Sheath liquid

Figure 11 CE/MS of alfalfa hay fortified with 0.25mgkg of paraquat (upper half) and 0.28mgkg of diquat (lower half). Buffer 50mM ammonium acetate at pH 4.5. Sheath liquid methanol-water-50mM ammonium acetate (5 4 1). Positive ESI...

Fig. 11.4. Different sprayers for ESI. (a) Pure electrospray, (b) ESI with sheath liquid, (c) pneumatically assisted ESI, and (d) ultrasonic nebulizer. Adapted from Ref. [5] (p. 109) by permission. John Wiley Sons, Inc. 1997.

Post-column introduction of a sheath liquid gives more flexibility to the selection of pH values for a mobile phase. Chiron et al. applied post-column addition of tripropylamine to reduce the acidicity of the mobile phase in their study of bentazone and chlorophenoxyacetic acids to enhance ESI negative ion signals. Apffel et al." on the other hand, introduced propionic acid/isopropanol to displace TEA in the mobile phase, and improved the ESI positive ion signals. 

An example qualification test for CE—MS involves the simple separation of two small pentapeptides Leu-enkephalin (miz 556) and Met-enkephalin (mlz 589). The sample mixture is composed of 100 pmol/pL of each component in 100% H20,1% acetic acid. The CE column is a 50 im ID x 65—80cm bare fused silica pre-washed with O.IM NaOH and H2O, and then pre-rinsed with the running buffer solution of 100% water, 1% acetic acid. The injection method is pressure at 50mbar for 10 s and an ESI sheath liquid in the ratio of 60/40 IPA/water, 1% acetic acid at 2 tL/min. The spectra are collected every 3 s over a 350—3000 mlz scan. 

Ammonium formate and phosphate buffers CEC-ESI-MS for the analysis of leucine enkephalin and substance E, which are respectively singly and triply charged peptides. The good mass spectra obtained for the peptides in both the volatile and non-volatile buffers (Figures 17 and 18) indicate the non-crystallization of the non-volatile buffer which is further diluted by the sheath liquid. 

Figure 17 shows the mass spectra obtained for leucine enkephalin (charged state -Fl at pH 2.8 and —0.5 at pH 9.5) and Figure 18 for substance P (charged state -F3 at pH 2.8, and -F1.5 at pH 9.5). The analytes undergo ionization at the interface to yield the positively charged ions as a result of the addition of the formic acid sheath liquid to the alkaline buffer. 

Bemet, P., Blaser, D., Berger, S., and Schaer, M. (2004). Development of a robust capillary electrophoresis-mass spectrometer interface with a floating sheath liquid feed. Chimia (Aarau) 58, 196-199. 

Nilsson, S. L., Bylund, D., Joernten-Karlsson, M., Petersson, P., and Markides, K. E. (2004). A chemometric study of active parameters and their interaction effects in a nebulized sheath-liquid electrospray interface for capillary electrophoresis-mass spectrometry. Electrophoresis 25, 2100-2107. 

Tseng, M. C., Chen, Y. R., and Her, G. R. (2004). A beveled tip sheath liquid interface for capillary electrophoresis-electrospray ionization-mass spectrometry. Electrophoresis 25, 2084—2089. 

Fig. 2 CE/MS interfaces, (a) coaxial sheath liquid, (b) liquid junction, (c) sheathless, and (d) direct electrode. See text for further explanation. (Adapted from Ref.

The unintended negative effects of high buffer-salt concentrations on the efficiency of the electrospray can be significantly reduced by adding an appropriate sheath liquid, thus usually improving the spray stability. However, since the sheath liquid acts as the terminal buffer reservoir, it must contain an electrolyte in order to maintain an efficient electrophoretic separation. Therefore, any choice of sheath flow composition represents a compromise between separation efficiency and spray stability. 

Another problem, the potential increase in background noise due to the addition of solvents and modifiers from the sheath liquid (e.g., volatile salts, acids, and bases), has been studied (19). Moreover, because of the different composition of the initial CE buffer reservoir and the sheath liquid, discontinuous and irreproducible conditions may result. These effects can potentially change migration times or even the migration order of the analytes (20). 

Many authors claim that the dilution by the sheath flow would not significantly affect the detection sensitivity, because it is completely evaporated in the spray process. Moreover, it has been discussed that in this layered-flow approach, preferably the inner layer of the spray enters the collector opening. If this were true, the composition of sheath liquid would be less important. Anyhow, it has to be stated that there is a dilution problem in the sheath-flow approach. In addition, it has been proven many times that ESI is a concentration-sensitive, not mass-sensitive, process. Knowing this, it makes sense to reduce the sheath liquid flow rate to the minimum required for stable spray conditions. 

Comparative studies indicate that a sheathless configuration offers improved sensitivity over sheath flow interfaces. However, this implies the need to use specialized, home-built CE capillaries and electrospray hardware. Therefore, it has not yet been as popular as the easily accessible sheath liquid system. 

Figure 22-18 shows tin electrospray interface for capillary electrophoresis. The silica capillary is contained in a stainless steel capillary held at the required outlet potential for electrophoresis. The steel makes electrical contact with the liquid inside the silica by a liquid sheath flowing between the capillaries. The sheath liquid, which is typically a mixed organic/ aqueous solvent, constitutes —90% of the aerosol. 

Lock Mass Similar to internal calibration. The lock mass compound is monitored during analysis of the unknown, and the mass calibration is adjusted based on the comparison of the measured m/z and the theoretical m/z for the lock mass compound. If multiple lock mass compounds are used across the m/z range, the process effectively becomes internal calibration. Lock mass compound(s) can be introduced into the LC-MS source via a tee into the LC flow or sheath liquid inlet or dedicated sprayer. 

What are the functions of the sheath liquid and sheath gas used in an ESI chip for interfacing with a mass spectrometer (Figure 7.30) [290,808] (2 marks)... 

The buffer (10 mM pH 3.0 ammonium formate /acetonitrile, 50/50 v/v) served both as mobile phase and sheath liquid, and was delivered at a flow rate of 3 pL/min using a syringe pump. A voltage of 20 kV was applied. The m/z =286 mass response of etodolac was detected at 18.3 min after injection, and detailed elution patterns of etodolac and its hydroxy metabolites were provided. 

---