Analysis of Coconut Oil-Diethanolamine Condensates by Gas Chromatography Arthur W. O’Connell J. L. Prescoti Co., Passaic, New Jersey 07055
A gas chromatographic method has been developed for the quantltatlve determlnatlon of glycerol, dlethanoiamine, and the distributions of dlethanolamldes and diesteramlnes present In coconut oil-diethanolamine condensates. of the possible products of side reactlons, monoesteramldes and N,N’-bls(2-hydroxyethyl)plperarlne were shown to be absent. Monoesteramlnes and diesteramldes are not determined but are not ilkeiy present In the flnlshed product.
Alkanolamides form an important class of nonionic surfactant that is widely used to control viscosity, corrosion, and foam properties of products used in the detergent, cosmetic, and textile industries. They are prepared from the reaction of an alkanolamine with a fatty acid, a fatty ester, or a triglyceride. The subject of this report, coconut diethanolamide (CDA), is formed as the condensation product of the reaction of coconut oil with diethanolamine (Equation 1). The product RCOOCH, CH,CH,OH I
RCOOCH
+ 3HN
I
\
RCOOCH, 657 g Coconut oil
100 OC
/
CH,CH,OH 315 g Diethanolamine CH,CH,OH /
EXPERIMENTAL HOCH, I
+
3 RCON
\
HOCH
(1)
I
CH,CH,OH
HOCH,
880 g 92 g Diethanolamides Glycerol is a light yellow oil consisting of glycerol, diethanolamides and a 6% weight excess of diethanolamine used to minimize some of the undesirable products formed in side reactions. These have been cited by Trowbridge, et al. (1) to include N,N’bis(2-hydroxyethyl)piperazine (I)formed by self-condensation of diethanolamine at elevated temperatures, monoesteramines (11), diesteramines (III), monoesteramides (IV), and diesteramides (V). R may stand for any linear saturated alkyl CH,CH, /
\
\
f
HOCH,CH,N
NCH,CH,OH CH,CH,
N,N’-bis(2-hydroxyethy1)piperazine(I) CH,CH,OCOR CH,CH,OCOR I
/
HN
HN
\
\
CH,CH,OH Monoesteramine (11) CH,CH,OCOR /
RCON \
CH,CH,OH Monoesteramide (IV)
group derived from coconut oil containing an even carbon number in the range C8-ClS. Also included is a small percentage of unsaturation at Cla; oleic, C18:., and linoleic, CI8:. moieties. Kroll and Lennon (2) have proposed a lengthy wet chemical scheme for the analyses of lauric acid-diethanolamine condensates which has also been widely used for alkanolamides prepared from fatty esters and glycerides. Shortcomings of this method, besides the amount of time involved, include the inability t o distinguish between monoand diesteramines or between mono- and diesteramides, as well as the inability to determine glycerol. In a critique of the method by Sanders (3),it was demonstrated by infrared spectroscopy that the ester content of various alkylolamides is much lower than that apparently indicated by chemical analysis. However, infrared also fails to distinguish the various ester types present. Infrared has been suggested as a means for determining total amide content and N,N’-bis(2hydroxyethy1)piperazine (4). A new gas chromatographic method will be described in the present communication that is suitable for the rapid and accurate analysis of coconut diethanolamide. All major components and most of the proposed possible impurities are unambiguously identified and quantitated.
CH,CH,OCOR Diesteramine (111) CH,CH,OCOR I
RCON \
CH,CH,OCOR Diesteramide (V)
Reagents. N,O-Bis(trimethylsily1)acetamide(BSA),96% pure, and trimethylchlorosilane(TMCS),99% pure, were obtained from Ohio Valley SpecialtyChemical, Inc. Silylation reagent is prepared by mixing BSA and TMCS in a 5:l v/v ratio. Glycerol puriss p.a. (Fluka AG), diethanolamine, 98% (Fisher Scientific Co.) and N,”-bis(2-hydroxyethyl)piperazine (Aldrich Chemical Co.) were used as standards. Lauroyl chloride, 98% (Eastman Organic Chemicals) was used in the preparation of reference standards not available commercially. Apparatus. A Perkin-Elmer 3920B gas chromatograph with flame ionization detectors was used for the analyses. Installed in the chromatograph were two 6 f t X 2 mm i.d. glass columns packed with 3% SP-2100on 100/120 mesh Supelcoport (Supelco, Inc.). Chromatographicconditions are set as follows: carrier gas, helium at a flow rate of 20 mL/min; detector air pressure, 50 psi; detector hydrogen pressure, 20 psi; injector temperature, 200 OC; detector temperature, 300 “C; oven temperature program, 175 “C isothermal for 1min followed by linear 8 “C/min increase to 300 “C which is then maintained for 4 min; amplifier range, X 10. Chromatograms were recorded by a Linear 260/mm recorder set to 1-mVsensitivity using a chart speed of 1cm/min. Retention times and peak areas were determined with a digital integrator (Autolab Minigrator, Spectra-Physics). Procedure. Using Drummond “Microcaps”, place 0.5 pL diethanolamine and 3 pL CDA into separate 3-mL screw cap top reaction vials. Pipet 0.2 mL of the 5:l v/v BSA/TMS silylation reagent into both vials, tightly close them, and heat at 60 “C for 30 min with occasional shaking. Obtain chromatograms of the reagent, diethanolamine, and CDA by injecting equal volumes (0.4 wL)of each into the chromatograph. Locate and delete those peaks contributed by the silylation reagent from the sample chromatogram. Sample peaks that have retention times common with reagent peaks are corrected by area subtraction. Calculate the diethanolamine correction factor from
d=
A I + A2 A2 ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
835
1
13
2
1
I
I
I
5
10
1s
20
TIME (minutes) Figure 1. Gas chromatogram of TMS-CDA derivatives
where AI and A2 are the areas of the partially and completely silylated derivatives obtained from the diethanolamine chromatogram. Corrected diethanolamine area is
and corrected glycerol area is
where AD and AG are the respective diethanolamine and glycerol areas reported on the CDA chromatogram. The FID response factor for glycerol is FG
amides
= AC,G
X 9.56
(5)
where the numerator is the sum of the diethanolamide areas. The diethanolamine response factor is
A response factor of unity is used for all diethanolamides and
diesteramines. Normalized areas, by means of which weight percentages are determined, are calculated by multiplication of the corrected areas by the response factors.
RESULTS AND DISCUSSION Many of the components present in CDA are moderately high molecular weight species containing hydroxyl and secondary amine groups. As a result, the product has a rather low volatility, thereby making chromatography at reasonable temperatures difficult. Since direct chromatography of CDA was not possible, conversion to a less polar derivative with increased volatility became an attractive alternative. A recent paper (5) reports on the successful conversion of ethanolamines 836
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
to trimethylsilyl derivatives. In this study, N,O-bis(trimethylsily1)acetamide(BSA) was found to react with both the hydroxyl and the amine groups of diethanolamine. BSA has the dual advantage of replacing all polar groups in a single step and allowing the use of high temperature nonpolar liquid stationary phases. In preliminary studies, BSA was used under the same conditions prescribed by Piekos. Peaks due to the glycerol and diethanolamine TMS derivatives formed were readily identified on the CDA chromatogram, Figure 1, by reference to pure standards. However, it was discovered that derivatization of pure diethanolamine by this method produces two peaks; one of which is nearly coincident and indistinguishable from the glycerol derivative. This peak amounts to 30-40% of the total area and was determined to be due to incompletely silylated diethanolamine in which only the two hydroxyl groups are silylated. Further experimentation with various silylating agents led to the adoption of a 51 v/v BSA/TMCS (trimethylchlorosilane) combination. With this mixed reagent, conversion to the completely silylated diethanolamine derivative was on the order of 98% when the reaction mixture was heated at 60 "C for 30 min. Neither longer heating times nor higher temperatures substantially improved the conversion enough to warrant exposure of CDA to possible alteration by these measures. Correction of this error imposed upon the glycerol and diethanolamine peak areas is made by obtaining a chromatogram of pure diethanolamine with each batch of reagent prepared to determine its silylation efficiency. Effects of concentration on efficiency determination are minimized by reacting an amount of diethanolamine equal to CDA on a mole basis, thus assuring similar concentrations of excess BSA in both instances.
~~~
Table I. TvDical Composition and Retention Times of P&cott CDA RetenFig. 1, tion peak time, s %, W I W No. Component 1 Glycerol 68 8.4 84 ... Reagent 99 Reagent ... 2 Diethanolamine 113 6.0 128 Reagent ... 3
9 10 11 12 13 14 15 16
C,?a ? C,, diesteramine ClO? C,, diesteramine Cl,? ? C,, diesteramine Cs diethanolamide C,, diesteramine C,, diethanolamide ClS? C,, diethanolamide C,, diethanolamide C,, diethanolamide '1S:l + '1B:Z
17
C,,:, diethanolamide
4 5 6 7 8
diet hanolamide
0.4 0.2 1.7 0.2 0.5
138 193 205 246 332 360 380 468 485 582 611 704 732 844 951
0.1 8.5 0.5 5.6 0.2 39.3 13.5 7.4
1036 1052
4.7 1.7
0.8 0.1
? = questionable series
The seven major peaks occurring after diethanolamine were tentatively identified as the diethanolamide group as they were present in proportions expected for coconut oil derivatives. All components are well resolved with the exception of the C18group which appears as a partially resolved doublet at the end of the chromatogram. The initial major component of the doublet consists of unsaturated oleic (C18:.) and linoleic (Clk2)diethanolamides eluting together while the second minor component is due to the saturated stearic analogue. The peaks appearing on the chromatogram are numbered in order of elution and are identified in Table I. Pure lauric diethanolamide containing about 2% C14impurity was prepared (1) and served to confirm the identity of the diethanolamide group by means of a semilog plot of retention time vs. carbon number for the homologous series. From 8 to 14 small peaks totaling about 5% of the total area may generally be found scattered throughout a CDA chromatogram. These were assumed to be due to compounds formed in side reaction processes of the type postulated in the introduction. Methods described by Trowbridgewere used to prepare reference standards of suspected compounds. These included lauric diesteramine (bis(2-lauroxyethy1)amine), lauric monoesteramide (lauric(2-lauroxyethyl)-(2hydroxyethyl)amide), and lauric diesteramide (lauric bis(21auroxyethyl)amide). Derivatization and chromatography of these materials indicate that the majority of impurities formed in CDA are diesteramines. The major impurity is located at 205-s retention time and is identical to that of lauric diesteramine. This would be expected from the preponderance of lauroyl moieties present in coconut oil. The remaining components in the diesteramine homologous series were located by a semi-log plot of retention times obtained isothermally at 200 "C vs. alkyl group carbon number. Two points, Clz and C14,were obtained from the lauric diesteramine standard by virture of a 2% C14impurity. These established a line by means of which the entire diesteramine series could be identified. C8 and Clo diesteramines were estimated to elute a t between 60-77 s and 110-125 s, respectively. Unfortunately, the glycerol derivative obscures the first and the diethanolamine derivative the second, making determination of these two diesteramines impossible. Inclusion of their small areas
within the glycerol and diethanolamine areas is not considered an error large enough to be of concern. The remaining minor peaks in the CDA chromatogram could not be identified with certainty but apparently form a related series. That this is so was indicated by construction of a semi-log plot taking the major peak at 360 s as a C12 compound. The remaining peaks fell on a straight line drawn through the 360-s point and with slope similar to those of the diethanolamide and diesteramine series. NflN'-Bis(2-hydroxyethy1)piperazine elutes a t 255 s but has never been found on any CDA chromatogram. Apparently, the reaction temperature is too low and the time too short for its formation (2). Lauric monoesteramide elutes a t 1131 s, considerably after the last C18 diethanolamide peak. It, too, has been absent from all CDA samples analyzed. The presence or absence of diesteramides in CDA could not be established by gas chromatography as no peak eluted from an injected lauric diesteramide reference. Neither would this compound elute from a short high temperature column (1% Dexsil300 on 100/120 Supelcoport, 18 inch X ' 1 8 inch steel operated at 350 "C) because of its high molecular weight and low volatility. However, since no monoesteramide is formed in the CDA reaction the probability of diesteramide being present is low. Difficulties in the preparation of pure lauric monoesteramine prevented direct location of its position on the chromatogram. Trowbridge has shown, however, that the monoesteramines are both thermally and temporally unstable and that they readily convert to diethanolamides in a few days a t room temperature. No indication of extraneous peaks that may possibly belong to monoesteramines has been found in freshly prepared CDA batches. It is doubtful that the unidentified series referred to above are monoesteramines because their expected position would be at much shorter retention times. These observations lead us to conclude that while monoesteramines may be formed during the reaction, their existence is transitory and they are not present in CDA that is more than one or two days old. The relative response factor could not be measured for every single component since pure standards of each were not available. It was necessary, therefore, to assume first, that the response per unit weight is constant within any given homologous series and, second, that all series have the same or equal response. These are fair assumptions based on past experience (6) with flame ionization detectors toward compounds with high numbers of carbon atoms per molecule. Glycerol and diethanolamine, however, contain relatively small numbers of carbon atoms and may be expected to exhibit greater response on an equal weight basis than the diethanolamides and diesteramines. Initial experiments with an equal weight mixture of glycerol, diethanolamine, and lauric diethanolamide indicated that the detector response to glycerol and diethanolamine were similar but about three times greater than for the amide. Careful repeated determinations of the relative responses by this approach established a reproducible glycerol to diethanolamine ratio of 1.08 but the glycerol to amide ratio had an unacceptably wide range (2.2-3.0). It is necessary, therefore, to determine response factors for each CDA sample analyzed. The glycerol response factor is calculated from
F=
A ,amides AC,G
(7)
where ZA,amidesis the total area for all diethanolamide peaks, AC,Gis the glycerol area corrected for the contribution of partially derivatized diethanolamine and K is the weight ratio of diethanolamides to glycerol formed in the reaction. The factor K is calculated from the stoichiometry of the reaction which for closely controlled compositions such as coconut oil ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
837
may be taken as a constant value of 9.56. Further refinement in the calculation of K becomes necessary for cases of high diesteramines content since, under these circumstances, Equation 1 is not representative of the reaction and less than theoretical weights of diethanolamides are formed. The equation for K then takes the form
K =
=
W,amides
w, glycerol ZA,,idesX
(zA,amides
880
-k zA,diesteramines)
92
(8)
not possible with the liquid phase used, this was not considered a serious deficiency. Certainly, less than 190 of the total information is lost because of components eluting together. Other, more polar phases may improve the separations obtained with SP-2100; however, these are temperature limited in general. For example, a phase such as DEGS could possibly resolve the CU group into its three components except that its upper temperature limit is only 200 "C; nearly 100 " C lower than necessary. A remaining possibility, Silar 1OC with a limit of 275 " C , is about the highest temperature polar phase available. However, it was not on hand a t the time of this study and could not be evaluated.
where W is weight. If the coconut oil composition is suspect, as indicated by an abnormal diethanolamide distribution on the chromatogram,or if a fractionated type is used, the average molecular weight may be determined from a saponification number determination and a new value of K calculated. The diethanolamine response factor is simply the product of the glycerol response factor with a constant (1.08). Response factors of all other components are taken as unity. Glycerol and diethanolamine areas are multiplied by their response factors to obtain weight' related areas. CONCLUDING REMARKS While complete separation of all components in CDA was
LITERATURE CITED J. R. Trowbridge, R. A. Falk, and I. J. Krems, J . Org. Chem., 20, 990 (1955). Harry Kroii and W. J. lennon, Toilet Goods Assoc., Cosmet. J., 25, 37 (1956). Herbert L. Sanders, J . Am. Oil Chem. Soc., 35, 548 (1958). "Manufacturing Diethanolamides From P 8. G Fatty Acids", Procter 8. Gamble Technical Service Report ( 197 1). Ryszard Piekos, Krzysztof Kobylczyk, and Janusz Grzybowski, Anal. Chern., 47, 1157 (1975). L. S. Ettre, J . Chromatogr., 8, 525 (1962).
RECEIVED for review December 9, 1976. Accepted January 21, 1977.
Correction for Interferences of Spectral Origin with Continuum Source, Echelle Wavelength Modulated Atomic Absorption Spectrometry Andrew T. Zander' and 1.C. Q'Haver" Chemistry Department, University of Maryland, College Park, Maryland 20742
Peter N. Keliher Chemistry Department, Villanova University, Villanova, Pennsylvania 19085
An atomlc absorption spectrometer based on a high-Intensity contlnuum source and a hlgkresolutbn, wavelengtkmodulated monochromator (the CEWMAA system) Is shown to be able lo correct for all major types of spectral Interferences In atomic absorption spectrometry. The performance of the system Is demonstrated by the analysis of trace levels of Zn, Pb, and Mg In nickel alloys (examples of nonspeclflc background absorption), Cd In nickel alloys (an example of background corrector error caused by a non-analyte absorptlon line In the spectral bandpass) and Zn In copper metal and In Iron metal (examples of direct spectral overlap).
We have previously described (1,2) an atomic absorption spectrometer which uses a high intensity continuum source and an echelle monochromator modified for wavelength modulated detection (CEWMAA). The analytical figures of merit of the CEWMAA system have been shown to be Present address, Department of Chemistry, Indiana University Bloomington, Ind. 47401. 838
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
comparable to those of a line source AA system which is background-corrected with a separate continuum source. Analytical calibration curves extend over useful dynamic ranges. The high resolution of the echelle monochromator affords sufficiently narrow spectral bandwidths so that characteristic concentrations are comparable to those of line source instruments (3). Flame detection limits are generally better than practically obtainable background corrected line source AA detection limits, and are within an order of magnitude of the best reported uncorrected AAL detection limits. This is achieved principally through the use of wavelength modulation (WM) which, as Snelleman (4) has pointed out, discriminates against low frequency additive noises and effectively discriminates against scattering and broadband absorption. It has been pointed out recently that obtaining corrections for background spectral interferences in AA can be difficult and sometimesquestionable for samples of complex or unusual matrices, when using the commonly employed means of background correction (5-8). Lovett, Welch, and Parsons (9) have reviewed the interferences of spectral origin which can occur in AA analysis; of these, the most important are (a) scattering of radiation and broadband absorption, (b) non-
