Anal. Chem. 1981, 53, 837-840
837
Determination of Sulfates in Diesel Particulates Dennis Schuetzle," Loretta M. Skewes, Gerald
E. Fisher, Steven P. Levine,
and Robert A. Gorse, Jr.
Analytical Sciences & Fuels and Lubricants Departments, Engineering and Research Staff- Research, Ford Motor Company, Dearborn, Michigan 48 12 1
Analytical techniques for the determination of total sulfates in dlesel particulates are! described utillzing the technlques of total combustion/nondlspersive infrared (NDIR) detection and modifled extraction/barlum perchlorate-thorln titration (BPT). The combustion technique utillzes a commercially available sulfur analyzer (LECO-lIR32) that requlres less than 15 mln for tripllcate analysls aind has a preclsion and accuracy of better than f 5 % for fillter samples containing from 100 to 10 000 pg of sulfate. The combustlon technique measures soluble and Insoluble sulfate such as BaSO, and is not affected by the presence of large quantities of carbon In the partlculate sample. The extraction efficlency for sulfates in diesel partlculates using standard procedures was found to average 67% for samples contalning only trace quantlties of anions and 59 % for saniples Containing stolchlometrlc levels of barium. These procedures have been modlfled to Include ultrasonication wlth 2-piroponaVwater in order to overcome the problem of incomplete sulfate extractlon. Extracted sulfate samples are then analyzed by using the BPT method. The extraction/BPT method showed an average recovery of 1.02 f 0.12 ( l o ) when compared to the combustlon technlque. ESCA studies shlowed that the chemlcal state of the surface sulfur species for the dlesel partlculates averaged 93 f 3 % sulfate and 7 f 3 % elemental sulfur.
There is increasing interest in the use of diesel engines as an alternative to gasoline powered internal combustion engines. However, the mass of particulate emissions from diesel engines are from 30 to 80 times higher than those from current catalyst-equipped spark. ignition gasoline engines (1,2). Diesel fuel contains from 0.1 to 0.5 wt % sulfur which is mainly converted to sulfur dioxide upon combustion (3). A portion (1-2%) of the sulfur is converted to sulfate and is present as a minor constituent in the particulates (4-7). The use of particulate abateinent hardware such as traps or catalytic control devices may convert a larger portion of the SO2 to sulfate. A number of techniques have been developed for the analysis of total sulfates in particulate matter collected on filter media (8-10). X-ray fluorescence (11) has been used to determine total sulfur in particulates and may be used to determine sulfate levels quantitatively if all of the sulfur exists as sulfate. The most widely used techniques involve extraction of the sulfate and analysis with a variety of classical analytical techniques including turbidimetry ( l a ) ,titration with barium perchlorate to a thorin end point (BPT) (13,14),ion exchange with barium chloranilate (15),and ion chromatography (16). Some work has been published on the thermal pyrolysis or volatilization of the particulate sample and detection of the SO2 and SO3 produced i~nthe process. The total sulfur in particulates has been measured by heating the sample to 1100 "C and converting the aerosol sulfate to SOz for subsequent detection by flame photometry (17). This technique has also been used for determining total soluble sulfur from sulfate in aqueous extracts of particulate matter collected on filters (18).
These techniques have not been shown to be entirely suitable for the analysis of sulfates in diesel particulates. Diesel particulates contain appreciable amounts of organic and inorganic carbon which could interfere with the quantitative extraction of sulfates. Barium compounds are sometimes added to the fuel as smoke-suppressants resulting in the formation of barium sulfate in the particulate emissions (7). It is not possible to analyze for barium sulfate (mp 1580 O C ) using the current flash vaporization techniques since the samples are only heated up to 1200 "C. The purpose of this work was to determine the chemical state of sulfur species in diesel particulates and develop suitable techniques for the analysis of total sulfates. EXPERIMENTAL SECTION Particulate Collection. Light-duty diesel exhaust particulate samples were collected on 142 mm diameter quartz fiber filters by use of an exhaust dilution tube. The filters were allowed to equilibrate in a constant-humidityroom and weighed to determine total particulate mass. The filters were placed on an aluminum foil template underlayered with polystyrene and were then radially cut into six equal pie-shaped parts along the extended template lines with a stainless steel razor-sharp straight edge. Round, punched pieces are not recommended due to possible radial inhomogeneities of filter loading. Total Combustion Analysis. Particualte laden 142 mm diameter quartz or glass fiber filters were combusted in oxygen using a LECO, Inc., induction furnace. The one-sixth portion of the filter sample or blank was placed in a combustion crucible by folding the edges inward. An accelerator (Vanodisc-LECO No. 761-933 plus one scoop Lecocel) was added to provide high plate current for the rapid release of sulfur. The sample was then rapidly heated from room temperature to approximately 1600-2000 "C for 20 s. The SO2 evolved from the sample was measured by using a LECO Sulfur (IR32) nondispersive infrared (NDIR) analyzer with a heated dust and moisture trap modification (IR33). Primary calibration of the instrument was made by using standard steel samples containing known quantities of sulfur. Blank filter disks measuring 380 mm2 were cut from quartz or glass fiber filters with a stainless steel template. Standards were prepared by spiking the blank disks with known quantities of sulfuric acid or barium sulfate. Modified Extraction-Titration Analysis. The BPT titration technique previously reported ( 1 1 ) was modified for the diesel particulate sample analyses. The filter samples were placed in a 250-mL beaker containing 100 mL of a 1:l mixture of deionized water and 2-propanol. The beaker was placed in a Bransonic 52 sonicator for 1 h at room temperature. After sonication was complete, 5 mL of freshly prepared Amberlite CG120,100-200 mesh cation exchange resin was added. The filter samples were then digested at 60 "C for 3 h with occasional stirring. The samples were allowed to cool and then filtered through a Millipore filter apparatus using a 5-pm Metricel GA-1 filter. ESCA Analysis. ESCA or X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical composition of the outermost 20-50 %, of the particulates using a Vacuum Generators ESCA 3 spectrometer. RESULTS AND DISCUSSION Initial work from this laboratory on the use of extractiontitration techniques for the analysis of sulfates in diesel particulates showed that there were significant variations in
0003-2700/81 /0353-0837$01.25/0 0 1981 American Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 6 , MAY 1981
Table I. A Comparison of Analytical Techniques for Sulfates in Diesel Particulates
sample no,
total combustiona
technique AC
489 460 477 459 26 2 261 276
357 4 99 517 673 6535 7750 86 20
231 (-35) 412 (-17) 293 (-43) 563 (-16) 3472 (-47) 4700 (-39) 5536 (-36)
sulfate found. u~g extraction-titration b technique Bd technique Ce 301 (-16) 460 (-8) 465 (-10) 632 (-6)
326 (-9) 479 (-4) 462 (-11) 692 (+ 3)
technique D f 323 (-10) 484 (-3) 458 (-11)
a Total combustion by LECO. Number in parentheses is percentage difference from total combustion technique. Technique A: HzO/2-propanol( 1 : l ) extraction at 60 "C for 3 h. Technique B: HzO/2-propanol( 1 : l ) extraction with sonication at 25 "C for 3 h. e Technique C: technique A followed by technique B. f Technique D: technique A followed bv 2.moDanol extraction with sonication at 60 "C for 3 h.
Table 11. A Comparison of Standard Vs. Measured Sulfuric Acid Levels as Determined by Total Combustion standard, measured,a' standard, pgof s pgof s Clg of s 8 9 + 2.0(+13) 160 16 32 64 80 96 112 128 144
1 5 + 1.8(-6) 31 + 3.4 (-3) 63 f 2.5 (-2) 74 f 8.1(-8) 9 5 + 4.8(-1) 115+6.9(+3) 129 rt 7.7 ( t l ) 147 + 4.4 ( t2)
192 224 26 1 272 290 320 1000
measured,asb fig of s 166 f 194 f 227 f 273 f 302 f 256 rt 343 + 1000 f
6.6 ( t4 ) 1.9 ( t1 ) 2.3 ( t1) 5.5 ( t5) 3.0 ( t11) 7.7 (-12) 3.2 ( t7 ) 24.0 (0)
a Averages for triplicate analysis are given. All measured values are blank corrected for response equivalent Number in parentheses is to 1 5 f 2.0 pg of sulfur. percentage difference from standard added.
sulfate recovery depending upon the techniques utilized. Table I summarizes the quantity of sulfate found in seven diesel particulate samples. The total combustion technique gives the highest values as anticipated since this technique measures all sulfur compounds that may be present in the sample. The extraction-titration technique commonly used for particulate emissions from catalyst equipped gasoline powered vehicles (9) gave low values (-33% average) for the seven diesel particulate samples. It was found that sonication of the filter sample at 25 O C in a 1:12-propanol-water mixture greatly increased the extraction efficiency of sulfate (Table I, techniques B and C). Utilization of techniques B, C, and D resulted in the extraction of 90, 93, and 92% (respective average) of the sulfate as compared to the results obtained from the total combustion technique. Further work was undertaken to determine the source of the 8% (average) discrepancy between the total combustion and these three techniques (B, C, and D). T o t a l Combustion Analysis. Work was undertaken to determine the accuracy, precision, sensitivity, and dynamic range of the combustion method and to compare results with those obtained by the extraction-BPT method (with and without sonication during extraction). A number of standards were prepared by spiking various amounts of sulfuric acid on quartz fiber filters. These samples were analyzed by use of the total combustion analysis technique (Table 11). The limit of detection using this technique when using whole 380-mm2 filter disks is approximately 30 pg of sulfate (as sulfur) at a signal/noise ratio of 2/1. The precision of the technique is approximately 4% for sulfate (as sulfur) at levels above 50 pg. These values are improved when using larger samples such as the 2670 mm2 (1/6 pie section) of the 142-mm filter.
Table 111. The Effect of Carbon on the Detection of Sulfate by the Total Combustion Procedure amt of sulfur, pg standard determined 0 0 0 32 32 32 64 64
7+2 172 2 33+ 5 32a 31'" 34a 64 a 60'
carbon added,b mg 0 100 200 0 100 200 0 100
a Corrected for blank value. Pulverized graphitic carbon was obtained from Ultra Carbon Corp., P.O.Box 747. Bas Cits. M I 48706.
The concentration of carbon (organic and elemental carbon) species in diesel particulates may be nearly 2 orders of magnitude greater than that of sulfate. Carbon may produce a reducing environment during the thermal conversion of sulfate to SOz during the combustion analysis and thus change the equilibrium concentraiton of S02/S03. Therefore, experiments were undertaken to determine the effects that carbon might have on the results obtained by the total combustion technique. Table I11 summarizes the effect of varying amounta of carbon on the analysis of sulfuric acid spiked on glass fiber filters. Pulverized graphitic carbon was analyzed initially to determine the blank values. The graphitic carbon was found to contain an average of 120 ppm of sulfur. The standard determinations were corrected for the blank values which corresponded to 7, 17, and 33 pg for 0, 100, and 200 mg of carbon added. I t can be concluded from these results that carbon has no significant effect even when present at several thousand times the concentration of the sulfate species. Similar results were found when carbon was added as an aliphatic hydrocarbon (C24). I t was not known if certain salts of sulfate such as barium could be quantitatively determined using the combustion technique, since barium sulfate has a melting point of 1580 "C. Barium sulfate was spiked (by weighing) a t various levels on the filter and analyzed as previously described. The measured and standard values were found to agree to within *2.0% ( l u ) as given in Table IV. Thus, barium sulfate can be analyzed with good accuracy by using this technique. The addition of excess barium (as barium carbonate) to the standard H2S04spiked filter did not have an effect on the results (Table V). The possible effect of the presence of moisture on the sample, which may cause sulfuric acid aerosol formation and possible loss in the LECO analyzer, was investigated. Two parts of water were added to one part of sulfuric acid (= 1000
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
Table IV. A Comparison of Standard Vs. Measured Barium Sulfate Levels as Determined by Total Combustion amt of sulfate, pg standarda measd
132 207 234 267 306 495 561 783 870
120 201 222 267 294 498 558 819 906
% difference
-3.7 -2.7 t 1.1 -2.8 c2.5 + 1.0 +2.7 -1.1 -0.4
a Barium sulfate was weighed directly into the combustion crucible by use of a imicrobalance in a humidity conCaltrolled room, Average of two determinations. calculated from linear regression. Slope: measured ( Y ) / standard (X)= 1.07;Y intercept = -25.1;correlation coefficient = 0.9996.
Table V. The Analysis of Sulfuric Acid by Total Combustion in the Presence of Barium Carbonate amt of sulfur, pg standard measda
amt of BaCO, added, pg
30 30 30
1392 6960
av
32 32 32
59 63 61
1392 6960
av
64 64 64
128 128 128 128 128 128 128 av 128
134 129 129 129 125 128 130 129
744 726 1470 1432 3480 3638 4820
3 20 320 320 3 20 3 20 320 320 320 av 320
352 346 349 3 51 339 341 319 332 341
770 712 1488 1594 3732 3436 7252 7072
839
Table VI. The Quantity of Sulfate in Diesel Particulates as Determined by Combustion and Modified Extraction-Titration Techniques sample no. 1143 1142 835H 1144 489 735c 460 477 809C 825H 807C 804C 867C 1157 C-3-1 728C 730C 721C 725C H3-4-1 H3-2
amt of sulfate,a pg com bustion extraction(Y ) titration ( X ) 125f 21 160 i: 0 19Of 14 390f 5 210 f 0 1901:14 340 f 0 390 f 0 357 f 15 317 f 13 770 f 100 443 f 20 499f 25 474 i: 13 517 rt 35 462 f 4 520 f 0 480f 14 710f 14 790 f 42 539 f 141 900f 57 810f 93 900 f 110 1450 i: 140 1160 f 85 1505f 35 1775 f 7 2435 f 320 2470 f 150 2503 i: 21 2840 rt 240 2952 rt 493 3010 f 10 3473f 262 3720f 57 3659 f 340 3580 f 99 8055 f 7 7600f 28 7333 f 115 6860 f 99
a Linear regression fit for X vs. Y gives: slope ( Y / X )= 1.06 i: 0.02;Y intercept = -134 ?: 56,and correlation coefficient = 0.997.
100
80
i!
60
>
e 5
40
f
20
a All measured values are blank corrected for response equivalent to 15 i: 2.0 pg of sulfur.
pg) and analyzed. A 50% recovery of sulfuric acid was found. However, no loss of sulfuric acid was noted for filter samples equilibrated a t constant humidity (45%). Comparison of Techniques. In addition to the data presented in Table I, a number of diesel particulate samples were collected for the purpose of further comparing the combustion vs. titration techniques. Each 142-mm filter sample was split into six equivalent sections, thus providing triplicate samples for the combustion and extraction-titration techniques. The result of these analyses are presented in Table VI. A linear regression analysis of the entire 22-point data set gives a slope of 1-06,an iintercept of -134, and a correlation coefficient of 0.997. The ratio of results from the titration vs. combustion techniques was calculated and found to average 1.02 f 0.12 ( l g ) . Three of the samples, no. l142,735C, and 807C gave abnormally high ratios (2.05,1.74, and 1.67) which
0 148.00
165.00
181.00
B I N D I N G ENERGY (ob')
Figure 1. Sulfur region of ESCA spectrum for diesel exhaust particulate sample no. 460 (Table I).
are greater than 5 standard deviations from the normal distribution of data. The reason for these anomolous results is not known. The linear regression analysis excluding these three points gives a slope of 1.05, an intercept of -90, and a correlation coefficient of 0.997. Therefore, it can be concluded from the above that the two techniques gave comparable results. Chemical State of Sulfate. The chemical state of the sulfur species present on the surface of diesel particulates was determined by using electron spectroscopy for chemical analysis (ESCA). The actual binding energy of the sulfur state on the surface was referenced with respect to the carbon (1s) spectrum a t 285.0 eV and compared to previously reported literature values (16). The only sulfur states detected on the surface were S6+ (sulfate) and So (elemental sulfur). SO2 (i.e., S4+) may not have been detected since ESCA is performed under high vacuum. Analysis of two samples (Table I, no. 460 and 477) showed that 10 f 2% of the total surface sulfur species was present as elemental sulfur. These results can be compared to the 8 f 3% (average) discrepancy between the two tech-
840
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
niques. Figure 1 shows the ESCA spectrum for sample no. 460. For this data set, it appears that the high sulfur values given by the total combustion technique as compared to the extraction-titration technique may be due to the presence of sulfur. The assumption made in this analysis is that the surface composition is similar to that of the bulk composition. ESCA analysis was employed to help explain the possible discrepancies between the combustion and extraction analysis results presented in Table VI. In this case, ESCA analysis for three of the samples (no. 804C, 728C, and 730C, Table VI) showed that 6 f 2% of the total sulfur was present as elemental sulfur. However, the extraction technique gave slightly higher results than the combustion technique for these three samples. Unfortunately, ESCA analysis of the other 12 particulate samples could not be undertaken because of the light particulate loading which resulted in interference of the sulfur (2p) spectra by the silicon (2s) spectra (from the quartz filter). ESCA analysis was used to determine the molecular composition of sulfate species in the cases where barium was present in the fuel as an additive. The ESCA analysis showed that all the sulfate was present as barium sulfate since the barium to sulfur ratio was 1.00 f 0.05. The elemental carbon to sulfur ratio for one of these samples was approximately 25/1. These results show that the surface composition given by the ESCA analysis was a reasonably good approximation of the bulk composition. The formation of elemental sulfur in flames has been previously confirmed by spectroscopic measurements, and similar mechanisms may explain the origin of this species in the diesel emissions. Previous ESCA (19) and mass spectrometric (20) studies have identified elemental sulfur in atmospheric particulates. Although the surface analyses were not correlated with bulk sample assays, this work suggesh that combustion processes may be a possible source for some of the elemental sulfur found in atmospheric particulates.
CONCLUSIONS The problem of incomplete extraction of sulfate from diesel particulates is overcome by extracting with a mixture of 1:l 2-proponallwater with sonication for 3 h. A number of analysis techniques then may be used to determine accurately total sulfate in the extract. The combustion technique accurately measures total sulfur (as sulfate) in diesel particulates even in cases when the sulfate
is present as barium sulfate. The presence of large concentrations of carbon in the particulate does not affect the accuracy of the measurement. The slightly higher sulfur (as sulfate) values given by the combustion technique when compared to the extraction/ titration technique in some cases may be accounted for by the presence of sulfur in the diesel particulates.
ACKNOWLEDGMENT The authors gratefully acknowledge the helpful suggestions of W. R. Pierson and E. L. Stokes and the ESCA spectra determinations by J. Hammond and J. Devries. Thanks is due to T. J. Truex who collected the samples.
LITERATURE CITED (1) Springer, K. J.; Baines, T. M. SA€ [Tech. Paper] 1977, 770818. (2) Barth, D. S.;Blacker, S. M. J. Air Pollut. Controi Assoc. 1978, 28, 769-771. (3) Hare, C. T.; Bradow, R. L. SA€ [Tech. Pap.] 1979, 790479. (4) Frisch, L. E.; Johnson, J. H.; Leddy, D. (3. SA€ [Tech. Pap,] 1979, 790417. (5) Funkenbusch, E. F.; Leddy, D.G.; Johnson, J. H. SA€ [Tech. Pap.] 1979, 790418.
D. E. fnviron. Scl. Techno/. 1980, 14, 1118-1121, (7) Truex, T. J.; Pierson, W. R.; McKee, D. E.; Shelef, M.; Baker, R. E. Environ. Scl. Techno/. 1980, 14, 1121-1124. (8) Tanner, R. L.; Newman, L. J. Alr. Poilut. Control Assoc. 1976, 26, (6) Truex, T. J.; Pierson, W. R.; McKee,
737-747. (9) Forrest, J.; Newman, L. J. Air Pollut. Control Assoc. 1973, 23, 781-768. (10) Katz, M. J. Air Pollut. Control Assoc. 1980, 30, 528-557. (11) Gilfrich, J. V.; Burkhalter. P. G.: Birks, L. S. Anal. Chem. 1973. 45. 2002-2009. (12) Coleman, R. L.; Shults, W. D.; Kelley, M. T.; Dean, J. A. Anal. Chem. 1972, 44, 1031-1034. (13) Butler, J. W.; Locke, D. N. J. fnviron. Scl. Hesnh Part A 1976, A 1 iflI.,, 79-92 - -(14) Budesinksy, Krumlova, L. Anal. Chim. Acts 1987, 39, 375-381. (15) TeJada, S. B.; Sigsby, J. E., Jr.; Bradow, R. L. I n “Analytical Proce\
dures for Characterizlng Unregulated Pollutant Emissions from Vehicles Using Middle-Distillate Fuels”; Interim Report, 1980,Environmental Protection Agency Report No. 60012-80-068. (18) Cadle, S. H.; Nebel, G. J.; Wllllams, R. L SA€ [Tech. Pap.] 1979,
790694. (17) Roberts, P. T.; Friedlander, S. K. Atmos. fnviron. 1978, 10, 403-408. (18) Husar, J. D.; Husar, R. 8.; Stubits, P. K. Anal. Chem. 1076, 4 7 , 2062-2065. (19) Craig, N. L.; Harker, A. B.; Novakov, T. Atmos. Environ. 1974, 8 15-21. (20) Schuetzle, D.; Crittenden, A. L.; Charbon, R. J. J. Air Poiht. Control Assoc. 1973, 23, 704-709.
RECE~VED for review October 20,1980. Accepted January 26, 1981.
