The gasoline additive methyl t-butyl ether (MTBE) has been leaking into groundwater ever since its introduction in the 1990s. 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 7-8). After microextraction, analytes are thermally desorbed from the fiber in the port of a gas chromatograph. The figure below 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/z 50 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.

Methyl t-butyl ether MTBE 88 Ethyl t-butyl ether ETBE 102 1-Amyl methyl ether TAME 102 Nominal mass: TAME 87 73* 71 ETBE 87 MTBE 1 57 57 on chromatogram 4 Unidentified Selected ion monitoring (miz 72.7 to 73.7) TAME MTBE 1 23 4 5 678 9 10 11 Time (min)