Using adduct ions

ESI and APCI produce predominantly pseudo-molecular ions [M + H] + species in a positive mode and [M — H] in negative ion mode, with variable fragmentation depending upon the cone voltage used and, in the case of APCI, on the temperamre used. Adduct ions are very often seen in ESI, and it is these that can cause most problems to inexperienced users, but they can be useful. Their formation depends on the coordinating properties, polarity and concentration of the analyte and to some extent on the solvent being used. 

ANALYSIS OF DRUG IMPURITIES Table 6.1 A list of adduct ions 

Ions appearing at approximately twice the mass of the parent ion are known as multimers. These usually appear at (2M + H) owing to the formation of a protonated dimer in the MS. 

Intensities of adducts can vary depending on different eluents and composition. Using both high and low cone voltages in all the above cases will help in the interpretation of the spectra. 

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Introduction of Other Nucleophiles Using Diazonium Ion Intermediates. Cyano and azido groups are also readily introduced via diazonium intermediates. The former involves a copper-catalyzed reaction analogous to the Sandmeyer reaction. Reaction of diazonium salts with azide ion gives adducts that smoothly decompose to nitrogen and the aryl azide.56... 

Oxidations Using Oxoammonium Ions. Another oxidation procedure uses an oxoammonium ion, usually derived from the stable nitroxide tetramethylpiperidine nitroxide, TEMPO, as the active reagent.31 It is regenerated in a catalytic cycle using hypochlorite ion32 or NCS33 as the stoichiometric oxidant. These reactions involve an intermediate adduct of the alcohol and the oxoammonium ion. 

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). 

One of the most serious drawbacks that has been observed in the ionisation processes with soft ionisation techniques is the very soft generation of ions. This process, which predominately leads to molecular ions or adduct ions but no fragments for identification, however, was also used to improve and speed-up MS analysis. 

With regard to quantitative measurements of APG surfactants in, e.g. environmental samples, the authors stressed that it was of crucial importance to promote the formation of the desired molecular (or adduct) ion in order to obtain reproducible mass spectra. If tuning of the ESI interface parameters did not suffice to yield abundant ions of the selected species, acquisitions of the mass spectrometric detector after negative ionisation in conjunction with appropriate selection of the mobile phase composition were used as an alternative despite the lower sensitivity in this mode [1,2],... 

Table 2.7.2. In (+ )-ionisation mode, apart from the [M + H]+ ion, the sodiated and potassiated ions could be detected. Furthermore, the fragment ion [M + H—H20]+ arising from the loss of one water molecule out of the carbohydrate moiety was formed. No ammonium adduct ion was observed although ammonia was used for pH adjustment of the eluent. In (+)-ionisation mode two fragments were detected, both containing the carbohydrate part of the molecule, [Gluc-NH2-CH3]+ and [(G1uc-H20)-NH2-CH3]+.

An industrial blend of AE surfactants with the general formula (CnH2n+iO-(CH2-CH2-0)xH n = 12, 14, 16 and 18) was examined using APCI-FIA-MS(-I-) for screening purposes (see Fig. 2.9.2(a)). According to the number of glycol units and the number of alkyl chain links, a series of homologue ammonium adduct ions ([M + NH4]+) equally spaced either with Am/z 44 (-CH2-CH2-0-) or Am/z 28... 

In a similar study with ESI the influence of different buffers was studied [12]. In the presence of acetic acid (HAc) only in the MeOH/H20 mobile-phase, a mass spectrum resulted with ion adducts of Na and K appearing as the most abundant ones. However, minor peaks could also be observed in the mass spectrum resulting from ammonium adducts (Fig. 4.3.2(A)). The respective ions could be suppressed or enhanced by changing the nature of the buffer used in the mobile-phase. For example, when a potassium buffer was used, sodium and ammonium adducts were suppressed, and the spectrum became less complicated with primarily the potassium adduct ion being visible (Fig. 4.3.2(C)). In addition, the signal-to-noise ratio improved by about a factor of 1.5-2. Similarly, sodium or ammonium acetate buffers enhanced the sodium and ammonium adduct ions, meanwhile suppressing other adducts (Fig. 4.3.2(B) and (D), respectively). 

The use of ammonia for the protonation of nitroarenes leads frequently to formation of aduct ions, e.g. [M + NH4]+, but not to the protonated species (MH+)112,113. The ammonia chemical ionization spectrum of nitrobenzene shows, in addition to a series of adduct ions, a dominant signal corresponding to the anilinium ion (m/z 94)112114115. Evidence for the isomerization of the [M + NR ]"1" adduct followed by successive loss of NO and OH or NH3 to give ions corresponding to the substitution products, e.g. the anilinium ion, has been given115 see Scheme 41. 

Example The Bunte salt [ 3( 2)158-803] Na yields a very useful negative-ion FAB spectrum from NBA matrix (Fig. 9.11). NBA forms [Ma-H] and Ma " ions. The salt anion contributes the base peak at m/z 337.3. [Ch-2A] m/z 697.5, and [2C-I-3A] m/z 1057.6, cluster ions are observed in addition, their isotopic patterns being in good agreement with theoretical expectation. It is noteworthy that the matrix adduct at m/z 513.4 is a negative radical ion. 

When G2-OH is mixed with a fourfold molar excess of Cu + ions the spectrum in Fig. 9b results. These data indicate that each G2-OH can sorb at least four Cu + ions. Moreover, the separation between adjacent copper adducts is 62.5, which indicates that the oxidation state of Cu inside dendrimer during the MALDI-MS experiments is -1-1. Reflectron-mode MS also confirms this assignment the mass differences between the monoisotopic peaks of protonated dendrimers, singlecopper adducts, and double-copper adducts are 61.96 and 61.93, respectively, which is consistent with the assignment of the adduct ions as [Mis + Cu(l)]+ and [Mis + 2Cu(I)-H] +. We speculate that the presence of Cu+ is a consequence of the photochemical reduction of Cu + during ionization. Such photoreduction in MALDI MS measurements has been observed previously when polymers or peptides are used as ligands for Cu + [117,118]. 

The formation of RDX cluster ions in LC/MS and the origin of the clustering agents have been studied in order to determine whether the clustering anions originate from self-decomposition of RDX in the source or from impurities in the mobile phase [19], IsotopicaUy labeled RDX ( C3-RDX and Ng-RDX) were used in order to estabhsh the composition and formation route of RDX adduct ions produced in ESI and APCI sources. Results showed that in ESI, RDX clusters with formate, acetate, hydroxyacetate and chloride anions, present in the mobile phase as impurities at ppm levels. In APCI, part of the RDX molecules decompose, yielding NO2 species, which in turn cluster with a second RDX molecule, producing abundant [M- -N02] cluster ions. 

Dunbar has added a deep theoretical imderstanding to the radiative stabilization process and has shown, using his standard hydrocarbon model, that the radiative cooling of chemically activated adduct ions, with energy e" in the /th level of the th normal mode, can be very accurately modeled by use of Equation (12). ... 

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