Background
Since the widespread commercial
introduction of ICPMS as an accepted
trace metal analysis technique in the
early 1980’s, the analytical limits of the
technique
are
continually
being
challenged.
Quadrupole ICPMS
maintains a large share of this analytical
market with its reputation for robust
hardware and quick multi-element
analysis. Continual development of new
applications for trace metal ICPMS
analysis in fields such as biotechnology,
semiconductor/integrated circuit design,
proteomics, and pharmacology ensures
the longevity of this technique.
About the time that ICPMS was
spreading into analytical laboratories
around the world, Seastar Chemicals Inc.
began developing our line of high purity
acids and ammonia. In the late 1970’s,
our foundations began with the redistillation of reagent grade acids for
sub-part-per-trillion (ppt) oceanographic
determinations. Realizing that other
scientists faced the same frustrations, we
began the research and development that
would culminate in our twenty-five
years of expertise purifying mineral
acids. Today, Seastar Chemicals designs
and manufactures our own distillation
apparatus and trace metal clean rooms,
as well as continually develops and
refines our analytical techniques and
methodologies. With these tools, we
are progressively expanding the lower
limit of analytical capabilities by ICPMS
to achieve our ultimate goal of
producing the highest purity acid for
trace metal analysis in the world. We
distribute these products through a
number of brand-name global partners.
The Challenge
Since Q-ICPMS is not actually a
simultaneous multielement analysis
technique (although it is very fast
sequential analysis) the limit of detection
is the critical measure of performance
(unlike for a multicollector mass
analyzer instrument). The detection
limit capability of ICPMS can be in
the 0.1 to 10ppt level for many
elements, but this is usually a blank
limited phenomenon. Poor blanks can
frustrate the determination of critical
elements. For ideal signal evaluation,
the blank should be no greater than 10%
of the sample analyte signals. Blank
values and RSD’s are not solely a
product of instrumental parameters.
Detection limits (3s blank) are ultimately
a function of the following three factors:
1) signal to noise ratio 2) net counting
rate (sensitivity, cps) and 3) counting
(dwell) time. The analyst’s key to
obtaining consistently low detection
limits is the analytical blank.
Blank analysis is critical for a number of
reasons. Many analyses require blank
subtraction for calculation of final
results, the blank is used to calculate the
method detection limit, and blank
control charting for reagents and
processes provides an early warning
system for errors. Poor blank analysis
ultimately
leads
to
increased
measurement uncertainty.
The uncertainties in measurements are
primarily caused by questionable reagent
purity (leading to inaccurate blank
measurement/correction
and
poor
MDL’s), sample handling (sample prep,
analyst
skill,
metrology
and
contamination), and instrument effects
(accuracy of the calibration, stability and
carryover).
After
appropriate
instrumental tuning and verification of
the calibration, the goal for our quality
control program at Seastar Chemicals is
maximizing the signal to noise ratio and
thereby achieving the best detection
limits and RSD’s in order to analyze and
certify our high purity reagents. We are
constantly trying to quantify the blank
therefore we also use another measure of
detection limit – the background
equivalent concentration (BEC). This is
often a more valid means of expressing
the instrumental detection limit. The
BEC is defined as the blank (cps)
divided by the sensitivity (cps/unit
concentration) for the isotope in
question. The BEC more accurately
accounts for all sources contributing to
the analytical blank such as the sample
introduction system, torch, cones,
interface, lenses and of course the
sample itself2. The LOD (3s blank) may
be an overly optimistic value if precision
(i.e. standard deviation) is good, even if
the actual blank counts are high.
Strategies & Techniques:
To reach or exceed the technological
capability of the instrument hardware it
is necessary to use the cleanest reagents.
Even using the highest purity reagents,
the analytical blank can be a function of
the cleanliness/contamination during
sample handling and matrix issues. The
most commonly contaminated elements
in a typical laboratory environment are
iron, zinc, calcium, potassium, sodium,
aluminum, and boron. We employ a
number of techniques to reduce sample
contamination. To effectively analyze
the variety of high purity acids matrices
in the quality control lab at Seastar, we
must
consider
issues
such
as
contamination
during
sample
preparation,
matrix
suppression,
instrument and equipment damage from
a concentrated acid environment, and
matrix dependent isobaric interferences.
From a sample handling point of view,
all our analytical processes occur in class
100 clean rooms with critical work
completed in class 10 exhausted laminar
flow fume hoods. We use Gore-Tex ™
Ultra Low Particulate Air (ULPA) filters
in the hoods and above sample prep
areas such as the analytical balance and
instrument autosamplers. All samples
are taken and prepared in pre-cleaned
Teflon PFA or FEP bottles. As certain
elements adsorb onto storage container
surfaces, the addition of a small aliquot
of high purity acid stabilizes the
elements in solution for accurate
concentration analysis if the solution is
not already acidic. Nitric acid is popular
for this application due to its chemical
compatibility, oxidizer capability in its
concentrated form and its high purity for
a reasonable cost relative to other high
purity reagents.
method detection limit which will be the
BEC
(background
equivalent
concentration) multiplied by the dilution
factor. Depending on the dilution factor,
the detection limit or BEC can become
quite large. Additional disadvantages of
dilution and direct injection are that
samples are more corrosive to the
instrument than those which have had
the concentrated acid matrix removed.
Recalibration and stabilization of the
instrument is required for each type of
concentrated acid matrix dilution.
Matrix Removal, Pre-concentration
and Reconstitution:
Methods of evaluation for low and sub
ppt measurements require a maximized
analyte signal to noise ratio.
For
matrices uncomplicated by isobaric
interferences, direct analysis may be
possible for the majority of elements
depending on the detection limits
required. For more challenging matrices
a method of matrix removal is the best
way to combat signal suppression,
matrix induced isobaric interferences,
and wear on the instrument.
Dilution with High Purity Water and
Direct Injection:
To control contamination by limiting
sample handling, sample dissolution
and/or dilution with high purity acids is
an acceptable technique. It provides the
advantage of a standard additions
calibration without the necessity for
internal standardization of all samples.
Also, volatile and refractory elements
(Hg, Se, Pt, and Pd) can be quantified
without loss of recovery.
But, as
previously stated, the quantification of
certain elements will be limited by the
Matrix removal has two main options:
open
beaker
and
closed
cell
evaporations. Both methodologies can
be
successfully
employed
to
preconcentrate by evaporation to dryness
followed by reconstitution. Even with
our virgin polypropylene laminar flow
clean hoods and cabinetry, as well as
ULPA filters to control contamination,
analytical blank from open beaker
evaporations can still be troublesome.
Elements such as Boron, Calcium,
Sodium, Iron, aluminum and zinc still
contaminate open beaker evaporations.
To address these issues, Seastar has a
continuously evolving QC project for the
past ten years. We’ve redesigned and
manufactured a closed cell evaporation
unit modeled originally from the first
one described at FACSS in 1994 1. In
ten years, our version has evolved from a
simple lamp with flexible foil heat
containment to a true cell design with
ceramic IR heat.
The cells can
accommodate 500mL of concentrated
acid and evaporate to dryness while
being purged with filtered nitrogen gas
to facilitate vapor removal. Sub-boiling
temperatures are maintained until
complete dryness occurs. The residue is
reconstituted in a small volume of warm
ultrapure acid diluted in ultrapure water.
Dilute nitric acid is a preferential matrix
due to it’s relative ease on the sample
introduction system, pumps and cones,
as well as the fact it won’t add additional
isobaric interferences not already
introduced with the argon plasma, air or
ultrapure water in the matrix. It is
necessary to deal only with the common
interferences associated with nitrogen,
oxygen, hydrogen, carbon, and argon.
For our applications at Seastar, we use
dilute nitric and hydrogen peroxide.
Dilute hydrogen peroxide added to the
matrix increases recoveries for certain
elements such as boron and titanium to
quantitative (80 - 120%) values.
With a concentration factor of up to 200
times, the method limits of detection are
improved
for
even
the
most
discriminating analysis as the signal to
noise ratio is increased accordingly. The
dilute acid matrix is less aggressive on
the instrumentation (increased longevity
of vacuum pumps and cones). Again, no
internal standards are required with this
technique. Pre-concentration is slower
than direct injection for sample
preparation and requires more sample
handling. Evaporation vessels must be
thoroughly cleaned and blank checked
and the recovery of volatile and
refractory elements will not fall within
the realm of quantitation.
Instrumental Analysis:
Another possibility for improving the
BEC is increasing sensitivity by
comparing different sample introduction
systems – a typical quartz cyclonic spray
chamber and the new APEX spray
chamber3.
The two tables provided,
compare the blank, sensitivity, BEC and
LOD for a short list of elements.
Instrument parameters and hardware for
this comparison are as follows:
PE/Sciex Elan 6000
Hot Plasma 1100W
Pt sampler and skimmer cones
100uL/min PFA concentric nebulizer
Ceramic Injector / Quartz Torch
Conclusions and Remarks:
For the most consistent analytical
results, the primary consideration of the
analyst must be the measurement of the
analytical blank. This requires control
of contamination sources, proper
instrument tuning and high purity
reagents for the best results. For the
quantification of elements at low trace
metal levels, the analyst may need to
exceed the technological capability of
instrumentation
and
additionally
increase the signal to noise ratio to
achieve
appropriate
blank
measurements.
To address this
application, pre-concentration using a
closed cell evaporation system to reduce
contamination is an effective technique.
For less discriminating analysis, open
beaker pre-concentrations or dilutions
and direct injections also offer analytical
advantages. Trials with alternative
sample introduction systems may also
increase the sensitivity sufficiently for
effective quantitation at low part-pertrillion levels.
References:
1. Barton Tillotson , Air Liquide,
Closed Cell Evaporation Unit,
FACSS 1994.
2. McKelvey, Brad and Shelley McIvor
et al. “Analyzing PPT and PPQ
Levels of Impurities in High Purity
Acids and Water.” Plasma Winter
Conference. Fort Lauderdale, USA,
January 2004.
3. Elemental Scientific Inc. Apex
sample introduction system page.
http://www.elementalscientific.com/p
roducts/apex.asp, 16Dec04