Anal. Chem. 1999, 71, 2241-2245
Technical Notes
Atmosphere Generation System for the Preparation of Ambient Air Volatile Organic Compound Standard Mixtures P. Pe´rez Ballesta,* A. Baldan, and J. Cancelinha
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European Commission, Joint Research Centre, Environmental Institute, Ispra Establishment, 21020 Ispra (VA), Italy
A dynamic generation system based on permeation for the preparation of volatile organic compounds at ambient air concentration levels is described herein. The performance of the equipment, sources of uncertainties, and overall uncertainty value are also evaluated. The system is capable of generating multicomponent mixtures at ppb levels and simulating different atmospheric conditions by changing the humidity, concentration level, temperature, and wind velocity. A minimized value for the overall uncertainty in the concentration generated by the system was determined to be (1.9% for higher weighing time intervals of 4 weeks. The evaluation of new analytical techniques for ambient air measurements requires the use of multicomponent standard atmospheres that allow the simulation of the field conditions where the techniques shall be used. This is, for instance, the case for diffusive samplers where most of the evaluation protocols foresee experiments at different atmospheric conditions with changes in temperature, humidity, and concentration levels.1-4 An ideal calibration system should satisfy the following minimum requirements: be capable of generating multicomponent atmospheres; be capable of reaching very low ambient concentration levels (of a few ppb); demonstrate traceability on primary state variables (mass, volume, temperature, pressure); display a wide dynamic concentration range from 0.1 to 100 ppb; exhibit stable concentration conditions over long periods (even months); have short stabilization times (1 h maximum); allow the control of humidity and temperature of the atmosphere; be capable of generating wind velocities in the range from 0 to 2 m/s. (1) CEN EN 838. Workplace atmospheressRequirements and test methods for diffusive samplers for the determination of gases and vapours. 1995. (2) CEN/TC 264 11. Ambient air qualitysDiffusive samplers for the determination of concentrations of gases and vapourssRequirements and test methods. Unpublished work item: 00264021. (3) Cassinelly, M. E.; Hull, R. D.; Crable, J. V.; Teass, A. W. Diffusive Sampling: An Alternative Approach to Workplace Air Monitoring; Berlin, A., Brown, R. H., Saunders, K. J., Eds.; Royal Society of Chemistry: London, 1987; pp 190-201. (4) Brown, R. H.; Harvey, R. P.; Purnell, C. J.; Saunder, K. J. Am. Ind. Hyg. Assoc. J. 1984, 45, 67-75. (5) Nelson, G. O. Controlled Test Atmospheres. Principles and Techniques; Ann Arbor Science Publishers Inc.: Ann Arbor, MI, 1972. 10.1021/ac981291l CCC: $18.00 Published on Web 04/09/1999
© 1999 American Chemical Society
In addition, the system should be equipped with an atmosphere exposure chamber that allows the simultaneous sampling of a representative statistical number of sensors under the same environmental conditions. In such a way, it is possible to use the system to determine the accuracy and precision of new measurement techniques. According to the characteristics described above, a dynamic system should be employed. Test atmosphere systems and methods for the preparation of gas calibration mixtures are reported in the literature.5-8 Nevertheless, the number of techniques is limited for the generation of stable VOC standard atmosphere mixtures in a one-dilution step. Typically, the following techniques can be considered: (a) syringe continuous injection,9,10 where the pollutant is injected continuously by means of a mechanically driven syringe at a constant rate; (b) capillary injection,11,12 where the pollutant is added to the dilution flow by means of a capillary line. A constant pressure drop controls the stability of the generation rate of the pollutant; (c) diffusion,13-15 where the pollutant is incorporated into the stream by means of a calibrated diffusion cell that contains a reservoir with the pollutant in the liquid state; (d) permeation,16,17 where the release of the pollutant is controlled by permeation through a polymer membrane. Poly(tetrafluorethylene) (PTFE) is commonly used for VOCs. This article describes an atmosphere generation system based on permeation for the preparation of VOCs at ambient air (6) Yao, C.; Krueger, D. C. Am. Ind. Hyg. Assoc. J. 1993, 54, 313-319. (7) Umbreit, G. R. In Chromatographic Analysis of the Environment; Grob, R. L., Ed.; Marcel Dekker Inc.: New York, 1983; Chapter 2, pp 98-118. (8) International Standard. ISO 6145/1. Gas analysissPreparation of calibration gas mixturessDynamic volumetric methods. Part 1 Methods of calibration. 1986. (9) Tipping, E.; Woof, C.; Hurley, M. A. Water. Res. 1991, 25, 425-435. (10) Rothweiler, H.; Wa¨ger, P. A.; Schalatter, C. Atmos. Environ. 1991, 25B, (2), 231-235. (11) Goelen, E.; Lambrechts, M.; Geykens, F.; Rymen, T. Int. J. Environ. Anal. Chem. 1992, 47, 217-225. (12) Horl, H.; Yanagisawa, Y. Environ. Sci. Technol. 1993, 27, 2023-2022. (13) Altschuller, P.; Cohen, I. R. Anal. Chem. 1960, 32, 802-810. (14) Fielden, P. R.; Greenway, G. M. Anal. Chem. 1989, 61, 1993-1996. (15) Wong, J. M.; Kado, N. Y. Kuzmicky, P. A.; Ning, H.-S.; Woodrow, J. E.; Hsieh, D. P. H.; Seiber, J. N. Anal. Chem. 1991, 63, 1644-1650. (16) Searingelli, F. P.; O’Keeffe, A. E.; Rosenberg, E.; Bell, J. P. Anal. Chem. 1970, 42, 871-876. (17) Gosjean, D.; Willians, E. L., II Atmos. Environ. 1992, 26A, 2923-2928.
Analytical Chemistry, Vol. 71, No. 11, June 1, 1999 2241
Figure 1. Standard atmosphere generation system.
concentration levels. The performance of the equipment, sources of uncertainty, and working range are also evaluated. EXPERIMENTAL SECTION Apparatus. The schematic of the whole system is shown in Figure 1. The system is composed of the following parts: Air Zero Generator. This consists of an air compressor (JunAir model 600, 3.3 kW, 15.8 bar maximum pressure) with a tank of 150 L volume with a MCZ GmbH purification system that consists of a reaction chamber with a UV lamp, a series of filters with active charcoal, molecular sieves, silica gel, and a hot Pt catalyst reaction chamber. The system is capable of producing a maximum flow of 100 L/min. Flow Controllers. Brooks mass flow controllers 5850S series, in the range of 0-200 mL/min and 0-30 L/min, control the flow of the permeation ovens and the dilution stream. The programming of the flows is done by computer software (Brooks Smart DDE) through a RS232/RS485 converter (ICC-11). Humidifier. The humidifier system consists of a gas-liquid surface contactor with a liquid level and temperature control. A water reservoir tank keeps the water level of the system constant. The relative humidity of the dilution flow can be regulated in the range from 5 to 95%. Permeation System. A gas standard generator system from KINTEK Laboratory Inc. forms the base structure of the three permeation ovens (a base mode 491M and two auxiliary modules). Mixture Chamber. A 1 L glass chamber allows the homogenization of the standard mixture. Exposure Chamber. This is a 20.7-L cylindrical glass chamber that is divided longitudinally by a glass tray, which allows the physical recirculation of the air inside the chamber and the generation of a controlled wind speed by a small fan installed inside. Sensors or samples can be introduced or taken through the 48 Scott 32GL entrances or be deposited on the glass tray. The pressure is measured and recorded continuously by a Druck (DPI 260) pressure indicator. An electronic sensor model 601 from 2242 Analytical Chemistry, Vol. 71, No. 11, June 1, 1999
Testo GmbH also records the temperature and relative humidity from inside the chamber. The room-temperature control is able to maintain constant temperatures in the system ranging from 10 to 40 °C with a variation of (1 °C. All the variables of the system are controlled and recorded by computer. An automatic gas chromatograph 755 series 600 with a PID detector from Air Instruments BV (Eumen, The Netherlands) measures alternatively the concentration in the mixture and exposure chamber. Data are integrated and recorded through an analog-digital converter (HP-IB 35900 C from HewlettPackard). The system thus described was tested for the generation of atmospheric standards of C6-C8 aromatic compounds, i.e., benzene, toluene, ethylbenzene, and p-xylene. Nonrefillable commercial permeation tubes type HRT and ELHRT from KIN-TEK were used. The tubes ranged from 20 to 100 mm in length, had a 6.35-mm external diameter, and 0.762-mm tetrafluorethylene (TFE) wall thickness with typical permeation rates between 100 and 400 ng/min. This range is suitable for generating concentrations between 1 and 100 ppb. For lower concentrations at the range of 0.1 ppb, permeation tubes at lower permeation rates of ∼20 ng/min are required: ELSRT- W model from KIN-TEK, with the following characteristics: 5-50 mm length, 4.76-mm external diameter, and 0.762-mm Teflon fluorinated ethylenepropylene (FEP) wall thickness. Procedure. The importance of the operational procedure resides in the reduction of uncertainties that affect the value of the final concentration in the chamber. The two main factors involved in the process regard the determination of flows and permeation rates. The stability and repeatability of flows are linked to the performance of the mass flowmeters. The manufacturer (Brooks Instrument B. V.) specifies an accuracy of (0.75%, a repeatability of (0.25%, and a temperature effect of less than 1% from 0 to 70 °C from the original calibration. The calibration was carried out in the laboratory by comparison with a primary
Figure 2. Permeation rates determined gravimetrically for different permeation tubes operating in the system: (b) benzene, P (ng/min) ) -0.0226t (days) + 172.84; (O) toluene, P (ng/min) ) -0.0109t (days) + 343.46; (9) ethylbenzene, P (ng/min) ) -0.0091t (days) + 108.13; (0) p-xylene, P (ng/min) ) -0.0293t (days) + 189.33.
mercury-sealed piston flowmeter (Brook Vol-U-Meter calibrator series 1060). Permeation tubes were periodically weighed by means of a Mettler AT21 comparator. The average experimental time required for the stabilization of the weight of the permeation tubes after being removed from the oven was about 1-2 h. To minimize uncertainties in the determination of the permeation rates, periods greater than 2 weeks were typically used. The weighing of the permeation tubes was always carried out under the same ambient conditions of humidity and temperature. To ensure such conditions, a balance was installed in the chamber under controlled conditions of temperature and humidity. The final concentration during an experiment is consequently known after determination of the material released from the permeation tubes by weighing. RESULTS AND DISCUSSION Four permeation tubes containing benzene, toluene, ethylbenzene and p-xylene were continuously present in the system and weighed periodically for almost two years. A typical standard deviation for repetitive weighings for the different permeation tubes was ∼8 µg. The uncertainty of the concentration can be estimated by using the expression18
u2f )
∑(∂f/∂x )u i
2 xi
(1)
where f is the function that defines the concentration, C, as
C ) m/Ft
(2)
and uxi represents the respective uncertainties for the mass delivered by the permeation tube, m, the global flow, F, and time between weighings, t. The uncertainty of the mass delivered by the tube, um, includes the uncertainty from the weighings, umw, (before and after the referred period), plus the uncertainty of the possible mass lost (18) Dietrich, O. Uncertainty, Calibration and Probability. The Statistic of Scientific and Industrial Measurement, 2nd ed.; Adam Hilga: Bristol, 1991; Chapter 8, pp 248-265.
Figure 3. Experimental increment on the flow with the humidity.
during the non-steady-state conditions, umnsc (mass delivered during the weighings and stabilization period). The period for nonsteady-state conditions of the permeation tube, ∆tnsc, was determined to be 3 h. Figure 2 represents the permeation rates determined for each compound during almost 700 days of continuing operation. A slight drift in the permeation rate with time was observed. Such a drift was only significant during approximately the first month of working. Afterward, the drift on the permeation rate, RPdrift, was lower than 0.1%/week for all of the compounds. The humidification step implies an increase in the dilution stream with respect to the calibrated dry flow. Under ideal conditions, the ratio between the humidified flow, Fwet, and dry flow of air, Fdry, can be estimated by the following equation:
[ ]
Vair + Vwater-gas Fwet P ) ) ) Fdry Vair P - pwater-gas P (3) P - pv(RH (%)/100)
where Vair and Vwater-gas represent the volumes corresponding to the air and water vapor, respectively. P is the global atmospheric pressure and pwater-gas is the partial pressure of water vapor in the air stream, which is a function of the relative humidity (RH) and the vapor pressure, pv. Nevertheless, under nonideal conditions, the real ratio Fwet/ Fdry will differ from the ideal ratio (Fwet/Fdry)ideal by a function of the partial pressures of the final humidified flow, i.e., of the relative humidity, f(RH). If the nonideal behavior is not very pronounced, this factor can be derived from a truncation of the virial equation of state19 and be expressed according to the following equation:
[ ]
Fwet Fwet ) f(RH) Fdry Fdry
ideal
(1 + RRH)
) (1 + RRH)
[ ] Fwet Fdry
) ideal
P 1 + aRH (4) ) P - pv(RH (%)/100) 1 + bRH
where a and b of eq 4 can be estimated by a nonlinear regression
(19) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; Chapter 3, pp 35-42.
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Table 1. Determination of the Overall Uncertainty relative uncertaintiesa
umw )
σw Prt
8 µg/(0.100 (µg/min) × 60(min/h) × 24 (h/day) × 7(day/week) × t(week)
7.93 × 10-3/t(week)
umnsc )
Pr(∆tnsc/2)/x3 Prt
(100(ng/min) × 60(min/h) × 3(h)/2/x3)/(100(ng/min) × 60(min/h) × 168(h/week) × t(week))
5.15 × 10-3/t(week)
u∆tnsc )
(∆tnsc/2)/x3 t
(3(h)/2/x3)/(168(h/week) × t(week))
5.15 × 10-3/t(week)
0.0075/x3 0.001206/1.035
4.33 × 10-3 1.25 × 10-3
uF
uhumd )
σh Fwet/Fdry
2.88 × 10-4t(week) 0.008
udrift ) (RPdriftt/2)/x3 0.001(week-1)t(week)/x3 udecay ) RSDr 0.008 uoverall ) (2u2mw + u2mnsc + u2∆tnsc + u2F + u2humd + u2drift + u2decay)1/2
a In the cases where only an interval of variation described the dispersion of the variable, a rectangular distribution of probability was assumed and the uncertainty was calculated according to the following equation: u ) (a2/3)1/2, where a represents the midpoint of the interval.
Figure 4. Concentration decay in the exposure chamber due to the presence of diffusive samplers. Conditions: average inlet concentration, 17.74 µg/m3, RSD 0.8%; exposure chamber average concentration, 13.78 µg/m3, RSD 0.9%; average ratio, 0.777, RSD 0.8; theoretical ratio, (C/CI) ) (F/(nUp + F)) ) (8.656/30 × 0.083 + 8.656) ) 0.777.
Figure 5. Average wind profile along the corridor of the exposure chamber.
following equation:
CIF - ( algorithms.20,21
method based on a least-squares Figure 3 represents the fitting of this equation to ratios measured in the system. An increase in the flow of ∼3% is observed when the stream is humidified from zero to almost saturation. This implies a consequent depletion in the concentration of the same range. The standard deviation of this correlation, sh, was estimated to be 0.001 206, the regression coefficients a and b being 6.06 × 10-3 and 6.20 × 10-3, respectively. The concentration in the exposure chamber can be affected by periods of non-steady-state operational conditions, for instance: changes in the flow, changes in the inlet concentration, opening of the chamber (introduction of samples), or the presence of concentration sinks inside the chamber as, for instance, for diffusive samplers. The modeling of these effects can be made by a material balance in the chamber,22 which leads to the (20) Dennis, J. E.; Gay, D. M. Weslseh, R. E. ACM Trans. Math. Soft. 1981, 7, 348-368. (21) Press: W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes. The Art of Scientific Computing, 2nd ed.; Cambridge University Press: New York, 1992; Chapter 15, pp 675-683. (22) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; Wiley & Sons: New York, 1999.
2244 Analytical Chemistry, Vol. 71, No. 11, June 1, 1999
∑U i
Pi
+ F)CO ) V
dCO dt
(5)
where CI is the concentration in the inlet flow, F; CO is the concentration in the outlet and/or inside the chamber, V is the chamber volume, UPi is the uptake rate of the sampler i, and t is the time. According to eq 5, when the system reaches steady-state conditions (i.e., dCO/dt ) 0), the concentration inside the chamber will differ from the inlet or reference concentration by the uptake rates and number of the diffusive samplers present inside the chamber. In the described system, this depletion can be observed and quantified by the gas chromatograph that is sampling alternatively from the inlet flow and the inside chamber concentration. Figure 4 shows an experiment where 30 diffusive samplers with an uptake rate of ∼83 cm3/min were exposed to an ambient air benzene concentration over 4.5 days, the flow in the system being 8.656 L/min. Experimental and theoretical ratios were in agreement. The relative standard deviation of this ratio, RSDr, measured during this period was ∼0.8%. From the integration of eq 5, it is also possible to estimate the stabilization time of the concentration inside the chamber.
Figure 6. Expanded overall uncertainty of the concentration as a function of the weighing time interval of the permeation tubes: (s) RPdrift ) 0.1%; (- - -) RPdrift ) 0.2%; (+) RPdrift ) 0.5%.
Therefore, the necessary time to reach 95% of the inlet concentration, t95%, when the initial concentration in the chamber is zero and no samplers were inside, is given by the following expression:
t95% ) -(V/F) ln(0.05)
(6)
As a general rule, stabilization periods are usually avoided during experiments or they are negligible in comparison to the whole duration of the experiment. The system is capable of generating wind velocities of ∼3 m/s. Wind velocities generated by recirculation of the air inside the chamber do not affect the concentration value and do not add any additional uncertainty to this. As a result of the air recirculation, the flow is of a turbulent nature. Homogeneous profiles of wind velocity were observed in the upper part of the chamber,
this section being used as the wind tunnel. Figure 5 shows the wind velocity profiles along the corridor of the exposure chamber at different set points. The overall uncertainty of the final concentration in the exposure chamber, uoverall, can be estimated and is displayed in Table 1. This overall uncertainty was calculated as a function of the weighing interval time. Figure 6 shows the expanded overall uncertainty for the 95% confidence level (2uoverall) versus the weighing interval time. The uncertainty due to the drift of the permeation rate can limit the maximum time between weighings and produce an optimal value. For instance, for a drift of 0.5%, the optimized time was ∼3 weeks. For the current system, the minimum weighing interval time can be fixed to greater than 4 weeks, which corresponds to a value of (1.9%. CONCLUSIONS The improvement in the overall uncertainty of the described system is mainly linked to the operational factors such as the repeatability in the weighing of the permeation tubes and the shortening of the weighing times. These are both related to the isolation of the weighing conditions from external disturbances such as changes in temperature, humidity, air streams, or electrostatic charges. The drift or decay in the permeation rate that is related to its long-term stability will limit the lapse between weighings because of the progressive increase of this uncertainty with time. Nevertheless, the minimum overall uncertainty obtained with the current equipment allows its use for the evaluation protocols referred to in this article. Received for review November 19, 1998. Accepted February 25, 1999. AC981291L
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