Chapter 27 Flashcards
26-1. How do gas-liquid and gas-solid chromatography differ?
In gas liquid chromatography, the stationary phase is a liquid that is immobilized on a solid. Retention of sample constituents involves equilibria between a gaseous and a liquid phase. In gas solid chromatography, the stationary phase is a solid surface that retains analytes by physical adsorption. Here separations involve adsorption/desorption equilibria.
26-3. What is meant by temperature programming in GC? Why is it frequently used?
Temperature programming involves increasing the temperature of a GC column as a function of time. This technique is particularly useful for samples that contain constituents whose boiling points differ significantly. Low boiling point constituents are separated initially at temperatures that provide good resolution. As the separation proceeds, the column temperature is increased so that the higher boiling constituents come off the column with good resolution and at reasonable lengths of time.
26-4. Define (a) retention volume, (b) corrected retention volume, (c) specific retention volume.
The retention volume, VR, is equal to the retention time times the average volumetric flow rate (tRF). The corrected retention volume, VR0, is the retention volume times the pressure drop correction factor, j. The specific retention volume, Vg, is equal to the difference between the retention volume of the species an the retention time of a species not retained by the column (VR0-VM0) over the mass of the stationary phase, ms, times 273K / temperature of the column, TC.
What is the difference between a concentration-sensitive and a mass-sensitive detector? Are the following detectors mass or concentration sensitive? (a) thermal conductivity, (b) atomic emission, (c) thermionic, (d) electron captive, (e) flamephotometric, (f) flame ionization.
A concentration-sensitive detector responds to the concentration of the analyte in the mobile phase, whereas a mass-sensitive detector responds to the number of analyte molecules or ions that come in contact with the detector. Peak areas for a concentration-sensitive detector increase as the flow rate decreases because the analyte is in contact with the detector for a longer period. Peak areas for a mass-sensitive detector are not greatly affected by flow rate. Using CS for concentration sensitive and MS for mass sensitive, we find for each of the detectors listed: (a) thermal conductivity (CS), (b) atomic emission (MS), (c) thermionic (MS), (d) electron capture (CS), (e) flame photometric (MS), (f) flame ionization (MS).
Discuss why the combination of GC and mass spectrometry is so powerful.
The combination of GC with MS allows the identification of species eluting from the chromatographic column. The total ion chromatogram gives information similar to a conventional GC chromatogram. By monitoring selected ions, information about specific species can be obtained. By scanning the mass spectrum during the chromatography experiment, species eluting at various times can be identified. Gas chromatography coupled with tandem mass spectrometry allows even more specific identifications to be made.
What properties should the stationary-phase liquid for GC possess?
Desirable properties of a stationary phase for GC include: low volatility, thermal stability, chemical inertness, and solvent characteristics that provide suitale k and α values for the analytes to be separated.
List the variables that lead to (a) band broadening and (b) band separation in GLC.
(a) Band broadening arises from very high or very low flow rates, large particles making up the packing, thick layers of stationary phase, low temperature and slow injection rates. (b) Band separation is enhanced by maintaining conditions so that k lies in the range of 1 to 10, using small particles for packing, limiting the amount of stationary phase so that particle coatings are thin, and injecting the sample rapidly.
One method for quantitative determination of the concentration of constituents in a sample analyzed by GC is the area-normalization method.
Here, complete elution of all of the sample constituents is necessary. The area of each peak is then measured and corrected for differences in detector response to the different eluates. This correction involves dividing the area by an empirically determined correction factor. The concentration of the analyte is found from the ratio of its corrected area to the total corrected area of all peaks.
For a chromatogram containing three peaks, the relative areas were found to be 16,4, 45.2, and 30.2 in the order of increasing retention time.
Calculate the percentage of each compound if the relative detector responses were 0.60, 0.78. and 0.88. respectively.
