Chem 155 Unit 1 Spring 2010 Page 1 of 5

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1 Chem 155 Unit 1 Spring 2010 Page 1 of 5 Quiz 1 preparation: Main learning objectives: 1. Computation and interpretation of confidence intervals 2. Computation and motivation for standard addition methods 3. Computation and motivation for internal standard methods 4. Experimental design (standard and sample prep.) 1. Design simple experiments given instrument dynamic range, expected sample composition, instrument sample volume requirement and given a set of standards. a. For the purposes of this exercise, we will pretend that the solvent density is around 1.0 g/ml. This allows us to define: %=g/0.1l, ppth=g/l, ppm=mg/l, ppb=μg/l. b. A GCMS can measure oleic acid in dissolved in diethyl ether in the range of 100 ppb to 50 ppm (0.1 to 50 ppm). The GCMS requires approximately 100 μl of sample. Butter contains about 5% oleic acid by weight. It dissolves in diethyl ether. You have pure oleic acid as a standard. It also dissolves in diethyl ether. You can prepare samples using automatic pipettors with a volume range of 20 to 1000 μl. You have volumetric flasks that are 1, 5, 10, 25, 50, 100, 250 and 500 ml to work with. i. Suggest a sample preparation that involves only weighing and dissolving the butter. The solution should have oleic acid within the instrument s dynamic range. ii. Propose three appropriate calibration standard concentrations. iii. Propose a method to prepare 1. A standard stock solution. 2. The three 3 calibration standards. 2. Calculate a confidence interval and interpret it. Given a set of data and a t-table, be able to asses the likelihood that there a given value differs from the computed one. a. For the following set of numbers: 2.78 kg, 2.66 kg, 2.76 kg calculate: i. average ii. sample standard deviation iii. 95% confidence interval b. The expected weight of this animal was 2.62 kg. i. Is there strong evidence that the measured weight differs from expectation? ii. Is there less than or greater than a 5% chance that the observed difference arises from random fluctuation.

2 Chem 155 Unit 1 Spring 2010 Page 2 of 5 c. Reflecting back on the temperature-camera question consider the following cases: i. temperature a b c d e 102 The error bars are 95% CI values for a given set of measurements on airline passengers. In which of the above cases can you conclude that the passenger clearly has an elevated or depressed temperature: Above 102? Below 102? What can you say in the remaining cases? For case a, which of the following is/are appropriate statement(s): i. there is less than a 5% chance that the observed difference between the average and the expectation value of 102 arises from random fluctuation. ii. there is at least a 95% chance that the patients true temperature exceeds 102 iii. this passenger s temperature exceeds 102 iv. it is highly likely that this patient s temperature exceeds 102 For case d, which statement is appropriate? i. This passenger does not have a fever ii. There is less than a 5% chance that this passenger has a fever iii. There is less than a 5% chance that this passenger s true temperature is equal to or greater than 102. For case b, which statement is appropriate? iv. This passenger definitely has a temperature of 102, v. There is a 95% chance that this passenger s temperature is 102. vi. There is a 95% chance that this passenger s temperature lies within the indicated boundaries. Since these boundaries include the threshold of 102, then there is a substantial likelihood that this persons temperatures equals or exceeds 102

3 Chem 155 Unit 1 Spring 2010 Page 3 of 5 Simulation Parameters: True mean: 3 Population standard deviation: Data and 95% CI 5 Data and +/- 1 standard deviation % CI 4.5 Sample Standard 4 Deviation N=5 2 N=5 1.5 N=4 N=3 1.5 N=4 N= Three experiments were done with the same instrument: the first had 5 replicates, the second 4 and the third 3. Data and 95% CI are on left, data and sample standard deviations are on right. These data all have the same mean and population standard deviation! 1. Why are the CI values larger than the standard deviations? 2. Rationalize why do the CI ranges get larger from left to right (as N goes from 5 to3) 3. Note that the sample standard deviation error bars do not always include the true population mean.

4 Chem 155 Unit 1 Spring 2010 Page 4 of 5 3. Do a simple standard addition calculation and understand why an experiment may be done that way: a. A urine sample was split into two 5.00 ml aliquots. The first aliquot was diluted to 100 ml with water. To the second, 1.00 ml of 2.00 mm urea was added and the mixture was diluted to the mark with water. The UV absorbance signal at the expected wavelength for urea was for the first sample and for the second. What is the concentration of urea in the urine sample? b. Why might this measurement be done in this way (i.e. by the method of standard additions)? c. For what does this method (standard additions) compensate? d. If there were another molecule, for example, glycine, in the urine that had some UV absorption at the same wavelength as the urea, would this method help us to cancel out the glycine signal? 4. Understand the function of an internal standard: a. In a GC experiment measuring the concentration of 2-butene in gasoline, a sample and 2-butene standards were prepared each at ml in sample vials. To each vial exactly 10.0 μl of a standard spike of 1,2-difluorooctane was added. b. The following results were obtained: Sample 2-butene signal 1,2 difluorooctane signal Gasoline ppm 2-butene std Calculate the concentration of 2-butene in the gasoline taking into account the internal standard. Corrected signal / / = conc 1.00 ppm x ppm x =

5 Chem 155 Unit 1 Spring 2010 Page 5 of 5 5. Understand the concept of method validation be able to answer simple questions about validation. a. Suggest a way to validate the following aspects of an analysis: i. The analyst ii. The sample preparation method: iii. The calibration method iv. The instrument [ b. What aspect of an analysis do the following measures validate: i. Spike recovery analysis. ii. Validation or quality control standard. c. For which of the following does an internal standard compensate? i. Accuracy of calibration standards. ii. Sample preparation method iii. Drift in Instrumental sensitivity iv. Matrix effects d. For which of the following does the method of standard additions compensate? i. Accuracy of calibration standards. ii. Sample preparation method iii. Drift in Instrumental sensitivity iv. Matrix effects