Reliable Measurements 16. ICP-MS Measurement By Paul Gaines, Ph.D. • Edited by Brian Brolin
ICP-MS Unit
This
section will focus upon considerations in developing an ICP-MS
measurement procedure for those using a spectrometer employing a
quadrupole mass filter. These instruments are relatively easy to
operate, have good stability, and are the most common instruments used
by trace elemental analytical laboratories. They also have a resolution
of something less than 1 amu (atomic mass unit). These instruments are
typically referred to as low resolution instruments.
Some
other types of spectrometers will not be addressed in this guide. These
include: a) spectrometers using magnetic fields to disperse the ion
beams can operate up to a resolving power of 1 part in 10,000; b)
time-of flight- spectrometers; c) and spectrometers utilizing ion-trap
principles. For more detail on the quadrupole as well as the other
types of ICP-MS spectrometers, I encourage you to refer to the
following references:
Plasma Source Mass Spectrometry - Developments and Applications; Holland, G., Tanner, S. D., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1997.
Inductively Coupled Plasma Mass Spectrometry; Mantaser, A., Ed.; Wiley-VCH: New York, 1998.
Taylor, H. E. Inductively Coupled Plasma Mass-Spectrometry, Practices and Techniques; Academic Press: New York, 2001.
Resolution
is a property that should be understood. The peaks are considered to be
resolved if the magnitude of the valley between two adjacent peaks in
less than 10 % of the mean of the magnitude of the peaks:
(Intensity peak 1 + Intensity peak 2) / 2 > 0.1 (valley intensity) NOTE: This is for neighboring peaks of the same intensity
Most
commercial quadrupole mass spectrometers are capable of 0.8 amu mass
resolution (at 10% of the valley definition and having equal adjacent
peak intensities).
Having adjacent peaks at the same
intensity is not a realistic or typical situation. Therefore, the
ability to measure a m/z peak at a low concentration adjacent to a high
concentration m/z peak is a situation that must be considered. This
very important consideration is referenced as Abundance Sensitivity.
The concern is that tailing from the larger peak into the smaller peak
will occur, giving false high results for the smaller peak. I
personally experience this problem on a daily basis while attempting to
measure ppb to ppt levels of impurities in our products that are
typically diluted to 100 - 200 �g/g for ICP-MS trace impurity analysis.
For
the quadruple mass filter, the abundance sensitivities for adjacent
peaks on the low and high mass are not equal. This is because the peaks
are asymmetric and tend to tail more on the low mass side. If we have a
high concentration element with a peak intensity of Ih at mass M and want to measure a low concentration element with a peak intensity of IL-1 on the low (M-1) mass side or IL+1 on the high (M+1) mass side, the ratios IL-1 / Ih and IL+1 / Ih, are referred to as the low-mass abundance sensitivity and high-mass abundance sensitivity, respectively. For a peak width of 0.8 amu, expected values of 1 x 10-5 and 1 x 10-6
for the low-mass and high-mass abundance sensitivities are not
unreasonable. This means the concentrations will be measured to be ~ 1
ng/g on the low-mass side and ~ 0.1 ng/g on the high-mass side,
adjacent to a mass with a concentration of 100 �g/g. If you are working
at dilution factors of 10 to 400, this translates into significant
errors. In these situations, I prefer to use axial view ICP-OES for the
low or high mass elements.
This
next section presents the most common interferences encountered in
ICP-MS using the quadrupole mass filter (hereafter referred to as
low-resolution ICP-MS). You can also find the major interferences for
the popular isotopes in our interactive periodic table.
Isobaric Interferences:
Isobaric
interference is a result of equal mass isotopes of different elements
present in the sample solution. Low-resolution instruments cannot
distinguish between the isotopes. There are many examples in the
intermediate mass regions where the second and third row transitions
and the rare earths appear. Fortunately, there are no elemental singly
charged isotopes that overlap with monoisotopic elements (9Be, 23Na, 27Al, 45Sc, 55Mn, 75As, 89Y, 103Rh, 127I, 133Cs, 141Pr, 159Tb, 165Ho, 169Tm, 197Au, and 232Th). However, for the monoisotopic elements, be aware of the other interferences to be discussed later.
For elements having more than one isotope, the quickest fix may be
('may' is used because other interferences could be encountered) to use
another isotope of that element. If the interference is from an isotope
with roughly the same or lower peak intensity, it is possible to
perform a correction by measuring the intensity of another isotope of
the interfering element and subtracting the appropriate correction
factor from the intensity of the interfered isotope. If you are working
with an unknown sample composition, a semi-quantitative analysis is
suggested with low-resolution instruments using a quick scan of the
sample and the rather sophisticated semi-quantitative programs
available on current instrumentation.
Polyatomic (Molecular) Interferences:
Molecular
interferences are due to the recombination of sample and matrix ions
with Ar and other matrix components such as O, N, H, C, Cl, S, F, etc.
The light elements (Li, Be, B) are not affected due to their small
masses.
Starting with 39K, this type of interference becomes a significant issue. For example, 39K is interfered with by 38ArH and 23Na16O. Some polyatomic interferences can be avoided by eliminating certain matrix elements such as the classic 40Ar35Cl interference upon the monoisotopic element 75As, where the use of HCl in the sample preparation is to be avoided. The isotopes 56Fe, 39K, and 44Ca or 40Ca are all interfered with by combinations of the Ar, O, and N isotopes.
As
we go to the heavier elements the major polyatomic interferences come
from isotopes that are 16 atomic mass units lower than the analyte
isotope through molecular oxide (MO) interference. The lanthanide element isotopes are especially prone to molecular oxide formation.
The
use of cool plasma techniques, reaction / collision cells, desolvation,
and chromatographic separations -- to name a few approaches -- have
resulted in reduction and, in some cases, complete elimination of many
polyatomic interferences. The severity of the MO interference can be
reduced through reduction of the sample argon gas flow rate. Mass
corrections may be an option in cases where the use of an alternate
isotope is not an option. Polyatomic interferences are particularly
troublesome in the determination of first row periodic table elements
(K thru Se) due to the vast number of combinations of Ar with matrix
components.
Doubly Charged Ion Interferences:
Doubly
charged ion interference is due to doubly charged element isotopes with
twice the mass of the analyte isotope. For example, interference from 206Pb++ (m/e = 103) upon 103Rh
is likely at high Pb concentration levels. Reduction in the sample Ar
will minimize this interference. Fortunately, this type of interference
is not as prominent in Ar plasmas, but care should be exercised in
matrices containing high levels of mid to heavy mass element isotopes.
The alkaline and rare earth elements form doubly charged ions to a
extent that is greater, relative to the other elements.
In addition to the matrix effects encountered for ICP-OES that are discussed in Section 15, ICP-MS suffers from space charge effects and salt buildup on the orifice of the interface sampler cone.
Space Charge Effects:
These
effects are thought to occur at the MS interface, the region between
the skimmer tip and ion optics and in the ion optics region. The net
result is a suppression of the signal in high concentrations of a
matrix element. The kinetic energy of the ion element matrix affects
the degree of suppression with larger masses (higher kinetic energy)
causing more depression than lower masses. Due to differences between
instruments in interface and ion optic designs it is difficult to
predict the conditions under which the effect is minimal. In my case,
working at matrix element concentrations of 100 - 200 �g/g will cause
only a slight reduction in the signal. Under 'cool plasma' conditions,
I have found this suppression effect to be more pronounced. The
approach taken in our laboratory is to attempt to keep the matrix
element concentration at or below the 100 �g/g concentration level.
Salt Buildup:
The
buildup of salts/oxides in samples containing high levels of matrix
elements (sea water is a common example) will result in partial or
total clogging of the sampler cone. Techniques used to help control
this effect include dilution to below 0.1% total solids, flow injection
analysis, or ion exchange removal of the matrix component(s).
Quantitative Analysis Measurement Techniques[ Back ]
What follows are some of the more popular techniques used for quantitative analysis using ICP-MS.
External Calibration using Calibration:
This
is the calibration technique that is most popular. Many analysts use
this approach for matrices that are known and can be matched. The use
of internal standards is helpful in accounting for drift. The choice of
the internal standard / isotope mass combination is reasonably well
understood. Finally, the use of spike recoveries on a split portion of
the sample allows the analyst to determine if space charge effects are
significant. The following are some brief notes on this topic:
Know
or learn about the sample composition. A semi-quantitative analysis
using a scanning approach for the entire mass range allows the analyst
to predict interferences and select internal standards and analyte
isotopic masses.
Perform
interference check analysis. Prepare for the variations in the matrix
and analyte composition and determine if corrections that have been
built into the procedure are capable of providing the required accuracy.
Use internal standards to help correct for drift.
Follow these basic guide lines for internal standard selection: 1. Avoid M2+ interferences 2. Avoid MO and other molecular interferences 3. Any naturally occurring internal standard element in your sample must be insignificant in comparison to the amount added 4. Use internal standard elements as close as possible to the masses of the analyte elements 5. Make sure the matrix doesn't react with the internal standard to alter (lower) it's concentration 6. Common internal standard elements are 6Li, Be, Sc, Ga, Ge, Y, Rh, In, Cs, Pr, Tb, Ho, Re, Bi, and Th - note many are monoisotopic.
Use peak hopping rather than scanning for the final analysis.
Peak hopping will save time and this capability is one of the major advantages of low-resolution systems.
At the beginning of the analytical day, optimize the instrument using 'optimization' standards.
I prefer to use a 10 ppb combination of Mg, U, Ce, and Rh. In addition, I like to optimize the instrument to obtain 140CeO / 140Ce and 140Ce+2 / 140Ce currents of < 0.5% relative. I routinely obtain a 'time scan' of 24Mg, 36Ar, 70Ce+2, 103Rh, 140Ce, 156CeO, 230BKG, and 238U
at the beginning of each analytical day. These scans are saved and
accompany the following analytical data. Torch alignment, sample argon
(nebulizer) flow, and ion optics settings are the parameters I most
often change (in the order listed) in the optimization process .
Make sure the introduction system is clean.
I
prefer using glass concentric nebulizers and cyclonic spray chambers. I
use dilute nitric acid for cleaning. It is often advantageous to change
the entire introduction system, sipper to torch, each analytical day.
In addition, the sample interface cones need to be rotated each
analytical day with cleaned cones. Cleaning the cones in a 1% solution
of nitric acid using an ultrasonic bath for 1-2 minutes is typically
all that is required. Carefully dry the cones in a drying oven before
reusing.
Periodically check the performance of the ICP-MS during the analytical 'run.'
I
prefer to split the sample and spike half of the sample with a known
low ppb addition of an assortment of analytes ranging from Mg to U.
After confirming the calibration by analyzing the standards, I like to
use an analysis sequence of blank, sample, and sample + spike. The
spike recovery allows me to determine if space charge effects from the
matrix element(s) have significantly lowered the analyte signal.
This
approach is common with ICP-OES but it may give the analyst a false
sense of security when using ICP-MS. It is a concern that this
technique has earned such a 'good reputation' in view of the fact that
it does not guarantee anything except a perfect matrix match. ICP-MS
has many more potentially serious problems than matrix matching. The
same interference issues discussed above must come into consideration
if you choose to use standard additions. For example, if you have a
molecular MO interference before the addition and do not use an
alternate mass or perform a correction, you will still obtain a false
high result. Spend the time to learn about the matrix and identify
potential interference issues. After you reach a high level of
confidence in the identification and correction for and/or elimination
of interferences, then the standard additions approach is a convenient
way to 'match' a complicated matrix.
Isotope Dilution:
The
technique of isotope dilution ICP mass spectrometry (ID-ICP-MS)
provides the analyst with the possibility of using a primary
(definitive) analytical method for the determination of trace metals in
a variety of sample types. Examples of primary analytical methods
are isotope dilution mass spectrometry (IDMS), ID-ICP-MS, gravimetry,
titrimetry, coulometry, differential scanning calorimetry and nuclear
magnetic resonance spectroscopy. ID-ICP-MS is of particular
interest to the Reference Material producer of materials for trace
metals content. Unfortunately, there are several of the elements that
are monoisotopic (9Be, 23Na, 27Al, 45Sc, 55Mn, 75As, 89Y, 103Rh, 127I, 133Cs, 141Pr, 159Tb, 165Ho, 169Tm, 197Au, and 232Th),
making ID-ICP-MS useless for these elements. Our laboratory has been
studying ID-ICP-MS along with the execution of accurate isotopic
abundance ratio measurements (another possible primary method) and will
publish these studies in the months to come.
