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Energy & Fuels 2008, 22, 410–415
Determination of Hydrocarbons Types and Oxygenates in Motor Gasoline: A Comparative Study by Different Analytical Techniques V. Bansal,* G. J. Krishna, A. P. Singh, A. K. Gupta, and A. S. Sarpal Indian Oil Corporation Ltd., Research & DeVelopment Centre, Faridabad-121007, Haryana, India ReceiVed March 9, 2007. ReVised Manuscript ReceiVed August 17, 2007
Various standard and published methods based on chromatographic and spectroscopic techniques are routinely used for hydrocarbon types (aromatics, olefins, oxygenates, etc.) in gasoline range fuel products for the assessment of product quality monitoring (PQM). The precision of data obtained by different techniques is of great significance in the PQM accomplished by various refining processes. The present work describes systematic studies for the statistical correlation of the results of total aromatics, olefins, and oxygenates (MTBE and ethanol) in motor gasoline (MS) obtained by the standard and published methods based on gas chromatography (GC, ASTM-D-5580, ASTM-D-4815), nuclear magnetic resonance (NMR) spectroscopy, and fluorescence indicator analysis (FIA, ASTM-D-1319). Results of a large number of samples from different sources were obtained by using these methods and treated statistically including two tests of significance—the Student t test and the analysis of variance (ANOVA)—in order to establish differences or similarities between the results obtained by these methods. The comparative study has proved that methods based on the 1H NMR spectroscopic technique for the estimation of aromatics, olefins, and oxygenates are comparable with the standard ASTM methods in terms of repeatability and accuracy. Results have indicated very good correlation between NMR, FIA, and GC based methods. The utility of NMR spectroscopic methods in real quality control situations is discussed in this paper.
Introduction The need for reliable and rapid analytical methods in energy research is essential, where fuel quality is correlated with their physicochemical properties and performance. Among the different classes of hydrocarbons in motor gasoline, the aromatics, olefins, and oxygenates content are identified as predominant characteristics that influence combustion performance. The determination of total aromatics, olefins, and oxygenates in gasoline are necessary to assess product quality in accordance with fuel specifications conforming to European Union (Euro standards II to V) and Indian (Bharat stage II to V) standards. Environmental regulations provide the limits for concentration of total aromatics, olefins, and oxygenates content in finished gasoline, which have been established for the year 1995 onward by European Standards. The limits of aromatics and olefins in gasoline have been specified to reduce ozone activity and toxicity of automotive evaporative and exhaust emissions. Olefins and oxygenates are good octane components of petrol, but higher olefins may lead to deposit formation and increased emissions of reactive hydrocarbons and undesirable compounds. Being thermally unstable, higher olefins may lead to gum formation and deposits in an engines intake systems. Higher aromatic content can increase engine deposits and increase tail pipe emissions including CO2, HC, and NOx. Oxygenates induce a lean shift in engine stoichiometry and reduce CO emissions. Ethers, alcohols, and other oxygenates are blended in gasoline to increase octane number as well as reduce exhaust gas emissions. Type and concentration of various oxygenates are specified and regulated to ensure acceptable commercial gasoline quality. Drivability, vapor pressure, phase * To whom correspondence should be addressed.
separation, exhaust, and evaporative emissions are some of the concerns associated with oxygenated fuels. Given the stringent environmental regulations, the optimization of product quality and performance becomes critical. The control specifications of motor gasoline (MS) as per BIS (Amendment No. 1 to IS 2796:2000 Motor Gasoline Specification) limit the total aromatic content to below 42.0 vol % (maximum), olefin content maximum 18.0 vol %, and oxygenates–ethanol maximum 5.0 vol %, and methyl tert-butyl ether (MTBE) maximum 15.0 vol %. With this, comprehensive legislation has imposed a tremendous technological challenge not only to refiners but also on the analytical methodologies, which can provide precise and reliable qualitative and quantitative information on total aromatics, olefins, and oxygenates in the commercial MS samples. Therefore, it has become very essential to monitor these hydrocarbons for quality control of final products and to comply with regulatory requirements for the control of pollution. Standard methods as recommended by BIS for the determination of total aromatics, olefins, and oxygenates content in motor gasoline are based on gas chromatography1 (ASTM-D6730, D-5580, D-3738, D-4815), mass spectrometry (ASTMD-2789), infrared spectroscopy (ASTM-D-5986), supercritical fluid chromatography2 (ASTM-5186), and fluorescent indicator adsorption3 (ASTM-D-1319) techniques. Methods based on nuclear magnetic resonance (NMR) spectroscopy for the determination of aromatics, olefins, naphthenes, and paraffins (1) American Society for Testing and Materials, ASTM-D-6730, D-5580, D-3738, D-4815. Annual Book of ASTM Standards, 1997, Vol. 05. 02. (2) American Society for Testing and Materials, ASTM-D-2789, ASTMD-5986, ASTM-5186. Annual Book of ASTM Standards, 1997, Vol. 05. 02. (3) American Society for Testing and Materials, ASTM D-1319. Annual Book of ASTM Standards, 1997, Vol. 05. 02.
10.1021/ef070121l CCC: $40.75 2008 American Chemical Society Published on Web 12/04/2007
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Energy & Fuels, Vol. 22, No. 1, 2008 411 Table 1. European/Indian Specifications for MS Fuels
Euro-II/Bharat stage II (1998)
characteristic
Euro-III/Bharat stage III (2000)
benzene (vol %) max
1.0
1.0
aromatics (vol %) max oxygen (wt %) max olefins (vol %) max
N/A N/A N/A
sulfur (wt , ppm) max
500
42 2.7 18 (Euro-III), 21 (Bharat stage III) 150
content (PONA) and oxygenates4–16 in commercial MS have gained prominence due to their ease and speed of determination. Because of nonavailability of any standard procedure, the methods developed by the NMR technique are generally validated by standard methods based on other analytical techniques. Many workers have coordinated the studies to compare the results of different analytical techniques for hydrocarbons types in naphtha, gasoline, kerosene, and diesel range fuel samples.17–19 The present work describes a systematic approach undertaken for correlation of results of total aromatics, olefins and oxygenates content in the MS samples determined by different analytical methods based on gas chromatography (ASTM-D-5580, ASTMD-4815), fluorescence indicator analysis (FIA, ASTM-D-1319), and nuclear magnetic resonance spectroscopic techniques.11,14,15 The data generated on more than 138 MS samples of varied nature (FCC, hydrocracking, etc.) have been subjected to detailed statistical analysis in order to establish differences or similarity between the results obtained by these methods. Experimental Section Samples. MS fuel samples (138 numbers) were collected from different Indian refineries and marketing terminals. Standards of ethyl alcohol and MTBE procured were 99.9% pure of AR grade. Twenty blends of varying compositions (% v/v) of ethyl alcohol and MTBE were prepared in MS. FIA Method. The fluorescent indicator adsorption method has been performed as per standard procedure ASTM-D-1319. GC Method. GC analyses as per ASTM-D-5580 and D-4815 have been performed on a CLARUS 500 GC analyzer equipped with both FID (flame ionization detector) and TCD (thermal conductivity detector). The system is a dedicated analyzer, which utilizes a 10-port valve, a PPC split injection port, an auxiliary PPC flow controller, a variable restrictor, and injectors. A micropacked polar column (56 cm × 1/16 in. SS) packed with 20% strongly (4) Kapur, G. S.; Sigh, A. P.; Sarpal, A. S. Fuel 2000, 79, 1023. (5) Roussis, S. G.; Fedora, J. W. S. A. Anal. Chem. 1997, 69 (8), 1550. (6) Fodor, G. E.; Kohl, K. B.; Mason, R. L. Anal. Chem. 1996, 68, 236. (7) Iob, A.; Buenafe, R.; Abbas, N. M. Fuel 1998, 77, 1861. (8) Bansal, V.; Kapur, G. S.; Sharma, V. K.; Sarpal, A. S Energy Fuels 2000, 14, 1024–31. (9) Sarpal, A. S.; Apparao, N. V. R. Res. Ind. 1986, 31, 64–69. (10) Bansal, V.; Kapur, G. S.; Sarpal, A. S.; Kagdiyal, V.; Jain, S. K.; Srivastava, S. P. Energy Fuels 1998, 12, 1223–27. (11) Kalsi, W. R.; Sarpal, A. S.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Energy Fuels 1995, 9, 574–79. (12) Sarpal, A. S.; Kapur, G. S.; Mukherjee, S.; Jain, S. K. Energy Fuels 1997, 1, 662. (13) Nagpal, J. M.; Joshi, G. C.; Rastogi, S. N. Fuel 1995, 74 (5), 720. (14) Kapur, G. S.; Singh, A. P.; Sarpal, A. S. Fuel 2000, 79 (9), 1023– 29. (15) Sarpal, A. S.; Kapur, G. S.; Mukherjee, S.; Tiwari, A. K. Fuel 2001, 80 (4), 521–28. (16) Meusinger, R. Fuel 1996, 75 (10), 1235–43. (17) Kosal, N.; Bhairi, A.; Ashraf Ali, M. Fuel 1990, 69 (8), 1012–19. (18) Mukherjee, S.; Kapur, G. S.; Sastry, M. I. S.; Tiwari, A. K.; Gupta, A. K.; Sarpal, A. S. DEW-Tech. Ber. 2003, 4, 37–42. (19) Lee, S. W.; Coulombe, S.; Glavincevski, B. Energy Fuels 1990, 4, 20–23.
Euro-IV/Bharat stage IV (2005)
Euro-V/Bharat stage V (2008/9)
1.0 (Euro-III) fotr Bharat stage III, 1.0 (NCR & Metros), 3.0 (other areas) 35 2.7 18
1.0 35 2.7 18
50
10
polar stationary phase tetracyanoethoxylated pentaerythritol (TCEPE) and a nonpolar capillary (WCOT) column of 30.0 m × 0.53 mm × 5.0 µm film thickness for aromatics, and 30.0 m × 0.53 mm × 2.6 µm film thickness has been used for oxygenates analysis. Programmed temperature operating conditions with injector and detector at 150 and 250 °C, respectively, as described in ASTMD-5580 and D-4815 have been used. Calibration has been performed using known concentration of hexan-2-one for aromatics and propan-2-ol for oxygenates analysis. NMR Measurements. 1H NMR spectra have been obtained on a 300 MHz NMR spectrometer under the following conditions: spectral width, 5000 Hz (0.0–12.0 ppm); spectral size, 16K; digital resolution, 0.49 Hz/point; 90° pulse, 13 µs; relaxation delay, 10 s; number of scans, 64; solution concentration, 5% in CDCl3 and tetramethylsilane (TMS) as internal reference. Integration has been done after baseline correction, and a mean of three integration values has been taken for each calculation.
Results and Discussion The latest European specifications (Euro-II to Euro-V) for gasoline have been given in Table 1, which are being adopted in toto by India as Bharat stage II to Bharat stage IV specifications for total aromatics, olefins, and oxygen content. As per Euro-IV and Bharat stage IV specifications for gasoline, total aromatics should be less than 35.0 vol % and olefin content should be less than 18.0 vol %. These parameters are required to be determined in each gasoline batch samples released by refinery and marketing as per ASTM-D-5580, D-4815, and D-1319 test methods. The limits for total oxygen content is specified as 2.7 wt % maximum, which will be sum of the oxygen content contributed by each oxygenated compound, and individual oxygenates are determined by the ASTM D-4815 test method. Estimation of Total Aromatics and Total Oxygenates by GC Method. Typical gas chromatograms of commercial MS sample showing total aromatic components and oxygenates (MTBE and ethanol) are shown in Figures 1 and 2, respectively. The sample is injected automatically into the injector of the GC system under the optimized conditions. The chromatogram is recorded and processed automatically. After identification of peaks, area of benzene, toluene, and internal standard from the first run (mode 1), and the internal standard, ethylbenzene, p-xylene, o-xylene, C9, and other aromatics were noted from the second run (mode 2). After completing calculations, the results are reported as per test procedures ASTM-D-5580 by the GC analyzer, and a similar procedure is applied with the incorporation of a different internal standard for the determination of alcohols and ethers and is reported as per standard method ASTM-D-4815. 104 number of MS samples have been analyzed for total aromatics by the ASTM-D-5580 method, and 54 number of samples have been analyzed for oxygenates by the ASTM-D-4815 method. Estimation of Total Aromatics and Total Olefins by FIA Method. The fluorescent indicator adsorption (FIA) method
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Figure 1. GC chromatogram of a commercial MS sample showing aromatics.
Figure 2. GC chromatogram of a commercial MS sample showing oxygenates.
Figure 3. 300 MHz 1H NMR spectrum of a MS sample containing oxygenates.
has been applied for the estimation of total aromatics and olefins as per ASTM-D-1319 for 27 numbers of commercial MS samples. Estimation of Total Aromatics, Olefins, and Total Oxygenates by NMR Method. Figure 3 shows the 1H NMR spectrum of a representative MS sample containing oxygenates (ethanol and MTBE). Various resonance signals assigned to different groups have been marked in the spectrum. The spectrum exhibits the structural specificity of the hydrogen type distribution associated with chemical shift regions that include hydrogens of aromatic rings (region A; 6.5–8.0 ppm), hydrogens
of olefinic carbons (region U; 4.5–6.2 ppm), hydrogen on carbon attached to oxygen of oxygenates (ether and alcohol, region O due to –OCH2/CH3), hydrogen on carbon alpha to aromatic rings (region B due to R-CH2; 2.45–3.0 ppm, 4.10–3.20 ppm), hydrogens on β-CH/CH2 groups to aromatic rings (region C due to R-CH3; 2.05–2.45 ppm) and –CH/CH2 groups of cycloalkanes and normal and isoparaffins (region E; 1.4.0–2.05 ppm), methylene groups of longer alkyl chains (region G; 1.05–1.40 ppm) and methyl hydrogens on γ, δ to aromatic rings, and methyl hydrogens of alkanes and cycloalkanes (region H; 0.5–1.05 ppm). The integral areas of these regions have been denoted by letters A, U, O, B, C, E, G, and H, respectively. Region “U” is characteristic of the presence of olefins and can be used to distinguish commercial gasoline sample (containing olefins) from SRN by just visual inspection. The region “O” is characteristic of the presence of oxygenates in the sample. The details for estimation of total aromatics from 1H NMR have been described elsewhere.14 After the complete assignment of the 1H NMR spectrum and the relative contribution of substituted aromatic (Arq) carbons have been realized, the total aromatic content of the samples has been calculated. For the estimation of total olefins, the absolute number of unsaturated protons attached to various types of double bonds (–CH3 branched, internal, and alpha) resonating in the chemical shift region of 4.5–6.2 ppm and average carbon alkyl chain length
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Energy & Fuels, Vol. 22, No. 1, 2008 413
Table 2. Repeatability/Reproducibility Estimates for Various Hydrocarbon Types by Different Techniques hydrocarbon type
range (% mass)
repeatability
reproducibility
total aromatics (GC) (ASTM-D-5580) total aromatics (FIA) (ASTM-D-1319) total aromatics (NMR)14 total olefins (FIA) ASTM-D-1319 total olefins (NMR)15 oxygenates (GC) ASTM-D-4815 total oxygenates (NMR)11
10.0–50.0 10.0–50.0 10.0–50.0 0.0–30.0 0.0–30.0 0.0–10.0 0.0–10.0
0.46 1.6 1.5 1.9 1.6 0.18 (MTBE), 0.24 (EtOH) 0.36
1.59 3.5 3.6 7.8 5.4 0.56 (MTBE), 0.86 (EtOH) 1.22
(n) have been utilized for the derivation of equation for their determination. The main steps involved in the estimation of total aromatic content are the estimation of total relative number of carbons (TC) and total group molecular weight (TW) of the sample. The TC for the sample is calculated by dividing the individual regions in the 1H NMR spectrum by the number of protons causing the signal. TW is then obtained by multiplying these numbers with the respective molecular weight of the groups (12 for C, 13 for CH, 14 for CH2, and 15 for CH3 group). Therefore, TC and TW are given as TC ) A/1 + 1.12U + B/2 + C/3 + E/2 + F/1 + G/2 + H/3 + Arq + O/2 Substituting the contributions of Arq as described in ref 14, the above equation simplifies to TW ) 13(A + F) + 14B + 10C + 7(E + G) + 5H + 14.36U + 7O For estimating the total aromatic content, the group molecular weight of the aromatics (AW) is estimated which is given by substituting the value of average alkyl chain length of aromatic substitutents Arq as described in ref 14; the equation simplifies to AW ) (A/1)13 + 7nB + 9C + 6B The total aromatic content of the sample (wt %) can then be estimated as described in our earlier publication.14 total aromatic content (TA) ) (AW/TW) × 100 The total olefin content (TU) in weight percentage has already been explained15 in detail as given in the equation TU ) KU′ where K is proportionality constant and K ) 8.8 and 7.3 for FCC and coker gasoline, respectively. U′ is the percentage integral intensity of the olefinic region “U” (4.5–6.2 ppm), and Itotal is the total integral intensity of the region 0.5–8.0 ppm. U′ ) U/Itotal × 100 The determination of oxygenates (TO) in wt % has been described,11 and the total oxygenates content has been given as TO ) OW/TW × 100 OW ) O/nM OW is the group molecular weight of oxygenates, O is the integral intensity of oxygenates, n is the number of protons in the alpha carbon group of oxygenates (n ) 2 for ethanol and 3 for MTBE ether or methanol type oxygenates), and M is the molecular weight of representative oxygenate type. The NMR method is independent of the source of sample and refinery process and applicable to all types of samples containing olefins and oxygenates. Besides, the method can also
yield total aromatics, olefins, and oxygenate content from the same 1H NMR spectrum. Repeatability of NMR, GC, and FIA Methods The following criteria are used for judging the acceptability of results (95% probability). The repeatability of the NMR method has been estimated on two numbers of samples having different average values of the total aromatic, olefinic, and oxygenates content. A single operator recorded each of the samples at least five times under similar experimental conditions on the same NMR equipment. Each time, a fresh sample solution was prepared in CDCl3 in a new NMR tube as per standard protocol. The total aromatic, olefinic, and oxygenate content have been estimated from the recorded NMR spectra by using developed equations. The repeatability of FIA and GC methods has been performed as given in the ASTM methods by checking on two numbers of samples performed by the same operator with the same apparatus under constant operating conditions thrice on each sample. The values of the repeatability (at 95% confidence level) for the samples by FIA and GC techniques as per ASTM-D-5580, D-4815, D-1319, and NMR methods are given in Table 2. The FIA and NMR methods have a higher value of repeatability as compared to the GC method. Comparison of Results by GC, FIA, and NMR Methods. The FIA method reports the total aromatic content and olefinic content in volume percentage contrary to GC and NMR methods, which give results as weight percentage. Therefore, a direct comparison of the GC and NMR methods with FIA method is not possible. However, as the density of the motor spirit fuel samples and those of the aromatic species is less than 1, the values by FIA method shall be on lower side compared to GC and NMR methods. For the purpose of comparison, the result of GC and NMR methods has been converted in to vol % by application of densities of the sample and the aromatics. It is assumed that similar types of aromatic species are present in the analyzed samples, and an average densities of 0.73, 0.89, and 0.70 have been considered for the sample as such, aromatics, and olefins present, respectively. A (vol %) ) A (wt %) × Fsample/Faromatics U (vol %) ) U (wt %) × Fsample/Folefins The data on aromatic, olefinic, and oxygenates content in all the samples obtained by the three techniques, viz., GC, FIA, and NMR, have been compiled and treated statistically. The percentages of total aromatics obtained by three methods have been plotted taking two techniques at a time. Figure 4 shows a correlation plot between the aromatic content obtained by GC (ASTM-D-5580) and FIA (ASTM-D-1319, Figure 4a), GC (ASTM-D-5580) and NMR (Figure 4b), and FIA (ASTM-D1319) and NMR (Figure 4c) methods. The values of R have been derived from regression analysis of the data set provided in the Supporting Information. As evident from the plots, results
414 Energy & Fuels, Vol. 22, No. 1, 2008
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Figure 4. Correlation plots of the aromatic content by different methods. Table 3. Pearson Correlation Coefficient (R) along with Other Statistical Data
methods
Pearson correlation coeff (R)
av diff
std dev
FIA vs GC (aromatics) GC vs NMR (aromatics) NMR vs FIA (aromatics) FIA vs NMR (olefins) GC vs NMR (oxygenates)
0.993 0.997 0.993 0.992 0.996
0.1074 1.014 0.163 1.178 -0.035
1.067 0.609 1.478 1.035 0.297
obtained by the three techniques are highly correlated, which is evident from the Pearson correlation coefficient (R g 0.99) between the three techniques. The values of R are given in Table 3 for pairs of techniques in the study along with the statistical data. Similar statistical treatment has been applied for olefins and oxygenates content. The percentage of total olefins obtained by FIA (ASTM-D-1319) and NMR is plotted in Figure 5a, and the percentage of oxygenates obtained by GC, ASTM-D-4815, method is plotted against NMR11 method shown in Figure 5b. The statistical data calculated above indicate that all the three methods give comparable results. The average difference calculated from the average of individual variations shows that both GC and FIA methods tend to overestimate the aromatic content compared to the NMR method. The standard deviations associated with these averages confirm that the data are more scattered when the FIA method is compared with GC and NMR methods; both show almost similar deviation in the results. However, GC and NMR methods show less deviation. The average difference calculated from the average of individual correlation in case of oxygenates shows that both GC and NMR methods give very comparable results. Both GC and NMR methods show very less deviation in case of oxygenates, whereas the deviation is toward the higher side in the case of olefins content. Overall, the differences between the values by these methods have been found to be within the repeatability and reproducibility of individual methods for the majority of the samples. Tests of Significance. Though the comparison of results on total aromatic content in MS samples determined by FIA, GC, and NMR methods has shown very high degrees of correlations, in view of the importance of this determination, it was thought appropriate to subject the total data to various tests of significance in order to evaluate the reliability of measurements by all methods. For such proposes, a visual inspection of the
results and simple correlations is usually inadequate. Therefore, two significance tests—the Student t test and the analysis of variance (ANOVA)—have been carried out on the set of data obtained by different techniques comparing two techniques at a time. Both the tests start with the null hypothesis H0: “there is no difference between the two techniques”. The t tests does the paired comparison, and the difference between the results is used and not the individual values. Similarly, ANOVA is a technique by which variations associated with defined sources may be isolated and estimated. These are the conditions, which an analyst may not be able to control during a determination by a particular technique. The “t test” and “two-way ANOVA without replicate” have been carried out for FIA-GC, GC-NMR, and NMR-FIA results. The calculated values of the t and the F ratio along with their critical values at 95% confidence level for particular degree of freedom (n) are in Table 4. It is very interesting to compare the two techniques. In the case of aromatics for FIA-GC and GC-NMR pairs, it was found that the calculated t and F ratio values are less than the corresponding critical values, and it can be concluded that there is no significant difference between the results of total aromatics obtained by GC-NMR and GC-FIA methods (Figure 5). For NMR-FIA pair, calculated t is slightly higher than the corresponding t critical values, indicating that there are differences in statistical terms in spite of very high correlation coefficient between the results obtained by NMR and FIA methods (Figure 5). In case of olefins and oxygenates content by FIA-NMR and GC-NMR pairs, the lower values of calculated t and F ratio than their corresponding values indicate a good relationship in statistical terms (Figure 5). Sources of Errors and Limitations. In view of very high degree of correlation coefficients obtained between FIA, GC, and NMR methods, the results obtained by significance tests also show good correlation in statistical terms as expected. During the course of this study, each method has been found to be associated with some sources of errors. • The ASTM-D-5580 and D-4815 methods are approved but time-consuming and cumbersome multidimensional based on GC. The main source of error can be during the preparation of calibration standards/curves. Sometimes, the peaks due to aromatics or oxygenates can be poorly resolved, leading to overestimation or underestimation the same. • The ASTM-D-1319 based on FIA is laborious and timeconsuming, and a lot of solvents are being used. Various sources
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Energy & Fuels, Vol. 22, No. 1, 2008 415
Figure 5. (a) Correlation plots of the olefin and (b) oxygenate content by two different methods. Table 4. Total Analysis Data for Various Tests of Significance for Different Techniques methods
calculate t
t (critical)
calculate F ratio
F (critical)
FIA vs GC (aromatics) GC vs NMR (aromatics) NMR vs FIA (aromatics) FIA vs NMR (olefins) GC vs NMR (oxygenates)
0.606
2.056 (n ) 26)
0.458
0.518
1.195
1.983 (n ) 103)
0.483
1.385
3.078
2.056 (n ) 26)
0.478
0.518
0.572
2.056 (n ) 26)
0.345
1.929
0.388
2.006 (n ) 26)
0.404
0.634
of errors can be from purity of solvents and adsorbent used; also the same should be dried properly before use. • The NMR based on methods are more versatile and faster but have relatively poor sensitivity. The main source of error is the spectral integration, which has manual intervention. The high cost of the instrument is justified in view of ease and speed of analysis and the host of information which is obtained from a single spectrum.
• The ASTM-D-5580 and D-4815 methods are cumbersome and time-consuming based on multidimensional GC for the gasoline range samples containing carbon range up to n-C11 only. • The FIA-based ASTM-D-1319 method, though standard, is tedious and slow and needs more caution from an analyst point of view while analyzing colored samples. Repeatability of the method is poor compared to other standard methods. • Methods based on 1H NMR techniques are reliable, convenient, and rapid compared to standard methods and therefore can be used as an alternative. The added advantage of NMR-based methods is that it provides simultaneous determination of PONA content along with oxygenates from a single 1H NMR spectrum. NMR can be used as a tool for quality monitoring on account of ease and convenience of analyses. • The results obtained by all the three methods are comparable when simply Pearson correlation coefficient R is compared. High degree of correlation (g0.99) has been obtained between FIAGC, GC-NMR, and NMR-FIA results. • Significance tests have shown that all the three methods correlate well in statistical terms. Depending upon the availability of technique and time, any of the methods can be used for the determination of aromatics, olefins, and oxygenates content.
Conclusions This study provides a detailed and systematic analysis of the results on aromatic, olefinic, and oxygenates content in MS range samples obtained by different analytical techniques. The methods used are ASTM-D-5580, D-4815, and D-1319, based on GC, FIA, and NMR11,14,15 techniques, respectively. From this study the following conclusions can be made:
Acknowledgment. The authors acknowledge the management of IOC, R&D Faridabad, for allowing to publish the present work. Supporting Information Available: Pearson correlation coefficient (R) derived from regression analysis. This material is available free of charge via the Internet at http://pubs.acs.org. EF070121L
