UMD CHEM 425 - AASLabreport (10 pages)

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AASLabreport



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AASLabreport

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Pages:
10
School:
University of Maryland, College Park
Course:
Chem 425 - Instrumental Methods of Analysis
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AAS Intro Atomic Absorption Spectroscopy AAS is an analytical laboratory technique that converts the analyte of study into a gaseous state for quantitative determination of the chemical species using the Beer Lambert Law1 AAS relies on the quantum principle that the ground state energy of an atom absorbs a discrete amount of electromagnetic radiation before it is excited to a higher state energy This radiation is absorbed in the form of a wavelength with each element corresponding to its own wavelength thus giving AAS a great degree of selectivity1 This is done by exciting the free atoms via optical radiation using a special hollow cathode lamp containing the element of interest allowing for the precision required for the excitation wavelength2 Another advantage of using AAS is it s easy use and relatively inexpensive cost when compared to other types of high precision spectroscopy However AAS also contains its disadvantages Since AAS relies on the nebulization of the analyte only solutions can be analyzed In addition AAS has less sensitivity than a graphite furnace which allows for analysis of smaller samples between 5 60 L whereas AAS requires samples in the range of 1 3 mL2 AAS can only be used for quantitative analysis of an element whereas atomic emission or atomic fluorescence are quantitative and qualitative instruments The first step in analyzing a chemical s atomic components is to atomize the sample1 For this lab the sample was atomized via a high temperature flame that was generated via a mixture of compressed air and fuel specifically an air acetylene flame at a temperature of about 2 300 C1 Once the liquid sample is decomposed into its atomic constituents the hollow cathode lamp is shined and allowed to pass through a collimating lens at the sample s corresponding absorption wavelength The absorption of light by the atomic components is then used to determine the sample s concentration once it passes through a monochromator wavelength selector This is because the absorption of the light from the hollow cathode lamp is linearly proportional to the sample concentration via the Beer Lambert relationship shown in Figure A4 up to a range of 0 1 to 0 8 units another disadvantage of AAS as our least concentrated standard had an absorbance below 0 12 A general scheme for the setup and instrumentation is illustrated in Figure B below5 Figure A Beer Lambert Law visual representation of incident light and relationship to concentration4 Figure B Atomic Absorption Spectrometer Block Diagram5 For this experiment we were interested in using the atomic absorption spectrometer in order to better understand the effectiveness of releasing agents towards overcoming matrix effects and chemical interferences This will be done by first completing a routine AAS analysis on trace amounts of the metal calcium in millipore water containing diluted HCl These results will then be compared with calcium solutions consisting of various added reagents that were suspended in a solution of either millipore water or diluted HCl This should allow for the determination of the effects of additional metals and pH on measured absorbance values for calcium Out of all of the reagents listed below in the methods section we hypothesize that the samples containing PO43 and Al3 will have the greatest matrix effects and will lead to a decrease in measured Ca2 absorbance with PO43 causing the most effect In addition we predict that samples containing the solvent HCl or the releasing agents Sr2 LaCl3 or Na2EDTA will indicate a mitigation of any matrix effects induced by PO43 and Al on the Ca2 metal Methods A SOLAAR atomic absorption spectrometer was used in the experiment to determine the chemical interference of different ions on calcium and unknown concentrations of calcium For each solution atomized the spectrometer took three samples each were recorded The average value was used to calculated the absorbance of the solution All solutions were forced through a Millipore water filter before be atomized by the spectrometer For part I II a series of 20 mL solutions with known calcium concentrations 0 ppm 10 ppm 20 ppm 30 ppm 40 ppm 50 ppm in Millipore water containing 2 12M HCl were used to create to a calibration curve with an R2 value greater than 95 Calibration curves after the samples were taken for both parts in the experiment the second calibration was performed after the samples due to the sensitivity of the machine In Part I a calibration curve was used to determine the calcium concentrations of unknowns A B and C The unknown concentrations were calculated using Beer s Law c A l In Part II 10 20 mL solutions were created to determine the interference of various chemicals all solution contained a 30 ppm concentration of calcium Again Beer s Law was used to calculate the unknown concentration of calcium in the solution2 Solution 1 Millipore water control Solution 2 100 ppm Al in Millipore water Solution 3 100 ppm Al in diluted HCl solution Solution 4 500 ppm PO43 in Millipore water Solution 5 100 ppm Al 2000 ppm Sr in HCl solution Solution 6 500 ppm PO43 2000 ppm Sr in Millipore water Solution 7 100 ppm Al 1 w v Na2EDTA in HCl solution Solution 8 500 ppm PO43 1 w v Na2EDTA in Millipore water Solution 9 100 ppm Al 5 w v LaCl3 in Millipore water Solution 10 500 ppm PO43 5 w v LaCl3 in Millipore water Results Part I Graph 1 Calibration Curve Part I Calibration measurements from day 1 the slope of the curve for Beer s Law 0008688 1 741E 5 R2 9866 Sample 1 Sample 2 Sample 3 Average Ca ppm Unknown A 0 00891 0 00748 0 00955 0 008647 9 952 02 Unknown B 0 00460 0 00471 0 00482 0 004710 5 421 02 Unknown C 0 00057 0 00053 0 00048 0 000527 0 6062 02 Table 1 Calculated concentrations of Unknowns The AAS samples of each solution were used to calculate the unknown s Ca concentration using the slope from the calibration curve as with a y intercept of zero concentration of A 008647 0008688 9 952268 ppm Part II Graph 2 Calibration Curve Part II 0069 1 349E 4 with an R2 of 989 calibration curve for part 2 using the standards described in the methods Our calibration curve only contains nine data points instead of ten as we ran out of our 20 ppm standard Also 30 ppm standard for the second calibration only has 2 data plots due to limited solution Solution Sample 1 Sample 2 Sample 3 Average Ca ppm 1 0 2125 0 2146 0 2141 0 2137 30 77 19 2 0 01663 0 01671 0 01663 0 01666 2 398 21 3 0 07348 0 07285 0 07228 0 07287 10 49 19 4 0 09628


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