Capillary Electrophoresis Mass Spectrometry: Practical Implementation and Applications

Transcription

1 2 Ross Capillary Electrophoresis Mass Spectrometry: Practical Implementation and Applications Gordon A. Ross, Agilent Technologies GmbH, Waldbronn, Germany. Introduction Mass spectrometry (MS) is becoming increasingly popular as a detection method for capillary electrophoresis (CE) (1 5). The combination of CE s high efficiency and high speed with the high sensitivity and high selectivity offered by MS detection is very attractive. CE is very tolerant of complex sample matrices, and therefore its combination with MS provides for highly selective detection of compounds in variously complex mixtures. MS detection also helps to improve the general sensitivity of CE analyses in appropriate instances. The power of combining MS detection with any separation technique is that it provides a second dimension of separation. Therefore, MS in combination with, for example, capillary zone electrophoresis (CZE), separates first on the basis of an analyte s charge-to-size ratio (CZE) and then on the basis of its massto-charge ratio (MS). A variety of ionization and coupling methods have been attempted since the first report of CE MS coupling by Olivares et al. (6). The most popular method for routine coupling of CE with MS is electrospray ionization (ESI) (7). ESI is a soft ionization process and produces ions in the gas phase from those in the liquid phase. Because ESI can generate multicharged ions it is of great benefit in bringing large molecules, for example, proteins, into the molecular mass range of most mass spectrometers. This article will concentrate on the practical aspects of coupling CE to MS via ESI and how this coupling affects the operational and optimizable parameters of a CE ESI MS method. CE ESI MS Coupling There are several factors that must be considered when coupling the CE instrument to an MS detector. Preparing the CE instrument: It may seem obvious that the CE capillary must exit the instrument, but the measures required to accomplish this are not so. The major suppliers of CE MS instrumentation provide specialized capillary cassettes and modifications to the CE instrument that allow the capillary to exit the instrument. Ultraviolet (UV) detection is generally facilitated at a short distance from the capillary inlet, and the total length of the capillary varies from 5 to 1 cm. CE ESI MS interface: The triple tube design as first developed by Smith et al. (8) is used on most commercially available instrumentation. This consists of a central tube (the CE capillary) surrounded by a second stainless steel tube the sheathliquid tube. The sheath-liquid flows between this tube and the inner CE capillary. Between the sheath-liquid tube and the third outer tube, or gas tube, flows the nebulizing gas that helps in the nebulizing process. The sheath liquid serves a dual function in the sprayer. First, it provides the means to complete an electrical circuit between the anode in the inlet vial and the metal of the sprayer, which is, in effect, the cathode. The sheath liquid contacts both the metal sprayer and the buffer flowing out of the CE capillary. Second, the electrospray process is optimal at flowrates in the µl/min range and because the electroosmotic flow (EOF) in CE is of the order of 2 2 nl/min, there is an obvious discrepancy between the EOF and the requirements of electrospray. In order to match the effluent flow to the requirements for electrospray, a make-up liquid is provided by the sheath liquid. The capillary must be placed in the triple tube sprayer and positioned in the mass spectrometer so that the capillary exit and spray are arranged to optimize transfer efficiency of the generated gas-phase ions into the MS capillary. The degree to which the CE capillary exits the tube is also important and should not be more than approximately.1 mm. Electrical interfacing: In the CE MS coupling there is a high voltage applied to the inlet side of the capillary and also a high voltage potential between the sprayer needle and the end-plates near the MS entrance capillary. The potential between the sprayer and the MS entrance is approximately 3 5 kv. If this potential is negative, then positive ions will enter the MS; this is called positive ion mode. If the potential is positive then negative ions will enter the MS and this is called negative ion mode. Therefore, the CE sprayer can be at a high voltage relative to a grounded end-plate, or the capillary tip can be held at ground and a voltage applied to the end-plates. The simplest arrangement is one in which the capillary end is at ground and the voltage is applied to the

2 Ross 3 end-plates. In this set-up the applied field strength will be the voltage applied by the CE instrument divided by the capillary length. This arrangement does not require any special electrical connections between the two instruments apart from a ground cable. Where the sprayer is at a high voltage, some consideration must be given to the coupling, and users are advised to consult the MS vendor for advice and guidance specific to the mass spectrometer being coupled. The essence of the problem is that an electrical current is being applied from the inlet and from the outlet at the same time. These two currents are of very different orders of magnitude and the CE separation current is frequently much higher than that generated by the electrospray. With nowhere to go, the smaller current and power source can be Figure 1: Effects of 2 psi and 1 psi nebulizing gas pressure on a separation of four peptides. Other conditions: buffer: 1 mm acetic acid, capillary: 8 cm 5 µm i.d., MS detection: scan 3 12, injections: 2 mbar*s, voltage 25 kv, temperature: 25 C, sheath liquid:.5% acetic acid in 5% methanol, 4 µl/min, drying gas: 1 L/min, 1 C, acquisition: positive ion mode Vcap 4 kv Figure 2: Effects of sheath flow delivery on baseline noise. ( ) Infusion pump with 5 ml syringe at 3 µl/min for overnight use. Arrow highlights the stick slip effect. ( ) LC pump with 1:1 splitter at 3 µl/min. overwhelmed by the incoming separation current. The solution is to provide an outlet for the currents by connecting the sprayer to ground via a resistor sink. Once such a coupling is achieved it should also be noted that the electrical field strength across the capillary will be the CE applied voltage minus the electrospray voltage on the sprayer tip, divided by the capillary length. Such an arrangement will also have some implications if switching polarity on the MS in order to detect cations and anions in one run. If the polarity is scanned then the field strength will vary every time the electrospray voltage changes polarity. Physical set-up: The physical siting of the instruments and the nature of their connection must be considered. For the minimal capillary length and therefore the maximum available field strength, the CE instrument and mass spectrometer should be placed as close as possible to each other. It should also be appreciated that the connection between the CE and the MS source is made via a hollow tube connected at one end to a liquid reservoir. For this reason the liquid level of the buffer in the inlet side of the set-up should be at the same height as the tip of the capillary at the outlet, in order to avoid siphoning. Any siphoning will be more pronounced with shorter, wider bore capillaries. The design of the triple tube sprayer includes a sheath of nebulizing gas that can also affect the liquid column within the capillary. The pressure of the sheath gas can create a suction effect so that liquid is pulled through the capillary. Figure 1 shows the effect of changing the nebulizing gas pressure on a separation of some peptides. The migration time is much faster with 2 psi, than if 1 psi is used. While this does not appear to cause great differences in peak efficiency or a deleterious effect on the separation, the analyst should be aware of the phenomenon. Sheath Liquid The sheath liquid should be sufficiently conductive to complete the CE circuit and to permit ESI, but not of such a high ionic strength that arcing or discharges are created in the ESI source. Delivery: The sheath flow can be delivered by a syringe pump or by a liquid chromatography (LC) pump with a splitter. Delivery should be as smooth and consistent as possible to ensure a stable electrospray and baseline. Figure 2 shows the baselines obtained from a syringe pump run at 3 µl/min and from an LC pump running at 4 µl/min and fitted

3 4 Ross with a 1:1 splitter with 99% of the liquid returning to the reservoir. Baseline noise is markedly improved by using the LC pump and splitter because the sheath liquid is more evenly delivered. The syringe pump generated baseline also shows a dramatic disturbance known as a stick slip effect, in which the plunger in the syringe barrel becomes stuck but is eventually freed by the increasing pressure from the syringe pump releasing a pulse of liquid into the sprayer. The syringe pump also has a finite lifetime; for example, a 5 ml syringe delivering liquid at 4 µl/min would have to be replaced after approximately 17 h. An LC pump with splitter, by contrast, can deliver such a flow-rate almost indefinitely thus allowing overnight or long sequence runs. Effects: Because the sheath liquid is in effect the buffer reservoir of the distal capillary end, it may be expected to have some effects on the analysis. While the running buffer affects the quality of the separation, both run buffer and sheath liquid affect the transfer of analytes from liquid phase into the gas phase. The sheath liquid is generally 5% or more organic with some acid or base addition depending on the ionization of the analytes of interest. The effects of changing the sheath liquid ph on peptide ionization have been reported previously (9). This demonstrated that, with increasing sheath liquid ph, the mass spectra of separated peptides revealed a decrease in their Figure 3: Comparison of volatile and non-volatile buffers for the analysis of four peptides (adapted from reference 9, Figure 7). Buffer: 1 mm acetic acid ph 3.4 or 2 mm phosphate ph 2.5, capillary: 75 cm (22 cm) 5 µm, injection: 15 mbar*s, voltage: 27 kv, temperature: 25 C, sheath liquid:.5% HAc in 5% MeOH 4 µl/min, nebulizing gas: 1 psi, drying gas: 1 L/min, 15 C, acquisition: positive ion mode Vcap 4 kv, MS: m/z 35 65, sample:.16 mg/ml 1 peptide mix. ( ) total ion electropherogram, ( ) extracted ion electropherogram. ionization state, indicating that the sheathliquid ph was having a direct effect on the ionization state of the peptide. Optimization: Optimization of the sheath flow involves optimizing the organic type and content, acid/base or salt content and flow-rate. These parameters are easily included into optimization experiments using the signal-to-noise of a selected peak as the optimization indicator. A good example of the use of chemometrics for optimization of a sheath liquid was reported by Varesio et al. (1) in developing a method for the analysis of amphetamines in urine. Non-EOF Effects In some instances there is no liquid flow within the capillary, that is, when using very low ph buffers or neutral coated capillaries and this can produce particular phenomena. This is discussed in some detail in Foret et al. (11). In this situation the buffer at the end of the CE capillary in the sprayer can become depleted of ions during application of a high voltage. These ions may be replaced by ions from the sheath liquid. This can result in a moving boundary of sheath-liquid ions, which travels down the capillary towards the inlet. If the buffer ions have a higher mobility than the incoming sheath-liquid ions, this zone can be very sharply defined. If the sheath-liquid ions have a different pk a, or other characteristics, then the liquid behind the moving boundary may be of a very different composition than that of the run buffer. This may have some implications if the sample is unduly affected by these conditions, for example, charge change or precipitation. In such an instance the sheath-liquid ions should be matched to the buffer ions both exactly or in terms of mobility and pk a. Buffers for CE MS Traditionally, the expectations of a buffer intended for CE MS are that it should be volatile and should contribute to, or at least not hinder, the electrospray process. The traditional enemy of electrospray ionization is non-volatile salt. Therefore, the choice of buffers for CE has been restricted to volatile buffers such as acetates and formate. Figure 3 compares the separation of a standard mixture of 1 peptides using a run electrolyte of either 1 mm acetic acid (Figure 3) or 2 mm phosphate buffer (Figure 3). The acetic acid electrolyte is much more volatile than the phosphate-based buffer, and this is reflected in the abundance obtained

4 Ross 5 Where sensitivity is not an issue then CE MS of peptides from protein digests can provide very rapid separation and detection. indicating that the ionization efficiency is much better with the volatile buffer than with the non-volatile phosphate. The total ion electropherogram (TIE) in Figure 3 is very noisy and the peptide signals very faint; however, the peptide signals can be observed using the extracted ion electropherogram. Although there are some disadvantages to using non-volatile buffers, they can be used provided the required data are not compromised. In the instance described, the masses of the peptides would need to be known in order to obtain an extracted ion electropherogram and sensitivity would certainly suffer, but if these were not critical issues then the phosphate buffer could be used. The following applications section details other examples of buffers that were appropriate to the analysis being undertaken. Volatile buffers and electrolytes such as acetic acid or formic acid and ammonium formate or ammonium acetate have been used, even at concentrations up to 1 M (12). These volatile buffers should be the buffers of choice unless they prove inadequate for the separation, in which case other buffers can be tried. The more traditional CE buffers TRIS, phosphate and borate can be used at more dilute strengths than are routinely used with CE: In this instance at less than 2 mm for TRIS and phosphate, and less than 1 mm for borate. It may also be advisable to use more volatile salts of these buffers, for example, ammonium phosphate rather than sodium phosphate. Applications Peptide analysis by CE MS has been previously discussed and compared with capillary-lc MS (9). For capillary LC, using 18 3 µm i.d. capillaries compared with traditional CE, the LC technique is about 1 times more sensitive, in concentration terms, because of the amount that can be loaded onto the column; that is, µl for LC and nl for CE. Several groups have taken various approaches to membrane preconcentration and solid-phase extraction to improve the CE sensitivity by increasing the amount loaded onto the column (13, 14). A nano-sprayer (15) has also been used to increase sensitivity as has on-line isotachophoresis (16). Isoelectric focusing coupled to MS via ESI has also been described providing concentration factors of 5 to 1 times (17). Where sensitivity is not an issue then CE MS of peptides from protein digests can provide very rapid separation and detection. The coupling of CE to MS MS has the additional benefit of providing fragmentation data that can then be compared against databases to identify an unknown peptide or protein. Amino acids have also been analysed by CE MS (12) and although the CE separation was not fully resolved, this was remedied by the MS. The sensitivity was sufficient for detection of amino acid levels in such complex matrices as soy sauce. In order to analyse stereoisomers by CE MS their separation is essential prior to MS detection. Because they have the same mass-to-charge they will not be discriminated by the MS. In conventional CE their separation would require the addition of a chiral selector to the CE run buffer. This is also possible in some instances for CE MS. Figure 4 shows the separation of scopolamine and two stereoisomers of hyoscyamine. After method development to select the chiral selector and its concentration, the final buffer conditions SIM m/z m/z 29 m/z 34 required 5 mm formic acid including 2 mm trimethyl-beta-cyclodextrin. Other pharmaceutical compounds have been analysed by CE MS using cyclodextrins as chiral selectors (18, 19). CE MS is also suitable for the analysis of drugs in various matrices. Ecstacy and derivatives in urine were analysed by Varesio et al. (1) for which they used the characteristic fragmentation of two otherwise very similar isomers in order to differentiate them and provide unequivocal identification. This was achieved using collision-induced dissociation on a single quadrupole instrument. The use of a more sophisticated MS MS technique using an ion trap MS has been described by Bach (2) for the analysis of methylphenidate in urine, in which a quantitative assay was developed capable of providing pharmacokinetic data. CE can provide great benefits in the analysis of components of natural products. Figure 5 shows the analysis of tetrandrine and fangchinoline, which are components of some traditional Chinese medicines. Although the separation is not fully resolved the use of selected ion monitoring enables the selective detection and quantification of these components in extracts of traditional Chinese medicines. CE is much more robust than LC in its ability to deal with the injection of samples with complex matrices. Unlike LC there is no stationary phase to contaminate and after detection of the analytes of interest any remaining components in the capillary can be flushed out rather than having to Figure 4: Separation of hyoscyamine iosomers by CE MS. Sample: 1 mg/l, buffer: 5 mm formic acid, 2 mm trimethyl-beta-cyclodextrin, capillary: 9 cm 5 µm i.d., injection: 5 mbar*s, voltage: 3 kv, temperature: 2 C, sheath: 5 mm ammonium acetate in 5% methanol 4 µl/min, nebulizer gas: nitrogen, 1 psi, drying gas: nitrogen, 6 L/min, 3 C, acquisition: positive ion mode Vcap 4 kv, fragmentor 4 V, gain 1, SIM (m/z 29.2, 34.1). Peaks: 1 L-scopolamine, 2 L-hyoscyamine, 3 D-hyoscyamine.

5 6 Ross mau (21 nm) (c) (d) Figure 5: Analysis of tetrandrine and fangchinoline. UV trace, total ion chromatogram, (c) selected ion monitoring 35 tetradrine, (d) selected ion monitoring 312 fangchinoline. Conditions: buffer: 1 mm formic acid, capillary 5 µm i.d. 76 cm (UV at 22 cm), voltage: 27 kv, temperature: 2 C, injection: 2 mbar*s, detection: UV 2 nm, sheath: 5 mm ammonium formate in 5% methanol, 5 µl/min, nebulizer gas: nitrogen, 1 psi, drying gas: 1 L/min nitrogen, 25 C, acquisition: positive ion mode Vcap 4 kv, fragmentor 7 V, scan: 3 m/z to 65 m/z. wait for them to elute. CE MS has been used to investigate adulteration of Chinese medicines (21) and for the characterization of plant species used in the production of traditional medicines (22). Summary The interfacing of capillary electrophoresis to mass spectrometric detection can be achieved using a triple-tube design sprayer that allows for routine and robust operation. With the use of such a sprayer the sheath liquid provides a further point of optimization that must be considered in method development. With regard to buffers that are suitable for use with CE MS, the applications presented above indicate that there is a more extensive range of buffer choice than initially expected. Unless preconcentration techniques are used, the sensitivity of MS detection coupled to CE is in the ppm range. With appropriate preconcentration techniques by either sample extraction or on-line concentration techniques, its sensitivity is greatly improved. The ability of CE MS to selectively detect components in very complex matrices is one of its strengths. Acknowledgements I would like to thank the following people: Maria Serwe, Agilent Technologies, for critical reading and for contributing Figures 1 and 2; Yi Li, Peking University, for contributing data on analysis of traditional Chinese medicines; Kaho Minoura, Yokogawa Analytical, Japan, for Figure 4; and Lori Keelty, Agilent Technologies, for critical reading. References (1) W.M.A. Niessen, J. Chromatogr. A, 794, (1997). (2) B. Mehlis and U. Kertscher, Anal. Chem. Acta, 352, (1997). (3) J.F. Banks, Electrophoresis, 18, (1997). (4) D. Figeys and R. Aebersold, Electrophoresis, 19, (1998). (5) R.D. Smith, D.R. Goodlet and J.H. Wahl, in Handbook of Capillary Electrophoresis, Landers, J.P., Ed. (Marcel Dekker, New York, USA, 1993), (6) J.A. Olivares et al., Anal. Chem., 59, (1987). (7) S. Cherkaoui, in Clinical and Forensic Applications of Capillary Electrophoresis, J.R. Petersen and A. Mohammad, Eds., Pathology and Laboratory Medicine Series, Humana Press Inc. in press. (8) R.D. Smith, C.J. Baringa and H.R. Udseth, Anal. Chem., 6, (1988). (9) M. Serwe and G.A. Ross, LC GC North America, 18, (2). (1) E. Varesio, S. Cherkaou and J-L. Veuthey, J. High Resol. Chromatogr., 21, (1998). (11) F. Foret et al., Anal. Chem., 66, (1994). (12) T. Soga and D Heiger, Anal. Chem., 72, (2). (13) C.J. Herring and J. Qin, Rapid Commun. Mass Spectrom., 13, 1 7 (1999). (14) A.J. Tomlinson, N.A. Guzman and S. Naylor, J. Capillary Electrophor., 2, (1995). (15) D. Figeys et al., Anal. Chem., 68, (1996). (16) T.J. Thompson et al., Anal. Chem., 65, 9 96 (1993). (17) Q. Tang, A.K. Harata and C. Lee, Anal. Chem., 67, (1995). (18) L. Wenzhe and R.B. Cole, J. Chromatogr., 714, (1998). (19) R.L. Sheppard et al., Anal. Chem., 67, (1995). (2) G.A. Bach and J. Henion, J. Chromatogr. B, 77, (1998). (21) Y.R. Chen, K.C. Wen and G.R. Her, J. Chromatogr., 866, (2). (22) Y. Li, H. Liu and G.A. Ross, submitted to J. Chromatogr. Gordon Ross works as an Applications Chemist for Agilent Technologies, Waldbronn, Germany. He obtained his BSc from the University of Glasgow, UK, and an MSc in Analytical Chemistry and PhD in Analytical Biochemistry from the University of London, UK. He was among the first graduates in the UK to obtain a PhD working on capillary electrophoresis. His interests include developing applications of capillary separation systems coupled to mass spectrometry.