United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/S4-89/043 Apr. 1990
Project Summary
Pre-Concentration Method for
Inductively Coupled Plasma-
Mass Spectrometry
J. T. Rowan and E. M. Heithmar
A semi-automated system is used
to pre-concentrate Ti, V, Mn, Fe, Co,
Ni, Cu, Cd, and Pb prior to analysis by
inductively coupled plasma-mass
spectrometry (ICP-MS). The pre-
concentration system accepts
digests with a broad range of acid
concentrations, neutralizes them, and
loads them onto a macroporous
imlnodlacetate resin. Alkali and
alkaline earth metals, along with
deleterious anions such as chloride,
are washed off the resin before the
concentrated analytes are eluted with
nitric acid. Measurement of a total of
13 Isotopes of the analytes, as well
as two Internal standard elements
added to the eluent stream, indicates
that the technique enhances the ICP-
MS response of the target metals.
Investigation of the nature of the
blank signals suggests that the
detection limits of several of the
isotopes could benefit by much
larger pre-concentration factors,
while those of copper, cadmium and
lead are currently limited by reagent
purity. Method performance data is
presented for several simple
synthetic matrices, synthetic sea
water, two waste waters and a natural
surface water.
This report was submitted in
fulfillment of contract number 68-03-
3249 by Lockheed Engineering and
Sciences Company under the
sponsorship of the U.S.
Environmental Protection Agency.
This report covers a period from
October 1, 1988 to September 30,
1989. Work is on-going.
This Pro/ect Summary was
developed by EPA'a Environmental
Monitoring Systems Laboratory, Las
Vegas, NV, to announce key findings
of the research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
The analysis of environmental samples
for trace elements poses two major
problems. First, the regulatory action
levels of several metals are in the low
part-per-billion range, and reliable
quantitation requires analytical methods
with detection limits 5-10 fold lower than
these threshold levels. Second, the
matrices encountered in environmental
samples are extremely diverse and often
highly complex. Because of the need for
low detection limits, graphite furnace
atomic absorption spectrometry (GFAAS)
and, more recently, inductively coupled
plasma-mass spectrometry (ICP-MS)
have been employed in environmental
analysis. ICP-MS has the advantage over
GFAAS of being a multi-elemental
technique, but there is incomplete
performance data for ICP-MS analyses of
target elements in all matrices of interest
to the U.S. Environmental Protection
Agency.
One of the major drawbacks of ICP-MS
is the interferences often exhibited with
the complex matrices encountered in
environmental analysis. Pre-concentration
can be used to separate analytes from in-
terferences prior to analysis. Separation
is effected by complexation of analytes to
an immobilized form of an iminodiacetate
chelating agent. The functional group is
covalently bonded to a highly cross-
linked divinyl benzene, polystyrene
copolymer resin. The iminodiacetate
strongly complexes transition metals and
certain main group metal ions, such as
lead, while showing very little affinity for
alkali and alkaline earth metals which can
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interfere with subsequent analyses. The
analytes can then be released by
changing the ionic form of the resin. This
approach allows the development of
semi-automated methods that make use
of resin-packed columns.
Procedure
Acidified samples are buffered to pH
5.5 by on-line mixing with 2-M
ammonium acetate and then fill a sample
loop of appropriate volume for the degree
of pre-concentration desired. Prior to the
pre-concentration step, the resin is
placed in the ammonium form using 2-M
ammonium acetate. When the pre-
concentration program is initiated, a high
pressure pump sweeps the sample to the
resin column with 2-M ammonium
acetate at 3 mL/min, pre-concentrating
the analytes, and passing the rest of the
sample matrix, including alkali metals and
anions, to waste. The ensuing wash-out
with additional ammonium acetate
replaces chelated Mg2+ and Ca2+ with
NH4 + . The high pressure pump then
switches to 1-M nitric acid, passing the
eluted trace metals to the ICP-MS. After
the analyte peaks have passed, 3 mL/min
of 2-M ammonium acetate is passed
through the column for 1 minute. This is
followed by successive 3-mL rinses with
nitric acid, ammonium acetate, nitric acid,
and finally ammonium acetate, again,
each at a rate of 3 mL/min
Mass spectra are obtained using the
"Multiple Elements" program of the
Perkin-Elmer ELAN ICP-MS. The
measurement routine allows the
observation of each peak every 0.67
seconds, and a temporal resolution of 2
seconds. The spectrum is converted to
an ASCII file and automatically sent via
the Kermit communications protocol to an
80386-based microcomputer for
processing. The spectrum is placed in a
commercial spreadsheet by a macro that
automatically finds the analyte peaks and
integrates each over the optimal
detection period.
Results and Discussion
Peak areas were used for quantitation.
Peak widths, and therefore peak heights,
sometimes varied considerably during
the day. Peak areas, conversely, were
very reproducible. Calibration curves
were determined for each of the analytes
from 0.3 ^g/L to 30 iig/L using a 2.5-mL
sample loop, and from 0.3 to 10 pg/L
using a 10-mL sample loop (the iron
calibration curves were determined using
ten times these concentrations). All
isotopic calibrations were linear, with the
correlation coefficients ranging from
0.998 to 0.9999. Generally, the long-term
precision of the method was good, but
the sensitivity for lead tended to drift
during the day. Lead calibrations and
analyses used the internal standard
intensity of bismuth to correct for this
drift.
Only in the cases of copper, cadmium
and lead, as well as 49Ti, would more
rigorous reagent purification and stringent
clean-room conditions significantly
improve detection limits over those
obtained in this study. Conversely, the
detection limits of the other isotopes
would definitely improve with larger
sample sizes. Even in the cases of
copper, cadmium, and lead, some
improvement with increased sample size
is still expected.
The efficiency of the pre-concentration
system was studied by comparing total
integrated signals for 10-^/L solutions of
each of the analytes, directly aspirated
into the ICP-MS, with the integrated
signals after pre-concentration.
Ammonium nitrate, which was present in
the elution front, apparently caused a
pronounced enhancement of signal that
gave higher sensitivities than would be
expected by pre-concentration alone.
The on-line buffering technique
adequately buffered standard solutions,
ranging from 0.8% to 1.4% (v:v) nitric
acid, to a pH of 5.4 ± 0.2. Analyte
recoveries from these solutions ranged
from 90% to 103% relative to the
recoveries from a standard solution in 1 %
nitric acid. The on-line mixing of samples
and buffer caused only a 20% dilution of
the samples, and eliminated hydrolysis
problems which can be caused by
prolonged exposure of analytes to non-
acidic conditions.
In early work, some analytes exhibited
elevated apparent concentrations in the
first blank after a high concentration
standard. It is not clear why vanadium,
and to a lesser extent, titanium, cobalt
and copper exhibited memory. It was
later determined that alternating the resin
between the hydronium and ammonium
forms three times between runs of the
pre-concentration program reduced the
effect. This cycling of ionic forms was
therefore added to the prescribed
procedure.
The degree of residual interferences
from molecular ions of alkali and alkaline
earth metals, as well as chloride, was
examined as a function of pre-elution
wash-out time. At a concentration of 2000
mg/L, only calcium produced residual
spectral interferences when the washi
times were at least two minutes. The o
spectral interferences remaining from
synthetic sea-water matrix after a tv
minute wash-out were relatively SIT
residual 4«ca + and CaOH+ peaks. Mi
spike recoveries for high concentrati
salt matrices fell between 85% a
115%.
A major drawback to pre-concentrati
methodologies is the low recover!
obtained in the presence of certi
organic chelators. Titanium, coppi
cadmium and lead showed very pc
recoveries in the raw waste wate
included in this study. A simp
microwave digestion procedure, howev
was sufficient to liberate the bou
metals and restore full recoveries.
Conclusions and
Recommendations
On-line pre-concentration with ;
iminodiacetate resin minimizes seve
spectral and physico-chemic
interferences in ICP-MS. Interferenc
from sodium, potassium, and magnesii
are largely eliminated, and that frc
calcium is greatly attenuated. Anio
which can cause severe spectr
interferences, such as chloride, are al
removed. Native organic chelatii
agents, which might reduce pr
concentration recoveries, are eliminat
from samples by a nitric acid digestion.
The efficiency and cost-effectivene
of this method could be improved w
expansion to an automated, multip
column system, wherein many sampl
would be in various stages of analysis
a given time. Unfortunately, the curre
status of ICP-MS software mak<
complete automation of the pr
concentration techniques problemati
Not only is satisfactory support f
transient signals lacking, but so is tl
hardware and software which would mal
interfacing a less daunting task.
Work on the pre-concentration metru
should continue, especially to extend i
application to ICP-AES. Further effc
should be expended to reduce tl
memory exhibited by some analyte
Alternate chemistries which might mal
possible the pre-concentration of arser
and selenium should be investigate
Finally, a multilaboratory evaluatic
should be conducted to assess tt
routine performance of the pn
concentration method coupled with bo
ICP-MS and ICP-AES.
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J. T. Rowan is with Lockheed Engineering and Sciences Company, Las Vegas,
NV 89119
£. M. Heithmar is the EPA Project Officer (see below)..
The complete report, entitled "Pre-Concentration Method for Inductively Coupled
Plasma-Mass Spectrometry," (Order No. PB 90-181348 ;Cost: $17.00 subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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