APCI general

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

In general terms, electrospray ionization is considered to be concentration-sensitive at Tow flow rates and mass-flow-sensitive at high flow rates, while APCI is considered to be mass-flow-sensitive. Low and high are both subjective terms and require investigation as part of method validation. 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

B-QITMS. LC-APCI-ToFMS is still in an early stage of development. LC-API-MS is more general purpose than LC-PB-MS these techniques are complementary. 

Since APCI is a chemical ionization, it needs a gas. Indeed, a nebulising gas (generally nitrogen) is introduced into the source. The gas molecules are ionized by a corona discharge (analogous to the fdament used in Cl) thus forming the primary ions, mainly composed of N2+" and N4+". In turn, the latter ionize the vaporized solvent molecules by... 

Liquid chromatography, coupled to the different ionization sources, is generally the technique most used to characterize the phenolic profile in fruit and vegetable products. With regard to the source ionization, it seems that ESI is used more frequently than other sources, such as APCI or APPI. Another important aspect of this technique is the ionization of phenolic compounds. Negative ionization seems to be more suitable... 

As a general rule, APCI is less likely to demonstrate matrix effects and ESI is more likely to be affected by matrix effects. Sample clean-up is another important factor—protein precipitation is more likely to result in matrix effects than is solid phase extraction. Matrix effects may be caused by sample constituents that are not parts of the biological matrix. Mei et al.126 129 showed that certain brands of sample tube containers can produce matrix effects. They also demonstrated that Li-heparin, a common anticoagulant for plasma samples, can produce significant matrix effects... 

Atmospheric pressure chemical ionization (APCI) was introduced in 1973 by Horning et al. [38, 42, 43] and coupled to GC. This is also the introduction of atmospheric pressure ionization (API) in general. The next year corona discharge was introduced for ion generation as well as successful coupling to LC [44, 45]. In APCI of a liquid, a pneumatic nebulizer induces the flow of liquid to form a spray at atmospheric pressure. The spray droplets pass a corona discharge electrode situated close to the orifice, which... 

Currently no comprehensive libraries have been developed for methods, such as ESI (see Section 2.1.15) and atmospheric pressure chemical ionization (APCI see Section 2.1.8), used to ionize compounds after LC separation. Generally these types of library are user generated for a specific purpose. 

Fig. 2.5.4. (a) APCI-FIA-MS(-) and (b) APCI-FIA-MS(+) overview spectrum combined with general structural formula demonstrating differences in the ionisation behaviour of NPEO-SO4 (C9H -0fiH4-0-(CH2-CH2-())r-S()3). 

As an example of an anionic surfactant mixture frequently contained in detergent formulations, an AES blend with the general formula C H2 i i—O—(CH2—CH2—O) —SO3 was examined in the negative FLAMS mode. Because of the considerable differences observed between both API ionisation mode overview spectra, the ESI—FIA—MS(—) and the APCI—FIA—MS(—) spectra are reproduced in Fig. 2.5.3(a) and (b), respectively. Ionisation of this blend in the positive APCI—FIA—MS mode, not presented here, leads to the destruction of the AES molecules by scission of the O—SO3 bond. Instead of the ions of the anionic surfactant mixture of AES, ions of AE can then be observed imaging the presence of non-ionic surfactants of AE type. 

Fig. 2.5.9. APCI-FIA-MS-MS(+) parent ion mass spectrum from product ion (m/z 344) observed in APCI-FIA-MS-MS(+) product ion spectrum as in Fig. 2.5.6(c) (inset general structure of polyglycol amine).

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 screening approach, non-ionic surfactants were monitored in the form of their [M + NH4]+ ions, equally spaced with Am/z 44 and identified by FIA-MS-MS(+) in combination with APCI or ESI interface [34,35]. Ci8-SPE was performed prior to selective elution by diethyl ether [35]. Ions of the non-ionics of AE type at m/z 350-570 (Am/z 44) were identified as surfactants with the general formula Ci3H27-0(CH2CH20)mH (m = 3-7). The complexity of the mixture confirmed the results using the diagnostic parent scans m/z 89 for aliphatic non-ionic surfactants of ethoxylate type necessary [35]. 

One of the most observed degradation pathways of non-ionic surfactants of ethoxylate type in the biochemical wastewater treatment process is the bond scission between the lipophilic alkyl chain and the hydrophilic ethoxylate moieties. The resulting ethoxylate compounds, PEG or PPG, are highly polar and are not quite easy to degrade, therefore often they can be observed in wastewater discharges. So, APCI— FIA-MS(+) product ion spectra of selected [M + NH4]+ ions, which were under suspicion as PEG (general formula HO—(CH2—CH2—0) H)... 

Fig. 2.9.5. APCI-FIA-MS-MS(-I-) (CID) product ion mass spectrum of selected [M + NH4]+ parent ion (m/z 476) of non-ionic surfactant metabolite identified as PEG homologue (general formula H0-(CH2-CH2-0)X-H) fragmentation behaviour under CID presented in the inset [35].

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