氢化物原子吸收在形态分析中的应用---砷形态分析

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 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 plasma-optical
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)

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
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 (H2S), 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.

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 acidwashed
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 Bio-
Rad.
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⋅6H2O 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.

嗯，不错。
现在国内做得也很好，不仅可以测定总量，三价，五价，而且有机砷也可以测定。

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
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 acidwashed
15-mL polyethylene bottle.
This sample was used to determine the
As(III) concentration.