Table VI. Use of Personal Sampler to Evaluate a Complex Mixture Concentration, pprn C ornp our1 d
D i r e c t analysis
CH,
co
C,H, CF,Cl, CH,=CHCl CFCl, CH,==CCI, C6H 6‘
Figure 6. Gas chromatogram of several contaminants sampled with t h e personal sampler
84
i
4
i
Personal sampler
4 0.5
9oi 5 922 5 6 t 0.5 651 3 4 i 0.5
6 I0.5 5 I0.5
I4
H6
86
i
4
5 92
?r
0.5 5 5 0.5 3 0.5
96 6
56 4 6 4
i i i i i i
t
0.5 0.5
chromatogram is shown in Figure 6. Results of these tests are shown in Table VI. Results to date with the sampler show that it is efficient and easy to use for low molecular weight compounds. The use of “inert” materials of construction could conceivably extend the usefulness for sampling atmospheres containing intermediate molecular weight and polar compounds.
ACKNOWLEDGMENT
The analytical column employed was a 10-foot X ’/4-inch stainless steel column packed with 10% DC-200(1200cstk)on chromosorb G, 45/60mesh
with less volatile contaminants, a sample mixture containing 50 ppm of methane, 10 ppm of hexane, and 70 ppm of n-decane was prepared. The data in Table V show that adsorption did occur, not only within the sampler but in the test chamber as well. During the period of sampling, the n-decane decreased to 64 ppm within the chamber. The initial analysis of the sampler indicated 21 ppm. The sampler was warmed and the resulting concentration was 55 ppm. Fabrication of a sampler lined with Teflon may relieve this problem. Atmospheres containing methane (CH4), dichlorodifluoromethane (CC12F2), trichlorofluoromethane (CCl,?F),ethane (CH~CH:I),n -hexane (CHz(CH2)4CH3), benzene (CSHG),vinyl chloride (CH*=CHCl), and vinylidene chloride (CHZ=CC12) have also been evaluated. A typical gas
The authors thank R. Gann for the computer program to calculate sampling times and J. Musick for designing the bottles.
LITERATURE CITED (1)C.L. Fraust and E. R. Hermann, Am. lnd. Hyg. Assoc. J., 27,68 (1966). (2)P. W. West, B. Sen, and N. A. Gibson, Anal. Chem., 30, 1390 (1958). (3)J. W. Anderson and R. Friedman, Rev. Scl. lnstrurn., 20,61 (1949). (4)J. P. Stone, H. G. Eaton, and F. W. Williams, Rev. Sci. Instrum., 46, 1288 (1975). (5) F. J. Woods and J. E. Johnson, Naval Research Laboratory Report 6606 of 22 September 1967. (6) K. Porter and D. H. Volman, Anal. Chem., 34, 748 (1962).
RECEIVEDfor review August 27, 1975. Accepted October 20, 1975. This work is being funded by the National Institute for Occupational Safety and Health Administration. This work was presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, in March 1975.
Moisture Determination in Polyester Polymers E. I?.Hoffmann Ethicon, lnc., Somerville, N.J. 08876
Moisture reacts quickly with a mixture of hexamethyldlsilaLane and trimethylchlorosilane ( 2 : l ) in the presence of pyridine to form hexamethyldisiloxane. This reaction was used as a new approach for the determination of moisture in polyesters and appears to have more general applicability. The sensitivity and specificity is good since gas-liquid chromatography is used to separate the products of reaction, and the derivative of water is amenable to flame ionization detection. The present limit of detection is 50 ppm and the relative standard deviation at the 200 ppm level is 13%.
The quest for a suitable method for the determination of water in polyester polymers has led to the development of a
new specific and sensitive method for water determination which has broader applicability. The new approach is based upon converting the moisture into an organic derivative and subsequently separating this derivative from any other side-reaction product by gas-liquid chromatography using flame ionization detection. This approach combines a high degree of specificity and good sensitivity. Water plays a central role on this blue planet, and methods for qualitative and quantitative determination abound in the literature. A critical review written by John Mitchell, Jr., ( I ) is an excellent source of work done prior to 1960 and includes many currently important methods. Among these, a commercial apparatus based upon a coulometric ANALYTICAL CHEMISTRY, VOL. 48, NO. 2 , FEBRUARY 1976
445
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Table I. Recovery of Water Added to Dry Polymer Known a d d i t i o n . pg
4
400
156 194, 228 267, 316 369
500
545
100
200 300
c
\
Figure 1. Separation of silyl derivatives
Peak identity: (1) Trimethylchlorosilane, (2) chloroform internal standard, (3) hexarnethyldisiloxane due to water, (4) pyridine, (5) hexamethyldisilazane
principle involving the Keidel (2, 3) cell was useful as a comparison method in this investigation. Water reacts with hexamethyldisilazane in the presence of trimethylchlorosilane to form hexamethyl disiloxane and ammonium chloride: 2(MesSi)2NH
+ 3H20 + 2MesSiCl3MesSiOSiMe3
+ 2NH4C1
The hexamethyldisiloxane is separated from other silanized active hydrogen compounds and determined using an internal standard.
commodate different instruments. Procedure. Use of a dry box is recommended for storage of apparatus and samples, and manipulations such as open transfer of reagents. Fill a large dry Serum vial with 4 ml Tri-Sil, 1 ml carbon tetrachloride, as internal standard, and 10 ml dried pyridine. Seal immediately with a Teflon-lined cap using a crimping tool. From this well-mixed reagent, a 1-ml amount is transferred to a 2-ml autoinjector vial by means of a syringe. From this smaller vial, 1-p1 amounts are injected into t h e gas chromatograph under conditions described above. (For sample chromatogram, see Figure 1.)Having established a stable blank value, inject 200-pl increments of water into the vial and, after each increment has reacted for a few minutes, inject into the gas chromatograph. I t is very desirable to use an automatic integration system. T h e ratio of the areas of the hexamethyldisiloxane peaks t o t h e carbon tetrachloride peaks is plotted vs. the pg water/ml. A calibration curve is thus constructed which is linear and intersects the ordinate a t the reagent blank ratio value. I t is recommended to carry out the analyses after standardization with no long delay, in order t o avoid building a blank value due to adsorbed water on the column. Blank values originally obtained on the column after long disuse are higher, b u t settle down quickly upon injecting the reagent. Samples of polymer (0.5-1.0 g) in ground or pellet form are weighed into a dry 2-ml vial and 1 ml reagent is added by syringe before crimping on the cap. Pick-up of water may be avoided by filling the vial in a dry box from a weighed amount of polymer, then weighing t h e residual polymer and calculating t h e weight difference. T h e samples are placed in the oven a t 105 O C for 40 min. After cooling to ambient temperature, 1-111 samples are injected into the gas chromatograph. T h e peak area ratios between sample and standard are converted to micrograms of water by means of t h e calibration curve.
RESULTS AND DISCUSSION
EXPERIMENTAL Reagents. Tri-Si1 silylation reagent was obtained from Pierce Chemical Co., P.O. Box 117, Rockford, Ill. 61105. Baker reagent grade pyridine was dried with Linde 5A molecular sieve, ?&inch, by magnetically stirring a pint reagent bottle containing about 50 ml molecular sieve and pyridine for a day. T h e clear supernate was carefully pipetted out as needed without stirring the contents. Spectrograde carbon tetrachloride was dried in the same manner as pyridine. ADoaratus. Auto injector vials. 1.9 ml, from Wheaton Glass wit; caps lined with Teflon TFE fluorocarbon resin were sealed with a crimping tool. Serum vials, 10 ml, Neutraglass with Teflonlined caps were also sealed with a larger crimping tool. T h e latter vials contain a maximum of 14 ml. Hamilton syringes, 1- and 10-11, were used. Larger glass Luer-Lok syringes with (No. 26) needles were used in 1- and 5-ml sizes. An oven was kept a t 105 "C. A dry box flushed with dry nitrogen from a liquid air supply was used on humid days. A Perkin-Elmer, Model 900 Gas Chromatograph was used with a Hewlett-Packard computer for peak integration. T h e detector on this Model 900 Chromatograph is particularly insensitive to the silica which is deposited on it during injection of silanizing agent. An automatic integration system is recommended for ease of operation, b u t peak height may be used within the usual limits of visual peak measurement. Gas Chromatograph Conditions. A 4-m, 6.4-mm 0.d.. 2-mm i.d.. glass column packed with 1%OV-1 on high performance Chromosorb W was used. T h e column temperature was set t o ambient, but the actual temperature was close t o 50 "Cbecause of heat from the manifold and injection block. T h e hydrogen flame detector was kept a t 150 "C,the injection port a t 100 "C, and the manifold a t 100 "C. Helium flow was 33 ml/min. T h e attenuation was lo3 X 2, and the chart speed 5 m i d i n c h . Conditions must be varied t o ac446
F o u n d , pg
The recovery of the method was checked in the following manner. Polymer was dried four days at 110 "C in a sealed vessel. Then the dry polymer was weighed into auto injector vials and varying amounts of water were injected into the sealed vial. After a day of standing, the samples were analyzed according to the above method. The results are tabulated in Table I. Note that the agreement between the added water and found water is quite good. The differences found reflect not only the error in analysis, but also the error involved in spiking the polymer samples with small quantities of water with a microliter syringe. With the exception of the first value, the absolute deviation from the nominal added values is f l l % . Samples of polymer were analyzed by the DuPont "Moisture Analyzer" and by the silylation method. The results are tabulated in Table 11. This work was done to compare the silylation method to an established method routinely used for moisture analysis in polymers. Note that the agreement is quite good in most cases. There are, however, several outlying values. The precision of the method can be judged from the replicate analyses run by the silylation technique. However, not enough replicates of any one sample were run so that a standard deviation could be calculated. Other work on a similar system gave a relative standard deviation of 13%at the 200-ppm level. The method is presently used for measuring moisture content of polymer pellets. The lower limit of detection at present is 50 ppm. It may
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
mizes surfaces and thus the adsorption of ever-present moisture.
Table 11. Comparison of Water Found in Polymer Granules by DuPont Moisture Meter and Silylation Procedure Silylation, ppm
287. 267 1501; 1550, 1240 180. 127 2631; 1959 123 135, 103 236 210, 262
ACKNOWLEDGMENT
Moisture meter, ppm
The author thanks J. R. McDivitt for valuable advice and John Sutherland for checking out the method and running innumerable analyses.
27 7 1430 133 1793 109 21 2 26 5 360
LITERATURE CITED (1) John Mitchell, Jr., "Treatise on Analytical Chemistry," Part 11, Vol. 1. I. M. Koithoff, P. J. Elving, and E. 6.Sandell, Ed., lnterscience Publishers, New York-London. 1961. (2) F. A. Keidel, U.S. Patent 2 630 945 (1958). (3) F. A. Keidel. Anal. Chern., 31, 2043 (1959).
be possible to reduce this limit through the use of very dry solvents and silylating reagents which have not contacted traces of moisture during manufacture and packaging. This static method for the determination of water mini-
RECEIVEDfor review September 17, 1975. Accepted November 10,1975.
Determination of Acetates and Acetyl Groups by Digestion of Samples in Perchloric Acid Followed by Either Nuclear Magnetic Resonance Spectrometry or Distillation and Potentiometric Titration A. A. Schilt" and G.
D. Martinie
Department of Chemistry, Northern Illinois University, DeKalb, Ill. 60 1 15
Methods are described for the determination of acetates and acetyl compounds based upon digestion with perchloric acid followed either by quantitative NMR analysis or by distillation and titration of the liberated acetic acid. Interference by codistilled perchloric acid is avoided by either of two methods. One utilizes ion-exclusion chromatography to remove perchloric acid prior to titration of acetic acid. The other involves a potentiometric titration after addition of dioxane to enable precise differentiation between acetic and perchloric acid contents. Applied to pure compounds, the methods gave results accurate to within 1% in most instances. Results are reported together with a study of digestion products for various samples.
Acetyl and acetate determinations commonly involve alkaline or acid hydrolysis to yield acetic acid, distillation of the liberated acetic acid, and titration of the acid with standard base ( I ) . Reagents most frequently used for the hydrolysis step include p -toluenesulfonic acid, sulfuric acid, and alcoholic potassium hydroxide. Many problems arise in such determinations including slow hydrolysis, oxidation of C-methyl groups to acetic acid, collection of volatile acids other than acetic in the distillate, and difficult distillation and inefficient recovery of the acetic acid ( I , 2 ) . Some procedures designed to overcome the problem of incomplete separations involve vacuum or steam distillation, use of traps or bubblers, or use of ion-exchange or other chromatographic techniques. Instrumental techniques have also been applied to circumvent, in certain cases, the need for distillation or even the initial hydrolysis step ( I ) .
The work reported here concerns an investigation of the effectiveness of perchloric acid digestion in place of the usual hydrolysis step for the determination of acetyl and acetate compounds. This approach seemed promising in view of the report by Smith ( 3 )that acetic acid is totally resistive to hot, concentrated perchloric acid, while formic acid is readily and completely oxidized. Thus formyl and formate groups should not interfere in acetyl and acetate determinations. Also, unlike the Kuhn and Roth method ( 4 ) which depends on the use of an oxidative mixture of chromic acid and concentrated sulfuric acid, the proposed method should be free of risk of oxidation of acetic acid. Inglis reports that recoveries of acetic acid are slightly low when distilled from chromic acid ( I ) . Furthermore, since the oxidation properties of hot, concentrated perchloric acid are generally superior to chromic-sulfuric acid mixtures, it seemed promising to explore perchloric acid digestion as an alternative method for liberating acetic acid quantitatively from C-methyl and acetyl compounds, As a part of the present study, some alternatives and refinements in the distillation and titration steps were also explored.
EXPERIMENTAL Reagents. All chemicals, including samples subjected to analysis, were reagent grade and used as received without further purification. Apparatus. A Varian A-60-A NMR spectrometer equipped with a variable temperature controller and drilled quartz sample tubes was used for recording NMR spectra. Potentiometric titrations were performed using a Corning Model 7 p H meter equipped with glass and saturated calomel electrodes. T h e same instrument and electrodes (inserted in a Teflon flow-through cell) were used to
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