Anal. Chem. 1995, 67,2042-2049
Liquid Droplet. A Renewable Gas Sampling Interface Shaorong Liu and Pumendu K. Dasgupta*
Department of Chemistly and Biochemistty, Texas Tech Univetsily, Lubbock, Texas 79409-1061
As a gas stream flows across a liquid droplet, the soluble constituents contained in the gas diffuse to and dissolve in the liquid drop. From the viewpoint of analytical chemistry, this droplet is thus a potential sampling interface for such soluble constituents. The problem that needs to be solved relates to the manner of its incorporation in a chemical analysis system. This paper describes the coupling of this sampling interface with a sequential analysis system, demonstration of its applicability for the measurement of parts-per-billionlevels of gaseous NHs, and a characterization of the performance of this experimental arrangement. Various aspects of the behavior of the liquid droplet as a collection interface are theoretically discussed and experimentally examined using NH3 and SO2 as test gases. It is often necessary to discriminate between gases and particles by some physical means because the same analyte may be present in both phases. Because the diffusion coefficient of a gas is typically 4 orders of magnitude greater than the smallest atmospheric aerosol of significance, diffusive discrimination provides a noninvasive means. Diffusion denuders, tubes with interior walls coated to function as a sink for the desired gases, were originally described by Townsend.' They have been used since the late 1970s for the collection of gases.2 The original implementation was labor-intensive,wetted denuders in which a (flowing) liquid is used as the collection surface and their membrane-based counterparts called diffusion scrubbers now permit automated continuous collector/analyzers;recent reviews are a~ailable.3.~ A droplet of water is a natural collector for soluble gases. Allusions to the freshness of air after drops of rain fall through the atmosphere has long been a favorite of the literati. The physical process of rainout of soluble gases like SOz, HNO3, etc., has been mathematically de~cribed.~ Additionally, droplets of atmospheric water, in the form of clouds, rain, fog, dew, etc., serve as efficient atmospheric reactors. An interesting aspect of using a droplet as a collector for gases is that as the liquid evaporates from the droplet surface, the flux of the molecules leaving the surface inhibits the approach of particles (but not of soluble gases). This is referred to as difisiophoresk due to Stefan flow in the aerosol science literature! (1) Townsend, J. S. Philos. Trans. R. Soc. London, Sect. A 1900,193,129-158. (2) Ferm, M.Atmos. Environ. 1979,13,1385-1393. (3) Dagupta, P. K. ACS Adv. Chem. Ser. 1993, No. 232,41-90. (4)Ali, Z.;Paul Thomas, C. L.; Alder, J. F. Analyst (London) 1989, 214, 759769. (5)Butler, J. D. Air Pollution Chemistry; Academic Press: New York, 1979;pp 102-106.
2042
Analytical Chemistry, Vol. 67,No. 73,July 7, 7995
The world of analytical chemistry increasinglyinvolves smaller scales and miniature devices. The use of capillary-basedanalysis systems is presently enjoying an exponential growth. Membraneinterfaced capillary systems have already been reported for gas A liquid drop, deployed as a collector at the end of a tube, may not only eliminate interference from particles, it may also constitute an effective interface for gas analysis by a capillarybased analysis system. S i c e such a droplet is readily renewable, there should be no fouling problems as with membranes. The design, characteristics, and deployment of a droplet-based gas analysis system is addressed in this paper. EXPERIMENTAL SECTION CollectionDevice. A droplet-based sampler is schematically shown in Figure 1. One end of a silica capillary tube is supported in the center of a larger tube; the latter serves as the sample gas conduit. A suitable solution is pumped through the capillary, and a droplet is formed at its tip for collection of the gas. As sample gas flows around the drop, gas molecules diffuse to its surface and are collected by the droplet. The analyte concentration can be determined by aspirating the droplet solution back to an inline automated analysis system. M e r one collection analysis cycle, a new droplet is formed and the cycle is begun anew. Trace Gas Generation and Sampling. Most of the work reported in this paper is related to the sampling and determination of ammonia at subparts-per-million @pm) levels. The standard NH3 gas generation system is schematically shown in Figure 2; The principle has been previously described?JO Compressed house air was fed through a pressure regulator (G), a particle ater (F1, Mini Capsule filter, 0.2 pm, P/N 12122, Gelman Sciences Inc., Ann Arbor, MI), and an NH3 filter (E?, a column filled with HzS04-washed silica gel). The flow rate of each line was controlled by a mass flowmeter and controller (FC1-3, Model FC-280, Qlan General, Torrance, CA). One line was connected to a porous Teflon tube (M, 60 cm x 5.5 mm i.d., Accurel, V8/2, mean pore size 0.2 pm, Akzo Inc., Wuppertal, Germany) which was completely immersed in the NH3 generation solution in a sealed plastic bottle. Ammonia was transported from the aqueous solution into the gas phase through the pores in the tube M. At the flow rate (0.12 Wmin) used in this experiment, Henry's law equilibration is achieved.I0 Thus, a constant NH3 concentration was generated in the gas phase as long as the [NH3(,d,lin the generation solution (6) Hinds, W. C. Aerosol Technology;Wiley: New York, 1982;p 161. (7)Bao, L.;Dasgupta, P. K. Anal. Chem. 1992,64, 991-996. (8)Liu, S.;Dasgupta, P. K. Anal. Chim. Acta, in press. (9)Hwang, H.; Dasgupta, P. K. Environ. Sci. Technol. 1985, 19,255-258. (10)Dasgupta, P. K.; Dong, S. Atmos. Environ. 1986,20,565-570. 0003-2700/95/0367-2042$9.00/0 0 1995 American Chemical Society
sample air
(a)
LED-Photodiode detector collection solution NH3 sample air ,transporting capillary
sealed with
7 \1
Jtion in
...................................................
collection solution
stainless sto Figure 1. Schematic diagram of the droplet collector construction. The capillary is 0.15 mm i.d. and 0.35 mm 0.d. The internal diameter of the PTFE tube is 8 mm.
1
steel tube
transporting capillary LED
TOP
view
Side view
.....-..__._____________________...__.._ I
compressed air
U U
0G F1 G
*r
I
LED-Photodiode detector Figure 3. Schematic diagram of the SIA detection scheme: DC, droplet collector; HC, collection solution holding coil; SV, selector valve; R1, R2, and R3, chemical reagents; aux., collection solution; W, waste; D, spectrophotometric detector with wavelength set at 630 nm. Detail constructions of the LED photodiode detector are schematically shown in the bottom panel.
vent
F1
to SIA
T
Dc%iT
plastic VP bottle Figure 2. NH3 gas generation system: G, pressure gauge and regulator; F1, aerosol filter; F2, ammonia filter; FC1, FC2, and FC3, mass flowmeter and controller; FR, flow resistor; V, pneumatic valve; soh., 5 mM NH&I at pH 7; M, porous PTFE membrane; N, needle valve; HS, humidity sensor; VP, vacuum pump; DC, droplet collector.
was maintained constant. A coiled PVC tube (1.5 m x 2 mm i.d.) served as a flow resistor to minimize the influences of flow fluctuations from the dilution air line on the NH3 source flow rate. The dilution air line was divided into two after the NH3 filter. One stream, passing through FC3, was fully humidifled by passage through two sequentialwater-filled bubblers. The relative humidity of the dilution airstream was thus controlled by the adjustment of the relative flow rates through FC2 and FC3. The concentration of NH3 directly in the source output was determined by sampling through a bubbler filled with 10 mM H2S04 and subsequent colorimetric analysis (vide infra) and agreed closely with previously published data.1° The concentrations of NH3 in the sample gas was varied by appropriate dilution. If the three-way valve (V) is directed to F2, all the sample is vented. When V is directed to the sample gas line, a portion of the sample is aspirated through the droplet collector (DC); the exact flow rate is controlled by (11) Huang, H.; Dasgupta, P. K. Anal. Chem. 1990, 62, 1935-1942.
the needle valve before the vacuum pump (VP). A microsized humidity sensorll was incorporated after V to determine the relative humidity of the sample gas through DC. Ammonia-bearing sample air was initially aspirated through DC in a bottom-to-top direction; later results were obtained with flow in the reverse direction as well. The collector droplet is formed as the collection solution is pumped out of the capillary tip. After the desired sampling period, the droplet is aspirated back to a capillary format sequential injection analysis (SIA) system12 for analysis. Sample carry-over was minimized by propelling the collection solution through the transporting capillary until a drop of the solution (-5 pL) dropped freely from the capillary tip. The volume of this drop is comparable to the volume of the capillary up to the SIA selector valve. A new droplet is then generated. The size of the droplet is controlled by the solution pumping time. An electroosmotic pump13 was used; however, this is not a requirement. Analysis System. The SIA system involves electroosmotic pumping and is schematically shown in Figure 3a. The main system was described previously.12 Briefly, the transporting capillary comprises two segments of capillaries, a -5 cm x 250 pm i.d. and a 15 cm x 150 pm i.d. capillary. The two segments are butt-jointed with a piece of PVC tubing as a sleeve. For this work, an LED photodiode detector was put on the transporting capillary to monitor when the meniscus of the (12) Liu, S.; Dasgupta, P. K. Talanta 1994, 41, 1903-1910. (13) Dasgupta, P. K.; Liu, S. Anal. Chem. 1994, 66, 3997-4004.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2043
Table 1. Protocol for Sample Collection and Detection for the Droplet Collection System
step0
time, s
HV,b kV
comments
90 330 -120
+20 0 -20
generation of a 90 s droplet collection of NH3 aspiration of the solutionC in the transporting capillary aspiration of NH3 collected droplet solution aspiration of R1 aspiration of R2 aspiration of R3 propulsion of the reaction product to detector and then cycle from steps 4 to 8 until all the droplet solution is consumed washing of the transporting capillary until a full dro of the collection solution is disidged. Return to
10
-20
3 5
-20 -20 -20 +20
15 240
-300
+20
step 1
This is the high voltage on the EOF pump. *The number in parenthesis indicates the selector valve position. The volume of the solution between the collection droplet and the selector valve should be minimized by minimizing the volume of the connecting tubing. From steps 8 to 9, selector valve stopped at position 1 for 0.5 s.
to the H2SO4 content. M e r collection, as the droplet is aspirated through the conductivity detector, the HzS04 concentration in the liquid can be directly monitored by conductivity as a function of aspiration time. Reagents. Reagents used for analysis have been described.'* For the NH3 generation solution, 0.26 g of NHdCl was dissolved in -100 mL of 0.1 M NazHPOs; 0.1 M KHzPO4 was used to adjust the pH to -7.0 and the solution was made up to a final volume to 500 mL. This solution has a C N Hof~10 mM and a Cpo43- of 0.1 M. The pH of the final solution was measured to be 7.10. SO2 Generation Solution. A solution of 5.0 mM NaHSO3 in 0.20 M phthalate buffer (PH -4) was prepared by dissolving 0.30 g of NaHS03 in 500 mL of 0.2 M potassium hydrogen phthalate aqueous solution. Collection Solution for NH,. Except as stated, 1.0 mM HzS04 was used. As the collection solution for SOz, 500 pM HzOz in 10 pM MnS04 was prepared by adding 29 p L of 3%H2Oz solution to 50 mL of 10 pM MnS04 solution. This solution is prepared immediately before use. NazB407 (2 mM), used as the electroosmotic pump fluid, was diluted from a 0.5 M stock solution. RESULTS AND DISCUSSION
collection solution passed across it (and thence to determine the aspirated droplet volume, vide infra). The detector is shown in Figure 3b. The plastic head of a T-13/4size red LED (650 nm, HLMP-8104, Hewlett-Packard) was filed to flatness. Then a U notch (-1 mm wide x -1 mm deep) was cut on top with the lightemitting source in the center of the notch. This was covered with a piece of aluminum film with a -200 pm diameter opening to allow the LED output to shine through. A stainless steel tube segment (-10 mm x -400 pm i.d. x -600 pm 0.d.) was filed with a hole (-1 mm long x -300 pm wide) each on opposing sides. This was placed in the U notch on the LED head with its holes in line with the opening of the aluminum film. A photodiode was put the other side of the stainless steel tube. The LED was powered with a constantcurrent source. The photocurrent was measured with a switched integrator-based d e t e ~ t 0 r . lFor ~ the present purpose, the polyimide coating on the transporting capillary need not be removed; even with the coating intact, there is sufficient photocurrent. S i c e the inner diameter of the stainless steel tube is slightly bigger than the outer diameter of the transporting capillary, the LED photodiode detector can be easily moved along the capillary and placed anywhere between the selector valve SV and the droplet collector DC. The chemistry involved in the determination of ammonia utilizes the Berthelot reaction using the sequential mixing of a phenol/sodium nitroprusside reagent (Rl, Figure 3a), alkaline EDTA (R2), and sodium hypochlorite W). The protocol is listed in Table 1. The absorbance of the indophenol blue product is measured at 630 nm using a linear PHD detector designed for capillary systems (Therm0 Separation Products). A set of experiments was also conducted with SO2 as the test gas. For the determination of SOz, a conductivity detector15was placed at the joint of two segmented capillaries in the transporting conduit. Using Mn-catalyzed oxidation by H2Oz in the collecting droplet, the collected SO2 was converted to sulfuric acid. The conductivity of the collecting droplet increases in direct proportion (14) Liu, H.; Dasgupta, P. IC;Zheng, H. J. Talanta 1993,40, 1331-1338. (15) Kar, S.; Dasgupta, P. IC;Liu, H.; Hwang, H. Anal. Chem. 1994,66, 25372543.
2044
Analytical Chemistry, Vol. 67,No. 13, July 7, 1995
Direction of Flow. Statistically, there was no significant difference between results obtained with the sample gas flowing downwards or upwards. Collection occurs by diffusion in a direction transverse to the flow direction in either case, and no difference is expected. However, if the capillary is not exactly centered and a dislodged wash droplet happens to end up on the wall, sample loss will occur with upward flow. With this in mind, a top downwards sampling arrangement was adopted. Diffusion and Collection of NH3. The collection of gaseous NH3 by the droplet involves the following sequential steps, similar to that outlined by Schwartz and Freibergl6for the collection of SO2 by a cloud droplet: (a) diffusion of NH3 molecules within the gas phase to the gadliquid interface; (b) Establishment of Henry's law equilibrium at the interface; (c) hydration of NH3 and partial ionization of NH40H (or protonation of NH3(,d by extant H+); (d) transport of NH4+ within the aqueous phase. Based on previous work on cloud droplets,16 steps a-c are fast relative to step d. The present experiments use droplet sizes close to 2.0 mm in diameter, substantially larger than cloud droplets, so step d can be safely assumed as the ratelimiting step for the absorption of NH3. As NH3 dissolves, the pH of the interfacial solution increases. Henry's law equilibrium at the interface is very quickly established. A steady state in NH4+ concentration in the interfacial solution can be assumed between its production by dissolution/ionization and transport from the interface. If we assume that the droplet is spherical and NH3 dissolution occurs with spherical symmetry around the droplet, and that the droplet solution is stagnant (no convective mixing),the NH4+ concentration withii the droplet can be described as'' r C,,! = C,R
2Co (-Qn + -cn,, n
nnr R
(
n i
sin -exp - -Dt
)
(1)
where C,,t is the NH4+ concentration within the droplet at a (16) Schwartz, S. E.; Freiberg, J. E. Atmos. Environ. 1981,15, 1129-1144. (17) Vergnaud, J. M. Liquid Transport Processes in Polymeric Materials, Modeling and Industrial Applications; Prentice Hall: Englewood Cliffs, NJ, 1991; p 32.
distance of r from the center of the droplet and after a sampling time of t, CO is the interfacial solution concentration, R is the droplet radius, and D is the diffusion coefficient of NH4+in the droplet. Normally, terms with n 2 2 are negligible; eq 1simplifies to
22
C,,=C---sin-exp ;0
;(- -9) t
(2)
The process of the aspiration of the droplet solution back into the capillary can be envisioned as the droplet being squeezed by the surface tension of the liquid. The solution in the center of the drop closest to the capillary tip is aspirated first while the solution at the surface of the droplet is the last to enter the capillary. For simplicity, we assume that any incremental aspiration results in a spherical volume of radius 1' being withdrawn into the capillary, the spherical volume being originally located tangential to the capillary tip (Figure 4). The volume elements 1,2,3,4,etc., in the original droplet may, for example, be aspirated in sequence as aspiration is commenced. In a spherical coordinate system, the NH4+ concentration at the exit can be expressed as a function of I-': Figure 4. Spherical coordinate system for the solution aspirated back: R,the radius of the whole droplet solution; r', the radius of the solution aspirated back; r, q, and 6 are the spherical coordinates.
is unaltered by the specific choice of CO.Numerical integration was carried out with a 1' step size of 2 pm, that is, 6 = 2 pm. This value was chosen after determining that, for R = 1000 pm, the results no longer change below a step size of 4 pm for 1'. Since 1' can be expressed in terms of experimental parameters from the relationship
where 6 is a small increment of 1'. For 21' IR
[C:, and for 21'
L
-
2
sin
exp( - $t)]2
dr (4a)
R
[
Co; -
2
sin
exp(- $It)]?
dr (4b)
For typical experimental parameters, R = -1.0 mm, t = 330 s (sampling time), Q = -1.5pL/min, and N&+ diflusioncoefficient in dilute aqueous solutions is 2.0 x cm/s (calculated from its inhite dilution equivalent conductance18at 25 "C); we can numerically integrate eqs 4a or 4b by iterative stepped integration methods. A value of '/znwas used for COfor computational purposes. Since both eqs 4a and 4b scale linearly with CO,the profile of C ( f l (18) Lide, D.R, Ed. CRC Handbook ofChemkttyandPhysics, 73rd ed.; CRC Press, Inc.: Boca Raton, FL, 1992; p 5-111.
where t' is the aspiration time and Q is the aspiration rate, the concentration profile C(fl can be converted as a function of the aspiration time t'. In our experiment, the sample requirements of the SIA system is such that a single droplet of 1mm radius allows many sequential aliquots to be taken for analysis. We aspirated the droplet in 18 successive aliquots, analyzing each aliquot separately. Figure 5 shows the comparison of the theoretical computations vs the experimental results. In general, at longer aspiration times, that is, for the results from the outer surface of the droplet, the experimental data match the theoretical expectations quite well. For earlier times, especially during the very beginning of the aspiration period, the solution represents mostly the volume element near the capillary tip; the experimental concentrations are increasingly below the theoretical model. This is readily understandable; the theoretical model does not account for the presence of the capillary tip on the surface of the sphere but assumes a uniform input flux of ammonia to the entire surface area of the sphere. In reality, a 0.35 mm diameter capillary occupies a significantportion of the surface area of a 1mm radius sphere and blocks any input flux over this area. Indeed, the Analytical Chemisty, vol. 67, NO. 13, July 1, 1995
204s
0
6'oo 5.00 n
3 4
i
P
/+
E 4.00
K
W
.-0
I T
-I-
0 0.02
1
2
.+ I
2.00
K
a, 0 K 0
Q)
2
3.00
I L
0
0
0
0 _c,
i
2.00
II Q
Aspiration time (second) I
0
'
'
'
5
'
I
'
~
10
'
J
I
~
15
1 .oo ~
~
J
20
I
'
Aliquot No. Figure 5. NH4+concentration profile as being aspirated back into the transporting capillary. The solid curve is computed from eqs 4a and 4b, corresponding to the left ordinate and the top abscissa. The points are experimental data, corresponding to the right ordinate and the bottom abscissa; one aliquot is equivalent to 10 s aspiration time. The experiment was done with a HV of 1 1 5 kV on the EOF pump and for a droplet size of 4.5pL. The experimental procedure is similar to that described in Table 1, with some changes in individual step intervals; 250 ppbv NH3, -80% relative humidity, and sampling rate 0.12 Umin.
volume element at the immediate tip of the capillary is virtually fully sheltered and the first aliquot contains almost no NH4+. A second feature of the experimental data becomes apparent after a closer examination of Figure 5. After the fist couple of aliquots, the absorbance increases nearly linearly until aliquot 12; then there is a virtual plateau until aliquot 14,and following this, the absorbance increases again linearly, with a greater slope than in the first step. This is a pattern we observe consistently. Close visual examination of the droplet during the sampling experiment clearly shows that the surface of the droplet is not stagnant. A circulatory motion of the surface layer, presumably caused by the frictional drag of the air moving past the droplet, results in better uptake than that predicted by a stagnant boundary layer. We believe this is responsible for the observed results. Effect of Droplet Si. The concentration of N&+ as a function of aliquot number was determined for different droplet sizes; the results are shown in Figure 6. Droplets with a radius of 0.65,0.81,and 0.93 mm were formed by pumping the absorbing solution for 1,2, and 3 min, respectively. Given the same analytical sample size, the number of aliquots that can be withdrawn from droplets of different size is naturally dependent on the size of the droplet. However, in all cases, the final aliquot represents the solution on the surface of the droplet. It is noteworthy that the final aliquot composition is virtually identical in all three droplet sizes. m e smallest droplet has a only slightly higher final aliquot concentration than the rest. This can be readily attributed to a sampling artifact: a longer time and a larger total aspirated volume are needed to get to the final aliquot for the larger droplets and this leads to greater mixing.) This validates that COis constant regardless of the droplet size. The area under each curve in Figure 6, hereinafter referred to as the integrated absorbance signal (IAS), is proportional to the total mass of ammonia collected. As long as the surface of the droplet still behaves as an effective sink for the exposed 2046 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
~
'
'
I
0.00
Aliquot no. from t h e droplet solution Flgure 6. NH4+ concentration profiles as a function of the aspiration aliquot number for different droplet sizes. Experimental conditions as in Figure 5.
surface area S of the droplet, which can be approximated as
where r, is the outer radius of the capillary,the IAS for the three cases in Figure 6 is indeed linearly related to the value of S with a linear correlation coefficient (12> value of 0.9936. Sampling Time and Initial Droplet Conditions. The total mass of an analyte gas collected will increase linearly with the sampling time as long as the sink efficiency of the droplet surface remains the same. In the present case, NH3(as)is protonated by H+ supplied by HzS04 originally present in the droplet; this removal of un-ionized NH3(,@by protonation keeps the droplet surface an effective sink for NH3. The initial amount of H+ available for this purpose is that present in the surface layer (the thickness of which is defined by the circulatory mixing), and as the H+ is consumed by the influx of NH3, it is also replenished by outward radial diffusion of H+ from the interior of the droplet. However, depending on the ammonia concentration and its flow rate, diffusive replenishment from the interior may be inadequate to maintain a sufficient H+concentration on the surface and thus limit the effectiveness of the surface to capture NH3. Figure 7 shows the amount of NH3 collected (at a constant NH3 concentration and sampling rate) as a function of sampling time for different concentrations of acid initially present in the droplet. The collection efficiency decreases with increasing sampling time, but better results are obtained with increasing initial acid content. Although not specifically shown here, it is intuitively obvious that, under otherwise fixed conditions, the collection efficiency will be less affected with a decreasing total flux of NH3 to the droplet, Le., with a lower NH3 concentration. A similar effect may be expected for the sample flow rate; however, in the present situation, increasing sample flow rate also results in greater circulatory mixing of the surface layer and may thus mitigate, at least partially, the effect of the increasing flux. Collection Efficiency. The absolute collection efficiency of the droplet was estimated by measuring the relative amounts of
40.0
-
N
E 0.66
n
-+
E
0 3
U .-0 30.0 *
c.l
0.64
f
W
3 Q
+-J
E
2 0.62 Q
W
- 20.0 U c
-
2
-W
.-0 m
LC
-lJ10.0
-
4
0
+J
P
-G
-
0.60
0
m
.--03 0.58
E3XXl0.1 mM H2S0+ in the droplet U M M 1.0 mM H2S04 in the droplet 4 A M A 10 mM HzS04 in the droplet
1 , , , , , 2.00 , , ~ ~ 1 , 4.00 , , , , , , , ,6.00 1,
0.0 0.00
1
1
~
1
1
1
!
I
(
F -
I
I
I
I
I
I
J
!
I
I
8.00
Sample collection t i m e (min)
0
W
Figure 7. Amount of NH3 collected as a function of sampling time. The experimental procedure is as in Table 1, except the time for step 2. Other conditions are as in Figure 5.
P
v
?20.0 1
-dm - - k ( l -
L
Y
40
10.0
60
80
100
%
Figure 9. Square of the final droplet radius as a function of the sample gas relative humidity.
.-CWu .-c
.-6
20
Relative humidity,
periods. Concerning the rate of evaporation from a sphericalwater droplet in a stream of flowing air, Frosslinglgpreviously derived the droplet radius R changes as a function of time, as
0 U
0
.-GC 0.56
dt
:
RH)&
u
0
0
7
&---"
1
L (
0.0 0.0 0.00 0.20 0.40 0.00 5.00 10.00 Sample gas flow rate (I/min) Reciprocal flow rate (min/l)
Figure 8. Collection efficiency as a function of sampling flow rate. The experimental procedure and conditions are as in Table 1 and Figure 5. See text for detail.
ammonia collected in the presence and absence of another identical droplet upstream. Referring to F i e 1,another capillary (not shown) was inserted into the top part of the DC so that a second droplet could be deployed, in series after the first one. The amount of NH3 collected by the principal droplet was measured with and without the second droplet. From these data, the NH3 removal efficiency of the auxiliary droplet (and thus the identical, principal droplet) is readily calculated. Figure 8a shows the collection efficiency as a function of sampling flow rate. For a diffusion-based collector, the collection efficiency f exhibits the general form3 h(l
-8
where k is a constant and Re is the Reynolds number. If the experimental conditions are fixed, integration of eq 8 yields
= @/&)
+ In a
(7)
where a and b are constants and Q is the sampling flow rate. Figure 8b shows that the h e a r behavior predicted by eq 7 is indeed observed. Effect of Sample Relative Humidity. The sample relative humidity (RH) can have two different effects on the amount of the analyte collected. The first is related to the evaporative loss of water from the droplet. This can be substantial, especially at low sample relative humidity values and for extended sampling
r: = r,2 - k(1 - RH)&
t
(9)
where r, and rf are the initial and final droplet radius. Under given experimental conditions, r? is therefore linearly related to the sample relative humidity. The experimental data (Figure 9) show that this relationship holds very well in our experimental system. The consequence of this behavior is that the droplet surface area decrease during sampling is directly proportional to the relative humidity of the sample gas. Because the gaseous analyte dissolution flux is proportional to the available surface, the amount of the analyte collected is expected to increase with the sample gas relative humidity. As detailed below, an experiment was designed to determine the collection of SO2 as a function of sample relative humidity with a fixed initial droplet size. As a result of 5.5 min sample collection (90, 70, 50, and 30% RH) at a flow rate of 0.12 L/min, the initial droplet volume (2.25 pL) decreased in size to 2.15,2.10, 2.025and 1.925pL, respectively. The integrated conductance signal, linearly proportional to the amount of SO2 collected, also decreased (0.287,0.285,0.272,0.268, respectively). These data show reasonable linear correlation (r = 0.96,0.99 with one point omitted) with the mean surface area value over the sampling period. In any case, these data also show that, for modest sampling periods (single digit minutes) and at flow rates typically used in this work, the relative humidity effect is not prohibitively large. (19) Frossling, N. Gerlands Beitr. Geophys. 1938,52,170.
Analytical Chemistry, Vol. 67, No. 13, July 7, 7995
2047
Sample relative humidity can also affect the collection of a gaseous analyte in an altogether different manner. In diffusion scrubber-based measurements of ammonia, it has been observed that sample collection efficiency decreases with increasing sample relative humidity; this presumably occurs due to the formation of ammonia hydrates, NH~(HzO),,in the gas phase.20The effective diffusion coefficient of NH3 as an analyte thus decreases with increasing relative humidity. The resulting decrease in collection efficiency may overshadow any droplet evaporation effects. The experimental results for the collection of NH3 as a function of relative humidity (Figure 10) shows this behavior; not only is this effect opposite to that observed for SOZ,it is substantially larger in magnitude. Nevertheless, at any given sample relative humidity, there is always a characteristic calibration curve. The relative humidity effects can therefore be corrected by establishing a set of calibration curves at different relative humidity levels. The NH3 content can be calculated based on the calibration curve for the appropriate relative humidity level. However, the relative humidity of the sample is not routinely measured. Since the extent of evaporation of the droplet is related to the relative humidity, the relative humidity of the sample gas can actually be obtained from the droplet size change during sampling. From eq 9, we have
!
(Q2l3=
(V0)2/3- (4~/3)’/~k(l- RH)&
50.0
7
.--W
0 45.0 -
*
3 Q
E
W
- 40.0 0 c .-m
-
Lil
35.0 0
I 0 +-
c 30.00.00
20.00
40.00
80.00
60.00
Relative humidity
100.00
(w)
Flgure 10. Amount of NH3 collected as a function of relative humidity. The experimental procedure and conditions are as in Table 1 and Figure 5. 50.0 n 4
t
(10)
where Vr and VO are the final and initial volumes of the droplet. vi can be easily determined based on the aspiration time, ta, needed to aspirate the final droplet solution back into the transporting capillary and the aspiration flow rate, F
(ta)2/3= k’RH + C
(11)
k‘ = (42~/3ZiJ~’~k&t
(12)
where
0
J
2 30.0 L
W
-u
O M X X , aspiration
time: time: 4M&baspiration time: QQQQO aspiration time:
10.0
ppPp0 aspiration
-w
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40
1 1
I I
crrq
r
50
I
, 1 4 1 ,
100
r v , , ,
, In/
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m , , !I
200
179.5 s
172.0 s
167.0 s 163.5 s 11,
250
( 1 1
r i
300
NH3 concentration (ppbv) For a given system and a set of fixed experimentalparameters, k’ and C are constants. They can be calculated from the experimental parameters and information such as that presented in Figure 9. In this work, the aspiration time is determined from the LED Photodiode detector (see Figure 3) while the NH3 absorbance signal is measured by the in-line SIA system. Alternatively and perhaps more conveniently, instead of designating different humidities, sets of calibration curves can be constructed, each corresponding to the length of time needed to aspirate the entire droplet. Such a set of data is shown in Figure 11. After the aspiration time and the NH3 absorbance signal are obtained, one can go directly to such calibration data for determining the NH3 concentration. CONCLUSIONS AND FUTURE POSSIBILITIES
We have demonstrated the feasibility of a new type of gaseous analyte collection system, a droplet collector. As may be apparent, (20) Genfa, Z.; Dasgupta, P. IC;Dong, S. Enuiron. Sci. Technol. 1989,23,14671474.
2048 Analytical Chemistry, Vol. 67, No. 73, July 7, 7995
Figure 11. Calibration curves of the droplet collector for NH3 at different relative humidity values. Aspiration times of 179.5f 2.4, 172.0f 1.8,167.03= 2.3,and 163.5f 1.3s,respectively, correspond to 90,70,50,and 30% relative humidity.
SIA is hardly the perfect system to measure the collected analyte. A more straightforward approach would be to deploy sensing probes in the form of microelectrodes, optical fibers directly in the droplet for continuous or periodic interrogation. If chemical treatment is required before detection, a multiplicity of capillaries bundled together can be used, with one of them transporting the collection solution and others for various reagents, all temini sharing a common droplet. For some analytes, reagents can also be introduced from the gas phase. For SO2 collection for example, oxidant reagents such as gaseous HzOz or O3may be introduced. When postsampling reaction with other reagents is used, satisfactory mixing can be attained with a simultaneously bundled empty capillary by repeated withdrawal and expulsion of the droplet solution. When the droplet needs to be discarded or replaced, more collection solution is delivered. As discussed, the collection
efficiency change resulting from different sample gas humidities can be corrected for on the basis of the experimental data. If the change is a function of only the evaporation of the droplet solution, it will be relatively small and correction may not be necessary. The "droplet"need not be spherical or ovoid. Such geometries are in fact disadvantageous if concentration, rather than the total amount collected, is measured (as may be the case with in situ probes) because of the low surface/volume ratio. Ongoing experiments with in situ probes in this laboratory utilize circular or U-shaped wire loops to form a filmlike "droplet"at the terminus of the liquid delivery conduit. The disadvantage of a droplet interface is that it is not a quantitative collector, except at very low flow rates. Consequently, some constancy of temperature, to maintain a stable collection efficiency, may be essential. The apparent advantages of the proposed technique over other types of gaseous analyte collection system are (1) its simplicity and low cost, (2) facile replacement or regeneration of the collection solution, (3) convenient introduction of reagent($ to carry out reaction($, (4) potential of real-time detection of the analyte of interest in the collection solution or in situ detection immediately after sample collection,and (5) facility of automation.
In terms of providing a readily renewable surface, the droplet interface for sampling gases has certain obvious similarities with a dropping mercury electrode @ME) or a hanging mercury drop electrode (HMDE). The present concept is obviously not limited to the gaseous samples; the sampling and sampled phases merely need be immiscible. It would be most gratifying to the authors if its usefulness attains even a small fraction of what the DME and HMDE have contributed to electroanalysis. ACKNOWLEDGMENT
This work was sponsored in part by the Office of Exploratory research, US Environmental Protection Agency through Grant R 821117-01-0. However, this paper has not been subjected to review by the agency and no official endorsement should be inferred. We are grateful to Steve Gluck, Dow Chemical Co., for his keen interest. Received
for
review January
13,1995. Accepted April 12,
1995.w AC9500460 @Abstractpublished in Advance ACS Abstracts, June 1, 1995.
Analytical Chemisfry, Vol. 67,No. 73,July 7, 7995
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