stant. Even if the first two terms remain constant, the selectivity observed could be due to a change in swelling pressure, x - xl, in addition to the solvated volume change of the cations. The osmotic pressure of the two cationic forms is undoubtedly different since the second solvation sphere composition is not the same as shown by the NMR data in Table I1 and the evaporation data in Figure 3; the x - x l term could vary with solvent composition. The third term could then determine the selectivity, but not necessarily due to volume differences alone. Because the data show the outer sphere solvation of the cations in both the external and resin phases to be different, one can question if the activity coefficient ratios would not change. No experiments were carried out in this study which would help resolve this problem. The problem is discussed by Helfferich ( 2 2 ) and Marinsky (19) where several methods of handling the activity terms are discussed. Previous data for the Cr(II1) system in ethanol-water mixtures indicate the magnitude of the problem (23).The values of Qn for reaction 1 (ethanol in place of methanol) show a definite dependence on the ratio of activities of ethanol to water. I t was concluded in the work that the cation activity ratio, TA/YB, depends on solvent composition. T h e value Qn shows no dependence on solvent activities for the methanol-water system (7). Thus, previous data would indicate that an assumption of constant activity ratio in the external phase would be valid, but there is no justification for this for the resin phase and it would leave open to question how much of the selectivity is due to activity ratio variation.
LITERATURE CITED (1) D. G. Howery. L. Shore, and B. H. Kohn. J. Phys. Chem., 76, 578 (1972). (2) D. Reichenberg and I. J. Lawrenson. Trans. Faraday SOC., 59, 141 (1963). (3) W. J. Casey and D. J. Pietrzyk. Anal. Chem., 45, 1404 (1973). (4) T. Sakaki, Bull. Chem. SOC.Jpn, 28, 217, 220 (1955). (5) G. L. Starobinets and A. B. Chizhevskaya, Russ. J. Phys. Chem.. 44, 1135 (1970). (6) A . R. Gupta, J. Phys. Chem., 75, 1152(1971). (7) C. C. Mills 111 and E. L. King, J. Am. Chem. SOC.,92, 3017 (1970). (8) J. C. Jayne and E. L. King, J. Am. Chem. Soc., 86, 3989 (1964). (9) R. J. Baltisberger and E. L. King, J. Am. Chem. Soc.,86, 795 (1964). (10) H. M. McConnell, J. Chem. Phys., 28, 430(1958). (11) V. S. Sastri. R. W. Henwood. S. Behrendt, and C. H. Langford, J. Am. Chem. Soc., 94, 753 (1972). (12) G.W. Haupt, J. Res. Natl. Bur. Std., 48, 414(1952). (13) D. C. Whitney and R. M. Diamond, J. Phys. Chem., 68, 1886 (1964). (14) R. W. Gable and H. A. Strobel, J. Phys. Chem., 60, 513 (1956). (15) A . M . Phipps, Anal. Chem., 40, 1769(1968). (16) R. J. Baltisberger and C. M. Melsa, Anal. Chem., 45, 2285 (1973). (17) K. W. Pepper, D. Reichenberg, and D. K. Hale, J. Chem. SOC., 1952, 3129. (18) H. D. Sharma, R. E. Jervis, and L. W. McMillen. J. Phys. Chem., 74, 969 (1970). (19) J. A. Marinsky, "Ion Exchange", Vol. 1, Marcel Dekker, New York, 1966, Chap. 9. (20) C. L. Knudson, M.S. Thesis, University of North Dakota, May 1972. (21) D. 8.Vanderheiden and E. L. King, J. Am. Chem. SOC.,95, 3860 (1973). (22) F. Helfferich, "Ion Exchange", McGraw-Hill, New York, 1962, Chap. 5. (23) D. W. Kemp and E. L. King, J. Am. Chem. SOC.,89, 3433 (1967).
RECEIVEDfor review July 28, 1975. Accepted October 27, 1975. Grateful acknowledgment is made to the National Science Foundation for a Graduate Traineeship to C.M.M.
Personal Atmospheric Gas Sampler Using the Critical Orifice Concept Frederick W. Williams,' Jack
P. Stone, and Harold G. Eaton
Chemistry Division, Code 6 180, Naval Research Laboratory, Washington, D . C. 20375
A small, light-weight, portable. personal atmospheric gas sampler designed for collecting time-integrated air samples has been developed. An evacuated container equlpped with a critical orifice is used to coltect the sample rather than a pump. The sampling interval is dictated by the size of the orifice and volume of the sample container. The personal sampler has been used to evaluate atmospheres containing the contaminants CO, CH4, Cop, CFpClp, CFCl3, &He, CH2=CHCI, CH2=CCIp, C6HI4, and CsHe.
A personal atmospheric sampler has been designed in response to industry's need to monitor and sample atmospheric contaminants over an eight-hour day in an individual's breathing zone. Design criteria for this sampling system encompassed weight and size in order that an individual might conveniently carry the sampler on his person. In addition, the system had to be: (a) on a personal basis; that is, the immediate environment of an individual must be sampled; (b) time-integrating for periods up to eight hours to measure the total exposure encountered; (c) capable of detecting concentration transients which might cause hazardous ex442
posures for short times; (d) quantitative so that reliable time-weighted averages can be determined for individual exposures. Current samplers which fulfill these requirements are the adsorptive type ( 1 ) . They usually consist of a small pump to draw the air through the device and an adsorptive material to trap atmospheric components. Activated carbon is widely used as a general adsorbent ( 2 ) .However, collection and storage of the more volatile contaminants, such as carbon monoxide and vinyl chloride, cannot be efficiently accomplished with carbon since the affinity of the adsorbent is low. When the affinity of a compound is high, adsorption techniques afford good sensitivity for qualitative results because large volumes of air are processed. For quantitative results, however, the adsorptive and desorptive efficiencies of a particular compound must first be determined. Another approach used to evaluate the environment is the collection of "grab samples" for subsequent analysis. This technique, however, does not fulfill the criteria of a continuous time-integrated sample. The atmospheric sampler described in this paper does not use a pump but rather an evacuated container
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
\
Figure 2. Environmental chamber OFF
Figure 1. Pocket size whole-gas personal sampler
I
equipped with a critical orifice. By this arrangement, th e gas flow into the container through the micron-size orific'e remains constant as long as a critical flow pattern exist.S ( 3 ) In order for this to occur, the internal pressure of th e container must remain less than one half of the external ai mospheric pressure during the sampling period. Thus, th.e length of sampling time depends on both the size of the or Ifice and the volume of the sample container. The equation that governs the gas flow across a critic211 orifice is as follows (3):
J where m = mass flow rate, g sec-'; A = orifice throat are24 cm'l; P = external pressure, dynes ern-?: y = specific hea.t ratio C,/C,; M = molecular weight, g: T = ambient t e n perature, K: R = gas constant, 8.31 X lo-? erg mole --I K-I and C = discharge coefficient (accounts empirically fo boundary layer effect having a range of 0.8 to 0.95 for smal orifices).
EXPERIMENTAL Sampler Design. The personal gas sampler as shown in Figur 1 is 6.35 cm wide by 7.32 cm high (less valve lengths) by 2.54 cr deep, and weighs 0.22 kg. The sampler body is fabricated from 0. em stainless steel with reinforcing in the middle to avoid d u m changes at reduced pressures. The internal volume is approximate ly 100 em4 The left valve, 5.41 cm high, is a simple off-on type and is use,d either to evacuate the sampler when connected to a vacuum SYE tern or to connect to a gas chromatograph for evaluation of the contaminants entrapped after B sampling period. The right valw 6.35 cm high, houses the critical orifice and two types of protectiv e filters through which the atmospheric sample enters. Complete de tails of the valve design can be found in reference (4). Glass woc placed at the top or entrance port is used to trap the larger parti des. To trap smaller particles, a Gelman glass fiber, type E filtei which has a retentivity of 99.8 i 0.3% for 0.3-u particles is usel and is located one on each side of the orifice. This filter arrange ment has proved very successful in the two years that the sample has been under development. It was even effective in protectin the orifice in sampling carbon monoxide in a fire environment. Th orifices were obtained from New England Laser Corporation, Low ell, Mass. This valve is actuated by turning counterclockwise one complete turn. The environmental test chamber, used to evaluate the sample, has been described elsewhere (5).but wa5 modified to handle th sampler as shown in Figure 2. The gas mixtures and pure gases were obtained from Mathesol and were mixed in the chamber an a partial pressure basis. Th, muffin fan (Rotron Mfg., Woodstock, N.Y.) in the chamber en sured a homogeneous mixture. The n-hexane and benzene wer,
i
SAMPLE LOOP
VENT
h PRESSURE TRPiNSDUCER
OIGITAL READOUT
FROM G C SAMPLE LOOP I2 " P O W E R
SUPPLY
T O SAMPLER
Figure 3. Schematic of gas handling system for personal sampler
obtained from Fisher Chemical Co. and trichloroethene from Matheson, Coleman and Bell. The continuously changing CO environment was created by metering in pure gas. The CO concentrations were followed by obtaining "grab samples" from the chamber periodically and analyzing by gas chromatography according to the method of Porter and Volman ( 6 ) . A small gas handling system was fabricated which served to evacuate the sampler or transfer the contents of the sampler to the gas chromatograph. A schematic of this system is shown in Figure 3. It is constructed of '&inch 303 stainless steel tubing. The stainless steel (316) valves are ball type manufactured by Hoke, the pressure transducer is Valedyne, 0-1600 Torr, the digital panel meter is from Analogic and the 12-volt power supply is from Lafayette. The vacuum pump used is a small direct drive Vae Torr 20 pump. Procedure. The sampler is prepared for use by connecting the off-on valve to the gas handling system and evacuating to less than 1 Torr pressure. The other valve which houses the critical orifice is used for sampling. When the sampler is received from the field, the pressure in the sampler is first tested to make sure the criterion for a critical orifice was not exceeded. If the pressure in the sampler is greater than 'iiambient, the sample is not valid. Following sample validation, a portion of the sample is introduced into the gas chromatographic sample loop. T h e limit of sensitivity of the chromatograph is dependent on the volume of the sample loop and the pressure of the sample. The level of sensitivity needed for a particular compound will depend on its OSHA established limit.
RESULTS AND DISCUSSION Although the two concepts, whole gas sampling and critical orifice, are well known, they have never been tested together for this type of application. T o test the ability of the orifice to remain critical in our configuration, the sampler pressure was monitored through the off-on valve while the other valve was open and sampling. T h e orifice was evaluated with and without a protective filter. A plot of pressure vs. time for a 4-rr diameter orifice on the 100 cm3 sampler is shown in Figure 4. As can he seen from the figure, the sampling rate is constant until the pressure reaches Y2 atm, then the rate begins t o decrease aspredicted. Orifices with diameters down t o 2 rr exhibit similar plots. T h e sampling ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976 * 443
4p ORIFICE 0 WITHOUT FILTERS W I T H FILTERS
” 76-
8
CI .-
25W’ cc
Od
IO
20 30 40 TIME-MINUTES
$0
50
Figure 5. Continuously changing carbon monoxide concentration within environmental chamber
4-
cc a 3-
Table 111. Sampling Precision of the Personal Sampler Run N o .
C O concn. ppm
Mean dev
1 18.6 0.3 2 18.3 0.0 3 18.1 0.2 Average 18.3 i 0.2 18.3 0.2 Precision 0.2118.3 = XilOOO. X = 11 parts per thousand.
TI ME-HOURS
Figure 4. Critical gas flow through a 4-p diameter orifice into the personal sampler
~
Table IV. Personal Sampler Bias Test with 5-p Diameter Orifice Using Hydrogen and Hexane
Table I. Sampling Time for Various Orifice and Sample Bottle Sizes0
Concentration, p p m
Orifice diameter, p Sampler volume, cm (S T P )
1
2
Compound
3 5 SamDline times. hr
I
Hydrogen n -Hexane
10
44.2 11.0 4.9 0.9 0.4 1.8 88.3 9.8 1.8 0.9 22.1 3.5 150 132.5 33.1 14.7 2.7 1.3 5.3 176.6 44.2 19.6 7.1 3.6 200 1.8 441.5 49.1 110.4 17.1 9.0 500 4.4 0 Gas = air, molecular weight = 28.95, C.,/C,, = 1.400, ambient pressure = 1.000 Atm, ambient temperature = 298.0 K, discharge coef. = 0.80.
Direct analysis, %
Personal sampler, %
0.68 i 0.01 0.35 i 0.01
0.66 i 0.01 0.36 i 0.01
50 100
Table 11. Accuracy of the Personal Sampler for Determining CO Chamber run
1
2 3
C O concentration. p p m Calcd
Direct analysis
Personal sampler
4 84 168
4 2 0.8 80 i 4 168i 8
4 k 0.8 851 4 164i 8
rate is the same with and without the filter; thus the filter is not impeding the gas flow. Using the critical orifice Equation 1 with the discharge coefficient of 0.8, Table I presents some typical sampling times for various orifice sizes vs. sample bottle volumes. To determine the accuracy of the sampler, three different concentrations of CO were established in the chamber. T h e concentration of CO was determined based on partial pressures, on direct sampling, and evaluation with the whole gas sampler. T h e results are given in Table 11. As seen in the table, the personal sampler is accurate over a 40-fold concentration range. To test the precision of the sampler in the chamber would have been difficult. This is because the errors in making the standards in the chamber, the reproducibility of taking “grab samples,” and the repeatability of the gas chromatographic method would have been additive. Therefore, a gas flow system was established using a standard gas mixture. The sampler was attached to this gas stream and allowed to sample for 0.8 hr. This was repeated three times and the precision calculated from these measurements. The results of this experiment are shown in Table 111. Consid444
~
Table V. Sample Bias Test with 5 - Orifice ~ toward Methane, Hexane, and n-Decane Concentration, ppm Compound
Calcd
Direct analysis
Personal sampler
Methane 50 55i 3 Hexane 10 11 i 0.6 n-Decane 70 64 F 3 a Sampler was then warmed and reanalyzed ppm.
58i 3 11 3 0.6 2 1 , a 55 t o obtain 5 5
ering the low concentration and multiple gas handling, the precision is good-11 parts in a thousand. To show the ability of the sampler t o follow a changing concentration, pure CO was metered continuously into the chamber. A muffin fan served to mix the gases in the chamber. The concentration of CO with time was followed by sampling the chamber periodically and analyzing the mixture by gas chromatography. For one experiment, this relation is shown in Figure 5 . A time weighted average for this curve, which is obtained by integrating the area under the curve, is 139.7 ppm, evaluation of the sampler’s contents showed 140.4 ppm. Two types of sample bias could occur in a device of this design, low molecular weight compound bias from diffusion considerations, and bias against high molecular weight, polar, or reactive compounds due to their adsorption in the walls. To answer the question of low molecular weight compound small orifice bias, a gas mixture of H:! and hexane in nitrogen was prepared in the chamber. This mixture was sampled with the 5-b diameter orifice sampler. T h e results of this test are shown in Table IV. As seen from this table, there is no low molecular weight compound bias for the orifice used. Additionally, this same gas mixture was stored for 30 days without any loss of either compound. T o evaluate the performance of the personal sampler
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Table VI. Use of Personal Sampler to Evaluate a Complex Mixture Concentration, pprn C ornp our1 d
Direct analysis
CH,
co
C,H, CF,Cl, CH,=CHCl CFCl, CH,==CCI, C6H 6‘
Figure 6. Gas chromatogram of several contaminants sampled with the 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 0.5 0.5
96 6 56 4 6 4
i i i i i i
t
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. T h e data in Table V show t h a t 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. T h e 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 ( C H ~ C H : 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
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