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ICP-MS与ICP-AES比较检测限要好：ICP-MS、ICP-AES与AAS比较可以进行多元素测量

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ICP-MS-1
April 1994
the selected ions reach the detector.
The ICP-MS provides information for each atomic
mass unit (amu or Daltons; more accurately, the
ratio of the mass of the ion to its charge is
displayed, and labelled m/z), in the mass region
3-250 amu. The isotope information can be used in
several ways; these include isotope ratio
measurements, often used for Pb and U which do
not have a constant natural abundance, and
analysis of samples having unnatural isotope
abundances.
Isotope dilution is a method of spiking the samples
with a known concentration of a pure isotope to
obtain a very accurate determination of the
concentration of the element. A pre-requisite of this
technique is that the element of interest must have
more than one isotope.
Detection Limits
ICP-MS detection limits are very impressive. Most
detection limits are in the 1-10 parts per trillion (ppt)
range for solutions. These are as good as, or better
than, GFAAS detection limits for most elements in
pure water. ICP-MS can also detect many elements
that have very poor GFAAS detection limits.
ICP-MS has typically two to three orders of
magnitude better detection limits than ICP-AES,
which has detection limits for most elements in the
1-10 parts per billion (ppb) range. Recently, some
ICP-AES spectrometers have shown impressive
detection limits in the sub ppb region for selected
elements in an optimized part of the spectrum for
clean samples.
It should be noted, however, that the comment
above about ICP-MS detection limits is for simple
solutions having low levels of other dissolved
material. For detection limits related to
concentrations in the solid, the advantage for
ICP-MS can be degraded by up to 50 times,
because of the poorer dissolved solids capability.
Some common lighter elements, (e.g. S, Ca, Fe, K,
Se) have serious interferences in ICP-MS which
degrade the detection limits considerably.
ICP-MS, or ICP-AES and AAS?—a comparison
Geoffrey Tyler
Varian Australia Pty Ltd
Mulgrave, Victoria, 3170, Australia
Introduction
The attractiveness of the inductively coupled plasma
atomic emission spectrometer (ICP-AES) has led
many analysts to ask whether it is wiser to buy an
ICP-AES or to stay with their trusted atomic
absorption technique (AAS)1. More recently, a new
technique, inductively coupled plasma mass
spectrometry (ICP-MS), has been introduced2.
The ICP-MS offers at first sight, albeit at higher
cost, the advantages of ICP-AES and the detection
limits of graphite furnace atomic absorption
spectrometry (GFAAS).
This article will briefly describe these three
techniques, and point out the important criteria by
which to judge their applicability to your own
analytical problems.
For many people with an ICP-AES background, ICPMS
is a plasma, with a mass spectrometer as a
detector. Mass spectroscopists would prefer to
describe ICP-MS as mass spectrometry with a
plasma source. Either way, the technique is capable
of giving isotope information. This information can
help to overcome many of the “spectral”
interference problems that can occur in the mass
spectrometer.
Basically, the sample compartments and plasma of
ICP-AES and ICP-MS look similar. In ICP-AES, the
optical spectrum (approximate range 165-800 nm),
is viewed and measured, either sequentially or
simultaneously. The simultaneous ICP-AES is
faster, but more expensive, than sequential
ICP-AES.
The ICP-MS extracts the ions produced in the
plasma into an interface consisting of a sampler
cone followed by a skimmer cone. This configuration
enables the pressure to be reduced differentially
from atmospheric pressure down to a final pressure
of between 10-5-10-7 Torr. The ions pass through the
interface into ion optics, which optimize the ion
paths. Neutral species are removed from the beam
at this stage, either by the vacuum pumps or by
collision with a photon stop. The ions then pass
through a mass filter, usually a quadrupole, before

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-MS
does 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.

2. Matrix effects
Like ICP-MS, ICP-AES can use internal
standards to overcome matrix effects such as
spray chamber effects and viscosity differences
between samples and calibration standards4.
3. Ionization
Interference from easily ionizable elements can
be minimized by careful choice of individual
element conditions6 or by adding an ionization
buffer, i.e. by adding an excess of a group I
element.
GFAAS interferences
The interferences in GFAAS have many sources.
They include:
1. Spectral
There are a few spectral interferences in
GFAAS when deuterium background correction
is used, but these can be eliminated by use of
Zeeman GFAAS.
2. Background
For many matrices careful programming of the
ash stage is required to minimize the
background signal during the atomization. The
use of chemical modifiers can be helpful in
increasing the allowable ash temperature. For
example, a Ni chemical modifier for Se
determinations allows ash temperatures of up
to 1000 °C before Se loss. The use of Zeeman
background correction can give an
improvement in accuracy compared with D2 arc
background correction in many GFAAS
applications.
3. Vapor phase interferences
These can be caused by the atomization of the
analyte into a cooler gas environment. These
interferences have been minimized in recent
years by isothermal tube design and use of
platforms so that the sample is atomized into a
hot inert gas environment.
4. Matrix effects
Matrix effects are caused by variable retention
of the analyte on the graphite tube, depending
on the sample type. The dry and ash stages
can have a dramatic effect on the shape of the
transient peak. The use of matrix modifers
(e.g. PdCl2) and hot injection can be quite
effective in minimizing these effects; also the
use of peak area measurement can be
advantageous in some cases5.
Ease of use
For routine analyses, ICP-AES has matured in
automation to the point where relatively unskilled
personnel can use methods created by the ICP-AES
specialist. Until recently, ICP-MS was still the
domain of the specialist chemist making fine
adjustments before performing routine analysis.
The trend to simplicity has been evident since 1993
and will continue in the future. One of the reasons
for this is full computer control of parameters stored
within a method. Another reason is the use of a
multitasking graphical user interface, to show the
operator several indicators of data integrity on the
same screen. The use of such software also has a
very positive effect on method development time.
Before this software became available, ICP-MS
method development was a highly complex and
time-consuming task. GFAAS, although relatively
simple for routine analysis, requires considerable
skill in setting up the methods.
Total Dissolved Solids (TDS)
Recent ICP-AES spectrometers have been able to
analyze routinely up to 10% TDS and even up to
30% for simple salt solutions. Although the analysis
of 0.5% TDS for ICP-MS may be possible for a
limited timescale, most chemists are happier with
0.2% maximum TDS. This should be borne in mind
when the original sample is a solid. The ultimate
detection limit for some elements in ICP-MS may
not be so impressive when expressed in the solid,
compared with ICP-AES. GFAAS can cope with
extremely high levels of dissolved solids.
Linear Dynamic Range (LDR)
ICP-MS can have a LDR in excess of 105. Various
methods for extending the linear range up to 107
include de-sensitizing one of the ion lenses, use of
detector analog mode, or use of a separate Faraday
cup as a second detector. These should be used
with caution, however, as high matrix component
concentrations may cause problems best solved by
dilution. For this reason, and because of the
problems with high levels of dissolved solids,
ICP-MS should be mainly the domain of trace/
ultratrace analysis. The exception to this is when
using isotope dilution. With this technique very good
results have been obtained with high concentrations.
GFAAS has a limited LDR of 102-103. It can be used
for higher concentrations if a less sensitive line is
selected.
Trace to major element analysis may be performed
by ICP-AES because of its 105 LDR. ICP-AES is
ideal for analysis up to and including percentage
levels. For this reason ICP-AES, in addition to
ICP-MS or GFAAS, is often needed to fulfil
laboratory requirements.
Precision
The short-term precision of ICP-MS is generally
1-3%. This is improved routinely by use of multiple
internal standards. The longer term precision (over a
period of hours) is still <5% RSD. The use of isotope
dilution can give results of very high precision and
accuracy, although the cost can be prohibitive for
routine analysis.
ICP-AES has generally precisions of 0.3-2% RSD in
the short term and again less than 5 % RSD over
several hours.

GFAAS, however, will generally have short term
precisions of 0.5-5% RSD. Longer term precision is
a function of the number of graphite tube firings,
rather than time.
Sample throughput
‘The ICP-MS has an incredible capacity to analyze a
vast number of samples for trace elements. Typical
analysis time is less than 5 minutes/sample, for the
whole suite of required trace elements. For some
applications this may only take a couple of minutes.
Consulting laboratories find the sample throughput a
major advantage.
While the speed of ICP-AES analyses will depend on
whether simultaneous or sequential instruments are
used, generally this can vary from 2 to 6 minutes
per sample. Simultaneous ICP-AES can be faster,
typically 2 minutes/sample, but sometimes its
accuracy can be compromised by spectral
interferences present with some types of samples
(e.g. rocks).
The speed of GFAAS is typically 3-4 minutes per
element per sample (assuming 2 replicates).
Automated overnight runs can be performed, and
this will improve throughput of samples.
Total sample throughput can be a major factor
favouring ICP-MS in the busy laboratory. The
following examples (expressed as solution
concentrations), will give a guide:
1. One to three elements/sample, at sub/low ppb
concentration will generally be better by
GFAAS, assuming the elements of interest can
be determined by this technique.
2. 5-20 elements/sample at sub ppm-% levels will
generally be better by ICP-AES.
3. 4 or more elements/sample at sub ppb and ppb
concentrations will generally be better by
ICP-MS, if the number of samples to be
analyzed is high.
Unattended operation
ICP-MS, ICP-AES and GFAAS can all operate
unattended overnight, because of the modern
automated designs and the safety inherent in the
use of inert argon gas in these techniques. For
highest productivity, overnight operation is
mandatory.
Cost of ownership
The running cost of ICP-MS is more than ICP-AES
because several components have a limited lifetime
and have to be replaced. These include the
turbomolecular pumps, the sampler and skimmer
cones and the detector. The torch and nebulizer
have similar lifetimes for both ICP-AES and ICP-MS
techniques. If ICP-AES is chosen instead of ICP-MS
the laboratory will probably require GFAAS as well.
Hence, the cost of graphite tubes for the latter has
to be taken into account. In all three techniques the
cost of argon is a significant budget item, with the
ICP techniques requiring more than GFAAS.
Capital cost
This is always a difficult subject to quantify because
it will depend on the amount of automation, the
accessories and the supplier. In very approximate
terms, you can estimate that an ICP-AES will cost
twice as much as a GFAAS and 2-3 times less than
ICP-MS. It should be noted, however, that the
accessories can distort these figures considerably.
Another cost that needs to be taken into account is
that ultra trace analysis requires a clean laboratory
and ultra pure chemicals. These are not cheap.
Accessories
Being a very rapid sequential method, ICP-MS can
utilize transient signals in multi-element mode. This
opens the way for a host of accessories.
Electrothermal vaporization, laser ablation, glow
discharge and spark ablation can obviate the need
to dissolve the sample. Some accessories provide
the means of separating the matrix from the sample
and/or to pre-concentrate. These include hydride
generation, and various forms of chromatography
(e.g. HPLC, ion chromatography, microcolumns).
The advantage of separation by chromatography for
speciation work has only been fully realized in
ICP-MS. This is due to the low concentration levels
of interest in environmental, toxicological, medical
and food samples.
Although ICP-AES can use some of the above
accessories, their cost and their marginal
advantages have meant that we have rarely seen
many of them routinely used.
Summary
To advise anyone what to buy is always difficult.
Look at your present and future needs, and answer
the check-list questions in table 1. This should help
you to decide.
It should always be remembered that no technique
will satisfy all your requirements. The techniques are
complementary. There will always be samples where
one technique is better suited for the analysis than
another.
Table 2 shows a simplified comparison of the three
techniques. Table 3 compares detection limits.

References
1 G.Tyler, AA or ICP - which do you choose?
Chemistry in Australia, Vol 59, No 4,
pp 150-152, April 1992.
2 A.R. Date and A.L. Gray, Applications of ICPMS,
Blackie, Glasgow, UK, 1989.
3 K.E. Jarvis, A.L. Gray, and R.S. Houk,
Handbook of ICP-MS, Blackie, Glasgow, UK,
1992.
4 M.Thompson, J.N. Walsh, Handbook of
Inductively Coupled Plasma Spectroscopy,
Blackie, Glasgow, UK, 1983.
5 J.E. Cantle, Atomic Absorption Spectroscopy,
Elsevier, 1982.
6 Analytical Methods for Liberty ICP
Spectrometer, Varian publication 85 100938 00,
Chapter 5, pp 81-82.
7 J.Olesik, Elemental Analysis Using ICP-OES
and ICP-MS, Anal. Chem. Vol 63 No 1, Jan 1
1991 pp 12A-21A.
Table 1. Checklist of analytical requirements
1. How many samples/week?
2. What are the sample types? (steels, rocks,
effluents, soils, etc)
3. What method of dissolution may be employed?
4. How many and what elements need to be
determined?
5. What are the concentration ranges?
6. What sample volume is typically available?
7. What other options/accessories are being
considered? Why?
8. How important is isotope information to you?
9. How much money is available to purchase or
lease costs/month?
10. What is the cost of ownership and running
costs for the techniques to fulfil the
requirements?
11. What skilled operators are available to you?