In previous sections, examples of molecular absorption spectroscopy were introduced. These give broad band spectral peaks due to presence of vibrational and rotational levels associated with each electronic level. On the contrary, atomic spectra are composed of separate lines due to transitions between different electronic levels. Atoms do not contain vibrational and rotational levels associated with electronic levels, thus leading to line spectra (Figure 1).
Atomic spectroscopy is divided into three types which are absorption, emission, and luminescence spectroscopy, All yielding line spectra.
Flame photometry is an atomic emission technique which may be regarded as the simplest of atomic spectroscopic methods and is very similar to the flame test which is applied for detection of alkali metals. Flame photometry is good only for elements that are easily excited and do not require very high temperatures (Na, K, Li, Ca are the most widely determined atoms by this technique).
Figure 1: Energy level diagram for atomic sodium (Only few lines are shown)
A flame photometer instrument is extremely simple where the sample in solution is aspirated through an aspirator or nebulizer into the flame which is usually a propane / air fuel or, even, a purified natural gas/air mixture. The sample matrix evaporates followed by atomization of the sample. Atoms present in the high temperature zone of the flame are excited to higher energy levels by absorbing energy from the flame. As excited atoms return to the ground state they emit radiation in definite wavelength depending on the energy level from which each atom drop. This gives rise to a line spectrum. However, in flame photometry a pre-selected filter (depending on the atom in question) is used and it is the intensity of the emission line that is practically measured and is related to the original concentration of the sample in solution. The detector is usually a phototube or a photomultiphier tube depending on the quality of the instrument. A schematic diagram of a simple flame photometer is shown is shown in Figure 2.
Figure 2: A schematic of a simple flame photometer instrument.
Filters can be changed or selected to suit the determination of the element in question.
Commercial instruments are usually equipped with capabilities to analyze for Na, K, L, and sometimes Ca.
Qualitative Versus Quantitative
Atomic emission based on flame photometry is used for quantitative determinations only. This is because quantitative analysis using atomic emission requires advanced equipments and measurement of exact location of each emission line followed by comparison with standard line sheets. However, it is possible to use the machine for the qualitative and quantitative determination of the elements of the first and second groups of the periodic table, since these elements exhibit good emission and very intense and few emission lines, using regular flames usually utilized in flame photometry.
Preparation of a Calibration Curve
A calibration curve should be constructed using standard solutions prepared from the finest grade available. Concentrations covering the regions around the expected analyte signal should be used. It is not wise to try to interpolate or extrapolate a calibration curve of this type since the relationship between the signal and concentration may not be exactly linear, as different factors affect the shape of the calibration plot.
Experiment 7: Flame Photometric Determination of sodium, Potassium, and Lithium
Analytical methods for the determination of sodium, potassium, and lithium are limited except those techniques based on atomic spectroscopy. The simplest versions of atomic spectroscopic methods are emission methods based on flames, using filters for wavelength selection. This is termed flame photometry and is widely used for routine analysis of samples containing species like Na, K, Li, and Ca. Emission signal at a specific wavelength is proportional to the concentration of analyte which emits at that wavelength.
Chemicals and Reagents
1. Standard Na, K, and Li solutions (1000 ppm each).
2. Sample of unknown concentrations of Na, K, and Li.
b. Need Preparation
1. Prepare standard Na, K, and Li solutions that are 1, 5, 10 ,20 ,30 ,40 ,50 ,60, 70, 80, 90, and 100 ppm of each metal ion.
1. Flame photometer equipped with Na, K, and Li filters.
1. Follow instructions for the correct operation of the flame photometer available.
2. Adjust the signal, using the Na filter, to zero using distilled deionized water.
3. Read the signal for the Na set of standards and then that of the unknown sample.
4. If the signal obtained for the sample is out of range, dilute a portion of the sample properly till a signal within the range is obtained.
5. Construct a calibration curve for Na in the sample and report your results in ppm.
6. Repeat steps 2-5 for K and finally for Li and find the concentration of each species in the sample. Results should also be reported in ppm analyte.
Note: The sample unknown can be a sample of any drinking supply. Therefore, each student is asked to bring his own sample and the class is asked to report an overview of water quality in the different areas of the city as compared to accepted values.