Heated nebulizer

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

The APCI interface uses pneumatic nebulization in an atmospheric pressure region to form fine spray droplets. Typically, these systems use a heated nebulizer (300-650 °C)... 

Fig. 11.28. Schematic of an APPl source, including the heated nebulizer probe, photoionization UV lamp and moimting bracket. Reproduced from Ref. [144] by permission. American Chemical Society, 2000.

FIGURE 8.5 Schematic representation of an API sonrce with a heated nebulizer interface for APCI. (Reproduced from Raffaelli, A., Atmospheric pressure chemical ionization (APCI), in Cappiello, A. (ed), Advances in LC-MS Instrumentation, vol. 72 Journal of Chromatography Library), Elsevier, Amsterdam, the Netherlands, 2007, 11-25. Copyright 2007. With permission from Elsevier.)... 

In the presence of traces of water, a series of ion-molecule reactions lead to the formation of proto-nated water clusters H30+(H20) . At atmospheric pressure, there is a significant interaction between reagent ions and analyte ions produced by the heated nebulizer and which will generate protonated analyte molecules [M + H]+. 

The in-line source depicted in Figure 8.7 was designed by Bruins et al. to be mounted on a PE-Sciex triple quadrupole and it was derived from the standard heated nebulizer of their APCI source. The corona needle is replaced by a discharge lamp. Nitrogen is used as the nebulizing and the lamp gas, while air is used as the auxiliary gas. A dopant improves the efficiency of ionization and it is supplied through the auxiliary gas line and vaporized together with the solvent in the heated nebulizer. 

Similar interface to that used for ESI. In APCI, a corona discharge is used to ionize the analyte in the atmospheric pressure region. Ions are formed by charge transfer from the solvent as the solution passes through a heated nebulizer into the APCI source... 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

In order to combine reversed-phase LC with atmospheric pressure chemical ionization (APCI)-MS (125), a commercially available heated nebulizer interface that can handle pure aqueous eluents at flow rates up to 2 ml/min in addition to nonvolatile buffers has been used (126). The heated nebulizer inlet probe consists... 

The success of API is due, at least in part, to the fact that both ion-spray and heated-nebulizer involve soft ionization processes that provide an abundance of molecular ions. This results in simple, easy-to-interpret spectra affording molecular weight information. Because of these features, this interface is perfectly suited to natural products chemistry. We use LC-MS on the material in those tubes identified as having biological activity. The combination of molecular weight and UV-visible data of biological active fractions is used for identification. 

APCI uses a heated nebulizer to facilitate solvent evaporation and obtain a fine spray of the mobile phase instead of a strong electromagnetic field as in electrospray. Unlike elec-... 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate and to facilitate solvent evaporation. In addition, a cross-flow of heated nitrogen gas is used to... 

The APCI interface uses a heated nebulizer to form a fine spray of the HPLC eluate, which is much finer than the particle beam system but similar to that formed during thermospray. A cross-flow of heated nitrogen gas is used to facilitate the evaporation of solvent from the droplets. The resulting gas-phase sample molecules are ionized by collisions with solvent ions, which are formed by a corona discharge in the atmospheric pressure chamber. Molecular ions, M+ or M , and/or protonated or de-protonated molecules can be formed. The relative abundance of each type of ion depends upon the sample itself, the HPLC solvent, and the ion source parameters. Next, ions are drawn into the mass spectrometer analyzer for measurement through a narrow opening or skimmer, which helps the vacuum pumps to maintain very low pressure inside the analyzer while the APCI source remains at atmospheric pressure. 

Kauppila et al. [28] developed a microfabricated heated nebulizer chip for atmospheric pressure photoionization-mass spectrometry. Various materials have been used to design and develop hyphenation of microfluidic devices and ESI-MS. These materials are photoresist SU-8 [29,30], polymers [31,32], and glassy carbon [33]. Thorslund et al. [34] developed a chip on which sample injection, separation, and ESI-emitter structures are integrated... 

Atmospheric pressure chemical ionization (APCI), is more efficient than ESI for nonpolar (hydrophobic) analytes, such as steroids, which do not readily form ions in solution [61], Since APCI requires use of a heated nebulizer, thermally labile compounds may decompose in the ionization source. 

S. Zhou and M. Hamburger, Application of liquid chromatography-atmospheric pressure ionization mass spectrometry in natural product analysis. Evaluation and optimization of electrospray and heated nebulizer interfaces, J. Chromatogr., A, 755, 189-204 (1996). 

The APPI source is one of the last arrivals of atmospheric pressure sources [80,81]. The principle is to use photons to ionize gas-phase molecules. The scheme of an APPI source is shown in Figure 1.34. The sample in solution is vaporized by a heated nebulizer similar to the one used in APCI. After vaporization, the analyte interacts with photons emitted by a discharge lamp. These photons induce a series of gas-phase reactions that lead to the ionization of the sample molecules. The APPI source is thus a modified APCI source. The main difference is the use of a discharge lamp emitting photons rather than the corona discharge needle emitting electrons. Several APPI sources have been developed since 2005 and are commercially available. The interest in the photoionization is that it has the potential to ionize compounds that are not ionizable by APCI and ESI, and in particular, compounds that are non-polar. 

In the APPI method of ionization, the solvent is vaporized in a heated nebulizer and the gaseous analytes are then ionized with photons from a lamp (Rivera et ah, 2011). It has been observed that certain solvents, called dopants, enhance the ionization of analytes via this technique. To date only one study has been published with carotenoids and APPI (Rivera et ah, 2011). APPI was compared to ESI and APCI as ionization techniques, and the authors observed that APPI positive produced approximately a 2- to 4-fold greater total ion signal for lycopene and (3-carotene as compared to APCI positive and ESI positive. In contrast, APCI positive outperformed APPI positive for a number of xantho-phylls and phytoene and phytofluene. 

Analogous to the ESI interface, APCI (Figure 13-2), also referred to as the heated nebulizer (HN), induces httle or no fragmentation to the analyte. 

This may be either a continuous process, used when the sample size is relatively large (1 ml or more), or a discrete process, used with samples of less than 20 /il. Continuous-flow systems are simpler to use and more precise, but they are less sensitive. They employ a nebulizer in association with a flame or gas plasma, and either a rotating electrode (Rotrode) or drip-feed to the electrode with the arc or spark. The pneumatic nebulizer has an efficiency of 5-10% and generates an inhomogeneous aerosol. Efiiciency can be improved by proper design of the nebulizer and spray chamber (N4), by use of heated nebulizer gas (R6) or ultrasonic devices (S23). The maximum improvement is a 5- to 10-fold increase in sensitivity. There is also an increase in the complexity and cost of the instrument which usually offsets these benefits. The effect... 

While in the TSP interface the heated solvent is nebulized into a medium-pressure ion source region, other systems have been described in which nebulization into an atmospheric-pressure system is performed. In an atmospheric-pressure spray system, as described by Sakairi and Kambara [94-95], a TSP nebulizer is used for the efficient introduction and evaporation of a mobile phase into an APCI source. In other systems, a heated nebulizer is used to achieve sample introduction in an APCI source. 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

Figure 5.13 General schematic diagram of an APCI heated nebulizer probe.

In APCI mass spectra of carbamates, fragment ions are observed, which are most likely due to thermal decomposition in the heated nebulizer interface and snbseqnent ionization of the thermal decomposition products [11, 14, 20-23]. For example, base peaks were observed at m/z 163 for oxamyl, due to the loss of methyl isocyanate, at m/z 168 for propoxur, dne to the loss of propylene, and at m/z 157 for aldicarb, due to the loss of HjS. The APCI mass spectra of aldicaib and two of its metabolites, aldicarb sulfoxide and aldicarb snlfone, showed significant fragmentation. Major fragments for aldicarb were dne to the loss of carbamic acid (to m/z 116) and due to charge retention at [CH3-S-C(CH3)2]. For aldicarb sulfoxide and aldicarb sulfone, the loss of carbamic acid resnlted in the base peaks of the spectra (at m/z 132 and 148, respectively). 

---