ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table VII. Comparison of the Semi-Micro Tube Method with the Semi-Automated Tube Method' no.
a
method
sample
semiautomated semimicro
wastewater wastewater
of COD, mg/L replistd. cates mean range dev. 11 10 9 11
26
4
1.3
270 34 175
12 6 13
4.6 1.8
4.7
Semi-automated tube method by Jirka and Carter ( 5 ) .
showed that the tube method recovery averaged 102.8% of the standard method on a variety of sample types (Table VI). On the average, the accuracy data collected fell within the limits (Table 11) of 95-100% recovery, with only the low level reference sample at a border 94.8%. Good recovery a t low levels, however, could be obtained by using reduced normalities of ferrous ammonium sulfate. Detection limits and precision on wastewater using a reduced titrant normality agreed favorably with data achieved by Jirka and Carter (Table VII). T h e acceptance of a new analytical method in water and wastewater is usually determined by one or more of at least five factors: precision, accuracy, cost, method simplicity, and safety. Without exception, the tube COD method, described herein, excels in these factors. Precision and accuracy of the tube method falls within the required limits and, in many respects, is a n improvement to the present method. By reducing the reagent volumes and using culture tubes in place of reflux apparatus, several advantages have been achieved. First, the elimination of boiling chips and the reduction of glasssample contact surface in the tube method minimizes the possibility of contamination. Secondly, a source of error introduced by heat generation during reagent mixing and the subsequent loss of volatile sample components is eliminated by adding the digestion solution down the side of the tube, capping, and mixing so that heat generation occurs only in closed tubes. Such losses are minimized in the standard method by submerging the flask in a n ice bath or adding reagents through a condenser. One further improvement effected by the tube method is the addition of mercuric sulfate in liquid form, as part of the digestion solution. In this way, the addition of this important reagent is more reproducible than when added as a powder as is done in the standard method. Probably the greatest asset of the tube method is cost reduction. Because of the high cost of mercuric sulfate and silver sulfate, the 75% reduction in reagent use makes the tube method very attractive t o t h e budget-minded laboratory. Furthermore, the initial equipment cost for the tube method is much less than the standard method. The estimated initial cost of glassware, for example, of one analysis by the standard method is $17.00 ($15.00, condenser; $2.00, flask), while the cost of one culture tube with cap, used in place of the condenser and flask, is $0.65. Other expenses such as condenser
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tubing and lattice framework, commonly used in the standard method, are eliminated in the tube method completely. The tube method is inherently simpler and less timeconsuming than other methods. The omission of much of the standard glassware eliminates condenser rinsing and reagent cooling plus glassware cleaning. Time is further saved because of smaller reagent additions, and the elimination of boiling chips and temperature control during reagent additions. One of the more attractive features of the tube method, however, is the conservation of bench space. In the standard method, the number of samples analyzed is usually determined by work space, condenser-hot plate accommodation, a n d water availability for condenser cooling. These limitations are all eliminated in the tube method, and existing laboratory equipment can be utilized with the purchase of only culture tubes. As many digestions as necessary can be done in an oven with essentially little space limitation. Block heaters may also be used to complete the digestion and are preferred. Although both the oven method and the block heater method work, the block heater method appears to be superior in several practical ways. After oven heating, it has been found that tube caps tend to stick and may require the use of pliers for removal. This does not occur when digestion is completed in a block heater since only the lower portion of the tube is heated. The solution level in the tube comes just to the top of the block and is completely surrounded by heat, producing even heating and excellent reflux. Lastly, cast aluminum heating blocks can be purchased a t low cost ($20.00) and heated on an existing laboratory hot plate. Finally, safety is a positive factor in the tube method. Mercury compounds are hazardous materials in the laboratory and mercuric sulfate is no exception. The standard method requires the addition of mercuric sulfate as a powder, usually using a measuring spoon. The prolonged use of this material and dust generation could make this procedure less than healthy. The tube method minimizes the manipulation of dry mercury salts by making the mercuric sulfate addition as part of the liquid digestion solution. The safety problem associated with the disposal of mercury compounds has generated recovery methods for mercury in the COD method which are not always followed (7).The smaller volumes used in the tube method reduce the amount of mercury released when mercury recovery is not practiced.
LITERATURE CITED "Chemical Analysis for Water Quality", Training Manual, Environmental Protection Agency, Cincinnati, Ohio, 1973. W. A. Moore, E. J. Ludzack. and C. C. Ruchhoft, Anal. Clem., 23, 1297 (1951). "Standard Methods for the Examination of Water and Wastewater", 14th ed., American Public Health Association, New York, 1975. R. A. Dobbs and R. T. Williams, Anal. Chem., 35, 1064 (1963). A. M. Jirka and M. J. Carter, Anal. Chem., 47, 1397 (1975). R. R. Himebaugh, M.S. Thesis, Wright State University, Dayton. M i , 1977. R. B. Dean, R. T. Williams, and R. N. Wise, Environ. Sci. Techno/.,5 , 1044 (1971). "Methods for Chemical Analysis of Water and Wastes", Environmental Protection Agency, Cincinnati, Ohio, 1974.
RECEIVED for review August 2 2 , 1978. Accepted cJanuary22, 1979.
Oxygen Plasma Asher J. E. Patterson Chemistry Division, DSIR, Lower Hutt, New Zealand
Electrodeless discharge lamps are used in many laboratories as light sources for the determination, by atomic absorption 0003-2700/79/0351-1087$01.OO/O
spectrometry (AA), of volatile metals or metals with volatile halides such as As, Cd, Pb, Sb, Se, Sn, T1, etc. Lamps powered 6 1979
American Chemical Society
1088
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979 A
To
C
1
,
8
1 ' 3 0
,
I'
mm ID
Figure 1. Plasma asher (plan view). (A) Isolating valve. (B) Air Bleed valve. (C) O2inlet valve. (D) Capillary or needle valve. (E) Quickfit Dreshsel bottle head MF 28131125, stem shortened to 90 mm. (F) Quickfit test tube MF 24/3/8, 75-mL capacity, 200-mm length, constriction 35 mm from bottom. Socket size 24/29. (G) BNC S301 Socket. (H) Shield, a 115 X 112 X 60 mm3, mild steel electrical junction box. (I) Coil 33 Turns Air wound 1.7-mm diameter copper wire, 30-mm i.d., 72 mm long, and tapped as shown. Five turns occupy 18 mm and the remaining 28 turns occupy 54 mm. (J) Sample compartment. (K) Optional vacuum gage 0-10 Torr. (L) Air filter for ambient air admission by a 27.12-MHq 30-W rf power supply are now commercially available. During periods when electrodeless lamps are not being used (e.g., overnight) the rf power supply can easily form the basis of a simple oxygen plasma ashing apparatus ( I ) . Samples containing the above elements require careful preparation for analysis and low rf power levels are necessary for complete recovery of As and Se (2). T h e use of open sample boats and high power levels in commercial equipment may be responsible for losses during ashing of samples. I n thia paper, a simple single-chamber plasma asher is described, which has a sample arrangement allowing ashing and dissolution t o be carried out in the same container. Components required, apart from the power supply, include: a vacuum pump, a tuning capacitor, a metal box, a connector, copper wire, some standard glass components, and plastic tubing.
EXPERIMENTAL The plasma asher is shown in plan view in Figure 1. The vacuum pump is a conventional laboratory unit capable of pumping down to 0.01 Torr. Operating pressure in the sample compartment is about 4 Torr. The vacuum gage is optional as observation of the discharge is a reliable guide to optimum operating pressure. An easily started discharge confined to the sample compartment (J) is required. Pressure is primarily determined by the capillary bore (D) and the pumping rate of the vacuum pump. The capillary is prepared from standard 0.3-mm i.d. glass capillary tubing reduced in bore by softening in a flame. (This is a trial and error procedure and one may wish to substitute a needle valve, but temperature-dependent creep may then be a problem for long term use). Valves A and C can be used for secondary control of pressure. The valves A, B, and C are Rotaflow Teflon stopcocks. The glass components are joined by short lengths of thick-walled polyethylene tubing and supported by laboratory clamps. Some care was taken in the design of the inductor (I) to ensure easy starting of the discharge and reliable running. The air wound inductor is a modified version of the helical resonator (3-9, with the rf feed a t the end of the coil instead of into a tap near the end (some commercial lamps also use this means of coupling). Better power transfer from the supply, under loaded conditions, and easy starting at very low power levels (less than 4 W) were
the main virtues. An innovation was the expanded winding pitch at the rf feed end of the coil, resulting in less critical sample positioning and better coupling to the power supply. The unloaded Q (quality factor) of the complete tuned circuit, enclosed in the shield, with the capacitor set to 25 pF and 3 m of connecting cable, was 160, compared to a Q of 200 for a commercial thallium lamp with 3 m of connecting cable. The sample compartment (J) is located in the expanded part of the coil. Insertion deep in the coil causes too much rf to be reflected back to the power supply because of poor matching, without any advantage in ashing efficiency. The coil is tuned by a 0-50 pF variable capacitor tapped down the coil to avoid high voltage discharges between the plates. Capacitive coupling to the shield (H) causes the open ended part of the coil to behave as an autotransformer, stepping up the supplied rf voltage into the kilovolt region. This provides the reliable starting characteristics ( 3 ) . This coil section could be considered analogous to the loading coil used in shortened transmitting antennas giving high rf voltages a t the antenna tip and good matching to the power supply. Some experiments were conducted to determine the effect of fewer than 23 turns on the open-ended part of the coil. For example, with 13 turns, starting would not occur below 9 W, while zero turns resulted in no discharge at all, even a t 30-W power dissipation. Using the full 23 turn section, the discharge can be sustained at power levels below 0.5 W. Some operating precautions need to be observed. Exposed glassware should be covered by plastic sleeves or tape, in case of breakage while under vacuum. Safety glasses must be worn. Tuning should be carried out quickly and at low power levels, e.g., 10 W, to avoid mismatching and excessive power dissipation in the power supply output stage. Radio-frequency interference with other equipment must be checked with a receiver and additional shielding added if necessary. If the oxygen pressure is too low, the discharge zone will extend outside the shield and radiate rf energy. Operation of the plasma asher is as follows: Samples weighing up to 10 g are introduced into the sample compartment. The joint is lightly greased with Apiezon L and the apparatus assembled. The air bleed valve (B) and the isolating valve (A) are closed and the O2inlet valve (C) is opened. The vacuum pump is started and the isolating valve (A) opened slowly to avoid mechanical loss. A trap could be added, as a precaution, to protect the pump and to collect condensable vapors by surrounding it with dry ice. Liquid samples are dried first with the isolating valve (A) only partially open to avoid boiling at room temperature. The sample
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
