce the gross sample to a small homogeneous sample for analysis
Place the following compounds in the order they will elute from the column when separated using reverse-phase liquid chromatography (RPLC). naphthalene anisol p-xylene phenol
In reverse-phase liquid chromatography, the stationary phase is nonpolar or very weakly polar and the mobile phase is more polar than the stationary phase. This means that nonpolar compounds will be more strongly retained on the column because they interact strongly with the stationary phase while polar compounds will be weakly retained as there is little interaction with the stationary phase. First to last: phenol, anisol, p-xylene, naphthalene
(Types of chromatography) adsorption
In this type of chromatography, solute molecules equilibrate between the mobile phase and the surface of the stationary phase.
(Types of chromatography) affinity
In this type of chromatography, specific groups are attached to the stationary phase to attract the solute.
A mixture of nonpolar compounds is separated by HPLC using a phenyl (-(CH2)3C6H5) stationary phase covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 90 vol% methanol and 10 vol% water to 35 vol% methanol and 65 vol% water?
Increase
A mixture of polar compounds is separated by HPLC using an amino (-(CH2)3NH2) stationary phase covalently bonded to the silica support. How would the retention times of the compounds change if the solvent composition was changed from 35 vol% acetonitrile and 65 vol% water to 70 vol% acetonitrile and 30 vol% water?
Increase
In a separate experiment, you are trying to separate a mixture of compounds using normal-phase liquid chromatography (NPLC). On the first run, with a mobile phase containing 35% toluene and 65% acetonitrile, the peaks again eluted quickly and were poorly resolved. To improve the resolution of the peaks, should you increase or decrease the amount of toluene in the mobile phase for the next run?
Increase
Suppose you are trying to separate a mixture of compounds using reversed-phase liquid chromatography (RPLC). On the first run, using a mobile phase containing 40% tetrahydrofuran and 60% water, the peaks all elute between 0 and 3 min and are too close together to be quantitatively resolved. To improve the resolution of the peaks, should you increase or decrease the amount of water in the mobile phase for the next run?
Increase
A young researcher set out to develop a method for quantifying the polycyclic aromatic hydrocarbon (PAH) content in a contaminated soil sample using open tubular gas chromatography. During the initial stages of method development, the researcher was unable to separate the early eluting compounds anthracene and naphthalene with baseline resolution. From the options below, select all possible modifications the researcher could make to improve the resolution.
Increase the stationary phase thickness Use a column with a smaller inner diameter Choose a different stationary phase
Which of the following statements are true if the particle size in a packed column used in HPLC is decreased?
Increased resolution Increased separation efficiency
Consider the van Deemter equation. Why does increasing the flow rate reduce the contribution of longitudinal diffusion to total plate height?
Increasing the flow rate will decreases the time the solute resides in the column, resulting in less diffusional broadening.
Components of gas chromatogram (give descriptions): -Injector port -Column oven -Column -Detector
Injector port-It is maintained approximately 50 °C above the boiling point of the least volatile analyte. Column oven-It is maintained at a temperature high enough to elute analytes in a reasonable amount of time. Column-It is packed or open tubular and is used to separate the analytes. Detector-Common ones include thermal conductivity, flame ionization, or electron capture.
(chromatography terms) linear flow rate
It describes the velocity of solvent as it travels through the column.
(chromatography terms) mobile phase
It is a liquid or gas solvent that carries the sample through the separation column.
(chromatography terms) retention time
It is the elapsed time between sample injection and detection.
(chromatography terms) eluent
It is the fluid that enters the column.
(chromatography terms) eluate
It is the fluid that exits the column.
What is the equation for the partition coefficient (K)?
K= Cs/Cm Cs= concentration of solute in the stationary phase Cm= concentration of solute in the mobile phase
Shown below are three van Deemter curves for columns containing a microporous stationary phase with particle diameters of 5 μm, 3 μm, and 1.8 μm. When compared to the other two curves, why is the curve for the 1.8 μm diameter particles nearly flat at high linear flow rates?
Once the particle size is small enough, the equilibration time between the mobile and stationary phases becomes very rapid which reduces the contribution of the C term to the plate height.
In gas chromatography, (a) why do open tubular columns yield greater solute resolution than packed columns? (Check all that apply.)
Open tubular columns eliminate the multiple path term (A) from the van Deemter equation.
What are the factors that lead to asymmetry in chromatographic peaks?
Overloading the column with solute Strong interaction between the solute and some sites of the stationary phase
define dry ashing
Oxidation of organic substances without the use of a liquid at high temperatures to leave behind any inorganic components for analysis.
What characteristic does a bromine mass spec have?
Peaks w/ approx equal intensity that are 2 m/z units apart
Describe the components of the high-performance liquid chromatography (HPLC) schematic: (a) pump (b) injection valve (c) column (d) detector
Pump- Used to move the mobile phase through the system. Injection valve- Used to introduce the sample to the system. Column- Contains the support and stationary phase and is used to separate the analytes. Detector- Used to monitor and measure the analytes as they move through the system.
Definition of relative retention (chromatography)
Ratio of the adjusted retention times or retention factors of two solutes. It is sometimes also called the separation factor.
In gas chromatography, what are the advantages of (a) temperature programming? (check all that apply.)
Resolution of low-boiling solutes is maintained. Retention times of high-boiling solutes are decreased.
In gas chromatography, what are the advantages of (b) pressure programming? (check all that apply.)
Resolution of low-boiling solutes is maintained. Retention times of high-boiling solutes are decreased. Thermal degradation of samples is less likely. It isn\'t necessary to wait for a hot column to cool before injecting another sample.
In gas chromatography, (b) why can more rapid flow rates be used with H2 and He carrier gases than with N2, without sacrificing column efficiency?
Solutes diffuse more rapidly through H2 and He than through N2.
(chromatography terms) stationary phase
The separation medium that is bonded to the surface of particles packed in a column.
Why is stir-bar sorptive extraction a more sensitive extraction technique than solid-phase microextraction?
The volume of stationary phase that coats the stir bar in stir-bar sorptive extraction is significantly greater than the volume of stationary phase that coats the fiber in solid-phase microextraction.
Definition of adjusted retention time (chromatography)
Time required by retained solute to travel through the column beyond the time of the unretained solvent
What is the purpose of cold trapping in gas chromatography?
To condense the analyte at the start of a gas chromatography column to insure a narrow injection band.
What is the purpose of the reflectron in time-of-flight mass spec?
To improve resolving power by allowing slower ions to catch up to faster ions so that all ions of the same mass reach the detector at the same time
It is the most common HPLC detector. It is useful for gradient elution with nonabsorbing solvents. Simple systems use lamps that produce discrete radiation, such as the 254-nm Hg vapor lamp, while more versatile systems use lamps that produce broadband radiation, such as xenon or tungsten lamps, and a monochromator to choose the wavelength of analysis.
UV detector
What HPLC detectors are compatible with separations performed using gradient elution?
UV detector Evaporative light-scattering detector Charged aerosol detector
Liquid chromatography detectors
UV detector refractive index detector
What is the purpose of the matrix in matrix-assisted laser desorption/ionization (MALDI)?
Upon irradiation, the matrix desorbs, carrying the analyte with it into the gas phase. The matrix is ionized by the laser and some of the charge is transferred to the analyte. To create a mixture of the analyte interspersed between fine crystals of the matrix once the solvent evaporates.
thermal desorption tube (defn)
Used to vaporize the analye after extraction in preparation for analysis by gas chromatography.
Definition of volumetric flow rate (chromatography)
Volume of solvent traveling through the column per unit time
Definition of retention volume (chromatography)
Volume of the mobile phase required to elute a solute from the column.
A sample containing acetic acid, benzene, propanoic acid, and octane is injected into a gas chromatograph whose column has a nonpolar stationary phase. Select the compound that will likely have the shortest retention time.
acetic acid
Best conditions for on-column injection
analyte can decompose above its boiling point, so the injector is not heated
Best conditions for splitless injection
analytes consist of less than 0.01% of sample
Best conditions for split injection
analytes consist of more than 0.1% of sample
Gold and platinum will not dissolve in nitric acid, but they will dissolve in..... which is a 3:1 (vol/vol) mixture of.....
aqua regia HCl:HNO3
(Sources of band broadening) mobile phase mass transfer
caused by different rates of travel of the solutes across a cross-section of the column
(Sources of band broadening) stationary phase mass transfer
caused by diffusion between the mobile phase and stationary phase
The dynamic range of this sensitive detector covers 4-5 orders of magnitude in analyte concentration. The eluate is first nebulized in a stream of N2 gas then a positive charge is added to the aerosol particles as they pass over a charged Pt needle. Small ions are attracted to the charged plates of a small ion trap, while the larger aerosol particles are too large to be deflected and pass to a collector where they are detected by an electrometer.
charged aerosol detector
Which of the following trapping types in splitless injection requires initially cooling the column temperature 150°C lower than the solute boiling points, and then raising the column temperature to vaporize solvent at the start of the column?
cold trapping
(Sources of band broadening) stagnant mobile phase mass transfer
diffusion of the solutes from the mobile phase outside the pores of the support to the mobile phase inside the pores of the support
Which of the following are benefits for choosing air from the atmosphere as a carrier gas in gas chromatography?
does not require a bulky supply tank readily accessible
(Sources of band broadening) longitudinal diffusion
due to diffusion of the solutes along the column length
Which type of ionization would you expect to produce the most fragmentation?
electron ionization
Which of the following interfaces used in liquid chromatography/mass spectrometry typically requires analyte ions to be in solution prior to reaching the interface?
electrospray
With this detector, no peaks from the solvent are produced and it is compatible with gradient elution. The eluate is nebulized, forming an aerosol that is then passed through a drying tube where the solvent is evaporated, leaving analyte aerosol particles. The particles scatter light from a diode laser, generating the signal for the chromatogram. The detector response is related to the mass of the analyte, not its structure or molecular mass. The use of low-concentration buffers is required with this detector.
evaporative light-scattering detector
Is longitudinal diffusion of the solute a more serious problem in gas or in liquid chromatography? What is the major reason for the difference?
gas chromatography Diffusion coefficients of gases are much greater than those of liquids.
What resolution is desireable for quantitative analysis?
greater than or equal to 1.5
A solid-phase microextractor (SPME) was used to extract decane from the headspace of a sealed vial containing an aqueous sample at 50 °C for a fixed amount of time. The SPME fiber is then transferred to the injection port of a gas chromatograph where the decane is desorbed for quantitative analysis. How will the size of the decane peak change if the extraction temperature is increased to 60 °C, but the extraction time remains the same?
increase
If the efficiency of a chromatographic column is doubled, by how much will the resolution change?
increase by a factor of the square root of two
Detectors for both gas and liquid chromatography
mass spectrometer
Which of the following molecules would a flame ionization detector (FID) be sensitive to after separation on a gas chromatography column? Choose all that apply.
methane n-pentane methylcyclopentane
Select the compounds to which a nitrogen-phosphorus detector is particularly sensitive.
o-anisidine pyrophosphoric acid
(Sources of band broadening) eddy diffusion (multiple paths)
produced by variable paths around support particles
What are the factors that affect a compound\'s rate of elution through a gas chromatography column.
the compound\'s boiling point and polarity
Choose the column performance chacteristics that can be used to indicate the degradation of the stationary phase in gas chromatography?
the retention factor of a known standard the number of plates peak asymmetry
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? Molecules with halogens, CN, NO2, conjugated C=O
thermal conductivity electron capture mass spectrometer
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? Carbon atoms bearing hydrogen atoms
thermal conductivity flame ionization mass spectrometer
What detector(s) can be used in gas chromatography for the detection of the following list of analytes: Sulfur or nitrogen containing compounds
thermal conductivity mass spectrometer electrolytic conductivity
What detector(s) can be used in gas chromatography for the detection of the following list of analytes: Halogenated compounds
thermal conductivity mass spectrometer electron capture electrolytic conductivity
What detector(s) can be used in gas chromatography for the detection of the following list of analytes: Hydrocarbons
thermal conductivity mass spectrometer flame ionization
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? P and S and other elements selected by wavelength.
thermal conductivity mass spectrometer flame photometric
What detector or detectors can be used in gas chromatography for the detection of the following analyte(s)? P and N (and also hydrocarbons)
thermal conductivity mass spectrometer nitrogen-phosphorus
