1388
Anal. Chem. 1987, 5 9 , 1388-1393
Liquid Chromatographic Separation and Indirect Detection of Organic Acids Using Iron(I I) 1, IO-Phenanthroline as a Mobile-Phase Additive Pantelis G. Rigas a n d Donald J. Pietrzyk*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Iron(I1) 1,lO-phenanthrollne [Fe(phen)t+] salts are used as mobliephase additives for the liquid chromatographic separation of organtc ackb as anlonlc analytes on a reverse statlonary phase. Major elution varlables that affect selectivity and reooIutkn are Fe(phen)t+ concenhatlon and tls retentlon on the statlonary phase, type and concentratlon of counteranlons, pH, buffer components and concentratlon, ionlc strength, organlc madlfler, and analyte anlon selectlvfty. Indirect detection Is used to detect nonabsorblng organlc analyte anlons by monltoring the effluent at 510 nm where Fe(phen):' exhibits maxlmum absorbance. Separatlons of mUnkompoMNlt mMures of alkylsulfonic, mono- and dlprotlc alkyl carboxyllc, and aromatic carboxylic acids are shown. Detection Ihnns, depending on analyte and condltions, approach 20 ng. A linear detector response was found for analyte anlon (propionate and lactate) concentratlons from 20 ng to over 10000 ng of InJectedanalyta.
The separation and determination of organic acids, such as carboxylic, phosphonic, and sulfonic acids, in complex mixtures is a difficult problem encountered in health, food, water, and environmental sciences and in related areas. Often the procedures used must be applicable to the separation of complex mixtures of simple organic acids including short alkyl chain mono- and diprotic acids. Modern chromatographic approaches for these kinds of separations are usually based on one of three major types of strategies. These are anion exchange (AExC), ion exclusion (IEC), and ion interaction (paired ion) (IIC) chromatography. A modern AExC strategy utilizes a strong, basic type anion exchanger as the stationary phase and an electrolyte containing mobile phase. The anion exchange selectivities of the analyte anions, the counteranions in the mobile phase, their concentration, and their competition for the anion exchange sites determines the quality of the separation (I,2). If a high capacity anion exchanger is used, then a more concentrated electrolyte containing mobile phase is required for a favorable elution time. This often causes detection problems since most simple, nonaromatic organic acids are nonchromophoric and are thus not easily detected at a favorable detection limit. Two strategies can solve this problem and allow favorable detection by conductivity. In one, a second column containing a high capacity cation exchanger is connected to the AExc column and suppresses the conductivity due to the electrolyte in the mobile phase (1-3). A similar more effective suppression is achieved with a membrane suppressor in place of the second column (4). The second general technique used to overcome the conductance detection problem due to the conducting mobile phase is to use a low-capacity anion exchanger column for the separation. Because of the low capacity, mobile-phase electrolyte concentration can be sharply reduced, which often eliminates the need for the suppression system (I, 2, 5). Collectively, these two ion exchange strategies are referred
to as ion chromatography (IC). They are widely used, and their areas of application have been reviewed extensively (I, 2,6). In general a poly(styrene-divinylbenzene) based anion exchanger is used in IC; however, bonded phase (1,2,6-8) and inorganic oxide stationary phase (9) anion exchangers can also be used with and without suppression. In IEC an ion exchanger and an electrolyte containing mobile phase are still used (I).The separation, however, is not dependent on ion exchange selectivities. Rather, the exchanger provides an electrolytic environment in which the analyte can distribute itself between the liquid held up on the stationary phase within the ion exchanger and the electrolyte containing mobile phase. While an anion exchanger can be used for organic acid IEC separations, a cation exchanger is preferred to avoid an anion exchange contribution. An aqueous mineral acid mobile phase is usually used to suppress ionization of the organic acids. Narrow peak widths are obtained in IEC because the retention process involves distribution rather than ion exchange. Because of this, IEC will often provide better resolution over AExC or IIC strategies. In the IIC separation of organic acid anions a hydrophobic ion, such as a tetraalkylammonium ion, is used as a mobilephase additive in combination with a reverse stationary phase (IO,11). An enhanced retention occurs because of interactions between the hydrophobic cation, its counteranion, the organic acid analyte anion, any other mobile-phase counteranions, and the stationary-phase surface. The type of hydrophobic cation and its concentration, the type and concentration of counteranion, the mobile-phase solvent, and the pH are the major parameters that can be altered to affect retention and resolution of analyte anion mixtures. We recently showed that iron(I1) 1,lO-phenanthroline, Fe(phen)32+, salts could be used effectively as mobile-phase additives for the IIC separation and detection of inorganic anions (12). This approach offers several major advantages. For example, nonchromophoric inorganic analyte anions are detected indirectly by absorbance in the visible region (510 nm), retention, efficiency, resolution, and detection limits were favorable, and calibration over several orders of magnitude was possible. This report focuses on the adaptation of this type of strategy toward the separation of simple organic acids, particularly those that are not chromophoric by themselves. EXPERIMENTAL SECTION Reagents. Instrumentation. Fe(phen)32+salts or concentrated solutions of the salts were obtained from GFS Chemical Co. Organic acids or their Na salts were purchased from Sigma Chemical and Curtin Matheson Scientific and were used as received. Prepacked 150 mm x 4.1 mm, 10 Fm spherical, macroporous, poly(styrene-divinylbenzene) columns (PRP-1) from Hamilton Co. were used. Buffer salts and ionic strength salts were analytical reagent grade. LC quality solvents were used to prepare mobile phases. Chromatographicinstrumentation used was described elsewhere (12). Procedures. Procedures for the preparation of analyte and mobile-phase solutions, column conditioning, and k'determination were discussed previously (12). Flow rate was 1.0 mL/min, column
0003-2700/87/035913SS$O 1.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
temperature was 30 "C, void volume was 1.0-1.3 mL, column inlet pressure depending on mobile-phase conditions was usually 800-1500 psi, and detection was monitored at 510 nm where the F e ( ~ h e n ) , ~salts + absorb. Organic acid analyte stock solutions were prepared by dissolving weighed quantities of the free acid or Na salt, if available, in LC water. Concentration for the general studies was usually about 1mg/5 mL. For preparation of the calibration curve, solutions were 500 or 1000 ppm, and these were used to prepare other standard solutions by successive dilution using class A type pipets (>20.0 mL) and volumetric flasks (>lo0 mL). A 20-rL fixed volume loop was used for sample injection (calibration curve) in combination with either a Micromeritics 725 automatic injector or a Rheodyne 7125 injector. Multiple determinations were performed, and calibration curves were determined by using a least-square linear regression method.
RESULTS AND DISCUSSION A major difficulty encountered in the LC separation and determination of simple nonchromophoric organic acids is their detection at favorable detection limits. Recently, we demonstrated that indirect photometric detection (IPD) using a Fe(phen)?+ salt as a mobilephase additive was both versatile and sensitive when applied to the separation of inorganic analyte anions (12). In this IIC strategy the F e ( ~ h e n ) salt ~~+ serves as both the ion interaction reagent and a chromophoric marker for the presence of the inorganic analyte anion. The pH range of F e ( ~ h e n ) salt ~ ~ +stability is broad and extends into the basic side. Thus, the conditions are favorable so that even weak organic acids will be dissociated and are candidates for separation as anions by this strategy. A PRP-1 column, which is a poly(styrene-divinylbenzene) copolymer that is stable in a basic pH environment, was used throughout these studies. The column is first equilibrated with the Fe(phen)32+salt containing mobile phase to produce a Fe(phen)32+salt equilibrated PRP-1 surface, where the amount of F e ( ~ h e n )salt ~~+ retained per column is dependent on the mobile-phase conditions (12). As the F e ( ~ h e n ) , ~salt + containing mobile phase passes through the absorbance detector, its absorbance is electronically offset by adjustment usually to zero absorbance. When the organic analyte anion passes through the column, it competes with other mobilephase counteranions for the retained F e ( ~ h e n )site. ~ ~ +If IIC is followed the overall dynamic process leading to the enhanced retention of the organic anion is the result of two major equilibria represented by eq 1 and 2, where C- is the mo-
PRP-1 + Fe(phen)2++ 2C- e PRP-l.-Fe(phen)2+.2C(1)
+
PRP-l-Fe(phen)2+.2C20A- + PRP-I -Fe(phe11)~+.20A-
+ 2C-
(2)
bile-phase counteranion and OA- is the organic analyte anion. As the organic analyte anion passes through the column, the equilibrium Fe(phen)32+salt concentration in the analyte zone is altered, and thus, when this zone passes through the detector, an absorbance difference relative to the background is recorded. The size of this difference is proportional to organic analyte anion concentration, and calibration is therefore possible via IPD. The Fe(phen)?+ salt mobile phase offers several advantages while providing both a favorable IIC and IPD condition. For example, F e ( ~ h e n ) salts ~ ~ +are highly retained by PRP-1 and thus the mobile-phase Fe(phen):+ salt concentration required to maintain a favorable equilibrium level of Fe(phen),2+ salt on the PRP-1 surface is low. Consequently, a photometric offset error that would occur with a more concentrated Fe(phen)? mobile phase is avoided. Secondly, a small alteration of the F e ( ~ h e n ) salt ~ ~ +concentration in the organic analyte anion zone is readily detected, since the Fe(phen)?+ salt molar
1389
a. Lactic b. Propionic o-tlydmmtyric
~oxlo-4Y He*rJ3So,
C.
d. SP-1 e.
K
SP-2
e
I' \
mL
mL rnL Ftgure 1. Effect of mobile-phase Fe(phen),SO, concentratton on the retention of organic acid analytes. A 150 mm X 4.1 mm, 10 fim, PRP-1 column and a Fe(phen)$O,, 0.50 mM acetate buffer (pH 6.20) mobile phase at 30 O C and a 1.0 mL/min flow rate with indirect photometric detection at 510 nm.
absorptivity at 510 nm is large. Several parameters will influence the retention of the organic and@ anions by affecting retention of the Fe(phen):+ salt on the PRP-1 (eq l),the anion exchange selectivity (eq 21, or both of these. These are concentration of Fe(phen),2+ salt, ionic strength, buffer concentration, the type and concentration of counteranions provided by these salts, mobilephase solvent composition, type of organic modifier, and pH. The range over which these parameters influence inorganic analyte retention, their optimization for favorable separation (12),and the influence and optimization of these parameters on IPD (13) are discussed elsewhere. Fe(~hen),~+ Salt. Figure 1 shows three chromatograms for the separation of organic analyte anions that illustrate the importance of adjusting the mobile phase F e ( ~ h e n ) , ~salt + concentration (only the extremes, Figure 1A,C, and optimum, Figure lB, of the range studied are shown). All organic acid analytes are in their anionic form due to nearly complete dissociation at the pH used in the mobile phase. Several conclusions are indicated in Figure 1. First, Figure 1 demonstrates that organic acid analyte anions are retained on PRP-1 in the presence of the F e ( ~ h e n ) , ~salt; + in its absence no retention is observed. The retention, which can be large, is dependent on organic acid structure. Selectivities, analyte peak shapes, column efficiency, and subsequently resolution are favorable. Second, Figure 1 reveals that IPD at 510 nm, where F e ( ~ h e n ) , ~salts + absorb, is both feasible and sensitive providing the mobile phase conditions are also optimized relative to an IPD strategy. Third, as the F e ( ~ h e n ) , ~salt + concentration increases, the amount of F e ( ~ h e n ) salt ~ ~ +retained (anion exchange capacity) on the PRP-1 surface increases (eq l), which should result in an increased analyte retention (eq 2). Accompanying the Fe(phen):+ salt, however, is a strong divalent counteranion, S042-,which also increases in concentration as Fe(phen),SO, mobile-phase concentration increases. This strong counteranion opposes increased anion exchange capacity effects because of its favorable anion selectivity, eventually dominates, and subsequently causes the retention of the organic analyte anion to decrease when the Fe(phen),S04 concentration becomes high enough. At the 0.10 mM Fe(phen)&304concentration (see Figure lB),17 pmol of Fe(phen)?+ salt is retained per column and corresponds to 34 pmol of apparent anion exchange capacity. This appears to be optimum and is consistent with the optimum Fe(phen)3z+ salt concentration found when studying inorganic analyte
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
Table I. Effect of Counteranion on Organic Acid Anion Retention organic acid analyte glycolic . formic acetic lactic propionic acetoacetic pyruvic a-hydroxybutyric chloroacetic isobutyric butyric SP*( C ) SPb (succinate)
"1
capacity factor, k' Fe(phen),W mobile phasesa C = SO4*- C = C10; C = Cl- C = F5.08 5.63 5.85 6.76 10.7 12.3 12.7 14.1 18.0 27 29
6.57 6.88 6.82 7.62 10.9 12.2 12.6 13.2 17.0 23 25
61
68
6.54 6.94 7.36 8.29 13.2 15.1 15.5 17.2 21 30 31 9.37 85
7.53 7.84 8.10 9.22 14.4 16.6 17.1 18.9 24 34 37 6.64 84
"A 150 mm X 4.1 mm, 10 p m , PRP-1column and an aqueous 1.0 lo4 M Fe(phen),W (where C = S042-, C104-, C1-, or P), 1.0 X lo4 M Na succinate buffer, (pH 6.10) mobile phase at 1.0 mL/ min. System peak.
01 0
I
I
I
I
I
2
3
4
5
6
Ionic Strength, [NaC104], M(X10e3)
X
anion retention (12). Fourth, the 0.10 mM F e ( ~ h e n ) ~ S O , concentration (see Figure 1B) is also an optimum concentration for IPD. At lower concentrations (see Figure 1A) the perturbation difference of F e ( ~ h e n )salt ~ ~ in + the analyte zone vs. the background is small while at higher mobile-phase Fe(phen)32+salt concentrations (see Figure IC) the perturbation difference is also small because of ionic strength effects; a discussion of ionic strength effects on IPD when using a Fe(phen)32+salt as a mobile-phase additive is provided elsewhere (13). The mobile phases used in Figure 1contain two counteranions, acetate (SP-1)and sulfate (SP-2). They are responsible for the system peaks, their location, and the type of analyte peak (absorbance increase or decrease relative to the background absorbance). Counteranion, Buffer, and Ionic Strength. Table I demonstrates how organic analyte anion retention is affected by the type of counteranion in the mobile phase. Four different counteranions were studied and introduced by using Fe(~hen)~~+.2Csalts. Even though the succinate buffer also provides a modestly strong divalent eluent counteranion at pH 6.10, the effect of the second counteranion C- is still observed. Of the four counteranions tested, eluent strength follows the order S042- 2 Clod- > C1- > F.This is also the retention order exhibited by these anions as analytes; a more modest change in eluent strength can therefore be achieved by using other inorganic or organic counteranions according to their anion exchange selectivities (12). In Table I all organic acids are analyte anions at the mobile-phase pH, and the data clearly indicate a favorable selectivity among closely related acids. Although not shown, chromatographic peaks, which were detected by IPD, were sharp and well-defined. If the ionic strength in Table I is increased by the addition of a NaC salt to the mobile phase, organic analyte anion retention decreases primarily because of mass action effects due to the increase in counteranion concentration (see eq 2) that occurs as ionic strength increases. This effect is illustrated in Figure 2 where NaC104 is added to the mobile phase. If NaF is used for the ionic strength salt as in Figure 2, a retention decrease also occurs, but the change is not as great as with the NaC104 even though ionic strength conditions are comparable. The reason for this difference is because F is a weaker eluent counteranion compared to C104-. A significant increase in mobile-phase ionic strength will also affect IPD; the limits of this effect is currently being studied. A decrease in retention also occurs if the buffer concentration is increased.
I
1
Figure 2. Effect of mobile-phase ionic strength on the retention of organic acid analytes. The conditions are the same as those in Figure 1 except 0.10 mM Fe(phen),(CIO,), (pH 5.30) and added NaCIO,.
301
k' 15-
10
-
5-
0-1 r 0
I
I
I
I
1
2
4
6
8
10
Percent MeOH Flgure 3. Effect of MeOH organic modifier on the retention of organic acid analytes. The conditions are the same as those in Figure 1 except 0.10 mM Fe(phen),(CIO,),, 0.10 mM succinate buffer (pH 6.1), and
added MeOH. A change in retention at a fixed buffer concentration can also be obtained by switching to a different buffer counteranion; the direction of change depends on the anion exchange selectivity of the new buffer counteranion. Since two counteranions are present in the mobile phase in Table I, two system peaks are possible. No attempt was made to quantitatively establish their identity or presence in the chromatograms in these studies. The parameters that affect system peaks and their significance in this chromatographic strategy are currently being evaluated. Organic Solvent. If organic modifier is added to the mobile phase (the effect is MeOH < EtOH < CH3CN), retention of the Fe(phen)32+salt on PRP-1 (eq 1) decreases. This reduces the apparent anion exchange capacity, and thus, organic analyte anion retention decreases. For example at 10% MeOH less than 6 pmol of F e ( ~ h e n ) ~ SisOretained ~ per column vs. 17 pmol a t 0% MeOH. This effect is illustrated in Figure 3. At the pH used, the organic acid analytes are anions and are not retained by the PRP-1 in the absence of
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
P M h
R Q M h
Pmplontc a
a 4-OH-Banrdc b 3-0H-6& L hnmk d. SP-1 0. 9 - 2
a
Lactlc
1. 9 - 2 0
Acotlc
4 Forinlc
O 4
4.5
5
5.5
6
7i
6.5
0
10
1
1
1
1
1
20
30
40
50
60
PH
rnL
Flgure 4. Effect of mobile-phase pH on the retentlon of organic acid analytes. The conditions are the same as those in Figure 1 except 0.10 mM Fe(phen),SO,, 1.0 mM Na2S0,, and 0.1 mM succinate buffer (pH 6.1).
Flgure 6. Separation of benzoic, chlorcacetic, and alkyl sulfonic acids: A, a 50 mm X 4.1 mm, 10 pm, PRP-1 column and a 0.10 mM Fe-
(pt-~enb(CIO,)~,0.10 mM citrate buffer (pH 6.8), 496 CH3CN/H20mobile phase at 30 OC and a 1.0 mL/min flow rate with indirect photometric detection at 510 nm; B, same as A except 100% H20; C, same as A except pH 7.15 and 5:95 CH,CN/H,O.
0
10
20
30
40 rnL
50
60
70
8
F w e 5. Separation of simple monoprotic organic acid analytes. The conditions are the same as those in Flgure 3 excluding MeOH.
the Fe(phen)32+salt even when the organic modifier is removed. Mobile-Phase pH, F e ( ~ h e n ) salt ~ ~ +solutions are stable over a wide pH range (14).When retention of several organic acid analyte anions was determined as a function of pH (see Figure 4), several major effects were noted. First, as long as the organic acid analytes are ionized, analyte selectivity remains constant. In Figure 4 pK, values for the organic acid analytes are in the range of 2.49-4.84 and above pH 5 retention and elution order is constant. An ionic strength salt that provides a strong eluent counteranion (SO:-) and a concentration at least 10 times the buffer was used in Figure 4 so that the effect of buffer counteranion and its concentration on the elution would be minimal. If either or both of these are altered appreciably as the pH is changed, analyte anion retention will be affected by counteranion selectivity and/or by the influence of these parameters on the retention of the F e ( ~ h e n ) salt ~ ~ +on PRP-1 (see eq 1). As the mobile-phase pH is lowered, organic acid analyte ionization is suppressed and several effects are apparent. (1) Succinate dissociation is also suppressed, and its strength as a counteranion decreases. (2) A second retention mode (reverse phase) is introduced whereby the undissociated organic acid analyte competes with the Fe(phen)32f salt for the PRP-I surface. Retention of undissociated carboxylic acids on PRP-1 has been reported (15).As the hydrophobic property of their alkyl side chain increases, retention increases; in Figure 4 the reversal in retention of propionic and pyruvic acids at lower pH is
I
01
mL
I
I
I
I
I
IO
20
30
40
rnL
Flgure 7. Separation of hmly retained mono- and diprotic organic acid analytes: A, same as Figure 1 except 0.10 mM Fe(phen),(CIO,), and 0.10 mM citrate buffer (pH 7.25); 8, same as Figure 6A.
probably due to this effect. (3) Indirect detection also becomes less sensitive since it is the anionic form of the analyte that is mainly responsible for the change in Fe(phen)32+concentration in the analyte band. Since detection is less favorable at low pH, this condition (pH 27OOO plates/m was calculated for the OAc- peak in Figure 5. The retention of organic acids in their anionic form is structure-dependent. However, the limited number of organic acid analyte anions studied does not permit a quantitative conclusion. While hydrophobicity of the side chain clearly influences retention, the trend is not always consistent and other factors are also important. For example, the polar hydroxy group causes a-hydroxybutyrate anion to be less retained than butyrate anion and lactate anion to be less retained than propionate anion while the nonpolar C1 group causes chloroacetate anion to be more retained than acetate anion. In contrast, the presence of the polar carbonyl group causes pyruvate anion to be more retained than propionate anion. Whether a quantitative correlation is possible or not requires the evaluation of many substituted acids. In Figure 5 the system peak is due to succinate. The chromatograms in Figure 6 are also examples of separations of monoprotic acids as anions except that the organic acid analytes are aromatic carboxylic acids or stronger acids. Since these organic acid analyte anions are strongly retained, the PRP-1 column length was significantlyreduced compared to that in Figure 5. In Figure 6A the benzoate anions are still detected by IF’D at 510 nm, and this separation demonstrates that this separation strategy is also applicable to a wide variety of other organic acids of more complex structures. However, in planning and optimizing these kinds of separations, it is important to consider that retention of more complex aromatic and hydrophobic acids may also involve retention by competing with the F e ( ~ h e n ) , ~salt + for the PRP-1 surface via reverse-phaseadsorption even though the analytes are in their anionic form (15,16). Parts B and C of Figure 6 demonstrate that strong organic acid anions can be separated and favorable selectivity differences are possible for even small structural changes. For example, increasing the number of C1 groups in acetic acid (see Figure 6B) and alkyl chain length in alkylsulfonic acids (see Figure 6C) causes a significant increase in their retention that permits favorable resolution of their mixtures. Excellent resolution is also obtained for higher alkyl chain sulfonic acids. However, their successful elution in a reasonable separation time requires a stronger eluent. This can be accomplished by a small increase in organic modifier concentration. Reduced retention is also possible by increasing counteranion concentration; however, this will also produce a less favorable detection limit because of its effect on IPD. In both cases excellent separations are obtained. In Figure 6 SP-1 is due to citrate and SP-2 (not shown in Figure 6C) is due to ClO,-. Figure 7 illustrates the separation of mixtures of more highly retained mono- and diprotic organic acids. At the mobile-
phase pH used the acids are completely dissociated. Thus, the diprotic acids are present as dianion analytes, which accounts for their high retention. A citrate buffer, which is also a trianion at the mobile-phase pH and providea a strong eluent counteranion, was used to reduce retention of these organic acid analyte anions. Column length was also reduced. Addition of salt or organic modifier to the mobile phase can also be used to reduce retention. The latter effect is illustrated in Figure 7B where CH3CNwas included in the mobile phase; the reduced analyte retention results because the CH3CN lowers the F e ( ~ h e n ) salt ~ ~ +retention on the PRP-1 (eq 1) producing less anion exchange sites on the surface. In Figure 7 SP-1 and SP-2 are due to citrate and C104-, respectively. All the separations shown in Figures 5-7 were performed at a mobile-phase pH where all organic acid analytes are dissociated. At lower pH where pK, values and reverse-phase retention are factors, selectivity can be changed. For example from Figure 4, pyruvate elutes before propionate at pH 6.00 while below pH 4.0 the reverse elution order is obtained. Previous studies (15,16) have shown that undissociated acids, particularly those with hydrophobic chains and/or aromatic groups are retained on PRP-1. Furthermore, this retention would be the largest in an aqueous mobile phase. However, indirect detection sensitivity is less favorable at pH conditions where association prevails. For this reason elution at low pH was not studied in detail. Calibration Detection Limits. If the mobile-phase conditions are optimized and held constant, calibration is possible by using IPD over a wide range of organic acid analyte anion concentration; the quantitative effects and limits of mobilephase variables, including ionic strength which is the major one, on peak heightlarea and location are currently being studied (13). With use of the same mobile- and stationaryphase conditions as in Figure 5, linear calibration curves were obtained for lactate and propionate analyte anions over the range of 20-10000 ng of organic acid as the Na salt. No attempt was made to determine linearity above 10OOO ng. At this analyte level approximately 0.5% of the available anion exchange capacity provided by the retained Fe(phen),2+ salt on the PRP-1 surface is utilized. At 20 ng peak height to base-line noise was about 51, and it appears that linearity is still possible at lower analyte loadings provided more sensitive, offset instrumentation is available. For lactate, the calibration curve fits the equation (peak area X = 0.308 (ng of lactate) - 12.509 with a correlation coefficient of 0.9997 while for propionate the equation is (peak area X lo-,) = 0.369(ng of propionate) - 5.70 with a 0.9999 correlation coefficient. If mobile-phase ionic strength or buffer concentration (also depends on the type of counteranion and its anion exchange selectivity) is increased, the slope for the resulting calibration curve for the new mobile-phase condition decreases. Also the detection limit will decrease.
LITERATURE CITED Fritz, J. S.; Gjerde, D. T.; Pohlandt. C. Ion Chromatography; Hiitkig, Verlag: Hledelberg, 1982. Smith, F. C., Jr.; Chang, R. C. The Practice of Ion Chromatography: Wiley-Interscience: New York, 1963. Small, H.; Stevens, T. S.: human, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. Stilllan, J. LC Magazine lS85, 3 , 802-812. Gjerde. D. T.; Fritz, J. S.; Schmuckler, G. J . Chromatogr. 1979, 786, 509-5 19. Haddad, P. R.; Heckenberg, A. L. J . Chromatogr. 1984, 300, 357-394. Stevenson. R. L.; Harrison, K. Am. Lab. (Fairfield, Conn.) 1981, 73(5),76-81. Matsushita, S.; Tada, Y.; Baba, N.; Hosako, K. J . Chromotogr. 1983, 259, 459-464. Schmitt. 0. L.; Pietrzyk, D. J. Anal. Chem. 1985, 5 7 , 2247-2253. Chromatogr. Scl. 1985, 37, 1, 27, 77, 141, 207, 259. Iskandarani, 2.; Pietrzyk, D. J. Anal. Chem. 1982, 5 4 , 1065-1071. Rigas, P. G.; Pletrzyk. D. J. Anal. Chem. 1986, 5 8 , 2226-2233. Pletrzyk, D. J.: Rigas, P. G. Presented at Pittsburgh Conference and Exposition, Atlantic City, NJ. March 1986: paper 986.
Anal. Chem. 1907, 59, 1393-1401 (14) Schilt, A. A. Analytllccll Applications of 1 , IO-PhenrmthrdIneand Relafed Compounds; Pergamon Ress: Oxford, 1969. (15) Lee, D. P. J . Chromtogr. Scl. 1982, 20,203-208. (16) Piet~~yk, D. J.; Chu, C. H. A M I . Chem. 1977, 49, 860-867.
RyecEIvED for review October 15,1986. Accepted February
1393
4,1987. Part of this work was supported by Grant AM28077 awarded by the National Institute of Arthritis, Diabetes, Digestive, and Kidney Diseases and was presented in part at the Tenth International Symposium on Column Liquid Chromatography, San Francisco, CA, May 1986, paper 1102.
Simulated Distillation of High-Boiling Petroleum Fractions by Capillary Supercritical Fluid Chromatography and Vacuum Thermal Gravimetric Analysis Herbert
E. Schwartz* a n d Robert G . Brownlee
Brownlee Labs, 2045 Martin Avenue, Santa Clara, California 95050 Mieczyslaw M. Boduszynski and F u S u
Chevron Research Company, 576 Standard Avenue, Richmond, California 94802
Caplllary supercrltlcal fluid chromatography (SFC) and vacuum thermal gravlmetrlc analysls (VTGA) were utlllzed for simulated dlstlllatlon (SIMDIS) of hlgh-bolllng petroleum fractions obtalned by short-path vacuum distllatbn. The SFC method covers the approxhrate bolllng range of 250-1400 OF. Under the present condltlons, even 42% of a nondlstlllable, nondeasphalted resldue was recovered from the column at a calculated 1428 OF atmospherlc equlvalent bolllng polnt. The Influence of temperature and pressure on resolution and retentbn was studled. SFCSIMDIS was performed by uslng linear pressure programming at 100 OC, as compared to 400 OC+ temperatures requlred for comparable samples when caplllary gas chromatography (GC) Is employed. Polysiloxane-coated, 50-pm-1.d. columns permltted fast analyses (30-mln run time), yet malntalned adequate resolution for SIMDIS analysls. VTGA-SIMDIS data of samples In the 500-1000 OF boiling range ylelded excellent correlatlon with actual dlstlllatlon data. However, thermal analysis revealed decomposnion of samples at temperatures exceedlng ca. 370 OC. The validity of the SFC method was demonstrated by comparlng SFGSIMDIS data with those obtained by GC and VTGA.
Distillation is the most widely used separation technique in the petroleum industry. Knowledge of boiling point distribution data has crude oils and refiied petroleum products is essential for process control and quality assurance. Distillation procedures, standardized by the American Society for Testing and Materials (ASTM), date back to 1926. The classical distillation procedures such as the ASTM D86 (I), the D1160 ( 2 ) ,and the D2892 (3) require large sample sizes and are generally less precise than simulated distillation (SIMDIS) methods based on gas chromatography (GC) ( 4 , 5 ) . In GC-SIMDIS, the gas chromatograph can be regarded as a highly efficient microdistillation unit, while, interestingly enough, chromatography under low resolution conditions is performed (6). The GC-SIMDIS for petroleum derived materials has been reviewed by Butler (7). The applicability of GC-SIMDIS to the analysis of coal-derived liquids has also been established (8,9).Green et al. (10) and Jackson et al. (11)have demonstrahd that GC-SIMDISdata are in excellent 0003-2700/87/0359-1393$01.50/0
agreement with actual distillation methods. GC-SIMDIS methods, adopted by the ASTM since 1973, involve the use of packed columns with a nonpolar silicone gum as a stationary p h . The upper limit of the boiling range covered by these methods is approximately 1000 O F atmospheric equivalent boiling point (AEBP) (In this paper, the Fahrenheit scale is used for boiling point distribution data in accordance with ASTM methods and most other publications. Experimental conditions, as is the custom, are cited in degrees Celsius). Efforts have been undertaken to extend the scope of the GC-SIMDIS methods by employing short, thin-film capillary columns (11,12,13).These columns are more favorable for the analysis of high-boiling fractions because of their increased phase ratio, which allows for a reduction of analysis time and column elution temperature. For instance, Trestianu et al. (13)found that elution temperatures on capillary columns were 100 "C lower than on corresponding packed columns. The authors demonstrated that capillary GC, combined with cold, on-column injection, is suitable for SIMDIS of petroleum fractions ranging from 300 to 1470 O F AEBP. In this approach, however, column temperatures of up to 430 "C are employed. In two other recent publications dealing with high-temperature GC, Lipsky and Duffy (14, 15) report on a new type of aluminum-clad,fused silica column for capillary GC. Crude oils were chromatographed with temperatures up to 440 "C. The high column temperatures far exceed the temperature limit (>350 "C) at which sample decomposition may occur and are of great concern to petroleum chemists. This problem will be discussed in more detail later in this paper. The search for milder operating conditions, i.e. lower temperatures, has led to the development of SIMDIS methods other than GC, e.g. gel permeation chromatography (GPC), vacuum thermal gravimetric analysis (VTGA), and supercritical fluid chromatography (SFC). In GPC, difficulties arise in the correlation of the "molecular size" distribution data to AEBP (16). VTGA techniques were described in the literature by Southern et al. (9)and by Mondragon and Ouchi (17). The first group employed the Maxwell-Bone11 equation (18) for calculation of AEBP, while the latter used a calibration method based on actual distillation data from the ASTM D1160 method (2). The VTGA method described in this paper utilized a different approach, i.e. VTGA temperatures are calibrated with n-alkane standards in a similar fashion as 0 1987 American Chemical Society
