2092
Anal. Chem. 1990, 62, 2092-2098
tained from the TOF-SIMS high mass fragmentation spectrum, a spectroscopy which satisfies these criterion. For this purpose we chose to examine a model homopolymer from the class of perfluorinated polyethers. We have demonstrated that quantitative estimates of the M , of a Krytox sample and of the composition of a Krytox sample can be obtained by using only the appropriately normalized intensities of specific negative or positive fragment ions. This possibility has not been generally recognized in SIMS analysis of polymers. These linear relationships are the result of the intensity of these fragment ions in TOF-SIMS being directly related to the polymer molecule’s length or to the concentration of a specific polymer molecule present in the PFPE sample. This conclusion seems to be relatively independent of the substrate the Krytox is on and the thickness of the Krytox film on the substrate. Although not discussed in this paper, the insensitivity to substrate and polymer thickness has been our general experience for other PFPE’s, in particular Fomblin which is the registered tradename for a group of random, copolymer PFPE’s manufactured by Monteflous (Spinetta Marengo, Alessandria, Italy) and represented by Montedison USA, Inc. The PFPE’s are particularly well suited to TOF-SIMS analysis, largely because of the highly reproducible nature of the relative intensities in the high mass fragmentation ion spectra. Thus, PFPE’s may be quite useful for further testing ideas about the mechanisms of forming ions in TOF-SIMS. Whether similar quantitative relationships, as were found in the present study, will be generally observed for other polymer classes is yet to be seen. There is some indication that not all polymer classes may be as suitable as PFPE for quantitative analysis by TOF-SIMS (13), but due to the currently limited understanding of the desorption/fragmentation/ionization process in SIMS of very large molecules, many more
experimental studies will be required to test the extent of generality. Nevertheless, the above results show there is potential for using TOF-SIMS for quantitative analysis of PFPE’s, and possibly other polymers, before, during, and following their actual use in real world applications.
ACKNOWLEDGMENT We wish to extend thanks to J. Patton of I. E. du Pont de Nemours and Co. for providing the Krytox samples. H. Hunziker deserves many thanks for extensive discussions about fragmentation. Thanks go to S. Hug who helped with some of the NMR measurements and data analysis. LITERATURE CITED (1) Clark, D. T. Advances in Po/ymer Science; Cantow, H. J., et al., Eds.; Springer-Verlag: Berlin, 1977; Vol. 24, p 125. (2) Briggs, D.; Wootton, A. B. S I A , Surf. Interface Anal. 1082. 4 , 109. 13) D. Anal. 1082. 4 . 151. ,., Briaas. - SIA. -~ , Surf. _ _ Interface .. _.. (4) Briggs, D.; Hearn, M. J.; Ratner, B D. SA, Surf. Interface Anal. 1084, 6 , 184. (5) Lub, J.; vanvroonhoven, F. C. 8. M.; Bruninx, E.; Benninghoven, A. Polymr 1060, 3 0 , 40. (6) Blestos, I . V.: Hercules, D. M.; vanleyen, D.; Benninghoven, A. Macromolecules 1087, 2 0 , 407. (7) Bletsos, I. V.; Hercules, D. M.; vanleyen, D.; Niehuis. E.; Benninghoven, A. Proc. Phys. 9 , Ion Formation from Organic SoMs I l l ; Benninghoven, A., Ed.; Springer-Verlag: Berlin, 1986; p 74. (8) Patton, J. D., E. I. du Pont de Nemours and Co., private communication, 1990. (9) Steffens, P.; Niehuis, E.; Friese, T.; Greifendorf, D.; Benninghoven, A. J. Vac. Sci. Technol. 1085, A3, 1322. (10) Secondary Ion Mass Spectrometry V ; Benninghoven, A,, CoRon, R. J., Simons, D. S., Werner, H. W., Eds.; Springer-Verlag: Berlin, 1986. (11) Fowler, D. E.; Johnson, R . D.; vanleyen, D.; Benninghoven, A. Unpub-
__..
~
lished results.
(12) Bletsos, I. V.; Fowler, D. E.; Hercules, D. M.; vanleyen, D.; Benninghoven, A. Anal. Chem. 1000, 62, 1275-1284. (13) Bletys, I . V.; Hercules, D. M.; Maglll, J. H.; vanleyen, D.; Niehuis, E.; Benninghoven, A. Anal. Chem. 1988, 6 0 , 938.
RECEIVED for review March 12,1990. Accepted July 11,1990.
Characterization of the Surface Composition of Alkyl Bonded Phases under Reversed-Phase Liquid Chromatographic Conditions Using Homologues of Alkanoate and Perfluoroalkanoate Esters as Solute Probes R. K. Gilpin,* M. Jaroniec,’ and S. Lin Department of Chemistry, Kent State University, Kent, Ohio 44242
A thermodynamic descriptlon is presented for the retention of homdogues separated under reversedphase conditions on chemkalty bonded phases. This desctiptlon leads to a slmple equation, whkh considers the Influence of the swface phase composltbn on a solute’s retention. AddluonaQ, this equation Is appled to retentlon data for ethyl alkanoate and methyl perfiuoroaikanoate esters chromatographed with either water-methanol or water-acetontrlle mobile phases on both octyl and octadecyl bornled phases. The parameters from the equation are used to interpret the effect of the surface phase composltlon on retention and to compare and contrast dlfferences in Interactions between the alkanoates and perfluoroaikanoates.
* To whom correspondence should be addressed.
Permanent address: Chemistry Faculty, M. Curie-Sklodowska University, 20031 Lublin, Poland.
INTRODUCTION Although many attempts have been made to explain solute retention mechanisms in liquid chromatography (e.g., refs 1-11, and references therein), exact theoretical descriptions are usually difficult because a variety of interactions exists which must be identified and taken into account. These include competitive interactions of the solute and solvents with the sorbent, as well as solute-solvent and solvent-solvent interactions which occur in both the mobile and surface (stationary) phases (7). For chromatographic systems where chemically bonded stationary phases are used, the interactions of solutes and solvents with the bonded ligands also must be considered. Likewise, structural effects can play a significant role in determining solute retention. The orientation, dynamics, and conformation of chemically bonded phases can change with the nature and composition of the mobile phase (e.g., refs 12-15).
0003-2700/90/0362-2092$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
Unified statistical thermodynamic models have been proposed to describe solute retention (9, 10, 16-19). These treatments incorporate the competitive interactions that arise between the solute and solvents for the sorbent and solutesolvent and solvent-solvent interactions which arise in the mobile and surface phases. However, the resulting equations are frequently quite complicated and thus inconvenient for interpreting retention data (8). Because of such difficulties, many simpler retention models have been suggested (see reviews (1-7) and references therein). Most of these consider the system in terms of two equilibria processes (20): (i) the formation of a combined solvent-urface stationary phase and (ii) the distribution of the solute between the mobile phase and the solvent-surface stationary phase. Generally, since the concentration of the solute is assumed to be infinitely dilute, its influence on the surface phase's composition is neglected. The composition of the surface phase is established by competitive sorption of the solvents which form the mobile phase. The stationary phase is assumed to be a thin layer of solvent the composition of which is determined via molecular interactions from the solid support. For solids like silica and alumina this competitive adsorption is for the higher energy sites on the surface. However, for sorbents with chemically bonded phases the surface composition is influenced significantly by the bonded ligands. Generally, the composition of the surface phase is a function of the mobile phase's composition. It may be estimated theoretically (7') or determined experimentally for the chromatographic packing by measuring the excess sorption isotherm under static or dynamic conditions using the same mixture of solvents as those which form the chromatographic mobile phase (21,22). The distribution of a solute between the mobile and stationary (surface) phases is controlled by the difference between the solute's and solvents' sorption energies, and/or by the differences in the interaction energies of the solute with the solvents in the mobile phase and with the solvents that are incorporated into the stationary phase. The first case is typical for liquid-solid chromatography (i.e., polar unmodified surfaces used in the normal-phase mode (NPLC)) where the solute's distribution between the mobile and stationary phases is governed by a displacement adsorption model. The second case, is for liquid-liquid chromatography where the solute's distribution between the mobile and stationary phases is governed by a partition mechanism. In many cases in reversed-phase LC the interaction of a solute with a chemically bonded phase (RPLC-CBP) has been treated in terms of a partition model (8, 201, analogous to classical partitioning of a compound between two different immiscible liquids. The distribution is determined by differences in the molecular interactions that arise in each phase. In some cases partition models have been used to represent solute retention even for NPLC (in which both phases contain the same solvents). It has been shown elsewhere (e.g., refs 23-31) that homologues are an attractive choice of solutes for studying molecular interactions in chromatographic systems. The methylene increment, which is obtained experimentally by measuring the retention of a homologous series of solutes, is an important parameter for characterizing hydrophobic properties of various chemically bonded phases (23,25,27,28,31).In the current work, the effect of the surface phase composition on the retention of homologues in RPLC-CBP has been studied by using two series of solutes. Experimental measurements have been carried out for ethyl alkanoate and methyl perfluoroalkanoate esters, in methanol-water and acetonitrile-water on c!3and c18 alkyl bonded phases. It has been shown in terms of a partition model that the methylene and perfluoromethylene increments for these homologues depend linearly
2093
on the surface phase composition. This linear dependence is more general than linearity of the methylene increment with mobile phase composition (31). A simple equation has been derived for analyzing the mobile phase composition dependence of both the methylene and perfluoromethylene increment. This relationship provides a better quantitative comparison of the chromatographic systems studied.
THEORY The general thermodynamic theory for the partitioning of a solute between two phases has been discussed elsewhere (32, 33). Accordingly, the logarithm of the capacity ratio, ki, of the sth solute in a hydro-organic mobile phase can be expressed as follows
+ $fWIn k IgCw) + Y
In k', = @,' In k Ig(o)
(1)
where k',,,) and k's(w) are the capacity ratios of the sth solute chromatographed on a chemically bonded phase in contact with either a pure organic solvent (0)or water (w). The volume fractions of organic solvent and water in the surface phase are respectively @, and & .,' The quantity Y, which is a complex expression taking into account solute-solvent interactions in the mobile phase and solvent-solvent interactions in both phases, depends on the compositions of these phases (32). Because the chemically bonded phase is not distinguished as a separate component of the surface phase and concentration of the solute is considered to infinitely dilute, the volume fractions
40,
+
=1
@UW
(2)
According to this treatment, the chemically bonded phase influences k',(o)and It',,,) as well as the surface phase composition. Experimental and theoretical studies (4,32,33)have shown that for nearly ideal phases the third term in eq 1 is negligible. Thus
In k', = qYo In k',,,,
+ #fw
In k',,,)
(3)
where
and
(5) In eq 4, q denotes the volume phase ratio (7,34),and pu8(i) and p18(i) are respectively the standard chemical potentials of the sth solute in the surface (a) and mobile (1) phases formed by the ith pure solvent (i.e., an organic solvent (0)or water (W)).
The interaction of alkyl solutes with a chemically bonded phase can be characterized by
+
Aps(i) = ApD(i) ncAp*(i)
for i = 0,w
(6)
where nc AF*(~)and Apofi)denote respectively the chemical potentials for the alkyl portion of the molecule with nc carbon atoms and for the remainder of the molecule. The quantity Ap*(i) is the incremental chemical potential associated with the -CX,-group in the solute, where X is a single-valent atom such as hydrogen or fluorine. A combination of eqs 3 and 6 gives In k', = In q - (RT)-l[@'oApo(o)
(RW1[V0
+ @',AP~(,)]
-
+VW A ~ * ( ~ ) l n(7) c
Equation 7 shows that the logarithm of the capacity ratio for a homologous series of solutes is linearly dependent on the number of carbon atoms in the alkyl chain. This relationship has been demonstrated to describe accurately the retention
2094
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1. 1990
behavior of solutes in reversed-phase chromatography (23,25, 27,31). It should be noted that the relationship given in eq 7 was derived by neglecting the third term in eq 1 and by assuming a linear dependence of Aps(i)on the number of carbon atoms, nc. For chromatographic systems with more complex solute retention mechanisms (e.g., for higher homologues, where configurational and steric effects may play significant roles), the dependence of ApS(i)on nc can be more complex than that given by eq 6. Differentiation of eq 7 with respect to nc gives the following equation
s
= d In k',/dnc = -(RT)-' A P * ( ~+) ( R ~ - ' ( A P * (-, )AP*(,))@',
(8)
Equation 8 shows that the slope s from plots of In k', vs nc (cf. eq 7 ) is linearly dependent on the volume fraction v, of water in the stationary phase s =a
+ b&',
(9)
where
and
For a series of alkyl or perfluoroalkyl homologues, s denotes the incremental change in the logarithm of the capacity ratio due to addition of a methylene or perfluoromethylene group to the alkyl chain of the solute. The volume fraction v, of the stationary phase is a function of the mobile phase composition, $I,, and may be evaluated independently by measuring the excess sorption isotherm using a mixture of solvents identical with the chromatographic mobile phase. This method of evaluating the surface phase composition in order to analyze retention measurements has been successfully used in normal-phase LC studies (7,21,22). An alternative method for modeling retention data to that discussed above is to assume a sorption model and to express $*, as a function of the mobile phase composition. Again if the third term in eq 1 is assumed to be neglible, $uw may be approximated by the following equation (35)
V, = 1 - e ~ p ~ - ~ , , ~ ~ , / ~ ~ (12) ,~ where $l, and $lo are respectively the volume fractions of water and organic solvent in the mobile phase and satisfy the condition: $', + $], = 1. K,, is generally a concentration-dependent parameter, which for nearly ideal adsorption systems can be considered as the equilibrium constant that characterizes the competitive sorption of solvents on a solid sorbent. A combination of eqs 9 and 12 gives s=a
+ b - b exp(-Kwo~lw/qblo)
For a pure aqueous mobile phase ($Iw gives
s=a
+b
= 1 and s,
$lo
(13) = 0) eq 13
(14)
In the current study, the quantity s, denotes either the methylene or the perfluoromethylene increment for a chromatographic system with pure water as the mobile phase. The value of s, may be evaluated experimentally by extrapolation of plots of s vs $l, to $Iw = 1. Because the ordering of a chemically bonded phase can change with the composition of the mobile phase (36), the value s, determined by extrapolation represents a hypothetical group increment which characterizes retention for the pure aqueous mobile phase by neglecting the influence of ordering and changes in ordering of the chemically bonded phase.
Combining eqs 3 and 4 gives s, - s = b exp(-Kw0q9~/q9,)
(15)
or
In (s, - s) = In b - Kwo~lw/$lo
(16)
where 4l0 = 1 - 41W
In this latter relationship since In (s, - s) is linearly dependent on $lw/$l0, eq 16 is potentially useful for evaluating s as a function of the mobile phase's composition, $I, and has been applied to experimental data obtained in the current study.
EXPERIMENTAL SECTION Reagents. The HPLC grade methanol and acetonitrile were obtained from Fisher Scientific (Pittsburgh, PA). The deionized water was purified with a Millipore (Milford,MA) Model Mill-& system. The hydro-organic mobile phases were prepared at different volume to volume compositions and degassed before using. The ethyl alkanoate esters [CH3(CH2)mCOOC2H5 for m = 0, 1, 2, 3, and 41 and methyl perfluoroalkanoate esters [CF3(CF,),COOCH:, for m = 0, 1,2] were purchased from the Aldrich Chemical Co. (Milwaukee, WI). Samples of these solutes were prepared in pure methanol at an approximate concentration of 1.0 mg/mL. Chromatographic Packings. The chromatographic packings were prepared in-house by the following procedures. Two grams of LiChrosorb Si-60 silica (E. Merck, Cherry Hill, NJ) was rinsed with deionized water, dried at 383 K for at least 4 h, and refluxed overnight in a mixture of 10 mL of a given monochlorosilane (i.e., octyl- or octadecylchlorosilane)and 50 mL of dry toluene. During the reaction, the solution was mixed and outgassed with a stream of dry nitrogen. The modified silica was washed sequentially with four 50-mL portions of dry toluene, two 50-mL portions of water-saturated toluene, and two 50-mL portions of diethyl ether. The modified silica was dried in an oven at 383 K for 4 h. The chemically bonded octyl or octadecyl phases were packed in upward fashion into 15 cm X 2.4 mm i.d. stainless steel column blanks by using a dynamic slurry procedure. The suspension solvent was 2-propanol and the delivery solvent was methanol. A Haskel (Burbank, CA) Model DSTV-52C air-driven fluid pump was used to pressurize the system. Chromatographic Measurements. All chromatographic measurements were carried out with a Laboratory Data Control (Riviera Beach, FL) Model ConstMetric I pump, a Rheodyne (Burbank, CA) Model 7120 injection valve fitted with a 10-rL loop, and an IBM Instruments (Danbury, CT) Model LC/9525 refractive index detector. Retention data were recorded and processed on an IBM Instruments Model 9000 data system. Column temperature was maintained at 303 K in a water bath using a Fisher Model 730 controller (Pittsburgh, PA). Retention measurements were made for the five ethyl alkanoate and three methyl perfluoroalkanoate esters on the C8 and C18columns at varying compositions of water-methanol and water-acetonitrile mobile phases at a flow rate of 1.0 mL/min. The void volume was determined with either D,O or methanol. The reported values of the capacity ratio k : are averages from at least duplicate injections.
RESULTS AND DISCUSSION Shown in Figures 1 and 2 are retention data for ethyl alkanoate (open symbols) and methyl perfluoroalkanoate (closed symbols) esters plotted as the natural logarithm of the capacity ratio, k',, vs the number of carbon atoms, nc, in the chain portion of the solute. These data were obtained by using various binary mixtures of water-methanol (Figure 1) and water-acetonitrile (Figure 2) as the mobile phases on the C8 (panels A) and C18 (panels B) columns. At a constant composition of the mobile phase, the plots are linear for each of the chromatographic systems studied, which is consistent with other experimental studies (23, 25, 27, 31). The In k'# is proportional to the free energy of transfer of a solute from the mobile phase to the stationary phase and thus is linearly
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
2095
4, A
3 --
,.
2 --
1 5
1
I
A
/
-cm
m
3 c
-
1-
0.3
--
0
0.2
--
-1
0.0 I eW=O.5 .A
3-
I
--
2
B
- ca ........ c1a
= 0.5
m
1--
-l"
-_
0
-1
--
-2
4
1
0
2
3
Carbon Number,
4
0.0
5
"t
t I
0,=0.6
I 9,=0.6
2 --
0.4
m
1-
-
0 --1
--
-2
7
Flgure 2. Experimental dependences of the natural logarithm of the capacity ratlo (In k'J on the number n, of carbon atoms in the alkyl chain of a homologous solute for ethyl akanoate (solid lines wlth open symbols) and methyl perfluoroalkanoate (dotted lines with closed symbols) esters In the water-acetonitrile mobile phase on C, (panel A) and C18(panel B) columns at 303 K measured at different volume fractions of water, $Iw.
0.0
0.6
I
1.o
e, I
Figure 3. Slope s = d In k',ldnc plotted against the volume fractlon
4 Iw of water in the water-methanol (panel A) and water-acetonitrile (panel B) mobile phases for ethyl akanoate (open symbols) and methyl perfluoroalkanoate (closed symbols) esters on C, (solid lines) and C, (dotted lines) columns. Table I. Calculated Thermodynamic Quantities ,s, K,, f o r the Chromatographic Systems Studied
chemically bonded phase
B
0.4
Volume Fraction,
capacity ratio (In k'J on the number nc of carbon atoms in the alkyl chain of a homologous solute for ethyl alkanoate (solid lines with open symbols) and methyl perfluoroalkanoate (dotted lines with closed symbols) esters in the water-methanol mobile phase on C, (panel A) and C, (panel B) columns at 303 K measured at different volume fractions of water, 4Iw.
3 --
0.2
nc
Figure 1. Experlmental dependences of the natural logarithm of the
-l
4
0.0
organic solvent'
homologous seriesb
CHSOH CHSOH CH30H CHSOH CHSCN CHSCN CHSCN CH&N
EtA MeFA EtA MeFA EtA MeFA EtA MeFA
s,
1.18 1.75 1.19 1.73 1.33 1.73 1.41 1.97
so, and
so
Kwo
0.14 0.08 0.24 0.20 0.07 0.03 0.18 0.16
0.62 0.70 0.68 0.69 0.28 0.30 0.28 0.25
Organic component of the hydroorganic mobile phases studied. EtA denotes ethyl alkanoate esters, MeFA denotes the perfluoroalkanoate esters.
dependent on the number of carbon atoms in the chain portion of a homologue. The plots shown in Figures 1 and 2 also satisfy eq 7, which predicts a linear relationship for In k', vs at a constant surface phase composition, i.e., duw= constant. The surface phase composition is related to but not necessarily the same constant composition as the mobile phase, i.e., 4lw = constant. The slopes, s d In k ',/dnc, of the lines shown in Figures 1 and 2, were calculated for each of the mobile phase compositions studied. These results are plotted in Figure 3 vs the volume fraction of water, 4Iw,in the mobile phase for the alkanoate and perfluoroalkanoate solutes. The watermethanol points fit a nearly linear relationship on both the C8and CI8columns (Figure 3A) whereas the watel-acetonitrile points deviate significantly from linearity (Figure 3B). Similar linearity of s vs $Iw for water-methanol mobile phases also has been observed in other investigations (25,28,31). Each of the s vs $Iw curves were extrapolated to $Iw= 1(pure water) in order to obtain the values of sw, which are summarized in
2096
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER , 1990
Table 11. Ratio of the Perfluoromethylene Increment to the Methylene Increment for Single-Component Mobile Phases
chemically bonded phase
one-component mobile phase water water methanol methanol acetonitrile acetonitrile
0.8
-0.6 - _
-0.8--
1.48" 1.30b 1.45' 1.40b 0.57
0.83 0.43 0.89
" Calculated from the values of s, for the perfluoromethylene and methylene groups in water-methanol mobile phases. * Calculated as above for water-acetontrile mobile phases.
Jr
5 -0.2-0.4-
ratio r
0
-ca
'"''''.. C 1 8
properties of perfluoro compounds such as their critical micelle concentration (cmc), phase inversion temperature, and wetting characteristics have been compared to corresponding alkyl surfactants. For a given head group the surface-active properties of fluoroalkyl surfactants are equivalent to those of structurally similar alkyl surfactant having 1.5 times more carbon in the hydrophobic chain of the molecule (37). On the basis of these results it is logical to assume that the ratio r - s,(F)/s,(H), where the subscripts F and H refer respectively to perfluoroalkanoates and alkanoates, should be about 1.5. By use of the values of s, listed in Table I the s w m / s ~ratios H) (Table 11) were calculated for the water-methanol (i.e,, r = 1.48 for water-C8, and r = 1.45 for water-CI8) and wateracetonitrile (i.e., r = 1.3 for water-C8 and r = 1.4 for water-C,d mobile phases. These ratios are close to but less than 1.5 obtained for pure water. This negative deviation is not surprising based a more favorable solute-chain (methylenemethylene) interaction for the alkylalkanoates compared to solute-chain (perfluoromethylene-methylene) interaction for the perfluoroalkanoates. These results and arguments also are consistent with the solute transfer data obtained in pure organics. The values of r obtained in pure organic solvent are all less than unity (Table 11),which indicates that the transfer of the alkanoates from the organic mobile phase to the stationary phase is preferable in comparison to perfluoroalkanoates. This is reasonable based on polarity/interaction arguments. Additionally, the r values listed in Table I1 seem to be dependent on the chain length of the bonded phase. For the CI8 phase the r values are above 0.8, whereas for the c8 these values are about 0.5. These latter results would seem to be opposite from what might be expected based on polarity arguments. The surface with the longer octadecyl chains should contain more alkyl character than the surface with the shorter octyl chains. Consequently, on a relative basis the hydrocarbon solutes compared to the perfluorocarbon solutes might be expected to have at least the same if not a greater affinity for the octadecyl surface, which was not the case. The surface phase composition was calculated by using eq 12 for the values of K,, summarized in Table I. Since concentration of the solute is considered to be infinitely dilute, the values of K,, evaluated by analysis of retention data for a given column and binary mobile phase should be independent of the nature of the solute. The results obtained in the current study satisfy this condition (Table I). Since the values of K,, for both the C8 and CI8 columns for a given binary mobile phase were nearly identical, the average values of K,, were calculated. For water-methanol K, was 0.67 and for water-acetonitrile K,, was 0.28. In Figure 5A the volume fraction of water in the stationary phase changes as a function of the mobile phase composition for water-methanol (open symbols) and water-acetonitrile (closed symbols) on the
ANALYTICAL CHEMISTRY, VOL. 62, NO. 19, OCTOBER 1, 1990
0.9 0 0,:
A b l 8
e e
CY'
0
3
e
.O*
e
sorption of water over the total concentration region. Since eq 1 2 accounts for solution nonideality and predicts an azeotropic point for K,, C 1,it is a good representation of the surface phase compositions for the water-methanol and water-acetonitrile on the C8 and C18 columns. Further, it is important to note that the results presented in Figure 5 for water-methanol and water-acetonitrile on the C8 and C18 columns qualitatively agree with NMR studies of the above systems (39)as well as with measurements of the distribution isotherms for organic modifiers used in RPLC (40).
-.'e
o,.'
6 0.6 LL
-5 P
0.3
0.0 -B 0
I b*3
0.0
11 i
e
80 -0.2
-0.4 0.0
0
e
e 0
e
0.3 Volume Froction,
e
0.6
2097
e
(
e,!,
Figure 5. Surface phase composition as a function of the volume fraction of water in the water-methanol (open circles) and wateracetonitrile (closed circles) mobile phases on CB and C,, columns. Panel A presents the volume fraction 4uwof water in the surface phase, whereas panel B shows the sorption excess of water, 4 uw r$Iw In this phase. The dashed line in panel A denotes line 4 uw = 4 Iw (identical composttions of both phases), which gives the zero sorption excess, Le., 4 uw - +Iw = 0 (dashed line in panel B).
C8 and C18 columns. Similarly, Figure 5B shows the sorption excess of water, (6", - $Iw as a function of the volume fraction dl, for the above-mentioned systems. It follows from Figure 5A that at lower concentrations of water in the mobile phase, methanol is sorbed preferentially in the surface phase i.e., $"I, - +lo > 0 because d", - #Iw C 0. However, at higher concentrations of water in the mobile phase the water concentration in the surface phase is slightly greater than that in the mobile phase and the sorption excess of water, 4", > 0. This implies that at higher water concentrations the sorption excess for methanol is negative, i.e., V , - $lo < 0. A general feature of water-methanol-alkyl bonded phase system is that the difference between the compositions of the mobile and surface phases is not significant. The sorption excess is small and has an azeotropic point, i.e., point in which the compositions in both phases are identical ($J", = &,). Because of the small sorption excess of water for water-methanol on C8 and C18 columns (small difference in the compositions of both phases), the dependence s vs q9, for solutes in the water-methanol mobile phase is nearly linear (cf. Figure 3A). In contrast to the water-methanol-alkyl bonded phase system, the sorption excess for water from acetonitrile is negative almost over the whole concentration region (Le., 6,- +Iw < 0) which indicates that acetonitrile sorbs preferentially on alkyl bonded phase in comparison to water and its sorption excess is positive (v0 - c$, > 0). The constant K,, for water-acetonitrile on alkyl bonded phase is about twice as small as that for watermethanol. Further it should be noted that the values of K,, for both mobile phases on C8and C18columns are smaller than unity. For ideal systems, which are described by an Everett's type equation (381,K,, C 1 indicates a preferential sorption of organic solvent over the total concentration region, K,, = 1 indicates the same compositions in both phases (sorption excess is equal to zero), and K, > 1indicates the preferential
CONCLUSIONS Thermodynamic considerations show that a proper analysis of the retention data for homologous series should take into account the surface phase composition. Also, it is shown that use of eq 12 for describing the surface phase composition as a function of the mobile phase composition leads to a simple eq 16, which permits evaluation of the parameters that characterize solute interactions with chemically bonded phase and solvents. The retention data measured for ethyl alkanoate and methyl perfluoroalkanoate esters in the water-methanol and water-acetonitrile mobile phases on C8 and C18 columns are well represented by eq 16. The parameters s, and so of this equation permitted a quantitative comparison of solute behavior (expressed per methylene or perfluoromethylene group) in pure solvents (water, methanol, or acetonitrile) contacted with a C8 or C18 chemically bonded phase. Another parameter of this equation, K,,, permitted estimation of the surface phase composition as a function of the mobile phase composition. It is shown that for C8 and C18 columns this functional dependence is nearly identical; however, it depends strongly on the chemical nature of the organic solvent forming the water-organic phase. While for watermethanol on alkyl bonded phases the difference in the compositions of both phases is small, for water-acetonitrile, this difference is significant. LITERATURE CITED (1) Melander, W. R.; Horvath, Cs. I n HPLC: Advances and Perspectives; Horvath, Cs., Ed.; Academic: New York, 1980; Vol. 2, p 113. (2) Snyder, L. R.; Poppe, H. J . Chromatogr. 1980, 784, 363. (3) Scott, R. P. W.; Simpson, C. F. Faraday Symp. Chem. SOC. 1980, 75, 69. (4) Jaronkc, M.; Oscik. J. J . High Resolut. Chromatogr. Chromatogr. Commun. 1982, 5 , 3. (5) Snyder, L. R. In HPLC: Advances and Perspectlves; Horvath, Cs.. Ed.; Academic: New York, 1983; Vol. 3, p 157. (6) Jaroniec, M.; Jaroniec, J. A. J . L I p . Chromatogr. 1984, 7 , 393. (7) Jaronlec, M.; Martire, D. E.; Borowko, M. Adv. Colloid Interface Sci. 1985, 22, 177. (8) Jaroniec, M.; Martire, D. E. J . Chromatogr. 1986, 357, 1. (9) Martire, D. E.; Boehm, R. E. J . Phys. Chem. 1987, 97, 2433. (10) Martire, D. E. J . Lip. Chromatogr. 1988, 7 7 , 1779. (11) Dorsey, J. G.; Dill, K. A. Chem. Rev. 1989, 89, 331. (12) Oilpin, R. K.; Squires, J. A. J . Chromatogr. Sci. 1981, 79, 195. (13) Yang, S. S.; Oilpin, R. K. J . Chromatogr. 1987, 394, 295; 1988, 449, 115. (14) Gilpin, R. K.; Gangoda, M. E.; Krishen, A. E. J . Chromatogr. Sci. W82, 20,345. (15) Hemetsberger, H.; Behrens-Meyer, P.; Henning, J.; Ricken, H. Chromatographia 1979, 72,71. (18) Martire, D. E.; Boehm, R. E. J . Li9. Chromatogr. 1980, 3, 753. (17) Boehm, R. E.; Martire, D. E. J . Phys. Chem. 1980, 84, 3620. (18) Martire, D. E.; Boehm, R. E. J . Phys. Chem 1983, 8 7 , 1045. (19) Martire, D. E. J . Chromatogr. 1988, 452, 17. (20) Jaronlec, M.; Martire, D. E. J . Chromatogr. 1987, 387, 55. (21) Jaronlec, M.; Rozylo, J. K.; Osclk-Mendyk, B. J . Chromatogr. 1979, 779, 237. (22) Jaroniec, M.; Oscik-Mendyk, B. J . Chem. Soc., Faraday Trans. 7 1981, 7 7 , 1277. (23) Dorsey, J. G.; Johnson, B. P. J . Lip. Chromatogr. 1987, 70, 2695. (24) Krstulovic, A. M.; Colin, H.; Tchapla, A.; Guiochon. G. Chromatographie 1983, 77, 228. (25) Johnson, B. P.; Khaledi, M. G.; Dorsey, J. G. J . Chromatogr. 1987, 384, 221. (28) Melander. W. R.; Horvath, Cs. Chromatographia 1982. 75, 86. (27) Colin, H.; Gulochon, G.; Yun, 2.; Diez-Masa, J. C.; Jandera, J. J . Chromatogr. Sci. 1983, 27, 179. (28) Karger, B. L.; Gant. J. R.; Hartkopf, A.; Weiner, P. H. J . Chromatcgr. 1976, 728, 65. (29) Kram, M. P.; Jeannaux, F.; LeBlanc, M.; Riess, J. G.; Berthod, A. Anal. Chem. 1988, 8 0 , 1989.
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(38) Dabrowski, A.; Jaroniec, M.; Osck, J. C o I M S u d . Sci. 1987. 14, 83. (39) Gllpln, R. K. Unpubllshed data. (40) McCormlck, R. M.; Karger. B. L. Anal. Chem. 1980, 52, 2249: J . Chromatogr. 1980, 199, 259.
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(34) Snyder, L. R. Principles of Adsorption Chromatography; M. Dekker: New York, 1968. (35) Dabrowski. A.; Jaroniec, M. Acta Chim. Hung., in press. (36) Gllpin, R. K.; Gangode, M. E. J . Chromatogr. sei. 1983, 21, 352. (37) Oengode, M. E.;Gilpin, R . K. J . Chromatogr. 1988, 488, 365.
REZEIVED for review March 20,1990. Accepted June 26,1990. Support from DARPA-ONR, Contract N00014-K-0766, is acknowledged.
Accelerator Mass Spectrometric Determination of Carbon-14 in the Low-Polarity Organic Fraction of Atmospheric Particles Ann E.Sheffield*J Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Lloyd A. Currie a n d George A. Klouda
Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Douglas J. Donahue, Timothy W.Linick, and A. J. Timothy J u l l
N S F Facility for Radioisotope Analysis, University of Arizona, Tucson, Arizona 85721
Pdycyclk aromatlc hydrocarbons and other low-polarlty organlc compoumle (LPCs) were Isdated from flne atmospheric partlclee collected In Albuquerque, NY, during December 1985. A procedure for removing solvent and oxldizlng the LPC samples to CO, was developed. Recovery for the most volatlle compound studied (phenanthrene) was >90 % , and the procedural blank was 0.98 IL: 0.06 pg of C (standard error, SE, for n = 5 replicates). Sixteen samples, each contalnlng the LPC fractlon from a dlfferent aerosol sample, were prepared by this method and converted to targets for accelerator mass spectrometry. The 14C/1aCratlo was measured for each target. Samples contahed 38-470 pg of C. Hlgh beam currents (0.4-3.7 PA) and good Poisson stattstks (>goo counts) were obtalnsd. The l’c data were used to calculate the contribution of resldentlal wood combustlon (RWC) to LPC levels ln the Albuquerque atmosphere. At a resldentlal d e , RWC contributed 81 f 1% (SE,n = 6) to the nlghttlme LPCs and 60 f 8% (SE, n = 3) to daythe LPCs. At a roadway Intersection, the RWC contribution was 74 f 3% (SE, n = 5 ) at night and 47 f 7% (SE, n = 2) during the day.
INTRODUCTION In recent years, radiocarbon (14C)has become widely accepted as an atmospheric tracer of biogenic emissions and biomass combustion products (1-14). Radiocarbon is present at known, approximately steady-state levels in all living materials. When a plant or animal dies, the 14C in its tissues begins to decay with a half-life of 5730 years. In fossil fuels, all of the 14Chas decayed to 14N. When modern fuels (e.g., wood, paper) or fossil fuels (e.g., coal, oil) are burned, the Present address: Department of Chemistry, Allegheny College, Meadville, PA 16335. 0003-2700/90/0362-2098$02.50/0
carbonaceous particles and organic compounds that are formed contain the same level of 14Cas the original fuel. Similarly, emissions from living vegetation contain the 14Clevel characteristic of the plants. Therefore, if only two sources of carbon (one modern, one fossil) are present, measurement of the ratio of ‘4c to stable 13Cor ‘2c in a sample of carbonaceous material reveals its origin. Radiocarbon analysis was first applied to the source apportionment of atmospheric particles in 1955 by Clayton and co-workers (15) and again in 1960 by Lodge et al. (16),but the need for very large samples (grams) discouraged further application of the method until more sensitive analytical techniques became available. The development of miniature, well-shielded counters (17, 18) made possible radiocarbon measurements of samples containing as little as 5 mg of carbon (19) and led to a resurgence of interest in radiocarbon as an atmospheric tracer (1-3, 13, 20, 21). Recently, the advent of accelerator mass spectrometry (AMS) has allowed samples containing only a few tens of micrograms of carbon to be analyzed (22-24). The importance of AMS for environmental applications has been stressed by Currie and co-workers (17,231. Results have been reported for small (
