A level chemistry help

Would love some help with this : Q6.A sample of ethanedioic acid was treated with an excess of an unknown alcohol in the
presence of a strong acid catalyst. The products of the reaction were separated and
analysed in a time of flight (TOF) mass spectrometer. Two peaks were observed at m / z =
104 and 118.
(a) Identify the species responsible for the two peaks.
ANSWER = [CH3OCOCOOH]+

Ethanedioic acid is just two carboxyl groups bonded together, e.g: HOOC-COOH.
When you react it with an excess of an alcohol, ROH, you esterify the carboxyl groups, so you would form a mixture of ROOC-COOH and ROOC-COOR (and water, but that’s irrelevant).
The peaks at 104 and 118 are going to be caused by cations with charges of +1 (at A level, the charge is always +1) and relative formula masses of 104 and 118, respectively.
It is probably quite safe to assume that in light of how little information you were given, the peaks you were told about are the molecular ion peaks and so they are respectively caused by [ROOC-RCOOH]^+ and [ROOC-COOR]^+.
We can use the fact that the relative formula mass of ROOC-COOH is 104 (or the Mr of ROOC-COOR is 118) to work out the Mr of the R group:
Mr of R + 4(16) + 2(12) + 1 = 104
Mr of R + 89 = 104
So Mr of R = 15
This suggests that the R group must have an Mr of 15 and since it is made up of C and H (remember ROH must be an alcohol - there must be carbon in there somewhere), you can deduce that R has to be CH3.
As such, your unknown alcohol is methanol and your molecular ions are [CH3OOC-COOH]^+ and [CH3OOC-COOCH3]^+

Thanks for your reply, is this an AS level question by the way? I'm not familiar with esterification 😭