Electrospray Stability

The electrospray stability of this device was studied with two different samples 1 pg mL 1 imipramine- 3 directly dissolved in 75% methanol, 25% water and 0.1% formic acid and extracted urine sample containing only methylphenidate-d3, which corresponds to the blank calibration standards. Figure 6.13A shows the total ion current of full mass range scan (m/z 250 to 350) of the imipramine-J3 sample over 15 min with a scan speed of 1.16 s per scan while Figure 6.13B shows the extracted ion current of the base peak (m/z 284.2) from Figure 6.13A. The RSD for the total ion current over the 15 min periods was 10.7% and the protonated molecule ion was even more stable with an RSD of 5.7%. Figure 6.13C shows the total selected ion current for the SRM data (m/z 234.2 > m/z 84.1, m/z 234.2 > m/z 84.1) from 150ngmL 1 methylphe-nidate-fi 3 sample. The SRM mode was also very stable with an RSD of 3.05% over a period of 5 min. 

Figure 6.13 Electrospray stability of the device. (A) Total ion current of m/z from 250 to 350 with solution of lpgmL-1 imipramine-d3 in 75% methanol, 25%water and 0.1 % formic acid. (B) Extracted ion current of imipramine-d3 (m/z 284.2) from (A). (C) Selected reaction monitoring of extracted urine sample containing 150ngmL 1 methylphenidate-<73 (from ref. 13).

The main factors affecting the stability island of an electrospray system is the fluid properties themselves in particular the surface tension and conductivity. The fluid surface tension directly affects the ability of a fluid to atomize an electrospray because it opposes the force applied by the ions at the fluid/air interface. Therefore, increasing the surface tension will increase the required field strength to establish a cone-jet mode. The conductivity of the fluid used has the effect of shifting the electrospray stability island to a narrower range and also to lower flow rates. This is shown in Fig. 32.12. 

Fig. 32.12 Electrospray stability island, effect of varying conductivity (Reprinted with permission [11])...

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

Figure 11.4 Schematic of a phospholipids unit (A) showing the polar head group that holds the charge in the electrospray (B) a phospholipids bilayer that is the possible origin of the signals at around 1400 mlz in the DIESMS spectra of bacteria (Figure 11.3) (C) the micellar arrangement that is stabilized in a polar solvent (D) the reverse micellar arrangement that is likely to be encountered in nonpolar environments.

Petty, I., Wisniewski, J.R., and Szewczuk, Z. (2001) Conformational stability of six truncated cHMGla proteins studied in their mixture by H/D exchange and electrospray ionization mass spectrometry. Acta Biochim. Pol. 48, 1131-1136. 

A reevaluation of molecular structure of humic substances based on data obtained primarily from nuclear magnetic resonance spectroscopy, X-ray absorption near-edge structure spectroscopy, electrospray ionization-mass spectrometry, and pyrolysis studies was presented by Sutton and Sposito (2005). The authors consider that humic substances are collections of diverse, relatively low molecular mass components forming dynamic associations stabilized by hydrophobic interactions and hydrogen bonds. These associations are capable of organizing into micellar structures in suitable aqueous environments. Humic components display contrasting molecular motional behavior and may be spatially segregated on a scale of nanometers. Within this new structural context, these components comprise any molecules... 

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