EC-APCI

Analysis of DNA adducts derived from lipid hydroperoxide bifunctional electrophiles has relied primarily on reversed-phase ESI—MS (Blair, 2005). Some of the relatively hydrophobic bifunctional electrophiles that result from lipid hydroperoxide decomposition cannot be analyzed under these conditions because they are poorly ionized. These electrophiles can be converted to their oxime derivatives to improve ESI efficiency however, this conversion results in syn- and anti-oxime isomers and extremely complex EC chromatograms (Lee and Blair, 2000). Therefore, normal-phase EC—APCI/MS has proved to be much more successful for the analysis of lipid-hydroperoxide-derived bifunctional electrophiles (Lee et ah, 2001, 2005b Williams et ah, 2005). 

Today, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are considered the standard ionization techniques for LC-MS/MS due to their predominant advantages in quantitative analysis of drug molecules in various sample matrices with high sensitivity, selectivity, reliability, robustness, and ease of operation. Other techniques, for example, atmospheric pressure photoionization (APPI), electron capture atmospheric pressure chemical ionization (EC-APCI), and high-field asymmetric waveform ion mobility mass spectrometry (FAIMS) serve as complements to the established ESI and/or APCI technical platforms whenever necessary for an enhanced sensitivity and/or selectivity of a bioanalytical assay [4,5]. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

The significant enhancement of ion formation by a corona discharge as compared to a Ni source has already been implemented in early API sources. [139,140] The nature of the APCI plasma varies widely as both solvent and nebulizing gas contribute to the composition of the Cl plasma, i.e., APCI spectra can resemble PICI, CECI, NICI, or EC spectra (Chap. 7.2-7.4) depending on the actual conditions and ion polarity. This explains why APCI conditions suffer from comparatively low reproducibility as compared to other ionization methods. 

Figure 2.4 Comparison of (a) sensitivity, (b) variability, (c) selectivity, and (d) pricing between various chemical and immunological analyses for the presence of PPCPs in the environment. FID = flame ionization detector and EC = electrochemical detection. Note that GC-MS-MS can have mass detectors such as triple quadrupole and ion trap with ionization from El = electron ionization or Cl = chemical ionization, whereas LC-MS-MS with ionization from ESI = electrospray ionization, APCI = atmospheric pressure chemical ionization, or APPI = atmospheric pressure photoionization. (Adapted from Ingerslev and HaUing-Sprensen, 2003.)...

A study was carried out for LEE by the Soxhlet method and microwave-assisted extraction for the determination of the priority phenols in soil samples. Recoveries varied from 67 to 97% with RSD between 8 and 14% for LEE, and >70% for the MAP, except for nitrophenols that underwent degradation when the latter method was applied. LOD was from 20 ngg for 2,4-dimethylphenol to 100 ngg for pentachlorophenol. The best detection method for EC was atmospheric pressnre chemical ionization MS (APCI-MS). The most abnndant ions obtained by this detection method were [M — H] for the lowly chlorinated phenols and [M — H — HCl] for tri-, tetra- and pentachlorophenols . 

APCI=atmospheric pressure chemical ionization AOAC=Association of Offical Analytical Chemists CaCl2=calciurn chioride cl=chemical ionization EC=electrochemical detection ECD=electron capture detection ED=electrochemical detection FID=flame ionization detection GC=gas chromatography ... 

As regards identification purposes, the use of exhaustive libraries in GC/MS became a routine operation in most laboratories. In contrast, these libraries are still scarce in LC-MS applications, mainly because ESI-MS and APCI-MS spectra are strongly influenced by the instrument settings, the EC conditions, and the sample type. Therefore, much more effort should be paid to define identification and confirmation criteria for herbicide analysis by these ionization techniques. In this way, a confirmation criterion was successfully evaluated on triazines by Flow Injection Analysis... 

Carbamates. A fast, sensitive and selective method for the concentration and analysis of 9 N-methylcarbamate pesticides was reported by Volmer et al. [507]. Three different SPME fibres combined with short-column ESI-LC-MS(-i-) and MS/MS were applied. The detection limits observed were 0.3-1.9 pg Signal intensities increasing by a factor of 2-7 were observed [508] using non-volatile buffers in the separation process prior to ESI-MS. After EC removal of the non-volatile buffers was essential. The results obtained by ESI and APCI-LC-MS and MS/MS for the analysis of the eight N-methylcarbamate pesticides and their degradation products were compared with results obtain with the application of TSP or PBI (cf. 15.3.3.1 TSP, carbamates) [108]. ESI-LC-MS and TSP-LC-MS were used for quantitative determination of 10 different carbamate pesticides which showed a broad variety in polarity. ESI-SIM detection limits were typically 10-60 pg which was 10-150 times better than using TSP-MS (cf. 15.3.3.1 TSP, carbamates) [509]. Interfacing a commercial ESI source to an ITMS allowed the determination of carbamates as well as triazines and azo dyes. Identification could be performed either by IT-MS/MS or by ESI-CID [424]. 

The quantitation of small-sized therapeutic proteins or peptides in plasma can be conducted with EC—MS/MS detection of the intact proteins. Electrospray ionization (ESI) is the most widely used ionization technique for quantitative EC—MS/MS analysis of proteins or peptides. There are some occasions where atmospheric pressure chemical ionization (APCI) was used to circumvent matrix effects (Volosov et al., 2001). ESI typically generates multiply-charged protein or peptide ions, depending upon the number of basic charges in the polypeptide backbone. For small proteins, high abundance peaks of multiply... 

There are two major types of ionization sources that operate at atmospheric pressure, ESI, and atmospheric pressure Cl (APCI). A modified version of the ESI source is the ion spray source. These sources are described in detail in Section I3.I.6.I, because they are used to interface EC with MS for the separation and mass spectrometric analysis of mixtures of nonvolatile high MW compounds, especially in the fields of pharmaceutical chemistry, biochemistry, and clinical biomonitoring. ESI will be described briefly so that its use may be demonstrated, but more detail will be found in Chapter 13. 

---