1154
Anal. Chem. 1980, 52, 1154-1157
T h e agreement was good. Moreover, t h a t comparison was found to be essentially the same as a similar comparison using data obtained by another set of workers utilizing t h e same two techniques on ambient samples collected on cellulose filters. This comparison is also discussed in t h e reference cited. We believe, therefore, t h a t we have provided evidence of t h e arsenic d a t a credibility. There is no reason to expect arsenic to be unique with respect to t h e application of a discrimination limit. In summary, we believe that we have documented credible d a t a a n d a practical procedure t o decide, prior t o sampling, t h e minimum elemental quantity which can be determined in t h e sample. T h e approach is applicable t o any filter medium and any suitable analytical technique. We encourage its consideration as a possibly way t o avoid frustrating ex-
periences which could tempt one to make sweeping nonquantitative generalizations.
LITERATURE CITED (1)
Walling, J
F , et al J A/r PoNut
Control Assoc
1978 28, 1134
J. P. F. L a m b e r t * F. W. W i l s h i r e
U.S. Environmental Protection Agency Trace Elements Analysis Section Analytical Chemistry Branch Environmental Monitoring and Support Laboratory Research Triangle Park, North Carolina 27711 RECEIVED for review March 20, 1980. Accepted March 20, 1980.
AIDS FOR ANALYTICAL CHEMISTS Polar Bonded-Phase Sorbents for High Performance Liquid Chromatographic Separations of Polycyclic Aromatic Hydrocarbons Jan Chmielowiec” and Albert E. George Energy Research Laboratories, c/o
555 Booth Street, Ottawa, Ontario K I A OG1, Canada
T h e increasing use of heavy oils, bitumens, coal liquids, and other nonconventional fossil fuels requires new methods for separation a n d characterization of higher molecular weight hydrocarbons. T h e polycyclic aromatic hydrocarbons (PAHs), which are abundant in these new fuel sources, are of particular interest because of their role in various process streams, as well as their potential carcinogenic and mutagenic properties. Compositional information acquired about aromatics will contribute to minimizing hydrogen consumption in fossil fuel processing a n d t o combatting coke formation in reaction systems. We believe t h a t separation of these hydrocarbons by liquid chromatography on bonded-phase sorbents will extend t h e possibilities of further instrumental characterization of aromatic fractions. High Performance Liquid Chromatography (HPLC) separations of PAHs on various sorbents were discussed by Wise e t al. ( I ) , who reported that silica with bonded amine groups yielded a separation sequence according to the number of condensed aromatic rings, regardless of the type of alkyl substitution. Although t h e reversed-phase H P L C systems show considerable selectivity (2-5) for the PAHs, alkyl substitution increases t h e retention of these PAHs considerably so t h a t they elute together with the higher number ring structures. This interferes with both the qualitative and quantitative characterization of PAHs. Also, the presence of water in t h e mobile phase makes gas chromatographic-mass spectrometric characterization of t h e collected fractions, or direct interfacing with MS, less practical because of the requirement to extract the organic material or to remove water. T h e r e is a shortage of d a t a on t h e selectivity of normalPresent address: Chemex Applied Research Associates Ltd., 174 Colonnade Road, Ottawa, Ontario K2E 755, Canada. 0003-2700/80/0352-1154501 .OO/O
Table I. Silica-Based Column Materials Studied (10-pm Particle Size) commercial name a n d supplier
bonded phase NH,
packing conditions
LiChrosorb NH, ready-t o-use (Altex, Berkeiey, Calif.) coiumn (250 X 3.2 m m )
CN
Chromegabond Nitrile (ES Industries, Marlton, N.J.) R(OH), Chromegabond Diol (ES Industries, Marlton, N.J.) ROR Chromegabond Ether (ES Industries, Marlton N.J.) Chro mega bond Diamine R( “2 )2 (ES Industries, Marlton, N.J.) RCN Vydac Polar Bonded Phase (TP 501) (The Separations Group, Hesperia, Calif.) quaternary Vydac Anion Exchange ammonium (TPB 301) (The Separations Group, Hesperia, Calif.)
heptane, 6000 psi (41.4 MPa
heptane, 6000 psi (41.4 MPa heptane, 6000 psi (41.4 NPa heptane, 6000 psi (41.4 MPa) methanol, 5000 psi (34.5 MPa)
heptane, 6000 psi (41.4 MPa)
phase systems for PAH separation. This study may help to overcome the above-mentioned drawbacks by the application of polar bonded-phase sorbents.
EXPERIMENTAL Columns (250 mm
X
4.6 m m ) were packed with particle size
‘G 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1155
Table 11. Capacity Factors ( h ' ) of Polycyclic Aromatic Hydrocarbons o n Polar Bonded-Phase Silicas Studied -R(",), -CN -NH, (n-hep(n-hexane) tane)
hydrocarbon naphthalene phenanthrene pyrene chrysene triphenylene per ylene ant han t hr ene dibenzo [ a , h ]anthracene dibenzo[ e , hlpyrene coronene a
s.r.
=
0.60 1.25 1.60 2.90 2.98 3.70 3.80 4.40 7.40 5.20
0.50 0.88 1.04 1.65 1.65 2.00 2.08 2.81 3.58 3.58
-WOW, (n-heptane)
-ROR (n-heptane)
0.38 0.77 0.96 1.46 1.46 2.00 2.00 2.42 2.88 2.88
1.81
-RCN -quaternary (n-hep- ammonium (n-hep(n-heptane) ta ne ) tane)
(6% methylene chloride in n-heptane)
0.19 0.50 0.77 1.58 1.58 2.96 2.96 3.46 6.35 3.46
2.34 2.88 4.31 4.61 6.46 7.08 9.23 12.50 9.50
0.61 1.61 2.08 4.08 4.08 8.08 8.08 14.23 21.92
2.38 2.38 3.38 3.38 3.38 3.38 4.12 4.19 3.38
15.00
3.84 9.23 l2.69 s.r.a
s.r. s.r. s.r. s.r. s.r. s.r.
-
strongly retained. S I L I C A - NH2
SILICA-CN
SILICA-R(OH)p
SILICA-ROR
n-HEXANE
n-HEPTANE
n - HEPTANE
n -HEPTANE
3
,'+ SILICA -(DIAMINE)
4
3
2,s
6 %CH,CL,in
n-Heptane
-
- Heptane
141
'?
E
t
0 N W V
z a
m
a
0 v)
m
a
I
I
1
1
0
IO
20
30
/ I 20 30
RETENTION VOLUME (mR)
Figure 2. Chromatograms of aromatic hydrocarbons on silica-R(NH,), column. Numbers refer to compounds in Table 111. Numbers in brackets refer to PAH standard mixture from Figure 1
0
20
40
so
10
20
R E T E N T I O N VOLUME, ( m i )
Figure 1. Chromatograms of PAH standard mixture on polar phases bonded to silicas. (1) Naphthalene; (2) phenanthrene; (3)pyrene; (4) chrysene, triphenylene; (5)perylene, anthanthrene; (6) dibenz [ a , h ] anthracene; (7) dibenzo[e,h]pyrene; and (8) coronene
10-pm silica-based sorbents (see Table I). The slurry packing technique was used with a DST-150A air-driven liquid pump (Haskel, Burbank, Calif.). HPLC grade n-hexane and n-heptane (Fisher Scientific, Fairlawn, N.J.) were equilibrated with activated molecular sieves before use to remove polar impurities which affect the PAH retention. The solvents were pumped by a Waters Associates Model 6000 pump (Mississauga, Ont., Canada) at a flow rate of 2 mL/min, at ambient temperature. Solutions of PAHs in n-heptane (10" M range) were injected by a valve (Valco Instruments Co., Houston, Tex.) with a home-made 7-pL loop. A UV spectroflow monitor (Schoeffel SF 870, Westwood, N.J.), cell volume 8 pL, was used as detector. The reversed-phase silica-CI8 system was used to obtain additional retention data, which had not been reported in (3).
RESULTS AND DISCUSSION In this work, HPLC systems consisted of sorbents with polar functional group-bonded phases, and hydrocarbons as t h e mobile phase. The retentive properties and selectivity of these systems were studied for a 10-PAH standard mixture. Chromatograms obtained for t h e 2 to 7 condensed aromatic ring structures are shown in Figure 1. Respectively, capacity ratio ( k ? values are presented in Table 11.
T h e classification of sorbent retentive properties in order of increasing strength might be done by an evaluation of t h e d a t a compiled in Table 11. Many unknown variables such as retention mechanism, volume of stationary phase, concentration of active sorption sites, etc., would be necessary for more complete sorbent property description. As far as t h e sorbent selectivity can be characterized using selectivity factors (ratios of capacity ratios), resolution factors, a n d retention indices, the sorbent retentive properties are described by the usefulness of capacity factor range for separation of compounds of interest. Within such a range, the values of capacity ratios reflecting t h e strength of retention can be evaluated for each sorbent. T h e elution order of t h e 5- to 7-ring structures was not based on t h e number of aromatic rings for all sorbents. Coronene, in particular, did not follow the same elution sequence on all the sorbents. The silica-NH2 based HPLC system with n-hexane as eluent, which has been previously described by Wise e t al. ( I ) , was found t o offer only limited selectivity for P A H mixtures. This would make its applicability t o PAH multicomponent mixtures, e.g., those from nonconventional fossil fuel origin, appear t o be limited. This was also stated by Dark a n d McFadden (6) when they applied this system to characterize a synthetic crude product. Only some segration of the aromatics based on the number of condensed rings was obtained, b u t no real resolution. The silica-R(NH2)2sorbent was studied in more detail. On this column, the mobile phase contained methylene chloride t o elute t h e higher ring structures. T h e chromatograms obtained are presented in Figure 2. Retention indices for various
1156
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
Table 111. Comparison of Retention Indices of Polycyclic Aromatic Hydrocarbons (PAH) on the Silica-R(NH,), and Silica-C,, Columns
ref. no. t o Figure 2 1.
2. 3.
4. 5.
6. 7.
8. 9.
10.
hydrocarbon benzene 1,2,3,4-tetrahydronaphthalene indene 3-methylindene naphthalene 2-methylnaphthalene 2-et hylnaphthalene 2,6-dimethylnaphthalene 1,6-dimethylnaphthalene 1,2-dimethylnaphthalene 2,7-dimethylnaphthalene 1,4-dimethylnaphthalene 2,3,5-trimethylnaphthalene
1,3,7-trimethylnaphthalene acenaphthene biphenyl biphenylene azulene fluorene 2-met h y l f l u o r e n ~ 1-phenylnaphthalene 1-benzylnapht halene o-terphenyl anthracene 9 ,lo-dihydroanthracene 2 -methy lanthracene 9 -meth y lan t h racene 2-phenylanthracene 9-phenylanthracene phenanthrene 3-methylphenanthrene 3,6-dimethylphenanthrene
9-n-dodecylphenanthrene 4,5-methylenephenanthrene
11.
12. 13. 14.
15.
16.
2 -phenylna pht halene 1,l'-dinaphthyl acenapht hy lene triptycene m-terphenyl 9,lO -diphenylant hracene p-terphenyl fluoranthene benzo [alfluorene pyrene 1-methylpyrene 3-met hy lpy rene 4-methylpyrene 3-n-decylpyrene benzo[ b ]fluorene benzo [clfluorene benzo[ blchrysene
silica-R(NH, ) 2 (n-heptane) log I
silica-C,, (3) ( 8 0 % acetonitrile in water)
1.000
1.000
1.218 1.782 1.757 2.000 2.076 1.9 07 2.208 2.179 2.109 1.907 1.907 2.171 2.0 55 2.384 2.252 2.300 2.424 2.500 2.592 2.484 2.587 2.731 2.916 2.950 3.042 3.000 3.225 3.290 3.000 3.063 3.198 2.779 2.816 3.152 3.167 3.199 3.104 3.330 3.470 3.488 3.586 3.723 3.433 3.506 3.524 3.506 3.018 3.827 3.869 4.000 -
silica-R(NH, ) 2 (6% methylene chloride in n-heptane) log I 16.
17.
18.
phenanthrene benzo[ blfluorene 1,3,5-triphenylbenzene 2,2'-dinaphthyl benz[a]anthracene 7,12-dimethylbenz[a] anthracene chrysene triphenylene 3-methy lcholant hre ne 20 -methylcholanthrene 2 2-methylcholanthrene 1,2,3,4-tetraphenylnaphthalene
19.
benzo[a Ipyrene
log I
-
-
2.000 2.670 3.128 3.211 3.211 2.951 3.154 3.086 3.355 3.591 2.587 2.401
-
2.774 3.460 3.237 3.128 3.225 3.225 -
3.723 3.560 4.299 4.038 3.000 3.417 3.823 ,5
3.517 3.391 3.860 2.2 30 -
3.474 4.682 3.851 3.474 3.922 3.691 4.106 4.106 4.173 >5 3.922 3.843 4.000 5.000 silica-C?, ( 3 )(80% acetonitrile in water) log I
3.000 3.469 3.864 3.879 4.000
3.000 3.922 4.169 4.169 4.000
3.834 4.000 4.169 4.178 4.118 4.690 4.402 4.425
4.376 4.000 3.851 -
-
4.571 4.591
1157
Anal. Chem. 1980, 52, 1157-1158
Table 111. (Continued)
-
silica-R(NH, ) * (6% methylene chloride ref. no. t o Figure 2
20.
21.
22. 23. 24. 25. -
hydrocarbon benzo[ b Ifluoranthene benzo[k Ifluoranthene perylene difluorenyl benzo[ b Ichrysene anthanthrene rubicene benzo [ghilperylene di benz [ a,h ]anthracene indeno[ 1,2,3-c,d]pyrene dibenz [ a , c ]anthracene picene coronene dibenzo [ e , h ] pyrene dibenzo[a,h]pyrene
aromatic hydrocarbons on this column were calculated from experimentally-determined retention volumes (Table 111) as previously described by Popl et al. (7). T o evaluate the effect of alkyl and phenyl substitution on t h e retention of fused-ring aromatics, representative derivatives have been included in the silica-R(NH,), column testing. Values of t h e appropriate retention indices (Table 111) corresponding to decreasing or increasing retentions due to alkyl a n d phenyl substitution of ring structures are included. Methyl substituents contributed insignificantly to changes in the retention of two-, three-, and four-ring types studied. Only n-decyl substitution of pyrene decreased the retention to any great extent. I n all cases phenyl-substituted hydrocarbons were retained more than parent ring structures. Table I11 also presents the retention indices for alkyl and phenyl substitutions calculated from earlier published results ( 3 ) and additional measurements on t h e silica-CI8 column. Increments in t h e retention indices are considerably higher than those corresponding to differences in selective behaviors on t h e silica-diamine column. I n t h e reversed-phase system, alkyl derivatives of PAHs overlap PAHs of higher ring number. T h e normal-phase diamine-based system studied here appears t o offer better ring-size selectivity because alkyl PAHs elute near the parent PAHs. Furthermore, separation based on t h e number of condensed rings was achieved up to 4-ring aromatics. Higher condensed ring structures did not follow t h e increasing ring-number sequence. In contrast t o the reversed-phase system (silica-CI8 and acetonitrile-water (80:20) as eluent). in this normal phase system phenyl-substituted structures tend not t o increase the parent aromatic retention to such a
in n-heptane) log I 4.435 4.435 4.559 4.852 5.000 >5
silica-C,, ( 3 ) (230% acetonitrile in water) ___ log I 4.376 4.461 4.404 3.954 5.000
-
>5 >5 >5
4.955 4.766
>5 >5
-
>5
-
-
>5 >5
-
>5
degree. Retentions of phenyl derivatives in the normal-phase diamine-bonded system seem t o be more dependent on molecule structural factors than on the number of x electrons or the number of molecular carbons.
CONCLUSIONS PAH separations achieved on the diamine sorhent are based on the number of fused aromatic rings up to 4. It differentiates this sorbent from classical liquid chromatography sorbents as silicas and aluminas offering selectivities based on general chemical affinities. I t also differentiates the diamine-bonded phase from the octadecyl-bonded phases. The silica-R(NH2)2 sorbent, because of its retentive properties, seems t o be superior t o other polar bonded phases studied for P A H chromatography. I t is very promising for t h e separation and characterization of PAH mixtures of various origins. Studies on developing separations schemes for P A H fractions from fossil fuel products are presently being performed.
LITERATURE CITED Wise, S. A.; Chesler, S. N.; Hertz, H. S . ; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 4 9 , 2306-2310. Thomas, R.; Zander, M. Erdol Kohle Erdgas Petrochem. 1977, 3 0 , 403-405. Chmielowlec, J.; Sawatzky, H. J . Chromatogr. Sci. 1979, 17, 245-252. Thomas, R. S.: Lao, R. C.; Wang, D. T.; Robinson, D.; Sakuma, T. "Carcinogenesis, Volume 3: Polynuclear Aromatic Hydrocarbons", Jones, P. W., Freudenthal, R. I . , Eds.; Raven Press: New York, 1978. Ogan, K.; Katz, E.; Salvin, W. Anal. Chem. 1979, 51, 1315-1320. Dark, W. A.; McFagden, W. H. J . chromatogr. Sci. 1978, 16, 289-293. Popl, M.; Dolansky, V.; Mostecky, J. J . Chromatogr. 1976, 177. 117-127.
RECEIVED for review September 14, 1979. Accepted January 29, 1980.
Determination of Ethyl Sulfate by Reversed-Phase Ion Pair Chromatography Grace Chiu' Monsanto Chemical Intermediates Company, P.O. Box 12830, Pensacola, Florida 32575
Almost all of the existing methods for the determination of ethyl sulfate ions are indirect in nature. They involve the conversion of ethyl sulfate t o sulfate ( I ) ,t h e determination of the hydrolysis products ( I ) ,the determination of the cation
'
Permanent address: Department of C h e m i s t q , rniversit? of West Florida, Pensacola, Fla. 32504. 0003-2700/80/0352-1157$01 O O / O
of the salt via ion-exchange ( 2 ) ,or indirect spectrophotometry ( 3 ) . A direct infrared method was reported by Abe et al. ( 4 ) ; but the method was not very sensitive, the limit of detection being 10% sodium ethyl sulfate in detergent powders. This paper describes a direct and sensitive method for the determination of ethyl sulfate by rek'ersed-phase ion Pair chromatography. C 1980 American Chemical Society
