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					                                        Lab No. 9
                                        ICP-AES

       Introduction Inductively coupled plasma atomic emission spectroscopy
represents a relatively new technique in analytical atomic spectroscopy, having begun to be
widely used only in the late 1970's. The analytical principles are similar to that encountered
in flame emission spectroscopy, the primary difference lying in the excitation source - the
inductively coupled argon plasma.
       Figure 1 illustrates a typical ICP torch. The source in ICP consists of 2 or 3
concentric quartz tubes positioned in the center of a radio frequency (rf) generating coil.
Alternating current (frequency is typically 27.12 MHz) is supplied to the coil by a radio
frequency generator. A stream of Argon, into which sample solution is aspirated, is
directed through the central quartz tube. A momentary high-voltage spark causes
ionization of some of the Argon atoms. The resulting cations and electrons are
accelerated by the rf field, and collide with other Ar atoms, resulting in further ionization.
With proper tuning, the rf field is able to sustain the ionization process, and a high
temperature plasma (~10000 °K at the core) results. The plasma temperature is high
enough that some means of taking heat away from the torch assembly is required. This
is accomplished by one or two streams of auxiliary Argon directed into the outer quartz
tubes. The auxiliary argon flows in a tangentail fashion around the plasma, and thus
serves to centralize the plasma in a confined region about the axis of the torch, as well
as carrying away excess heat.




                                             Hottest Region of Plasma



                                                Induction Coil (Copper tubing)




                                     Coolant Argon
                                  Plasma Argon

                            Sample Aerosol

                                     Figure 1
                                                                        9-2



Question 1: Emission from excited sample atoms are viewed in a region just above the
the top of the torch, where temperatures are typically in the 5000 to 8000 °K range,
rather than in the higher temperature core region itself. What advantage does this
have?

Question 2: The primary advantages of a plasma derive from the high temperature of
the plasma and relative inertness of the Argon carrier gas. Briefly summarize and
explain the relative advantages and disadvantages of ICP-AES as compared to Flame
Emission Spectroscopy. In your answer you should consider capital and operating
costs, detection limits, various types of interference, and dynamic range.

        In the first part of this experiment, a Thermo Jarrel-Ash AtomScan 16 ICP-AES
spectrometer will be used to determine the intensities of several emission lines from Ar
atoms. The relative intensities of these lines allows, at least in principle, determination
of the temperature of the plasma at the point of observation. Theoretically, the intensity
of an emission line is given by equation 1, where h is Planck's constant, k is
Boltzmann's constant, A is the Einstein probability coefficient for spontaneous emission,
c is the speed of light, λ is the wavelength of the emitted radiation, gk and go are
statistical weights associated with the upper excited state from which the transition
arises and the ground state, respectively, and ∆E is the energy difference between the
upper excited state and the ground state.

                I = h(c/λ)ANo(gk/go )exp(-∆E / kT)                      (1)


Equation 1 can be rearranged to give:

                  Iog(Iλ/Agk) = constant - ΔE/2.303kT                   (2)


        Provided that the Einstein coefficients and statistical weights of the observed
transitions are known, a plot of the left hand side of equation 2 vs ΔE should be linear
with slope inversely proportional to the temperature, T. If ΔE is given in wavenumbers
(cm-1) the slope is given by -0.625/T. Note that it is implicit in equation 1 that the
number of atoms in excited states is negligible compared to the number of atoms in the
ground state. This is not always true for easily excited atoms, such as the alkali metals,
at the higher temperatures associated with ICP-AES. Equation 1 also assumes that
local thermodynamic equilibrium exists in the plasma. This assumption is probably not
true. Nonetheless, equation 2 has been widely employed to estimate the temperature of
plasmas.
                                                                          9-3

      In this experiment, the intensities of six emission lines from Ar will be measured
and used to estimate the plasma temperature. These lines, along with A, gk, and ΔE
values, are listed in the table below.

            λ / nm                ΔE / cm-1            gk                    A / 108s-1
            426.629               117,184              5                     0.0265
            427.217               117,151              3                     0.0688
            430.010               116,999              5                     0.0318
            433.356               118,469              5                     0.0506
            433.534               118,460              3                     0.0308
            434.517               118,407              3                     0.0273


         In the second part of this experiment, the ICP will be applied to the qualitative
and quantitative determination of some of the elements in a sample of misch metal.
Misch metal is a rare earth alloy composed of ~60% cerium, 20% lanthanum, 15%
neodymium, and 5% praesodymium. In addition, it may contain trace levels of a variety
of transition metals. It finds use as flint for lighters and gas strikers. (Note that natural
flint is not composed of misch metal, but rather is a particular variety of quartz.) When
misch metal is alloyed with certain other metals, the resulting material is capable of
absorbing large quantities of hydrogen gas. These materials find use in hydrogen
storage applications and are known commercially as HydroStor's.

Experimental Argon gas is expensive. In order to minimize the amount you use, be
sure to complete ALL of the solution preparation before proceeding to light the
plasma.

Initial Preparations

I. Solution preparation

Preparation of unknown

1.   Obtain a striker flint from your instructor and accurately determine its weight.

2.   Place it into an Erlenmeyer flask and cover it with ~50 mL of 1 M HNO3. Bring it to
     a slow boil on a hot plate until dissolution is complete. There will be some black
     particulate matter which will not dissolve. This arises from a coating on the flint
     which prevents air oxidation of the underlying misch metal.

3    After dissolution is complete, quantitatively transfer to a 100 mL volumetric flask,
     and dilute to volume with deionized water. Filter approximately half of this solution
     into a clean Erlenmeyer flask. Take a 10 mL aliquot of the filtered solution and
                                                                         9-4

     dilute to 100 mL in a volumetric flask with deionized water. This is the test solution
     you will use for analysis.

Preparation of Cerium, Lanthanum standard solutions

4.   Prepare standard solutions from the 2000 ppm Ce + 800 ppm La stock solution by
     pipetting the indicated quantities into 100 mL volumetric flasks and diluting to
     volume with deionized water:

     Standard A (100 ppm Ce + 40 ppm La) - 5 mL stock solution
     Standard B (200 ppm Ce + 80 ppm La) - 10 mL stock solution
     Standard C (400 ppm Ce + 160 ppm La) - 20 mL stock solution


II. Turning on the Computer and Inspecting the Line Libraries

      The Hard- disk drive on the computer contains two line libraries. The analytical
library lists the so-called primary lines for all elements analyzable by ICP-AES. These
are the most intense and most frequently analyzed lines. The Peak Search library is
much more extensive, and lists some 24,000 known emission lines of the elements.
This can be useful for identifying possible interfering species.

1.   Turn on the power switches on the computer, the monitor, and the printer. The
     software package "ThermoSPEC", which controls the instrument, will automatically
     load and commence operation. Note that there are several menus displayed along
     the top of the screen which are used to select a course of action. These are, from
     left to right, Operation, Development, IMS (information mangaement system),
     Setup, and Exit. Utilizing the left, right, up, and down arrows of the keyboard,
     different menus may be highlighted. Note that under each menu, a variety of
     courses of action may be highlighted, again using the arrow keys on the keyboard.
     Once a particular course of action is highlighted, it may be initiated by pressing the
     enter key. Initiating a course of action will, in general, cause a new window to be
     displayed on the screen. The lower left corner of the screen displays instructions on
     how to further proceed, and the lower right corner of the screen indicates different
     operations associated with the function, or F keys, along the top of keyboard.

2.   Under the IMS menu, use the arrow keys to highlight the Analytical Library, and
     press ENTER. The computer will query you as to which element you wish to search
     the analytical line library for. Type in Nd (for Neodymium), press ENTER, and then
     press the F1 (search) function key. The computer will now display the wavelengths
     of the primary lines for Nd. Record these in your notebook for reference later on in
     the lab. When you are finshed viewing the list, press ESC (escape). This returns
     you to the previous window. You may now enter Pr ( for Praesodymium), and
     search the analytical library for primary lines of this element as before. Record the
                                                                       9-5

     wavelengths of the primary lines listed for Pr. Then press ESC twice to return you
     to the IMS menu. (Note that wherever you are within ThermoSPEC, you may
     always return to the main menu by repeatedly pressing ESC).

3.   It should now be obvious to you how to enter and view lines for a particular element
     in the Peak Search library. Enter the Peak Search library and view the line listing
     for Neodynium. Scroll through to the end of the listing by repeatedly pressing the
     Page Down key. You can similarly return to the beginning of the listing by pressing
     the Page Up key. Scroll through the listing for Neodynium using the Page Up and
     Page Down keys and record the number of lines listed in your notebook. (Each
     page contains 16 lines). This is only a fraction of the total number of lines emitted
     by Nd when excited in a Plasma. All of the rare earths give emission at a similarly
     large number of wavelengths. Note that the Peak Search library also contains
     information as to the relative intensities of the emission lines, and the state from
     which the line arises. After viewing the listing for Nd return to the main menu by
     pressing ESC.

Question 3: Experienced analysts are normally very cautious when undertaking ICP-
AES analysis of samples known to contain rare earths. Why?

Question 4 (Preliminary): The state listed for most of the Nd lines in the peak search
library is II. What does this indicate? (You can find the answer in the explanation to the
"Persistent Lines of the Elements" tables in any recent edition of the CRC handbook of
Chemistry and Physics).

Question 5 (Preliminary): Monochromators used in ICP-AES spectrometers normally
have much greater resolution than those used in Atomic Absorption Spectrometers.
Why is this so?.



III Calibrating Wavelength Accuracy and Lighting the Plasma

1.   Before proceeding to light the plasma, the monochromator should be calibrated for
     wavelength accuracy. To do this, first turn on the main valve of the Argon
     supply cylinder. The regulated pressure should read 50 to 80 psi. Under the
     Setup Menu, enter Wavelength calibration, and proceed as per the instructions on
     the computer screen. The instrument utilizes a pneumatic servo to move a low
     pressure Hg vapour lamp in front of the monochromator entrance slit, and bases its
     wavelength calibration on known emission wavelengths of Hg. The calibration
     process takes about 5 minutes. Once completed, save the calibration information
     by pressing the F9 (Done/Keep) function key.
                                                                           9-6

2.   Ensure that the sample inlet tube of the instrument is fully immersed in a beaker of
     deionized water.

3.   In the torch compartment, lock the 3 plattens of the peristaltic pump in the down
     position.

4.   Under the Setup Menu, enter the Plasma Control Panel, press F1 (startup), and
     then press F9 (Continue) when the computer requests you to do so. The plasma
     will not light if oxygen is present in the torch, instrument first purges the torch for 90
     seconds prior to ignition.

5.   Once the plasma is lit, the computer will prompt you to press ENTER. Do so, and
     then ask your instructor to assist you in adjusting the tension on the peristaltic pump
     plattens.

6.   Press F2 (Levels) and set the peristaltic pump rate to 100 RPM. Leave all other
     Levels (Torch Gas, Auxiliary Gas, Nebulizer Gas, RF Power, and Slit Height) at their
     default values. Press F9 (Done) and then escape (ESC) back to the main menu.


IV Determing the Presence of Nd and Pr in Misch Metal

1.   Under the Operation Menu, enter Wavelength Scan. Select 'Acquire' by pressing
     the appropriate F key, and then select 'Instrument' to instruct the computer that a
     new scan is to be acquired from the instrument, rather than displaying an old scan
     retrieved from the hard disk.

2.   After pressing 'F1 (Acquire)', enter the element name (e.g. Nd), and then the
     wavelength of the first primary line for this element . Then press 'Done/Keep'. You
     now have the option of setting the plasma operation and data acquisition
     parameters. Set the RF power to 950 watts, and the PMT high voltage to 600 volts.
     Note that when editing scan parameters you can also toggle between several
     available scan intervals, or windows, as indicated in the top right section of screen,
     by pressing the F6 (Scan Intrval) key. Press F6 until you have the largest window
     possible.

3.   Immerse the sample intake tube in the misch metal solution you prepared earlier.
     After allowing ~ 20 seconds to flush the system with this solution, press 'Run'. The
     instrument will scan the selected range centered around the emission line, and
     display the resulting intensity/wavelength plot. The intensity at the strongest peak
     will be displayed on the right side of the screen. (Note that this is not necessarily
     the correct peak, and that the spectrum is rich in lines. Use the arrow keys to move
     the cursor to the peak which lies closest in wavelength to the line you are seeking.)
     Now repeat the scan of the Nd line using the smallest possible scan interval. This
                                                                        9-7

     allows greater precision in determining the wavelength. (First press F9, and then
     select acquire, element name, wavelength, instrument, etc. as in step 2 above).
     Record the intensity of the selected peak in your notebook. Also record the
     apparent wavelength of the peak and compare it to the known wavelength for the
     line you have identified as due to Nd. Use the left and right arrow keys to scroll the
     cursor to a background position to one side of the emission line. The intensity
     displayed on the right side of the screen is that determined at the cursor position.
     Note that this value is not zero. Record the background emission intensity in your
     notebook, and report it in your lab writeup.

Question 6 (Preliminary): What processes contribute to the background emission in
ICP-AES?

4.   Press F9 (Done).

5.   Select 'acquire' and 'instrument' again and determine the intensities, corrected for
     background emission, as well as the apparent wavelengths, of all of the primary Nd
     and Pr lines, again using as small a scan interval as possible. Remember to set
     the PMT High Voltage to 600 V and the RF power to 950 Watts in each case.
     Return to the Operation Menu after recording this value and immerse the sample
     inlet tube in deionized water to conserve your sample.

Question 7: Comment on your observations in steps 3 to 5 above. What do
your observations suggest about the potential for interference in ICP
spectroscopy, and the resolution required for a reliable ICP-AES instrument.

Question 8: Plot the apparent wavelengths found for the primary Nd and Pr lines
against the theoretical values found in the line library. From your data, estimate the
wavelength reproducibility of the instrument. (This will be given approximately by the
standard error of regression associated with a least squares calculation). Also
determine whether the 95% confidence levels of the slope and intercept of your plot
contain the expected values.

Question 9: Why do experienced analysts always check for the presence of more than
one line when confirming the presence of an element using ICP spectroscopy?




V    Determination of the Plasma Temperature.

1.   Make sure that the printer is turned on.
                                                                        9-8

2.   Under the Operation Menu, select Analysis. When queried for a method name, type
     in PLSMATMP and press ENTER.

3.   Press F1 (Analyze). When queried for a sample name, type in Argon (or whatever
     you wish), and press ENTER.

4.   Ensure that deionized water is being pumped into the nebulizer and press F1 (Run).
     The instrument will scan and measure peak intensities of all of the Argon lines listed
     in the table in the introduction. It also measures background intensities adjacent to
     the emission lines, and subtracts these values from the peak intensities. When
     complete, the results will be displayed on the screen. If acceptable, press F9
     (Done/Keep) to obtain a printout. Then escape back to the main menu.

Question 10: From your data, prepare the appropriate plot, in accordance with
equation 2, and from the least squares slope calculate the temperature of the plasma.
How would you expect the plasma temperature to vary as a function of RF power?
(Note: The method PLSMATMP employs an RF power of 1150 watts, but the
instrument can employ powers of between 750 and 1750 watts). How would you expect
the temperature to vary as a function of distance above the torch?

Question 11: For many elements, the detection limits afforded by ICP are superior to
those obtainable by atomic absorption spectroscopy, but the ICP detection limits for the
alkali metals are inferior to the AA detection limits. Explain why this is so.



Vi Quantitative Analysis of Ce and La

1.   Under the Operation Menu, enter Analysis. When queried for a method name, type
     in 515LACE and press ENTER.

2.   Press F3 (Stndrdize). The names of all of the standard solutions appear on the
     screen. Immerse the sample introduction tube into Solution A, and press F1 (Run).
     The instrument will carry out three replicate analyses for the intensities of primary
     lines of La and Ce, and display the results on the screen. Provided the results look
     acceptable (RSD of less than one percent) press F9 (Done/Keep).

3.   Proceeding as in the step above, run standard solutions B and C.

4.   Once all of the standards have been run, press F9 (Done/Keep) again, and the
     calibration curves, as well as all of the individual results will be printed.

5.   Press F1 (Analyze). When queried for a sample name, type in Flint (or whatever
     you wish), and press ENTER.
                                                                      9-9



6.   Introduce the unknown, and press F1 (Run). The Computer will display the results
     of the analysis on the screen when complete. If these are acceptable, press F8
     then F2 (Done/Keep) to obtain a printout of the results.

Question 12: Report the concentration of cerium and lanthanum in your misch metal
solution. Also calculate and report the percentage of each of these elements in your
original striker flint.

VI Observing colour zones in the Plasma

1. Aspirate dionized water into the plasma. Prepare a sketch of the plasma, noting any
    differently coloured zones. Repeat with your standard C solution, and then with a
    0.1 M LiCl solution.

Question 13: Explain the difference in the plasma appearance for these three solutions.

VII Shutting Down the Plasma

1.   Remove your unknown, and introduce deionized water to the plasma for two
     minutes before proceeding to the next step.

2.   Escape back to the Main Menu. Under the Setup Menu, Enter the Plasma Control
     Panel. Using the appropriate F key, select Shutdown. After the shutdown is
     complete ( the computer will notify you), turn off the power to the computer,
     monitor, and printer. Then turn off the main valve of the Argon Supply cylinder, and
     release the plattens of the peristaltic pump.


References:

1    "Principles of Instrumental Analysis", D. A. Skoog, F.J. Holler, and T.A. Nieman,
     5th edn., Saunders, Philadelphia, 1984.
2    Spectrochemical Analysis. James D. Ingle, Jr., and Stanley R. Crouch, Prentice-
     Hall, New York, 1988. (A copy is kept in the prep-room).
3.   "Inductively Coupled Plasma Optical Emission Spectroscopy" G.A. Meyer, Anal.
     Chem. 59, 1345A (1987).
4.   "Elemental Analysis Using ICP-OES and ICP/MS, J.W. Olesik, Anal. Chem. 63,
     12A (1991).
5.   "Inductively Coupled Plasma Emission Spectroscopy", P.W.J.M. Boumans, ed.,
     Parts 1 and 2, QD 96 P62I52 1987. (Vol. 90 in Chemical Analysis, John Wiley,
     N.Y.)

				
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