A downhole optical chemical compound monitoring device (30) arranged to be integrated in a mandrel (20) of a drilling system, for monitoring a chemical compound in a drilling fluid (14B) circulated through a well (7, 8), the downhole optical chemical compound monitoring device (30) comprising: - an analysis cell part (31) comprising an open cavity (40, 140) into which the drilling fluid (14B) is free to flow; - a first optical probe (41 A, 141 A) coupled to a light source (157) and arranged to transmit a light energy into the open cavity (40, 140); - a second optical probe (41 B, 141 B) coupled to a detector (158) and arranged to produce in use a signal resulting from an interaction of the drilling fluid (14B) present in the open cavity (40, 140) with said light energy, indicative of a quantity of chemical compound present in the drilling fluid (14B); - each of said optical probes is mounted through a wall (38A, 38B, 138A, 138B) of the analysis cell part (31) in a sealed manner and has a tip (45A, 45B), the tip being needle shaped and having an external diameter less than 1 mm; and - the first optical probe (41 A, 141 A) and the second optical probe (41 B, 141 B) are arranged such that the first and second tips of the respective optical probes project into the open cavity (40, 140) and such that the first tip (45A) of the first optical probe (41 A, 141 A) faces the second tip (45B) of the second optical probe (41 B, 141 B) at a defined distance ranging from 10 µm to 10 mm.
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