Interferences
The three techniques present different types and
complexities of interference problems. For this
reason, we will look at each technique separately.
ICP-MS interferences
1. Spectral
The spectral interferences in
ICP-MS are
predictable and number less than 300.
Polyatomic and isobaric interferences are found
where a species has a similar mass to the
analyte, whereby the resolution of the
spectrometer (generally around 0.8 amu) will
not resolve it, e.g. 58Ni on 58Fe, 40Ar on
40Ca,40Ar16O on 56Fe, or 40Ar-Ar on 80Se.
Element correction equations (similar in
principle to inter-element correction in
ICP-AES) can be used. In many cases
alternative isotopes with lower natural
abundances may be employed. The use of
mixed gases (small percentages of other gases
such as nitrogen, added to the main argon gas)
can sometimes be effective in reducing
interferences.
2. Matrix acids
It should be especially noted that HCl, HClO4,
H3PO4 and H2SO4 can cause considerable
spectral problems. Polyatomic interferences are
caused by Cl+, P+, S+ ions in conjunction with
other matrix elements like Ar+, O+, H+.
Examples are, 35Cl40Ar on 75As and 35Cl16O on
51V.
The avoidance of HCl, HClO4, H3PO4 and
H2SO4 in
ICP-MS is paramount for most
analyses. Where this is not possible, separation
chromatography (microcolumns) may be used
before the sample is introduced into the
plasma. This is a method many favour to get rid
of the unwanted species, and it also creates an
opportunity to preconcentrate at the same time.
Other techniques used to overcome these
problems are: electrothermal vaporization
(ETV), and mixed gases. Another very
expensive alternative is a high resolution
magnetic sector
ICP-MS which can resolve
masses less than 0.01 amu apart. This enables
many of the spectral interferences to be
eliminated.
Solutions for
ICP-MS analysis are normally
prepared in nitric acid.
3. Doubly charged ions
A doubly charged ion will cause a spectral
interference at half the m/z of the singly
charged ion, e.g.138Ba++ on 69Ga+ or 208Pb++ on
104Ru+. These interferences are few and can be
considerably minimized, or effectively
eliminated, by optimizing the system before
proceeding with the analysis.
4. Matrix effects
Transport effects include spray chamber effects
and differences in viscosity between sample
solutions and calibration standards. This will
change the efficiency of aerosol production
from one solution to another. Matrix matching is
usually required, although internal
standardization can be used as an alternative
method. The rapid scanning speed of
ICP-MSdoes give superior results when using an
internal standard.
5. Ionization
Ionization effects can be caused by samples
containing high concentrations of group I and
group II elements. Matrix matching, sample
dilution, standard addition, isotope dilution,
extraction or separation by chromatography
may be necessary.
6. Space charge effects
Space charge effects occur mainly behind the
skimmer cone, where the net charge density
becomes significantly different from zero. The
high ion density leads to interaction between
ions present in the ion beam causing
preferential loss of the light ions in the presence
of heavy ions, e.g. Pb+ on Li+ 3. Matrix
matching, or careful choice of internal
standards across the mass range of analytes,
will help to compensate for these effects,
although this may prove difficult in practice.
Isotope dilution will be effective though
expensive, but the simplest and most effective
method is to dilute the sample.
ICP-AES interferences
1. Spectral
ICP-AES spectral interferences are more
numerous and are more difficult to solve. There
are more than 50,000 ICP-AES spectral lines
documented, and the matrix can cause
considerable problems, which makes a high
resolution spectrometer mandatory for the
analysis of samples such as steels, chemicals
and rocks. Inter-element correction, used
extensively in simultaneous ICP-AES, can have
only limited success.
The background in ICP-AES may be elevated
or structured, requiring an off line background
correction. Sophisticated dynamic background
correction, if available, is very useful to improve
accuracy. Different molecular species such as
OH give peaks or bands which can cause
analytical problems at low analyte
concentrations, degrading the detection limits in
real samples.
The background in
ICP-MS is so low, typically
<20 counts/second, that it doesn’t pose a
problem. This is a major reason for the superior
detection limits of
ICP-MS.