Atomic spectroscopy has experienced remarkable growth and diversity in the past few years, making it more difficult for analysts to keep up with developments in the field. Atomic spectroscopy is not one but three techniques: I) atomic absorption, 2) atomic emission, and 3) atomic fluorescence. The first two are the most common and widely used techniques. The linear relationship between the amount of light absorbed or emitted and the amount of species of interest is called the Beer-Lambert Law. It can be used to find unknown concentrations by measuring the light emitted or absorbed.
1) Atomic Absorption is the process where vaporized atoms absorbs light and is measured.
The basic instrument for atomic absorption requires a light source, an atom source, a monochrometer to isolate the specific wavelength of light, a detector, some electronics to treat the signal, and a data display. The light source is usually a hollow cathode lamp.
2) Atomic Emission is a process in which the light emitted by excited atoms or ions is measured.
The basic instrument for atomic emission is similar to atomic absorption except that it has no primary light source. The critical component in emission is the atomization source because it must provide all the energy to excite as well as atomize the atoms. Previously many sources were tried but the ICP eliminates most all problems associated with past emission sources. This has revolutionized the utility of atomic emission spectroscopy.
The ICP is an argon plasma maintained by the interaction of an RF field and ionized argon gas. The ICP can reach temperatures around 10,000 K with sample temperatures between 5,000 and 8,000 K. These temperatures allow complete atomization of elements thus minimizing chemical interference effects.
In ICP-MS, the function of the Mass Spectrometer is similar to that of the monochrometer in Atomic Absorption or ICP Emission systems. In ICP-MS, rather than separating light into wavelengths, the mass analysis separates the ions from the ICP according to their mass/charge ratio. The ICP-MS combines the multielement capabilities and broad linear range of ICP with the exceptional detection limits of graphite furnace AA.
There are four techniques normally Suited for analytical determinations and they are:
1) Flame Atomic Absorption
2) Graphite Furnace Atomic Absorption
3) Inductively Coupled Plasma Emission
4) Inductively Coupled Plasmal Mass Spectrometry
clear understanding of the analytical problems and the capabilities provided
by the different techniques is necessary. Some important criteria for selecting
a particular technique include 1) detection limits, 2) analytical working
range, 3) sample throughput, 4) cost, and 5) ease of use.
Without adequate detection limit capabilities, lengthy preparation may be required prior to analysis. Typical detection limits for the major atomic spectroscopic techniques are shown in Table 1. Generally, the best detection limits are attained using ICP-MS and Graphite Furnace Atomic Absorption as shown below.
Analytical Working Range
The analytical working range is just the concentration range over which the quantitative results can be obtained without having to recalibrate the system (the linear relationship between amount of light measured and concentration).
For the GFAA there is approximately 2 order of magnitude for its working range where the ICP-AES has a 5th order of magnitude as its working range and the ICP-MS has a 6th to 8th order of magnitude as its working range.
Element AA Hydride GFAA Emission ICP-MS Element AA Hydride GFAA Emission lCP-MS
Ag 1.5 0.05 1.5 0.003 Mo 45 0.2 7.5 0.003
Al 45 0.3 0.006 Na 0.3 0.05 6 0.05
As 150 0.03 0.5 30 0.006 Nb 1500 5 0.0009
Au 9 0.4 6 0.001 Nd 1500 0.002
B 1000 45 3 0.09 Ni 6 0.8 6 0.005
Ba 15 0.9 0.15 0.002 Os 120
Be 1.5 0.02 0.09 0.03 P 75000 320 45 0.3
Bi 30 0.03 0.6 30 0.0005 Pb 15 0.15 30 0.001
Br 0.2 Pd 30 2 1.5 0.003
C 75 150 Pr 7500 <0.0005
Ca 1.5 0.03 0.15 2 Pt 60 5 30 0.002
Cd 0.8 0.02 1.5 0.003 Rb 3 0.08 0.003
Ce 15 0.0004 Re 750 30 0.0006
Cl 10 Rh 6 30 0.0008
Co 9 0.4 3 0.0009 Ru 100 3 6 0.002
Cr 3 0.08 3 0.02 S 75 70r
Cs 15 0.0005 Sb 45 0.15 0.4 90 0.001
Cu 1.5 0.25 0.003 Sc 30 0.3 0.02
Dy 50 0.001 Se 100 0.03 0.7 90 0.065
Er 60 0.0008 Si 90 2.5 5 0.7
Eu 30 0.0007 Sm 3000 0.001
F 10000 Sn 150 0.5 60 0.002
Fe 5 0.3 1.5 0.45 Sr 3 0.06 0.075 0.0008
Ga 75 15 0.001 Ta 1500 30 0.0006
Gd 1800 0.002 Tb 90 <0.0005
Ge 300 15 0.003 Te 30 0.03 1 75 0.01
Hf 300 0.0006 Th <0.0005
Hg 300 0.009 1.5 30 0.004 Ti 75 0.9 0.75 0.006
Ho 60 <0.0005 Tl 15 0.4 60 0.0005
I 0.008 Tm 15 <0.0005
In 30 45 <0.0005 U 15000 35 <0.0005
Ir 900 7 30 0.0006 V 60 0.3 3 0.002
K 3 0.02 75 1 W 1500 30 0.001
La 3000 1.5 0.0005 Y 75 0.3 0.0009
Li 0.8 0.15 1.5 0.03 Yb 8 0.001
Lu 1000 <0.0005 Zn 1.5 0.3 1.5 0.003
Mg 0.15 0.01 0.15 0.007 Zr 450 1.5 0.004
Mn 1.5 0.09 0.6 0.002
All detection limits are given in micrograms per liter and were determined using elemental
standards in dilute aqueous solution. All detection limits are based on a 98% confidence
level (3 standard deviations).
Atomic absorption and ICP emission detection limits were determined using
instrumental parameters optimized for the individual element. ICP
emission detection limits obtained during multielement analyses will typically be within a
factor of 2 the values shown.
Cold vapor mercury detection limits were determined with a FIAS-200 flow injection
system with amalgamation accessory. Hydride detection limits were determined using a
MHS-10 Mercury/Hydride system. Fumace AA (Model 5100 Pc with 5100 ZL Zeeman
furnace Module or Model 4100 ZL) detection limits were determined using STPF conditions
and are all based on 20 microliterL sample volumes and use of a L'vov platform.
ICPMS detection limits were determined using an ELAN 5000. Letters following an ICP-MS
detection limit value refer to the use of a less abundant mass for the determination as
follows:a-C 13 b-Ca 44 c-Fe 54, d-Ni 60, e-S 34, f-Se 82
Sample throughput is the number of samples which can be analyzed or elements to be determined per unit
time. Analyses near the limit of detection or where absolute precision is required are more time consuming
than other less demanding analyses.
GFAA is basically a single element technique because of the need ~ thermally program the system
to remove solvent and matrix components prior to atomization. The GFAA has a relatively low sample
throughput where a typical determination (single burn) requires 2-3 minutes per sample per element.
ICP Emission is a true multielement technique with exceptional sample throughput. IC? emission
systems typically can determine l~80 elements per minute in individual samples. For few elements the ICP
is limited by the time needed to equilibrate the plasma which is typically 15-30 seconds (0.25-0.5 minutes)
ICP-MS has the same multielement capabilities and time requirements as IC? but can get much
better detection limits like those in GFAA.
To run one sample by GFAA it takes 4 minutes to precondition the graphite tube, 8 minutes to calibrate
using a four point calibration method. Another 8 minutes are taken up by internal analysis checks and
blanks. that is 22 minutes just for calibration without any complications. To analyze the unknown sample
takes 2-3 minutes providing it is in the right working range or it will need to be diluted which takes still
more time. And to do eight elements it will take approximately 4 hours (240 minutes). On the ICP it takes
10 minutes to calibrate, in a typical case 23 elements, and then l minute per burn but all 23 elements are
determined simultaneously. That is a total of 11-12 minutes. That is 20 times faster and there is three times
Instrumentation for single element atomic spectroscopy (flame AA and GFAA) is generally less costly than
that for the multielement techniques such as ICP and ICP-MS as the former are less complex systems There
is cost differentials among instruments for the same technique. Instruments which offer versatility frequently
offer a greater degree of automation than that of the basic instrument. Cost of the instrument is only one
issue to look at regarding the total cost of what the data determines. Other things may include cost of down
time, the value of real time data in solving critical excursions, cost of sample delivery. and cost of
Ease of Use
Other comparison criteria for analytical techniques include ease of use, operator skill level, and availability
of documented methodologies.
GFAA applications are well documented. GFAA has exceptional detection
limit capabilities but
with a limited working range. Sample throughput is less than that of other atomic spectroscopy techniques.
Operator skill requirements are more than minimum expertise to obtain best results, especially on the more
difficult samples we encounter.
ICP Emission is the best overall multielement atomic spectroscopy
technique with excellent
sample throughput and very wide analytical range. Good documentation is available for numerous
applications. Operator skill requirements are intermediate.
ICP-MS is a relatively new technique with exceptional multielement
capabilities at trace and
ultratrace concentration levels. Isotopic determinations can also be performed on the ICP-MS. Good basic
documentation for interferences exists. Application documentation is limited but growing rapidly. ICP-MS
requires operator skill similar to those for ICP and GFAA.
Most often the selection of technique is based on analyte concentrations, flame AA and ICP emission are favored
for moderate to high levels while graphite furnace AA and ICP-MS are favored for low levels. ICP and
ICP-MS are multielement techniques favored where large numbers of samples are to be analyzed.