  A pilot project has been proceeded to map 1 deg$^2$ on the Galactic plane for
radio recombination lines (RRLs) using the Five hundred meter Aperture
Spherical Telescope (FAST). The motivation is to verify the techniques and
reliabilities for a large-scale Galactic plane RRL survey with FAST aiming to
investigate the ionized environment in the Galaxy. The data shows that the
bandpass of the FAST 19 beam L-band is severely affected by radio frequency
interferences (RFIs) and standing wave ripples, which can hardly be corrected
by traditional low order polynomials. In this paper, we investigate a series of
penalized least square (PLS) based baseline correction methods for radio
astronomical spectra that usually contain weak signals with high level of
noise. Three promising penalized least squares based methods, AsLS, arPLS, and
asPLS are evaluated. Adopting their advantages, a modified method named rrlPLS
is developed to optimize the baseline fitting to our RRL spectra. To check
their effectiveness, the four methods are tested by simulations and further
verified using observed data sets. It turns out that the rrlPLS method, with
optimized parameter $\lambda = 2 \times 10^8$ , reveals the most sensitive and
reliable emission features in the RRL map. By injecting artificial line
profiles into the real data cube, a further evaluation of profile distortion is
conducted for rrlPLS. Comparing to simulated signals, the processed lines with
low signal-to-noise ratio are less affected, of which the uncertainties are
mainly caused by the rms noise. The rrlPLS method will be applied for baseline
correction in future data processing pipeline of FAST RRL survey. Configured
with proper parameters, the rrlPLS technique verified in this work may also be
used for other spectroscopy projects.
