ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
cools on evaporation and valve A can then be opened further until the sample dries. The end of the exciting coil (I) is introduced over the sample compartment (J) and the power supply turned on. Perkin-Elmer 067 861 or Westinghouse Model 185-230electrodeless discharge lamp power supplies were both found to be suitable. The power control knob is set to about one third of its maximum setting and the tuning capacitor adjusted for maximum forward power. Altering the sample tube position in the coil also affects the power used. The power level is then adjusted to about 10 W. Periodic agitation of the sample is carried out by rotation of the tube around the joint. A change of discharge color from blue to pink is a reliable indication that ashing is complete. After the completion of ashing the isolating valve (A) is closed and the air bleed valve (B) slowly opened to release the vacuum.
RESULTS AND DISCUSSION A 2-g sample of dry, powdered human liver was ashed, a t 10 W, in 12 h with 90% recovery for added arsenic (as Na2HAs047H20). A 5-g sample of wet human liver took 48 h t o ash a t the same power level. A 2-mL urine sample took 6 h including drying under vacuum. A l - m L blood sample took a similar time. A l-g sample of powdered coal took about 70 h t o ash, although this time was heavily dependent on the degree of agitation as a protective ash layer slowed the oxidation rate. The modified test tube is a versatile container for a variety of samples. T h e chances of mechanical loss of powders are reduced when compared with open boats, and rotation around the test tube joint allows agitation of the sample without releasing the vacuum. T h e walls of the test tube outside the discharge zone provide a condensation surface a t room
1089
temperature, for volatile inorganic compounds. For example, during the ashing of human liver samples, a white ring formed close t o the constriction. After ashing is completed, wet dissolution of the sample may be carried out in the same test tube, eliminating losses and contamination from sample transfer. Inductive heating and heating due to dissipation in the coil windings were not excessive. At 10 W, the temperature of the sample was close to 100 "C. This could be reduced by simple forced air cooling. T h e sample chamber is small, allowing a dense plasma to be produced a t low power levels; 10 W is sufficient for most purposes. Samples high in graphite or other electrically conductive material showed evidence of self-heating due to eddy currents. T h e condensation surface tends t o catch volatile material released from such samples. Sample heating could be reduced by a more elaborate geometry where the discharge zone and the sample zone are separated. I t is not intended that this equipment should in any way replace commercial devices, but it should be a worthwhile addition to any laboratory dealing with small numbers of organic samples for inorganic analyses.
LITERATURE CITED (1) (2) (3) (4) (5)
C. E. Gleit and W. D. Holland, Anal. Chem., 34 1454 (1962). C. E. Mulford, At. Absorpt. News/.,5 , 135 (1966). F. C. Gabriel, Rev. Sci. Insfrum., 47, 484, (1976). W . W . Macalpine and R. 0. Schildknecht, Proc. IRE, 47, 2099 (1959). W. W . Macalpine and R. 0. Schlldknecht, Electronics, 33, 140 (1960).
RECEIVED for review September 12, 1978. Accepted January 17, 1979.
Drier for Field Use in the Determination of Trace Atmospheric Gases B.
E. Foulger"
Admiralty Marine Technology Establishment, Holton Heath, Poole, Dorset, BH 16 6JU, England
P. G. Simmonds International Science Consultants, The Pines, The Chase, Hurn Road, Ringwood, Hants, BH24 ZAN, England
The accurate measurement of trace gases in the atmosphere is often complicated by the ubiquitous presence of water vapor. T h e determination of a particular gas can be perturbed either directly by the presence of water vapor which represents about 3% of the atmospheric pressure ( a t 28 "C and 80% relative humidity), or indirectly where it may have an adverse effect on detector resolution or sensitivity ( I , 2). Where entrapment of gases onto adsorbents is required t o enhance sensitivity, water vapor may effectively compete for the adsorption sites, although the recent use of hydrophobic porous polymers, such as Tenax-GC, reduces this problem (3-6). Nevertheless, concentration of atmospheric gases by cryogenic freezeout techniques is often plagued by the accumulation of excessive water in the collected sample. T h e use of conventional desiccants, including molecular sieves, to predry the air sample is seldom practical as the component of interest may be partially or completely adsorbed by the desiccant (7). In this study we report the use of a permaselective membrane of Dupont Nafion (a copolymer of tetrafluorethylene and fluorosulfonyl monomer) t o effectively dry ambient air prior t o the determination of trace halocarbons. Water vapor is removed selectively by diffusion through the membrane without loss of the halocarbons of interest. Furthermore, this method has two advantages: namely, low 0003-2700/79/0351-1089$01.00/0
dead volume and constant pressure drop compared with conventional bed desiccants.
EXPERIMENTAL Nafion was obtained from E. I. Dupont de Nernours, Plastics Division (Wilmington, Del.) as type 813 tubing with nominal internal dimension of 0.045 inch (1.14 mm) and wall thickness of 0.005 inch (0.13 mm). Although drying tubes fabricated from Ndion can be obtained commercially (Perma Pure Products, Inc., Oceanport, N.J.), they rely on a countercurrent flow of dry gas for their successful operation. In our experiments, we required a field portable unit where static drying using a desiccant was more attractive because of a limited gas supply. However, where an adequate purge gas is available, this should serve equally well. Driers for field use were fabricated as tubular elements from a l-m length of Nafion 815 tubing and enclosed in a plastic container with removable nylon end caps, which could be filled with desiccant as illustrated in Figure 1. Two types of desiccant were investigated, 13X molecular sieve in the form of 1/16-inch(1.6-mm) pellets (previously activated by heating at 250 "C for 4 h) and magnesium perchlorate. The efficiency of the drier was determined by drawing air by means of a small pump at two different flow rates (nominally 50 and 100 mL/min) through two water bubblers placed in series. The water-saturated air was then drawn through the Nafion drier and finally into a minihygrometer (Shaw moisture meters, Model M, C 1979 American Chemical Society
1090
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979 MODIFIED
Table I. Analysis of Halocarbons in 5-mL Samples of Ambient Air without Removal of Water Vapor ( A ) and after Removal of Water Vapor by Sampling the Air through a Nafion drier (B) integrator response compound analyzed A B trichloromonofluorome thane 54830 54700 54030 54890 54840 54270 54390 54320 mean 54522 54545 carbon tetrachloride 52030 51000 51860 50960 51730 52050 52000 51970 mean 51905 51495 methyl chloroform 10760 10980 10970 10990 10780 11050 11100 10850 _mean 10902 10967
UNION
b k 8 1 5 NAFION TUBING
.t~
PERSPEX TUBE(2.250.0.)
R EM OVA B LE NYLON E N D C A P
Figure 1. Schematic of Nafion drier. Nominal dimensions, 8 inches X 2.25 inches 0.d. Nominal desiccant weight, approx. 250 g (13X molecular sieve)
-13
0
i.
b 20
LO
60
80
100
120
TIME(hours1
Figure 2. Effect of flow rate and desiccant on drier performance for air saturated at 22 'C. (A) Magnesium perchlorate, 50 rnL/min. (B) 13X molecular sieve, 50 mL/min. (C) 13X molecular sieve, 100 mL/min Bradford, U.K.) to determine relative humidity. The mean ambient temperature during these measurements was 22 "C. The performance of the drier was monitored continuously to determine the practical lifetime of the drier before the desiccant need be replaced. In other experiments, the concentrations of several halocarbons in ambient air were determined using a Hewlett-Packard Model 5840A gas chromatograph equipped with a B3Nielectron capture detector and automatic gas sampling valve. The gas chromatographic column was a 1.8m by 6.3 mm 0.d. aluminum tube filled with 12% w/w OV-101 silicone on 100-120 Mesh Chromosorb P, operated at a temperature of 50 "C. Five-mL samples of ambient air were analyzed for halocarbons both before and after passage through the Nafion drier. In these experiments, the ambient temperature and relative humidity were 20 "C and 65%, respectively.
RESULTS AND DISCUSSION Figure 2 illustrates the performance of the drier, expressed in terms of dew point, over about a four-day period with water saturated air entering the drier continuously for two different flow rates. Magnesium perchlorate (curve A) as a desiccant is slightly more effective than the 13X molecular sieve (curve B) a t the same flow rate of 50 mL/min. As expected, the efficiency of drying is less at the higher flow rate (curve C; 100 mL/min and 13X sieve).
In all three experiments, the efficiency of drying decreases quite rapidly after about 100 h of continuous use as the desiccant becomes exhausted. However, in actual field use where the relative humidity is usually in the range of 20-80%, the drier maintained the dried gas a t a low dew point (less than -30 "C) for as long as 7 days. The effective lifetime of the drier will obviously depend on the ambient temperature and relative humidity, and the amount of desiccant used. Although magnesium perchlorate is a slightly better desiccant than the 13X sieve, the latter was selected for routine use as it was easier to replace and reactivate, whereas the magnesium perchlorate generally was difficult to remove from the drier, once wet. The behavior of the drier toward several atmospheric trace halocarbons is listed in Table I, where the integrator area of the response for trichloromonofluoromethane, carbon tetrachloride, and methyl chloroform as determined by the gas chromatograph are listed for air samples analyzed before and after passage through the Nafion drier. There is no loss of any of these particular compounds within experimental error, although the water content of the sampled air was reduced from a relative humidity of 65% to less than 1%(dew point -36 "C). Baker (2) has reported that most inorganic gases, hydrocarbons, esters, and aldehydes are unaffected by passage through driers fabricated from Nafion tubing, and therefore the present design of drier should be suitable for the analysis of these classes of compounds. However, tests should be made to establish the suitability of the Nafion drier where the analysis of more polar compounds is contemplated.
ACKNOWLEDGMENT We thank Russell Dor6 for expert technical assistance in construction of the permaselective driers.
LITERATURE CITED (1) J. Hsiao and W. L. Chiou, J . Pharm. Sci., 63, 1776 (1974). (2) B. B. Baker, Am. Ind. Hyg. Assoc. J., 735-740 (1974). (3) A. Zlatkis, H. A . Lichtenstein. and A. Tishbee, Chromatographia, 6, 67 (1973). (4) M. Novotony, M. L. Lee, and K. D.Bartb, Cluomatographk,7,333 (1974). (5) E. D. Pellizzari, J. E. Bunch, B. H. Carpenter, and E. Sawicki, Environ. Sci. Techno/.,9, 552 (1975). ( 6 ) J. Janak. J. Ruzickova, and J. Novak, J . Chromatogr., 99, 689 (1974). (7) J. W. Russell and L. A . Shadoff, J . Chromatogr., 134, 375 (1977).
RECEIVED for review October 13, 1978. Accepted January 29, 1979.
