Alkaline Adducts

2 Alkaline Adducts PC species can readily form alkaline (Aik = Li, Na, K) adducts. The formation of alkaline adducts depends largely on the availability or the concentration of the alkaline ions in the lipid solution. Therefore, if lithium adducts would like to be used for the analysis of PC species, lithium hydroxide or a lithium salt should be used as a modifier in the lipid solution. A sodium adduct is always displayed in a mass spectrum if there is no any other modifier added to the lipid solution since sodium ions are essentially present everywhere.  

A common fragmentation pattern yielded from alkaline adducts of PC species after CID as demonstrated [3, 4, 7, 17, 22] can be summarized. This pattern includes the following  

The fragmentation patterns of alkaline adducts of plamenylcholine (pPC) and plasmanylcholine (aPC) species contain all the fragment ions related to phospho-choline head group as those resulted from dPC species, including [M+Alk-59], [M+Alk-183]+, and [M+Alk-(Alk+182)]+. However, the fragmentation patterns of alkaline adducts of pPC and aPC species are also distinct from that of dPC species in three aspects as follows  

The fragmentation pattern of anion adducts of PC species after CID contains three types of fragment ions as follows  

Many studies have shown that the peak intensity of the carboxylate anion resulted from the sn-2 FA chain is approximately three times more intense than that arising from the sn-l FA chain of dPC species [17, 22, 29, 30]. This fact has been used to identify the location of fatty acyls and thus regioisomers of dPC species. However, this ratio becomes smaller than three if the sn-2 FA chain is a polyunsaturated FA substituent [22]. It is now clear that this reduced ratio is due to the fact that the resultant polyunsaturated FA carboxylate anion can go further fragmentation to yield an ion corresponding to the neutral loss of carbon dioxide (i.e., [carboxylate anion-44] ) [31-33]. This continuous loss of carbon dioxide has recently been exploited to identify the location of double bond(s) of fatty acyls [33]. With the recognition of carbon dioxide loss, the combined ion intensity of the carboxylate anion and [carboxylate anion-44] ion arising from sn-2 polyunsaturated FA chain is still approximately three times more intense than that of the sn-l FA carboxylate ion [32]. 

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Distinction of alkaline adducts of pPC species from aPC species is the presence of a unique feature to the fragmentation pattern of pPC species, that is, the presence of an abundant fragment ion corresponding to [M-i-Alk-(182-i-Alk)-R2C02H]+ in the product-ion ESI-MS spectra of pPC alkaline adducts, but not in those of both aPC and dPC counterparts [7]. This prominent ion arises from the further neutral loss of the sn-2 FA moiety from the ion corresponding to ([M-l-Alk-(182-l-Alk)] ). For example, the tn/z 279 ion ([M-l-Li-(182-i-Li)-R2C02H]+) arises from lithiated 16 0-18 2 pPC species (Figure 7.1d). 

Alkaline Adducts PE species can essentially form adducts with any alkaline (e.g., Li, Na, K) ion in the positive-ion mode when a modifier carrying the alkaline ion is added to the hpid solution, but sodium adduct is formed if no other alkaline ion is added. Similar to the formation of proton adducts, the ionization efficiency of PE alkaline adducfs is much lower in comparison to that of PC counterparts. Therefore, utilization of these adducts for analysis of PE species present in biological samples is suppressed by PC species by shotgun lipidomics. Application of these adducts for the analysis of PE species with LC-MS is also minimal. 

Fragmentation pattern of PE alkaline adducts has been well characterized [5,22]. Fragmentation pattern of PE alkaline adducts, which is similar to that of PC alkaline adducts, contains the following fragment ions ... 

As discussed in Chapter 2, ionization of anionic GPL species in the positive-ion mode is not favorable, but still can be formed in the presence of alkaline ion(s) in the solution or under acidic conditions. Generally, the sensitivity for ionization as alkaline adducts is markedly lower than what is observed as the [M-FH]+ ions in the positive-ion mode. Characterization of lithium and dilithium adducts of PI species (i.e., [M-FLi]+ and [M-H-l-2Li] ) or their protonated species has been performed [1]. 

Characterization of choline lysoGPL species as alkaline adducts is well studied in the positive-ion mode after low-energy CID [6, 7, 23, 26], The fragmentation pattern of alkaline adducted lysoPC species contains three groups of informative fragments as follows ... 

The fragmentation pattern of lysoPE species as alkaline adducts contains the following fragments [6] (Figure 7.7b) ... 

The sn-1 and sn-2 position isomers of acyl lysoPE alkaline adducts can be distinct from the differential intensities of these fragment ions as discussed in Section 7.1. For example, these isomers can be substantiated by the significant difference of the ratio between the fragment ion pair at [M-i-Alk-( 140-1-Alk)]+ and [M-i-Alk-61]+ as previously discussed [6]. Finally, a few fragment ions resulting from the phospho-ethanolamine head group are present (Figure 7.7b). For example, the m/z 148 ion corresponds to the lithiated phosphoethanolamine as previously described [5]. 

SM species can be readily ionized as alkaline adducts in the positive-ion mode when alkaline ions are available in the matrix. Again, sodium adducts of SM species are readily formed if no other modifier(s) are added to the lipid solution. The fragmentation pattern of SM alkaline adducts is well studied after low-energy CID (Figure 6.4) [14, 15], This pattern includes the following ... 

In the positive-ion mode, cerebroside (HexCer) species including both GluCer and GalCer can be readily ionized as their proton or alkaline adducts (i.e., [M-I-XJ+, X=H,... 

Li, Na, K), depending on the availability and affinity of the small cation(s) [12, 16, 17]. The fragmentation pattern of alkaline-adducted HexCer species yield three types... 

The ions at [180-bAlk] or [162-i-Alk]+ corresponding to the alkaline-adducted hexose derivatives. 

Lysosphingomyelin (lysoSM) can be readily ionized to form proton or alkaline adducts in the positive-ion mode. As with other choline-containing glycerophospho-lipids, product-ion mass spectra of protonated lysoSM species display the following fragment ions [26] (Figure 8.4c) ... 

Since a prominent charge site is absent in the glyceride species, the structure of FA substituent(s) significantly contributes to their ionization, particularly in the case of sodium adducts [10]. Therefore, the instrument response factors for ionization of these glyceride species as ammonium and alkaline adducts are apparently very different, depending on both the number of carbon atoms and of double bonds in the FA chains [10],... 

The major fragmentation process arising from the cleavage of inositol monophosphate residue, yielding a prominent ion corresponding to the elimination of inositol monophosphate residue and an ion at miz (260-l-Alk)+, corresponding to an alkaline-adducted ion of inositol monophosphate. 

Other ions equivalent to the alkaline-adducted ceramide ion and its further dehydrated ions. 

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