Concentration Of Copper Isotopes In An Unknown Solution Biology Essay

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In this experiment, sample solution #11-067 containing two different isotopes of copper was analyzed using inductively-coupled plasma mass spectrometry (ICP-MS). The unknown was spiked with a stock copper solution containing a high concentration of 65Cu in the isotope dilution mass spectrometric technique. This technique was used to determine the relative concentrations of both 63Cu and 65Cu. In addition to the unknown sample, a sample of a copper solution containing a natural abundance ratio of 63Cu to 65 Cu was analyzed in the mass spectrometer to which the isotopic ratio of the unknown solution was compared.

The sample is mixed with an acid to act as its matrix and is introduced into the ICP torch through the bottom into the centre tube by a peristaltic pump.1 The acid with which the sample is diluted does not change the composition of the sample, but rather the signal intensity produced by the instrument4. The vapour portion of the sample passes through the argon plasma and the liquid portion evaporates, leaving any solids that may have been present in the liquid. In this experiment, these solids are copper atoms. These atoms are unstable in such high temperature conditions, so they ionize to form copper ions and electrons.

These ions can then be detected by the instrument by their amount of change and mass, and the readout displays the atom's concentration in the original sample. The natural copper sample used in this experiment contains both 63Cu and 65 Cu atoms, so the IDMS technique can be used to differentiate between the two species by spiking the sample with a stock solution with a high concentration of 65Cu and a low concentration of 63Cu. This is the perfect internal standard because it will produce a signal from the instrument of the same intensity and in the same way that the unknown copper solution would. IDMS works independently of any drift or matrix effects3 in the instrument while using it over a long period of time, making it a reliable technique5 to precisely determine ratios of elemental isotopes in an unknown solution.

Results and Calculations

Table 1: Intensity Values corrected for background

Sample

1

2

3

4

5

Corrected Mean Measured Intensity

Corrected Mean Measured Intensity

Corrected Mean Measured Intensity

Corrected Mean Measured Intensity

Corrected Mean Measured Intensity

62.930g

0

49661.745

27.401

50946.487

42471.777

64.928g

0

23672.973

62093.644

88136.011

20165.390

2: Purity of Enriched 65Cu Solution

0.044109% 63Cu

99.9559% 65Cu

3: k from solution 5

0.933

4: R from solution 4

0.58268796

5: Concentration of Cu by IDMS

318 ppb

6: Concentration of Cu by ratio

575 ppb

Table 2: Calculation results Parts 2-6

See the following pages for the printouts of each Trial

Discussion

The purpose of this experiment was to determine the relative concentrations of two copper isotopes, 63Cu and 65Cu, in an unknown solution using both an isotope dilution mass spectrometric method and a ratio method. In the process, the purity of the enriched 65 Cu was also assessed as compared to the purity certificate provided with the chemical upon purchase.

5 separate solutions were analyzed using the ICP spectrometer, as outlined in the Appendix, Table 5. Solution 1 was used as a blank, and its intensities for 63Cu and 65Cu were subtracted from each subsequent run as a background correction. Solution 4 was the unknown spiked with an enriched 65Cu solution, where solutions 2, 3 and 5 were the unknown, enriched standard and natural solution respectively, all diluted to a volume of 50 mL with 1% HNO3. The instrument was pre-preprogrammed to run 5 mass analyses of the solutions and output the individual runs and the average masses for both isotopes. These printouts have been attached to the end of the Appendix of this report.

The biggest obstacle to overcome during this experiment was the deviation in intensity results for each solution. While the nebulizer was rinsed with 1% HNO3 for approximately 2 minutes between each solution, it was determined that the longer the sample solution was drawn into the spectrometer, the higher the intensity of the isotopes. In fact, doing a second run of the unknown sample solution resulted in a significant increase for both the 65Cu and 63Cu isotopes: a difference of 4694.166 and 10254.037, respectively. This drift may indicate that doing the analyses of the same solutions at a different point in time may result in different relative concentrations of both copper isotopes, as is apparent by the large deviation between the IDMS and ratio method results of 318 and 575 ppb, respectively.

For the ratio determination of the concentration of the copper solution in part 6 of the calculations, the count data from the 63Cu isotope was used. This data was used because, judging from the ratio of isotope intensities in the natural copper solution of which the unknown solution was made, there is a higher concentration of 63Cu in the unknown. For that reason, the 63Cu data should result in a more reliable determination of the copper concentration in the unknown than the 65Cu data. By this method, 63Cu and 47Ti are used for this type of ratio determination because they are the most reliable isotopes of their respective atoms and provide the most consistent results as far as ICP analysis.

The alleged purity of the enriched copper solution was 99.9% 65Cu and 0.1% 63Cu according to the manufacturer's purity certificate. This purity was found to be accurate, as is shown in the results table of this report, with the composition of the solution being 99.95% 65Cu and 0.044% 63Cu.

Conclusion

The concentration of the unknown copper solution was determined to be 318 ppb by the isotope dilution mass spectrometry (IDMS) method, and 575 ppb by the ratio of isotopes method. These two results do statistically agree, as is seen from the calculation in the Appendix of this report. In addition, the purity of the enriched copper solution reported by the analyte's purity certificate was proven to be accurate.

References

Chemistry Department. Chemistry 311: Instrumental Analysis Laboratory Manual; University of British Columbia: Vancouver, 2010W.

Skoog, D.A.; Holler, F.J.; Crouch, S.R. Principles of Instrumental Analysis, 6th ed.; Brooks/Cole: Belmont, CA, 2007.

Gaines, P. ICP Operations- Internal Standardization and Isotope Dilution.

http://www.inorganicventures.com/tech/icp-operations/internal-standardization-and-isotope-dilution (Accessed February 13, 2011).

Gaines, P. ICP-Operations- Calibration curve and Standard Additions Techniques.

http://www.inorganicventures.com/tech/icp-operations/calibration-curve-and-standard-additions (Accessed February 13, 2011).

Bradford, T.; Cook, M.N. Inductively Coupled Plasma (ICP)

http://www.cee.vt.edu/ewr/environmental/teach/smprimer/icp/icp.html#Combining%20ICP%20with%20Mass%20Spectrometry (Accessed February 13, 2011).

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