Anal. Chem. 1988, 58, 2581-2583
54 represents about 300 ng of trimethylpyrazine. Indeed a comparison of the spectrum of Figure 6 with a spectrum of 59 ng of injected trimethylpyrazine showed this to be the case. Thus, very weak absorbers such as trimethylpyrazine can be quite nicely concentrated by the SDE technique to give excellent infrared spectra when the solvent is eliminated by the injector/trap. Still weaker GC peaks presently in the noise in the chromatogram of Figure 3 can be intensified for identification by injecting more Freon solution or carrying out the SDE for longer periods of time. There is no indication of column degradation or alteration to date. ACKNOWLEDGMENT We are grateful to J. Anast and T. H. Eichhold for constructing the injector/trap and writing the software for its control. We thank L.'V. Haynes, G. P. Rizzi, and P. A. Ro~
2581
driguez for many helpful discussions and for supplying some of the materials in this work.
LITERATURE CITED (1) Griffiths, P. R.; de Haseth, J. A.; Azarraga, L. V. Anal. Chem. 1983, 55, 1361A-1387A. (2) Yang, P. W. J.; Grlffiths, P. R. Appl. Spectrosc. 1984, 38, 816-821. Rodriguez, P. A.; Eddy, C. L.; RkMer, G. M.; Culbertson, C. R. J . Chm(3) matogr. 1982. 236, 39-49. (4) Griffiths, P. R. Appl. Spectrosc. 1977, 3 1 , 284-288. (5) Azarraga, L. V. Appl. Spectrosc. 1980, 3 4 , 224-225. (6) RiJks, J.; Curvers, J.; Noy, T.; Cramers, C. J . Chromatogr. 1983, 279, 395-407. (7) de Haseth, J. A.; Isenhour, T. L. Anal. C b m . 1977, 4 9 , 1977-1981. (8) Shankaranarayana, M. L.; Abraham, K. 0.; Raghavan, B.; Natarajan, C. P. CRC Crlt. Rev. Food Sci. Nub. 1975, 6 , 271-315.
RECEIVEDfor review March 3,1986. Accepted May 22,1986.
Characterization of Mixtures of Organic Acids by Ion-Exclusion Partition Chromatography-Mass Spectrometry Frank Pacholec,' David R. Eaton,* and David T. Rossi2
Monsanto Co., 800 North Lindbergh Boulevard, St. Louis, Missouri 63167 With the advent of thermospray as a means for mass spectrometric sample introduction (I), liquid chromatography-mass spectrometry (LC-MS) has increased in popularity (2). Many applications of thermospray LC-MS have already been documented, and a review of some of these applications has recently been published (3). Most thermospray LC-MS applications involve the reversed-phase high-performance liquid chromatographic (HPLC) separation of neutral species, with subsequent ionization in the thermospray process, and monitoring by mass spectrometry. In the past several years, there has been a notable increase in the amount of literature pertaining to the chromatography of ionic species. For the determination of organic acids, ion-exclusion partition chromatography (IEPC) offers some advantages over other liquid chromatographic techniques with respect to sensitivity, capacity, and simplicity of the chromatographic system (4). In IEPC, the pH of the mobile phase is adjusted so that the acids of interest are partially protonated. Separation occurs by the Donnan exclusion principle, whereby neutral species can enter the pores of the column packing and be retained, while ionic species are excluded from the pores. Because acids with lower pK, values spend more time in an ionized form, they elute from the column earlier than acids with higher pK, values. This work demonstrates for the first time the combination of IEPC with mass spectrometry through a thermospray interface, facilitating the acquisition of mass spectral data of the organic acids. Four monoprotic acids were studied in this preliminary work, ranging in molecular weight from 60 to 88 m u . Limits of detection were evaluated for thermospray and discharge ionization, in the positive-ion mode.
EXPERIMENTAL SECTION Reagents. Mobile phase was prepared with Ultrex hydrochloric acid (J. T. Baker, Phillipsburg, NJ). Acetic and glycolic acids were reagent grade (Fisher Scientific, Fair Lawn, NJ). Propionic Present address: Suprex Corp., SFC Research Center, 125 William Pitt Way, Pittsburgh, PA 15238. Present address: Adria Laboratories,5000 Post Rd., Columbus, OH 43216.
0003-2700/86/0358-2581$01.50/0
and butyric acids were obtained from the PolyScience Corp. (Evanston,IL), as part of standard kit 65A. Deionized water used in mobile phase preparation and dilution of standards was obtained from a Milli-Q water purification system (Millipore Corp., Bedford, MA). Apparatus. The ion chromatography system used to perform the initial organic acid separation was a Model 16 ion chromatograph equipped with a conductivity detector (Dionex Corp., Sunnyvale, CA). The IEPC/MS system consisted of a Model 8700xR pump and Rheodyne 7125 injector (Spectra-Physics,San Jose, CA), a Model 5985B GC/MS (Hewlett-Packard, Palo Alto, CA), and a Vestec thermospray LC/MS interface (Vestec Corp., Houston, TX). For all separations, a Dionex HPICE-AS1 column was used. The column was connected to the Rheodyne valve and thermospray interface with Knurl-Lok adapters (Alltech Associates, Deerfield, IL). When the separation was monitored by conductivity detection, a Dionex packed bed HPICE suppressor column was placed in series with the separator column. The suppressor column served to remove protons and chloride ions from the mobile phase, thereby lowering the background conductivity. (This suppressor column was not used in the IEPC/MS work.) The thermospray ion source should be cleaned approximatelyonce a week to remove nonvolatile chloride salts resulting from the use of unsuppressed 1mM HC1 mobile phase. Also, water was flushed through the entire system at the end of each day. As long as these cleaning and flushing procedures were followed, the HC1 mobile phase appeared to have no detrimental effect on the thermospray or chromatographic equipment.
RESULTS AND DISCUSSION IEPC was chosen for preliminary experimentation due to its high sample capacity relative to other types of analytical ion chromatography (IC) and the high volatility of its mobile M HC1) relative to other types of IC (Na2C03, phase (1.0 X etc.). Ion chromatography columns on which compounds separate by an ion-exchange mechanism can be overloaded at solute concentrations as low as 100 ppm. In IEPC, retention is due to an exclusion-partition mechanism and good peak shape is maintained for solute concentrations up to at least 0.10%. Therefore, if initial thermospray experiments were performed under nonoptimal conditions resulting in poor ionization efficiencies, some mass spectral data might still be 0 1986 American Chemical Society
2582
ANALYTICAL CHEMISTRY, VOL. 58, NO. 12,OCTOBER 1986
A
iA I'
hB
C
Time (mln)
Time (mln)
Figure 1. (A) Chromatogram of 10 ppm mixture each of glycolic (A), aceti (B), propionic (C), and butyric (D) acids on an HPICE-AS 1 cdumn with a packed bed suppressor; conductivity detection at 10 pS full scale; 1.0 X M HCI mobile phase at 0.8 mL/min. (B) Same as A but each acid is present at 1000 ppm and full scale conductivity = 1000 MS. If SPECTRUM DISPLPY,EDIT $ 0 0 0 PPM 6 I C 1 0 STD I C I M S D I S C H R R C E O N P S I I M M HCL 8 . 8 M L l M l N 2 4 0 0 V 3 1 D C T 8 5 D R E
**
FR!I 2 3 0 7 2 I S T SC,Pti 74 50 Y - 4 . 0 8
>-
Flgure 3. Total ion ICE/MS chromatogram of a 10 ppm mixture each of glycolic (A), acetic (B), propionic (C), and butyric (D) acids on an HPICE-AS1 column without a suppressor. Thermospray is operated under same conditions as in Figure 2.
3 100
Acetlc Acld
M
+ H' + ?Ha0
amu I
I
[
";
l M + H+
BuIyrlc Acid
a B
amu
Figure 4. Mass spectra of 100 ppm acetic and butyric acids obtained with discharge ionization and poshe ion scanning from 75 to 250 amu. Chromatographic and thermospray conditions are given in Figure 2. Time (Mln)
Flgure 2. Total ion IEPC/MS chromatogram of a 1000 ppm mixture each of glycolic (A), acetic (B), propionic (C) and butyric (D) acids on an HPICE-AS1 column without a suppressor column. Thermospray is operated in discharge ionization mode and positive ion scanning from 75 to 225 amu. Thermospray T , temperature = 107 O C , T , temperature = 150 O C . obtained at these higher solute concentrations. Figure 1 shows two conductivity chromatograms for the IEPC separation of aqueous mixtures of glycolic, acetic, propionic, and butyric acids. In Figure 1A each acid is present at 10 ppm, and all componenk! are base-line resolved. Figure 1B shows a chromatogram of the same separation, but with each component present at lo00 ppm. Comparison of the two chromatograms reveals no decrease in the quality of the separation or change in peak shapes at higher solute concentrations. Initially, the thermospray interface was operated in the thermospray ionization mode. In 1 mM HCl solution, however, thermospray does not appear to be a very efficient form of positive ionization for the carboxylic acids studied, and very poor limits of detection (500-2000 ppm) were obtained. Subsequent work in the discharge ionization mode yielded detection limit improvements of 20-200-fold over thermospray ionization. Figure 2 is a total ion chromatogram of the same fourcomponent acid mixture as in Figure lB,using the discharge ionization mode. The peaks produced by each of the acids can be observed down to the 10 ppm level in a total ion chromatogram. (See Figure 3.) Note the change in the
Table I. Selected Acids and Relevant Data
analyte acid
pK,
mol wt
glycolic acetic propionic butyric
3.83 4.75 4.87 4.81
76
analytically useful ions, amu 77,95 79,97 75,93 89, 107
60 74 88
relative height of peak A in Figures 2 and 3, due to the nonlinear ionization efficiency behavior of this hydroxy acid upon dilution. Selected ion monitoring for the (M 1) and (M 19) species could lower detection limits to approximately 1 ppm for each acid. Table I lists the major ions observed for each organic acid studied. In each case, except acetic acid, the (M + 1)ion from protonation and the (M + 19) ion from the proton-bound complex of acid and water were observed. The mass spectral scan was started at 75 amu to avoid interference caused by water ion clusters appearing at 19,37,55,and 73 amu. Thus for acetic acid, the (M + 1)ion at 61 amu was below the scan range; however, the (M + 19) and (M 37) (acid + Hf 2H20) ions were observed with good sensitivity. Figure 4 shows the mass spectra of acetic and butyric acids obtained in the discharge ionization mode for 1000 ppm standards. In addition to the four organic acids discussed to this point, lactic, succinic, malonic, and oxalic acids were studied. None of these produced usable mass spectral information in either discharge or thermospray ionization modes. Positive ionization
+
+
+
+
Anal. Chem. 1988, 58,2583-2585
occurs through proton addition, and donation of an extra proton to these stronger acids may be difficult. These acids may be detectable with negative ionization monitoring. This work demonstrates the feasibility of using an IEPC/MS system to obtain mass spectral data which provide valuable molecular weight information for unknown components of a monoprotic organic acid mixture. Ongoing and future work involve studying a wider variety of monoprotic and diprotic acids to determine the molecular parameters necessary for successful application of IEPC/MS, and optimization of mobile phase, column type, and thermospray and
2583
mass spectral operating conditions for various acids. Registry No. Glycloic acid, 79-14-1; acetic acid, 64-19-7; propionic acid, 79-09-4; butyric acid, 107-92-6.
LITERATURE CITED (1) Garteiz, D. A.: Vestal, M. L. LC Mag. 1085, 3 , 334-348. (2) Arpino, P. J. J. Chromatogr. 1085, 323,3-11. (3) Schubert, R. GITFachz. Lab. 1084, 28,323-325. (4) Fritz: J. S.; Gjerde, D. 7.; Pohlandt, C. Ion Chromatography; Dr. Alfred Huthig Veriag: New York, 1982; pp 185-199.
RECEIVED for review March 10,1986. Accepted June 6, 1986.
Quantitative Determination of Aluminum-27 by High-Resolution Nuclear Magnetic Resonance Spectrometry P. M. Bertsch,*' R. I. Barnhisel, and G. W. Thomas Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546
W. J. Layton and S. L. Smith Department of Chemistry, University of Kentucky, Lexington, Kentucky 40546 Quantitative determination of A1 in aqueous solution not containing interfering substances can be made by a variety of spectroscopic and potentiometric methods reliably and with relative ease ( I ) . Aluminum is, however, an element that is frequently analyzed in the study of geologic materials, sediments, sludges, and plant and/or animal tissues and in many industrial processes. For most of these analytical applications, the presence of interfering substances is ubiquitous, often resulting in the introduction of tedious, error-inducing pretreatments that can complicate the analysis. In addition, the reagents commonly used to solubilize and/or extract A1 in many of the aforementioned applications often cause matrix effects which make the analysis even more cumbersome. Many of the methods utilized for the quantitative determination of soluble A1 in different matrix solutions attempt to eliminate the various interferences encountered ( I ) . Although many of these methods are quite good, there still does not exist a rapid method for the routine determination of A1 overa wide m m mrange %is freefrom intmfermce of other elements or matrix complications. In this communication, we describe the use of high-resolution 27AlNMR spectrometry as a rapid, nondestructive analytical tool for the quantitative determination of aluminum over a wide linear dynamic analytical range. The method is useful not only when total A1 determinations are needed but also when the concentration of a particular soluble A1 complex is desired.
EXPERIMENTAL SECTION Reagents. Standard aqueous A1 solutions were prepared by serial dilution of a Fisher Scientific 1000 wg/mL certified atomic absorption standard solution with COz-freedouble-deionized H,O. Standard solutions with A1 concentrations ranging from 1.00 to 1000.O g / m L were prepared in this manner and used to construct the calibration curves. During the course of this investigation it became apparent that acidification of certain solutions containing relatively low concentrations of the A~(HQO),~+ cation (ca. 1-10 pg/mL) resulted in significant decrease in the line widths of the "A1 signals. The Fisher standard A1 solution contains a dilute hydrochloric acid media with a pH of
