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ANALYTICAL CHEMISTRY, VOL. 51,
NO. 14,
DECEMBER 1979
Determination of Poly(ethy1ene terephthalate) Oligomers in Refrigeration Oils by Adsorption Column Chromatography-Gel Permeation Chromatography Satoru Shiono Central Research Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo, Japan 66 I
Poly(ethy1ene terephthalate) (PET) is known to contain small amounts of low molecular weight oligomers which mainly consist of cyclic n-mers ( 1 3 ) . Because PET films are always exposed to refrigerant and/or refrigeration oils in refrigeration compressors, it is unavoidable that some amounts of oligomers are extracted from PET films and dissolved in refrigerant-oil mixtures. PET oligomers dissolved in refrigerant-oil mixtures have some unfavorable effects on refrigeration compressors, as discussed in recent papers ( 7 , 8 ) . In most cases six oligomers were found in used refrigeration oils. a cyclic pentamer (I), a cyclic tetramer (11), a cyclic trimer (111), his (2-hydroxyethy1)terephthalate(IV), a cyclic dimer containing one diethylene glycol unit (V), and a cyclic dimer (VI) (7-9);
Table I. Properties and Sources of Oil Studied
a
0.916
158
B C
0.872
149 106
0.888
Pennsylvania.
naphthenic, mineral alkyl benzene paraffinic, mineral
Delaware.
source Sun Oila Du Pontb MitsubishiC Petroleum
Tokyo. Japan
oligomers adsorbed on the silica gel were recovered with 25 mL of chloroform-methanol (1:l). The chloroform-methanol solution was evaporated to dryness under reduced pressure. The oligomen were then dissolved in 1 mL of tetrahydrofuran and, after centrifugation, the tetrahydrofuran solution was chromatographed by GPC. PET Oligomers. PET oligomers were extracted from a commercial P E T film (Toray, Tokyo, Japan) with chloroform using an ordinary Soxhlet extractor in an atmosphere of nitrogen for a period of 24 h. GPC analyses showed that the chloroform extract contained six oligomers: I, 1.2%; 11, 4.0%; 111, 83.6%; IV, 0.7%; v, 4.690 ; VI, 5.9%. I11 was isolated from the crude extract using preparative GPC. Analytical GPC analysis showed that the purity of the isolated I11 was higher than 99%. The isolated I11 was used as a standard material to obtain a calibration curve between peak heights and amounts of I11 injected and for the reproducibility study of the AC-GPC method. Refrigeration oil samples containing known concentrations of P E T oligomers were prepared as follows. A known amount of the PET extract or I11 was dissolved in chloroform and then a known amount of a refrigeration oil was added to the chloroform solution. The chloroform was removed from this mixture under reduced pressure at 50 "C for 5 h. Reagents. Analytical grade tetrahydrofuran (Wako Pure Chemical, Osaka, Japan) was used as a GPC solvent. Benzene, chloroform, and methanol of spectroquality were purchased from Dojindo, Kumamoto, Japan. Silica gel as 100/200-mesh Wakogel C-200 was obtained from Wako Pure Chemical. Silica gel was dried at 180 O C for a period of 3 h prior to use. Refrigeration Oils. Three commercial refrigeration oils, Oil A, Oil B, and Oil C were used. Some properties of these oils are listed in Table I, together with their sources. Infrared analysis (10) showed that Oil C contained 0.5% (w/w) tricresyl phosphate.
n =I
Though PET contains oligomers with higher molecular weights (6),these olgiomers were rarely found in used refrigeration oils. In order t o elucidate the behavior of PET oligomers in refrigeration compressors, it is prerequisite to develop a n accurate and precise method for the determination of PET oligomers in refrigeration oils. A semiquantitative method for this purpose has already been described in a previous paper (7), but this method required a large amount of oil samples, about 20 g, because it was used for qualitative analysis as well as quantitative analysis. Consequently it suffered the disadvantage that the number of oil samples to be withdrawn from a single refrigeration compressor was limited. I n this report the author describes a new method which requires only 1-mL oil samples for the determination of PET oligomers in refrigeration oils by AC-GPC.
EXPERIMENTAL Apparatus. A Toyo Soda (Shinnanyo, Japan) 802 UR liquid chromatograph equipped with a 254-nm ultraviolet detector was used a t 40 "C. Tetrahydrofuran was used as a mobile phase at a flow rate of 1.0 mL/min. GPC separations are made using a set of four Toyo Soda TSK-gel type HI0 columns (40,250, 1500, 10000 A). A Toyo Soda six-port valve with a 100-pL external sample loop was used to inject samples dissolved in tetrahydrofuran onto the analytical columns. A preparative GPC instrument was used to isolate a cyclic trimer (111)from P E T oligomeric mixtures extracted from PET films. A detailed description of the preparative GPC was given elsewhere (9). AC Clean-up Procedure. Concentration of oligomers by adsorption chromatography (AC)was carried out on a glass column (9.6-mm i.d.) slurry-packed with 1 g of silica gel. With a 1-mL hypodermic syringe, 1 mL of an oil sample to be analyzed was injected at the top of the column. The column was washed with 140 mL of benzene and the benzene effluent was discarded. The 0003-2700/79/0351-2398$01.00/0
A
O
I , n=4,III n = 3 .
Ill, n.2.V.
oil
viscosity, (38.8 sp gr3 C) compositional features SUS g/cm
RESULTS AND DISCUSSION Figure 1 shows the GPC chromatogram of a chloroform extract from a commercial PET film. Chloroform extracts yield six oligomer peaks, denoted by I-VI, whose behavior in refrigeration compressors is t o be elucidated ( 7 , 8 ) . T h e identification of these peak eluates was described in a recent paper (9). A calibration curve was established between peak heights and amounts of I11 were injected over the range of 0.2 to 10 pg. When the amount of I11 injected was less than 6 kg, the linearity of the calibration curve was good and the plot passed through the point of origin. Calibration curves for the other oligomers were not obtained, because it was difficult t o isolate sufficient amounts of them. Calibration curves for the other oligomers were assumed to have the same slope as that obC
1979
American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2399
W
m 0
50
Q
60
m
e l u t i o n volume, counts
L
chromatogram of PET oligomers. Conditions: sample, 1 pg of a chloroform extract from a PET film: mobile phase, tetrahydrofuran; flow rate, 1.0 mL/min; temp., 40 OC: column, 40, 250, 1500, 10 000 A. Peak identity: (I) cyclic pentamer, (11) cyclic tetramer, (111) cyclic trimer, (IV) bis(2-hydroxyethyl)terephthalate,(V) cyclic dimer containing one diethylene glycol unit, (VI) cyclic dimer Figure 1. GPC
> J
.
I
50 60 elution volume, counts
70
chromatograms of refrigeration oils containing 10 ppm of a chloroform extract. (For conditions and peak identity, see Figure 1) Figure 3. AC-GPC
___
Table 11. Reproducibility of the AC-GPC Methoda
u
E
concentration \ ~ _ _ _ _ _ _ added, _ _ _ppm i concentration no. determined, ppm of I11
~
OO
100
200
5
300
10
25 of I11
volume o f eluent(benzene),mL
Flgure 2. Relationship between recoveries of PET oligomers and amounts of benzene made to flow in an AC clean-up procedure. Sample: 1 mL of benzene containing 30 ppm of a chloroform extract
tained for I11 (IO, I I ) , although ultraviolet detector weight responses of the others might be slightly different from that of 111. The relationship between recoveries of each oligomer, and the amounts of benzene made to flow through an AC column was studied to determine an optimum amount of benzene, using 1 mL of a benzene solution containing 30 ppm of a chloroform extract instead of a refrigeration oil containing known amounts of P E T oligomers. T h e results of this experiment are shown in Figure 2. When an AC column is washed with less than 300 mL of benzene, every oligomer is recovered completely except VI. On the other hand, the recovery of VI decreases, if more than 150 mL of benzene is made to flow through an AC column. With the AC-GPC method, the oligomers in refrigeration oils appear superimposed on a broad peak attributable to polar compounds of oils (Figure 3). T h e more benzene is made to flow, the smaller this broad peak becomes, Therefore it is preferable for an AC column to be washed with as large an amount of benzene as possible. Consequently, it is concluded that the optimum amount of benzene is 140 mL for an AC clean-up procedure. Figure 3 shows the AC-GPC chromatograms of refrigeration oils containing 10 ppm of a chloroform extract. Six oligomer peaks, I-VI, appear superimposed on a broad peak. In the case of Oil A, this broad peak elutes from 47 to 70 counts. The shape of the broad peak is found to be dependent upon the amounts and molecular weights of polar compounds contained in the refrigeration oils. Generally speaking, the broad peak becomes larger as the deterioration of oils proceeds (7). In the AC-GPC chromatogram of Oil C (Figure 3c), there is a tricresyl phosphate (TCP) peak interfering with the determination of V. The interference from some additives is discussed later.
1 2 3 4 5
mean, ppm accuracy, 5% precision, 5% RSD
-
5.17 5.29 5.09 5.11 5.25 5.18 3.6 1.67
9.76 9.66 9.81 10.00 9.74 9.79 -2.1 1.27
24.76 24.89 24.50 24.35 25.03 24.71 -1.2 1.13
Refrigeration oil: Oil A. The reproducibility of the AC-GPC method was evaluated, using Oil A samples containing 5,10, and 25 ppm of 111. The results are summarized in Table 11. The detection limit was 0.04 ppm for each oligomer (signal-to-noise ratio, 3:l). Refrigeration oils are usually used without additives, but in some cases refrigeration oils contain some additives. Dimethylpolysiloxane as anti-foaming additive (approved concentration: 2-100 ppm) does not interfere with the determination of P E T oligomers by the AC-GPC method. Almost all kinds of hindered phenols also cause no interference, because hindered phenols were found to elute thoroughly from an AC column with more than 100 mL of a benzene eluent (12). Tricresyl phosphate (TCP) used as extreme pressure additive yields a GPC peak a t almost the same counts as V. As shown in Figure 3c, T C P interferes with the determination of V. In the cme of Oil C containing 0.5% (wjw) TCP, about 60 wg of T C P was recovered with P E T oligomers after the AC clean-up procedure. Figure 4 shows the relationship between the amounts of T C P recovered and the amounts of benzene with which an AC column was washed. The amount of T C P recovered becomes negligibly small when an AC column was washed with more than 200 mL of benzene. Figure 4 indicates that when more than 200 mL of benzene is made to flow through an AC column, V can be determined without interference from T C P a t the sacrifice of the determination of VI. In order to determine all oligomers in refrigeration oils con-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
I
I
avoiding the interference from these aromatic amines. Refrigeration oil samples to be analyzed are usually withdrawn from refrigeration compressors with chlorofluorocarhon refrigerant dissolved in them. However, the removal of refrigerant is not necessary prior to the AC-GPC procedure, since neither dichlorodifluoromethane nor chlorodifluoromethane with which refrigeration oils were saturated a t atmospheric pressure was found to interfere with the determination of oligomers in the oils.
ACKNOWLEDGMENT T h e author thanks T. Imamura and J. Enomoto for their fruitful discussions. volume of eluent(benzene), mL Figure 4. Relationship between recoveries of TCP and amounts of benzene made to flow in an AC clean-up procedure. Sample: Oil C
which contains 0.5% (w/w) TCP taining TCP, the determination of oligomers should be done twice: first, the determination of oligomers I-IV and VI by the ordinary AC-GPC method described in the Experimental section and, second, the determination of oligomer V by the modified AC-GPC method described here. Aromatic amines added as antioxidants are another example of potential interference. Low molecular weight aromatic amines such as N-phenyl-@-naphthylaminedo not interfere, because they can be separated from P E T oligomers by GPC. However, aromatic amines with higher molecular weights than that of N,N’-diphenyl-p-phenylene-diamine do interfere with the determination of oligomers. A similar procedure to the elimination of the interference caused by T C P is thought to be effective for
LITERATURE CITED S.D. Ross, E. R. Coburn, W. A. Leach, and W. B. Robinson, J . Po/ym Sci., 13, 406 (1956). R. Giuffria, J . Polym. Sci., 49, 427 (1961). I. Goodman and E. F. Nesbitt, Polymer, 1, 384 (1960); J . Polym. Sci , 48, 423 (1960). L. H. Peebles, M. W. Hoffman, and C. T. Albett, J. Polym. Sci., Pari A- 1 , 7 , 479 (1969). H. Zahn and P. Kusch, T e x L l n d . (Moenchen-Gladbach,Ger.), 69, 880 (1967). D. R. Cooper and J. A. Semlyen, Polymer, 14, 185 (1973). S. Shiono, J. Enomoto, K. Shimamura, and K. Aiba, Reito, 53, 23 (1978). S. Shiono, J. Enomoto, K. Shimarnura, and S. Watanabe, presented at the annual meeting of American Society of Heating, Refrigerating and Air-Conditioning Engineers, Detroit, Mich., June 1979. S. Shiono, J . Polym. Sci.. Part A- 1 , in press. G. W. Recktenwaid, Anal. Chem., 31, 1742 (1959). S.Mori, S.Iwasaki, M. Furukawa, and T. Takeuchi, J , Cbromafogr., 62, 109 (1971). L. M. Zaborsky 11. Anal. Cbem., 49, 1166 (1977). M. Fujita and R. Tsuda, Shinku Kagaku, 16, 101 (1969).
RECEIVED for review June 5 , 1979. Accepted August 17, 1979.
Ohm’s Law, Fick’s Law, and Diffusion Samplers for Gases E. D. Palmes” and R. H. Lindenboom New York University Medical Center, Institute of Environmental Medicine, 550 First Avenue, New York, New York 10016
T h e first demonstration that molecular diffusion could be used for the quantitative collection of atmospheric gases was made by Palmes and Gunnison ( I ) . The theoretical transfer of the test gas through a tube of known dimensions was estimated using Fick’s First Law. T h e results clearly showed t h a t rates of atmospheric sulfur dioxide and water vapor transfer through tubes having an efficient absorbent on the closed end and the other end open to the contaminated atmosphere could be predicted accurately by applying Fick’s law and using coefficients of diffusion taken from the literature. In the ideal situation, working a t a fixed temperature and using at the closed end of the tube an absorbent which is 100% efficient, Le., concentration at absorbent surface = 0, the rate a t which gas is transferred through the tube by diffusion can be stated by the equation:
JA = D(A/L)c (1) where J = diffusion flux (mol/cm2/s), A = cross sectional area of tube (cm,), I) = coefficient of diffusion (cm2/s),L = length of tube (cm), and c = concentration of contaminant gas (mol/cm3). Using this equation to calculate theoretical transfer rate and exposing samplers to known gas concentrations for various times showed that the predictions can be confirmed with great accuracy over a wide range of tube 0003-2700/79/035 1-2400$01 O O / O
lengths and diameters. A more detailed discussion of the principle of this type of sampler and its application to NO, measurement was given by Palmes et al. (2). When we adapted the NO, sampler for the measurement of NO, (31, it became desirable to reduce the sampling rate. The solution adopted was to seal a full-length, smaller diameter tube inside the larger one and thereby reduce the effective cross section of the tube and the sampling rate. Another possibility, not tried prior to publication, was to insert shorter lengths of smaller tubing into the sampler and to produce, in effect, two tubes in series; this, of course, raised the question as to the manner in which two resistances to diffusional flux could be added. It has been demonstrated by others that there is a strong similarity between Fourier’s law of heat flow, Ohm’s law of electrical current, Poiseuille‘s law of liquid flow, and Fick’s law of diffusion; the similarities between these laws are treated quite comprehensively by Castellan ( 4 ) . We shall concentrate on the similarity between Fick‘s law and Ohm’s law. It should be noted that Nadeau et al. ( 5 )had mentioned this similarity in a discussion of a sampler based on permeation through a membrane; their treatment was only descriptive, however, since permeation constants, which include membrane thickness, must be determined empirically. In the diffusion SamC 1979 American Chemical Society
