Lab Report Basic Instrumental Exp 4.docx

Experiment 4: AAS INTRODUCTION In atomic absorption spectroscopy (AAS) , light is absorbed by the atoms of an analyte. If a solution containing a metal is aspirated in the form of aerosol into a hot flame, the liquid/solvent is evaporated droplets and the metal is vaporized mainly as atoms. Alternatively an atomic vapour can be produced by rapid electrochemical heating of a graphite rod or tube on which a drop of the sample has been placed. If the sample contains particular elements, its atom will selectively absorb some of the radiation thereby attenuating the beam and causing the detector signal to fall. This absorbance is proportional to the concentration of that element in the vapour state and hence in the original sample. In the flame AAS, we aspirate a sample into a flame using a nebulizer. The flame is lined up in a beam of light of the appropriate wavelength. The flame(thermal energy) cause the atom to undergo a transition from the round state to the first excited state. When the atoms make their transition, they absorb some of the light from the beam. The light beam is generated by a lamp that is specific for a target metal. The lamp must be perfectly aligned so the beam crosses the hottest part of the flame and travels into the detector. The detector measures the intensity of the beam of the light. When some of the light is absorbed by a metal, the beam intensity is reduced. The detector records that reduction as an absorption. That absorption that is shown on a read out by data system. We can find the concentrations of metals in a sample running a series of calibration standards through the instrument. The instrument will record the absorption generated by a given concentration. By plotting the absorption versus the concentrations of standards, a calibration curve can be plotted. We can then look at the absorption for a sample solution and use the calibration curves to determine the analyte concentration in the sample. Atomic absorption is a very sensitive technique compared with ordinary UV-Visible spectrophotometry and can therefore be used for trace analysis. Flame atomic absorption is a very common technique for detecting metals and metalloids in environmental samples. It is very reliable and simple to use where the amount of light absorbed can be measured against a standard curve to determine the concentration of the metal present.

OBJECTIVES

1. To study how to operate a flame AAS instrument. 2. To plot a standard calibration curve for determination of Ca in the sample. 3. To determine the amount of Ca in the sample using the standard calibration curve.

APPARATUS Beaker Burette Retort Stand Dropper Volumetric flask, 50ml CHEMICALS Deionized water 1000 ppm Ca/Cu/Cr/Zn stock solution Unknown solution PROCEDURES A. Preparation of Standard Solutions 1. 5.00ml of 1000 ppm stock solution was pipetted. It was transferred to 50.0ml volumetric flask and diluted with deionized water. The solution was labeled ‘100 ppm’. 2. By using the 100 ppm solution and a 50.0ml volumetric flask, a series of standard solutions having the following concentrations were prepared : 1 ppm , 3 ppm , 5 ppm , 7 ppm , 9 ppm 3. The unknown solution was obtained. 4. The absorbance of the standard solutions were measured, followed by the unknown.

B. Operation of the AAS(referred on lab manual)

Question In a standard addition method, you will prepare a series of solution in which you add different increments of standard solutions to fixed aliquots of an unknown sample X i.e. 10.00ml of X. All solutions were prepared using 50.00ml volumetric flasks. Suppose that the analysis of the solutions of X gave the following results. * 1. Plot the standard addition graph. 2. determine the concentration of X (in ppm) in unknown sample.

Discussion Atomic absorption spectroscopy is designed for the quantification of metal elements , and trace metal and trace inorganic elements present in environmental samples. It is done so by measuring absorbed radiation of free atoms of the element of interest and reading of the spectra produced when the sample is excited by radiation. The atoms absorb ultraviolet visible light and make transitions to a higher energy level. Atomic absorption methods measure the amount of energy in the form of photons of light that are absorbed by the sample. A detector measures the wavelengths of light transmitted by the sample, and compares them to the wavelengths which originally passed through the sample. One technique that should be done for every analysis is to obtain the signal of a blank; a substance prepared in the same manner as the analyte, except it contains no analyte. You can calibrate the instrument you are using to be set to a signal of zero for the signal of the blank. You can also subtract the signal produced by the blank from all the signals obtained throughout the analysis, generating “corrected” values. The corrected values can then be used for further data manipulation such as generating calibration curves to solve for unknown concentrations. Subtracting the blank signal, from the absorbance value, will correct for some of the interferences resulting from the matrix of the solution. Instrumental settings can also be manipulated to counter matrix effects. Adjusting fuel-tooxidant ratios in flames, or using a different oxidant, can reduce certain matrix interferences. If incomplete combustion is causing organic substances within the matrix to produce a signal, increasing the temperature of the flame can ensure complete combustion, reducing or eliminating the effects of organics. In method of background correction, the signal from a continuum source, such a deuterium lamp, is subtracted from the signal from a line source, such as the analyte’s hallow cathode lamp. A chopper alternates the radiation passing through the instrument between the deuterium continuum and the analyte source. The signal from the continuum source indicates when radiation is attenuated by something other than the analyte. When the continuum source is passed through the atomizer, the atoms of interest only absorb the resonance wavelength. Because this is a very small fraction of the total radiation, the effect of the analyte on the continuum signal is negligible. Aside from the analyte, the continuum source can be attenuated by scattering or broadband absorption. By subtracting the signal from the continuum source from the analyte’s line source, results in an analyte signal that is corrected for these attenuations. This method does, however, have flaws. Some systems “over” or “under” correct. The signal-to-noise ratio often decreases with the addition of another source because it cannot correct for background specific to the incident radiation’s interaction with the system. It is also limited in its wavelength range, since deuterium is an ultraviolet source.

Both the beam from the deuterium lamp and the beam from the hollow-cathode lamp, hit the chopper, which is constantly spinning. This produces alternating pulses of radiation from the deuterium source and the hollow cathode lamp. When the beam from the deuterium lamp passes through the atomizer, and the beam from the hollow cathode lamp is reflected off the mirror. Conversely, when the beam of the hollow cathode passes through the atomizer, the beam of the deuterium lamp is reflected off of the mirror. In a Zeeman Effect instrument, a magnetic field is applied to atoms, splitting the electronic energy levels. This causes multiple absorption lines to be present, and the sum of these absorption lines is equal to the original line that formed them. In it, the absorption line is split into two components: the pi component, which is present at the original wavelength, and sigma component which is both negatively and positively shifted so that two absorption lines are present. One is present at the right of the pi line and the other is present at left of the pi line. Absorption by the pi and sigma lines corresponds to different components of polarized light. Polarized light waves are light waves in which the vibrations are occurring in the same plane. Pi lines correspond to radiation that polarized parallel to the magnetic field, while σ lines correspond to radiation polarized perpendicular to the field. The Zeeman Effect is preferred over most other forms of background correction, as it tends to be more accurate. AC Zeeman systems tend to be more expensive than DC Zeeman systems, but they are more sensitive and have larger linear working ranges. The experiment need to plot a standard calibration curve for determination of Ca in the sample. By running the FAAS, the standard calibration curve of all the standards have been plotted. Based on the experiment conducted, the amount of Ca in the sample obtained is 3.077mg/L. The calibration curve correlation coefficient was obtained through the experiment which is 0.999. This show that the dilution steps of the standards was run precisely.

Conclusion In conclusion, the experiment is exposed techniques to operate a flame AAS instrument. Besides, the standard calibration curve for determination of Ca in the sample was plotted and the amount of Ca concentration was identified which is 3.077 mg/L. Reference https://books.google.com.my/books?id=4539BAAAQBAJ&pg=PA79&lpg=PA79&dq=integration+of+a as+spectroscopy+spectrum&source=bl&ots=MJSE1EBZ70&sig=peBAuYj5mbEJqQuPBDLk8ZwL5_0&hl =ms&sa=X&ved=0ahUKEwiatuighP3XAhVEuI8KHdiEBtM4ChDoAQhcMAg#v=onepage&q=integration %20of%20aas%20spectroscopy%20spectrum&f=false http://www.labs-services.com/product/atomic-absorption-spectroscopy-aas/ http://www.sciencedirect.com/science/article/pii/0022407374901137