ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Solid Sorbent for Sampling Acrolein in Air Avram Gold," C. E. Dub6, and I?. B. Perni Department of Environment Health Sciences, Harvard School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 02 1 15
Activated 13X molecular sieves were found to be an excellent solid sorbent for sampling acrolein in air, providing quantitative trapping and recovery and stability of stored samples. Atthough water vapor decreases acrolein breakthrough times, a sample volume of 8 L/g sieve sorbent is permissible at 100% relative humidity, allowing determination of acrolein at sub-ppm levels even under such unfavorable conditions. The dynamic adsorption isotherm of acrolein on 13X sieves was determined and found to be adequately described by a Langmuir curve. Statistical Moments Theory was applied to breakthrough data to estimate breakthrough times of acrolein over a range of concentrations. Field trials indicate that 13X sieves would also be suitable for sampling other low molecular weight aldehydes, short chain alcohols, and volatile ketones.
To date, the available methods for air sampling of low molecular weight, water soluble aldehydes involve the use of liquid absorbing solutions ( I , 2). These aldehydes, especially acrolein, are of increasing interest in industrial hygiene and environmental analysis, and a solid sorbent would be highly desirable for convenience in making field measurements. Molecular sieves have been employed as a gas chromatographic medium for volatile hydrocarbons (3-5). Reports on the adsorptive properties of sieves with regard to a number of polar and nonpolar organic compounds (6-8) and widespread industrial use of sieves for gas purification (9, 10) suggest the possible utility of molecular sieves for air sampling. The adsorption and breakthrough characteristics of aldehyde vapors on sieves have not been reported; therefore this investigation was undertaken to determine the suitability of activated molecular sieves as a sorbent for acrolein vapor. Large pore size in the sieves was desirable to ensure rapid mass transfer and efficient adsorption of acrolein; therefore 13X molecular sieves were selected as sorbent for the study. Activated 13X sieves were found to be an excellent sampling medium for acrolein, providing quantitative trapping and recovery and stability for storage of adsorbed samples. Although water vapor decreases acrolein breakthrough times, 1 g of sieves has sufficient capacity to d o w adequate sampling volumes for determination of sub-ppm concentrations even a t 100% relative humidity. Field trials indicate that the 13X sieves should also prove suitable for sampling formaldehyde, acetaldehyde, methanol, ethanol, and acetone. EXPERIMENTAL Reagents. Acrolein (Eastman) was freshly distilled before use. Standards were made up by addition of appropriate amounts of liquid acrolein to distilled water. Linde 13X molecular sieves, 12-30 mesh, were activated by slow heating under vacuum to 400 "C and maintaining that temperature overnight. The sieves were packed into 8 mm i.d. X 6 cm Pyrex sampling tubes and held in place by glass wool plugs. The tubes were capped by serum stoppers. Analysis. Sieves were desorbed by slow addition to distilled water (2 mL/g sieves) in polyethylene stoppered glass vials chilled in ice. Desorbate, liquid standards, and gas mixtures were analyzed for acrolein by gas chromatography (GC) using a flame 0003-2700/78/0350-1839$01,00/0
ionization detector. Blank runs on activated sieves desorbed with distilled water showed no peaks at lowest attenuation settings. A stainless steel column 11 ft X inch packed with Tenax GC was operated at 85 " C with a nitrogen carrier flow of 20 mL/min. Recovery Experiments. Measured volumes of air containing acrolein vapor in known concentration were added to sampling tubes by syringe. The tubes were desorbed and the desorbate was analyzed. Breakthrough and Dynamic Adsorption. The apparatus shown in Figure 1 was capable of providing constant concentrations of acrolein for periods of 3-4 h over the range of concentrations indicated in Table I. Nitrogen flow rate through the acrolein reservoir was -20mL/min and dilution air (0,30,100%, relative humidity) was adjusted to provide the desired acrolein input concentration ci. Breakthrough was followed by periodic monitoring of carrier gas downstream from the sieves. Points on the dynamic adsorption isotherm were established by (i) gravimetric and/or desorption analysis for determination of sieve capacity and calculation of input concentration ci by Equation 2 with the breakthrough time of ci/2 or (ii) GC determination of ci and calculation of capacity by Equation 2 with the appropriate breakthrough time of c,,i2. Combinations of these procedures were simultaneously applied to some experiments as indicated in Table I. Saturation Capacity by Static Adsorption. Sieves (0.5 to 1 g) were weighed into a small container fabricated from stainless steel screen which was suspended in a closed vessel over pure liquid acrolein at 23 "C. Acrolein uptake was measured by weight gain and/or analysis of desorbate.
RESULTS AND DISCUSSION Essentially quantitative recovery was achieved with both high and low loadings of acrolein on sieves. Recovery for 11 runs with nominal loadings of 3-8 Kg acrolein/g of sieves was 97 f 11%,while for higher loadings of 60-200 pg/g, recovery was 90 f 7% for six runs. Sampling tubes loaded with acrolein were analyzed a t intervals over four weeks with no observable loss of aldehyde when stored at 0 "(2. The quantitative recovery of acrolein is noteworthy since this compound is reactive and irreversible adsorption (8)or transformation (12-23) of reactive compounds has been observed on sieves even a t low temperatures. The performance of sieves as a trap for acrolein a t relatively high concentrations in dry air was evaluated by breakthrough experiments. Breakthrough curves (Figure 2) show to a good approximation the sigmoid form of the normal probability distribution and each curve can be adequately reproduced from a breakthrough time tclc1(concentration c < input concentration ci) and standard deviation u. The experimental breakthrough curves, tabulated in terms of o, and relative standard deviation o* = a/to,5are presented in Table I along with experimentally determined sieve capacities and corresponding ci. Within experimental uncertainty, t.he values of g* (-0.20) are independent of concentration. The conformity of the breakthrough curves to t h e normal probability distribution and the invariance of u* suggest that the behavior of acrolein on 13X sieves can be described by Stat,istical Moments Theory (SMT) (14-16) despite the nonlinearity of the adsorption isotherm (Figure 3). When mass transfer is controlled by intraparticle diffusion, a* may be approximated by the ratio 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Table I. Breakthrough Data for Acrolein and Water on 13X Sieves acrolein mg/L desorption
A , mglg
Ci,
gravimetric
1.66" 2.79" 3.53" 4.81" 5.0ga 6.31"
2.73 3.22" 3.39 4. 6ga 6.94
GC 0.903" 2.45 1.96" 2.25 3.08" 3.68 3.52 5.55 5.11 4.24 6.42
gravimetric desorption 121" 169"
166 148"
179"
171
151" 166" 174" 179"
197
GC 7 6" 178 147" 136 127" 170 179 178 176 146 182
f,,,, min
u* = u/t,.,
0.18 0.18 0.15 0.21 0.24 0.20 0.21 0.21 0.19 0.39 0.21
F , Llmin
84 69 67 54 39 46 43 31 32 33 25
1.1 1.1 1.1
1.1 1.1 1.1
1.1 1.1 1.1
1.1 1.1
water 21.3 260 0.18 22 1.1 6.6 230 0.29 18.3 2.0 " Data used in least squares f i t t o Langmuir isotherm. Accuracy of results was assumed t o be gravimetry > desorption > GC.
1 Sampling
pump
I!
I
o U - - - - - 20
40
60
J
_L1I 80
I00
120
140
160
Id0
200
c,(mgLl
Figure 3. Dynamic adsorption isotherm of acrolein at 21 Figure 1. Dynamic dilution apparatus
O C
Table 11. Breakthrough Times of 5% of Acrolein Input Concentration in Dry Air
I
el, m d L 4.51 2.29 0.229 0.023 0.0023 0.0002
c/c,
to.,,, min
20 43 72
82 82 82
Substitution into Equation 1 of the values for the parameters used in the experimental work ( R = 0.06 cm, U = 30 cm/s, H = 4 cm, and Di 0.1 cm2/s) yields a value of 0.19 for c*, in agreement with the values in Table I and confirming the validity of SMT in interpreting the acrolein breakthrough data. From SMT, the breakthrough curve a t any point on the isotherm may be constructed from to which is calculated by Equation 2 and the standard deviation u (= u* t o & .
-
Time ( m i n i
Figure 2. Representative breakthrough experiments for the following input concentrations compared to the normal probability distributions (solid curves) generated from values of to,5 and corresponding standard deviations in Table I: (a) 6.31 mg/L, (b) 4.68 mg/L, (c) 3.08 mg/L, (d) 2.79 rng/L, (e) 1.66 mg/L
of the central statistical moment AI2and the ordinary moment MI (16):
where R = radius of adsorbent particles, U = linear velocity of carrier, H = bed length, and Di = diffusion coefficient.
=
a W -.ci F
where a = sieve capacity (mgig), c , = input concentration (mg/L), W = weight of sieves (g), and F = flow rate (L/min). Table I1 shows breakthrough times t o a t a sampling rate of 1 L/min with various c, for 1 g of 13X sorbent, indicating that in dry air, large sampling volumes are permissible. Capacities and corresponding concentrations from Table I show a good fit by least squares to a Langmuir isotherm in the linear form ( l / a ) = m(l/c,) + (l/asat);coefficient of correlation 0.94, standard error of estimate 19%. The isotherm replotted in conventional form is shown in Figure 3. The value
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978 .
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Figure 4. Acrolein breakthrough, 100% relative humidity
0
I
4
I 8
I
12
I
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Ttme (min)
Figure 6. Gas chromatogram of a 2.5-L field sample collected over 12 min at a sampling rate of 208 mL/min. The following concentrations were calculated on the basis of a comparison of peak heights with those of standard solutions: (1) methanol, 110 ppm; (2) acetaldehyde, 9.7 ppm; (3)ethanol, 3.3 ppm; (4) acrolein, 16 ppm; (5)acetone, 66 ppm
volumes a t lower relative humidities, sensitivity increases correspondingly. Sampling tubes containing 2 g activated sieves were included in a sampling package for combustion gases. T h e chromatogram in Figure 6 is typical of samples collected a t residential fires. Formaldehyde did not elute from the Tenax GC column and consequently does not appear in the chromatogram in Figure 6. Formaldehyde and acrolein have been verified (but not quantitated) by colorimetric methods ( I , 2) while the identities of the remaining compounds were nonrigorously established by comparison of retention times and spiking experiments. :,me
I m P I
Figure 5. Acrolein breakthrough, 30 % relative humidity
of 236 mg/g calculated for saturation capacity asatagrees well with the value of 226 mg/g obtained by static adsorption experiments. Figures 4 and 5 show the effect of water vapor on the breakthrough of 1500 ppm acrolein in air at relative humidities of 30% and l o o % , respectively. The ratio of breakthrough concentration to input concentration (c/c,) plotted vs. time is superimposed on the calculated breakthrough curve of water. T h e water curves were generated for the required conditions from experimentally determined relative standard deviations, and to,svalues estimated from Equation 2 applied to experimental water breakthrough curves (Table I). Figures 4 and 5 indicate that acrolein is displaced by the water front and emerges from the sieve bed in much the same form as an elution GC peak, with a concentration maximum occurring a t approximately to of water and a variance similar to that of the water curve. Recalculating the curves for a flow rate of 1 L/min yields breakthrough time to,o5of 8 and 20 min per gram of sieves for 100% and 30% relative humidities, respectively. Since 2 pg acrolein/g of sieves can be detected readily by desorption and GC analysis, the 8-L sample volume at 25 "C and 100% relative humidity permits detection of 0.11 ppm acrolein/g of sorbent. With larger allowable sample
ACKNOWLEDGMENT T h e authors thank 0. Grubner for helpful discussions. LITERATURE CITED (1) "NIOSH Manual of Analytical Methods,"2rxj ed.,U S Deparbnent of Health, Education, and Welfare, Public Heatth Service, Center for Disease Control. NIOSH, Cincinnati, Ohio, Publication #77-157-A. P and CAM 118, 125. (2) Intersociety Committee,"Methodsof Air Sampling and Anabsis," American Public Health Association, Washington, D.C., 1972, pp 187, 194, 199. (3) C. F. Nightingale and J. M. Walker, Anal. Chem., 34, 1435 (1962). (4) W. M. Graven, Anal. Chem., 31, 1197 (1959). (5) A. Amano and M. Uchiyama. J . Phys. Chem., 67, 1242 (1963). (6) R. M. Barrer, F. W. Buttiiude, and J. W. Siltherland, Trans. Faraday SOC., 53, 1111 (1957). (7) R. M. Barrer and J. W. Sutherland, Proc. R. SOC.London, Ser. A , 237, 439 (1956). (8) R. M. Barrer and P. J. Reucrott, R o c . F?. SOC.London, Ser. A , 258, 431 (1960). (9) H. Lee, Adv. Chem. Ser., 121, 311 (1973). (10) R. A. Anderson, "Mol Sieves II", ACS Symp. Ser., 40, 637 (1977). (11) P. B. Venuto and P. S. Landis, Adv. Catal., 18, 259 (1968). (12) M. L. Poutsma, "Zeolite Chem. and Catalysis", ACS Monogr. 171, 529 (1976). (13) R. M. Barrer and D. W. Brook, Trans. f;araday SOC.,49, 940 (1953). (14) 0 . Grubner, A. Zikanova, and M. Ralek, J . Chromatogr.,28, 209 (1967). (15) 0. Grubner, Adw. Chromatogr., 6, 173 (1968). (16) 0. Grubner and D. Underhill, Sep. Sci., 5 , 555 (1970).
RECEIVED for review June 2 2 , 1978. Accepted August 8, 1978. Supported by NIOSH Grant 5 R 0 1 OH 00369-05, National Fire Protection and Control Administration Contract NFPCA 7X 008, and the Society of Plastics Industries.
