One approach —
Data = {1729167932375 '13:25:32.375' 405650.13
1729167932475 '13:25:32.475' 400621.04
1729167932575 '13:25:32.575' 398764.17
1729167932675 '13:25:32.675' 400905.63
1729167932775 '13:25:32.775' 400606.19
1729167932875 '13:25:32.875' 400141.29};
T1 = cell2table(Data);
Time = datetime(T1{:,2}, InputFormat='HH:mm:ss.SSS', Format='HH:mm:ss.SSS');
ElapsedTime = seconds(Time - Time(1));
T1 = removevars(T1,2);
T1 = addvars(T1, ElapsedTime, 'Before',2)
figure
plot(T1{:,2}, T1{:,3})
grid
title('Spectrometer Plot')
I suspect that you will import your file (not provided) with readtable. This approach should work with it.
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