Herein, we report
on surprisingly efficient photochemical vapor
generation (PVG) of Ru, Re, and especially Ir, achieved from very
dilute HCOOH media employing a thin-film flow-through photoreactor
operated in flow injection mode. In the absence of added metal ion
sensitizers, efficiencies near 20% for Ir and approximately 0.06%
for Ru and Re occur in a narrow range of HCOOH concentrations (around
0.01 M), significantly higher than previously reported from conventionally
optimized HCOOH concentrations (1–20 M). A substantial enhancement
in efficiency, to around 9 and 1.5%, could be realized for Ru and
Re, respectively, when 0.005 M HCOONa served as the PVG medium. The
addition of metal ion sensitizers (particularly Cd2+ and
Co2+) to 0.01 M HCOOH significantly enhanced PVG efficiencies
to 17, 2.2, and 81% for Ru, Re, and Ir, respectively. Possible mechanistic
aspects occurring in dilute HCOOH media are discussed, wherein this
phenomenon is attributed to the action of 185 nm radiation available
in the thin-film flow-through photoreactor. An extended study of PVG
of Fe, Co, Ni, As, Se, Mo, Rh, Te, W, and Bi from both dilute HCOOH
and CH3COOH was undertaken, and several elements for which
a similar phenomenon appears were identified (i.e., Co, As, Se, Te,
and Bi). Although use of dilute HCOOH media is attractive for practical
analytical applications employing PVG, it is less tolerant toward
dissolved gases and interferents in the liquid phase due to the likely
lower concentrations of free radicals and aquated electrons required
for analyte ion reduction and product synthesis.