ATOMIC EMISSION SPECTROSCOPIC DETERMINATION OF LITHIUM

ATOMIC EMISSION SPECTROSCOPIC DETERMINATION OF LITHIUM

Lithium and its compounds have several industrial applications, including heat-resistant glass and ceramics, high strength-to-weight alloys used in aircraft, lithium batteries and lithium-ion batteries. These uses consume more than half of lithium production.
Trace amounts of lithium are present in all organisms. The element serves no apparent vital biological function, since animals and plants survive in good health without it. Nonvital functions have not been ruled out. The lithium ion Li⁺ administered as any of several lithium salts has proved to be useful as a mood-stabilizing drug in the treatment of bipolar disorder, due to neurological effects of the ion in the human body.  Lithium affects the flow of sodium through nerve and muscle cells in the body. Sodium affects excitation or mania.  Lithium is used to treat the manic episodes of manic depression. Manic symptoms include hyperactivity, rushed speech, poor judgment, reduced need for sleep, aggression, and anger. It also helps to prevent or lessen the intensity of manic episodes.
THEORY:Atomic spectroscopy is an instrumental method that takes advantage of the fact that every atom has unique quantized energy levels for electrons in a particular atom. If a source of energy is applied to a sample that can produce an atom of an element in an atomic state, several techniques are available to probe these atomic states. Flames are a commonly used source of excitation since their high temperature atomizes most materials. In aqueous solutions, the solvent water is stripped from the ions and atoms in the gaseous state are obtained. Since flames are a rich source of electrons, the large majority of the atoms will be in the atomic ground state.

Two common analytical methods are then used to determine the amount of a particular atom in the flame. Since the majority of the atoms will be in the atomic ground state, a the light from a high energy source made from the element to be determined can be passed through the flame. Atoms of that specific element will absorb the light resulting in a decrease of light intensity reaching the detector as compared to a distilled water blank containing none of the atoms of that element. This technique is known as atomic absorption spectrophotometry. A block diagram of this instrument is shown in Figure 1. Light from the hollow cathode lamp source is passed through the flame and then through a monochromator which isolates the particular wavelength selected for analysis. The change in intensity of the light striking the detector generates a voltage signal which is read out on a digital-panel-meter or recorder.

A simpler technique which gives high sensitivity for a few elements, notably the alkali metal elements, takes advantage of the fact that the flame can supply enough energy for these elements to reach an excited atomic state. The atom will quickly return to the ground state by emitting a photon of light that exactly corresponds to the energy difference between the excited state and the ground state. Quantitative analysis can then be performed by measuring the intensity of the emitted light which is proportional to the number of atoms of the particular element in the flame. The block diagram of the instrument is exactly the same as in atomic absorption spectrophotometry except that no light source is used. The sample response is then measured as the difference in signal between the solution being measured and a distilled water blank.

This particular experiment involves the determination of lithium in a prepared sample. The most intense analytical line is the 670.8 nm transition which will be used in this experiment.

Reagents:
	1000 ppm certified lithium standard

EXPERIMENTAL PROCEDURE

Preparation of Standards and Determination of a Calibration Curve
You will be divided into groups to prepare the following solutions: 

A standard solution containing 1000 ppm (1000 mg/mL) of Li⁺ will be available in the laboratory  (Pipet these standards directly from the bottle - do not waste this solution).  Standard dilutions will be made with an adjustable 1000 μL pipet.  See your laboratory instructor for directions on how to use these pipets.  Six standards should be prepared by making the following dilutions:

Solution	mL 1000 ppm Li standard	Total Volume
1	100	100 mL
2	200	100 mL
3	300	100 mL
4	500	100 mL
5	700	100 mL
6	1000	100 mL
check	200 - 400	100 mL

In addition, a check standard of known concentration should be prepared (use a value different from your standards). Each student should present the instructor with a clean 100 mL volumetric flask for the unknown with a tape label with your initials (it does not have to be dry.) Dilute the sample to the mark, mix thoroughly, and pour the solutions into 15 mL conical centrifuge tubes.
The atomic emission spectrometer will be available in the prep room. The instructor will ignite the oxygen/acetylene flame and optimize the instrumental parameters.  The instrument should be in emission mode. We will attempt to use the auto-sampler accessory for this experiment.  The Instructor will show you how to load the vials into the racks.
Manually you would zero the meter while aspirating distilled water into the flame. The most concentrated Li solution is aspirated into the flame and the instrument automatically adjusts the signal for optimal analyisis. A method will be set up to run your calibration standards and your samples. Run the Li solutions from lowest to highest concentration.  When running your samples, dip the tip in distilled water for about 5 seconds and wipe with a Kimwipe before running each sample solution. Obtain a printout of your data.
LAB CLEAN-UP
 When you have finished running your solutions, rinse all flasks, centrifuge tubes, caps and stoppers with distilled water.  Place them upside down in racks to dry.  Do not dry volumetric flasks upside down.

DATA ANALYSIS:
Obtain a printout or a digital copy of the spectrometer signal versus lithium concentration data.  Alternatively, you may read the signal directly from the spectrometer.

Use a spreadsheet to prepare a calibration plot of the standard solutions. Check your standards for linearity. Discard any points that significantly differ from a straight line. Use regression analysis to obtain the equation for a straight line. The slope and intercept of the calibration line can be used to determine the concentration of the unknown.

Calculations:
Use a spreadsheet to generate a plot of your data. Use the standards to prepare a calibration curve by plotting signal (y) versus lithium concentration (x). Perform a least-squares regression analysis of your data to determine the slope and intercept. If you are using Excel, you can use the "TRENDLINE" feature to draw your best regression line and choose the "show equation" option to obtain the slope and intercept. Try fitting the data with both a linear fit and a polynomial fit.   Check the linearity of the curve and omit any points that fall significantly outside the best straight line  (You can use CHART/SOURCE DATA/ADD to select a new set of x-y values to plot from the CHART menu).

Atomic Emission Determination of Lithium  —  Sample Data Solution Li  conc   (ppm) signal 1 1 0.110 2 1 0.139 3 2 0.227 4 2 0.258 5 3 0.341 6 3 0.365 7 5 0.635 8 5 0.652 9 7 0.867 10 7 0.849 11 10 0.980 12 10 1.021 unknown ? 0.418

The new spectrometer may be much more sensitive than older instruments.  The atomic spectroscopy techniques are inherently non-linear at higher concentrations due to self absorption in the flame.  Light emitted by an excited atom on one side of the flame may be absorbed by a ground state atom of that element on the other side of the flame and therefore not reach the detector.  This effect becomes more pronounced at higher concentrations where the atom density in the flame is increased.  The net effect of this self-absorption will be a curved calibration curve.   Many instruments can correct for this effect by using a polynomial calibration curve ( y = a + bx + cx² ).  The following example shows how this data can be evaluated in Excel.

			intercept	slope
fitting parameters			a	b	c	R2
Linear Fit (all points)			0.0599	0.1022	---	0.9646
Linear Fit (selected points)			-0.0163	0.1299	---	0.9912
Polynomial Fit (all points)			-0.0717	0.1754	-0.0067	0.9923
unknown mean			0.418	signal for your unknown
Linear Fit (all points)			3.50	ppm	=(y-a)/b
Linear Fit (selected points)			3.34	ppm	=(y-a)/b
Polynomial Fit (all points)        the calculated unknown signal is evaluated from the guessed value using the polynomial fitted parameters  from using TRENDLINE (polynomial fit) on the graph.			guessed unknown concentration	measured unknown signal	calculated unknown signal from guessed value	difference between observed and calculated
			choose Tools/Goal Seek from menu
			by changing the value in this cell			SELECT THIS CELL and set the desired value to 0
			3.00	0.418	0.394	0.024
		Results	3.18	0.418	0.418	0.000

Note that all 3 estimates give different values.  This is a choice the analysts must face.  One observation that might be used is where the plot intersects the y-axis at zero concentration.  The linear fit for all points is biased high while the polynomial fit curves away from the origin.  For trace analyses, the difference between 3.50, 3.54, and 3.18 might not be significant but the analyst needs to make a rational choice.