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We present a study of the CO isotopologues and the high-density tracersH$_{2}$CO, HCO$^{+}$, and CS in Class 0/I proto-brown dwarfs (proto-BDs). Wehave used the IRAM 30m telescope to observe the $^{12}$CO (2-1), $^{13}$CO(2-1), C$^{18}$O (2-1), C$^{17}$O (2-1), H$_{2}$CO (3-2), HCO$^{+}$ (3-2), andCS (5-4) lines in 7 proto-BDs. The hydrogen column density for the proto-BDsderived from the CO gas emission is $\sim$2-15 times lower than that derivedfrom the dust continuum emission, indicating CO depletion from the gas-phase.The mean H$_{2}$CO ortho-to-para ratio is $\sim$3 for the proto-BDs andindicates gas-phase formation for H$_{2}$CO. We have investigated thecorrelations in the molecular abundances between the proto-BDs and protostars.Proto-BDs on average show a factor of $\sim$2 higher ortho-to-para H$_{2}$COratio than the protostars. Possible explanations include a difference in theH$_{2}$CO formation mechanism, spin-selective photo-dissociation,self-shielding effects, or different emitting regions for the ortho and paraspecies. There is a tentative trend of a decline in the HCO$^{+}$ and H$_{2}$COabundances with decreasing bolometric luminosity, while the CS and COabundances show no particular difference between the proto-BDs and protostars.These trends reflect the scaled-down physical structures for the proto-BDscompared to protostars and differences in the peak emitting regions for thesespecies. The C$^{17}$O isotopologue is detected in all of the proto-BDs as wellas the more evolved Class Flat/Class II BDs in our sample, and can probe thequiescent gas at both early and late evolutionary stages.

Chemical tracers in proto-brown dwarfs: CO, ortho-H$_{2}$CO, para-H$_{2}$CO, HCO$^{+}$, CS observations