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 An Overview of Atomic Spectroscopy

         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.


 Selecting the Proper Atomic Spectroscopy Technique

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

    A 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.

Detection Limits
         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.

   Table 1. Atomic Spectroscopy Detection Limits (micrograms/liter)

                        Flame    Hg'                       ICP                                               Flame        Hg'                         ICP
         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
             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
         more information.

             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.