Passage
Fatty acids (FAs) are lipids that play important roles in cellular function, including energy storage and membrane structure. Most polyunsaturated fatty acids (PUFAs) are made in the body; however, omega-3 alpha-linolenic acid and omega-6 linoleic acid must be obtained from the diet. These two PUFAs are precursors to other omega-3 and omega-6 long-chain FAs that are necessary for physiological processes such as neurological development, normal growth, and immune response. Insufficient omega-3 and omega-6 long-chain FAs can lead to an increase in omega-9 fatty acids such as mead acid, and to essential fatty acid deficiency (EFAD) , which can cause infertility, decreased growth, and increased susceptibility to infection.Total FA levels in plasma and serum can be used to diagnose EFAD. Plasma and serum samples from several individuals were hydrolyzed to release FAs from triacylglycerols and phospholipids. Once isolated, the free FAs were converted to pentafluorobenzyl (PFB) derivatives and separated by gas chromatography (GC) . Total levels of each type of fatty acid were quantified by negative chemical ionization-mass spectrometry (NCI-MS) immediately after GC. NCI is an ionization technique that creates negatively charged ions with little to no fragmentation. The NCI-MS revealed that two of the peaks from GC had the same m/z ratio (Figure 1) . They were both confirmed to be forms of an unsaturated, unbranched 22-carbon FA called omega-9 docosenoic acid.
Figure 1 Selected ion chromatogram of docosenoic acidResearchers have proposed that an elevated ratio of the triene omega-9 mead acid to the tetraene omega-6 arachidonic acid, known as the triene/tetraene (TT) ratio, can signify EFAD. Using GC and NCI-MS, they measured the TT ratio, concentration of mead acid, and individual concentrations of omega-3 and omega-6 FAs in plasma and serum samples from three patients and compared their results to normal ranges. An increased TT ratio and mead acid concentration and decreased omega-3 and omega-6 FA concentrations were observed in the patient samples compared to normal ranges.
Adapted from E. Kish-Trier et al., "Quantitation of total fatty acids in plasma and serum by GC-NCI-MS." Clinical Mass Spectrometry. ©2016 Elsevier.
-What can be inferred from Figure 1 about the relationship of Peaks A and B?

Question

Passage
Fatty acids (FAs) are lipids that play important roles in cellular function, including energy storage and membrane structure. Most polyunsaturated fatty acids (PUFAs) are made in the body; however, omega-3 alpha-linolenic acid and omega-6 linoleic acid must be obtained from the diet. These two PUFAs are precursors to other omega-3 and omega-6 long-chain FAs that are necessary for physiological processes such as neurological development, normal growth, and immune response. Insufficient omega-3 and omega-6 long-chain FAs can lead to an increase in omega-9 fatty acids such as mead acid, and to essential fatty acid deficiency (EFAD) , which can cause infertility, decreased growth, and increased susceptibility to infection.Total FA levels in plasma and serum can be used to diagnose EFAD. Plasma and serum samples from several individuals were hydrolyzed to release FAs from triacylglycerols and phospholipids. Once isolated, the free FAs were converted to pentafluorobenzyl (PFB) derivatives and separated by gas chromatography (GC) . Total levels of each type of fatty acid were quantified by negative chemical ionization-mass spectrometry (NCI-MS) immediately after GC. NCI is an ionization technique that creates negatively charged ions with little to no fragmentation. The NCI-MS revealed that two of the peaks from GC had the same m/z ratio (Figure 1) . They were both confirmed to be forms of an unsaturated, unbranched 22-carbon FA called omega-9 docosenoic acid.

Figure 1 Selected ion chromatogram of docosenoic acidResearchers have proposed that an elevated ratio of the triene omega-9 mead acid to the tetraene omega-6 arachidonic acid, known as the triene/tetraene (TT) ratio, can signify EFAD. Using GC and NCI-MS, they measured the TT ratio, concentration of mead acid, and individual concentrations of omega-3 and omega-6 FAs in plasma and serum samples from three patients and compared their results to normal ranges. An increased TT ratio and mead acid concentration and decreased omega-3 and omega-6 FA concentrations were observed in the patient samples compared to normal ranges.
Adapted from E. Kish-Trier et al., "Quantitation of total fatty acids in plasma and serum by GC-NCI-MS." Clinical Mass Spectrometry. ©2016 Elsevier.
-What can be inferred from Figure 1 about the relationship of Peaks A and B?

Quizplus · Accepted Answer

11ecba2d_1297_1ac0_a3bf_cdd17d199ded_MD0008_00 Isomers are molecules with the same molecular formula but different structural arrangements. They can be generally classified as constitutional isomers and stereoisomers. Constitutional isomers differ in atom connectivity whereas stereoisomers have the same atom connectivity but differ in spatial arrangement. Stereoisomers that arise from disubstituted double bonds are called geometric isomers. They are classified as cis or trans based on the relative positions of the nonhydrogen substituents. Geometric isomers can be separated by gas chromatography because they have slightly different boiling points. Docosenoic acid is an unsaturated fatty acid, meaning it contains a disubstituted double bond, and can be in the cis or trans form. Both isomers have the same molecular weight and m/z ratio, but will come off the GC column at different times, yielding two distinct peaks. Therefore, it can be inferred that Peaks A and B are cis/trans isomers of docosenoic acid. (Choice A) Information is given in the passage to infer the relationship of Peaks A and B, including the fact that both peaks correspond to docosenoic acid and that docosenoic acid is unsaturated, meaning it has cis and trans forms. (Choice B) The passage states that little to no fragmentation occurs in NCI-MS, so it is unlikely that docosenoic acid fragmented. (Choice C) Although constitutional isomers have the same molecular weight, they have different atom connectivity, making them different compounds. The passage states that both compounds are forms of docosenoic acid, so they cannot be constitutional isomers. Educational objective: Isomers are molecules with the same molecular formula but different structural arrangements, and are either constitutional or stereoisomers. Disubstituted double bonds give rise to cis or trans isomers, depending on the relative positions of nonhydrogen substituents. These isomers have slightly different boiling points and can be separated by gas chromatography.

Passage Fatty Acids (FAs) Are Lipids That Play Important Roles in in Cellular