Environ. Sci. Technol. 1005, 19. 831-835
Haines, T. A. Trans. Am. Fish. SOC.1981,110, 669-707. Muniz, I. P.; Leivestad, H. Ecol. Impact Acid Precip. Proc. Int. Conf. 1980, 320-321. Baker, J.; Schofield, C. Water,Air, Soil Pollut. 1982,18, 289-309. Tandjung, D. S. Ph.D. Thesis, Fordham University, Bronx, NY, 1982. Lowe, T. P.; May, T. W.; Brumbaugh, W. G.; Kane, D. A. Arch. Environ. Contam. Toxicol. 1985, 14, 363-388. Shrader, D. E.; Voth, L. M.; Covick, L. A. Am. Lab. 1983, 15 (8)) 66-70. Zief, M.; Mitchell, J. W. "Contamination Control in Trace Element Analysis";Wiley-Interscience: New York, 1976; p 85. Julshamn, K.; Anderson, K.; Willasen, Y.; Braekkan, 0. R. Anal. Biochem. 1978,88, 552-559. Slavin, W.; Manning, D. C.; Carnricki, G. R. Anal. Chem. 1981,53, 1504-1510. Meinert, D. L.; Miller, F. A,, 111; Ruane, R. J.; Olem, H. A. TVA, Chattanooga, TN, 1981, Review of Water Quality Data in Acid Sensitive Watersheds Within the Tennessee Valley, Project Completion Report. Olem, H., Tennessee Valley Authority, Chattanooga, TN, unpublished data, 1983. Krishnan, S. S.; Gillespie, K. A.; Crapper, D. R. Anal. Chem. 1972,44,1469-1470. Manning, D. C.; Slavin, W.; Carerick, G. R. Spectrochim. Acta, Part B 1982,37B, 331-341. American Chemical Society Subcommittee on Environmental Analytical Chemistry Anal. Chem. 1980, 52, 2242-2249. Flegal, A. R.; Martin, J. H. Mar. Pollut. Bull. 1977,8,90-91. Buergel, P. M.; Soltero, R. A. J. Freshwater Ecol. 1983,2, 37-44. Kosta, L. Talanta 1982,29, 985-992.
Table VI. Precision of Duplicate Aluminum Determinations on Smallmouth Bass Carcass Samples sample no. 0876 0877 0878 0879 0880 0881 0882 0883 0884 0885
aluminum concn, Mg/g wet weight run 1 run 2 meann 4.6 2.2 1.4 1.2 2.2 10.8 2.0 1.2 2.1 1.1
5.3 2.5 2.3 1.2 2.6 9.6 2.4 1.0 2.7 1.3
5.0 2.4 1.8 1.2 2.4 10.2 2.2 1.1 2.4 1.2
Mean concentration f SD = 3.0 f 2.8 pg/g. ence & SD = 18 f 12%. (I
% differenceb (A/? X 100)
14 13 49 0
17 12 18 18 25 17 Mean % differ-
Acknowledgments We thank Mike Van Den Avyle and personnel of the Georgia Cooperative Fishery Research Unit for collecting the samples, Ray Wiedmeyer of the analytical support group at the Columbia laboratory for assistance in the digestion study, and US.Fish and Wildlife field station leaders James Wiener, La Crosse, WI, and Parley Winger, Athens, GA, for their helpful comments. Registry No. Al, 7429-90-5.
Literature Cited (1) Dickson, W. Ecol. Impact Acid Precip. Proc. Int. Conf. 1980, 75-83. (2) Schindler, D.; Hesslein, R.; Wagerman, R.; Broecker, W. Can. J.Fish. Aquat. Sci. 1980, 37, 373-377. (3) Almer, B.; Dickson, W.; Ekstrom, C.; Hornstrom, E.; U. Miller Ambio 1974, 3, 30-36.
Received for review August 13,1984. Revised manuscript received March 15, 1985. Accepted April 12, 1985.
Effect of Environmental Factors on Filter Alkalinity and Artifact Formation Samuel Wltz South Coast Air Quality Management District, El Monte, California 91731
Artifact sulfate formation, estimated from filter alkalinities determined by a hot water extraction method (ASTM D-202)) may be low compared to experimental values. This arises from the fact that the alkalinity available for reaction with acidic atmospheric gases depends on the extent of exposure to heat, moisture, and acidity. The rate of alkali diffusion to the filter surface increased with increasing acidity of the extraction medium and increasing filter pH. The leaching rate exhibited a positive temperature coefficient which became less pronounced with decreasin filter pH. The reaction kinetics showed a linear (time)l dependence, suggesting a diffusion-controlled mechanism. Exposure of seven different filters, with pHs of 5.9-9.9, to an aqueous environment of pH 2 for 2 h at ambient temperature resulted in artifact formation in all cases, with the extent of artifact decreasing with decreasing filter pH.
B
Introduction Artifact formation resulting from sorption by alkaline glass-fiber filters of acidic precursors such as SO,, NO,, and HN03 with subsequent conversion to particulate sulfate and nitrate has been well documented (1-6). This positive bias in the sulfate and nitrate measurements can 0013-936X/85/0919-0831$01.50/0
result in higher total suspended particulate (TSP)values as well. In one parallel field study comparing glass-fiber filters of pH 6.5 and 11,the more alkaline filter exhibited an average increase of 18% in TSP, 40% in sulfate, and 60% in nitrate (6). Artifact formation thus makes it difficult for those engaged in pollution trend analysis or checking compliance with existing state and federal air quality standards such as those for sulfate and TSP. The basicity of glass-fiber filters is due to alkali or alkaline earth metal compounds (oxides of salts) that are either present in the original formulation or added in posttreatment either to assist in the processing of the fibers or to improve their chemical resistance or physical properties. For these filters the alkaline content can constitute 1-10 wt 9%. The high purity silica or quartz filters generally have less than 1 wt % of these alkaline salts. A chemical model (eq 1for estimating the amount of artifact
-
S042- (pg/m3) =
%[
0.25
+ (1.57 X
X
sulfate formed by the absorption of ambient SO2 on the
0 1985 American Chemical Society
Environ. Sci. Technol., Vol. 19, No. 9, 1985 831
Table I. Comparison of Observed vs. Theoretical Artifact Sulfate
filter type Whatman EPA 1000‘ (glass) S & S 197gb (glass) Whatman QM-Ab (quartz) Pallflex QASTb (quartz)
artifact sulfate, 4 m 3 theor theor obsd/theor obsd (CoutantJd satn Coutant 10.9
6.6
5.2
1.7
2.1
15.0 5.2
11.9 0.72
7.5 0.44
1.3 7.2
2.0 11.8
1.5
0.15
0.09
10.0
16.7
a Based on parallel field sampling comparing the Whatman against a Pallflex tissue quartz (2500 QAO) filter (4). bBased on SOz sorption studies by Appel (8). CBasedon reaction of filter alkalinity with equivalent amount of SOz and subsequent oxidation to sulfate. dBased on Coutant’s chemical model (1).
moist basic surface of a filter has been proposed by Coutant ( I ) , where F = air volume per unit area (m3/cm2)in a 24-h sample, A = filter alkalinity (pequiv/cm2), T = temperature (K), P = partial pressure of SO2 (ppb), RH = relative humidity (decimal fraction), and 2 = ratio of S(IV) to concentration of metal ion leached from filter into a sorbed water layer. In this equation, the amount of artifact sulfate formed is directly proportional to the filter alkalinity available for absorption of ambient SOz. Filter alkalinity was determined by Coutant using an ASTM (D-202)procedure in which a hot water (-100 “C) extract of the macerated filter is titrated with standard acid. Under normal Hi-vol sampling conditions the artifact sulfate is generally in the range 0.3-3 pg/m3. While Coutant’s chemical model is basically correct and gives reasonable estimates of the artifact sulfate formed under normal operating conditions, there is some evidence that the artifact calculated by using eq 1tends to be low compared to observed values (1,2,4). Illustrative data are shown in Table I. The theoretical values shown were calculated by two different methods, one by using Coutant’s model and the other a limiting case in which it is assumed that all of the filter alkalinity is taken up by reaction with an equivalent amount of SO2 (with subsequent oxidation to sulfate) or H2S04 to form artifact sulfate, viz. artifact SO-: (pg/m3) = (AE X 48)/V (2) where A = filter alkalinity (pequiv/cm2), E = effective filter area (cm2),48 = 1 pequiv of sulfate (pg), and V = 24-h sample air volume (m3). The latter case should provide an estimate of the maximum amount of artifact sulfate that could be formed for a particular filter. The observed values in the table are based on the artifact formed either by a field comparison of a glass and quartz filter or by laboratory sorption studies in which SOz is passed through a filter at varying concentrations, temperatures, humidities, and flow rates. An examination of the ratios of observed/theoretical values shows that in all cases they are greater than unity. Since the artifact sulfate is directly proportional to filter alkalinity, this would suggest that the ASTM hot water extraction method may, under certain sampling conditions, underestimate the amount of surface alkali available for reaction. The ratios are also seen to be much higher for the quartz than for alkaline glass-fiber filters. The present study was undertaken in an attempt to find an explanation for this inconsistency, as well as the reparted anomalous observation of artifact formation with near-neutral or acid quartz filters (7,8).The result of this 832 Environ. Sci. Technol., Vol. 19, No. 9, 1985
study also served to explain why small differences in filter pH can lead to substantial differences in artifact formation, as recently reported by Witz et al. (5). Since the magnitude of the artifact is directly proportional to filter alkalinity, the influence of several variables on the latter parameter was also investigated.
Experimental Section Filter alkalinity was determined by pH measurements using an EPA procedure (9) and by titration of aqueous filter extracts at ambient or elevated temperatures (ASTM D-202). For the EPA procedure, a 3 in. X 3 in. filter aliquot was extracted in 15 mL of aqueous 0.05 M KCL for 10 min at ambient temperature. In the ASTM method for determination of alkalinity, a l-g filter aliquot was macerated by stirring in 100 mL of pH 7 water for 5 min at -100 “C and filtered rapidly and the filtrate titrated with standard acid. The filter alkalinity is expressed in microequivalents of alkali per gram of filter, unless otherwise indicated. All other determinations of filter alkalinity not employing the ASTM procedure were made under conditions specified, typically using 0.6 g of filter in 60 mL of aqueous media. The seven filters examined in the present study included Schleicher & Schuell’s (S & S) 1*HVand Whatman’s EPM 2000 glass-fiber filters used in the NAMS and SLAMS networks in 1981 and 1982, respectively, Pallflex’s Teflon-coated glass-fiber filter T60A20, and four quartz-fiber filters (Pallflex’s 2500 QAST and 2500 QAO and Whatman’s QM-A and QM-B). Results Filter Alkalinities as a Function of Extraction Procedure. The alkalinities of the various filters examined in the present study are summarized in Table 11. The alkalinities in the upper half of the table were obtained by the EPA and ASTM procedures. The average values for pH determined by the EPA method range from about 5.9 to 9.9 and follow the same order as the alkalinities determined by the ASTM procedure. The last four filters exhibit an alkalinity of less than 5 pequiv/g, the tentative maximum selected by the EPA and CARB for their specifications of an acceptable filter. However, in view of the wide differences in filter weights, the total alkalinity might be better defined if the alkalinity were expressed as pequivalents per total exposed filter area rather than pequiv per gram of filter. To obtain the values for alkalinity shown in the lower half of Table 11, weighed filter aliquots (-0.6 g) were immersed in aqueous media (60 mL) of varying pH for 2 h at 23 f.2 “C under a nitrogen atmosphere. The variable pH was obtained by adding known amounts of H2S04or NaOH solution to pH 7 water and back-titrating an aliquot with standard acid or alkali after a 2-h extraction. For the S & S filter, results with HN03 and HC1 were identical with those obtained with H$04 (data not included). Data are shown for alkaline glass-fiber, quartz-fiber, and Teflon-coated glass-fiber filters with pHs ranging from 5.9 to 9.9. The “available”alkalinity, calculated from the amount of alkali extracted, is expressed in microequivalents per squared centimeter (rather than microequivalents per gram) to be consistent with the filter pH measurement which, according to the EPA procedure, is made with samples of equal area rather than equal weight. The results indicate that, in all cases, including the near-neutral quartz and acid filters, a substantial increase in available alkalinity occurs with increasing acidity of the extraction medium. I t is evident that, on exposure to an acid environment, all of these filters would be susceptible to artifact formation to varying degrees. An interesting phenomenon
Table 11. Effect of Extraction Procedure on Filter Alkalinity S&S 9.87 (S = 0.15)b 143 (S = 18)
filter pH" alkalinity, pequiv/gc filter weight, g alkalinity, pequiv/filterd
4.1 586
Whatman EPM 2000 9.40 (S = 0.11) 71 (S = 9.5) 4.3 305
Whatman QM-A 8.59 (S = 0.27) 23 (S = 0.4)
Whatman QM-B 7.70 ( S =
Pallflex QAST 7.69 (S =
Pallflex Tef-Ctd 6.57 (S =
0.05) 4.6 (S = 0.2)
0.17) 3.9 (S = 5.8)
0.11) 3.4 (S = 2.3)
4.3 99
8.4 39
3.2 12
32 18 1.4 (acid)
24 6.4 4.3 (acid)
14 3.7 1.9 (acid)
Pallflex QAO 5.89 ( S = 0.07) 36 (acid)' 3.4 122 (acid)
1.8 6
alkalinity, pequiv/cm2, X102f
a t extr pH 2 a t extr pH 7 a t extr pH 10.7
134 40 38
63 27 17
EPA method. * S is the standard deviation. "C for various pHs of the extraction medium.
1
ASTM-D202 procedure.
8 in. X 10 in. filter.
0 SSS,
2.3 0.3 (acid) 12.4 (acid)
10 0.1 3.2 (acid)
e Single
determination. f Two hours at 23
PH 7.0
0 WHAT OM-A,
PH 7 . 0
0 WHhT. EPR 2000
85
5
10
15
20
25
30
35
'
2 9
RECIPROCAL
YTiKiTr
Flgure 1. Filter alkalinity as a function of the square root of the leaching time (at 23 f 2 "C).
occurs in an alkaline medium with the filters of lower pH. For the quartz and Teflon-coated glass-fiber filters a reversal occurs at high pH with these filters displaying acidic properties (as indicated by a decrease in alkalinity of the extracting media). I t would appear that, in general, the lower the filter pH, the more readily this uptake of alkali by the filter occurs. Effect of Extraction Time on Filter Alkalinity. The effect of extraction time on the amount of alkalinity leached from the Whatman EPM 2000 and S & S filters on immersion in pH 7 water at 23 f 2 "C for times ranging from 5 min to 24 h is shown in Figure 1. An approximately linear relationship is observed in a plot of the available alkalinity vs. the (time)1/2. The slope of the line when Q, the alkalinity (in pequivlg), is plotted vs. (time)ll2 may be considered as a rate constant for the reaction. It is evident from Figure 1 that 5 & S, the more alkaline of the two (pH of S & S = 9.87 vs. pH of Whatman = 9.48), exhibited a steeper slope than the Whatman. It is to be noted that the difference in alkalinity between the two filters increased from 12 pequiv/g at 5 min to 27 pequiv/g after 24 h. The latter would correspond to 2.3 pg/m3 artifact sulfate for a 24-h Hi-vol sample. Effect of Temperature on Filter Alkalinity. The effect of the medium extraction temperature on the alkalinity of a quartz (Whatman QM-B) and glass-fiber filter (S & S) is shown in the Arrhenius plot of log Qt1I2 vs. the reciprocal of the absolute teinperature in Figure 2. For both types of filters the available alkalinity increases with temperature; however, the effect of temperature becomes less pronounced with decreasing filter pH. The temperature coefficients or apparent activation energies obtained
3 1
30
3 2
3 3
OF ABSOLUTE TEMPERATURE
3 4
3 6
3 5
'
x 103
Flgure 2. Relationship between alkalinity and extraction temperature for Schleicher & Schuell and Whatman QM-A filters at pH 7 for the extraction medium. Alkalinity is in microequivalentsper gram and time is in hours.
from the slopes of these plots indicate a lower value for the Whatman QM-A quartz than the S & S glass-fiber filter.
Discussion Filter Alkalinity. The pH dependence for the extraction of alkali observed with the filters in the present study (lower half of Table 11) is consistent with the results obtained by others in their investigation of the hydrolysis of alkali-containing silicate glasses by aqueous solutions (10-13). When a soda-silica glass is exposed to an aqueous environment, two main reactions occur on the surface: (1) extraction of alkali from the glass with replacement by H+ ion from solution and (2) the dissolution of the silica network due to splitting of the siloxane bonds on the glass surface by reaction with OH- ions. For solutions of pH Si-O-R
t Ht
.--)Si-OH
+
R+
(3)
It is evident that the position of the above equilibrium is determined by the pH of the solution and the alkali content of the glass. The equilibrium is shifted to the right with decreasing pH or increasing alkali content of the glass, in agreement with the results of the present study. Thus, the rate of alkali extraction was found to increase with decreasing pH of the solution and with increasing alkali content (Table 11) of the filter. Since the driving force for Environ. Sci. Technol., Vol. 19, No. 9, 1985
833
Table 111. Effect of Leaching Conditions on Artifact Sulfate Formation
leaching conditions ext pH t , "C time 7 2 2 2
100 23 23 43
5 min" 2 hb 24 hc 24 hd
Whatman EPM 2000
S&S 12.3 14.5 17.0 33.7
(5.9) (19.1)
6.4 7.0 8.6 14.6
estimated artifact sulfate, pg/ms Whatman Whatman Pallflex QM-A QM-B QAST 2.1 3.5
0.81 2.6
0.26 1.5
Pallflex Tef-Ctd
Pallflex QAO
0.13 1.1
(acid) 0.25
" ASTM D202 extraction conditions. Filter alkalinities used in estimating the artifact sulfate under these conditions shown in Table 11. bAlkalinities obtained from extraction at pH 2 used (see Table 11). CExperimentallydetermined alkalinities were 197 and 96 pequivlg for S & S and Whatman, respectively. dFilter alkalinities for S & S and Whatman were 391 and 162 pequivlg, respectively. diffusion of alkali would be the concentration gradient across the glass solution interface, in strongly basic solution and low filter alkalinity, uptake of alkali from solution by the filter is to be expected. This is what was actually observed for the quartz and Teflon-coatedglass-fiber filters (Table 11). A gellike hydrated layer (silica rich) is formed on the glass surface as the original Si-0-R bond is converted to a Si-OH bond. This process is believed to be diffusion controlled with the amount of alkali extracted proportional to the square root of time (10). Reference to Figure 1in the present study indicates that on exposure to an aqueous environment, the kinetics for the alkali extraction process of the indicated filters also exhibited a linear t1/2dependence. The longer the reaction time, the deeper the layer affected by the extraction process. Through the use of modern surface analytical techniques, it has been shown that the composition of a glass surface is usually significantly different from the bulk composition (14). The formation and stability of surface layers on glass depend not only on bulk composition but also on such factors as environment, fabrication variables, and thermal history. Thus, it was recently shown (11)that as little as a 5-min exposure of the clean bulk surface of a typical soda-lime glass to atmospheric moisture at room temperature resulted in migration of sodium and calcium from the interior to the glass-air interface where these cations formed a thin surface layer 50 A thick. The alkali-depleted layer extended for 500 A below this alkali-rich layer on the surface. Evidence was presented to show that the loss of alkali was due to a hydrogen ion-alkali exchange mechanism. Continued exposure of this glass to pure water for 1h a t 37 OC resulted in a silica-rich layer of nearly 1000 A in depth. In alkaline media (pH >lo), hydrolysis with splitting of the Si-0-Si is the dominant reaction. The hydroxyl ion serves as a catalyst, with the first step being adsorption of the OH- ion by the silicon. This increases the coordination number of the silicon atom on the surface to more than four, thereby weakening the oxygen bonds to underlying silicon atoms. Breaking of the Si-0-Si bonds then occurs with the silicon atoms going into solution as a silicate ion, Si(0H)C. If the pH is much below 11,the silicate ion hydrolyzes to soluble silica, Si(OH)4and OH-. This depolymerization process is repeated until the concentration of Si(OH)4 reaches a steady state in the depolymerization-polymerization equilibrium. Above pH l l , Si(OH)4is converted to silicate ions, keeping the solution unsaturated (with respect to Si(OH),) so that silica continues to dissolve (12). Whereas the process of alkali-H+ ion exchange under neutral or acid conditions exhibits a t1J2dependence, the process of network dissolution which takes place on exposure of a glass surface to an alkaline environment is controlled by an interface reaction with a linear t' dependence (11). The extraction of alkali ions also becomes linearly dependent with time as the reaction proceeds, and the pH of the solution exceeds a value of 834
Environ. Sci. Technol., Vol. 19, No. 9, 1985
approximately 10. Since no attempt was made to analyze for silica in the present study, data are not available to illustrate the time dependence for removal of silica from the filter surface under alkaline conditions. In this study, the extraction rate of alkali from the S & S filter was found to be independent of the type of acid used (Le., H2S04,HN03, and HCI). Whether this is true for the other filters was not investigated. For sodaglass-containing lime, it has been reported (15)that the extraction rate for calcium was found to depend on the type of anion present in solution, being greatest for chloride and nitrate ions (which form highly soluble salts with calcium) and least for sulfate ion (which forms a sparingly soluble salt with calcium).
Artifact Formation In contrast to Coutant's chemical model which regards available filter alkalinity as a fixed quantity in estimating artifact sulfate formation, results of the present study indicate that several environmental factors (e.g., heat, moisture, and acidity) can have a substantial effect on the magnitude of this parameter. It was shown that the ASTM hot water extraction method used by Coutant in determining filter alkalinity tends to underestimate the amount of artifact, particularly if the filter had been exposed to an acidic environment prior to analysis. It was found that contact of a filter with an acidic environment appears to increase the rate of migration of alkali from the interior to the surface. The greater the acidity of the liquid phase, the greater will be the alkali extracted from the filter in a given time. It was shown that on exposure to an acidic environment of pH 2 for 2 h at ambient (23 "C) temperature, even quartz and Teflon-coated glass-fiber filters with pHs ranging from 5.9 to 8.6 do exhibit some degree of leaching of alkali. Although uncommon, atmospheric environments of pH 2 do occur occasionally. At Corona Del Mar, a coastal site in the Los Angeles basin, an acid fog of pH 1.7 was recorded in Dec 1982 (16). There have been a number of instances of near pH 2 acid fog reported in the L.A. basin. An estimate of the artifact sulfate that may result from exposure of a filter to a condensed phase of varying acidity and temperature is shown in Table 111. The alkalinity, extracted under the stated conditions, was used to calculate the maximum amount of artifact sulfate which might form based on a Hi-vol operated at 45 cfm (1.27 m3/min) for 24 h. It was assumed that, as a limiting case, all of the available alkalinity would react with an equivalent amount of SO2 which was then oxidized to particulate sulfate. The amount of artifact sulfate was then estimated by using eq 2. The data shown on the first line of Table I11 indicate the amount of artifact sulfate that might form for each of the filters on the basis of a filter alkalinity determined by the ASTM hot water extraction method. The amount of artifact estimated for the Whatman and S & S filters is seen to be quite substantial but small or negligible for the quartz and Teflon-coated
glass-fiber filters. The data on the second line indicate the artifact that might form on exposure of the filter for 2 h to an acidic environment (pH 2) at ambient temperature. All show higher artifact values than those derived by using as a basis alkalinity determined by the ASTM method. The last two lines in Table I11 were included to show the effects of increased exposure time and temperature on the potential artifact. Referring to the values in parentheses, which represent the difference in artifact of the Whatman EPM 2000 and S & S filters, the increase from 5.9 to 19.1 ~ g / m serves ~ to illustrate that, under the combined influence of acidity, temperature, and exposure time, small differences in filter pH can lead to appreciable differences in artifact formation, as recently reported by Witz et al. (5). The results shown in Table I11 are consistent with our earlier findings that the “available” filter alkalinity increases with increasing acidity, temperature, and exposure time. They also help explain some of the observations noted in Table I, particularly the observed/theoretical ratios > 1 which are undoubtedly caused by leaching of alkali to the filter surface in excess of that determined by the ASTM hot water extraction procedure. In general, the higher observed/theoretical ratios displayed by the quartz relative to the glass-fiber filters are also indicated by the data in Table 111. As noted earlier while Coutant’s model based on alkalinities determined by the ASTM procedure is adequate for predicting artifact sulfate in most cases, it is deficient in not being able to specify the available alkalinity under all operating conditions. The discrepancy between observed and theoretical artifact becomes most evident in dealing with the more weakly basic filters. To truly define the amount of alkalinity available on the surface of a filter, the model would have to incorporate the effects of temperature and acidity on the diffusion rate of the various alkaline components (i.e., Na+, K+, Ca2+,Mg2+,etc.), assuming the composition of the filter medium was known. Formulating such a model and validating it under actual field conditions would present a formidable task, well beyond the scope of the present investigation. Finally, it should be noted that any volatile acidic component in the atmosphere may react with the alkali in the filter, and this could include SO,, HN03, HCl, COz,or even formic, acetic, or propionic acids. Salts of the latter organic acids have ben isolated by Stevens (7) from blank quartz filters. If all of the filter alkalinity were consumed by reaction with propionic acid, the amount of artifact formed would be about 50% higher than the values shown in Table 111. Summary and Conclusions The higher values often observed experimentally for artifact sulfate compared to theoretical estimates result from exposure to an acid environment which increases the rate at which alkali is leached to the filter surface. The leaching rate was found to be pH dependent, with the rate increasing with acidity of the extraction medium and with increasing filter pH. The rate of extraction exhibited a positive temperature coefficient, but the effect of temperature became less pronounced with decreasing filter pH. Kinetics for the alkali extraction process was linear with respect to the square root of time, suggesting a diffusioncontrolled mechanism. Exposure of seven different filters, with pHs ranging from 5.9 to 9.9, to an aqueous acid environment of pH 2 for 2 h at ambient temperature resulted in artifact in all cases, with the extent of artifact decreasing with decreasing filter pH. It was also shown that under the combined
influence of acidity, temperature, and exposure time small differences in filter pH can produce substantial differences in artifact formation. By use of 5 Mequiv/g maximum filter alkalinity as a basis for acceptability, only the Tefloncoated glass filter (Pallflex) and three of the quartz filters (Whatman QM-B, Pallflex QAST, and Pallflex QAO) were found tohmeet this requirement. However, the Pallflex QAST and QAO filters are quite brittle, and although they may be suitable for anion or cation analysis, they would require special handling for mass loading determinations. Of the two remaining filters, both appear to be satisfactory for mass loading measurements, with the Whatman QM-B obviously the filter of choice in determinations involving carbon (elemental, primary, and secondary). An alternative to the use of either the quartz or Teflon-coated glass-fiber filters would be a Teflon membrane filter. However, apart from the lower flow rates that would be required, negative artifact formation (i.e., loss of collected nitrate through volatilization or interaction with more acidic sulfates (17,18) would still remain a problem with this filter. Acknowledgments
The useful discussions with Margil W. Wadley are gratefully acknowledged. Registry No. S042-, 14808-79-8; quartz, 14808-60-7; Teflon, 9002-84-0. L i t e r a t u r e Cited Coutant, R. W. Enuiron. Sci. Technol. 1977,11,873-878. Appel, B. R.; Tokiwa, Y.; Wall, S. M.; Hoffer, E. M.; Haik, M.; Weslowski, J. J. Air and Industrial Hygiene Laboratory, California Air Resources Board, Contract No. ARB 5-1032, 1978, Final Report. Spicer, C. W.; Schumacker, P. M.; Kouyournjian, J. D. W. Battelle Columbus Laboratories, U.S. Environmental Protection Agency Contract No. 68-02-2213, 1978, Final Report. Witz, S.; Wendt, S. G. Enuiron. Sei. Technol. 1981, 15, 79-83. Witz, S.; Smith, M. M.; Moore, A. B. J. Air Pollut. Control ASSOC. 1983, 33, 988-991. Mueller, P. K.; Twis, S.; Sanders, G. 13th Conference on Methods in Air Pollution and Industrial Hygiene Studies, University of California, Berkeley, CA, Oct 1972. Stevens, R. C., U S . Environmental Protection Agency, Research Triangle Park, NC, personal communication, 1982. Appel, B. R., Air and Industrial Hygiene Laboratory, Berkeley, CA, personal communication, 1983. Rhodes, R. C., U S . Environmental Protection Agency, Research Triangle Park, NC, personal communication, 1982. El-Shamy, T. M.; Lewins, J.;Douglas, R. W. Glass Technol. 1972,13, 81-87. Hench, L. L.; Clark, D. E. J . Non-Cryst. Solids 1978,28, 83-105. Iler, R. K. “The Chemistry of Silica”; Wiley-Interscience: New York, 1979. Wright, B. W.; Lee, M. L.; Graham, S. W.; Phillips, L. V.; Hercules, D. M. J. Chromatogr. 1980,199, 355-379. Sanders, D. M.; Hench, L. L. J . Am. Ceram. SOC.1973,56, 373-377. El-Shamy, T. M.; Morsi, S. E.; Taki-Eldin, H. D.; Ahmed, A. A. J . Non-Cryst. Solids 1975, 19, 241-250. Hoffman, M. R. Enuiron. Sci. Technol. 1983, 17, 117A120A. Appel, B. R.; Tokiwa, Y.; Haik, M. Atmos. Enuiron. 1981, 15, 283-289. Forrest, J.; Tanner, R. L.; Spandau, D.; D’Ottavis,
