Anal. Chem. 1982, 54, 1871-1874
its use as a screening method would be justified. As can be seen from the data in Table IV, this method suffers from some background interference from bromine containing materials in control tissues. The nature of this material is also of interest, but the identification of this material is beyond the scope of this work. However, for this reason the NAA method alone would not be useful for determining residue levels of PBBs in environmental Eiamples, but a t higher levels as used in this validation work it is a useful ancillary method. These findings further emphasize the need to design and develop tissue residue methodology with attention to answering the basic question of how aclcurate are the results in addition to how reproducible and reliable is the method. Interlaboratory studies may show good reproducibility but this does not necessarily mean the reciults are "accurate". ACKNOWLEDGMENT The experimental portion of this project concerned with polybrominated biphenyl analysis was performed by Analytical Bio Chemistry Laboratories, Inc., Columbia, MO, and that concerned with total bromine analysis by the Department of Nuclear Engineering, North Carolina State University, Raleigh, NC.
1871
LITERATURE CITED Kay, K. fnviron. Res. 1977, 13 74. Hass, J. R.; McConnell, E. E.; Harvan, D. J. J . Agric. Food Chem. 1978, 2 6 , 94. Stratton, C. I.;Moussa, J. J.; Bursey, J. T . "Analysis for Polybrominated Biphenyls (PBB) in Environmental Samples"; EPA Publicatlon No. EP-560/13-79-001, March 1979. Burse, V. W.; Needham, L. C.; Liddle, J. A.; Bayse, D. D.; Price, H. A. J . Anal. Toxicol. 1980, No. 4 , 22. Gupta, B. N.; McConnel, E. E.; Harris, M. W.; Moore, J. A,, Toxicol. Appl. Pharmacol. 1981, 57, 99. Watts, R. W. "Manual of Analytical Methods for the Analysis of Pesticide Residue In Human and Environmental Samples"; Prepared by U.S. EPA Environmental Toxicology Divlsion, Section 5, A (1); (a) EPA 60018- 80-038, June 1980. Albro, P. W.; Corbett, 8. J. Chemosphere 1977, (7). 381. Fries, G. F.; Marrow, G. S.; Cook, R. M., EHP, €nv/fon. Health f e r spect. 1978, 23, 43. Fehrlnger, N. W., J. Assoc. Off. Anal. Chem. 1975, 58, 978. Albro, P. W. "Environmental Health Chemlstry", 1st ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Chapter 8. Albro, P. W. Ann. N . Y . Acad. Sci. 1979, 320, 19. Mes, J.; Campbell, D. S. Bull. fnviron. Contam. Toxicol. 1978, 76, 53. Folch, J. M., Lees, M.; Sloane-Stanley, G. H. J . Biol. Chem. 1975, 236, 497, Bligh, E. C.; Dyer, W. J. Can. J . Biochem. Physiol. 1959, 37, 911. ~
RECEIVED for review December 10,1981. Accepted June 11, 1982.
Charact erizait ion of MiddIe PetroIeuim Fractions by Nuc Iear Magnetic Resonance Spectrometry Jasenka Muhl,
Vlastn SrlEa, Branka Mimlca, and MlllvoJ Tomaikovld
INA-Industrija naffe, Research and Development, Zagreb, Yugoslavia
A method for characterization of petroleum fractions (220-500 " C ) Is presentisd. The structural1characterlstlc and the parafflnlc and aromirtlc hydrocarbon content are determlned on the bask of thle 'H NMR spectral data and bolllng range data. The method Is tested ON middle fracllons (220-390 "C), as well 21s on a number of Yugoslav and imported samples wlth wltler bolllng range and hlgher bolllng points (up to 500 "C). The aromatic carbon content was compared wlth the resullts obtalned by the I R method. The aromatlc and parafflnlc lhydrocarbon content was compared with the results obtalned by ASTM D-1319-70 and D-2549-68. The standard devlatlon and the difference of the average for the results obtalned by the NMR method were computed, as opposed to the results of the forementloned referent methods.
In the analysis of complex hydrocarbon compounds in petroleum with respect to the types of functional groups, the technique of NMR spectrometry has a very distinctive position, due to its specific characteristics. Numerous methods have been developed (I-ZO), based on the lH and 13C NMR spectrometry data. Any analysis is significantly improved by application of 13C NMR spectrometry, provided that all the conditions for obtaining quantitative results are satisfied ( I 1, 12). However, the NMlZ technique is rarely applied to hydrocarbon-type analysis of petroleum. A successful method of gasoline analysis is discussed (13). Von Deutsch (14) has established relationships by which the aromatic and paraffinic hydrocarbons content, as well as the carbon content in 0003-2700/82/0354-1871$01.25/0
functional groups, could be determined for petroleum fractions above 220 OC, using the lH NMR spectral data of the analyzed sample and of the aromatic fraction of examined petroleum. The obtained results were not compared in his paper with the results of any alternative method for verification purposes. In this paper, the relations established by Von Deutsch are used as the starting point, and a method for determination of aromatic and paraffinic (n-,iso-, cyclo-) hydrocarbons content, as well as the carbon content in functional groups, is developed. This is based, on one hand, on the lH NMR spectral data and, on the other hand, on boiling-range value of the analyzed fraction. The equations introduce also ring junction carbons, which are not derivable directly from the 'H NMR spectrum. This follows from the assumed structure of aromatic hydrocarbons in ]petroleum, taking into account the hydrogen content in condensed aromatic rings. The assumption is made that the average structure of aromatic hydrocarbons in various petroleum types does not significantly differ within a given boiling-point range. Therefore, the established dependence between a boiling-point range and the corresponding a-alkyl to L3,y-alkyl ratio is used to determine the portion of aromatic alkyls in the section of spectrum, which is overlapped by corresponding paraffinic signals of the analyzed fraction. The method is tested on middle fractions (220-390 "C) of Yugoslav petroleum with narrow boiling point range, as well as on a number of Yugoslav and imported samples with wider boiling range and higher boiling points (up to 500 OC). The aromatic and paraffinic content is compared with the results obtained by standard liquid chromatography methods (ASTM 0 1982 Amerlcan Chemical Society
1872
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
D-139-70 and ASTM D-2549-68). The aromatic carbon content is compared with the results obtained by IR spectrometry. Also, the reproducibility of the proposed NMR method results is considered and tested. THEORY The proton spectra of petroleum fractions were divided into four regions (A, B, C, D) for the following functional groups: aromatic ring (A), 6.5-9.0 ppm; a-alkyl to aromatic ring (B), 1.8-3.8 ppm; P,yCH,CHz to aromatic ring and paraffinic CH,CHz (C), 1.0-1.8 ppm; P,yCH3to aromatic paraffinic CH3 (D), 0.5-1.0 ppm. The areas of these regions were denoted HA-D. The aromatic resonances area consists of monoaromatic ring proton signals (A' from 6.5 to 7.05 ppm) and condensed aromatic ring proton signals (A" from 7.05 to 9.0 ppm) (3, 8 ) , therefore
HA = HA,
+ HA,,
(1)
The aromatic ring carbons are substituted either by hydrogen atom or by alkyl groups, or they are ring junction carbons. In the first case, they are equivalent to aromatic hydrogen (HA). In the second case, they are equivalent to a-alkyl hydrogen divided by 2.5 (H~/2.5).This is true because, on the average, 2.5 a-alkyl hydrogens correspond to one a-alkyl carbon (15),i.e., to a ring a-alkyl carbon substituted. It is not possible to determine the ring junction carbons directly from an l H NMR spectrum. In this paper an attempt is made to determine these carbons indirectly, based on the hydrogen content of condensed aromatic rings (HA"). Since monoaromatic substitution percentage is approximately 50% and the average number of substituents in monoaromatic and condensed rings equal to one another (8), than in bicyclic structures approximately 40% of the carbons, which are not substituted by alkyl groups, are ring junction carbons, while in tricyclic structures this number amounts to approximately 60%. Since in aromatic petroleum fractions less than 5 wt % of the cyclic systems contain more than two condensed aromatic rings (16), we should multiply the hydrogen signal intensity for the condensed aromatic rings (HA,,) by 1.4, in order to account for the ring junction carbons. The 'H NMR spectrum areas denoted by HCand H D contain paraffinic hydrogens and also P,y-alkyl groups hydrogens of aromatic hydrocarbons which could be expressed as ~ H B . Approximately 61% of this ~ H signal B belongs to the area HC and the remaining 39% to the area H D , as was proven experimentally (14). Because of the low concentration of methine groups (15),it is assumed that two hydrogen atoms (Hc) are equivalent to one carbon atom. Three hydrogen atoms in the area H D are equivalent to one carbon atom. From the above we can now formulate the following equations: X1 = H A 1.4HAn (2)
+
Xz = HB/2.5
(3)
X3 = (Hc - 0,61fHB)/2
(4)
X4 = (HD - 0.39fHB)/3
(5)
In eq 2-5 symbol X1 stands for the aromatic ring carbon (except for the carbons substituted by alkyl groups), Xz denotes a-alkyl carbons, X3 stands for the paraffinic CH2 carbons, and X4 denotes the paraffinic CH3 carbons. Symbol f expresses the ratio HB,y/H,.
f
= Ho,,/H,
(6)
It describes the relationship between the P,y-alkyl groups and
the a-alkyl groups of the aromatic hydrocarbons. It is impossible to determine the value f from NMR spectral data, because the P,y-alkyl group signals of aromatic hydrocarbons are overlapped by the paraffinic signals. In this paper, we derive the value f for various petroleum types without prior separation of their aromatic fractions. This is based on the assumption that the average hydrocarbon structure does not differ significantly for various petroleum samples at a given boiling-point range. On the basis of some research studies (14),as well as on our own experimental data, we established the dependence of value f on the boiling-point value, using the minimum square method. The dependence diagram is a linear function f = A,T + Ao. The temperature range (7') is between 220 and 500 OC, while the parameters are A, = 0.05 and A. = -0.1309. The obtained relationship is applied to all of the analyzed petroleum samples. The total carbon content (S,) is the sum of the following: aromatic carbons (X, + Xz); a-alkyls ( X z ) ;P,y-alkyl aromatic carbons ((0.61/2)fHB + (0.39/3)fHB = (0.61/2)f2.5Xz + (0.39/3)f2.5Xz = 1.0875fX2); CHz paraffinic (XJ; CH3 paraffinic ( X 4 ) .
Si = X i
+ (2 + 1.0875f)Xz + X3 + X4
(7)
Using eq 2, 3, 4, and 5, as well as the total carbon content, eq 7, we obtain the carbon percentages (C) as shown in eq 8-11. Here, CA stands for the aromatic carbon percentage;
+ x2)/s,x 100 c, = x,/s, x 100 cz = x,/s, x 100
(10)
C3 = X4/S1 X 100
(11)
CA = (XI
(8) (9)
a-alkyl carbon, CHz group paraffinic carbon, and CH3 group paraffinic carbon percentages are denoted by C,, Cz, and C3, respectively. The aromatic ring substitution percentage is derived from the relationship between the carbons, substituted by alkyl groups, and the total aromatic ring carbons, substituted by alkyl groups and by hydrogen atoms.
s = c,/c* x 100
(12)
Here, the CA value does not account for the ring junction carbons. This is computed as X l = HA'+ HA,,. The carbon percentage in paraffinic methyl groups is derived from the relationship between CH3 paraffinic group carbons and the total paraffinic carbons
Since the average weight for aromatic and paraffinic functional groups is thus known, we can compute the weight percent of aromatic ( A ) and paraffinic (P)hydrocarbons from the following equations:
A=
+
1 2 . 5 C ~ (14.5
+ 14.4f)C,
100
(14)
s2= 12.5CA + (14.5 + 14.4f)C, + 1 4 c z + 15c3
(16)
X
s 2
EXPERIMENTAL SECTION Samples. The petroleum samples from Yugoslav sources
(Slavonia, Moslavina) were obtained by fractional distillation (ASTM 2892-73) on Podbielniak 3700. The analyzed fractions were characterized with distillation range from 20 to 30 "C within a range between 220 and 390 O C . In Tables I and I1 the samples of the Slavonic origin are denoted by numbers 1 to 5 , and the samples of the Moslavic origin by numbers 6 to 12. Other pe-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
1873
-
Table I. Aromatic ancrparaffinic Hydrocarbon Content; Carbon Functional Group Distribution in Middle Petroleum Fractions Obtained b y NMR sample (bp range, "C) l(230-260) 2 (260-290) 3 (290-320) 4 (320-350) 5 (350-380) 6 (220-240) 7 (240-260) 8 (260-280) 9 (280-300) 10 (300-330) 11 (330-360) 1 2 (360-390)
A , wt %
P,wt %
% CA
% c,
% c,
%
31.2 33.2 26.2 32.1 35.9 31.0 31.8 31.7 35.0 31.6 35.1 38.4
68.8 66.8 73.8 67.9 64.1 69.0 68.2 68.3 65.0 68.4 64.9 61.6
17.2 17.9 13.2 15.6 17.7 19.2 17.6 17.3 17.6 15.6 16.4 16.9
7.2 7.3 5.8 6.8 7.0 6.5 7.2 7.0 7.9 6.9 7.4 8.0
51.9 52.9 57.2 55.4 53.9 51.4 52.4 53.8 51.8 54.7 53.5 52.0
15.1 12.1 15.1 10.9 8.4 15.7 14.0 12.8 11.5 12.1 9.4 7.9
Table 11. Comparison of Aromatic Hydrocarbon Percentage Values, Obtained by NMR Method and the ASTM D-1319.70 and D-2549-68, and Comparison of Aromatic Carbon Percentage Values, Obtained by NMR and IR, of Middle Narrow Boiling Point Range Fractions sample (bp range,%) l(230-260) 2 (260-290) 3 (290-320) 4 (320-350) 5 (350-380) 6 (220-240) 7 (240-260) 8 (260-280) 9 (280-300) 10 (300-330) 11 (330-360) 12 (360-390) ' Obtained by the
A , wt %
KJMR 31.2 33.2 26.2 32.1 35.9 31.0
31.8 31.7 35.0 31.6 35.1 38.4 ASTM
ASTM
% C*
NMR
27.4' 17.2 33.7' 17.9 22.8 13.2 27.7 15.6 30.5 17.7 31.4' 19.2 29.0' 17.6 28.0 17.3 28.4 17.6 26.9 15.6 32.5 16.4 35.3 16.9 D 1319-70 method,
IR 15.8 14.0 10.3 11.8 13.1 18.3 13.5 17.2 16.0 10.9 15.1
troleum samples with various boiling-point range values of either Yugoslav or foreign oriigin were not prepared in any special manner. In Table I11 thie samples of Slavonic origin are denoted by numbers 13 to 15 and the samples of Moslavic origin by numbers 16 to 20. The siunples of Voivodinian origin are denoted by numbers 21 to 24 and the foreign petroleum samples by numbers 25 to 27. NMR Spectrometry. The discussed 'H NMR spectra were obtained by Varian EM-360 NMR spectrometer, using chloroform-d, as solvent. The internal standard was Me3Si. The signal areas HA-D were obtained by an electronic integrator, while the aromatic signal areas were obtained by counting (HA'HA"). IR Spectrometry. 'The comparative analyses of all liquid samples were done by 113 spectrometry on a Perkin-Elmer IR spectrometer, Model 421. The aromatic carbon content (CA)is determined by the ASA method (17),based on the measurement of intensity of the aromatic C=C ring stretching band at 1610 cm-'. Liquid Chromatography. The comparative analyses of the samples, with respect to the hydrocarbon types, were performed by using the standard methods of liquid chromatography (ASTM D 1319-70 and ASTM-11 2549-68).
RESULTS AND DISCUSSION The hydrogen signal intensity in A, B, C, and D regions is determined from the NMR spectrum. The value f for the boiling point range middle of an analyzed fraction is determined from the diagram. The carbon content percentage in aromatic rings (CA),in cu-alkyl groups (CJ, in CH2 paraffinic groups (C2),and in CH3 paraffinic groups (C3), as well as the weight percent of aromatic ( A ) and paraffinic (P) hydrocarbons is computed, using 2-5 and 7-16. The results for middle petroleum fractions with narrow boiling point range are presented in Table I.
c,
S 44.4 44.3 47.5 48.3 44.4 36.5 45.4 44.4 48.5 48.3 50.7 52.6
% CCH,
22.5 18.6 20.9 16.4 13.5 23.4 21.1 19.2 18.2 18.1
14.9 13.2
-
Table 111. Comparison of Aromatic Hydrocarbon Percentage Obtained by NMR and ASTM D-2549-68 and of Aromatic Carbon Percentage Obtained by NMR and IR Method, of Higher (up t o 500 "C) Wider Boiling Point Range Fractions A , wt %
% CA
sample (bprange,"C)
NMR
ASTM
NMR
IR
1 3 (230-350) 14 (380-410) 15 (410-440) 16 (230-350) 17 (235-350) 18 (381-528) 19 (233-459) 20 (300-350) 21 (350-495) 22 (300-450) 23 (393-479) 24 (428-508) 25 (233-459) 26 (370-478) 27 (328-492)
27.3 28.2 33.4 31.4 29.4 43.2 35.0 47.9 29.6 36.6 31.7 35.8 37.6 27.6 64.5
30.5 34.2 38.3 30.4 26.6 48.2 31.8
13.4 14.1 14.1 16.6 16.0 18.3 16.7 23.3 15.9 15.2 14.3 15.6 17.7 9.4 27.4
12.1
34.4 31.6 35.7 35.7 32.3 60.2
15.2 14.7 13.3 22.5 15.7 12.5 13.2 15.3 6.9 31.9
The results for aromatic hydrocarbons content, obtained by the NMR method and by the forementioned standard methods, as well as the percentage of CA values (obtained by NMR and IR method) are presented in Table 11. The standard deviation (0) for the results obtained by NMR method, as opposed to the results of the referent methods (RM), is computed by using the following equation (13): / "
2 (NMR;-RMJ2
i=l
u=
n-1
(17)
and the difference of the average ( d ) is computed as
~ ( N M R-, RMJ From eq 17 and 18 we computed the following values for the percentage of aromatic hydrocarbons: u = 4.03%; d = +3.3%. Therefore, it is obvious that the NMR method results are on the average characterized by higher values, as opposed to the results of the standard methods. Since the compared methods are basically different in nature and since the separation by standard methods, dealing with mixed structures of compounds, may affect the results, we consider the obtained consistency between the NMR and the RM results to be very good. It should be noted that some comparative analyses of standard methods for aromatic and paraffinic hydrocarbons content determination (ASTM-D 2549-68 (18),GUD 427/75
1874
Anal. Chem. 1982, 5 4 , 1874-1876
(19), DIN 51384 (20))show that the ASTM gives consistently 1 to 3% lower values, as compared to the other two methods (21).
As far as the CApercentage is concerned, we obtained the following values for standard deviation and difference of the average of the NMR results (as opposed to the IR spectrometry results): u = 3.25%; d = +2.68%. This is in accordance with the statement (22)that the IR method gives lower values when the aromatic rings make up less than 50% of a sample and higher values when the aromatic hydrocarbons make up more than 50% of a sample. Therefore, the obtained match is also considered to be very good. The results for the wider boiling-point range samples, as well as for samples above the middle fractions, are presented in Table 111. As far as the aromatic hydrocarbon content is concerned, the following values are computed: u = 3.67% and d = -0.63%. It should be noted that the standard deviation is approximately the same as previously, while the difference of the average is negative. This leads to the conclusion that, for the wider boiling point range and for the higher fractions, the errors, inherent to both methods, compensate for one another-their effect is nullified. The values in Table I11 suggest that, independent upon the boiling point range width, the difference of the average is less for samples including middle fractions (d = +1.74%) than for the samples with boiling range above middle fractions (d = -2.4%). In the latter case, the difference of the average is higher and has negative sign. As far as the CA percentage is concerned, the obtained deviation values from the IR method results are u = 2.65% and d = +1.3%. Although these values are less than in the analysis of middle fractions, values obtained by the NMR method are still higher. The only exception is sample no. 27. This is because its aromatic ring content is >50%, which is in accordance with the previously discussed characteristics of the NMR/IR value relationship. In order to test the reproducibility, we have performed 12 measurements of the same petroleum sample. For the aromatic (paraffinic) hydrocarbon content, we obtained the standard deviation: u = 1.24% and ur,l = 3.54% for the aromatic hydrocarbons and urel = 1.91% for the paraffinic hydrocarbons. As far as the aromatic ring carbon content is
concerned, the obtained values are u = 0.82% and are]= 4.47%. For any analysis of this kind, the exhibited level of reproducibility is considered to be very satisfactory. LITERATURE CITED (1) Williams, R. B. "Characterization of Hydrocarbons In Petroleum by Nuclear Magnetic Resonance Spectrometry"; ASTM: Philadelphia, PA, 1957; STP 224, pp 168-194. (2) Wllllams, R. B. Spectrochlm. Acta 1959, 14, 24-44. (3) Williams, R. B. "New Developments In Hydrocarbon Type Characterlzation Using Nuclear Magnetlc Resonance"; 6th World Petroleum Congress, 1963; Section V, No. 17, p 217. (4) Hlrsch, E.; Altgelt, K. A. Anal. Chem. 1970, 42, 1330-1339. (5) Oelert, H. H. Z . Anal. Chem. 1971, 225 177-185. (6) Knight, B. A. Chem. Ind. (London) 1967, 45,1920-1923. (7) Knight, S. A. Erdol Kohle 1972, 25, 9, 522-526. (8)Clutter, D. R.; Petrakis, L.; Stenger, R. L., Jr.; Jensen, R. K. Anal. Chem. 1972, 44, 1395. (9) Dorn, H. C.; Wooton, D. L. Anal. Chem. 1976, 4 8 , 14. (10) Yashina, N. S.: Bogdashkina, V. I. Neflekhimya 1979, 79, 7-16. (11) Shoolery, J. N.; Budde, W. L. Anal. Chem. 1976, 48, 1458. (12) Shoolery, J. N.; Jankowski, W. C. "Quantitative Aspects of C-13 NMR Spectroscopy"; Varian Appllcation Note, 1973; NMR-73-4. (13) Myers, M. E., Jr.; Stollsteimer, J.; Wims, A. M. Anal. Chem. 1975, 47, 2010. (14) Von Deutsch, K. J. Prakt. Chem. 1977, 379, 439-443. (15) Von Deutsch, K.; Janeke, M.; Zligan, D. J. Prakt. Chem. 1977, 319, 1-7. (16) Jeweli, D. M.; Ruberto, R. G.; Davls, B. E. "Abstracts of Papers"; 163rd National Meeting of the American Chemical Society, Boston, MA, 1972: American Chemlcal Society: Washington, DC; Petroleum Division. (17) Berthold, P. H.; Stande, E.; Bernhard, U. Chem. Tech. (Leipziz)1975 27,234. (18) "Standard Method for Separation of Representative Aromatic and Non-aromatics Fractions of High-Boiling Oils by Elution Chromatography"; ASTM D 2549-68. (19) "Method for Determination of Non-aromatic Hydrocarbons in Mineral Oils with a Boillng Point Above 315 OC (Modified "Smit method"); GUD1472175 also GUD 476-7013, 1045172. (20) "Prufung von Mineralol-Kohlenwasserstoffen und anllchen Produkten. Bestimung des Gehaltes an nichtaromatischen und arornatischen Bestandteiien in hohersieden den Kohlenwasserstoff Gemischen (Chromatographie and Kieseigei)"; DIN Normvoriage 0051 38411975. (21) Karcher, W.; Glaude, P.; Nagy, E.; Van Eijk, I. "Evaluation of Some Standard Methods for the Determlnatlon of Aromatic and Non-aromatic Content in Hlgh boiling Mineral 011s"; Commission of the European Comrnunlties, 1978. (22) Oelert, H. H.; Hemmer, E. A. Erdol Kohle 1972, 25,437-439.
RECEIVED for review January 11,1982. Accepted May 25,1982. Paper was presented in Section "Composition of Feedstock and Products" on "Symposium on Crude Oil and Gas Processing into Fuels and Petrochemicals" in Zadar (Yugoslavia) March 24-26, 1980.
CORRESPONDENCE Exchange of Comments on Inaccuracies Encountered in Sulfur Determination by Particle Induced X-ray Emission Sir: A recent paper of Hansen et al. (1) on sulfur losses during X-ray analyses has caqed significant controversy. This is reflected in the comments of Shaw et al. (2) and Hegg and Hobbs (3),while a rebutal by Hansen et al. ( 4 ) raises new issues. The importance of sulfur analyses by particle induced X-ray emission (PIXE) and X-ray fluorescence (XRF) to the field of atmospheric chemistry (among others) demands a better resolution of the question than that achieved to date. Since our laboratory has used both PIXE and XRF for sulfur analyses since 1970, participated in both of the interlaboratory comparisons briefly mentioned by Hansen et al. (I), and 0003-27OOl82IO354-1874$01.2510
generated the data cited by Hansen et al. (I) in Table IV of their paper, we would like to assist in resolving the serious misunderstandings that appear prevalent at this time. Our experience is that both PIXE and XRF have been proven again and again to be both accurate and precise in analysis of sulfur-containing ambient atmospheric particles. The effects seen by Hansen et al. (I) at Brigham Young are understandable in terms of three factors. (1) The energy deposition in their targets is high due to their low proton energy and high beam fluxes, giving target temperatures as high as 225 "C. 0 1982 Amerlcan Chemical Society
