Passage
Electrospray ionization mass spectrometry (ESI-MS) employs a soft ionization technique that, unlike traditional ionization via electron bombardment, results in little to no fragmentation of target molecules. During electron bombardment, electrons collide with the analyte and cause the molecule to lose an electron whereas during ESI-MS, ions are produced by passing the analyte solution through an electrospray needle that has a potential difference applied with respect to a counterelectrode. This leads to the formation of charged droplets, which are repelled from the needle toward the counterelectrode and mass spectrometer, as shown in Figure 1.
Figure 1 ESI-MS ionization mechanismDuring the droplet's flight, the solvent is evaporated off analyte molecules. Ionization occurs through the protonation of basic sites on target molecules. Reaction 1 shows the number of protons n added to molecule M.M → (M + nH) ⁿ⁺Reaction 1Researchers used ESI-MS to quantify the formation of four pyridyloxobutyl-DNA (POB-DNA) adducts, shown in Figure 2. POB-DNA adduct formation involves highly mutagenic pyridyloxobutylating agents, which are formed from cytochrome P450-mediated hydroxylation of 4-(methylnitrosamino) -1-(3-pyridyl) -1-butanone (NNK) , an abundant N-nitrosamine carcinogen found in cigarette smoke. Adducts were produced in vitro by treating samples of calf thymus DNA with NNK. The DNA adducts were then separated by reversed-phase high-performance liquid chromatography (RP-HPLC) and analyzed via ESI-MS.
Figure 2 Molecular structure of POB-DNA adducts
Adapted from Lao Y, Villalta PW, Sturla SJ, Wang M, Hecht SS. Quantitation of pyridyloxobutyl DNA adducts of tobacco-specific nitrosamines in rat tissue DNA by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. Chem Res Toxicol. 2006.
-Which of the following changes could be made to this experiment to increase the signal intensity of peaks that correspond to smaller mass-to-charge (m/z) values relative to larger m/z values?

Question

Passage
Electrospray ionization mass spectrometry (ESI-MS) employs a soft ionization technique that, unlike traditional ionization via electron bombardment, results in little to no fragmentation of target molecules. During electron bombardment, electrons collide with the analyte and cause the molecule to lose an electron whereas during ESI-MS, ions are produced by passing the analyte solution through an electrospray needle that has a potential difference applied with respect to a counterelectrode. This leads to the formation of charged droplets, which are repelled from the needle toward the counterelectrode and mass spectrometer, as shown in Figure 1.

Figure 1 ESI-MS ionization mechanismDuring the droplet's flight, the solvent is evaporated off analyte molecules. Ionization occurs through the protonation of basic sites on target molecules. Reaction 1 shows the number of protons n added to molecule M.M → (M + nH) ⁿ⁺Reaction 1Researchers used ESI-MS to quantify the formation of four pyridyloxobutyl-DNA (POB-DNA) adducts, shown in Figure 2. POB-DNA adduct formation involves highly mutagenic pyridyloxobutylating agents, which are formed from cytochrome P450-mediated hydroxylation of 4-(methylnitrosamino) -1-(3-pyridyl) -1-butanone (NNK) , an abundant N-nitrosamine carcinogen found in cigarette smoke. Adducts were produced in vitro by treating samples of calf thymus DNA with NNK. The DNA adducts were then separated by reversed-phase high-performance liquid chromatography (RP-HPLC) and analyzed via ESI-MS.

Figure 2 Molecular structure of POB-DNA adducts
Adapted from Lao Y, Villalta PW, Sturla SJ, Wang M, Hecht SS. Quantitation of pyridyloxobutyl DNA adducts of tobacco-specific nitrosamines in rat tissue DNA by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. Chem Res Toxicol. 2006.
-Which of the following changes could be made to this experiment to increase the signal intensity of peaks that correspond to smaller mass-to-charge (m/z) values relative to larger m/z values?

Quizplus · Accepted Answer

11ecba2d_1280_10f1_a3bf_f30b2cd77d4b_MD0008_00 In MS analysis, signal intensity, or the height of a data peak relative to other peaks, corresponds to the relative quantity of ions at each mass-to-charge ratio m/z. During electrospray ionization (ESI), particles are charged via protonation within droplets. Ions that have multiple charges have a smaller m/z ratio because the denominator (charge) has increased. In spectrometry data, this translates into greater signal intensities for these ions. Lowering the pH of the solvent increases the concentration of protons and results in a greater number of multiply charged molecules (z> 1). An increase in multiply charged molecules would therefore result in greater signal intensity for smaller m/z values. (Choice A) An increase in the solvent's pH would cause a decrease in the concentration of protons and therefore would not increase the number of multiply charged molecules or the signal intensity of the smaller m/z values. (Choice C) Decreasing the concentration of the sample would decrease the amount of the sample present and cause a decrease in the overall signal intensity. (Choice D) Increasing the concentration of the sample would increase the amount of the sample present, but this would not affect the relative heights of the peaks because all the peaks would increase. The concentration of protons would affect how often a molecule gets protonated. Educational objective: Signal intensity of a mass spectrometry peak corresponds to the relative quantity of ions at a given mass-to-charge ratio m/z. Increased charge yields a decreased m/z ratio; therefore, an increase in the number of multiply charged particles increases signal intensity at lower m/z ratios.

Passage Electrospray Ionization Mass Spectrometry (ESI-MS) Employs a Soft Ionization Technique