1. X signifies either Leu or Ile. Numbers in parentheses indicate the combined mass of two amino acids, where the composition is not known. Letters in parentheses indicate the single letter code of amino acids known to be present, but where the sequence of the two are not known. For the ion trap data (Problems #1 and #2) the mass accuracy is not sufficient to distinguish Gln from Lys nor oxidized Met from Phe; however, for the sake of brevity Q is used for Gln/Lys and F indicates Phe/ox-Met. Residues marked in red indicate errors in reading the sequence, and black indicates regions where the sequence was correctly deduced. "NA" indicates that no answer was given.
2. Years of experience.
3. Had the participant ever had the occasion to deduce a correct sequence in "real life".
4. Did the participant have a preference for data obtained from the Qtof or the ion trap?
5. Type of laboratory where the participant is employed. "A" indicates an academic facility; "P" indicates a pharmaceutical company; "B" indicates a biotech company; "S" indicates a graduate student; "C" indicates a consultant; "U" indicates unemployed or in transition.

Many of the errors were anticipated. Primarily these involved confusing Gly-Gly with Asn (same mass) or Gly-Ala with Gln/Lys (same mass); for these errors, certain fragment ion(s) were overlooked. A few errors were unexpected, but some can be explained as follows:

Person #1: I think Person #1 went astray on Problem #4 ((D,P)XGFAGAQGGXTFR instead of the correct XPDGFAGAQGGXTFR) by confusing some internal fragment ions for b-type ions. There is a pair of ions at m/z 213 and 185 that look enticingly like a pair of b- and a-type ions containing two amino acids, D and P, but these in fact are internal fragment ions. These internal fragment ions then connected up with the correct b-type series using X.

Person #2: Person #2 apparently thought that the ions at 175.118, 276.135, 389.183, and 536.318 corresponded to a series of y-type ions that delineated the C-terminal sequence to be "FXTR". However, the mass differences between these ions were 101.02, 113.05, and 147.135, whereas the actual masses for T, X, and F are 101.05, 113.08, and 147.07, which are outside of the Qtof tolerance of +/- 0.02 u.

Person #3: Person #3 seemed to think that the ions at m/z 175.118, 276.135, and 423.235 in Problem #4 were part of the y-type ion series delineating the C-terminal sequence "FTR". However, the mass differences between these ions are 101.02 and 147.10, whereas the residue masses of T and F are actually 101.05 and 147.07. These mass differences are only 0.03 u, which are quite close to the anticipated tolerance of +/- 0.02. The correct C-terminal sequence of "TFR" is supported by the presence of other ions with lower errors; however, I would say that in a real experiment that it would be tough to distinguish between these two choices (TFR vs. FTR). I don't know why this person assigned "GX" to the sequence, instead of the correct "XG", since there are no b- or y-type ions present to support this assignment. Perhaps it was a typer.

Person #5: Same error as Person #3.

Person #6: Mixed up the two N-terminal amino acids, which is usually easy to do. In this case there is a small singly-charged and a more abundant doubly-charged y-type ion present at m/z 1393.699 and 697.354 that delineate the N-terminal sequence in Problem #4.

Person #7: For both Problems #2 and #4 this person got the sequence mostly correct, but perhaps got confused by the presence of an extra proton in the b-type ions. Anyway, the N-termini of both peptides was called as "N", which is 1 u higher than the correct "X".

Person #8: I have no idea where the "HA" in the sequence comes from. This sequence does not add up to the correct peptide MW, so maybe it was another typa.

Person #9: This person made the same errors as Person #2 in Problem #4.

Person #10: This person made the same errors as Person #2 in Problem #4. In addition, "APF" was called in Problem #3 instead of the correct "DEA". Perhaps they thought that the ion series m/z 262.141, 409.283, 506.299, and 577.249 corresponded to a continuous series of y-type ions. However, the mass differences between these ions are 147.14, 97.02, and 70.95, which are outside of the tolerance of +/- 0.02 u for the residue masses of F, P, and A (147.07, 97.05, and 71.04 u).

Person #11: This person did the easy part of deducing the N-terminal sequence from the fragment ions found at m/z above the precursor ion in Problem #4. He also found an ion at m/z 175 suggesting the presence of a C-terminal Arg. He then ran out of time and was unable to sort through the multitude of ions below the precursor and above m/z 175, acknowledging the presence of an impressive series of internal fragment ions.

Person #13: I'm not sure how this person derived the "FDX" part of the sequence. The calculated mass of the peptide deduced by this person is off by 1 u.

Person #14: This person got the C-terminal "EK" wrong. EK could give the y-type ion at m/z 276.1, the correct C-terminal sequence "TR" is a better choice, since there is a tiny b-type ion at m/z 1332 that suggests the presence of Arg at the C-terminus.

Person #17: Made the same minor mistake on Problem #2 as Person #14.

Person #20: Called the N-terminus of Problem #1 as "QN" for some reason, which has the same mass as "EL" (242 u).