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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Determination of Nitrogen Compound Distribution in Petroleum by Gas Chromatography with a Thermionic Detector D. Kendall Albert Standard Oil Company (Indiana), Amoco Research Center, Naperville, Illinois 60540
A selective thermlonic nitrogen detector was used in combinatlon wlth a flame-ionlzatlon detector lor the gas chromatographic determination of nitrogen compound distribution in light catalytic cycle oil and light vacuum gas oil. Distribution of the dominant nitrogen compound types-pyridines, qulnolines, indoles, and carbazoles-was determined by compound type and carbon number in the cycle oil and by bolllng polnt in the vacuum gas oil. Accuracy and precision of the method were judged from analysis of reference blends, comparison to nitrogen distribution obtained by gas chromatography with a microcoulometric detector for a cycle oil, and comparison to dlstrlbutlon obtained from elemental nitrogen analyses of distlllatlon fractions of a light vacuum gas oil. With reference blends, accuracy approached precision-about 3 YO relatlve. For gas oils, accuracy was generally about 7 to 8% relatlve. The analyses particularly demonstrate the feasibility 01 employing the thermionic detector to determlne nitrogen compound distribution by boiling point.
Nitrogen compounds in petroleum adversely affect many important catalytic processes and product stability. For example, they cause catalyst poisoning and are involved with gum and color formation in products. To overcome these effects, sensitive and accurate analytical methods are needed to determine the distribution and the different types of nitrogen compounds, e.g., pyridines, quinolines, indoles, and carbazoles. Gas chromatography with a selective nitrogen detector is a sensitive, rapid, and accurate analytical approach to the analysis of nitrogen compounds in petroleum, and the advantages of this technique over slower, complex fractionation methods have been discussed ( I , 2 ) . There are four major types of nitrogen-selective detectors: thermionic or alkali flame, microcoulometric, electrolytic conductivity, and the chemiluminescent detector. The thermionic detector is a flame-ionization detector that is modified with some means for vaporizing a suitably volatile alkali metal, such as sodium, potassium, or rubidium, so that the vapors are present in the flame. The resulting flame shows an enhanced response to halogen, phosphorous, and nitrogen organic compounds. With proper choice of alkali metal and experimental conditions, the detector can be made specific for the different elements. The other detectors employ some means of chemical conversion of organo-nitrogen compounds to a detectable species. With the microcoulometric and electrolytic conductivity detectors, nitrogen compounds undergo hydrogenolysis to form ammonia. With the chemiluminescent detector, chemically-bound nitrogen is converted to metastable nitrogen dioxide (NO2*) which produces a photoemission. All of these detectors have been described in detail and are well documented in the literature, e.g., (3-7). Of these detectors, the microcoulometric (1, 8) and electrolytic conductivity (2,9)detectors have been the most widely used in gas chromatographic analyses of petroleum. An application of the newer chemiluminescent detector to gas 0003-2700/78/0350-1822$01.00/0
chromatographic analysis of fossil fuels was recently reported (10). The thermionic detector has been used relatively little in petroleum analyses, although it has advantages over the electrochemical detectors of higher sensitivity, faster response, and simpler apparatus. Because of these advantages, the thermionic detector would seem to be the most attractive detector for petroleum analysis, and in this paper our studies to determine this applicability are described. Historically, the major drawback of the thermionic detector has been its instability ( 3 , 11). As a result, it has not been generally as suitable as the electrochemical detectors for routine quantitative work (2). This instability has been generally attributed to aging of the alkali metal source and t o the method employed to generate the thermionic emissions. In earlier detector designs, the hydrogen flame was employed for both combusting the sample and heating the alkali metal source. The thermionic emission or detector sensitivity could not be controlled independently of the flame. However, in newer detector designs that are now available, e.g., (12-16), the instability problem has been minimized and made controllable by employing more thermally stable alkali metal sources, such as rubidium silicate, and providing an electrical heating source so that the level of thermionic emission can be precisely controlled. These newer detector designs are receiving increasing attention in the analysis of many complex materials. Most reported applications have been in nonpetroleum areas; for example, analysis of nitrogen-containing drugs ( 17, 18) and amino acids (19) in biological fluids. Applications to petroleum have been mainly qualitative, such as obtaining nitrogen compound profiles or “fingerprint” chromatograms of petroleum fractions. Such applications have been reported for oil spills (20), engine oils (21),and distillate fuels (6). This method has also been used to follow changes in nitrogen distribution upon hydrotreating of shale naphthas (22). This paper extends previous work by evaluating the quantitative performance of the thermionic detector for analyses of nitrogen compound distributions in gas oils. The work includes a study of detector response characteristics, analyses of reference blends, comparison of nitrogen distribution analyses of a light catalytic cycle oil with that obtained by microcoulometry, and a comparison of distribution analyses of a light vacuum gas oil with that obtained from elemental nitrogen analysis of distillation fractions.
EXPERIMENTAL Materials. Reference nitrogen compounds were obtained from commercial sources except for two compounds, 2,3,5,7-tetrawhich were methylcarbazole and 1,2,4,6,7-pentamethylcarbazole, synthesized (23). Hydrocarbons were Phillips pure grade. Other reagents were reagent grade. Solutions were prepared with isooctane or acetone solvents. Petroleum fractions included a hydrocracked recycle oil, a light catalytic cycle oil, and a light vacuum gas oil. The boiling ranges, nitrogen, and sulfur contents are shown in Table I. Apparatus. The gas chromatograph was a Perkin-Elmer Model 3920 equipped with a nitrogen phosphorus detector (NPD) and a flame-ionization detector (FID). The detectors were operated in parallel with a dual-pen 1-mV recorder. Data reduction
a 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978 lx
Table I. Petroleum Fractions Employed in Study light hydrolight catalytic vacuum cracked recycle oil cycle oil gas oil boiling range, ’C nitrogen, ppm sulfur, wt%
200-345 5 0.0155
200-325 240 0.31
245-455 390 0.38
Table 11. Chromatographic Operating Conditions 60 cm’imin, helium column flow rate effluent split“ 29 cm3/min to NPD 31 cm’/min to FID NPD hydrogen, 4.0 cm’/min air, 100 cm3/min bead voltage, adjusted as required FI D hydrogen, 35 cm3/min air, 350 cm’imin injector (glass liner), 275 to 300 “ C tempera tures manifold (detectors), 350 “ C column, (see text) “ Achieved with two Perkin-Elmer B, capillary restrictors. was accomplished with an on-line computer. A polar and three nonpolar packed columns were employed. The polar column was 6 ft of ‘/,-inch ad.. 2-mm id., glass tubing packed with 10% Carbowax 20M on 80-100 mesh Gas Chrom Q. The nonpolar columns contained 20% polyethylene (mol wt 12 000) on 80-100 mesh, non-acid washed Chromosorb W, which was pretreated with 3% potassium carbonate. The columns were prepared from 8 ft x ’/* inch o.d., 2-mm id., stainless steel tubing and 6 ft and 3 ft lengths of inch o.d., 2-mm i.d., glass tubing. Polyethylene was chosen as the high temperature liquid phase because nit,rogen background from column bleed was less than that obtained with conventional silicone liquid phases ( 8 ) . Life of the polyethylene columns was equivalent to that of SE-30 columns. Operating Conditions. The gas chromatographic operating conditions are summarized in Table 11. The column flow rate and effluent split ratio were maintained at the values shown for all columns. A hydrogen flow rate of 4.0 cm3/min was optimum for maximum sensitivity without sacrificing selectivity. Incremental adjustments were made, as appropriate, in the bead current to compensate for loss in sensitivity due to aging of the bead. The dependence of the detector performance on the intrarelationships between the flow rates of the carrier gas, hydrogen, and air and the bead heating current have been described in detail ( 2 4 ) . Various isothermal and temperature programmed column temperatures were employed depending on the boiling range of the sample and the separations desired. Isothermal column temperatures are indicated in the text. For temperature programmed runs, the sample was charged to the column at 80 “C and after 4 min programming was initiated at a rate of 4 “C/min and was continued to a maximum of 250 “C for the polar column and 310 “C for the nonpolar columns. Elution was continued, if needed, a t the maximum temperature until completion of the chromatogram. Procedures. At the beginning of each day of operation, the chromatographic system was “conditioned” by heating the column to an elevated temperature, e.g., 200 “C, and while at that temperature charging at least two successive 1-pL injections of a solution containing 1000 ppm N, as 2,4,6-trimethylpyridine (TMP),in isooctane. Subsequently, the column temperature was re-equilibrated as required for calibration and analysis. In general, the NPD bead voltage was such that the system background or bead current was no greater than 30 picoamperes (PA) (24). With well-conditioned columns, the bead current was generally of the order of 5 pA or less. The electrometer range setting was 10 for both detectors. Calibration of the chromatographic system was made with a reference solution containing 95 ppm of TMP, which corresponded to 11 ppm N, and 10000 ppm of 1,2,3,4-tetramethylbenzene
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lx
+ Ln
5
lx
0
8
2
1x
lx
1
0.1
I
1 .o
I
L
I
10
100
1000
I
10,000
PPm N
Figure 1. Linearity of NPD and FID. Polyethylene column, 8 f i stainless steel: column temperature, 110 ‘C. Sample: 1 .O pL; 2,4,6-trimethylpyridine in isooctane
(TMB), which corresponded to 8950 pprn C. Tht: composition of this solution was chosen to represent a reasonable test for both sensitivity and selectivity. As criteria for acceptable system performance, an NPD sensitivity-expressed as the NPD/FID area response ratio for TMP-of at least 8 and a nitrogen-tocarbon selectivity of at least 8000 was maintained. Selectivity (SI was calculated as follows.
s = -‘41X gz g1
x A2
where AI = NPD area response to TMP, A2 = NPD area response to TMB. g, = grams of nitrogen, as TMP, charged to the NPD, and g2 = grams of carbon, as TMB, charged to the NPD. Calibration was made (following conditioning of the column) at an isothermal column temperature generally in a range of 100-125 “C with a sample size of 1pL. The sensitivity and selectivity of the NPD are further discussed in the subsequent section. For distribution analysis of petroleum fractions, sample sizes were generally 1pL. Distributions were calculated by normalizing the total area of the chromatogram to 100% or the total nitrogen content of the sample. For fractions with low concentration levels of nitrogen, e.g., 5 ppm N, as for the hydrocracked oil, distribution analyses were not feasible because of the inability to distinguish nitrogen compounds at sub-ppm nitrogen concentration levels from hydrocarbon background response. [Such distributions could be expedited, however, by concentrating the nitrogen compounds by liquid chromatography, e.g., (I),prior to gas chromatographic analysis.] The methodology employed for the niicrocoulometric distribution analyses has been reported ( I ) .
RESULTS AND DISCUSSION Detector Response Characteristics. Detector response characteristics that were determined were linearity, lower limit of detection, selectivity, and response factors. The FID was used qualitatively to help interpret peak identifications in NPD chromatograms and quantitatively to help judge the performance of the chromatographic system. Therefore, response characteristics of the FID were also determined. Satisfactory operation of the dual detectors was indicated by a linear response for each detector, as shown in Figure 1. This shows a log-log plot of detector response vs. concentration of nitrogen in ppm. The slopes were calculated from a linear regression fit of the log-log data, and they agree within experimental error with a slope of unity for a linear response. The NPD response was linear from a lower detection limit of about 0.2 ppm N to loo0 ppm. The FID response was linear to a t least 10000 ppm N. Tailing of the solvent peak into the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Table 111. Lower Limit of Detection for 2,4,6-Trimethylpyridine (TMP) LLD = gis for 2 : l signal/noise ratio stainless steel columna glass columnb 4.12 x 10'l2 (TMP) 1.73 x (TMP) 4.75 x 10-1~ (N) 2.00 x 10-13 (N) FID 3.55 x lo-" (TMP) 7.96 x lo-'? (TMP) a 8 ft, polyethylene packing, column temperature 125 "C. 6 ft, polyethylene packing, column temperature 125 "C. NPD
X
r
x
'
r
d
ii
0
5
10
15
Retention time, minutes Figure 2. Illustrative chromatogram of NPD sensitivity and selectivity. Polyethylene column, 6-ft glass; column temperature, 125 OC
TMP peak prevented a better lower detection limit, which was 10 ppm N, for the FID. Separations were satisfactory with either stainless steel or glass columns, but better sensitivity was obtained with the glass columns. This is illustrated in Table 111, which compares lower limits of detection for both detectors on a stainless steel and a glass column. For the NPD, the glass column was more favorable by a factor of 2.4; for the FID, the glass column was better by a factor of 4.5. In comparing the sensitivities of the detectors for each column, the lower limit of detection for the N P D was better than that of the FID by factors of 8.6 and 4.6 for the stainless steel and glass columns, respectively. The N P D sensitivity is of the same order of magnitude that has been reported for this detector for other types of nitrogencontaining compounds (13). T h e sensitivity and selectivity of the NPD are illustrated in Figure 2, which shows a chromatogram that was obtained with a glass column for a blend of TMP and TMB, representing approximately 0.1 ppm N and 90 ppm C, respectively (a 1OO:l dilution of the calibration standard). Although the C to N ratio was 900 to 1,the NPD response to the T M B was not measurable. On the other hand, the NPD response to the TMP was about nine times greater than that of the FID. Analyses of the undiluted calibration blend with 11 ppm N and 8950 ppm C for which the NPD hydrocarbon response was measurable, showed the nitrogen selectivity was generally in the range of 8000 to 11000 on a g N/g C basis. This
Table IV. Effect of Alkylsubstituents on Detector Response compound NP D pyridine 1.000 2-methylpyridine 1.167 4-methylpyridine 1.006 1.020 3-eth ylpyridine 4-tert-butylpyridine 0.9160 2,6-dimethylpyridine 1.235 2,4,6-trimethylpyridine 1.202 1.000 quinoline 2-methylquinoline 1.301 6-methylquinoline 1.015 2,6-dimethylquinoline 1.266 2,4-dimethylquinoline 1.296 indole 1.000 3-methylindole 1.020 2,3-dimethylindole 1.090 2,5-dimethylindole 1.117 1,2-dimethylindole 0.7408 carbazole 1.000 1,2,3,4-tetrahydrocarbazole 0.9566 6-methyl-1,2,3,4-tetrahydrocarbazole0.9520 N-ethylcarbazole 0.7924 1.156 2,3,5,7 -tetramethylcarbazole 1,2,4,6,7-pentamethylcarbazole 1.179
FID 1.000
0.997 1 0.9902 0.9549 0.9347 0.9522 0.9530 1.000 1.036 0.9873 1.014 1.033 1.000 0.9544 0.9493 0.9971 0.9318 1.000 0.9818 0.9626 0.9010 not det. not det.
compares with reported values of 5000 to '7500 (12, 13). For petroleum analyses it is also important that the detector have good nitrogen selectivity relative to sulfur. Benzothiophenes, for example, are a major sulfur compound type in petroleum. Analysis of a blend containing 120 ppm N as 2,4,6-trimethylpyridine and 250 ppm S as benzothiophene, in acetone, showed a nitrogen to sulfur selectivity of 2100 on a g N/g S basis. Little comparative information is available, although the N / S selectivity has been indicated to be of the same order of magnitude as the N/C selectivity (12). However, in our experience, the N / S selectivity is of an order of magnitude of lo3, as compared with an order of magnitude of lo4 for the N / C selectivity. Although lower, the N / S selectivity is still satisfactory. The NPD also responds to phosphorus when it is operated in the nitrogen mode. However, the phosphorus content of petroleum appears to be low enough that the N P D response to phosphorus can be assumed to be negligible. For example, in the oils used in this study, analysis by X-ray fluorescence showed less than 5 ppm phosphorus. Gas chromatographic analyses with the N P D operated in the phosphorus mode showed no measurable phosphorus. The lower limit of detection was 0.1 ppm P as determined with tricresylphosphate in dodecane. The response of the NPD is approximately proportional to the nitrogen content of a compound, but the response is also dependent on the compound structure. The detector mechanism is not completely understood, but it is based on the formation of cyano radicals ( C r N : ) in a cool flame zone (12). In general, those compounds having structures that would be expected to be favorable for this reaction would be expected to give the larger response. The effect of alkylsubstituents on detector response is shown in Table IV. The responses are relative to 1.00 for the parent compound of each class. The NPD factors are the ratios of weight percent of nitrogen that was blended divided by the corresponding peak area percent. The FID factors are ratios of weight percent of compounds that were blended, divided by the corresponding peak area percent. Analyses were made with a polyethylene column (8-ft stainless steel) with temperature programming. The data indicate that for most of the compounds the alkyl substituents affect the FID response by no more than 6% relative; whereas, for the NPD, the effect is more widespread,
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Table V. Relative Responses of NPD and FID to Different Types of Nitrogen Compounds relative area blended weight ratio response ratio compound' nitrogen carbon NPD FID pyridine 2,6-dime thylpyridine 4-methylpyridine 2,4,6-trimethylpyridine 4-tert-bu tylpyridine quinoline 2-methylquinoline 6-methylquinoline 2,6-dimethylquinoline 2,4-dimethylquinoline indole 3-methylindole 2,3-dimethylindole N-ethylcarbazole N-methylcarbazole 5,6-benzoquinoline 1,2,3,4-tetrahydrocarbazole
1.278 0.9494 1.076 0.8720 0.7551 1.008 0.9857 0.7314 0.6445 0.9146
0.7988 1.617 0.8308 1.120 0.8082 1.564 0.8759 1.109 0.8515 1.145 1.133 1.101 1.235 1.054 0.9154 0.8450 0.8853 0.6985 1.257 0.9732 1.000 1.000 1.000 1.017 1.145 0.9628 0.7061 0.8815 0.6260 0.5466 0.9567 0.5942 0.6129 0.9981 0.6596 0.7819 1.272 0.8492 0.6224 0.9360 0.5204 6-methyl-1,2,3,4-tetrahydrocarbazole 0.5481 0.8928 0.4831 carbazole 0.7472 1.120 0.6621 Listed in order of elution from the Carbowax 20M glass column (see text).
0.7122 0.8450 0.7534 0.9438 0.8995 1.037 1.252 0.8880 0.9255 1.318 1.000
1.184 0.8730 0.9525 0.9257 1.284 0.9078 0.8889 1.072
relative response factor,
- weightlarea NPD 0.7904 0.8477 0.6880 0.7863 0.6595 0.9155 0.9352 0.8656 0.9227 0.9398 1.000 1.056 1.128 0.9199 0.9292 0.9207 1.196 1.135 1.128
FID 1.122 0.9832 1.073 0.9281 0.9466 1.093 0.9864 1.031 0.9566 0.9537 1.000 0.9671 1.010 1.004 1.078 0.9907 1.031 1.004 1.045
Table VI. NPD Analysis of Blend of Nitrogen Compounds in a Hydrocracked Gas Oil compound 4-tert-butylpyridine quinoline 6-methylquinoline 2,4-dimethylquinoline indole 3-methylindole 2,3-dimethylindole N-methylcarbazole carbazole
PPm N blended 22.0 32.5 28.1 25.9 29.1 28.0 24.2 18.7 21.6
1 21.6 33.4 28.8 26.5 28.1 28.8 24.2 22.3 21.4
ppm N determined 4 av
2
3
21.4 33.9 29.2 26.8 27.8 28.7 23.7 22.3 21.7
20.9 32.9 28.6 26.5 27.5 28.5 23.3 21.8 20.9
e.g., as high as 30% relative for the alkylquinolines. Notable are the relatively low factors for 1,2-dimethylindole and N-ethylcarbazole. In both of these compounds, an alkyl group is bonded t o the nitrogen atom, which is especially favorable for the formation of cyano radicals. These compounds also produced the largest deviations in the FID response. In general, the compound structure is more favorable for the formation of cyano radicals as the response factor decreases. As a further comparison of differences in response among the different classes of nitrogen compounds, a blend of several compounds representing pyridines, quinolines, indoles, and carbazoles was chromatographed with temperature programming on the Carbowax column, and response factors relative to 1.00 for indole were calculated for both the NPD and FID, Table V. The area response ratios for each detector are compared with the corresponding ratios of nitrogen and carbon that were blended and then a weight-to-area ratio or response factor was calculated. It will be noted that the FID response follows the carbon content much closer than does the NPD response with the nitrogen content. The data are summarized in Figure 3, which shows the range of factors for each class of compounds vs. the boiling range that was covered for each class. The data show significant response differences among the different types with the largest differences for the pyridines. Quinolines, indoles, and carbazoles are the dominant types in the oils used in this study. Blend of Nitrogen Compounds i n Hydrocracked Oil. To determine the reliability of the NPD in component analysis of a petroleum fraction, a blend of nitrogen compounds in a hydrocracked oil was analyzed on the Carbowax column with temperature programming. An illustrative chromatogram is
21.1 33.1 28.9 26.4 27.4 28.7 23.0 22.0 21.4
b 1.10 U 2 1.00 2 0.90 -
std dev
error
0.31 0.43 0.25 0.17 0.32 0.13 0.52 0.24 0.33
-0.7 0.8 0.8 0.7 -1.4 0.7 -0.6 3.4 -0.2
21.3 33.3 28.9 26.6 27.7 28.7 23.6 22.1 21.4
120
0
-
Carbazoles Indole reference
Cluinc,lines
-d
Pyridines
o,60
0 501 110
150
200
250
300
1
360
Boiling range, "C
Figure 3. Comparison of NPD responses for different types of nitrogen compounds
shown in Figure 4, which shows both the NPD and FID responses. The prominent peaks in the FID trace are nparaffins, which are present in an approximate carbon number range of CI1 to Cs0. The NPD trace shows the blended nitrogen compounds, which were added a t a concentration level corresponding to 20 to 30 ppm N each. The absence of significant peaks between those of the blended compounds illustrates the selectivity of the detector and shows the absence of other nitrogen compounds. The first component, 2,4.6-trimethylpyridine (TMP), is an internal standard. Four consecutive analyses were made with the detector bead voltage at the same setting for all runs. Peak areas were corrected with response factors (Table V). A reference run on the unspiked oil showed that the area count of the background for each of the nitrogen compound peaks
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1
I
I
I
1
0
15
30
45
65
Retention time, minutes Figure 4. Chromatogram of blend of nitrogen compounds in hydrocracked gas oil
_ -
I
0
10
I
I
I
30
50
75
Quinolines
2
C2
c,
Indoles
Q1C
-Jg z$ g8 -, J. K. Baker, Anal. Chem., 49, 906 (1977). R. F. Admas, F. L. Vandemark, and G. J. Schmidt, J . Chromatogr. Sci.. 15, 63 (1977). G. A. Flanigan and G. M. Frame, Res..IDev., 28, (9), 28 (1977). M. L. Lee, K. D. Bartle, and M. V. Novotny, Anal. Chem.. 47, 540 (1975). C. E. Kennard, S. M. Sonchik, and M. P. T. Bradley, "Detailed Analysis of Shale Oil Using Specific Detectors", presented before the Analytical Division 174th National Meeting, American Chemical Society, August 28-September 2, 1977, Chicago, IIi. I. Puskas, E. K. Fields, and E. M. Banas. Div. Pet. Chem., Am. Chem. SOC.,Prepr., 17 ( l ) , 856, (1972). J. A. Lubkowitz, J. L. Glajch, B. P. Semonian, and L. B. Rogers, J . Chromatogr., 133, 37 (1977). American Society for Testing and Materials, "1977 Annual Book of ASTM Standards", Part 24, p 830, Method D2896-73 (1977). M. A . Muhs and F. T. Weiss, Anal. Chem., 30, 259 (1958). American Society for Testing and Materials, "1977 Annual Book of ASTM Standards", Part 24, p 770, Method D2887-73. ~
(18) (19)
CONCLUSIONS
(20) (21) (22)
In conclusion, this work has shown that (1)with response factors and an internal standard technique, the thermionic detector or NPD can be used for accurate component analyses of petroleum without prior separations; (2) the NPD is applicable to the determination of nitrogen compound distributions in petroleum and results compare favorably with those obtained by microcoulometry or elemental analyses of fractions; and (3) nitrogen compound distribution by boiling point is feasible with the NPD-FID system. Further development of this technique is being done in our laboratory.
(23) (24) (25)
(26) (27)
ACKNOWLEDGMENT I am grateful to R. B. Armstrong of Amoco Oil Company who provided the light vacuum gas oil distillation fractions and to J. J. Miskovich of Standard Oil Company (Indiana) who assisted with the experimental work.
RECEIC'ED for review June 13,1978. Accepted August 17,1978. Presented before the Division of Analytical Chemistry, 175th National Meeting, American Chemical Society, Anaheim, Calif., March 12-17, 1978.
Cubic Spline Interpolation for the Calculation of Retention Indices in Temperature-Programmed Gas-Liquid Chromatography Wolfgang A. Halang," Rolph Langlais, and Ernst Kugler Coca-Cola GmbH, Research & Development Department, Kaninenbergstr. 66, 4300 Essen 1, West Germany
Temperature-programmed gas-liquid chromatography (GLCj is extensively used in our laboratories for the analysis of essential oils and other mixtures of volatile oils and substances. The methods applied for the collection, processing, and evaluation of our GLC data have been described earlier (1-3). All GLC retention data are converted into Kovgts indices for further handling and reference. Hence, the indices need to be calculated as accurately as possible. This paper describes the method we employ, which has been
A method is described for the calculation of retention indices based on nonlinear calibration data in temperature-programmed gas-liquid chromatography. It is the method of choice when the indices serve as input for statistical analyses and identification procedures. A comparison of this technique with the polygonal technique is discussed. For easier implementation of the calculation procedure Into existing gasliquid chromatography computer programs, a detailed description of the spline Interpolation procedure is given. 0003-2700/78/0350-1829$01 .OO/O
G
1978 American Chemical Society
