Atmospheric pressure chemical ionization

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo- 

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 

Atmospheric pressure chemical ionization (Bruins, 1991) was developed starting from the assumption that the yield of a gas-phase reaction depends not only on the partial pressure of the two reactants, but also on the total pressure of the reaction environment. For this reason, the passage from the operative pressure of 0.1-1 Torr, present inside a classical Cl source, to atmospheric pressure would, in principle, lead to a relevant increase in ion production, which consequently leads to a relevant sensitivity increase. Furthermore, the presence of air at atmospheric pressure can play a positive role in promoting ionization processes. 

The needle generates a discharge current of -2-3 pA, which ionizes air producing primary ions (mainly NT, Of, H20 and NO1 in the positive mode, Or, O-, NOf, NOT, Oj and COf in the negative mode). Primary ions react very rapidly (within 10 6s) transferring their charge to solvent molecules, in a reaction controlled by the 

In atmospheric pressure chemical ionization (APCI) ion-molecule reactions occurring at atmospheric pressure are employed to generate the ions, i.e., it represents a high-pressure version of conventional chemical ionization (Cl, Chap. 7). The Cl plasma is maintained by a corona discharge between a needle and the spray chamber serving as the counter electrode. The ions are transferred into the mass analyzer by use of the same type of vacuum interface as employed in ESI. Therefore, ESI ion sources can easily be switched to APCI instead of an ESI sprayer, a unit comprising a heated pneumatic nebulizer and the spray chamber with the needle electrode are put in front of the orifice, while the atmospheric pressure-to-vacuum interface remains unchanged. [48,138] 

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. 

It is the great advantage of APCI that it - different from ESI - actively generates ions from neutrals. Thus, APCI makes low- to medium-polarity analytes eluting from a liquid chromatograph accessible for mass spectrometry. In contrast to its development as an ionization method, the application of APCI has a backlog behind ESI. The use of APCI rapidly grew in the mid-1990, perhaps because 

When it is believed that it could be better to add an electrolyte to improve sample detection, one should think about the electrochemical process when selecting it. We have seen, for instance, that adding an acid can improve the detection of negative ions. But the ESI process is not simple, and many trials are often needed. 

Generally, the evaporated mobile phase acts as the ionizing gas and reactant ions are produced from the effect of a corona discharge on the nebulized solvent. Typically, the corona discharge forms by electron ionization primary ions such as N2 + or 02 +. Then, these ions collide with vaporized solvent molecules to form secondary reactant gas ions. 

The electrons needed for the primary ionization are not produced by a heated filament, as the pressure in that part of the interface is atmospheric pressure and the filament would burn, but rather using corona discharges or (3 particle emitters. These two electron sources are fairly insensitive to the presence of corrosive or oxidizing gases. 

As the ionization of the substrate occurs at atmospheric pressure and thus with a high collision frequency, it is very efficient. Furthermore the high frequency of collisions serves to thermalize the reactant species. In the same way, the rapid desolvation and vaporization of the droplets reduce considerably the thermal decomposition of the analyte. The result is production predominantly of ions of the molecular species with few fragmentations. 

The technique of atmospheric-pressure chemical ionization (APCl) also serves to analyze LC effluents by mass spectrometry. It is applicable to relatively less polar and thermally stable compounds with an upper mass range of 1500 Da. The principle of ionization in APCI is identical to that described for conventional Cl, with the difference that APCI is performed at attnospheric pressure, at which many more ion-molecule collisions can occur between the sample molecules and reagent ions. Therefore, the ionization efficiency and detection sensitive are improved significantly. 

Ion-molecule reactions occurring at atmospheric pressure are employed for analyte ion production in atmospheric pressure chemical ionization (APCI). Basically, APCI represents an atmospheric pressure variant of vacuum chemical ionization (Cl, Chap. 7). In APCI the reagent ion plasma is maintained by a corona discharge between the sharp tip of a needle and the spray chamber serving as the counter electrode. 

The corona discharge ionizes the solvent molecules in the tiny droplets as they pass by a tip held at very high voltage. In turn, the charge is transferred to any analyte molecules present in the solvent stream. 

A positive APCI mass spectrum usually contains the quasi-molecular ion, [M + H]+. However, APCI is a less-soft ionization technique compared to ESI and generates more fragment ions. 

This technique is used as an LCMS (liquid chromatographic-mass spectrometric) interface because it can accommodate very high (1 mL/min) liquid flow rates. 

APCI is best suited to relatively polar, semivolatile samples. 

The sample dissolved in a solvent is introduced directly into the ion source cavity. Solvent vapor (S) is reacted with primary ions to generate secondary ions which act as proton donors 

Both positive and negative analyte ions can be obtained as a result of ion-molecule reactions. Protonated and deprotonated pseudomolecular ions usually dominate the positive-ion and negative-ion mode mass spectra, respectively. Overall, ion-molecule reactions involved in the APCI include proton transfer, charge transfer, and hydride abstraction. 

APCI is a relatively soft ionization technique. In fact, only few fragment ions are normally recorded. Nevertheless, analyte decomposition may occur due to heating. It can be used in the analysis of polar and low polar analytes with molecular weights up to 1500 Da [29] as long as the proton affinity of the analytes is higher than that of the solvents. On the other hand, ionization techniques such as ESI and matrix-assisted laser desorption/ionization (MALDI) are mainly used in the analyses of polar, less volatile, and thermally labile analytes, for example, large biomolecules. These two ionization techniques are discussed in the following (Sections 2.4 and 2.6). 

The mobile phase enters the interface through a capillary, which is heated up to typically 400-500 °C. Under these conditions, all solvents and solutes are vaporized and at the ouflet they are usually mixed with nitrogen gas. Through a high potential applied on a needle (corona discharge needle), a plasma of ions is created around 

With APCI, the sensitivity is related to the mass flow, not to the concentration. With some instruments, APCI can be used for flow rates up to 2 ml min Thus, unlike with ESI, there is little to gain from using narrow-bore columns. 

---

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

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). 

The LC/TOF instmment was designed specifically for use with the effluent flowing from LC columns, but it can be used also with static solutions. The initial problem with either of these inlets revolves around how to remove the solvent without affecting the substrate (solute) dissolved in it. Without this step, upon ionization, the large excess of ionized solvent molecules would make it difficult if not impossible to observe ions due only to the substrate. Combined inlet/ionization systems are ideal for this purpose. For example, dynamic fast-atom bombardment (FAB), plas-maspray, thermospray, atmospheric-pressure chemical ionization (APCI), and electrospray (ES)... 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

Electrospray Ionization (ES) and Atmospheric Pressure Chemical Ionization (APCI)... 

Thus, either the emitted light or the ions formed can be used to examine samples. For example, the mass spectrometric ionization technique of atmospheric-pressure chemical ionization (APCI) utilizes a corona discharge to enhance the number of ions formed. Carbon arc discharges have been used to generate ions of otherwise analytically intractable inorganic substances, with the ions being examined by mass spectrometry. 

Samples containing mixtures of peptides can be analyzed directly by electrospray. Alternatively, the peptides can be separated and analyzed by LC/MS coupling techniques such as electrospray or atmospheric pressure chemical ionization (APCI). 

The ion guides are frequently used to transmit ions from an atmospheric-pressure inlet/source system (electrospray ionization, atmospheric-pressure chemical ionization) into the vacuum region of an m/z analyzer. 

AIR. (atmospheric) air, a standard for nitrogen and chlorine isotopes APCL atmospheric-pressure chemical ionization, also called plasmaspray API. atmospheric-pressure ionization... 

A liquid chromatography-mass spectrometry (LC-MS) method that can quantitatively analyze urinar y normal and modified nucleosides in less than 30 min with a good resolution and sufficient sensitivity has been developed. Nineteen kinds of normal and modified nucleosides were determined in urine samples from 10 healthy persons and 18 breast cancer patients. Compounds were separ ated on a reverse phase Kromasil C18 column (2.1 mm I.D.) by isocratic elution mode using 20 mg/1 ammonium acetate - acetonitrile (97 3 % v/v) at 200 p.l/min. A higher sensitivity was obtained in positive atmospheric pressure chemical ionization mode APCI(-i-). 

An on-line chromatography/atmospheric pressure chemical ionization tandem mass spectrometry (LC-APCI/MS/MS) methods was developed for rapid screen of pharmacokinetics of different drugs, including 5 (98RCM1216). The electron impact mass spectrum of 5 and ethyl 9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-7Ff-pyrido[l,2,3- fe]-l,4-benzoxazine-6-carboxylate was reported (97MI28). Electron impact/Fourier transform... 

I. Fener, V. Pichon, M-C. Hennion and D. Barcelo, Automated sample preparation with exti action columns by means of anti-isoproturon immunosorbents foi the determination of phenylurea herbicides in water followed by liquid chi omatography-diode aixay detection and liquid cliromatogi aphy-atmospheric pressure chemical ionization mass spectrometiy , 7. Chromatogr. 777 91-98 (1997). 

C. Aguilar, I. Feirer, R Bonnll, R. M. Marce and D. Barcelo, Monitoring of pesticides in river water based on samples previously stored in polymeric cartridges followed by on-line solid-phase extraction-liquid cliromatography-diode array detection and confirmation by atmospheric pressure chemical ionization mass spectrometry . Anal. Chim. Acta 386 237-248 (1999). 

S. Lacorte and D. Barcelo, Determination of parts per trillion levels of organophospho-rus pesticides in groundwater by automated on-line liquid- solid extraction followed by liquid chr omatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation . Anal. Chem. 68 2464- 2470 (1996). 

D. Puig, L. Silgoner, M. Grasserbauer and D. Barcelo, Part-per-trillion level determination of priority methyl-, nirto-, and clilor ophenols in river water samples by automated online liquid/solid exrtaction followed by liquid chr omatography/mass spectr ometry using atmospheric pressure chemical ionization and ion spray interfaces . Anal. Chem. 69 2756-2761 (1997). 

I. Eeirer, M. C. Hennion and D. Barcelo, Immunosorbents coupled on-line with liquid chi omatography/atmospheric pressure chemical ionization/mass specti ometiy for the part per trillion level determination of pesticides in sediments and natural waters using low preconcenti ation volumes . Anal. Chem. 69 4508-4514 (1997). 

Atmospheric pressure chemical ionization (APCI) Chemical ionization at atmospheric pressure. 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

Ionization methods that may be utihzed in LC-MS include electron ionization (El), chemical ionization (Cl), fast-atom bombardment (FAB), thermospray (TSP), electrospray (ESI) and atmospheric-pressure chemical ionization (APCI). 

Cl is not the only ionization technique where this aspect of interpretation must be considered carefully fast-atom bombardment, thermospray, electrospray and atmospheric-pressure chemical ionization, described below in Sections 3.2.3, 4.6, 4.7 and 4.8, respectively, all produce adducts in the molecular ion region of their spectra. 

It was also the first of a number of interfaces, with the others being electrospray and atmospheric-pressure chemical ionization, in which ionization is effected directly from solution within the interface itself, i.e. the mass spectrometer was not nsed to prodnce ions from the analyte simply to separate them according to their m/z ratios. 

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

Figure 4.21 Schematic of an atmospheric-pressure-chemical-ionization probe. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Particular emphasis has been placed upon electrospray and atmospheric-pressure chemical ionization (APCI) which, in addition to being the currently most widely used interfaces, are ionization techniques in their own right. 

Figure 5.1 Pesticides included in the systematic investigations on APCI-MS signal response dependence on eluent flow rate the parameter IsTow represents the distribution coefficient of the pesticide between n-octanol and water. Reprinted from J. Chromatogr, A, 937, Asperger, A., Efer, 1., Koal, T. and Engewald, W., On the signal response of various pesticides in electrospray and atmospheric pressure chemical ionization depending on the flow rate of eluent applied in liquid chromatography-mass spectrometry , 65-72, Copyright (2001), with permission from Elsevier Science.

---