Title Experiment 7: Gas Chromatography and Mass Spectrometry: Fuel Analysis

Transcription

1 Title Experiment 7: Gas Chromatography and Mass Spectrometry: Fuel Analysis Name Manraj Gill (Partner: Tanner Adams, Lab Section: 102) Introduction In this experiment, we use chromatography and mass spectrometry to analyze constituents of a sample. Chromatography, in the general sense, is used to separate chemical compounds mixed together. In this experiment, we use a variation of the general concept of chromatography that relies on a tube (~30 meters long) layered on the inside with a stationary phase (in this case, 5%-phenyl-methylpolysiloxane) and with the molecules of fuel (liquid when injected but vaporized in the chamber), traveling in a gaseous medium (the mobile phase). Due to this use of a gaseous mobile phase, this type of chromatography is called Gas Chromatography (GC). The stationary phase we use has x = 0.05 (or 5%) making it retain molecules with nonpolar Van der Waals interactions. The principle of GC is to separate the constituent gas species based on their chemical properties. The chemical property of interest in this case is the partition coefficient. The partition coefficient is the measure of the ratio of a compound s concentration in two different phases. Therefore, the separation in GC happens as a result of the difference in partition coefficients of the constituents as they flow through the interphase of the stationary and mobile phase. If a compound s partition coefficient dictates that it would have a higher concentration in the mobile phase than in the stationary phase, this compound would be detected earlier out of the GC column. In contrast, a species that interacts more (relative terms) favorably with the stationary phase, it would take longer to exit the column. Data and Results As mentioned in the introduction above, the different partition coefficients lead to the compounds being detected at different times. This time until detection, i.e. the time that the compound is in the column, is called the retention time, tr. Many factors, other than the inherent partition coefficient, influence the retention time of a species and it is for this reason that we perform standardization of the instrument with individually purified species. These standard results can then be used to determine their presence in mixtures. The boiling points and identities of the possible compounds in our unknown sample is given in table on the following page. Our unknown mixture was Sample 1

2 Another thing to note in regards to our data analysis is the coupling of the data from the Gas Chromatographer to a detector. We use two instruments to denote concentration and mass based identities to the chemical species that are eluted out sequentially from the GC column. In this experiment, we use a flame ionization detector (FID) and a mass spectrometer (MS). A flame ionization detector measures the concentration of gaseous species. It relies on hydrogen flame combustion to ionize the species and then correlates the number of ions detected to the concentration of the species. Mass spectrometry, on the other hand, relies on correlating the amount a compound can be charged and then measuring the mass of the molecule based on the amount of charge detected. We manually injected Sample 1 into the GC-FID instrument and obtained the following result: A non-manual FID (using an autosampler) was performed by the lab technicians for Sample 1 and this overnight sampling resulted in:

3 Retention Time (minutes) The immediate discrepancy between the two is observed in the presence of a detection at a retention time of minutes in our manual injection. This was a result of a lag in injection after the GC column began detecting. i.e. the injection was not performed simultaneously with the start of the detection. To analyze this off-set, we ignore the 1 st peak detected in the manual injection and compare the retention times observed between the two methods: 7 Off-Set in Manual Injection Autosampler Manual Injection Peak Number Therefore, it is evident that the off-set essentially (though it seems to be less significant at longer retention times) is present in the manual injection retention times (all values of

4 manual > values of autosampler). This, at the very least, confirms that the same peaks are observed in both methods because the retention times would be very comparable in the absence of the off-set. However, considering that the comparison (in regards to assigning the identities) is made with standards that are also run using the auto-sampler (shown below), it would be most reliable to compare the autosampler retention times to minimize any error in the analysis derived from this very evident discrepancy between a manual and automated injection. The off-set not being equal and diminishing at longer retention times makes it difficult to modify or correct the manual injection retention times as no two points have a similar off-set! Now, in order to assign and label the peaks, we need to augment the table of the possible compounds by adding their respective retention times on a GC-FID column that was run on the same day as our samples were. This allows for a minimization in the error derived from the instrument. The augmented table is as follows: Compound Boiling Point (Celcius) Retention Time 2,2,4- Trimethylpentane Benzene Cyclohexane n-decane n-heptane n-hexane 69 1 st Peak: nd Peak: (highest peak) 3 rd Peak: n-octane n-pentane o-xylene p-xylene Toluene (It is evident, again, that the off-set from the manual injection would skew analysis considering how close some of the retention times of unknowns are to each other) Based on this data, there is a clear correlation (a linear dependence) between the retention times observed and the boiling points of the compounds. This linear dependence can be confirmed with the near 1.0 R 2 value when the Retention Times are plotted against Boiling Points for the standards:

5 Retention TImes (minutes) Retention Times vs Boiling Point R² = Boiling Temperatures (Celcius) This clear correlation informs us of the chemical property that is discriminating between the species. The lower the boiling temperature of a species, the lower is its retention time. This can be explained based on a understanding of how the molecules interact with the stationary phase. A higher boiling point refers to a higher molecular weight. And a larger molecule has a higher number of Van der Waals interactions with the stationary phase. And as explained in the introduction, the specific stationary phase in use is dependent upon such interactions. Therefore, this correlation can be understood in this context! Now that the standards are established, we can infer the identities of the species present in our unknown by comparing the retention times. In this analysis, due to the offset observed in our manual injection (explained above), I use only the retention times from our sample s overnight autosampler data. This can be summarized as follows: Peak Number (#) Retention Time in Mixture (min) Comparable Retention Time of Standard (min) Difference in Retention Times (delta-min) Identity of Standard / of Unknown in Mixture n-pentane Cyclohexane Toluene p-xylene n-decane The only outlying ambiguity in the analysis is the determination of the identity of the species in Peak 2 as the observed retention time of minutes is close to not only

6 the retention time of Cyclohexane (2.899 minutes) but also to that of Benzene (2.915 minutes). To clear up this ambiguity, we rely on GC-Mass Spec (GC-MS) data. The GC-MS was also performed by the lab technicians and the data obtained from that is as follows for Sample 1: The initial graph (above) in the GC-MS data shows the total ion chromatogram and this representation is quite comparable to the chromatograms observed in the GC-FID data. Since we use the same GC column, the sorting/separation is still equivalent and the

7 eluents are still detected at a retention time dependent upon their boiling point. The actual retention times, however, are not comparable but the retention times observed from the GC-MS are shorter for each of the five peaks than their equivalents in the GC-FID chromatograms. The ambiguity is in the identity of the second peak. The retention time of this peak in the GC-MS chromatogram is minutes. The two possible compound this corresponds to, as described above, are Benzene and Cyclohexane (differ from each other by 1 C in boiling point). The MS is averaged over minutes (1.003 min to min). We do not have mass spectrum of specific peaks. Therefore, the ambiguous peak from the GC data is not correlated to the mass spectrum directly because the mass spectrum is composite of all 5 GC peaks. To analyze the mass spec, we look to the base peaks of each of the known compounds to narrow down on a base peak that would help us distinguish between cyclohexane and benzene. The mass spectrums of the known (unambiguous) GC peaks of the individual components are given on the following page. These were obtained from the National Institute of Standards and Technology (NIST) database. The 100% relative intensity peaks, the highest peaks, and their m/z (mass-charge ratio) are characteristic and comparable to the mass spectrum of our mixture. The base peaks can be summarized as such: Compound Base Peak s m/z Value Pentane 43 Toluene 91 p-xylene 91 Decane 57 The base peaks of the two possible compounds, cyclohexane and benzene (mass spectrums from the database given below), are 56 and 78 m/z respectively. An additional attribute of the GC MS analysis is the relationship of the peaks to their abundances. The abundances in the GC retention-time based analysis shows that the 1 st Peak, attributed to Pentane, seems the least abundant. This, perhaps, explains the absence of Pentane s base peak of 43 in the mass spectrum. That is, it is out competed (made negligible) in terms of relative intensity when compared to the more abundant species present. Based on these reference data, we can assign the mass spectrum of Sample 1 (see below after the reference mass spectrums)

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9 The assigned mass spectrum of Sample #1 (excluding the assignment of Pentane): The base peak of 78 m/z (benzene) is not abundant whereas the base peak (56m/z) of cyclohexane is apparent in Sample 1 s mass spectrum. As seen in the assignments above, the other sized ionic species that are also characteristic of the compounds going through

10 mass spectrometer are labeled. This is based on the less than 100% relative intensity peaks observed on the reference mass spectrums of the individual compounds. The GC-MS analysis allows us to clear the ambiguities present in the GC-FID analysis and thereby be more certain of the presence or absence of a particular species in our mixture. Discussion In conclusion, this experiment has allowed us to accurately determine the identity of chemical species present in Sample 1, an unknown mixture. In the following image, the combined analysis from GC-FID and GC-MS data shows an assignment of the peaks to the chemical compounds they arise from. An overall remark to be made is not only on the consistency of measurements arising from an autosampler (the lack of an off-set facilitates comparison of retention times from two different runs) but also the benefit of appending a GC column with a mass spectrometer that can aid in distinguishing the chemical species present when their boiling points are very similar. The similarity in the boiling points leads to a similarity in retention times observed on the GC-FID chromatogram (as explained by the linear relationship between the two). A few inconsistencies observed are of the relative abundances observed from the peaks between the GC-FID and GC-MS analysis. The GC-MS reflected a significantly low abundance of the compound detected in the first peak whereby the GC-FID showed roughly similar amounts of each of the compounds. This could perhaps be attributive to the means by which the chromatograms are generated for the two instruments, i.e. the FID relies on combustion of ions to detect the concentrations that perhaps brings about a loss of sensitivity to minutely different concentrations. Regardless, we can conclude the benefit of not using a manual injection and appending the GC column with a mass spectrometer!