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D - question4info - 722 Part Eight The Techniques 60 exit...

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Unformatted text preview: 722 Part Eight The Techniques 60 exit —> —> Carrier gas gases Coolant Figure 15.11 Collection trap. Another method for collecting samples is to connect a cooled trap to the exit port of the column. A simple trap, suitable for microscale work, is illustrated in Figure 15.11. Suitable coolants include ice water, liquid nitrogen, or dry ice—acetone. For instance, if the coolant is liquid nitrogen (bp —196°C) and the carrier gas is helium (bp w—269"C), com pounds boiling above the temperature of liquid nitrogen generally are condensed or trapped in the small tube at the bottom of the U-shaped tube. The small tube is scored with a file just below the point where it is connected to the larger tube, the tube is broken off, and the sample is removed for analysis. To collect each component of the mixture, you must change the trap after each sample is collected. 15.11 QUANTITATIVE ANALYSIS The area under a gas-chromatograph peak is proportional to the amount (moles) of com- pound eluted. Hence, the molar percentage composition of a mixture can be approximated by comparing relative peak areas. This method of analysis assumes that the detector is equally sensitive to all compounds eluted and that it gives a linear response with respect to amount. Nevertheless, it gives reasonably accurate results. The simplest method of measuring the area‘of a peak is by geometrical approximation, or triangulation. In this method, you multiply the height h of the peak above the baseline of the chromatogram by the width of the peak at half of its height wm. This is illustrated in Figure 15.12. The baseline is approximated by drawing a line between the two sidearrns of the peak. This method works well only if the peak is symmetrical. If the peak has tailed or is unsymmetrical, it is best to cut out the peaks with scissors and weigh the pieces of paper on an analytical balance. Because the weight per area of a piece of good chart paper is reasonably constant from place to place, the ratio of the areas is the same as the ratio of the weights. To obtain a percentage composition for the mixture, first add all the peak areas Approximate area = h x w ,2 Figure 15.12 Triangulation of a peak. Technique 1 5 Gas Chromatography 723 (weights). Then, to calculate the percentage of any component in the mixture, divide its in- dividual area by the total area and multiply the result by 100. A sample calculation is illus- trated in Figure 15.13. If peaks overlap (see Fig. 15.7), either the gas-chromatographic conditions must be readjusted to achieve better resolution of the peaks or the peak shape must be estimated. There are various instrumental means, which are built into recorders, of detecting the amounts of each sample automatically. One method uses a separate pen that produces a trace that integrates the area under each peak. Another method employs an electronic de- vice that automatically prints out the area under each peak and the percentage composition of the sample. ‘ Most modern data stations label the rep of each peak with its retention time in minutes. When the trace is completed, the computer prints a table of all the peaks with their reten- tion times, areas, and the percentage of the total area (sum of all the peaks) that each peak represents. Some caution should be used with these results because the computer often does not include smaller peaks, and occasionally does not resolve narrow peaks that are so close together that they overlap. If the trace has several peaks and you would like the ratio of only two of them, you will have to determine their percentages yourself using only their two areas or instruct the instrument to integrate only these two peaks. For the experiments in this textbook, we have assumed that the detector is equally sen- sitive to all compounds eluted. Compounds with different functional groups or with widely varying molecular weights, however, produce different responses with both TCD and FID gas chromatographs. With a TCD, the responses are different because not all compounds have the same thermal conductivity. Different compounds analyzed with a FID gas chro- matograph also give different reSponses because the detector response varies with the type of ions produced. For both types of detectors, it is possible to calculate a response factor for each compound in a mixture. Response factors are usually determined by making up an Area PeakB = 19 x 122 = 2320 mm2 AreaPeakA = 17x 40 = 680mm2 Total area = 3000 mm2 680 %A = —3000x 100 = 22.7%Composition of mixture Total 100.0% Ratio 5 = ——2320 = 1% A 680 Figure 15.13 Sample percentage composition calculation. 724 Part Eight The Techniques equimolar mixture of two compounds, one of which is considered to be the reference. The mixture is separated on a gas chromatograph, and the relative percentages are calcu- lated using one of the methods described previously. From these percentages you can determine a response factor for the compound being compared to the reference. If you do this for all the components in a mixture, you can then use these correction factors to make more accurate calculations of the relative percentages for the compounds in the mixture. Consider the following example, which illustrates how response factors are determined. In this example, an equimolar mixture of benzene, hexane, and ethyl acetate is prepared and analyzed using a flame-ionization gas chromatograph. The peak areas obtained are Benzene 966463 Hexane 831 158 Ethyl acetate 1449695 In most cases, benzene is taken as the standard, and its resPonse factor is defined to be equal to 1.00. Calculation of the response factors for the other components of the test mix- ture proceeds as follows: Benzene 966463/966463 = 1.00 (by definition) Hexane 831158/966463 = 0.86 Ethyl acetate 1449695/966463 = 1.50 Notice that the response factors calculated in this example are molar response factors. It is necessary to correct these values by the relative molecular weights of each substance to obtain weight response factors. When you use a flame-ionization gas chromatograph for quantitative analysis, it is first necessary to determine the response factors for each component of the mixture being ana- lyzed. For a quantitative analysis, it is likely that you will have to convert molar response factors into weight response factors. Next, the chromatography experiment using the un- known samples is performed. The observed peak areas for each component are corrected using the response factors in order to am've at the correct weight percentage of each com- ponent in the sample. 15.12 GAS CHROMATOGRAPHY— MASS SPECTROMETRY (CC-MS) A recently developed variation on gas chromatography is gas chromatography—mass spectrometry, also known as GC-MS. In this technique a gas chromatograph is cou- pled to a mass spectrometer (see Appendix 6). In effect, the mass Spectrometer acts in the role of detector. The gas stream emerging from the gas chromatOgraph is admitted through a valve into a tube, where it passes over the sample inlet system of the mass spectrometer. Some of the gas stream is thus admitted into the ionization chamber of the mass spectrometer. The molecules in the gas stream are converted into ions in the ionization chamber, and thus the gas chromatogram is actually a plot of time versus ion current, a measure of the number of ions produced. At the same time that the molecules are converted into ions, they are also accelerated and passed through the mass analyzer of the instrument. The instru- ment, therefore, determines the mass spectrum of each fraction eluting from the gas- chromatography column. ...
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