System optimization for determination of cobalt in biological samples by ICP-OES using photochemical vapor generation

Abstract

An optimized photochemical vapor generation (PVG) approach for efficient synthesis of volatile cobalt species is described. Solutions containing Co(II) in a pH 3.3 medium of 50% formic acid were exposed to a source of deep UV (254 and 185 nm) radiation generated within a 19 W flow-through low pressure mercury discharge lamp. Following efficient phase separation, the analyte was transported to an ICP-OES system for detection at the 238.892 nm emission line of Co I. Several variables were investigated, including the type of UV lamp and gas-liquid separator, identity and concentration of the low molecular weight organic acid, solution pH, sample flow rate and exposure time to the UV irradiation as well as transport gas flow and mode of introduction of sample (continuous or segmented) to the ICP. In continuous mode, an optimum generation efficiency of 42 AE 2% was achieved with an irradiation time of 10 s, providing a 27-fold improvement in sensitivity compared to pneumatic nebulization and a limit of detection of 0.4 mg L À1 with a precision of 3% at 100 mg L À1 . Direct analysis of acid digested biological tissues (NRC TORT-2 and TORT-3) was hampered by strong matrix interferences from the presence of nitrate and other ions which could be circumvented by longer irradiation time and sufficient dilution such that accurate analysis of real samples by the method of additions could be achieved while maintaining high generation efficiency.

Efficient Photochemical Vapor Generation of Molybdenum for ICPMS Detection

“…In both cases, simultaneous nebulization of liquid IS is possible, creating more robust (wet plasma) conditions in the ICP and permitting the monitoring/correction for any sensitivity drift due to changes in the plasma or interface transmission efficiency. 33,35 Efficient generation of volatile molybdenum species was achieved at rather higher concentrations of formic acid in the reaction medium, as shown in Figure 2, in agreement with typical conditions optimized for other transition metals that form volatile metal carbonylsNi, Fe, and Co. 15,17,22,23,36 Due to its high concentration, a significant volume flow of gaseous byproducts (CO, CO 2 , and H 2 ) is generated in the inner quartz tube of the photoreactor by direct photolysis of the formic acid. 37 As a result, a segmented flow of the reaction medium arises due to bubbles of these gases, and the supply of analyte to the GLS is thus not uniform.…”

“…The sensitivity gradually decreased from its maximum for 30% formic acid (pH 1.1) to around 50% response for 30% formic acid containing 1.5 M ammonium formate (pH 3.2). This observation is not consistent with that noted for PVG of other transition metal carbonyls, namely, Ni, Fe, and Co. ,− The addition of acetic acid in the range 0.5–5% to the 30% formic acid did not enhance PVG efficiency, and in a mixture containing 10% acetic acid, sensitivity decreased to around 60%.…”

“…Apart from elements typically amenable to HG and nonmetals (Br, Cl, and I), − PVG has been successfully achieved with several transition metals, including Ni, − Fe, − Co, ,, Cu, Cd, and Os . PVG of Rh, Pt, Pd, Ag, and Au was briefly reported in very early work and has only recently been followed up with a more detailed investigation, although absolute efficiencies were not estimated .…”

“…9 Since UV light has only to pass the thin synthetic quartz wall of the inner channel, this photoreactor permits ready access to intense 185 nm radiation, efficiently photolyzing the sample without concurrent ozone generation. 5 Apart from elements typically amenable to HG 10 and nonmetals (Br, Cl, and I), 11−13 PVG has been successfully achieved with several transition metals, including Ni, 14−20 Fe, 20−22 Co, 14,23,24 Cu, 25 Cd, 26 and Os. 27 PVG of Rh, Pt, Pd, Ag, and Au was briefly reported in very early work 28 and has only recently been followed up with a more detailed investigation, although absolute efficiencies were not estimated.…”

Cadmium Assisted Photochemical Vapor Generation of Tungsten for Detection by Inductively Coupled Plasma Mass Spectrometry

“…Apart from those of the p-block of the Periodic Table (As, Sb, Bi, Se, Te, Pb, Sn, and Tl) and Hg, − whose volatile species remain most frequently generated by chemical reduction with tetrahydroborate for routine applications, PVG of nonmetals (Br, Cl, and I), − and that of transition metals can be also successfully achieved. The latter include Ni, − Fe, − Co, ,− Cu, Cd, Os, Rh, , Pt, , Pd, , Ag, Au, , and Ir . Among them, PVG of Ni, Fe, and Co from a formic acid based medium has garnered special attention, resulting in several analytical applications, due to its high efficiency and the intrinsic stability of the resultant volatile metal carbonyls that have been identified among the gaseous products. ,, …”