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

Water-Resources Investigations Report 03-4117 

U.S. Department of the Interior 

U.S. Geological Survey

i
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 

By R. Blaine McCleskey, D. Kirk Nordstrom, and James W. Ball 

Water Resources Investigations Report 03-4117 

Boulder, Colorado 

2003 

ii
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. geological Survey. 

For additional information write to: Copies of this report can be purchased from: 

Chief, Branch of Regional Research U.S. Geological Survey 

U.S. Geological Survey` Branch of Information Services

Denver Federal Center Denver Federal Center 

Box 25046, MS-418 Box 25286 

Denver, Colorado 80225 Denver, Colorado 80225-0286 

iii
Contents 

Abstract............................................................................................................................................... 1 

Introduction......................................................................................................................................... 1 

Methods of Investigation .................................................................................................................... 3 

Sample Collection and Preservation................................................................................................ 3 

Reagents........................................................................................................................................... 3 

Analytical Apparatus....................................................................................................................... 3 

Analytical Procedures...................................................................................................................... 3 

Determination of Arsenic(T)........................................................................................................ 4 

Determination of Arsenic(III)............................................................................................................. 4 

Removal of Interfering Metals Prior to Arsenic(III) Determination............................................ 4 

Results and Discussion ....................................................................................................................... 5 

Selective Reduction of Arsenic(III)................................................................................................. 5 

Potential Metal Interferences on the Determination of Arsenic(T)................................................. 5 

Potential Metal Interferences on the Determination of Arsenic(III) ............................................... 6 

Cation Exchange Separation of Iron(III) and Copper ..................................................................... 8 

Accuracy and Time Stability of Arsenic Redox Species in Acid Mine Waters.............................. 8 

Accuracy of Arsenic(T) Determinations...................................................................................... 8 

Accuracy of Arsenic(III) Determinations .................................................................................. 11 

Time Stability of Arsenic Redox Species .................................................................................. 11 

Summary........................................................................................................................................... 12 

References Cited ............................................................................................................................... 12 

Figures 

Figure 1. Effect of NaBH4 concentration on the calibration curve in solutions containing only 

As(III) ................................................................................................................................................. 5

Figure 2. Effect of NaBH4 concentration on the reduction of As(V) to As(III) in solutions 

containing 10 •g/L As(III) and varying As(V) concentrations. .......................................................... 5

Figure 3. As(T) determinations in the presence of potential interfering metals. ................................ 6

Figure 4. As(T) determinations in the presence of hydride-forming metals....................................... 6

Figure 5. As(III) determinations in the presence of potential interfering metals................................ 6

Figure 6. As(III) determinations in the presence of interfering metals............................................... 7

Figure 7. As(III) determinations in the presence of hydride-forming metals..................................... 7

Figure 8. Additive interference in the determination of As(III) in a solution containing 10 

micrograms per liter As(III) and 5 milligrams per liter Cu(II). .......................................................... 7

Figure 9. Spike recovery determinations of As(T) using hydride generation atomic absorption 

spectrometry........................................................................................................................................ 9

Figure 10. As(T) determinations by inductively coupled plasma-optical emission spectrometry and 

hydride generation atomic absorption spectrometry for 20 samples collected from Summitville 

Mine, Colo., 6 samples collected from Richmond Mine, Calif., and 3 samples collected from the

Penn Mine, Calif. ................................................................................................................................ 9

Figure 11. Spike recovery determinations using hydride generation atomic absorption spectrometry 

for As(III) in samples separated by cation exchange and not separated........................................... 11

iv
Figure 12. As(III) determinations using hydride generation atomic absorption spectrometry for 

samples collected from the Summitville Mine. ................................................................................ 11

Figure 13. Time stability of As(III) for 45 surface and ground water samples from Yellowstone 

National Park, Wyo.; Questa Mine site, N. Mex.; Summitville Mine site, Colo.; Richmond Mine,

Calif.; Penn Mine, Calif.; Ester Dome, Alaska; Fallon, Nev.; Mojave Desert, Calif.; and 

Kamchatka, Russia reanalyzed 3 to 15 months after the initial determination. The curved line is a 

Gaussian Fit. ..................................................................................................................................... 12 

Tables

Table 1. Cation exchange separation of Cu and Fe(III)...................................................................... 9

Table 2. Composition of acid mine water samples collected from Summitville Mine, Colo.; 

Richmond Mine, Calif.; and Penn Mine, Calif. ................................................................................ 10

Table 3. As(T) determinations in U.S. Geological Survey standard reference water samples......... 10 

v
Explanation of Abbreviations 

--- (not analyzed or not measured) 

°C (degrees Celsius) 

AAS (atomic absorption spectrometry) 

As(III) (arsenic(III) or arsenite) 

As(V) (arsenic(V) or arsenate)

As(T) (total dissolved arsenic (As(III) plus 

As(V)) 

cm (centimeter) 

conc. (concentration) 

EDL (electrodeless discharge lamp) 

FIAS (flow injection analysis system) 

g (gas) 

HCl (hydrochloric acid) 

HGAAS (hydride generation-atomic 

absorption spectrometry)

HNO3

 (nitric acid) 

KI (potassium iodide) 

ICP-OES (inductively coupled plasmaoptical emission spectrometry) 

ISE (ion-selective electrode) 

μg/L (micrograms per liter) 

M (moles per liter) 

meq/L (milliequivalents per liter) 

meq/mL (milliequivalents per milliliter) 

mg/L (milligrams per liter) 

min. (minute) 

mL (milliliters) 

mv (millivolt) 

MPV (most probable value) 

μL (microliter) 

μm (micrometer) 

μS/cm (microsiemens per centimeter at 25 

degrees Celsius) 

n (number of analyses) 

NaBH4 (sodium borohydride) 

nm (nanometer) 

RSD (relative standard deviation) 

s (standard deviation)

SRWS (standard reference water sample)

v/v (volume per volume) 

w/v (weight per volume) 

w/w (weight per weight) 

vi
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 

By R. Blaine McCleskey, D. Kirk Nordstrom, and James W. Ball 

Abstract 

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). 

Introduction 

Accurate determination of the redox 

state of dissolved As is important for 

interpreting its toxicity and mobility in the 

environment. Dissolved As can form aqueous 

species in several oxidation states, but in 

1
natural waters occurs dominantly as the 

inorganic oxyanions As(III) and As(V). 

Several analytical methods use hydride 

generation to selectively measure As redox 

species, but high metal concentrations can 

interfere with the determination of As(T) 

(As(III) plus As(V)) and As(III) (Smith, 

1975; Welz and Melcher, 1984; Welsch and 

others, 1990; Creed and others 1996; 

Hageman and Welsch, 1996). Accurate 

determination of arsenic redox species in acid 

mine waters is challenging because of 

elevated metal concentrations frequently 

found in this type of water (Nordstrom and 

Alpers, 1999). Sixteen metal species 

including Al, Cd, Co, Cr(III), Cr(VI), Cu(II), 

Fe(II), Fe(III), Mn, Ni, Pb, Sb(III), Sb(V), 

Se(IV), Se(VI), and Zn were evaluated as 

possible interferents on the determination of 

As(T) and As(III). Because As(III) and As(V) 

exist as oxyanions in solution, interfering 

metal cations can be removed from solution 

using cation-exchange while maintaining the 

existing As(III) and As(T) concentrations. 

Pre-analysis separation of Fe(III) using 

cation-exchange resin for the determination 

of As redox species by ion chromatography in 

iron sulfate-sulfuric acid media has been 

successfully performed (Tan and Dutrizac, 

1985). Cyanide also has been used as a 

complexing agent to eliminate interferences 

by metals in the determination of As using 

hydride-generation (Jamoussi and others, 

1996). Interferences usually associated with 

atomic absorption analysis are negligible 

because arsine (AsH3

(g)) is separated from 

the sample matrix. 

Results of accurate and precise 

analytical methods can be invalidated by 

deteriorated samples. Proper filtration and 

preservation are critical for maintaining the 

existing As(III)/As(T) ratio prior to analyses. 

Wing and others (1987) observed that 

filtration (0.1-μm or 0.45-μm), acidification 

with HCl, and storage in the dark at 4°C 

effectively preserved the As redox species for 

up to 8 months in ground waters collected 

near Fallon, Nev. Except for Fe(III) and 

hydrogen sulfide (H2

S), laboratory studies 

demonstrate that rates of oxidation of As(III) 

and reduction of As(V) in aqueous solution 

with redox agents common to natural waters 

are slow (Cherry and others, 1979). Filtering 

the sample removes colloidal material and 

bacteria that can affect the dissolved

As(III)/As(T) ratio (Gihring and others, 2001; 

Wilkie and Hering, 1998). Acidification 

prevents precipitation of Fe hydroxides that 

can coprecipitate or adsorb As (Gao and 

others, 1988; Wilkie and Hering, 1996). 

Excluding light inhibits potential 

photochemical reactions (Emett and Khoe, 

2001; Hug and others, 2001) and storing the 

samples at 4°C slows chemical reactions. 

To address the need for more accurate 

determinations of As(T) and As(III) by 

hydride generation atomic absorption 

spectrometry (HGAAS), the U.S. Geological 

Survey (USGS) evaluated metal interferences 

and their removal in acid mine waters. This 

report (1) describes sample collection, 

preservation, and analytical procedures for 

the accurate determination of As(T) and 

As(III) using HGAAS; (2) identifies metals 

and the concentration at which each interferes 

in the determination of As(T) and As(III); (3) 

demonstrates that Cu(II) and Fe(III), both of 

which interfere in the determination of As(III) 

by HGAAS, can be removed from solutions 

using cation-exchange while maintaining 

existing As(III)/As(T) ratios; (4) applies the 

method to acid mine waters collected from 

the Summitville Mine, Colo.; Penn Mine, 

Calif.; and Richmond Mine, Calif.; and (5) 

presents As redox species time stability data 

for 45 surface- and ground-water samples 

collected during 1999-2002 from 

Yellowstone National Park, Wyo.; Questa 

Mine site, N. Mex.; Summitville Mine site, 

Colo.; Richmond Mine, Calif.; Penn Mine, 

Calif.; Ester Dome, Alaska; Fallon, Nev.; 

Mojave Desert, Calif.; and Kamchatka, 

Russia. 

2
Methods of Investigation 

Sample Collection and Preservation 

Water samples collected in the field 

were pumped from the source through a 0.1 

μm tortuous-path filter, acidified to pH less 

than 2 with hydrochloric acid (typically 2 mL 

6-M HCl per 250 mL sample), and stored in 

acid-washed opaque bottles at 4°C (To and 

others, 1999). 

Reagents 

All reagents were of purity at least 

equal to the reagent-grade standards of the 

American Chemical Society. Double-distilled 

water and re-distilled or trace metal grade 

acids were used in all preparations. The 

following reagents were used for the As(T) 

and As(III) HGAAS procedure: 10 percent 

(w/v) KI from Aldrich; 10 percent (w/v) LAscorbic Acid from Aldrich; NaOH from 

Fisher; NaBH4

 from Fisher; trace metal grade 

HCl from Fisher, Assay w/w 35 to 38 

percent; 1,000 mg/L As(III) from High Purity 

Standards; 1,000 mg/L As(V) from High 

Purity Standards; and cation-exchange resin, 

AG 50W-X8, 20-50 mesh, H+

 form from BioRad. 

The following single-element 

standards (1 to 10,000 mg/L) were prepared 

for interference studies: CuSO4⋅5H2O was 

used to prepare Cu(II); FeSO4

⋅7H2O was used 

to prepare Fe(II); Fe2(SO4)3 was used to 

prepare Fe(III); Zn(NO3)2⋅6H2O was used to 

prepare Zn(II); MnSO4⋅H2O was used to 

prepare Mn(II); Al(NO3)3⋅9H2O was used to 

prepare Al(III); Ni(SO4)⋅6H2O was used to 

prepare Ni(II); CrCl3⋅6H2

O was used to 

prepare Cr(III); K2Cr2O7 was used to prepare 

Cr(VI); CoCl2⋅6H2O was used to prepare 

Co(II); Cd powder was used to prepare 

Cd(II); NaSeO4

 was used to prepare Se(VI); 

Sb2O3

 was used to prepare Sb(III); SbCl5 was 

used to prepare Sb(V); and Pb metal was used 

to prepare Pb. The Se(IV) standard was 

prepared by adding an equal volume of 

concentrated HCl to a portion of Se(VI) 

standard and heating at 90°C for 20 minutes. 

Analytical Apparatus 

An atomic absorption 

spectrophotometer (Perkin-Elmer (PE) – 

AAnalyst 300) with an electrically heated 

quartz cell having a path length of 15-cm 

inline with a flow injection analysis system 

(FIAS; PE – FIAS 100), an autosampler (PE 

– AS90), and an arsenic electrodeless 

discharge lamp (EDL) attached to an EDL 

power supply (PE – EDL System 2) were 

used. The following spectrometer parameters 

were used: EDL current: 380 mv; 

wavelength: 193.7 nm; slit: 0.7 nm. Peak 

height was used for data processing. The 

following FIAS parameters were used: carrier 

gas: Ar; cell temperature: 900°C; sample 

loop: 500 μL; carrier solution: 10 percent 

(v/v) HCl; reducing agent: 0.25 percent 

(As(T)) or 0.03 percent (As(III)) NaBH4 in 

0.05 percent NaOH; carrier solution flow 

rate: 10 mL/min; reductant flow rate: 5 

mL/min. Sodium borohydride was prepared 

daily and filtered through a 0.45 μm 

polyvinylidene fluoride filter membrane using 

a vacuum pump. 

Analytical Procedures

Hydride formation occurs when 

As(III) in an acidic solution reacts with 

NaBH4 according to the following reaction:

3NaBH4 + 4H3AsO3 → 

4AsH3(g) + 3H3BO3 + 3NaOH (1) 

For the determination of As(T), As(V) is prereduced to As(III) using KI and L-ascorbic 

acid. 

All water samples and standards were 

prepared in 25-mL volumetric flasks. The 

3
method detection limits of the HGAAS 

analytical procedure used at the USGS 

National Research Laboratory in Boulder, 

Colo., are 0.1 μg/L for As(T) and 0.8 μg/L 

for As(III). 

Determination of Arsenic(T) 

(1) A solution containing 10 percent (w/v) KI 

and 10 percent (w/v) L-ascorbic acid was 

prepared. 

(2) To each 25-mL volumetric flask, 5 mL 

concentrated HCl and 2.5 mL of the 

KI−L-ascorbic acid solution prepared 

from step 1 were added. 

(3) The As(III) standard stock solution was 

used to prepare standards ranging in 

concentration from 0 to 10 μg/L As. 

(4) To demonstrate completeness of 

reduction, synthetic samples containing 

varying As(III)/As(V) ratios were 

prepared. 

(5) A volume of sample (up to 16.5 mL) that 

will yield 1 to 10 μg/L As(T) when 

diluted to volume was transferred to a 25-

mL volumetric flask. 

(6) Appropriate spike-recovery samples were 

prepared by adding both As(III) and 

As(V) to selected samples to demonstrate 

reduction of As(V) and AsH3(g) 

production. 

(7) The HGAAS analyses were performed a 

minimum of 30 minutes after sample 

preparation. 

Determination of Arsenic(III) 

(1) Twenty-five mL of concentrated HCl was 

added to each 25-mL volumetric flask. 

(2) As(III) standards ranging in concentration 

from 0 to 20 μg/L were prepared. 

(3) To demonstrate that As(V) was not 

reduced, synthetic samples were prepared 

containing several As(III)/As(V) ratios. 

(4) A volume of sample (up to 24.75 mL) 

that will yield 2 to 20 μg/L As(III) when 

diluted to volume was transferred to a

25-mL volumetric flask.

(5) Appropriate spike-recovery samples were 

prepared by adding both As(III) and 

As(V) to demonstrate that As(V) is not 

reduced and that adequate AsH3

(g) 

production occurs. 

(6) The HGAAS analysis was performed 

immediately.

Removal of Interfering Metals Prior to 

Arsenic(III) Determination 

For samples having a Cu(II)/As(III) 

molar ratio greater than 120, Fe(III)/As(III) 

molar ratio greater than 70, Cd/As(III) molar 

ratio greater than 800, or for samples with 

low As(III) spike recoveries, the sample was 

mixed with cation-exchange resin (AG 50WX8, 20-50 mesh, H+

 form) to remove 

interfering metals prior to determination of 

As(III). The following procedure was used to 

remove interfering metals and regenerate the 

resin:

(1) Approximately 100 grams dry resin was 

transferred to a 250-mL polyethylene 

bottle. The resin was washed 2 times

with 100 mL 2-M HCl followed by 5 

washes with distilled water. The excess 

water was decanted. 

(2) The resin was dried in an oven at 100°C. 

(3) To a 50-mL polyethylene bottle, 12.5 

grams dried resin and 25 mL sample 

were added. The mixture was 

periodically shaken for 30 minutes. 

(4) The sample was separated from the resin 

by decanting it into an acid-washed 15-

mL polyethylene bottle. This sample was 

used to determine the As(III) 

concentration. 

4
0.25 0.125 0.0625 0.031 0.016

-10

-8

-6

-4

-2

0

2

4

6

8

10

ARSENIC(V) R

EDUCTION,

IN PERCENT

SODIUM BOROHYDRIDECONCENTRATION,

IN PERCENT

As(V) concentration

 10 micrograms per liter

 20 micrograms per liter

 40 micrograms per liter

Results and Discussion 

Selective Reduction of Arsenic(III) 

During the As(III) determination, 

As(V) can be reduced to AsH3(g) by the 

NaBH4 reductant causing an overestimation 

of As(III) concentration. At a constant carrier 

concentration and FIAS flow rate, increasing 

the NaBH4 concentration increases the 

sensitivity of the As(III) determination (fig. 

1); however, when the NaBH4 concentration 

exceeds 0.0625 percent, As(V) is reduced, 

causing an overestimation of As(III) 

concentration (fig. 2). To ensure selective 

reduction of As(III), the NaBH4

 concentration 

is maintained at 0.03 percent for the As(III) 

determination for analyses described in this 

report. 

Figure 1. Effect of NaBH4

 concentration on 

the calibration curve in solutions containing 

only As(III) 

 Potential Metal Interferences on the 

Determination of Arsenic(T) 

Sixteen metal species including Al, 

Cd, Co, Cr(III), Cr(VI), Cu(II), Fe(III), Fe(II), 

Mn, Ni, Pb, Sb(III), Sb(V), Se(IV), Se(VI), 

Figure 2. Effect of NaBH4

 concentration on 

the reduction of As(V) to As(III) in solutions 

containing 10 μg/L As(III) and varying As(V) 

concentrations. 

And Zn were evaluated as possible 

interferents on the determination of As(T). 

The interference of each metal was evaluated 

by spiking synthetic samples containing 6 

μg/L As(III) and 6 μg/L As(V) with 0.002 to 

1,000 mg/L of the potential interfering metal 

and then determining the As(T) concentration 

using the HGAAS procedure (without cation 

exchange treatment). Arsenic(T) recoveries 

ranged from 92 to 102 percent for individual 

solutions spiked with Al, Cd, Co, Cr(VI), 

Cr(III), Cu(II), Fe(II), Fe(III), Mn, Ni, or Zn 

(fig. 3). For solutions spiked with a 

combination of metals including Al, Cd, Co, 

Cr(III), Cr(VI), Cu(II), Fe(II), Fe(III), Mn, 

Ni, and Zn, each up to a concentration of 500 

mg/L, As(T) recoveries ranged from 97 to 

102 percent. Dissolved Cu(II) was reduced to 

Cu metal during the pre-reduction step. The 

precipitated metal is visible at Cu 

concentrations greater than 100 mg/L and the 

liquid phase was decanted and analyzed with 

little change in As(T) concentration. 

Antimony(III) and Sb(V) were the only 

hydride-forming species found to interfere 

with the formation of AsH3(g) (fig. 4). 

Antimony(III) and Sb(V) substantially 

suppressed AsH3(g) formation by reacting 

0 5 10 15 20

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

NaBH4 Concentration

 0.250 percent

 0.125 percent

 0.063 percent

 0.031 percent

 0.016 percent

ABSORBANCE

ARSENIC(III) CONCENTRATION,

IN MICROGRAMS PER LITER

5
10 100 1000

70

75

80

85

90

95

100

105

 Al Cd

 Co Cr(III)

 Cr(VI) Cu

 Fe(II) Fe(III)

 Mn Ni

 Zn All 11 metals combined

RECOVERY, IN PERCENT

INTERFERENT CONCENTRATION,

IN MILLIGRAMS PER LITER

Figure 3. As(T) determinations in the 

presence of potential interfering metals. 

Figure 4. As(T) determinations in the 

presence of hydride-forming metals. 

with the NaBH4

 when the Sb(III)/As(T) molar 

ratio exceeded 4 or the Sb(V)/As(T) molar 

ratio exceeded 2. In most waters, 

concentrations of Sb(III) and Sb(V) are much 

lower than As(T) concentrations and are 

likely to be diluted (As(T) less than 10 μg/L) 

to a concentration that will not interfere with 

the As(T) determination. Alkali and alkaline 

earth elements do not interfere with the As(T) 

determination (Smith, 1975). 

Potential Metal Interferences on the 

Determination of Arsenic(III) 

The interference of 16 metal species 

on the determination of As(III) was tested by 

spiking synthetic samples containing 10 μg/L 

As(III) and 10 μg/L As(V) with 0.0001 to 

1,000 mg/L Al, Cd, Co, Cr(III), Cr(VI), 

Cu(II), Fe(III), Fe(II), Mn, Ni, Pb, Sb(III), 

Sb(V), Se(IV), Se(VI), and Zn and 

determining the As(III) concentration using 

the HGAAS procedure (without cationexchange treatment). Arsenic(III) recoveries 

for samples spiked with Al, Co, Cr(III), 

Fe(II), Mn, Ni, and Zn ranged from 84 to 107 

percent over the entire concentration range 

tested (fig. 5). 

0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

 Pb

 Sb(III)

 Sb(V)

 Se(IV)

RECOVERY, IN PERCENT

 Se(VI)

INTERFERENT CONCENTRATION,

IN MILLIGRAMS PER LITER

0 200 400 600 800 1000

50

60

70

80

90

100

110

Al Cr(III)

Co Fe(II) 

Mn Ni

RECOV

Zn All 7 metals combined

ERY, IN PERCENT

INTERFERENT CONCENTRATION,

IN MILLIGRAMS PER LITER

Figure 5. As(III) determinations in the 

presence of potential interfering metals. 

Low As(III) recoveries occurred when

Cd exceeded 25 mg/L, Cr(VI) exceeded 0.03 

mg/L, Cu(II) exceeded 2.0 mg/L, or Fe(III) 

exceeded 1.0 mg/L. Cadmium and Cu(II) are 

likely reduced by NaBH4

 in the FIAS reaction 

chamber and, consequently, arsine production 

is limited by the available NaBH4. 

Chromium(VI) and Fe(III) inhibit arsine 

production by either competing with As(III) 

for the available NaBH4 or oxidizing As(III) 

to As(V). Arsenic(III) is quickly oxidized as 

6
Fe(III) undergoes photoreduction (Emett and 

Khoe, 2001). These synthetic samples were 

prepared in clear volumetric flasks and 

exposed to light. Therefore, the low As(III) 

recovery in the presence of only Fe(III) may 

be due to photochemical reactions with 

Fe(III) prior to the As(III) determination. 

However, when Fe(II) concentrations were 

twice the concentration of Fe(III), low As(III) 

recoveries occurred only when Fe(III) 

exceeded 10 mg/L (fig. 6). Iron(II) relieves 

the interference from Fe(III), within a limited 

Fe(III) concentration range, by reacting with 

free radicals generated as Fe(III) undergoes 

photoreduction (Emett and Khoe, 2001). 

Figure 6. As(III) determinations in the 

presence of interfering metals. 

Among the hydride-forming metals, Sb(III) at 

concentrations greater than 0.1 mg/L, Sb(V) 

greater than 0.4 mg/L, and Se(IV) greater 

than 0.02 mg/L interfered with As(III) 

determinations, whereas Pb and Se(VI) did 

not (fig. 7). Antimony(III), Sb(V), and Se(IV) 

suppress AsH3(g) production by reacting with 

the available NaBH4. An additive effect of 

interfering metals on the determination of 

As(III) was demonstrated by adding 

increasing amounts of Fe(III) to a solution 

containing 0.5 mg/L Cu and 10 μg/L As(III) 

(fig. 8). Low As(III) recoveries occurred 

were: Cu(II) greater than 120, Fe(III) great

than 70, Cr(VI) greater than 2, Cd greater 

than 800, Sb(III) greater than 3, Sb(V) grea

than 12, or Se(IV) greater than 1. The sample 

could not be diluted to an As(III) 

concentration (As(III) less than 20

below which these interferences were abse

when the molar ratios of metals to As(III) 

er

ter

μg/L) 

nt.

Figure 7. As(III) determinations in the 

(III) and 

Copper(II) and Fe(III) are of primary concern 

because water generated from acid mine 

0.001 0.01 0.1 1 10

0

20

40

60

80

100

 Pb

 Sb(III)

 Sb(V)

 Se(IV)

 Se(VI)

RECOVERY, I

N PERCENT

INTERFERENT CONCENTRATION,

IN MILLIGRAMS PER LITER

1E-3 0.01 0.1 1 10 100 1000

0

20

40

60

80

100

 Cd

 Cr(VI)

 Cu

 Fe(III)

RE

 Fe(III) (with Fe(II)=2x Fe(III) added)

COVERY, IN 

P

E

R

CEN

T

INTERFERENT CONCENTRATION,

IN MILLIGRAMS PER LITER

presence of hydride-forming metals. 

0 2 4 6 8 10

50

60

70

80

90

100

110

 Fe(III) without Cu

RE

 Fe(III) with 0.5 milligrams per liter Cu

COVERY, IN PERCEN

T

IRON(III) CONCENTRATION,

IN MILLIGRAMS PER LITER

Figure 8. Additive interference in the 

determination of As(III) in a solution 

containing 10 micrograms per liter As

5 milligrams per liter Cu(II). 

7
drainage potentially contains high 

concentrations of Cu(II) and Fe(III). 

Cation Exchange Separation of Iron(III)

and Copper

ring metals can be removed 

using cation-exchange resin prior to the 

determ

nge

1.0 for

m

s

his

onstrated that AG 50W-X8 cationexchan

tion is

a

ccuracy and Time Stability of Arsenic 

Redox Species in Acid Mine Waters 

s

1

major metal species with concentrations that 

can var

the

the

s, as

as

ard addition

on sam

to

T)

ride 

ns

ge

S)

0)

Interfe

ination of As(III). The relative 

selectivity for AG 50W-X8 cation-excha

resin is 2.95 for Cd, 2.9 for Cu(II), and

H+

(Bio-Rad, 2003). Therefore, as long as the

capacity of the resin (1.7 millieqivalents 

(meq) per mL of resin) is not exceeded, Cd 

and Cu(II) are expected to be removed fro

solution. The relative selectivity for Fe(III) i

not published in the AG 50W instruction 

manual. 

Laboratory experiments done for t

report dem

ge resin in the H+

 form removed large 

amounts of Cu(II) and Fe(III) while 

maintaining the existing As(III)/As(T) ratio 

(table 1). When the As(III) concentra

high relative to that of the interfering metal, 

the sample may be diluted before analysis to

Cd, Cu(II), Cr(VI), Fe(III), Sb(III), or Se(IV)

concentration that does not interfere with the 

As(III) determination. Chromium(VI) and the

Se and Sb oxyanions will not be removed by 

cation exchange. 

A

Accuracy of Arsenic(T) Determination

Acid mine water may contain up to 3

y over several orders of magnitude

(Nordstrom and Alpers, 1999). Consequently,

interferences are likely during As redox 

determinations by HGAAS for acid mine 

waters. Samples of water collected from

Reynolds Adit and a monitoring well near

Chandler Adit on the Summitville Mine site,

the Richmond Mine, and the Penn Mine were 

analyzed for many dissolved constituents 

including As(T) and As(III). Concentrations 

of potential interfering metals for these 

samples are listed in Table 2. The samples 

contain elevated concentrations of metal

determined using inductively coupled plasm

optical emission spectroscopy (ICP-OES) and

colorimetric determinations for Fe redox 

species (To and others, 1999). 

Accuracy of the As(T) method wa

evaluated by performing a stand

ples collected from the Summitville 

Mine, Colo. (Reynolds Adit and a monitoring

well near Chandler Adit); the Richmond 

Mine, Calif.; and the Penn Mine, Calif. Spike 

recoveries for the samples ranged from 96

101 percent (fig. 9). Twenty samples from the

Summitville Mine site, 6 from the Richmond 

Mine, and 3 from the Penn Mine were 

analyzed by ICP-OES for As(T). 

Concentrations compared well with As(

concentrations determined by hyd

generation (fig. 10). All As(T) determinatio

were performed without cation-exchan

separation. As another measurement of 

accuracy for As(T) determinations, USGS

standard reference water samples (SRW

AMW4, T143, T155, and T159 (Farrar, 200

were analyzed as unknowns along with the

acid mine water samples. Measured 

concentrations compare well to the most 

probable values (MPV) (table 3). 

8
Table 1. Cation exchange separation of Cu and Fe(III)

[mg/L, milligrams per liter; %, percent; ---, not measured; > greater than] 

Initial Sample Concentration, mg/L % Recovery % Removal 

As(III) As(V) Cu Fe(III) Fe(II) As(III) As(T) Cu Fe(T)

0.010 0.010 --- --- --- 96 99 --- ---

0.010 0.010 25 --- --- 96 97 >99 ---

0.010 0.010 --- 100 200 99 102 --- >99

Figure 9. Spike recovery determinations of 

As(T) using hydride generation atomic 

absorption spectrometry.

0 1 2 3

0

1

2

3

 Summitville Mine (20 samples)

 Richmond Mine (6 samples)

 Penn Mine (3 samples)

ARSENIC(T) ANALYZED BY INDUTIVELY

COUPLED PLASMA-OPTICAL EMISSION

SPECTROMETRY, IN MILLI

GRAMS PER LITER

ARSENIC(T) ANALYZED BY HYDRIDE GENEREATIONATOMIC ABSORPTION SPECTROMETRY,

IN MILLIGRAMS PER LITER

70

80

90

100

Penn MineRichmond Mine

Chandler Adit - MW 1

Chandler Adit - MW 2

Reynolds Adit - 2

Reynolds Adit - 1

ARSENIC(T) RECOVERY,

IN PERCENT

ACID MINE WATER SAMPLES

Figure 10. As(T) determinations by inductively 

coupled plasma-optical emission 

spectrometry and hydride generation atomic 

absorption spectrometry for 20 samples 

collected from Summitville Mine, Colo., 6 

samples collected from Richmond Mine, Calif., 

and 3 samples collected from the Penn Mine, 

Calif.

9
Table 2. Composition of acid mine water samples collected from Summitville Mine, Colo.; Richmond 

Mine, Calif.; and Penn Mine, Calif. 

[As(T), As(III) plus As(V); HGAAS, hydride generation atomic absorption spectrometry; ICP-OES, inductively

coupled plasma-optical emission spectrometry; ISE, ion-selective electrode; mg/L, milligrams per liter]

Constituent 

mg/L Technique 

Chandler Adit

Monitoring Well Reynolds Adit Richmond Mine Penn Mine

pH ISE 2.7 2.8 0.9 3.2

As(T) ICP-OES 2.6 2.1 33 0.14

As(T) HGAAS 2.4 2.1 35 0.12

Al ICP-OES 170 270 1,300 5.0

Cd ICP-OES 0.088 0.52 14 0.023

Co ICP-OES 0.50 1.1 0.47 0.048

Cr ICP-OES 0.13 0.088 0.27 0.070

Cu ICP-OES 31 130 350 <0.01

Mn ICP-OES 13 39 16 8.2

Ni ICP-OES 0.73 1.7 1.4 0.044

Pb ICP-OES 0.19 1.1 4.1 0.022

Zn ICP-OES 80 35 2,100 12

Fe(II) Colorimetric 246 707 20,700 551

Fe(III) Colorimetric by

difference 

205 54 1,300 4.0

Table 3. As(T) determinations in U.S. Geological Survey standard reference water samples 

[μg/L, micrograms per liter; n, number of analyses; s, standard deviation; SRWS, standard reference water samples; 

MPV, most probable value; %RSD, percent relative standard deviation]

 

SRWS n As(T), μg/L s %RSD MPV, μg/L 

AMW4 25 170 4 2 168

T143 13 15 1 6 15.2

T155 6 33 1 2 32.9

T159 23 27 1 3 28.4

10
Accuracy of Arsenic(III) Determinations 

Accuracy of As(III) determinations, 

both with and without cation exchange, was 

estimated by spiking samples collected from 

the Summitville Mine (Reynolds Adit and 

Chandler Adit), the Richmond Mine, and the 

Penn Mine with As(III). Iron(III) and Cu(II) 

interfere with the determination of As(III) in 

samples collected from Summitville, but not 

with the Richmond or Penn Mine samples (fig 

11). Spike recoveries for Summitville Mine 

samples not separated by cation exchange 

were 44 to 53 percent, whereas recoveries for 

the same samples separated by cation 

exchange were 102 to 103 percent. The 

Richmond and Penn Mine samples were not 

separated with cation exchange resin because 

the ratios of Fe(III) and Cu(II) to As(III) were 

low enough that samples could be diluted to 

Fe(III) and Cu(II) concentrations that did not 

interfere with the determination. The As(III) 

concentrations were underestimated in the 

Summitville Mine samples not separated by 

cation exchange (fig. 12). No USGS standard 

reference water sample exists for As redox 

species. 

Figure 11. Spike recovery determinations 

using hydride generation atomic absorption 

spectrometry for As(III) in samples separated 

by cation exchange and not separated. 

Time Stability of Arsenic Redox Species 

The time stability of As(III) was 

monitored in 45 surface and ground water 

samples from Yellowstone National Park, 

Wyo.; Questa Mine site, N. Mex., 

Summitville Mine site, Colo., Richmond 

Mine, Calif., Penn Mine, Calif., Ester Dome, 

Alaska, Fallon, Nev., Mojave Desert, Calif., 

and Kamchatka, Russia. Samples were 

filtered through a 0.1-μm membrane, acidified

to pH <2 with HCl, and stored in opaque 

bottles at 4°C. Samples were reanalyzed as 

many as 15 months after the initial 

determination. The samples containing Cu(II) 

or Fe(III) concentrations that interfered with 

the As(III) determination were separated 

using cation exchange. The change in As(III) 

is represented by plotting percent difference 

in As(III) concentration as a function of the 

initial As(III)/As(T) ratio. The equation used 

to calculate percent difference (∆%) is: 

Initial As(III) conc.

(Initial As(III) conc.- Final As(III) conc.) 100 ∆ × % = (2) 

Reynolds Adit - 2

Reynolds Adit - 3

Chandler Monitoring Well

0.00

0.02

0.04

0.06

0.08

0.10

ARSENIC(III) CONCENTRATION

IN MILLIG

RAMS PER LITER

 Cation exchange separated

 Not separated

Reynolds Adit - 2

Reynolds Adit - 3

Chandler Monitoring Well

Richmond Mine

Penn Mine

0

20

40

60

80

100

120

 Cation exchange separated

 Not separated

ARSENIC(III) RECOVERY, 

IN PERCENT

Figure 12. As(III) determinations using hydride 

generation atomic absorption spectrometry 

for samples collected from the Summitville 

Mine. 

11
The As(T) concentration ranged from 0.006 to 

33 mg/L and the initial As(III)/As(T) ratio 

ranged from 0.01 to 1.0 (fig. 13). The curved 

line is a Gaussian Fit. The average percent 

difference (∆%) for all samples was 0.2 with a 

standard deviation of 7. 

-50 -40 -30 -20 -10 0 10 20 30 40 50

0

2

4

6

8

10

12

Reduction Oxidation

DIFFERENCE IN ARSENIC(III) CONCENTRATION,

IN PERCENT

 Frequency

Figure 13. Time stability of As(III) for 45 

surface and ground water samples from 

Yellowstone National Park, Wyo.; Questa 

Mine site, N. Mex.; Summitville Mine site, 

Colo.; Richmond Mine, Calif.; Penn Mine, 

Calif.; Ester Dome, Alaska; Fallon, Nev.; 

Mojave Desert, Calif.; and Kamchatka, Russia 

reanalyzed 3 to 15 months after the initial 

determination. The curved line is a Gaussian 

Fit. 

Summary 

Hydride generation atomic absorption 

spectrometry (HGAAS) is a sensitive and 

selective method for the determination of 

As(T) and arsenic(III); however, it is subject 

to metal interferences for acid mine waters. 

Antimony(III) and Sb(V) interfere with the 

HGAAS As(T) determination when the molar 

ratios of Sb(III) and Sb(V) exceed 4 and 2, 

respectively. Low As(III) recoveries occurred 

when the molar ratios of metals to As(III) 

were: Cu(II) greater than 120, Fe(III) greater 

than 70, Cr(VI) greater than 2, Cd greater 

than 800, Sb(III) greater than 3, Sb(V) greater 

than 12, or Se(IV) greater than 1. Samples 

could not be diluted to an As(III) 

concentration (As(III) less than 20 μg/L) 

below which these interferences were absent. 

Copper(II) and Fe(III) are of primary concern 

because water generated from acid mine 

drainage potentially contains high 

concentrations of Cu(II) and Fe(III). 

Separation of Fe(III) and Cu(II) from the 

sample while maintaining the existing 

As(III)/As(T) ratio can be achieved using 

cation exchange. 

Some acid mine waters contain high 

Fe(III) and Cu(II) and accurate measurements 

of As(III) is problematic without removing 

the Fe(III) and Cu(II) prior to hydride 

generation. Proper sample collection and 

preservation are critical to maintain the 

existing As(III/V) ratio prior to the analysis. 

Filtering samples through a 0.1 μm filter, 

acidifying with HCl to a pH less than 2, and 

storing in an opaque bottle at 4°C inhibits 

changes in the As(III)/As(T) ratios for up to 

15 months after sample collection. 

References Cited 

Bio-Rad, 2003, AG 50W and AG MP-50 

cation exchange resins, instruction manual: 

accessed April 28, 2003 at http://www.biorad.com/webmaster/pdfs/9118_AG_50.pdf

Cherry, J.A., Shaikh, A.U., Tallman, D.E., 

and Nicholson, R.V., 1979, Arsenic species 

as an indicator of redox conditions in 

groundwater: Journal of Hydrology, v. 43, 

p. 373-392. 

12
Creed, J.T., Magnuson, M.L., and Brockhoff, 

C.A., 1996, Arsenic determination in saline 

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gas-liquid separator for hydride generation 

inductively coupled plasma mass 

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Emett, M.T. and Khoe, G.H., 2001, 

Photochemical oxidation of arsenic by 

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K.M., Ball, J.W., and McCleskey, R.B. 

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Welsch, E.P., Crock, J.G., and Sanzolone, R., 

1990, Trace-level determination of arsenic 

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Welz, B. and Melcher, M., 1984, Mechanisms 

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13
Wing, R., Nordstrom, D.K., and Parks, G.A. 

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14