Achievements APCI

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

The applicable HPLC flow rate with ESI is lower than that with thermospray or APCI, usually below the O.SmLmin range. The typical flow rate is 0.10-0.20 mL min for ESI, which means that the effluent flow introduced into the electrospray must be reduced by splitting when using a conventional HPLC column (4.6-mm i.d. x 250 mm). Currently, narrower columns (e.g., 2.1-mm i.d.) and slower flow rates are commonly used to achieve the desirable flow rates. The advantage of this approach is that improved separation efficiency and faster separations are also achieved (at the cost of sample capacity). 

In the case of carbamate insecticides, both ESI and APCI can be used. However, in this study, the sensitivity of APCI was 3-5-fold less than that of ESI. In this case, the Z-spray configuration was used with APCI, which gives a lower efficiency of ions reaching the mass analyzer than is achieved with other instrumental configurations. 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

APCI is suitable for the ionisation of small molecules that are polar to non polar in nature particularly those containing heteroatoms. Samples that are ionic/charged in solution or are very thermally unstable or photosensitive are better suited to ESI where ionisation is achieved in solution, prior to detection... 

Figures 10 and 11 show the structure of the hindered phenolic antioxidant Irganox 1010 (Ciba) and its negative ion APCI mass spectra, respectively. Separation was achieved under the following LC conditions Column Aqua Cl 8 (Phenomenex) 3 pm, 150x2.00 mm, 15% carbon loading, proprietary end capping. Column Temp 50°C. Injection volume 5 pi.

This ESI(+) TIC, however, is dominated by strong and broad signals that eluted between 17 and 31 min, neither observable under APCI(+/—) nor ESI(-) conditions. Even under gradient RP-C18 conditions a strong tailing effect was observed while isocratic RP-C18 failed. The information obtained by ESI—LC—MS(+) was that the compounds could be ionised in the form of [M]+ ions at m/z 230, 258 and 286. ESI-LC-MS-MS(+) resulted in product ion spectra which, by means of a MS-MS library, were found to be characteristic for the amphoteric amine oxide surfactants. These compounds not yet observed in household formulations will be presented later on with the RIC of LC separation (cf. Fig. 2.5.11(d)). After identification as amine oxides, the separation and detection of this compound mixture now could be achieved by an isocratic elution using a PLRP-column and methane sulfonic acid and ESI(+) ionisation with the result of sharp signals (RT = 4-6 min) as presented in Fig. 2.5.11(d). 

In the absence of any added salts, the APCI-MS spectra were dominated by the Na+ adducts, as shown in Fig. 2.8.5. The NH4 and K+ adducts were present at lower intensities, the latter especially for the higher molecular weight analogues. Addition of CH3CO2NH4 did not simplify the adduct formation to [M + NH4]+ species as observed in ESI-MS and the best results for APCI-MS analysis were obtained without addition of any salt solutions. Application of this method to determinations of M2D-C3-0-(E0)n-Me recovery from solid substrates was achieved, using triethylene glycol monohexyl ether [C6(EO)3] as the internal standard (Fig. 2.8.5) [29],... 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

APPI is a relatively recent development compared with the other techniques. Here, ionisation is achieved photochemically, either directly or mediated by a dopant such as acetone added to the eluent. Both even- and the less stable odd-electron ions (e.g. M ) may be formed. At the time of writing, the mechanisms involved and scope of the technique are still not fully understood. What is apparent is that it provides a complementary technique to ESI and APCI. 

Both ESI and APCI spectra can look relatively simple in most cases, just showing the pseudo-molecular ion MH or adduct ion in the positive mode, and deprotonation or adduct ions in the negative mode. With API techniques we are dealing with even-electron (non-radical) MH ions as opposed to odd-electron M species that result from electron ionisation. Once an ion has achieved an even-electron state, it is unlikely to revert to an odd-electron state, as this is energetically unfavourable. This means that fragmentations from MH should... 

The real breakthrough in LC/MS development was achieved with the broad introduction in the 1990s of atmospheric pressure ionization (API) techniques, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), which enable the analysis of a wide variety of molecular species. The spectrum of available API techniques has been amended meanwhile by the introduction of sonic spray ionization (SSI) and atmospheric pressure photoionization (APPI). 

Dne to their estrogenic activity, steroids have been inclnded in preliminary lists of EDCs. These chemicals are even more diffnsely fonnd in waters, also dne to sensitivities nowadays achieved in advanced LC/MS and LC/MS/MS instrumentations. The interfaces most widely nsed for the LC/ MS determination of steroids, drngs, surfactants, and organic pollntants in an aqnatic environment are ESI, which is particnlarly well suited for the analysis of polar componnds, and APCI, that is more effective in the analysis of medium- and low-polarity snbstances. LC/MS and LC/MS/MS have been mostly applied in the SIM mode and in the MRM mode. 

Peroxide explosives are potent explosives that can be made starting from common and easy to obtain raw materials. The analysis of triacetone triperoxide (TATP) and hexamethylenetriper-oxidediamine (HMTD) was successfully carried out by HPLC-APCI-MS in a powder sample as well as in post-blast extracts originating from a forensic case [134]. After RP separation on a C18 column using a methanohwater (75 25 v/v) mobile phase containing ammonium acetate (2.5mM) at a 0.4mL/min flow rate, detection was carried out in positive ion mode. MS-MS analysis of [TATPh-NH4]+ and [HMTD - H]+ as precursor ions was necessary in order to achieve the required sensitivity in the analysis of postblast extracts (LOD 0.8 and 0.08 ng on column, respectively). 

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