Negative mode

Description ring cleavage charge retained on nonreducing end infrequent pathway in the positive-ion mode, but a major pathway in the negative mode, because a stable, enolate anion results from loss of the enolic hydrogen atom ions are 42 mass units higher than those formed in Pathway C. 

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. 

Chiron et al. used HPLC/ESI-MS in the negative mode for the determination of acidic herbicides in environmental waters. The acidic herbicides investigated were benazolin, bentazone, 2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-... 

Databases Commercially molecular ions, fragment ions pos./neg. mode Only user-generated... 

LC-MS-MS was also the method of choice for the analysis of UV filters in solid matrices. Both LC and UPLC have been applied in three out of the four methods available for the determination of UV filters in sludge. Separation was performed on C8 and C18 LC-chromatographic columns (Zorbax, Eclipse, Vydac, and Purosphere) using binary gradient elution of mobile phases consisting of water/ methanol or water/acetonitrile. MS-MS detection was performed in SRM with ESI and atmospheric pressure photoionization (APPI) in both positive and negative modes. For each compound, two characteristics transitions were monitored. In addition to MS and MS-MS, diode array detection (DAD) was occasionally applied to the determination of OT. Spectra were recorded between 240 and 360 nm and discrete channels at 310 nm. 

The time-of-flight mass spectrometer was usually operated in the positive mode in order to detect the cationic sodium adducts of the analyte molecules. The anions of H-acidic substances were detected in the negative mode. This mode significantly enhanced the signals of acidic compounds, e.g. free fatty acids compared with ester signals in the mass spectra of beeswax and tung oil. 

The LC-MS/MS technique has been used to quantify and identify phenolic compounds. In order to quantify, multiple reaction monitoring (MRM), in which there is a combination of the precursor ion and one of its daughter fragments, is used to characterize a particular compound. This behavior should be as specific as possible in samples with a complex mixture of phenolic compounds. This technique has been largely used to quantify phenolic compound metabolites in urine and plasma (Urpf-Sarda and others 2005, 2007). In this context, LC-ESI-MS/MS with negative mode has been applied for the identification of a variety of phenolic compounds in a cocoa sample (Sanchez-Rabaneda and others 2003 Andres-Lacucva and others 2000). 

Compounds were optimized in positive ionization mode and in negative mode if necessary. Automaton can also perform automatic MS method development from solutions containing multiple compounds to increase throughput. When mixture solutions are used, Automaton injects a mixture once to determine all precursor ions and DP values and then injects once per compound to determine product ion and CE value. This approach allows automatic and unattended optimization of MS parameters for hundreds of compounds. The optimized parameters are stored in a compound database that permits fast and efficient retrieval of information about a specific compound and allows a compound to be used in multiple assays, eliminating the need to re-optimize the LC/MS/MS conditions. 

Figure 7. Deep UV sensitivity curves of SPP positive node (A) compared with that of SPP negative mode (B). In positive mode, a 0.4 izm thick SPP layer was exposed to deep UV and then dip-developed in a 1.6 wt% TMAH solution for 60 s at 25 °C. In negative mode, SPP was exposed to deep UV followed by a flood exposure using near UV radiation and then dip-developed in a 0.7 wt% TMAH solution for 60 s at 25 C.

Comparison of the results in the ionisation of AES obtained from the different API techniques in positive or negative mode, which were presented in Fig. 2.5.10 proved that neither APCI nor ESI can be termed as a universal interface. 

APCI and ESI-FIA-MS-MS examinations in the positive and negative mode were performed. The product ions of the [M + NH4]+ parent ion m/z 394 from the feed extract allowed an unequivocal identification of the homologue as ion of the AE molecule with the formula Ci3H27—O—(CH2—CH2—0)4—H. The product ions m/z 45, 89, 133 and 177, known as ethoxylate fragments of the PEG chain, and the alkyl fragments m/z 57, 71, 85, 99,113 and 127 were also characteristic [7] for this AE type, which is presented with its fragmentation scheme in Fig. 2.9.4 [22],... 

A mixture of sorbitan derivatives named Tween 81 specified as ethoxylated sorbitan esters containing oleic acid, was examined by APCI-FIA-MS and —LC—MS in the positive and negative modes. APCI-MS ionisation was supported by the addition of ammonium acetate resulting in equal spaced (Am/z 44) [M + NH4]+ ions which, in parallel, suppressed [M + Na]+ ions. The FIA—MS(+) spectrum contained ions with m/z between 358 and 974 while negative ionisation led to a series of ions from 399 to 971, Am/z 44 equally spaced, too. 

The qualitative determination of anionic surfactants in environmental samples such as water extracts by flow injection analysis coupled with MS (FIA-MS) applying a screening approach in the negative ionisation mode sometimes may be very effective. Using atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI), coupled with FIA or LC in combination with MS, anionic surfactants are either predominantly or sometimes exclusively ionised in the negative mode. Therefore, overview spectra obtained by FIA—MS(—) often are very clear and free from disturbing matrix components that are ionisable only in the positive mode. However, the advantage of clear... 

Octadecyl sulfate sodium salt (Ci8H37-0-S03 Na+) was examined by laser desorption (LD) Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) in the negative mode resulting in [M - H] ions. Little fragmentation was observed under these conditions [28]. 

AESs with the general structural formula CnH2n+1-0-(CH2-CH2-0)X-SC>3 (cf. Fig. 2.11.6) were determined qualitatively using ESI-LC-MS(-). TSI analysis of these compounds in the negative mode was only possible with reduced sensitivity [22,25], whereas positive ionisation led to the cleavage of the SO3 moiety, imagining... 

For comparison of the different ionisation methods and detection modes, the results obtained as FIA overview spectra are presented in Figs. 2.11.7 and 2.11.8. Reconstructed ion chromatograms (RIC) of APCI and ESI combined with selected mass traces of all LC separations and, in parallel, the selected standardised mass traces of the C42 and C14 homologues containing three ethoxy chain links recorded in the negative mode are presented in Fig. 2.11.9. These results again demonstrate the quite large variation in the ionisation efficiency of... 

Triton X-200 is known to be hardly biologically degradable. To confirm this behaviour, the compound characterised as NPE0-CH2-S03 containing two EO units in the ethoxy chain (NP(E0)2-S03) was submitted to a LC separation. RT comparison in APCI-LC-MS(+/-) experiments and APCI-FIA-MS-MS in the product ion mode were examined (Fig. 2.11.20(a)) in positive and negative modes [15]. In the positive CID spectrum of NP(E0)2-S03, the parent ion 420 disappeared and the product ion m/z 291 (cf. inset in Fig. 2.11.20(a)) known as the characteristic NPEO product ion was observed as base peak [22]. Application of CID furthermore led to alkyl fragments (m/z 57, 71, and 113) and ions at m/z 165 and 121 resulting from the abstraction of the alkyl chain of ion 291 or from the abstraction of the alkyl chain and one... 

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