Capillary electrophoresis - a fast and universal tool in soil analysis

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1 Universität Potsdam Markus Howald, Helmut Elsenbeer, Endre Laczko, Urs Peter Schlunegger Capillary electrophoresis - a fast and universal tool in soil analysis first published in: Analytical Methods and Instrumentation. - 2 (995), 4, pp ISSN Postprint published at the Institutional Repository of the Potsdam University: In: Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe ; Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe ; 70

2 Capillary Electrophoresis a Fast and Universal Tool in Soil Analysis Markus Howald, Helmut Elsenbeer, Endre Laczko, Urs Peter Schlunegger* Keywords: c a p i l l a r y e l e c t r o p h o r e s i s ; soil a n a l y s i s ; U V - d e t e c t i o n ; i n o r g a n i c ions; m o n o s a c c h a r i d e s Introduction 2 Inorganic ion analysis, however, has received relatively little attention. This is due to the existence of other sensitive techniques (ion chromatography) and the lack of a direct detection method. The latter problem has been solved by introducing the indirect UV absorption * Author for correspondence: University of Berne, D e p a r t m e n t of Organic Chemistry Frcicstrassc 3 Acknowledgements: (Switzerland) This w o r k w a s s u p p o r t e d by t h e Swiss N a t i o n a l Science F o u n d a t i o n, grant n o ,6 4 Jones et al. " ' found the background electrolyte chromate to be the most useful for highly mobile inorganic anions. Incorporated in this electrolyte there is a n additive that reverses the direction of E O F in the capillary, so that the E O F augments the mobility of the analytes. We adapted this electrolyte system for a clear separation of chloride, sulfate and nitrate. The interest in soil analysis of carbohydrates is restricted to glucose, mannose, arabinose and xylose. Due to the low absorbency of these monosaccharides, indirect UV detectionl -- ) or pre-column derivatisation' ' are recommended. Oefner et a/.' ' proposed sorbic acid as both carrier electrolyte anion and c h r o m o p h o r e with indirect UV detection, but glucose and arabinose could not be separated. Recently, incolumn derivatisation of reducing monosaccharides' ' has been demonstrated. Regarding simplicity, reproducibility and sensitivity, we adapted a pre-column derivatisation with 4-aminobenzonitrile and separation by micellar electrokinetic capillary chromatography ( M E C C ) with a carrier containing borate and sodium dodecylsulfate ( S D S ) ' l This method makes sensitive, direct UV detection possible and allows fast selectivity for the separation of oligo- and monosaccharides. The large peak caused by the excess concentration of the precolumn reagent is well separated from all analytes of interest [2 U.P. Schlungcggcr C H Bern m e t h o d ' ". Weston et al. investigated the factors that affect the separation of metal cations and they optimized the detection sensitivity of indirect photometric detection. Forct et alp^ utilized indirect UV detection to demonstrate an efficient separation of fourteen lanthanide cations within 5 min with the aid of hydroxyisobutric acid (HIBA) as a complexjng agent. But no electrolyte system was found that was capable of clearly separating all the inorganic cations pedologists were interested in: N H, Ca, M g, M n, N a, Al, K, Fe(m). In this study, we investigated the use of creatinine as a UV-absorbing background electrolyte with ls-crown-6 as the complexing agent. This agent can selectively modulate the mobility of the cations (NH4, K), and with creatinine as the co-ion even aluminium is sharply detectable. This electrolyte allows a clear separation of the usually interesting eight inorganic cations. 3 Until now in soil analysis different time consuming techniques and expensive methods [ion chromatography (IC) and high performance liquid chromatography (HPLC)] have been used for most different species of molecules (inorganic ions, carbohydrates). Capillary electrophoresis (CE) represents a low cost and timesaving alternative to the commonly used techniques for the analysis of small molecules and ions' '. The availability of narrow bore fused silica capillaries and the recent introduction of commercially available CE units, have once again brought electrophoresis techniques to the attention of analytical chemists. Although the main input for developing elcctrophoretic methods has come from the biological applications, C E offers, in connection with UV detection and its high resolution (N > ), the possibility to separate almost any type of compound. When high voltage is applied between the ends of the capillary tube filled with an electrolyte solution, the surface of the inner wall of the tube is negatively charged. If a sample is injected into the tube from the anionic end (cationic mode), all compounds of the sample are transported to the cathode at a uniform velocity, anionic compounds are pulled back by electrostatic force (electrophoresis). In addition, the clectroosmotic flow ( E O F ) in uncoated fused-silica capillaries is significantly greater than the electrophoretic mobility of the ions. Consequently, an E O F modifier that reverses the direction of the flow is necessary in anionic mode. Prof Abstract: Fast a n a l y s i s of d i f f e r e n t s p e c i e s of m o l e c u l e s in s o i l s is i n v e s t i g a t e d by c a p i l l a r y e l e c t r o p h o r e s i s (CE). S e v e r a l CE t e c h n i q u e s for t h e a n a l y s i s of i n o r g a n i c ions and c a r b o h y d r a t e s have been t e s t e d. With r e g a r d to t h e intents of p e d o l o g i s t s a n d t h e u s u a l l y l a r g e n u m b e r of soil a n a l y s e s a b u n d l e of CE s y s t e m s is p r o p o s e d, c a p a b l e of effecting t i m e - s a v i n g soil a n a l y s e s. A d a p t e d electrolyte systems recently published and new s e p a r a t i o n s y s t e m s a r e d e s c r i b e d. E x a m p l e s of t h e a p p l i c a t i o n of t h e s e m e t h o d s to t w o d i f f e r e n t soil samples are presented. 20

3 CH-,OH CH OH 2 NaCNBH, OH V V HO HO I CN OH CN OH \=/ F i g u r e. R e a c t i o n s c h e m e for the reductive a m i n a t i o n of g l u c o s e with 4 - a m i n o b e n z o n i t r i l e and s o d i u m c y a n o b o r o h y d r i d e. 2. Experimental 2.. Chemicals Reference solutions were prepared from chromatographic or analytical reagent grade chemicals (Fluka, Buchs, Switzerland) in deionized water for CE. Running electrolyte solution for analysis of cations was prepared by dissolving appropriate amounts of creatinine, acetic acid and 8-crown-6 in doubly-distilled water and similarly for anions of sodium chromate and tetradecyldimethylammonium bromide, respectively. For the analysis of carbohydrates the borate running buffer for M E C C was purchased (Fluka) detection the pherograms were inverted by a self-made computer program written in TurboPascal for M S - Windows" ". All pherograms were integrated with the J C L 6000 Chromatography Data System (Jones Chromatography L T D, Hengoed, UK) Capillary Conditioning Each new fused silica capillary was flushed with M N a O H f o r I h followed by 0. M N a O H for l O m i n a n d by running buffer for 5 min. Between runs, the capillary was flushed with running buffer for 2 min. Finally, the system was pre-run for lomin in order to obtain a stable background signal. The capillary was stored in 0. M N a O H overnight. Apparatus Analyses were performed on a Prince (Lauerlabs, Emmen, Netherlands), the programmable injector, combined with a 206 P H D U VIS-detector (Linear Instruments, Reno, U SA), which were equipped with a 75 cm fused silica capillary of I.D. 50 pimanda 00 cm fused silica capillary of I.D. 75 /mi, respectively. Detection was carried out by oncolumn measurement of UV absorption at different wavelengths at 5 cm from the cathode. Pherograms were recorded on an IBM-compatible with the UVIS206 Computer Software for M S - W i n d o w s (Linear Instruments, Reno, USA). In the case of indirect UV 2.4. Sequential Extraction Methods of Organic from Soils - Matter 26 The air-dried soil samples were shaken in «-pcnlane for 7 2 h. The undissolved part was extracted with a mixture of chloroform, methanol and water ( : 2 : 0. 8 ) for 2 4 h. The two phases were separated on adding chloroform and water ( : : 0. 9 ). The residue was again shaken intensively with 2 5 % N N H under argon for 3 J + A u t h o r s : M. H o w a l d, M, Saegesser, M. Gaelli: University of Berne time F i g u r e 2. P e a k Identities, c o n c e n t r a t i o n s a n d a r e a s : () a m m o n i u m, 6 p p m, 08; (2) c a l c i u m, 20 p p m, 204; (3) m a g n e s i u m, 6.5 p p m, 08; (4) m a n g a n e s e, 7 p p m, 03; (5) s o d i u m, 6 p p m, 48; (6) a l u m i n i u m, p p m, 244; (7) p o t a s s i u m, 3.5 p p m, 322; (8) iron{iii), 28 p p m, 55. I7l

4 ,000-0,0003-0, F i g u r e s. S i m u l t a n e o u s s e p a r a t i o n of a l k a l i, a l k a l i n e e a r t h, transition metal cations a n d a m m o n i u m. F u s e d silica capillary: 75 x 6 0 c m, 50 / / m I.D. C a r r i e r electrolyte: 3 0 m m o l c r e a t i n i n e, 3 0 m m o t acetic acid and 4 m m o l 8 - c r o w n - 6, ph 3. C u r r e n t : 7. 3 / j A. V o l t a g e : v 2 5 k V. S e p a r a t i o n t e m p e r a t u r e : 35 C. S a m p l i n g lor 6 s at kv. Indirect U V detection at 24 n m. A: Peak identities a n d c o n c e n t r a t i o n s : () a m m o n i u m, 67 p p m ; {2) c a l c i u m, 26 p p m ; (3) m a g n e s i u m, 5 p p m ; (4} m a n g a n e s e, 6 p p m ; (5) s o d i u m, 3 p p m ; (7) p o t a s s i u m, 5 p p m ; (8) iron(m), 6 p p m. B ( s a m e c o n d i t i o n s a s A ) : () a m m o n i u m, 70 p p m ; (2) c a l c i u m, p p m ; (3) m a g n e s i u m, 6 p p m ; (5) s o d i u m, 5 p p m ; (7) p o t a s s i u m, 0 p p m ; (8) iron(m), 4 p p m. 24 h. All extractions were performed at room temperature. The dark coloured extract was separated from the corresponding insoluble residues by centrifugation at 6000 g for 5 min. The clear supernatant was acidified with 5 M HC to ph 4, diluted with M e O H and allowed to stand at room temperature for 48 h. Soluble fulvic acid was then separated from the coagulate (humic acids) by centrifugation at 6000 g for 0 min Hydrolysis Analysis" - * of Oligosaccharides for Component m L of each water sample w^ere taken in a capillary vial 72 and 00/tL of 2 M triffuoroacetic acid were added. The solution was flushed with nitrogen for a few minutes. The vial was sealed and heated at 9 5 " C in an oven for 2h Pre-column Derhatisation of Reducing Monosaccharides to 4-Aminohenzonhrt'le Derivatives This was done according to a procedure published by Schweiger et al) '. The reagent was freshly prepared before derivatisation by dissolving lomg of sodium cyanoborhydride in ml methanol containing 4-aminobenzonitrile and acetic acid to a common concentration of 0% (w/v). m L of the mixture of reducing

5 V " W ~ V, t t F i g u r e 4. T w o e l e c t r o p h e r o g r a m s of the t h r e e i m p o r t a n t a n i o n s in soil a n a l y s i s. F u s e d silica c a p i l l a r y : 75 x 6 0 c m, SO/im I.D. C a r r i e r electrolyte: 5 m u s o d i u m c h r o m a t e, O.Smmol t e t r a d e c y l d i m e t h y l a m m o n l u m b r o m i d e, p H C u r r e n t : - 2 m A. V o l t a g e : 22 kv. S e p a r a t i o n t e m p e r a t u r e : 35 C. S a m p l i n g lor 6s at kv. Indirect UV detection at n m. A: P e a k identities a n d c o n c e n t r a t i o n s : {} c h l o r i d e, 3 p p m ; (2) sultate, 0.4 p p m ; (3) nitrate, 34 p p m. B { s a m e conditions a s A ) : () c h l o r i d e, 3 p p m ; (2) sulfate, 0.2 p p m ; (3) nitrate, p p m. monosaccharides was taken in a capillary vial and m L of the reagent solution was added. The rate of the selected precolumn derivatisation is determined by relatively slow reaction kinetics. To reach a maximum yield, the mixture was maintained at 95 C in an oven for h. The reaction mixture was directly injected into the capillary system (see Figure ). 3. Results and Discussion Two soil samples from the Valle Onsernone, Ticino, Switzerland were chosen to demonstrate the sensitivity. rapidity and capability of the CE technique for the detection, separation and determination of small ions and molecules. Soil A represents an agriculturally used and fertilized, grass-covered area. Soil B has lain fallow for about ninety years and is totally overgrown by trees. The soils were extracted in the manner described above, all the cations, anions and reducing carbohydrates being dissolved in the methanol/water phase of the second extraction step. The concentrations of each molecule were correlated with the measured peak area. Figure 2 shows as an example the reference electrophcrogram for cations with the concentrations and peak areas. 73

6 R c.oooi.yv\j\ F i g u r e 5. M i c e l l a r e l e c t r o k i n e t i c c h r o m a t o g r a p h y of 4 - a m i n o b e n z o n i t r i l e d e r i v a t i v e s of h y d r o l y s e d m o n o s a c c h a r i d e s. F u s e d silica c a p i l l a r y : 75 x 6 0 c m, 75 j i m I.D. C a r r i e r e l e c t r o l y t e : 20 mw borate with 20 m m o I S D S. ph 7.0. C u r r e n t : 58 ; J A. V o l t a g e : + 22 kv, S e p a r a t i o n 35 a r a b i n o s e 9 p p m ; ( 3 ), xylose 4 p p m, (R) 4 - a m i n o b e n z o n i t r i l e. 3.. Capillary Zone Electrophoresis of Inorganic A : () g l u c o s e 2 p p m. Cations Different soils can be classified according to their content and concentration of inorganic ions. Besides alkali and alkaline earth cations, pedoiogists are additionally interested in ammonium, aluminium and iron(ni). Creatinine as the co-ion offers the possibility of detecting the eight interesting cations simultaneously. Since creatinine also acts as a highly absorbing background, the running buffer becomes very simple. Additionally, the compiexing agent 8-erown-6 is needed to separate potassium from ammonium. Varia- 74 temperature; C, S a m p l i n g for s at 0 m b a r. D i r e c t UV detection at 265 n m. P e a k identities and c o n c e n t r a t i o n s : () g l u c o s e 6 p p m ; (2) m a n nose p p m ; tion of the concentration of this agent makes it possible to control the shift of the potassium peak. We chose a concentration which shifted the potassium peak to a point behind the first group of cations. This electrolyte allows a clear, sensitive separation of the eight cations within 4.5 min. Figure Capillary Zone Electrophoresis of Inorganic Anions F o r the three anions of interest, the often used and tested electrolyte containing chromatc as a background

7 was selected. In the case of the anionic mode used for the separation of anionic molecules, an additive is necessary in order to obtain a reverse E O F that pushes the anions to the anionic end of the capillary. With tetradecyldimelhylammonium bromide as the E O F modifier, all three anions were clearly separated and detected within 3min, Figure Mice liar Eiectrokinetic of Monosaccharides Capillary Chromatography Component analysis of monosaccharides offers pedologists a further approach to the classification of a soil. Therefore, the carbohydrate material has first to be hydrolysed. The analysis of the four monosaccharides demanded (arabinose, glucose, mannose, xylose) led. among a large number of separation systems with different derivatisation methods, to reductive amination with 4-aminobenzonitrile. The derivatives are separated by the distribution of the aqueous electrophorctic and the micellar phase. This method allows an efficient separation of the four derivatized molecules within 9 min and simultaneously an excellent sensitivity and sharp direct UV detection at 285 nm. Due to the fact that in the two different soils only glucose was detected, the reference electropherogram is presented in addition, Figure Conclusion A p p l i c a t i o n s " in Journal Chromatography Library. Elsevier, Molecules IS ( ) 780. [4] W. J. W i l d m a n, P. E. J a c k s o n, W. R. J o n e s a n d P. G A l d e n : J. Chromatogr. 546 ( 9 9 ) [5] A. W e s t o n, P, R. B r o w n, P. J a n d i k, A L. H c c k e n b c r g, W. R. J o n e s : J. Chromatogr., 60S ( ) [6] A. W e s t o n, P. R. B r o w n, P. J a n d i k, W. R. J o n e s, A, L. H e c k e n b e r g :.. Chromatogr., 593 ( ) [7] N. W u, W. J. H o r v a t h, P. S u n, C. W. H u i e : /. Chromatogr., 635 (993) 307, [8] T. - I. L i n, Y. - H. L e e, Y. - C. C h e n : J. Chromatogr. A. 654 ( ) 67. [9] G. B o n d o u x, P, J a n d i k ; W. J o n e s : /. Chromatogr., 602 ( ) 7 9. [0] B. J. W i l d m a n, P. E. J a c k s o n, W. R. J o n e s, P. G. A l d e n : /. Chromatogr., 546 ( 9 9 ) [] P. J a n d i k, W. R. J o n e s. /, Chromatogr., 546 ( 9 9 ) 4 3. [2] A, E. Vorndran, P.J. Ocfncr, H. Scherz, G,K. Bonn; Chromatographia, 33 ( ) 6 3. [3] P. J, O e f n e r, A, E. V o r n d r a n, E. G r i l l, C. H u b e r, Bonn: G.K. Chromatographia, 34 ( ) [4] E. G r i l l, C. H u b e r, P. J. O e f n e r, A. E. V o n d r a n, G. B o n n : Electrophoresis, 4 ( ) [5] A. E, V o n d r a n, E. G r i l l, C. H u b e r, P. J. O e f n e r, G. B o n n : Chromatographia, 34 ( ) 0 9. [6] S. H o n d a, K. Y a m a m o t o. S. S u z u k i, M. U e d a, K. K a k e h i :./. Chromatogr., 5SS ( 9 9 ) [ 7 ] S. H o n d a S. I w a s e, A. M a k i n o, S. F u j i w a r a : Anal. Biochem., (989) 72. [8] T. A k i y a r a a : J. Chromatogr., 588 ( 9 9 ) 5 3. [9] S. H o f s t e t t e r - K u h n. A. P a u l u s, E. G a s s m a n n, H. M. Anal. Chem., 63 ( 9 9 ) 5 4. [20] H. S c h w e i g e r ; Optimierung kapillarelektrophoretischer nmethoden C E of soil components fulfils the postulated major advantages: the analyses are made in a very short time and, the different techniques developed in the course of the experiment are highly sensitive. Moreover, the samples can be handled very easily: they may be directly injected into the tube, even after pre-column derivatisation. Regarding the intents of the pedologists and the normally large number of soil samples, CE offers a very efficient and simple method of analysis. of A m s t e r d a m (992). vol. 52. [2] P. J a n d i k, G. B o n n : Capillary Electrophoresis of Small and Ions, V C H N e w Y o r k ( ). [3] F. F o r e s t, S. F a n a h, A N a r d i. P. B o c c k : Electrophoresis, fur Kohlenhydrate, Diplomarbcit, 76 Widmer; Innsbruck Tren(992) (in G e r m a n ). [2] M. S c h n i t z e l, P. S c h u p p l i : Soil Sci. Soc. Am. J., 53 ( ) 4 8. [22] J. M. S e r r a, G. T. F e l b e c k J r. : Proc. In. meet. Humic Subts., Nieuwersluis P u d o c, Wagcningen (972). [23] M. S c h n i t z e r : Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties (982), A g r o n o m y M o n o g r a p h, no. 9 (2nd Edn). [24] C M. P r e s t o n, M. S c h n i t z e r : Soil Set. Soc. Am. / 48 ( ) [25] W. Z i c e l i m a n n : Huminstoffe Probleme, Methoden, Ergebnixse, Vcrlag, Basel (980). [26] D. J, G r e e n l a n d, M. H. B. H a y e s : The Chemistry of Soil Constituents, W i l e y, C h i c h e s t e r ( ) p References [] S.F.Y. Li: "Capillary Electrophoresis: Principles, Practice and 75