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Anal. Chem. 1985, 57,362-364
A;
IO
20
30
LO
50
60
0 8KeV.5 l d 9 A cni2
70
80
90
100
110
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1LO
150 amy
Figure 1. Static SIMS spectrum of ammonium sulfate recorded in the positive mode after an ion dose of 1013 ionscm-2.
dependent on the metal-oxygen (M-O) bond and thus on the nature of the cation. For instance, the valence state of the metdoid could be not reflected in the negative SIMS spectra of calcium, lead, and magnesium salts in which the S-0 bond is weaker than the M-0 bond. In the recombination model, particles properly correlated in time, space, and velocity recombine in the gas phase to give the most stable species if the attractive potential energy is greater than the differences in their relative kinetic energies (9). As sulfate is the most stable, it results that as the lattice atomization becomes more severe, the more intense are the positive cluster ions containing hexavalent sulfur. It is reasonable to assume that, in SIMS, the amount of sample fragmentation is smaller than in LAMMA. It can be understood if one takes into account the way the energy is deposited and the abundance of low-energy tails in both techniques. In static SIMS, each of the discrete impact points is surrounded by a zone where the low-energy tails of the cascade are emerging. On the contrary, in LAMMA, energy is deposited in a single step and processes requiring a low power can only take place during the tail of the laser pulse. This discrepancy toward the energy deposition explains perhaps why, in LAMMA, Na3S04+is always higher than Na3S03+whereas, in SIMS, the Rs ratio is higher or lower than one depending if the salt is sulfite or sulfate. Regarding the formation of positive ion clusters, the nature of the cation also plays an essential role. Indeed, either the recombination of particles into cationized salt molecules (Na3S0,+) or the cationization of ejected intact molecules compete with the formation of oxides. Therefore, a salt such as calcium sulfate, in which the metal has a high oxygen affinity, does not exhibit in positive SIMS (IO) and LAMMA (11) spectra the pseudomolecular ions. Likewise, no MgzS04+or Pb2S04+is found in MgS04 and PbS04 (IO) because the metal makes prefer-
entially oxide ions. In the same respect, sulfates, in which the cation has a low oxygen affinity (Ag+,NH,+), show no ion cluster containing sulfite according as most of the oxygen is available for the formation of the sulfate species. As an example, we show in Figure 1 the positive static SIMS spectrum of ammonium sulfate; the only pseudomolecular ions present are [(NH4),H3,S041,+ (n = 0-3). In a comparison of the methods, it is also worth discussing the possible artifacts of both techniques. Whereas redox phenomena due to preferential sputtering and recoil implantation have been already pointed out (5) in SIMS, the contribution of the support in the LAMMA analysis of single micrometer-size particles could perhaps affect the ion distribution. For instance, in the experiments reported in ref 1,the oxygen contribution from the Formvar foil is estimated to be about 5%. Such an amount of oxygen could likely decrease the ratio RL. Indeed we have demonstrated (5) that the adsorption of less than 10% of a monolayer of oxygen on NaZSO4lowers the RS,13value from 0.70 down to 0.28; the latter value being the same as the one observed in LAMMA. In conclusion, the above observations suggest that similar ion formation mechanisms occur in SIMS and LAMMA. Nevertheless, the balance between the two models, direct ejection and atomization followed by recombination, is different. The process of direct ejection is likely less represented in LAMMA than in SIMS, as the deposited energy density is higher. Registry No. S, 7104-34-9; NazS03, 7157-83-7; Na2S04, 1757-82-6.
LITERATURE CITED (1) Bruynseeis, F. J.; Van Grieken, R. E. Anal. Chem. 1984, 5 6 , 871-873. (2) Marien, J.; De Pauw, E. Bull. SOC.Chim. Be@. 1979, 66,115-121. (3) De Pauw, E.; Marien, J. Springer Ser. Chem. Phys. 1979, 9 , 139-141. (4) De Pauw, E.; Marien, J. Bull. SOC. Chlm. 88@. 1980, 89, 83-84. (5) De Pauw, E.; Marien, J. Int. J. Mass Spectrom. Ion Phys. 1982, 43, 233-247. (6) Ganjel, J. D.; Colton, R. J.; Murday, J. S. Int. J . Mass Spectrom. Ion Phys. 1981, 3 7 , 49-65. (7) “Handbook of Chemistry and Physics”, 59th ed.; CRC Press: Boca Ratan, FL, 1979. (8) Plog, C.; Wiedman, L.; Benninghoven, A. Surf. Scl. 1977, 6 7 ,
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565-580.
(9) Murray, P. T.; Rabaiais, J. W. J. Am. Chem. SOC. 1981, 703, 1007-1013. (10) De Pauw, E. Doctoral Thesis, University of LiBge, 1981. (11) Bruynseeis, F. J.; Van Grieken, R. E. Spectrochlm. Acta 1983, 38, 853-858.
Jose Marien* Edwin De Pauw Laboratory for the Physicochemistry of Surfaces Institute of Chemistry (B-6) University of Libge B-4000-Liege,Belgium RECEIVED for review June 25,1984. Accepted September 28, 1984. The authors are grateful to the National Fund for Scientific Research (Belgium) for its continuous support.
Effect of Dilution by Achiral Solvents on Enantiomer Separation with N -Lauroy I-L-vaIine fert -Butylamide Sir: The chiral diamides, R1CONHCH(R2)CONHR3,are useful stationary phases for the resolution of optical isomers, particularly those containing amine functional groups. They are used extensively for the enantiomeric analysis of amino
acids by gas chromatography (GC) (I). Diamide molecules can interact with each other to form hydrogen-bonded associates (“dimers”, ”trimers”, etc.), for instance, of a type analogous to the pleated sheet @ structure
0003-2700/85/0357-0362$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
A
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B
o lo-
Figure 1. (A) Pleated sheet @ structure of N-lauroyl-L-valine terf-butylamlde. (B) Schematic presentation of a "C5-C7" hydrogen-bonded ring between an N-TFA-L-a-amino acld isdpropyl ester and N-lauroyl-L-valine terf-butylamide. I
Leucine
L%
I .2
100-
05
IO
05
IO
X
X
Figure 3. Plots of resolution coefficient (aexp anal ad) of amino acid enantiomers vs. weight fraction [ X I of N-huroyk-valine tert-butylamide in the mixed phase: (A) on phase I mixed with SQ; (B) on phase I mixed with diethylene glycol succinate polyester (DEGS). The symbols are as follows: (A) Leu, (0)Ala, (0)Val. Equation 1 gives a plot of aadd vs. x, which Is concave with respect to the straight line passing through the polnts x = 1.00, a = a' and x = 0, a = 1.00 when D V t > V: [see Flgure 2, curve 8,and Figure 38, curve cu,(Leu)]. Also for the derivatives of Ala and Val, the values of ' V : are larger than those of V:, at 95 O C , on DEGS: Ala, 0.605 > 0.502; Val, 1.02 > 0.496. All ru,curves converge at the point (Y = 1.00, x = 0. Column conditions are as follows: Whisker-walled glass capillary (20 m length X 0.35 mm i.d.) coated by dynamic method; temperature 95 O C ; carrier gas, nitrogen (1 mL/mln).
X
Figure 2. Plots of the calculated resolution coefficient (aadd, based on eq 1) and the experimental values (ae,) for leucine vs. weight fraction x of N-lawoyk-vallne fert-butylamide In the mlxed phase with squalane [SQ]. At x = 0.95, apparently, a nonhomogeneous film Is formed on the capillary, leading to a reduction of aBxp by Increased absorption on the glass surface.
known to occur in certain proteins, as shown in Figure 1. The question has been raised (2),whether self-association affects the stereoselective behavior of the diamides. To answer this point, a study was undertaken of the chromatographic behavior of N-lauroyl-L-valinetert-butylamide (I),diluted with either squalane (SQ) or diethylene glycol succinate polyester (DEGS). The experimental resolution coefficients (aeq)on I and its mixtures with SQ and DEGS, respectively, were determined at 95 "C on whisker-walled Pyrex glass columns of 20 m X 0.35 mm i.d., coated by the dynamic method with 10-20% solutions of the phase in CH2C12. The capillary columns yielded accurate a values with a minimum of phase; they were not, however, suitable to yield good data for the specific retention volumes (V,). These were, therefore, measured on columns (2 m X 3 mm i.d.1 packed with 80-100 mesh of either Chromosorb W(AW) or G(AW DMCS) and coated with about 5-10% of phase. The solutes resolved were D,L-alanine, D,Lvaline, and D,L-leucine, which had been previously derivatized to their N-trifluoroacetyl (TFA) isopropyl esters. The experimental data represent the average of three measurements with a mean deviation of f0.5%. If the retentitivity due to each of the components in the mixture is assumed to be linearly additive, the corresponding resolution coefficient ( a a d d x ) on a mixed phase of composition ( x ) can be calculated by
where a1is the resolution coefficient on 100% pure I ( x = l), "Vp" is the specific retention volume of the D-amino acid enantiomer on phase I, VgAis the specific retention volume of the amino acid derivative on an achiral phase, and x is the weight fraction of I in the phase.
't-
- 0
10
05
00
X
Flgure 4. Dependence of specific retention volume ( V,) on weight fraction of N-lauroyl-L-valine red-butylamide in the mixed phase with SQ: (0)L-Leu; (0)o-Leu. Slmllar plots have been obtained for the other N-TFA esters, extending for the derivatives of D-Val and M a l from V , = 0.235, for x = 0, to V, = 0.607, respectively, V , = 0.794 for x = 1.O; and for the derivatives of D-Ala and L-Ala from V , =: 0.589, for x = 0, to V, = 1.021, respectively, V, = 1.348 for x = 1.0.
The plots of the resolution coefficient ( a ) against the diamide diluted with SQ (Figure 2) show that the experimental values (aexp) are higher than the calculated ones (aadd) based on eq 1. It can be further seen (Figure 2 and 3A) that the resolution coefficients for the various solutes tested are unchanged throughout or even slightly higher than those on the pure diamide in the range of x = 0.5-1.0. In the range of x = 0.0-0.5, cyexp must decrease to 1.00, since by definition for x = 0 there can be no separation. On the other hand, the specific retention volume (VJ of the various solutes, as measured, e.g., on columns packed with 80-100 mesh Chrom G (AW DMCS) coated with 5-10% of phase, has been found to be approximately constant in the range of x = 0.5-1.0. In Figure 4 the corresponding plots are given for the D- and L-N-TFA-leucineisopropyl esters; for data on the derivatives of alanine and valine see Figure 4,legend. It would have been expected that on adding squalane, which has lower retentivity than the diamide for the amino acid derivatives, the Vgvalues would decrease. The findings can be explained by the solute-solvent association model (Figure l), which is assumed to explain the stereoselective effects. In the associated solvent molecules (Figure lA), part of the CO and NH groups, which can form solute-solvent associates, are blocked. However, on dilution with increase of the monomeric form (Figure lB), the number of hydrogen bonding sites
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Anal. Chem. 1985, 57, 364-366
available per unit weight of the diamide becomes larger, leading to stronger retentivity by the polar component of the mixture. As to the constancy of the aexpvalues over the range x = 0.5-1.0, it can be explained in part, but not entirely, by the increase of the specific retention volume (V,) on dilution, which counteracts the expected decrease of the measured resolution coefficient on adding SQ. In addition, however, it is necessary to postulate that the diamide phase in its diluted state also has a higher effective resolution coefficient (estimated at about 1.6 for the leucine derivative at 95 “C)than the pure compound. Entirely different results were obtained for the diamide/ DEGS mixtures (Figure 3B). If the effect of DEGS were only to introduce an additional interaction with the solute, the result would be expressed by the curve of vs. x (see Figure 3B). If in addition, a mere diluting effect would operate, as for squalane, the experimental values of the resolution coefficients would be above the a,dd plot. The experimentally found values are, however, lower than those calculated according to eq 1 and fall to 1.000 (Le., no chiral recognition) for x = 0.5. To explain the effect of DEGS, it has to be assumed that the carbonyl functional groups of this added achiral phase block the NH groups of the diamide and, thus, the formation of the stereoselective associates (Figure 1B) between the diamide and the solute is first reduced, and then annuled for x = 0.5. These experiments have thus demonstrated that the “monomeric” form of the diamide tends to have both higher
retention and somewhat higher resolution coefficients than the associated solvent, with respect to solutes such as the N-TFA amino acid esters. On the other hand, the addition of achiral phases which can hydrogen bond with the diamide is detrimental to their stereoselectivity. Registry No. DEGS,25569-53-3;SQ, 111-01-3;N-lauroyl-Lvaline tert-butylamide, 33105-36-1. LITERATURE CITED (1) Chang, Shu-Cheng; Charles, R.; Oil-Av, E. J . Chromfogr. 1082, 235, 87, and references thereln. (2) Felbuah, B.; Balan, A,; Altrnan, B.; Oil-Av, E. J . Chem. SOC.,Perkln Trans. 2 1070, 1230.
Toshiyuki Hobo Shigetaka Suzuki Faculty of Technology Tokyo Metropolitan University Fukasawa, Setagaya-Ku Tokyo, Japan Katsunori Watabe* Emanuel Gil-Av* Department of Organic Chemistry The Weizmann Institute of Science Rehovot 76100, Israel RECEIVED for review May 21, 1984. Accepted September 4, 1984.
Unified Model for Transient Potentials in Ion-Selective Electrodes at Zero Current Sir: The transient potentials observed during measurements using ion-selective electrodes have recently attracted considerable attention (1,2).The models suggested to account for such transients have been critically examined (3-5). The approximate expressions derived from the models are mainly of the following two types:
+ A exp(-Bt) E , = E , + A/(1 + Bt) E, = E,
(exponential behavior)
(1)
(hyperbolic behavior)
(2)
In eq 1and 2 E, is the emf a t time t, E, is some constant value at the final steady state, and A and B are parameters. However, the fit obtained with eq 1 and 2 is generally poor (I, 3). It should be noted that some authors have obtained equations slightly different from eq 1and 2 and, thus, have improved the fit in certain cases (4,5). Shatkay (3) suggested a model expressed by eq 3.
E , = E,
+ AI exp(-B,t) + A2 exp(-B2t)
(3)
Equation 3 allows a reasonable fit in most cases when eq 1 and 2 fail. It has been recommended by standard texts as practically helpful (6),but its theoretical basis requires further justification. While all three equations can account at least qualitatively for a monotonic change of the emf with time, eq 1 and 2 cannot possibly account for a passage through a local extremum. Now it is quite a common experience to encounter in transients an “overshot”. Such overshots are regarded as irrelevant and
rarely are reported in the literature. Even when they appear clearly in the experimental results (7),they are not discussed. A more detailed appearance of an overshot is given in Figure 1. Obviously a correct model of a transient should be able to account for an overshot. Equation 3 is formally capable of expressing a passage through a local extremum, when AI and A2 have opposite signs. This is a necessary but not a sufficient criterion. An even more complicated behavior of transients has been observed; sometimes a transient exhibits a series of strongly damped, or even sustained, oscillations. Oscillating potentials coupled with imposed current have been discussed in considerable detail (8-11).Isolated cases of oscillations without any external current have been reported (8,11, 12) with little comment. To the best of our knowledge, no effort has been made to connect such oscillationswith the kinetics of response of ion-selective electrodes. Careful consideration of Figures 5 and 9 of a previous publication (3) reveals a slight oscillation of the emf, which might be dismissed as a random fluctuation. Similar behavior can be observed in some other experimental results, with the same limitations. However, we present in Figures 2 and 3 some further results which illustrate better our above statements. A result very similar to that of our Figure 3 has been reported by Umezawa (12)for a very different system of electrodes.
0003-2700/85/0357-0384$01.50/00 1984 American Chemical Society
