Metal interferences and their removal prior to the determination of As(T) and As(III) in acid mine waters by hydride generation atomic absorption spectrometry

Hydride generation atomic absorption
spectrometry (HGAAS) is a sensitive and
selective method for the determination of total
arsenic (arsenic(III) plus arsenic(V)) and
arsenic(III); however, it is subject to metal
interferences for acid mine waters. Sodium
borohydride is used to produce arsine gas, but
high metal concentrations can suppress arsine
production.

This report investigates interferences of
sixteen metal species including aluminum,
antimony(III), antimony(V), cadmium,
chromium(III), chromium(IV), cobalt,
copper(II), iron(III), iron(II), lead,
manganese, nickel, selenium(IV),
selenium(VI), and zinc ranging in
concentration from 0 to 1,000 milligrams per
liter and offers a method for removing
interfering metal cations with cation exchange
resin. The degree of interference for each
metal without cation-exchange on the
determination of total arsenic and arsenic(III)
was evaluated by spiking synthetic samples
containing arsenic(III) and arsenic(V) with
the potential interfering metal. Total arsenic
recoveries ranged from 92 to 102 percent for
all metals tested except antimony(III) and
antimony(V) which suppressed arsine
formation when the antimony(III)/total
arsenic molar ratio exceeded 4 or the
antimony(V)/total arsenic molar ratio
exceeded 2. Arsenic(III) recoveries for
samples spiked with aluminum,
chromium(III), cobalt, iron(II), lead,
manganese, nickel, selenium(VI), and zinc ranged from 84 to 107 percent over the entire
concentration range tested. Low arsenic(III)
recoveries occurred when the molar ratios of
metals to arsenic(III) were copper greater than
120, iron(III) greater than 70, chromium(VI)
greater than 2, cadmium greater than 800,
antimony(III) greater than 3, antimony(V)
greater than 12, or selenium(IV) greater than
1. Low recoveries result when interfering
metals compete for available sodium
borohydride, causing incomplete arsine
production, or when the interfering metal
oxidizes arsenic(III).
Separation of interfering metal cations
using cation-exchange prior to hydridegeneration
permits accurate arsenic(III)
determinations in acid mine waters containing
high concentrations of interfering metals.
Stabilization of the arsenic redox species for
as many as 15 months is demonstrated for
samples that have been properly filtered and
acidified with HCl in the field. The detection
limits for the method described in this report
are 0.1 micrograms per liter for total arsenic
and 0.8 micrograms per liter for arsenic(III).