Chapter 25: QDE (page 706)

After poisonous melamine and cyanuric acid appeared in milk in China (Box $11 - 3$ ) in $2008$ , there was a flurry of activity to develop methods to measure these substances. An analytical method for milk is to treat 1volume of milk with 9 volumes of $H_{2} O_{2} {CH}_{3} CN (20 : 80 vol / vol)$ to precipitate proteins. The mixture is centrifuged for $5 \min$ to remove precipitate. The supernatant liquid is filtered through a $0.5 - μm$ filter and injected into a HILIC liquid chromatography column (TSK gel Amide-stationary phase) and products are measured by mass spectrometry with selected reaction monitoring (Section $22 - 5$ ). Melamine is measured in positive ion mode with the transition $m/z127→85$ . Cyanuric acid is measured in negative ion mode with the transition $m / 2128 \to 42$ .
(a) Write the formulas for the four ions and propose structures for all four ions.
(b) Even though milk is a complex substance, only one clean peak is observed for melamine and one for cyanuric acid spiked into milk. Explain why

Short Answer

Expert verified

a)The formulas are

b) There aren't many other species in milk that can produce the same two ions as these two. Melamine and cyanuric acid have just one peak in the mass spectrum as a result.

Step by step solution

Definition of chemical formula.

A chemical formula is a technique of expressing the chemical quantities of atoms that make up a certain chemical compound or molecule.

Step 2: The formulas for the four ions and propose structures for all four ions.

Melamine is a molecule with a molecular weight of 126 . It decomposes in positive ion mode to generate an -m/z fragment. This implies that melamine $H^{+}$ loses a $42 Da$ . This is most likely related to the loss of ${CN}_{2} H_{2}$ .

$If {CN}_{2} H_{2} is lost, then the remaining cation is C_{2} H_{4} {H_{5}}^{+} . The structure for the same can be proposed as,$

[Cyanuric acid- $H$ ] decomposes to $m / z = 42 .$

This is more likely to be a cyanate ion $N \equiv C - O^{-} .$

Step 3: Explanation of why only one peak is absorbed for melamine and cyanuric acid in milk.

Melamine is a molecule with a molecular weight of 126 . Through collisionally induced dissociation, it decomposes in positive ion mode to generate an $85 - m / z$ fragment. This implies that melamine $H^{+}$ loses a 42Da.

role="math" localid="1654933425262" ${[Cyanuric acid ‐ H]}^{-}$ decomposes to $m / z = 42$ .

This is more likely to be a cyanate ion $N = C^{-} O^{-}$

There aren't many organisms in milk that can produce the same two ions. As a result, the mass spectra for melamine and cyanuric acid only shows one peak.