shape for minimum error will shift toward sharper peaks (1). This shift in turn suggests some decrease in optimum r. The over-all effect of increasing base line uncertainty will be to increase the error with which the peak area can be measured, but not to significantly change the fractional height at which the peak width is measured. The above analysis was derived for Gaussian peaks. As real peaks frequently show tailing, which increases the width near the base, a slightly larger value of r may be preferable to that predicted for this generalized case. Practical Choice of Fractional Height. It is clear that no single value of fractional height r will produce minimum relative error in area for all peak shapes and conditions. The choice of r for peaks of small area is more critical than for those of large area. Similarly, the choice of r for sharp peaks is more critical than for flat peaks. Nevertheless, from a practical point of view, a single fractional height (or a restricted number of fractional heights) at which to measure the width of all peaks in a chromatogram is desirable. The
choice of a single r necessarily involves a compromise to minimize the relative error in area for varying peak shapes. This compromise will depend in part upon the measurement conditions and analytical requirements of the individual laboratory, Considering Figure 1, however, we can see that if a full range of smooth Gaussian peaks is normally encountered, then a single value of r should be chosen near 0.25. Such a choice means a slight loss in precision for flat peaks, but a considerable gain for sharp peaks. For those types of analyses yielding smooth, sharp, well resolved peaks, r would be better chosen in the neighborhood of 0.1. Under conditions of serious peak tailing, r would be better chosen at perhaps a value as high as the popular fractional height of 0.5. RECEIVED for review January 15, 1968. Accepted February 29, 1968. Paper presented in part at the 153rd National Meeting of the American Chemical Society at Miami Beach, Fla., 1967. Financial support to D. L. B. by the National Research Council of Canada is gratefully acknowledged.
Application of Gas Chromatography to Qualitative and Quantitative Copolyamide Analysis Anthony Anton Carothers Research Laboratory, Textile Fibers Department, Experimental Station, E. I . du Pont de Nemours and Co., Inc., Wilmington, Del. A method is described for the gas chromatographic separation of the diacids (as the dimethyl esters) recovered from hydrolyzed copolyamides prepared from a single diamine-i.e., hexamethylenediaminewhich will give both qualitative and quantitative results. The method requires only 50.2-gram samples, and has a relative error in accuracy of 15%. The per cent 6 nylon in a copolyamide must be determined by difference, and with copolyamides made from more than one diamine, a calibration curve for each diamine must be prepared as well as for the diacids.
TECHNIQUES PREVIOUSLY DESCRIBED for the analysis of copolyamides are either time-consuming or give only qualitative or quantitative information, but not both. For example, paper chromatography, infrared spectrometry, and pyrolytic analysis cannot be used to quantitatively establish the concentration ratio in such copolymers as 66/610 or 66/610/612 nylon. Even if the components are known, a melting point-composition relationship is not reliable. In the 66/610 case for example, a melting point of 210 “C characterizes both a 60/40 and a l0/90 copolyamide composition (1). DTA, however, can be used to determine whether the sample is a block or random copolymer. The titration of diacids or diamines recovered by extraction or ion exchange from an acid hydrolyzate may produce quantitative data but a tedious separation and characterization is required to establish the nature of the copolyamide (2). The method described in this paper, which involves the gas chromatographic resolution of the polymer hydrolyzate, (1) W. E. Catlin, E. S. Czerwin, and R. H. Wiley, J . Polymer Sci., 2, 412-19 (1947). (2) Haslam and Willis, “Identification and Analysis of Plastics,” Chap. 4, Van Nostrand, New York, 1965. 1 1 16
ANALYTICAL CHEMISTRY
will give both qualitative and quantitative results. The liberated diacids in the hydrolyzate are esterified with BF3methanol and the diesters are recovered in diethyl ether, dried, gas chromatographed, and the retention time is measured to identify the corresponding diacid. A second hydrolyzate is made caustic, extracted with n-butanol which is then removed by atmospheric distillation, and the residue is gas chromatographed to identify the diamine. Assuming one diamine is present, the copolyamide cornposition can be determined by converting the peak height ratio of each diacid to the corresponding polymer weight. The method is applicable to 0.05- to 0.2-gram samples and has a relative error in accuracy of 10 %. Similar techniques for identifying carboxylic acids in plasticizers, in polyesters fibers, and in alkyd coating resins have been described (3-5). In these techniques, isophthalic, terephthalic, adipic, and sebacic acids have been recovered and converted to methyl esters from their products via transesterification with sodium methoxide and separated by gas chromatography. However, these techniques are not applicable to polyamides which are relatively resistant to aqueous caustic hydrolysis. EXPERIMENTAL
Apparatus. An F & M Model 1609 with a flame ionization detector was used for all gas chromatographic separations. Diacids were separated on a 6-ft, ‘i4-inch 0.d. X 3/16-in~h i.d. stainless steel column packed with 60/80-mesh acid-washed Chromosorb W (a product of Hewlett-Packard) (3) G. G. Esposito and M. H. Swann, ANAL.CHEM., 34,1048 (1962). (4) S . J. Jankowski and P. Garner, ibid., 37, 1709-11 (1965). (5) D. F.Percival, ibid., 35, 236 (1963).
SPEC.: HEAVY WALL GLASS FOR PRESSURE MEASUREMENTS IN cm.
- 1001bs. MAX.
b Figure 1. Pressure hydrolysis set-up
TO APPARATUS
GROOVED TO RING
which was coated with 5 % diethylene column was conditioned at 200 "C 1 injection port was maintained at 260 at 2x0 "C. and the column was progrt ..~ 220 "C at a rate 01 . S"C/min. For di column of identical dimensions but pacKeo witn oujau-mesn glass heads coated urith 1 % Apiezon L (a product of Hewlett~ > \ ~~..> racraru) wab USCU. Conditioning, injection, and detector temperatures and programming rates were identical to those specified for the diester separation. Polymer samples were acid hydrolyzed under 40-lb nitrogen pressure in the apparatus which is described in Figure 1. Materials. The adipic (C6), sebacic (GO), undecanedioic (Cl,), dodecanedioic (Ct2) acids, and the azelaic (G) acid which was used as the internal standard, were obtained from commercial vendors and had a minimum assay of 99.7z. The Cointernal standard was prepared by dissolving 3.0 grams of the acid in 100 ml of methanol (3.0% solution). The esterification reagent, BFrmethanol, was purchased from Applied Science L aboratories, Inc., StateCollege, Pa. Procedure. A 0.05- to 0.2-gram sample of polymer or :"-^a..uerl..,: I.=IIuAcy n.tb 25 ml of 6N HCI either under atmospheric pressure for at least 24 hr; under 40 Ib of pressure the reflux time can be reduced to about 4 hr. The hydrolyzate is transferred to a 100-ml beaker and evaporated to dryness on a steam bath or hot plate. The sample is divided, or a second hydrolysis performed (evaporation to dryness not required), if both the diamine and diacid have to be identified. To analyze for the diacids, 2 ml of CI internal standard and 10 ml BFrmethanol solution are added to the hydrolysis residue. The solution is heated to boiling (aromatic diacids require refluxing for at least 2 hr) over the steam bath, cooled, transferred to a separatory funnel containing 50 ml of water, and extracted with two 30-1111 portions of diethyl ether. The ethereal solution is dried over anhydrous magnesium sulfate, filtered, and evaporated to about 0.5-1.0 ml total volume. The second hydrolyzate solution, which is used for identifying the diamines, is treated with a 50/50 NaOH/HIO solution until a pH of 10-12 is attained. The solution is then extracted with two 30-ml portions of n-butanol and the excess n-butanol is removed from the extract by atmospheric ~~~
..^__
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~
nu .ux E -
\
EVAPOR4TE
~
YI*IIIII"L>
DIAMINE
I I""
+
1 -1
**I"
HYDROLYZ4TE. DRY
>*LI=
1
EXTRACT WITH n-BUTANOL
DIESTERS, Dl4MINE.ZHCl
n-BUT4NOL
DIAMINE IRESIDUEI WITH TRACE BUT4NOL
DIESTERS ~
GAS CHROM4TOGR4PHY
645 CHROMPITOGRAPHY
IDENTIFY
II
EXTRACT WITH DlETHIL ETHER
DISTILL
1
ESTERIFY WITH BF,. METHANOL
IDENTIFY AND QUANTIT4TIVELY AN4LYZE
Figure 2. Analysis of copolyamides via gas chromatography distillation. About a 10-pl sample of the 1-3 ml of diaminebutanol residue remaining in the flask is used for the chromatographic analysis. The diamine is identified by comparing its retention time and separation temperature with known diamines. Calibration for Acid Mixtures. A calibration curve is constructed by weighing out 10, 20, 50, and 100 mg each of C 6 , Cia, C,,, and CL2diacids into 125-ml Erlenmeyer flasks. Two milliters of the C , internal standard solution are added and the mixture is esterified by the BF8-methanol procedure and is then gas chromatographed. The peak height of each acid is ratioed with the internal standard and plotted with respect to weight of acid. The slopes of the curves yield factors giving the milligrams of diacid per unit area ratio. The following calculation takes into account the determination of diacid, conversion of milligrams of diacid to milligrams of corresponding polymer, and hydrolysis correction if atmospheric, reduced-time, hydrolysis is used; as in cases where only a quantitative analysis of a known system is required:
'Z
,"Polvmer
.
X.. Y. . etc. in coDolyamide . .
=
diacid X, Y,etc. peak height ratio . internal standard sample wt. (grams) x H e x . *tO. x 10
FX.Y a b . X
VOL 40, NO. 7, JUNE 1968
1 117
SAMPLE: ESTERIFIED HYDROLYZATE OF MIXTURE OF 66, 610 AND 612
'6
I2
INT.s'
:lo
CS e 66
612 x610
0
"
4
8
20
12 16 TIME (hrs.)
24
.Cll IMPURITY IN 612 64~128x
Figure 3. Hydrolysis rates of 66, 610, and 612 nylon Atmospheric pressure solvent, 6N HCI. Concentration: 0.1 g/25 ml
Factors: F converts milligrams of diacid to milligrams of polymer F66 = 144.2 mg, F 610 = 86.5 mg, F612 = 83.5 mg HC, hydrolysis correction, takes into account per cent polymer hydrolyzed at a specified time--e.g., after 16 hr HC66 = 1 ; Hcsio = 0.94; Hc61z = 0.92. DISCUSSION
A schematic describing the qualitative and quantitative analysis of a copolyamide is shown in Figure 2. If atmospheric hydrolysis is used, a minimum of 24 hr of refluxing with 6N HC1 is required to cleave 610 and 612 nylon. However, shorter hydrolysis times can be used if the per cent
Table I.
Copolyamide Analysis
ACCURACY Synthetic polymer mixtures A B
C
66 610 612 66 610 612 66 610 612
Actual random copolymer D E F
Concentration, mg Added Founda 30.0 27. 33. 30.0 30. 30.0 50. 50.0 50.0 49. 50.0 100
Re1 error, % -7.0
+10.0 $1.7 +0.4 -1.0
-1.0 +10.0 $6.0
45.
110 26. 5 24. 7
25 25
-3.0
Per cent Given 30170 40160 60140
Found 35/74 39/61 62/41
PRECISION % 610 in 60/40 661610 random copolymer a. 38.5 b. 40.6 c. 40.5 d. 39.2 e. 39.7 Av 39.7 u a
0.8
Corrected for hydrolysis rate.
1 1 18
ANALYTICAL CHEMISTRY
RETENTION TIME (min.7 BASED ON INT STD.
-?-
{
A -5
-
Figure 4. Gas chromatographic separation of adipic, sebacic, and dodecanedioic acids recovered from a mixture of corresponding polymers
polyamide hydrolyzed at any given time is known (Figure 3). Under 40 lb of NZ pressure, complete hydrolysis is accomplished in less than 4 hr. Sealed-tube pressure hydrolysis techniques could also be employed (2). A typical separation of a mixture of 66, 610, and 612 polymer is given in Figure 4. Because of the sensitivity of gas chromatography, impurities in the diacids used to prepare the polyamide could be readily detected, as in the case for 612. The data presented in Table I indicate that the method has an average relative error of about 10% in accuracy and a CJ of =t0.8zabsolute for the analysis of 610 in the 40% copolymer range. The method described is applicable to products resulting from the copolymerization of a variety of diacids with hexamethylenediamine. If two or more dissimilar diamines are used to prepare a copolyamide, a calibration curve for each diamine must be constructed in order to derive quantitative data. The per cent 6 nylon in a 66/6 copolyamide is determined by difference. The aminocaproic acid, which is the hydrolysis product of 6 nylon, cannot be resolved according to the procedure outlined in this paper. Its presence is detected by the use of paper or thin-layer chromatography (2). A copolymer of 610/6T poses a problem since terephthalic and sebacic acids have the same retention times on this column. This is circumvented by taking advantage of the large difference in rates of BF3 esterification between aliphatic and aromatic diacids; sebacic acid is esterified within10 min whereas a minimum of 2 hr is required for complete esterification of the terephthalic acid. The peak area ratio difference between the two chromatograms is then related to the terephthalic acid content. ACKNOWLEDGMENT
The author thanks C. H. Roberts of the Carothers Research Laboratory for his technical assistance in the preparation and analysis of some of the copolyamide samples. RECEIVED for review February 8, 1968. Accepted March 21, 1968. Paper presented at 3rd Middle Atlantic Regional Meeting, ACS, Philadelphia, Pa., February 1968.
