Negative ion ESI

The instrument must be properly tuned and mass calibrated in the negative ion ESI mode. Typically, two ions are monitored for each analyte, one transition (parent) ion for each metabolite, and one quantitation (fragment) ion for each metabolite. The typical transition and quantitation ions for the analytes are shown in Table 5. Alternative ions may be used if they provide better data (sensitivity and/or specificity). 

Also, a brief note has appeared concerning electrospray ionization mass spectrometry of mixtures of -carotene with ft- and with y-cyclodexlrin in aqueous methanol solutions. Whereas negative ion ESI produced 1 1 adduct ions of -carotene with both of the cyclodextrin isomers, positive ESI gave these adducts only in the case of ft-cyclodextrin302. 

Fig. 11.10. (a) Partial negative-ion ESI spectrum of 2-(trismethylstannyl)pyrrole-A-carbamate (A ) from tetrahydrofurane at low nozzle-skimmer voltage drop and (b) dependence of the [A-C02]7A ratio variation of this voltage. 

Fig. 11.25. Negative-ion ESI spectrum of an industrial cooling lubricant dissolved 1 1000 in 1-propanol. The dominant ions belong to aUcylbenzene sulfonates. The inset expands the mJz 420-555 range, the most intensive peaks belonging to saturated alkyl chains. By courtesy of OMTEC GmbH, Eberbach.

Fig. 11.26. Negative-ion ESI spectrum of the ohgonucleotide dTio in the presence of imidazole and piperidine. Here, H /Na exchange is not completely suppressed. By courtesy of T. Kruger, University of Heidelberg.

Gross, J.H. Eckert, A. Siehert, W. Negative-Ion ESI Mass Spectra of Carbon Dioxide-Protected N-Heterocychc Anions. J. Mass Spectrom. 2002, 37, 541-543. 

Cole, R.B. Harrata, A.K. Solvent Effect on Analyte Charge State, Signal Intensity, and Stability in Negative Ion ESI-MS Implications for the Mechanism of Negative Ion Formation. J. Am. Soc. Mass Spectrom. 1993,4,546-556. 

Figure 18 Positive- and negative-ion ESI mass spectra of ImM sodium perchlorate in methanol-water (50 50), at heated capillary of 220 C. [Reproduced from X. Zhao and J.Yinon, Rapid. Commun. Mass Spectrom., 16, (2002) 1137. Copyright 2002, with permission fromJohnWiley Sons].

Negative-ion ESI mass spectrometry has been employed to investigate the mechanisms of the synthesis of the heterobimetallic clusters [((THF)Li)2(MeZn)2(OBu-f)4] (24), [((THF)2K)2(MeZn)2(OSiMe3)4l (25) and [K(MeZn)3(OBu-t)4] (26) in solution". Compounds 24-26 possess distorted heterocubane skeletons Li2Zn204, K2Zn204 and KZn304, respectively. 

In this way, the reaction mixtures for the synthesis of the 24-26 clusters have been subjected to negative-ion ESI MS analysis and several negatively charged intermediates have been detected. Some identified species were cluster anions [C36H8oLi308Zn2] = [(Me2ZnOBu-f)2 + Li + (LiOBu-t)2 + 4THF] (for compound... 

Derivatized polyethers such as polyether sulfate have been investigated for both positive- and negative-ion calibration [11]. Although poly ether sulfates are not commercially available, they are easily synthesized. Lauryl sulfate ethoxy-lates were also used as calibrants for negative-ion ESI. Polyether amines and quaternary ammonium salts were used as positive-ion calibration solutions [11]. These commercially available compounds do not exhibit significant sodium or potassium adducts, and they are more easily flushed out of the mass spectrometer ion source than are nonderivatized polyethers. In addition, doubly charged poly ether diamines can produce reference peaks at low m/z values. 

Fredline et al. [18] report an MRM negative-ion ESI method for aldosterone using flumethasone as an internal standard. The MRM transition used was 359—>331. The assay was linear over a 15- to 5000-pg/ml range and the limit of quantitation was 15 pg/ml. A manual extraction was used and recovery and accuracy were excellent. 

An LC-MS method for quantification has been available for many years [79] using 13C2 cholesterol sulfate as an internal standard. The deuterated material is now commercially available (CDN or ISOTEC). Negative-ion ESI-MS is used for quantitation and m/z 365 (M-II, analyte) and 367 (M-H", internal standard) are monitored. The mean cholesterol sulfate level in 166 cases of RXLI was 4859 1589 pg/dl and for normal individuals 186 112 pg/dl (n = 109). 

An example of the LC/MS analysis of aplant extract is shown in Figure A.3A.3. In this case, negative ion ESI-MS was used in combination with C18 reversed-phase HPLC separation. Extracts of the botanical Trifolium pratense (red clover) are used as dietary supplements by menopausal and post-menopausal women (Liu et al., 2001). The two-dimensional map illustrates the amount of information that may be... 

The phosphonic acids are strong acids (pKA 2) and under typical LC conditions exist mostly in the deprotonated form. They are therefore well suited to both negative and positive ionization. Kosti-ainen et al. (24) reported negative-ion ESI (ionspray) spectra for alkyl methylphosphonic acids (alkyl MPAs), 0-ethyl methylphosphonothioic acid, and... 

Siegenthaler (34) reported a screening procedure for phosphonic acids, which included ethyl methyl-phosphonothioic acid and ethyl isopropylphospho-nothioic acid, based on negative ion ESI under conditions that promoted in-source CID. LC separation was on a 250 x 3mm C18 column, eluted with a water-methanol-20 mM NH4OAc gradient,... 

In 2007, negative-ion ESI-MS was used to detect the boron species responsible for transmetallation in the Suzuki reaction [PhB(OCH3)3] (Fig. 4D) [46]. Soon after, ESI-MS allowed the in situ observation of small catalytically active palladium clusters during a Suzuki reaction that could be precursors to catalytically active palladium nanoparticles. [(lL)sPd3(H20)], [(lL)3Pd3(H20)7] and the... 

P A. Shipkova, L. Heimark, P. L. Bartner, G. Chen, and B. N. Pramanik, High-resolution LC/MS for analysis of minor components in complex mixtures Negative ion ESI for identification of impurities and degradation products of a novel oligosaccharide antibiotic, J. Mass Spectrom. 35 (2000), 1252-1258. 

M. Moini, Ultramark 1621 as a calibrant/reference compound for MS. II. Positive- and negative-ion ESI, Rapid Commun. Mass Spectrom., 8 (1994) 711. 

M. Moini, B.L. Jones, R.M. Rogers, L. Jiang, Sodium trifiuoroacetate as a tune/calibration compound for positive- and negative-ion ESI-MS in the mass range of 100-4000Da, J. Am. Soc. Mass Spectrom., 9 (1998) 977. 

The appearance of ESI mass spectra depends on the solvent conditions, especially on the extent of sodium contamination. Sodiated molecules were observed as the base peak for chlortoluron, isoproturon, diuron, linuron, and diflubenzuron [34], while protonated molecules and only weak sodiated molecules were observed for monuron, diuron, and neburon [14], Some phenylureas can also be analysed in negative-ion ESI, where deprotonated molecules as well as acetate or formate adducts can be observed, depending on the mobile-phase composition [9],... 

Chlorinated phenoxy acid (CPA) herbicides can analysed by GC-MS only after derivatization. For LC-MS, negative-ion ESI is the method of choice. Deprotonated molecules are detected as the most abundant ions under these conditions, often next to a phenolate fragment ion due to the loss of the acid side chain. Next to the deprotonated molecule, weak formic acid adducts [M+HCOO] were observed for MCPA, 2,4-D, MCPP, andMCPB [41]. 

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