Analysis of Lead (Pb) in Antacids and
Calcium Compounds for Proposition 65 Compliance
Ruth E. Wolf
The Perkin-Elmer Corporation
761 Main Avenue, Norwalk, CT 05859-0324 USA
INTRODUCTION
The Safe Drinking Water & Toxic
Enforcement Act of 1985, or the
law more commonly referred to as
“California Proposition 65”, applies
to all companies with more than
10 employees doing business in the
state of California (1). Proposition
65 is a voter initiative passed to
address citizen concerns regarding
exposure to chemicals which cause
cancer or reproductive toxicity.
Under this law, businesses are prohibited from: (a) discharging listed
chemicals to potential sources of
drinking water; and (b) exposing
people in the state to listed chemicals without prior warning.
Lead is one of the elements
identified by the State of California
as both a cancer-causing agent and
a reproductive toxin (2). Under
California Proposition 65, the “no
significant risk level” or NSRL established by the Office of Environmental Health Hazard Assessment for
lead exposure is 0.5 µg/day (3).
One of the more common areas
of concern is that people who routinely use calcium-containing
dietary supplements and antacids
may exceed this limit. Since
antacids and dietary supplements
are commonly ingested by people
in significant amounts on a daily
basis, many manufacturers of supplements and the calcium-containing compounds used in them are
now testing these materials for lead
content. Since the actual dose may
vary due to intake rate, the level of
lead present in a material is generally reported in units of µg lead per
gram material (µg/g).
Since the NSRL level established
for lead is given as a total exposure
of 0.5 µg/day, it is necessary to
determine what detection levels are
AS
ABSTRACT
Compliance with California
Proposition 65 requirements
for the monitoring of lead in calcium-containing compounds used
in dietary supplements requires
the analytical methodology used
to have detection limits below
0.05 µg/g in the solid material.
The analytical capabilities of
Inductively Coupled Plasma Mass
Spectrometry (ICP-MS) make it
one of the techniques of choice
for performing lead analyses at
these low levels. Data are presented showing the accuracy and
precision of analyses performed
using ICP-MS. Interferences that
can occur during sample analysis
and the suitability of simple acid
dissolution techniques for various
calcium-containing matrices are
also discussed.
needed for monitoring purposes.
For example, the U.S. RDA (Recommended Daily Allowance) for calcium in the adult diet is 1000 mg.
If the entire RDA were to be
obtained from a single calciumcontaining supplement, the lead
concentration in that supplement
must be less than 0.5 µg/g. In order
to state that a material has a Pb
concentration less than 0.5 µg/g,
the detection limit of Pb by the
selected analytical technique must
be significantly below 0.5 µg/g to
ensure reliable and accurate results.
Ideally, the Limit of Detection
(LOD) should be a minimum of
10 times lower than the required
quantitation limit. For this example,
an analytical detection limit of 0.05
µg/g or lower would be necessary
to perform reliable quantitation of
lead at the required level. As a
result, suggested methods for analysis of lead at these low levels
include Graphite Furnace Atomic
Atomic Spectroscopy
Vol. 18(6), November/December 1997
169
Absorption (GFAA) and Inductively
Coupled Plasma Mass Spectrometry
(ICP-MS). Of the two methods,
comparison work done by a commercial laboratory has shown that
ICP-MS has better detection limits,
precision, and accuracy than GFAA
(4).
This paper will discuss the
ICP-MS determination of lead in
a variety of calcium sources, including calcium carbonate, anhydrous
dicalcium phosphate, dicalcium
phosphate dihydrate, and tricalcium
phosphate.
EXPERIMENTAL
Instrumentation
For this work, the Perkin-Elmer
SCIEX ELAN® 6000 ICP-MS (PerkinElmer SCIEX Instruments, Concord,
Ontario, Canada), equipped with a
Ryton® spray chamber, cross-flow
nebulizer, and a Perkin-Elmer®
AS-91 autosampler, was used to
perform the analysis of several calcium compounds after dissolution
in nitric acid. The instrument conditions used are given in Table I.
For most of the analyses
reported in this work, only lead
was determined in order to
illustrate the applicability of ICP-MS
for this particular analysis and the
speed with which it can be
performed. Additional analytes
including arsenic, cadmium,
chromium, and others may be
determined simultaneously using
ICP-MS and will be the topic of
a future publication. The total
acquisition time for each sample
was 18.5 seconds. Including sample
uptake, stabilization, and washout
time, a new sample was analyzed
approximately every 2 minutes.
The use of the autosampler and
automated quality control software
allowed complete unattended
operation, including monitoring
of replicate precision, check standards and internal standards.
Reagents and Standard
Solutions
Optima® (Fisher Scientific,
Pittsburgh, PA USA) nitric and
hydrochloric acids were used to
prepare all samples and standards.
Reagent water (18 MΩ or better),
prepared by mixed-bed ion
exchange (Continental Water Systems), was used to prepare all dilutions, blanks, and standards. Stock
multi-element standard solutions
including PE Pure AS Standards
from Perkin-Elmer (Norwalk, CT
USA), High-Purity Standards
(Charleston, SC USA), and from
Inorganic Ventures (Lakewood, NJ
USA) were used to prepare all standards, spikes, and internal standard
solutions. Certified standard reference materials SRM 1400 Bone Ash
and SRM 1486 Bone Meal were
obtained from the National Institute
TABLE I
Operating Conditions
ICP RF Power:
1200 W
Cones:
Nickel
Nebulizer
gas flow:
0.92–0.96 L/min
Analog detector voltage: –2200 V
Pulse stage detector
voltage:
1200 V
Lens voltage:
AutoLens™
Sample uptake rate:
1 mL/min
++
Ba /Ba ratio:
<3.0%
CeO/Ce ratio:
<3.0%
159
Isotopes monitored:
Tb, 165Ho,
209Bi, 206Pb, 207Pb, 208Pb
Scanning mode:
Peak hopping
Dwell time:
100 ms
Total integration time:
1 s per
isotope
Number of replicates:
3
Total analysis time:
18.5 s
of Standards and Technology (NIST,
Gaithersburg, MD USA). All standards
were prepared in pre-cleaned
polypropylene autosampler tubes
(Sarstadt, Germany) using prerinsed metal-free tips and mechanical air displacement pipettes (Fisher
Scientific, Pittsburgh, PA USA).
Sample Preparation
Samples of antacid tablets,
calcium carbonate, anhydrous dicalcium phosphate, dicalcium phosphate dihydrate, and tricalcium
phosphate were obtained from
various manufacturers for testing.
The samples were prepared using
a simple acid dissolution procedure.
Although some of the samples were
heated gently for 5–10 minutes to
aid dissolution, they were not rigorously digested for an extended
period of time. The samples were
either obtained in powder form
or crushed to produce a fine powder. For the antacid A, antacid B,
calcium carbonate, tricalcium phosphate-Lot A, NIST SRM 1400, and
NIST SRM 1486 samples, a 0.5-g
portion was accurately weighed
into a pre-cleaned, acid-soaked
Erlenmeyer flask. A second portion
of the tricalcium phosphate-Lot A,
antacid A, SRM 1400, and SRM 1486
samples was weighed out for the
purpose of performing a pre-dissolution spike. A small amount of
water was used to rinse the samples
into the flask and 5 mL of concentrated nitric acid was added to each
flask. To prepare the spikes, 500 µL
of a 10-mg/L stock standard solution was added to the second portion of tricalcium phosphate-Lot A,
antacid A, SRM 1400, and SRM
1486, giving a final spike concentration of 10 µg/g. The flasks were
put onto a hot plate and gently
warmed for 5–10 minutes to aid
dissolution. The antacid A, calcium
carbonate, tricalcium phosphate Lot A, and NIST SRM 1400 dissolved
immediately. The antacid B sample
required an additional 5 mL of nitric
acid before dissolving and SRM
170
1486 Bone Meal required both an
additional 5 mL of nitric acid and
5 mL of hydrochloric acid to go
into solution. After the samples
were dissolved, the flasks were
cooled and transferred into cleaned
polypropylene 50-mL autosampler
vials and diluted to 50 mL. The
total dissolved solids content of
these samples after dissolution
was approximately 1%.
The samples of anhydrous
dicalcium phosphate, dicalcium
phosphate dihydrate (Lots A, B,
and C), and tricalcium phosphateLot B were obtained in a powdered
form from the manufacturer. A 2.5-g
portion of each powder was accurately weighed and transferred into
a pre-cleaned 50-mL autosampler
tube using reagent water to rinse
the powders into the tube. Approximately 20 mL of reagent water and
10 mL of nitric acid were added
to each tube. After the powders
dissolved, the samples were taken
up to 50 mL using reagent water.
The total dissolved solids content
of these samples was approximately
5%.
Interferences
Two of the significant advantages
of ICP-MS include the sensitivity
and selectivity of the technique.
Any isobaric spectral overlaps that
occur when elements have isotopes
at the same nominal mass-to-charge
ratio are predictable and can generally be avoided by proper isotope
selection or corrected for using
interference correction equations.
Lead is a naturally occurring
element that has four isotopes:
204 Pb (1.4% abundant)
206 Pb (24.1% abundant)
207 Pb (22.1% abundant)
208 Pb (52.4% abundant)
The abundance of the Pb isotopes
may vary depending on the source
of the lead. Isotopes that originated
as part of the formation of the
galaxy are considered “stable”
and their isotopic composition has
AStomic
pectroscopy
Vol. 18(6), Nov./Dec. 1997
remained constant throughout geologic time (5). The second source
of lead is radiogenic lead or lead
that results from the radioactive
decay of an unstable parent. Three
of the isotopes of lead, 206Pb, 207Pb,
and 208Pb, are radiogenic decay
products of either uranium or thorium. The fourth lead isotope,
204
Pb, is considered “stable”, but is
rarely used for quantitation because
of its low abundance and isobaric
overlap with the 204Hg isotope.
208
Pb is the most abundant and
most frequently used isotope for
quantitation; however, different
sources of lead may have slightly
different 206Pb, 207Pb, and 208Pb
abundances. In order to avoid problems caused when the isotope
abundances of 206Pb, 207Pb, and
208Pb vary due to differing sources,
the isotopes of 206Pb, 207Pb, and
208Pb were added together using
the interference equation in Table
II to perform the lead calculations.
This is a commonly used elemental
correction and is recommended by
the U.S. EPA in Methods 200.8 and
6020 for regulatory analysis of environmental samples by ICP-MS (6,7).
TABLE II
Interference Equations
Element Interference Equation
Pb 208c = Pb 208u +Pb 207u+Pb 206u
where c = corrected
intensity
and u = uncorrected
intensity
Another type of interference that
can occur in ICP-MS includes signal
enhancement or suppression due
to the presence of matrix species.
This type of interference can be
corrected for by using internal standards and/or dilution of the sample
matrix. One such interference discovered in the course of this work
was that the calcium phosphate
sample matrix caused a delay in
achieving stable analyte and inter-
nal standard signals. It was found
that even in the diluted samples,
the presence of high levels of Ca
and POx (x=1–4) ions in the sample
matrix caused a delay in the stabilization of the analyte signal. One of
the results of this delay, if the analyst is not aware of its occurrence,
is that the analyte and internal standard element signals appear to be
significantly suppressed. This can
lead to over-correction of the
results by the internal standard
response and poor reproducibility
between individual replicates during the analysis, because the signal
is not at a steady state. Indeed, this
kind of temporal suppression was
observed due to the calcium matrices and the initial indications were
of severe (up to 70%) suppression
of the internal standards. Upon
closer examination, it was shown
that the signal maximum was slow
to occur and that the analytical
readings were taken during the stabilization period, leading to low
average signal levels and poor replicate standard deviations. The solution in this case was a simple one.
The read delay time or the waiting
time between the introduction of a
new sample and the reading of the
signal level by the ICP-MS was
increased from 10 seconds to 20
seconds. The result of the extra
read delay time was excellent precision between replicates and the
internal standard signals were no
longer artificially suppressed. The
cause of this type of matrix-induced
interference is beyond the scope of
this work, but has been recently
researched and discussed by others
(8).
Sample Analysis
A 1:20 dilution was performed
on all the dissolved samples using
reagent water as the diluent. After
this dilution, the total dissolved
solids content of the samples was
0.1–0.5% depending on the dissolution procedure used. All calibration
blanks, standards, and diluted sam-
171
ples were spiked with 20 µg/L of
Bi, In, Tb, and Ho as internal standards. The samples were capped
and shaken well to mix before placing on the autosampler. SRM 1486
had some grease residue formation
after cooling. The aliquot taken
from this sample for dilution was
obtained by inserting the pipette
tip well into the autosampler vessel
and withdrawing an aliquot away
from the surface layer of residue.
This aliquot was diluted 1:20 with
reagent water.
The instrument was calibrated
for Pb at 0.1, 1, 5, and 10 µg/L
using calibration standards
prepared in 1% nitric acid. The
correlation coefficient obtained
for the Pb calibration curve using
a linear through zero curve type
was 0.9999. Check standards were
run every 10–15 samples and the
internal standard levels were monitored throughout the course of the
analysis by the automated native
ELAN NT quality control software.
RESULTS AND DISCUSSION
Detection Limits
The detection limits were calculated by analyzing 1% nitric acid
blanks, tricalcium phosphate
(Ca3PO42), and calcium carbonate
(CaCO3) matrices. The Instrument
Detection Limit (IDL) is a measurement of the best achievable detection limit of an instrument and is
defined by IUPAC (International
Union of Pure and Applied
Chemists) as the concentration that
produces a net intensity equivalent
to three times the standard deviation of the background signal. Since
IDLs are typically measured in clean
solutions, such as nitric acid blanks,
they may be unrealistically low for
most practical analyses. They may,
however, be useful in comparing
different techniques and instrumentation. The IDL was calculated by
the following:
IDL = 3(σ1% nitric acid Pb concentration )
where σ is the standard deviation of a minimum of seven replicate measurements.
Since most of the measurements
made for this application are on
solid samples, it is appropriate to
determine the detection limit in the
solid. The IDL in the solid was calculated based upon the analytical
dilution (1:20) used during the
analysis and the sample preparation
method. In this case, the IDL (µg/g)
was calculated as:
IDL in µg/g =
(IDL in µg/L)(20)(1L/1000 mL)
(50 mL final volume/2.5 g sample)
Since the determination of lead
in calcium-containing matrices is
the goal of this study and the detection limits may differ from that in
a clean nitric acid solution, the
Method Detection Limits (MDLs)
for the calcium phosphate and calcium carbonate matrices were
determined. The Pb concentration
in eight individual aliquots of each
matrix was measured. The MDL
was calculated as follows:
MDL =
3(σblank matrix Pb concentration )
The MDL in the solid was calculated based upon the 1:20 analytical
dilution used during the analysis
and the sample preparation
method. In this case, the MDL
(µg/g) was calculated as:
MDL in µg/g =
(MDL in µg/L)(20)(1L/1000 mL)(50
mL final volume/2.5 g sample)
Based on the data in Table III,
it can be concluded that the detection limits for Pb using ICP-MS are
well below the limit necessary for
the analysis of Pb in calcium matrices, assuming the adult dosage of
1000 mg/day. In fact, when the
ratio of the NSRL of 0.5 µg/g to the
MDL is calculated, the method
detection limits are between 60 and
250 times below the NSRL.
Method Validation and Accuracy
In order to assess the accuracy
of the method, two NIST Standard
Reference Materials (SRMs) were
prepared and run along with the
samples. SRMs are useful in assessing overall method accuracy and
evaluating potential method bias,
because they are prepared as homogeneous materials and are certified
for element concentrations using
more than one analytical technique.
The two SRMs used for this work
were SRM 1400 Bone Ash and
SRM 1486 Bone Meal. The results
obtained are shown in Table IV.
Each result is the mean value of
three replicate measurements of
the prepared sample and the error
listed is the standard deviation of
the replicate measurements in concentration units.
The agreement between the
ICP-MS results for SRM 1400 and
the NIST certified value for lead is
excellent, indicating that the dissolution and analytical methods used
will give an accurate, non-biased
result. Since the matrix in SRM
1400 is essentially an inorganic
matrix composed primarily of
calcium phosphate, it can be concluded that the ICP-MS method
discussed here will work well for
this type of sample. In addition, the
pre-dissolution and post-dissolution
spike recoveries are excellent, indi-
cating that no analyte was lost in
the dissolution procedure and that
no matrix effects have biased the
results.
Although the values obtained
for SRM 1486 are not within the
stated confidence levels, they are
within 90% of the stated mean
value. It is believed that the result
is lower than the stated value due
to the sample preparation method
used. It is possible that some of the
Pb remained in the residue which
was not sampled when the digestate was prepared for analysis.
Again, the pre-dissolution and postdissolution spike recoveries for
SRM 1486 are excellent. It can be
concluded from this data that for
materials having some organic
components, a more rigorous
digestion procedure will have to
be employed to obtain more accurate results. Further studies are
planned to verify this conclusion.
The results for the nine individual samples are shown in Table V.
Each result shown is the average
of three measurements. The Relative Standard Deviation (%RSD) is
also given as a measurement of the
precision of the analyses. In all
cases, better than 3% RSDs were
obtained for each matrix, indicating
the excellent precision obtained
using ICP-MS. Two samples were
selected to have pre-dissolution
TABLE III – Detection Limits for Lead
Calcium Phosphate
Calcium Carbonate
Matrix IDL =(3)(σ1% nitric acid) MDL =(3)(σsample matrix) MDL =(3)(σsample matrix)
Solution
0.0008 µg/L
0.005 µg/L
0.008 µg/L
Solid
0.0003 µg/g
0.002 µg/g
0.003 µg/g
Sample ID
SRM 1400
SRM 1486
TABLE IV – SRM Results
Measured
NIST Certified 10 µg/g preconcn
value
dissolution
(µg/g)
(µg/g)
spike
recovery
9.10 ± 0.11
9.07 ± 0.12
106%
1.207 ± 0.008
172
1.335 ± 0.014
101%
0.1 µg/g postdissolution
spike
recovery
109%
99%
AStomic
pectroscopy
Vol. 18(6), Nov./Dec. 1997
spikes performed on them. These
samples were dissolved with the
addition of acid and mild heating
as discussed in the sample preparation section. The spike recoveries
are excellent and are generally
within ±10% of the spiked value.
The post-dissolution spike recoveries are also excellent and are all
within ±11% of the spiked value,
indicating that the matrix is not
affecting the detection of small
amounts of lead or changes in concentration to any significant extent.
Long-Term Stability
One of the known limitations
of ICP-MS is the ability of the instrumentation to handle high levels of
dissolved solids for a long period
of time. Since the commercial
development of the technique in
1984, instrument manufacturers
have recommended that users keep
the total dissolved solids content of
the samples run on ICP-MS below
0.1–0.2% total dissolved solids for
best instrument performance and
long-term stability. Because of the
use of interface cones with small
orifices (0.8–1.1 mm) between the
Table V – Sample Results
Sample
Tricalcium Phosphate-A
Tricalcium Phosphate-B
Calcium Carbonate
Antacid - A
Antacid - B
Anhydrous Dicalcium
Phosphate
Dicalcium Phosphate
Dihydrate-A
Dicalcium Phosphate
Dihydrate-B
Dicalcium Phosphate
Dihydrat -C
10 µg/g predissolution
spike
recovery
(%)
0.05 µg/g postdissolution
spike
recovery
(%)
Measured
concn
(µg/g)
%RSD
n=3
0.105
0.108
0.315
0.114
0.259
0.88
0.60
1.03
2.84
1.28
0.089
0.62
89
0.093
0.70
90
0.159
0.89
101
0.168
0.75
102
99
99
92
90
93
106
90
Percent of original value
Internal Standard Response
120%
100%
80%
60%
Ho 165
40%
20%
0%
0:00
1:12
2:24
3:36
Elapsed Time (hours:minutes)
4:48
Fig. 1. Internal Standard Response vs Time.
173
ICP and the entrance to the ion
optic region of the mass spectrometer, running samples with high
levels of dissolved solids can eventually cause buildup and blockage
of the cone orifices. For this reason,
the use of internal standards
to correct for instrument drift as
the material slowly builds up on
the interface cones is a commonly
used and necessary practice in
ICP-MS analysis. For this work,
holmium (Ho) was used as the
internal standard for Pb because
it is similar in mass to lead and was
not already present in the samples.
Figure 1 shows the Ho signal measured in a 0.1-ppb calibration check
solution as a function of elapsed
time as the calcium samples were
analyzed. The response in a check
standard is used for this drift determination because there is no matrix
in the check standard that may
cause additional suppression of the
signal, giving an indication of the
degree of material present on the
interface cones. The y-axis shows
the Ho signal as a percent of the
original signal level measured in
the calibration blank. This value is
automatically calculated and monitored by the ELAN NT quality control software. The dissolved solids
levels of the samples analyzed in
this study varied between 0.1–0.5%
which is at the upper limit of what
commercial ICP-MS instrumentation
is designed to tolerate. A total of
123 samples were analyzed during
the nearly five-hour time period
shown in Figure 1. As illustrated
by Figure 1, there was indeed a
gradual drop in the internal standard signal as material built up on
the interface cones. This decrease
in internal standard intensities
down to approximately 80% of
the original value measured in the
calibration blank is typical for the
analysis of these types of samples
over several hours. Indeed, the
internal standard is still well within
its ability to measure low level
concentrations accurately as is
illustrated by Figure 2.
CONCLUSION
It has been shown that the use
of ICP-MS can provide accurate and
precise results for the determination of lead in a variety of calcium
matrices. Selection of suitable sample uptake and read delay rates was
critical in obtaining reproducible
and accurate data. Compared to
clean nitric acid matrices, it was necessary to increase read delay times
in order to deal with delayed signal
response times caused by the sample matrix. Even with the use of
long sample uptake and read delay
times, the total time for each sample analysis was less than two minutes. Simple acid dissolution with
0.1 ppb Calibration Check Recovery
Recovery (%)
120
Check Standard
Figure 2 shows the percent
recovery of the 0.1 ppb calibration
check standard over the same time
period as Figure 1. As illustrated by
Figure 2, even though the internal
standard response is only 80% of
the original value, the measured
concentration of the check
standard remains stable at ±10%
of the true value for the duration
of the analysis. In fact the measured
values stay within 99–107% of the
actual value of the 0.1 ppb standard
for nearly five hours. The internal
standard response in the samples
varied during this time period from
60–80% of the value measured in
the calibration blank. The additional
suppression seen during the analysis of the samples indicates some
suppression of signal is also occurring due to the sample matrix.
Although the overall signal intensities dropped over the course of
the four hours, the precision of the
three replicate measurements was
not degraded. Furthermore, the
excellent spike recoveries obtained
in the sample matrices indicate that
this suppression is being correctly
compensated for by the internal
standards and that the accuracy
of the results is not being adversely
affected by the matrix suppression.
110
Pb 208
100
90
80
0:00
1:12
2:24
3:36
4:48
Elapsed time (hours:minutes)
Fig. 2. Check Standard Recovery vs. Time.
and without heating was shown to
be an effective sample preparation
method for all matrices except
SRM 1486 Bone Meal which
appeared to have significant
organic content present as fats.
The detection limits obtained in
calcium phosphate and calcium
carbonate are suitably low for the
determination of lead in calcium
supplements below the NSRL of
0.5 µg/day.
6. “Method 200.8, Determination of
Trace Elements in Water and Wastes
by Inductively Coupled Plasma Mass
Spectrometry,” in Methods for the
Determination of Metals in Environmental Samples, Supplement I,
EPA/600/R-94/111, USEPA Monitoring Systems Laboratory, Cincinnati,
OH (May 1994).
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8. “Fundamental Investigations of
Steady State and Transient Acid
Effects in ICP-AES and ICP-MS,” Ian
Stewart and John W. Olesik, The
Ohio State University, Paper No. 662,
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Spectroscopy Societies (FACSS),
Providence, RI (October 1997).
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Enforcement Act of 1986,” California
Code of Regulations, Sections
25249.5 and 25249.6.
2. “List of Chemicals Known to the
State to Cause Cancer or Reproductive Toxicity,” California Code of
Regulations, Title 22, Section 12000,
August 26, 1997.
3. “Method of Detection Argument in
the Crystal Glassware Case,” Prop 65
News 9 (7) (July 1994).
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Application Note, West Coast Analytical Services Inc., 9840 Alburtis
Avenue, Santa Fe Springs, CA 90670
USA (1997).
5. Handbook of Inductively Coupled
Plasma Mass Spectrometry, K. E.
Jarvis, A.L. Gray, and R.S. Houk,
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7. “Method 6020, Inductively Coupled
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