Chemistry 321, Experiment 8: Quantitation of caffeine from a beverage using gas chromatography

Transcription

1 Chemistry 321, Experiment 8: Quantitation of caffeine from a beverage using gas chromatography INTRODUCTION The analysis of soft drinks for caffeine was able to be performed using UV-Vis. The complex sample matrix associated with soft drinks required sample preparation to be used in order to separate caffeine from matrix components such as caramel and syrup. The sample preparation (or separation ) used is referred to as solid-phase extraction (SPE). In many instances, the sample matrix is still too complex even after using SPE. For instance, the analysis of drugs in blood or urine cannot simply be measured by UV-Vis because numerous other compounds would interfere with the analysis. In addition, oftentimes a more sensitive method of analysis is required. The analysis of caffeine in beverages such as coffee, tea and numerous energy drinks cannot be accomplished by similar SPE methods with UV-Vis. Other components are present in the drink that will also be extracted with caffeine and also have absorbance bands that would interfere with caffeine. In these sample matrices, further separation is required after SPE. We refer to these separation methods as chromatography. In this laboratory, you will use SPE (using disposable pipette extraction (DPX)) for sample preparation for the analysis of caffeine in coffee and energy drinks using gas chromatography with mass spectrometry (GC/MS). Gas Chromatography Sample Carrier gas cylinder Gas filters, pressure control, and flow control Injection system Data system Detector Column oven Figure 1. A block diagram of a GC showing all of the essential components. A gas cylinder provides the mobile phase, a flow controller monitors its flow, the injection system introduces the sample solution, the chromatography column is contained inside an oven where the temperature is controlled, a detector provides an electrical signal proportional to the concentration of analyte, and a data system is used for data acquisition and analysis.

2 A Separation of gases; principle of gas chromatography B C To detector Figure 2. As the sample of 2 components (represented by circles and squares) comes in contact with the stationary phase (A), some of the compounds will partition into the polymeric film. As the temperature is raised, the more volatile compounds will move out of the film and into the open tubular space where the gas flow is moving in one direction (B). After a certain length (or time) as the temperature continues to rise, the analytes are separated and pass to the end of the column to be detected (C). Chromatography methods involve some sort of stationary phase and a mobile phase. A chromatography method you are probably very familiar is thin chromatography (TLC) or paper chromatography, where the stationary phase is silica gel or cellulose and the mobile phase is some solvent, such as methylene chloride or methanol. One of the most popular types of chromatography is gas chromatography (GC). In gas chromatography, the mobile phase is an inert gas (such as helium) and the stationary phase is a polymeric film contained inside of an open tubular capillary column (or solid substrate in a packed GC column). A block diagram of GC is shown in Fig. 1 (taken from CHEM 621 lab experiment). The GC is used to separate the components and detect the intensities of the separated compounds. The resulting chromatogram is a plot of signal intensity versus time (or retention time). The detector used in this GC laboratory experiment is a flame ionization detector (FID) or mass spectrometer (MS). The FID gives a response that is based on C-H groups

3 (formation of CHO + groups), and this type of detector is therefore very useful in the analysis of organic compounds. The MS is the most popular detector, which gives a fragmentation pattern (or fingerprint ) of the chemical compounds separated by GC. This fingerprint of the compounds provides high specificity for compound identification and quantitation. We will use GC/MS for this experiment. Quantitation of caffeine The concentration of caffeine will be determined by making a calibration plot of known concentrations of caffeine, and then comparing the areas obtained from the calibrators with that of your sample solution. The use of an internal standard will be used to provide better quantitative data. This type of standard is added to the calibrator solutions (for making the calibration plot) as well as the sample solution. By adding the same amount of this standard to each of the solutions, a ratio of the areas of caffeine with respect to the area of internal standard will be plotted versus concentration of caffeine. The reason for using an internal standard is that some errors in the analysis can be compensated by injecting a known amount of the standard; ie, if the volume of solution that is injected or the volume that is introduced into the column differs slightly from one sample injection to another, the amount of standard will also vary in proportion. This also helps to correct for detector response, which may be an issue for many types of detectors (where the response drifts over time). Most internal standards are deuterated analogues of the analyte ( heavy ), so the internal standards have the same chemical and physical characteristics as the analyte of interest. Keep in mind that these internal standards can only be used with mass spectrometry (GC/MS). In this laboratory experiment, the internal standard will be 13 C 3 -caffeine, which is another heavy caffeine standard. The main advantage of using a heavy analogue of caffeine as an internal standard is that its chemistry is exactly the same as that of caffeine. If the extraction efficiency is low for caffeine for one of the samples, then it will also be low for the internal standard. By using the ratio of the analyte to internal standard for the quantitation, the internal standard corrects for this error. Conventional GC (GC-FID) cannot be utilized with deuterated internal standards because the retention times would be the same (ie, d 3 -caffeine or 13 C 3 -caffeine would have the same retention time as caffeine and would interfere with its detection). PROCEDURE A. Calibrators of caffeine 1. Add 20 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100

4 GC vial and label as S Add 40 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100 GC vial and label as S Add 50 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100 GC vial and label as S Add 60 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100 GC vial and label as S Add 80 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100 GC vial and label as S Add 100 µl of 1,000 ppm caffeine standard to 5 ml volumetric flask, and add 100 µl of 13 C 3 -caffeine internal standard. Bring to volume using methanol. **Make sure to mix thoroughly by inversion (at least 10 times). 7. Transfer app. 1 ml to a GC vial and label as S-6. B. SPE of caffeine from coffee or energy drink: Solution preparation (same as above for calibrators): 1. Pipette 1 ml 70% IPA and 1 ml DI water into test tube Pipette 2 ml DI Water into test tube Pipette 100 µl coffee or energy drink (your choice) into test tube 3. Add 1 ml DI water to the same test tube. 4. Add 100 µl 13 C 3 -caffeine internal standard into test tube 3 this is the internal standard. Mix test tube 3 by swirling the test tube or vortex mixing. 5. Pipette 2.0 ml methanol into test tube 4. DPX extraction procedure: 1. Aspirate solution 1, mix with air (app. 7 ml), wait 10 seconds, dispense. 2. Aspirate solution 2, mix with air (app. 7 ml), wait 10 seconds, dispense. 3. Aspirate the beverage solution (test tube 3), mix with air (app. 7 ml), wait 30 seconds, dispense. 4. Add about 0.5 to 1 ml of DI water to the top of the DPX tip using a water bottle, and dispense into the same test tube to discard the sample matrix. 5. Aspirate solution 4, mix with air (app. 7 ml), wait 10 seconds, dispense into a clean test tube. THE CAFFEINE (and internal standard) FROM THE BEVERAGE IS IN THIS TEST TUBE.

5 6. Transfer all of the contents of this solution into a 5 ml volumetric flask. Dilute the flask to mark using methanol. **Make sure to mix thoroughly by inversion (at least 10 times). 7. Transfer app. 1 ml of the solution into a GC vial clearly label the vial! Suggestion: Mark as G-1-1 for group 1 sample 1, or G-2-3 for group 2 sample 3. Make sure you identify each sample number in your notebook. C. Submit each vial (calibrator and sample) to be analyzed by GC. D. Obtain peak areas for caffeine and 13 C 3 -caffeine for each of the calibrator standards and your samples. These will be provided on blackboard later in the week. DATA ANALYSIS 1. Make a table of peak areas for caffeine and 13 C 3 -caffeine for each chromatogram (calibrators and sample solution). 2. Make a calibration plot of area of caffeine v. concentration. a. Determine the best fit linear equation for the calibration plot. b. Determine the R 2 value, which provides a basis of the linear quality of the data. c. Calculate the concentration of caffeine (ppm) in the sample solution from this calibration plot. d. Calculate the concentration of caffeine in the drink. 3. Make another calibration plot using the area ratio of caffeine to 13 C 3 -caffeine v. concentration of caffeine. a. Determine the best fit linear equation for the calibration plot. b. Determine the R 2 value from this plot. c. Calculate the concentration of caffeine (ppm) in the sample solution using this calibration plot (ie, incorporating the internal standard). d. Calculate the concentration of caffeine in the drink. 4. Convert the concentration of caffeine in the drinks analyzed by your group into units of % (mass/volume)? Use both calibration plots for this determination. QUESTIONS 1. Which method for the calibration plot provided the best R 2 value? There may not be much of a difference using an autosampler, but if the injections were performed manually, there would be a big difference. Explain why. 2. If the concentration of caffeine is supplied for the energy drink, determine the % error in the analysis using both calibration plots. Which calibration plot had the least error? 3. Which drink from your group has the highest concentration of caffeine?