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Chapter 22: Q32 P (page 602)
32:Why is direct electron ionization in Figure 22-14 permissible for gas chromatography, but only for capillary columns in liquidchromatography in Figure 22-29?
A technique for coupling liquid chromatography and mass spectrometry
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(This is a long exercise suitable for group work.) Relative intenities for the molecular ion region of several compounds are listed in arts (a)-(d) and shown in the figure. Suggest a composition for each nolecule and calculate the expected isotopic peak intensities.
a)
b)
c) m/z
d)
Quantitative analysis by selected ion monitoring. Caffeine in beverages and urine can be measured by adding caffeine- as an internal standard and using selected ion monitoring to measure each compound by gas chromatography. The figure shows mass chromatograms of caffeine (m/z,194)and caffeine-D, which have nearly the same retention time.
Suppose that the following data were obtained for standard mixtures:
(a) Compute the mean response factor in the equation
role="math" localid="1664874599903"
(b) For analysis of a cola beverage, 1.000mL of beverage was treated with of standard solution containing 1.11 g/Lcaffeine- in methanol. The combined solution was passed through a solid-phase extraction cartridge that retains caffeine. Polar solutes were washed off with water. Then the caffeine was washed off the cartridge with an organic solvent and the solvent was evaporated to dryness. The residue was dissolved in of methanol for gas chromatography. Peak areas were 1144 for m/z197 and 1733 for m/z194. Find the concentration of caffeine (mg/L ) in the beverage.
Phytoplankton at the ocean surface maintain the fluidity of their cell membranes by altering their lipid (fat) composition when the temperature changes. When the ocean temperature is high, plankton synthesize relatively more 37:2 than
After they die, plankton sink to the ocean fl oor and end up buried insediment. The deeper we sample a sediment, the further back into time we delve. By measuring the relative quantities of cell-membrane compounds at different depths in the sediment, we can infer the temperature of the ocean long ago. The molecular ion regions of the chemical ionization mass spectra of 37:2 and 37:3 are listed in the table. Predict the expected intensities of M, M11, and M12 for each of the four species listed. Include contributions from C, H, O, and N, as appropriate. Compare your predictions with the observed values. Discrepant intensities in these data are typical unless care is taken to obtain high-quality data.
Chlorate , chlorite , bromate , and iodate can be measured in drinking water at the 1-ppb level with 1% precision by selected reaction monitoring. Chlorate and chlorite arise from used as a disinfectant. Bromate and iodate can be formed from or when water is disinfected with ozone . For the highly selective measurement of chlorate, the negative ion selected by Q1 in Figure 22-33 is m/z 83 and the negative ion selected by Q3 is m/z 67. Explain how this measurement works and how it distinguishes from , , and
Explain what is meant by spectral, chemical, ionization, andisobaric interference.
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