Question

The gasoline additive methyl t-butyl ether (MTBE) has been leaking into groundwater ever since its introduction in the. MTBE can be measured at parts per billion levels by solid-phase microextraction from groundwater to which 25 % (wt/vol) NaCl has been added (salting out, Problem 8-9). After microextraction, analytes are thermally desorbed from the fiber in the port of a gas chromatograph. The figure on the next page shows a reconstructed total ion chromatogram and selected ion monitoring of substances desorbed from the extraction fiber.

(a) What is the purpose of adding NaCl prior to extraction?

(b) What nominal mass is being observed in selected ion monitoring? Why are only three peaks observed?

(c) Here is a list of major ions above m/z50 in the mass spectra. The base (tallest) peak is marked by an asterisk. Given that MTBE and TAME have an intense peak at m/z,73 and there is no significant peak at m/z,73 for ETBE, suggest a structure for m/z,73. Suggest structures for all ions listed in the table.

Short answer

(a) The inclusion of sodium chloride increases the percentage of organic compounds that will be transported to the fibre extraction process.

(b) Selective ion monitoring is used to measure ion abundance for m/z73, and only three chemicals are stated to have significant intensity in the extract at m/z73 .

(c) The structures shown below can be used as examples.

Step-by-step solution

Definition of NaCl, MTBE, TAME and ETBE

• The chemical name for salt is sodium chloride. Sodium is an electrolyte that controls how much water your body holds. Nerve impulses and muscle contractions are also affected by sodium.
• Dehydration, excessive sweating, and other causes of sodium loss are treated or prevented by sodium chloride.
• The volatile organic compound (VOC) methyl tertiary butyl ether (MTBE) is extensively utilised as a gasoline additive.
• MTBE is also known as 2-methoxy-2-methylpropane, methyl t-butyl ether, and methyl tert butyl ether.
• TAME (tert-amyl methyl ether) is a kind of ether that is utilised as a fuel oxygenate.
• Ethyl tert-butyl ether (ETBE) is a substitute for methyl tert-butyl ether (MTBE) that combines the blending capabilities of MTBE and the renewable nature of ethanol.

Determining the purpose of adding NaCl prior to extraction

(a)

• It is necessary to explain why sodium chloride was added to the extraction earlier.
• In water, sodium chloride lowers the solubility of non-polar molecules such as ethers. The inclusion of sodium chloride increases the percentage of organic compounds that will be transported to the fibre extraction process.

Determining the nominal mass being observed in selected ion monitoring and why only three peaks are observed

(b)

• The nominal mass seen in selected ion monitoring must be provided, as well as the explanation for the detection of three peaks.
• The nominal mass seen in chosen ion monitoring, as well as the explanation for the detection of three peaks, must be provided.
• Selective ion monitoring is used to measure ion abundance for m/z 73, and only three chemicals are stated to have significant intensity in the extract at m/z 73.

Determining the structures for all ions listed in the table

(c)

For the values in the table, the peaks must be suggested.

To make suggestions for the peaks for the values in the table

m/z 73 Base peaks for MTBE and TAME have been discovered. This mass relates to M-15 (loss of mass) in the case of MTBE ${\mathrm{CH}}_3$TAME's mass, on the other hand, relates to M-29 (a reduction in ${\mathrm{C}}_2{\mathrm{H}}_5$) . The loss of the ethyl group in TAME, which is connected to carbon, suggests that the methyl group lost in MTBE is also tied to carbon, rather than oxygen.

If the methyl group related to oxygen was easily lost from MTBE and TAME, the ethyl group linked to oxygen should be easily lost from ETBE. There isn't a prominent crest at M-29( m/z 73 in ETBE.

The structures shown below can be used as examples.