There are several options, however the one I usually use is first to calculate the Fourier transform of the signal using the fft function (to get an idea of the spectral content of the signal) and then using either the bandpass or highpass functions (depending on whether there is high-frequency noise to be rejected) to design the filter and do the actual filtering. Use 'ImpulseResponse','iir' for best results with those functions.
Another option is the detrend function. This may be preferable if the baseline drift is easy to model with a polynomial function. If this does not provide the desired result, then first use the islocalmin function to detect the minima of the signal, and then detrend the result of that operation (indexing into the signal using the logical vector resulting from the islocalmin call).
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