Which pair of amino acids is indistinguishable in peptide sequencing by tandem mass spectrometry?

The correct answer is Leucine (Leu) and Isoleucine (Ile).

Because the amino-acids leucine and isoleucine have the same mass they don’t differ by tandem mass spectrometry.

Tandem MS (MS/MS) is a technique which includes the breakage of the selected molecules (precursor ions), such as proteins into fragments (product ions), such as peptides. The fragments then reveal aspects of the chemical structure, such as amino acids of the precursor ion. The process have a few steps:

•Ionization of the samples to generate a mixture of ions,  

•Selection of precursor ions of a specific mass-to-charge ratio (MS1)

•Fragmentation of precursor ions (MS2) to generate a product ions for detection.

Val and AlaSer, ThrPhe and TryAsn, Gln, Leu and lie are the pairs of amino acids which are indistinguishable in peptide sequencing by tandem mass spectrometry. Amino acids are organic compounds which combine to form proteins. These are building blocks of our Life.                  

i think its the first one