Positive ESI

In positive ion mode, M + H (M + 1) and M + Na (M + 23) ions may be formed from 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 4.12 Positive ESI mass spectrum of acidified water and methanol (normal background spectrum). 

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Triethylamine (TEA) is sometimes used as an additive for signal enhancement. However, in the positive ESI mode, TEA readily ionizes to give an intense [M + H]+ ion at miz 102. This then suppresses the ionization of the less basic compounds in the positive ESI mode. In the negative mode, TEA can enhance ionization for certain compounds because of its basic properties. 

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...

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

Analytical methods for the detection of residues of semicarbazide use derivatisation with 2-nitrobenzaldehyde and LC-MS detection. Figure 18 shows the positive ESI response for a 1 ppm solution of semicarbazide after derivatisation and concentration. The main peak 2 at 16 min shows the expected 209 (M+H)+ ion of the 2-nitrobenzaldehyde derivative of semicarbazide together with its sodium adduct ion at m/z 231 (Figure 19). 

Figure 21 LC-MS separation (in elution order) of myristic, palmitic, stearic and eicosanoic acids using positive ESI.

Figure 21 shows the LC-MS separation of fatty acid lubricants detected using positive ESI with specific ion monitoring as (M—H). The following conditions were employed. 

Schmitt [17] in his book on the analysis of surfactants includes details of a number of HPLC-based procedures. LC-MS can be used for positive identification. Figure 29 shows the molecular ion mass spectrum for the surfactant lauryl hydrogen sulfate detectable as its (M—H) ion by positive ESI. 

Ionic surfactants such as sodium dodecyl sulfate can also be detected by ESI. Figure 30 shows an overlay of sub ppm concentrations detected using ion pair chromatography with specific ion LC-MS detection (positive ESI at m/z 265). Gradient elution from 50.0% water containing 5 mM acetic acid triethylamine) to 50.0% 80/20 acetonitrile/water (5mM acetic acid triethylamine) was employed. 

Figure 29 Mass spectrum for lauryl hydrogen sulfate (positive ESI).

Using the Tomtec Quadra 96 workstation, 0.1 mL of the ethyl acetate layer was transferred to a 96-well collection plate containing 0.4 mL of acetonitrile in each sample well. The solution was mixed 10 times by aspiration and dispersion on the Tomtec. The plate was then covered with a sealing mat and stored at 2 to 8°C until LC/MS/MS analysis. The HILIC-MS/MS system consisted of a Shimadzu 10ADVP HPLC system and Perkin Elmer Sciex API 3000 and 4000 tandem mass spectrometers operating in the positive ESI mode. The analytical column was Betasil silica (5 fim, 50 x 3 mm) and a mobile phase of acetonitrile water formic acid with a linear gradient elution from 95 5 0.1 to 73.5 26.5 0.1 was used for 2 min. The flow rate was 1.0 mL/min for the API 3000 and 1.5 mL/min for the API 4000 without any eluent split. The injection volume was 10 jjL and a run time of 2.75 min was employed. 

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. 

In Ref. 42 a similar approach was chosen as in Ref. 39 using stereoisomers of the type Fmoc-L-Asp-L-Asp-D-Xaa-D-Xaa (Xaa = Gly, Ala, Phe, His, Ser, Tyr). Interestingly, in part the findings are different. The ACE/MS hyphenation caused a number of practical problems affecting the reliability of the system. Surprisingly, the authors faced problems with positive ESI and were forced to use negative ionization. Because of the use of the nonvolatile Tris buffer, crystallization problems occurred frequently. Only high-EOF conditions prevented this knockout scenario. However, the description of problems and related solutions is very instructive. 

Fig. 4 Epitope mapping by ACE/MS in the positive ESI mode. (A) Tryptic digest, 4.6 pmol//j,E, 10-s pressure injection (B) tryptic digest, 10-s injection followed by 25-s injection of antibody (4.7 pmol//xL). Selected ion electropherograms for the m/z. indicated and the total ion electropherograms, respectively. It is obvious that peptide 1 (Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys) is captured by the antibody. See text for further explanation. (Reprinted with permission from Ref. 40. Copyright 1997 American Chemical Society.)... 

Table 7.1.2 MS settings for pyrimidine degradation screening in the positive ESI mode ... 

Fig. 2 Simultaneous LC-MS/MS analysis of diverse tropane alkaloids. Analysis of plasma samples was performed according to John et al. [50] using an Atlantis T3 C18 column (150 mmx4.6 mm I.D., 5 pm) in gradient mode (as indicated) with solvent A (0.1 %v/v FA) and solvent B (ACN/water 80 20 v/v 0.1% v/v FA) at 1 ml/min and 30 °C. Detection was done by positive ESI MS/MS in MRM mode...

Most often positive ESI and only to a small extent positive atmospheric pressure chemical ionization (APCI) were used as ion source (interface) to generate desol-vated free ions of TTA or QTA suitable for MS or MS/MS detection. TTA were detected as their proton adducts [M+H]+, whereas QTA were simply monitored as the original cations [M]+ thus not requiring adduct formation (Table 9). 

Specific analytical methods have not been developed for dialkyl alkylphosphonates. They are usually sufficiently hydrophilic to be partially extracted by water from matrices such as soil and they are detected by the screening methods described above under positive ESI and APCI conditions. They are easily differentiated from isomeric alkyl alkylphosphonic acids by their lack of response under negative ionization conditions (Section 6.2.3, Figure 8) (14). 

LC/MS/MS is an important technique for the analysis of free metabolites and covalent adducts of sulfur mustard in urine and blood. In the case of TDG and TDGO, LC/MS has not yet been able to achieve the LODs obtainable with GC/MS after derivatization. LC/MS/MS has, however, been used successfully to analyze the metabolites (20, 21) derived from an initial reaction of sulfur mustard with glutathione (see Chapter 16). The two metabolites (20), derived from the 3-lyase pathway, can be isolated from urine by SPE on a hydroxylated polystyrene-divinylbenzene polymeric cartridge. Using a sensitive triple sector quadrupole LC/MS/MS system, detection limits of O.lng/ml have been achieved using positive ESI and MRM (56). This provides a useful alternative to GC/MS/MS, which requires reduction of the sulfoxide functions with titanium trichloride. An LC/MS/MS method (detection limit lng/ml) has been developed for the analysis of the hisf N - ace ly I cysteine) metabolite (21) in urine (57). Concentration from acidified urine was achieved on... 

Aflatoxins, for example, Bj (36) and their biological markers have been determined using ESI (90,91). Aflatoxins B1 B2, G1 G2 were analyzed in food samples by LC/ESI/MS using a 150 x 2-mm C18 column eluted isocratically with acetonitrile-MeOH-10 mM NH4OAc (2 16 15) (90). The positive ESI spectra were dominated by the protonated molecules, which were used for SIM. The method enabled concentrations down to lppb to be detected in various food materials. LC/ESI/MS/MS has been used for the detection of aflatoxin DNA adducts as urinary biomarkers of exposure (91). 

An alternative to GC/MS/MS of the reduced product (10) is direct FC/ESI/MS/MS of (2) and (3). The analytes can be isolated from urine by SPE on a hydroxylated polystyrene-divinylbenzene (PS-DVB) polymeric cartridge. Using a sensitive triple sector quadrupole LC/MS/MS system, detection limits of O.lng/ml have been achieved using positive ESI and MRM (30). 

Pruvost, A. et al. Specificity enhancement with LC-positive ESI-MS/MS for the measurement of nucleotides. J. Mass Spectrom. 2008,43, 224-233. 

In positive-ion APCI, either protonated or ammoniated molecules are observed, and fragments due to the loss of nonene. Low-mass NPEO suffer from enhanced background noise [30]. For quantitative analysis, electrospray LC-MS is preferred, while the protonated molecules generated in APCI can be readily fragmented in MS-MS for structure elucidation [28]. The positive ESI and APCI mass spectra of an NPEO with an average EO number of four are shown in Figure 8.3. 

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. 

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