Negative ESI

In negative ion mode, M - H (M - 1) is formed from acidic organic analytes. Since this is a very soft type of ionization, fragmentation is rarely seen, and the spectrum is dominated by these molecular ion species. 

FIGURE 4.15 Negative ESI spectrum of water and methanol acidified with acetic acid. 

---

The TIC recorded by negative ESI after RP-Cis (Fig. 2.5.11(f)) also contained just one broad signal with an RT of 2.0-5.5 min. Here, the anionic AES formed the [M] ions as also observed under APCI(—)... 

While APEO cannot be ionised successfully under negative conditions and consequently for identification MS-MS(—) is not informative, the identification of all anionic APEO derivatives is possible in the negative ionisation mode. For some derivatives, negative as well as positive ionisation can be applied. The loss of the anionic moiety, however, must be taken into account if ionisation is performed in the positive mode. Di-NPEC surfactant homologues submitted to negative CID resulted in the prominent di-alkyl-phenolate ion at m/z 345 (Fig. 2.11.18(a) as shown with the homologue m/z 799 under CID conditions). Therefore, the application of the parent ion scan of m/z 345 in the negative ESI and APCI-FIA-MS-MS mode is very specific for the detection of all anionic derivatives of di-NPEO comparable in... 

Fluorinated phosphinic and phosphonic acid derivatives Perfluoro derivatives of alkyl phosphonic acid CnF2n+1-P(0)(0H)2 and alkyl phosphinic acid CnF2n+i(CmF2m+1)-P(0)0H (n = m or n m) shown with their general structural formulae in Fig. 2.11.29(1) and (II) were examined by negative ESI- and APCI-FIA-MS. These anionic surfactant compounds contained perfluoro alkyl chains [2,22,25]. By analogy with their behaviour in the TSI-FIA-MS(—) process [25], the phosphonic acid formed [M — H] ions at m/z 399 and 499... 

Figure 6.10 Mass spectra of an impurity run in (a) APCI mode and (b) negative ESI mode.

Due to their acidic nature, flavonoids usually give higher ion abundances upon deprotonation in the negative ESI mode than via protonation in the positive mode. 

Water Cluster. Numerous groups have used water clusters successfully as calibration solutions [10,11,20-22]. Water clusters do not produce any source contamination in ESI-MS and provide closely spaced reference peaks with a calibration range up to m lz 1000. In positive-ion mode, protonated water clusters with up to 70 water molecules are observed. In negative ESI singly deprotonated water clusters are observed [OH (HjO), with n > 20], as well as solvated electrons [(H20)m with m > 11]. 

Acetate Salts. Sodium acetate and sodium trifluoracetate clusters were used and produce useful reference peaks for both positive and negative ESI [10,11,23] ... 

Table 7.1.1 Mass spectrometry fMSj settings for purine and pyrimidine screening in the positive electrospray ionization (ESI) mode, orotic acid in the negative ESI mode ... 

Figure 10.5 Typical LC-MS/MS trace by negative ESI of isoflavonoids from a urine extract. Measured concentrations in bracket. F-axis units in relative intensity EL = enterolactone, ED = enterodiol, other abbreviations see figure.

The MS data (negative ESI mode) provide information on the molecular mass of the partially deuterated molecules due to the use of D20 in the eluent. The MS experiments, however, were carried out via flow injection analysis (FIA-MS) on peaks that were cut from a separate chromatographic run in which H20 was used instead of D20. In this way, in addition to the fragmentation, the... 

The thus prepared ligands la-le-H4 are introduced into coordination studies with titanium(IV) ions in the presence of base (M2C03 M = Li, Na, K) with methanol as solvent. Negative ESI-MS (electron spray ionization mass spectrometry) of the obtained orange-red solids in methanol reveals that coordination compounds of general formula M2[l2(OCH3)2Ti2] are formed (Scheme 1.3.2) [20, 21]. 

Acetate Salts. Sodium acetate and sodium trifluoracetate clusters were used and produce useful reference peaks for both positive and negative ESI [10,11,23] 0.5% acetic acid in ammonium acetate solutions can be used for calibration in ESI-MS. This calibration solution, which is volatile, produces cluster ions up to m/z 1000. Therefore, it does not produce any source contamination or memory effects. Replacing acetic acid by trifluoroacetic acid (TEA) furtlier enlarges file mass range to m/z 4000, but TEA produces some memory effects and ion suppression, especially in negative-ion mode. 

E. Rathahao, A. Page, I. Jouanin, A. Paris, L. Debrauwer, LC coupled to negative ESI ion trap MS for the identification of isomeric glutathione conjugates of catechol estrogens, Int. J. Mass Spectrom., 231 (2004) 119. 

E. Hvattum, D. Ekeberg, Study of the collision-induced radical cleavage of flavonoid glycosides using negative ESI-MS-MS, J. Mass Spectrom., 38 (2003) 43. 

M.K. Moe, T. Anderssen, M.B. Strom, E. Jensen, Total structure characterization of unsaturated acidic GPL provided by vicinal di-hydroxylation of FA double bonds and negative ESI-MS, J. Am. Soc. Mass Spectrom., 16 (2005) 46. 

ESI is most commonly associated with the analysis of large biomolecules of medium to high polarity, and it is a major tool for proteomic analyses,17 but it can also be used for the MS analysis of small molecules provided they contain basic groups (e.g., amino, amide) for positive ESI or acidic groups (e.g., carboxylic acid, hydroxyl) for negative ESI. 

FIGURE 13.6 Color-coded summary of results from processing of pos/neg ESI data. 

---