Hydride generation atomic absorption spectrometry (HGAAS) is a sensitive and selective method for the determination of and 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 metals 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 to1,000 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 and 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 are the result of interfering metals competing for available sodium borohydride, causing incomplete arsine production or oxidation of arsenic(III) by the interfering metal prior to the determination. Separation of interfering metal cations using cation-exchange prior to hydride-generation 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 method detection limits for the method describe in this report are 0.1 micrograms per liter for total arsenic and 0.8 micrograms per liter for arsenic(III).
Contents
Figures
Tables
Explanation of Abbreviations and Symbols
Abstract
Introduction
Methods of investigation
Sample Collection and Preservation
Reagents
Analytical Apparatus
Analytical Procedures
Determination of Arsenic(T)
Determination of Arsenic(III)
Removal of Interfering Metals Prior To Arsenic(III) Determination
Results and Discussion
Selective Reduction of Arsenic(III)
Potential Metal Interferences on the Determination of Arsenic(T)
Potential Metal Interferences in the Determination of Arsenic(III)
Cation Exchange Separation of Iron(III) and Copper
Accuracy and time stability of Arsenic Redox Species in Acid Mine Waters
Accuracy of Arsenic(T) Determinations
Accuracy of Arsenic(III) Determinations
Time Stability of Arsenic Redox Species
Summary
References Cited
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For more information on the investigation of baseline and pre-mining ground water quality, Red River Valley Basin, NM visit our website at:
http://wwwbrr.cr.usgs.gov/projects/GWC_chemtherm/questa.htm
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