Anal. Chem. 1989, 61, 1993-1996
respect to the removal of passivating films. On a final note regarding practicality, the Nd:YAG laser employed for this work is a versatile research laser with high cost compared to that of the LCEC detector. However, initial experiments with a small Nz laser have demonstrated activation with a much lower cost laser (ca. $3500).
ACKNOWLEDGMENT We acknowledge Jan Pursely of BAS for valuable advice on chromatographic conditions and crucial equipment repairs.
LITERATURE CITED Shoup, R. E. In High Performance Liquid Chromatography;Academic Press: 1986; Vol. 4, p 91. Stulik, K.; Pacakova, V. CRC Crk. Rev. Anal. Chem. 1984, 74, 297. Knecht, L. A.; Guthrie, E. J.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479. Lunte, C. E.; Kissinger, P. T.; Shoup, R. E. Anal. Chem. 1985, 5 7 , 1541. White, J. G.; St. Claire, R. L.; Jorgenson, J. W. Anal. Chem. 1988, 58, 293. Lavrich, C.; Kissinger, P. T. Chromafogr. Sci. 1985, 32, 191. Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1417A. Hu, I . F.; Karweik, D. H.; Kuwana, T. J . Electroanal. Chem. Inferfacia1 Electrochem. 1985, 786, 59. Kumau, G. N.; Wiliis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545. Thorton, D. C.; Corby, K. T.; Spendei, V. A,; Jordan, J.; Robbot, A,; Rustrom, D. J.; Gross, M.; Ritzler, G. Anal. Chem. 1985, 5 7 , 150. Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632.
1993
Fagan, D. T.; Hu, I. F.; Kuwana. T. Anal. Chem. 198$, 5 7 , 2759. Kovach, P. M.; Kuhr, W. G.; Stutts, K. Wightman, R. M.; Deakin, M. I?.; J. J . Electrochem. SOC. 1984, 737, 1578. Miller, C. W.; Karweik, D. H.; Kuwana, T. Anal. Chem. 1981, 53, 2319. Evans, J.; Kuwana, T. Anal. Chem. 1979, 57, 358. Engstrom, R. C. Anal. Chem. 1982, 5 4 , 2310. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136. Wang, J.; Hutchins. L. D. Anal. Chlm. Acta 1985, 5 0 , 1056. Moiroux, J.; Eiving, P. J. Anal. Chem. 1978, 5 0 , 1056. Falat, L.; Cheng, H. Y. J . Elecfmanal. Chem. I n t e r f a c i a l E l e c t m . 1983, 757, 393. Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386. Wang, J. Anal. Chem. 1981, 53, 2280. Wang, J.; Tuzhl. P. Anal. Chem. 1986. 58, 1787. Wang, J.; Lin. M. S. Anal. Chem. 1988, 6 0 , 499. Hershenhart, E.; McCreery, R. L.; Knight, R. D. Anal. Chem. 1984, 56, 2256. Poon, M. J.; McCreery, R. L. Anal. Chem. 1988. 58, 2745. Poon, M. J.; McCreery, R. L. Anal. Chem. 1987, 5 9 , 1615. Poon, M. J.; Engstrom, R. C. McCreery, R. L. Anal. Chem. 1988, 6 0 , 1725. Allison, L. A.; Shoup. R. E. Anal. Chem. 1983, 55, 8. Neuburger, G. C.; Johnson, D. C. Anal. Chem. 1987, 5 9 , 203. Bowling, R. J.; Packard, R. T.; McCreery, R. L. Langmuk 1989, 5 , 663. Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 6 0 , 1459.
RECEIVED for review December
30, 1988. Accepted May 1, 1989. This work was supported by the Air Force Office of Scientific Research and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
Diffusion Apparatus for Trace Level Vapor Generation of Tetramethyllead P. R. Fielden* and G . M. Greenway' Department of Instrumentation and Analytical Science, UMIST, P.O. Box 88, Manchester M60 1 8 0 , U.K. The generation of vapor standards and measurement of the diffusion coefficients of vapors is an important aspect of the development and calibration of methods for trace vapor analysis in atmospheres. Dynamic vapor generation systems can be based on a wide range of production methods such as gas stream mixing (l),injection methods, evaporation and chemical reactions ( 2 ) ,permeation devices (3), diffusion apparatus ( 4 ) , electrolytic methods (5),and gas-phase titrations (6). The toxicity of the compound must be considered when deciding which method should be employed. Usually for toxic compounds the vapor standards required will be of very low concentrations. In this work vapor samples of tetramethyllead (TML) were prepared near the Occupational Exposure Limit of 0.15 mg m-3 (7). Permeation devices and diffusion apparatus are the most appropriate methods for the production of low vapor concentrations; however permeation devices are difficult to develop and construct for toxic compounds because of their basic design (8). Diffusion apparatus provides a simple method for preparing mixtures of vapor-containing atmospheres and determining diffusion data ( 4 ) . The apparatus works by maintaining the liquid phase of a vapor in a reservoir which is kept a t constant temperature. The liquid is then allowed to evaporate and the vapor diffuses through a capillary tube into a flowing gas stream. If the rate of diffusion of the vapor and the flow rate of the diluent gas are known, the vapor concentration in the resultant mixture can be calculated. Present address: Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7 R X , U.K. 0003-2700/89/0361-1993$01.50/0
More recent designs allow for rapid changes in the concentration of the vapor standards by altering the diffusion path length (9,lO). These methods require syringes or taps to alter the volume of liquid present. This is not suitable for toxic compounds because there is a danger of the pure liquid leaking, and the need for significant volumes of liquid renders such an approach potentially hazardous. Another important consideration when handling toxic compounds is the method of measuring the diffusion rate. The gavimetric method is not always practicable. Alternatively the diffusion rate can be found by monitoring the position of the liquid meniscus in an open precision capillary tube as a function of time (11). The gradient (X)of a graph of the square of the variation in diffusional path length (1 2, vs time is given by
where p is the density of the liquid a t temperature T, P is pressure in diffusion cell at the open end of the capillary (Pa), p is the partial pressure of the diffusing vapor at temperature T (Pa), M is relative molecular mass of the vapor, R is the gas constant (8.314 J K-l mol-'), D is the diffusion coefficient (m2s-l), and T i s the temperature (K). At a fixed temperature and pressure the diffusion rate can be calculated from
S = XAp/21
(2)
where (S) is rate of diffusion of vapor out of the capillary tube (kg s-l), and 1 is diffusion path length (m). A is the cross sectional area of the diffusion tube (m2). 0 1989 American Chemical Society
1994
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989 TWO-WAY TAP
AIR INLET
E;
Ra ' c-
STANDARD OUT VAPOUR LET
,
1 ~1 1:
T
SCREW-CAP SEAL
I
I
+/ . . I WATER ! /
Figure 2. Diagram showing design of the dilution manifold.
CIRCULATI NG PUMP PRECISION-BORE
T
:
I
; CAP ; SILICONE I L L A; R Y +
RUBBER B UI N G 4 I
BASE
S U PP 0R T '
_1
: Flgure 1. Diagram to show the construction of the diffusion apparatus
The diffusion rate can be determined at any time from a knowledge of X , A , T , and 1. For eq 2 the change in path length ( I ) with time is negligible in comparison to the actual diffusion path length and therefore does not affect results. T o obtain precise results, control of the temperature to within f0.2 "C is considered necessary (9). A difference of 0.4 "C would cause an error in the diffusion rate of 0.01 X kg s-l. Once the system has been calibrated, the vapor concentration produced can be altered by varying the temperature of the system or by altering the diffusion path length. Sufficient time must be allowed for equilibrium to be obtained after adjustment. Changing the flow rate of the diluent gas will also alter the concentration but should not exceed ca. 1 dm3 min-' or turbulence will occur (12),which would reduce the effective diffusion path length. Since the diffusion rate is also governed by the length and diameter of the capillary column, a judicious selection of these parameters will contribute to the range of concentrations that may be obtained.
EXPERIMENTAL SECTION Apparatus. In the design of the apparatus, eq 1 and 2 were utilized to set the parameters so that a suitable diffusion rate could be obtained. To use the equations, it was necessary to obtain an approximate value for the diffusion coefficient of the compound in question. When this is not readily available in tables, it can be calculated by using (13)
-
-
0.0043T3I2 - + -
D =
[ VA1!3
+AL: VB1/3]*P ;E31
(3)
where MA is molecular mass of vapor, MBis molecular mass of diluent gas, VA is molal volume of A (MA/Pat boiling point) of vapor (m3),V, is molal volume of B (MB/pa t boiling point) of diluent gas (m3),and P is total pressure (Pa). An approximate value of 6.3 x lo* m2 s-l was calculated for TML by using the available data (the density of TML a t 25 "C and the density of nitrogen a t its boiling point). This approximation could then be used to construct the apparatus and find the experimental diffusion coefficient. The apparatus that was constructed is illustrated in Figure 1. The pure tetramethyllead was obtained from Associated Octel (Port Sunlight, U.K.). One end of a precision capillary tube ( 2 mm i.d.) was sealed and the compound was introduced directly into the capillary bore. This meant the liquid was contained and could only escape up the capillary bore. Introduction was accomplished by a glass syringe with a 60 ern length of poly(tetrafluoroethylene) (PTFE) tube attached which could be passed
down the capillary tube. All glassware was previously silanized with hexamethyldisilazane to prevent loss of sample on the glass walls. The temperature of the system was controlled by enclosing the capillary tube in a water jacket, and a water circulating pump (Churchill, Surrey, U.K.) was used to keep the capillary tube a t the required temperature. When the apparatus was designed, it was necessary to ensure that the drop in the meniscus would be sufficiently large to be monitored. If the system was set up so that there was a diffusion path length of 40 cm and the temperature was 50 OC,then, using eq 3, the theoretical diffusion rate was calculated to be 4.87 X lo-" kg s-'. This value was then used to estimate the drop in meniscus height over an 8-h period. With this diffusion rate, 1.403 x IOe3g would have diffused from the tube over an 8-h period. at 25 "C, so the volume of TML The density of TML is 1.99 g that would be lost from the tube over 8 h is 7.04 X lo4 em3. With the cross-sectional area of the capillary being 3.14 X 1r2cm2,the drop in the meniscus would be 0.22 mm. This change in meniscus level could be monitored precisely to within f 5 % by using a cathetometer (Precision Instruments, Ltd.), which could be read to fO.01 mm. Calibration of the Diffusion Apparatus. The system was assembled by using the capillary dimensions described in the calculation given above and the temperature was maintained a t 50 "C. The diluent gas was air, which was first pumped through a glass microfiber filter tube (grade 10 minifilter, Whatman) to remove particulate matter. A mass flow controller (flow range, 281-1160 cm3 min-', Brookes, Stockport, U.K.) was utilized to alter the flow rate enetering the dilution chamber. The position of the meniscus was then monitored with time. The cathetometer was placed on a stable base as near as possible to the meniscus. Dilution of Vapor Standard. The apparatus used for dilution is shown in Figure 2. Stainless steel valves are often used in gas dilution apparatus but with the very low concentrations that were involved in this experiment, the sample could not be allowed to come in contact with such surfaces. The system was designed so that the sample was only in contact with glass walls that had been silanized previously with hexamethyldisilazane. The dilution occurred in two stages and was achieved by using mass flow controllers, (Brooks, Stockport, U.K.). The errors in this type of system are typically f 5 % (11). At each mixing stage a proportion of the gas flow was removed leaving 100 cm2 min-' to dilute, otherwise high flows would have resulted. The flow was split with a mass flow controller, which meant that the amount removed was always constant and not affected by back pressure; an adsorption tube or gas bubbler was required at the end of the system to generate a back pressure or the gas would take the path of least resistance and travel straight out of the open end. The diluent gas flow was also controlled by a mass flow controller, and therefore when all the flow controllers were fixed at the correct values, a known dilution was obtained. These flow controllers were shown to have a maximum error of *3% by observing the fluctuations of the float and by taking into account the scale reading errors. The dilution process occurred in the following steps: (i) 730 cm3min-' standard atmosphere entered the dilution apparatus; (ii) split 630 cm3 min-' standard atmosphere, leaving 100 cm3 min-'; (iii) added 500 cm3 min-l air thus, diluting by 6; (iv) split 500 cm3min-' diluted standard, leaving 100 cm3 min-'; (v) added 400 cm3min-' air thus diluting by 5; (vi) 500 cm3m i d standard atmosphere, which had been diluted 30 times, left the dilution apparatus. The diluted vapor standard was.collected either on adsorption tubes packed with Porapak Q or in a gas bubbler containing nitric
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989 2.70
?
1
I
$ l e /
0
5
>O
15
20
,
25
T I M E IO'S
Figure 3. Graph of diffusion path length squared vs time, for determination of the diffusion rate at 50 OC.
acid (70% (w/v)). With the system set up in this manner, a total dilution of 30 was achieved. The system could easily be adjusted to give different dilutions. The baffle mixers incorporated after each split/dilution stage, ensured adequate mixing between the standard vapor and diluent gas. Dilutions of up to 400 times would be possible with two dilution steps. Validation o f Diffusion M e t h o d b y Anodic Stripping V o l t a m m e t r y . Anodic stripping voltammetry was used for the
determination of the lead in the vapor standards because of ita inherent sensitivity. For the analysis a static mercury drop electrode was utilized (Model 303 static mercury drop electrode, Princeton Applied Research) with a Ag/AgCl reference electrode. The experiment was controlled by a polarographic analyzer/ stripping voltammeter (Model 264, Princeton Applied Research). The instrument was set up as follows: medium drop size; scan rate 5 mV s-l; pulse repetition interval 0.2 s; and pulse amplitude 50 mV. Under these conditions,the potential at peak maximum (differential pulse stripping voltammetry mode) for lead was found to occur between -0.67 and -0.73 V vs Ag/AgCl reference electrode. The system then had to be calibrated for lead determination. For trace analysis in electrochemistry it is esaential to use ultrapure water to keep the blank value low. The water was prepared by circulating doubly distilled water through a water purifier containing a mixed bed ion exchange column (Water-I Gelman Sciences, Inc., Ann Arbor, MI) until ita resistance was above 18 MR. The standards were prepared from lead nitrate (AnalaR Grade, BDH, Doxset) and were made up in the pure water to which nitric acid (70% (w/v) Aristar Grade, BDH, Dorset) had been added (5% (v/v)). A stock solution of 1000 ppm Pb2+was prepared from which fresh standards were made daily. The standards were stored in nitric acid washed polythene bottles.
RESULTS AND DISCUSSION The results obtained for the calibration of the diffusion apparatus were plotted as a graph of diffusion path length squared vs time (Figure 3). It can be seen from Figure 3 that the precision of the results improves after 1.18 X 106 s. After this point to reduce errors due to slight movement of the apparatus, the readings were taken relative to a fixed mark on the capillary tube. The equation for the line after 1.18 X lo6s was y = 1 . 0 2 ~+ 1620 and the correlation coefficient was 0.998 (the standard deviation of the (y) residuals was 2.7). It was decided to take the gradient of the final part of the line, (1.02 f 0.01) X 10-8m2 s-' (95% confidence limits) where there waa greater precision in the readings for the calculation of the diffusion rate from eq 1 and 2. The diffusion rate was found as (7.97 f 0.14) X lo-" kg s-l. The diffusion coefficient was calculated to be (1.03 f 0.02) X lob m2 s-'. When the diffusion rate was calculated, care had to be taken to use the correct diffusion path length. This was because the graph covers readings taken over 31 days. The meniscus dropped approximately 1mm per day and therefore over this time period the diffusion path length would alter considerably (ca. 31 mm). By use of the diffusion coefficient calculated, a difference of
1995
0
I
,
,
,
,
2 L5
0
50
100
150
200
T I M E ?05S
Flgure 4. Graph of diffusion path length squared vs time, for determination of the diffusion rate at 30 OC.
1mm would cause the diffusion rate to alter by 0.13 x lo-" kg s-l. This experiment also showed the stability of the system to be good because the error in the gradient of the line over a 31-day period was only f1.5%. The experiment was repeated with the temperature set at 30 O C , Figure 4. The equation of the line was y = 0 . 5 2 ~+ 2500 and the correlation coefficient was 0.981 (the standard deviation of the y residuals was 6.7). The results obtained a t this temperature were less precise because it was more difficult to keep the temperature constant to within f0.2 "C and the change in the meniscus level was smaller. The percent relative standard deviation for the gradient at 50 O C was 1.3% compared to 4.5% for 30 OC. The diffusion rate obtained a t 30 "C was 3.26 X lo-" kg s-l. The temperature of the system was returned to 50 "C and the diffusion rate was calculated by using the experimental diffusion coefficient and a diffusion path length of 49.5 cm. The diffusion rate was calculated to be (6.44 f 0.01) x lo-" kg s-l. This meant that when the flow rate of the diluent gas was 730 cm3 min-', the concentration of TML in air was 5.3 f 0.18 mg m-3. This concentration is approximately 30 times greater than the occupational exposure limit for TML (0.15 mg m-3 as lead). The concentration could be altered by changing the diffusion rate, but this was undesirable as the only easily variable parameter was the diffusional path length, which meant handling the toxic tetramethyllead. A more acceptable solution was the quantitative dilution of the gas standard using the apparatus described. In order to assess the dilution process, the gas standard collected in the bubblers was determined for total lead content by anodic stripping voltammetry. The value obtained by this method was compared with the calculated value. For analysis, the polarographic conditions were as previously described and the electrode cell was cleaned with dilute nitric acid. A blank of 10 mL of ultrapure water was then run with a supporting electrolyte of 5 pL of Aristar nitric acid. Purging was carried out for 4 min with "oxygen free" nitrogen. This resulted in a low blank signal with less than 10 nA variation in stripping current over the electroactive region for lead. For this initial calibration, 5-pL aliquots of a 100 ppm solution of lead were successively added to the blank and after each addition a run was taken. Figure 5 shows these data as a series of stripping peaks of increasing magnitude. All results were triplicated to check reproducibility (within 2%) and the blank value was subtracted from each result. A linear calibration curve was obtained for 0 to 40 ng cm-3 and the equation of the line was y = 1 . 0 4 ~- 0.4; the correlation coefficient was 0.999 and the standard deviation of the y residuals was 0.43. Once a suitable calibration curve had been obtained, the vapor standard could be analyzed. TML was collected in concentrated nitric acid. To ensure that no TML was lost,
Anal. Chem. 1989. 67.
1996
I
A = BLANK
-
20
I -1.0
1
-0.9
I
I -0.7
-0.8
POTENTIAL
/
-0.6
-
I -0.5
-0.L
V fi Ag/AgCI
Figure 5. Figure to show the stripping voltammograms obtained for the blank and a series of lead calibration solutions.
three bubblers in series were used to collect it. In each bubbler there was 100 cm3 of acid. In subsequent determinations most of the lead was found in the first bubbler and none was found in the last bubbler. The nitric acid destroys the TML
+ HX (CH3)SPbX + HX (CH3)dPb
+
+ CHI (CH3)2PbX + CHk
(CH3)SPbX
----*
Under extreme conditions inorganic lead(I1) compounds are formed (14). It is essential that all the organolead be converted to inorganic lead, because anodic stripping voltammetry only detects inorganic lead. After several experiments it was found that all the TML could be converted to inorganic lead by placing it in a sealed digestion vessel, lined with Teflon (Berghol, West Germany) and heating it in an oven at 100 "C for 3 h. Once it was known that all the lead could be converted to inorganic lead, a sample of the diluted vapor standard could be analyzed. The standard atmosphere was generated with an air throughput of 730 cm3 min-'. After dilution, the sample flow rate through the collection solution was 500 cm3 min-'. A 0.5-cm3 portion of the sample that had been collected in acid and digested was diluted to 10 cm3. This was then determined and the concentration of lead in the acid was found
1996-1998
to be 6.75 ng ~ m - The ~ . sample had been collected for 5 min and the diffusion rate was 6.44 X lo-'' kg ss1. There was a 30 times dilution before collection of the sample in 100 cm3 nitric acid. This meant that theoretically there should have been 6.44 ng cm-3 lead present. The error in the result was therefore 4.8%. The ASV analysis confirmed that the vapor generator was operating successfully. The stability of the system was also shown to be acceptable with the error in the slope of the calibration curve being 1.5% over 31 days. CONCLUSIONS The apparatus described here is capable of providing very low vapor concentrations of toxic compounds. For this type of apparatus with a diffusion path length of 49.5 cm, a calibration range of approximately 4 mg m-3 to 10 pg m-3 TML would be possible at 50 "C. At 30 "C the range would be 2 mg mW3to 5 pg m-3 TML. The equipment ensures that the toxic liquid is safely contained but, rapid production of different vapor concentrations is still possible by gas stream mixing. In using this method the diffusion coefficient of the compound may also be determined without making an extra measurement. Although the system was designed for tetramethyllead, it can be easily adapted to other compounds by altering the appropriate parameters. R e g i s t r y No. Tetramethyllead, 75-74-1.
LITERATURE CITED Lucro, D. P. Calibration in Air Monitoring, 7 ; A.S.T.M.S.. Special Techniques Publication 568; ASTM: Philadelphia, PA, 1976; p 301. Nelson, G. 0. Controlled Test Atmospheres, Principles and Techniques; Ann Arbor Publishers: Ann Arbor, MI, 1971. O'Keefe, A. E.; Ortman, G. C. Anal. Chem. 1986, 3 8 , 760. Fortuin, J. M. Anal. Chim. Acta 1986, 75, 521. Methods for the Preparation of Gaseous Mixtures; BS4559; British Standard Institution: London, U.K., 1970. Selected Methods of Measuring Air Pollutants ; WHO Offset Publication No. 249; World Health Organisation: Geneva, Switzerland, 1976. Farmer, D.; Humphrey, J. Safe to Beathe; Klngwood Publications Ltd.: 1984; p 78. Lucero. D. P. Anal. Chem. 1971, 4 3 , 1744. Barratt, R. S.Analyst (London) 1981, 106, 187. Savitsky, A. C.; Sigga, S. Anal. Chem. 1972, 4 4 , 1712. Desty, D. N.; Geach, C. J.; Goldup, A. Gas Chromatography;Butterworth: London, 1960. Atshuiler, A. P.; Cohen, I. R. Anal. Chem. 1960, 3 2 , 802. Gilliland, E. R. Ind. Eng. Chem. 1934, 26, 681. Willemsens, L. C. Organolead Chemistry; International Zinc Research Organisation: New York, 1964.
RECEIVED for review November 30, 1988. Revised May 10, 1989. Accepted May 15, 1989. The authors wish to acknowledge the Occupational Medicine and Hygiene Laboratories of the Health and Safety Executive (Sheffield, U.K.) for supporting this research.
Preparation of Organic Matter for Stable Carbon Isotope Analysis by Sealed Tube Combustion: A Cautionary Note Michael H. Engel* and Rick J. Maynard
School of Geology and Geophysics, The Energy Center, 100 East Boyd Street, The University of Oklahoma, Norman, Oklahoma 73019 INTRODUCTION The utilization of sealed tube combustion methods (1,2) continues to increase in popularity as a more time-efficient and less costly method for the conversion of organic carbon to carbon dioxide for stable carbon isotope analysis. Com-
* Author to whom correspondence should be addressed. 0003-2700/89/0361-1996$01.50/0
parisons of dynamic combustion and sealed tube combustion have demonstrated that, with appropriate precautions, both methods provide comparable results (1-3). In the course of preparing several samples of reference material NBS 22 for stable carbon isotope analyses using sealed tube combustion, it was observed that with increasing elapsed time subsequent to combustion, the 613C values of 0 1989 American Chemical Society
