Chemical ionisation

The positive ion chemical ionisation process using ammonia as reagent gas 

Negative ions can also be formed using Cl processes. The source is similar, although the polarities are reversed to expel negative rather than positive ions. Two types of reaction can occur. The high pressure of a modulating 

Using a species such as ammonia, positive/negative switching can be used. In such cases alternative scans detect either positive or negative ions. A useM 

Like El, Cl is often coupled to a gas chromatograph. As a lower energy process than El the protonated species are often more stable giving intact molecular ions. This can be very useful when uncertainty exists about the molecular weight, especially as the protonated molecule is often accompanied by the corresponding cluster ion. Thus when using positive ion ammonia Cl, the presence of two ions 17 u apart is probably due to [M + H] and M + NH/. 

Separation is more usually carried out in the liquid, rather than gas phase. Neither El nor Cl can deal very effectively with samples in solution and a number of techniques to overcome this have arisen over the last few years. The first widely used method was thermospray [16], However, this has been largely superseded by atmospheric pressure chemical ionisation (APCl). The other commonly applied technique is electrospray. These techniques are described in detail below. 

CI-MS and EI-MS address the same compound class. Cl is used mainly when the molecule fragments so completely in El mode that no M+ ions are observed or when the problem is only knowledge of the molecular weight of the sample component. In fact, El and Cl are usually both carried out on the same sample, as the two ionisation methods produce complementary information of value for the determination of structure and MW of a compound. The detection limits of Cl tend to be better than El, as the latter technique divides the ion current between molecular and fragment ions. A few ng of sample may be detected. 

Cl and El are both limited to materials that can be transferred to the ion source of a mass spectrometer without significant degradation prior to ionisation. This is accomplished either directly in the high vacuum of the mass spectrometer, or with heating of the material in the high vacuum. Sample introduction into the Cl source thus may take place by a direct insertion probe (including those of the desorption chemical ionisation type) for solid samples a GC interface for reasonably volatile samples in solution a reference inlet for calibration materials or a particle-beam interface for more polar organic molecules. This is not unlike the options for El operation. 

Harrison [57] distinguishes Br0nsted acid and base chemical ionisation, charge exchange (CE) and electron capture chemical ionisation (ECCI). 

Among the separation methods coupled to El and CI-MS on-line GC-MS is outstanding. Various groups [58-60] have examined the feasibility of characterising volatiles evolved in TG by Cl mass spectrometry. 

Numerous articles and reviews [57,61,62] and a book [38] deal with chemical ionisation with permanent gases. 

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A. G. Harrison, Chemical Ionisation Mass Spectrometry, CRC Press, Boca Raton, Fla., 1983. 

P Dugo, L. Mondello, E. Sebastian , R. Ottana, G. Eirante and G. Dugo, Identification of minor oxygen heterocyclic compounds of citi us essential oils by liquid chromatography-atmospheric pressure chemical ionisation mass specti ometiy , J. Liq. Chromatogr. 22 2991-3005 (1999). 

B. Herbreteau, A. Salvador, M. Lafosse and M. Dreux, SFC with evaporative lightscattering detection and atmospheric-pressure chemical-ionisation mass specti ometiy for methylated glucoses and cyclodextiins analysis, Analusis 27 706-712 (1999). 

The product ions generated by chemical ionisation are stable even-electron species with relatively little excess energy compared to those generateg electron impact. The chemical ionization mass... 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

Flow injection Chemical ionisation (Cl) Quadrupole ion trap (QITMS) Diode array detector... 

Desorption chemical ionisation (DCI) Fourier transform (FTMS) MicroChannel plate... 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

Atmospheric pressure chemical ionisation (APCI) MH+ (M - II) M, Q, ToF, FTMS Polar and some nonpolar organics <1 500... 

Principles and Characteristics Electron impact (El) ionisation is the original ionisation method (1918). Before 1980, mass spectrometry was merely restricted to electron impact (El), with chemical ionisation (Cl) being applied mainly for those samples which resist generation of satisfactory El data. Nowadays, El is still a widely used universal and nonselective ionisation method. In El, the sample is introduced as a vapour... 

Table 6.13 lists the main characteristics of chemical ionisation. The use of Cl overcomes some of the limitations of EI-MS. CI-MS has the advantage of ease of interpretation and of being able to operate at higher input pressures. Cl restricts the fragmentation... 

Table 6.13 Main characteristics of chemical ionisation Advantages...

Applications Desorption chemical ionisation has proven potential in the analysis of thermally labile, nonvolatile and polar compounds [40,67,68], for the identification of unknown polymers and the study of the thermal degradation mechanisms of polymers. Considering the overall ease of DCI operation, the capability of analysing nonvolatile compounds, and the selectivity provided by choosing different reagent gases, DCI has found surprisingly few practitioners in the analysis of polymer additives. 

FD-MS is a very effective technique for determining molecular weights of thermally labile and nonvolatile compounds, such as polymer additives which do not give good molecular ion spectra during electron impact or chemical ionisation [108], In order to enhance the structural information of the technique, MS/MS approaches must be used [96], Hyphenated chromatography-FD/FT-MS techniques appear to be restricted to on-line GC-MS. 

Gas-phase ionisation-atmospheric pressure chemical ionisation... 

Produces chemical ionisation mass spectra (M + H)+ at atmospheric pressure... 

Smith and Udseth [154] first described SFE-MS in 1983. Direct fluid injection (DFT) mass spectrometry (DFT-MS, DFI-MS/MS) utilises supercritical fluids for solvation and transfer of materials to a mass-spectrometer chemical ionisation (Cl) source. Extraction with scC02 is compatible with a variety of Cl reagents, which allow a sensitive and selective means for ionising the solute classes of interest. If the interfering effects of the sample matrix cannot be overcome by selective ionisation, techniques based on tandem mass spectrometry can be used [7]. In these cases, a cheaper and more attractive alternative is often to perform some form of chromatography between extraction and detection. In SFE-MS, on-line fractionation using pressure can be used to control SCF solubility to a limited extent. The main features of on-line SFE-MS are summarised in Table 7.20. It appears that the direct introduction into a mass spectrometer of analytes dissolved in supercritical fluids without on-line chromatography has not actively been pursued. 

Chemical ionisation is the most frequently used ionisation technique in SFC-MS. It provides better sensitivity compared with charge exchange. All standard Cl reagent gases can be used in SFC-MS. In the absence of a modifier, CO2 can act as reagent gas giving real molecular ion spectra (eq. 7.2) ... 

PCI, positive chemical ionisation El, electron impact CID, collisionally induced dissociation. 

Different options are available for LC-MS instruments. The vacuum system of a mass spectrometer typically will accept liquid flows in the range of 10-20 p,L min-1. For higher flow-rates it is necessary to modify the vacuum system (TSP interface), to remove the solvent before entry into the ion source (MB interface) or to split the effluent of the column (DLI interface). In the latter case only a small fraction (10-20 iLrnin ) of the total effluent is introduced into the ion source, where the mobile phase provides for chemical ionisation of the sample. The currently available commercial LC-MS systems (Table 7.48) differ widely in characteristics mass spectrometer (QMS, QQQ, QITMS, ToF-MS, B, B-QITMS, QToF-MS), mass range m/z 25000), resolution (up to 5000), mass accuracy (at best <5ppm), scan speed (up to 13000Das-1), interface (usually ESP/ISP and APCI, nanospray, PB, CF-FAB). There is no single LC-MS interface and ionisation mode that is readily suitable for all compounds... 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

The use of ionisation techniques such as El and Cl for TLC stationary phases has generally been limited to relatively nonpolar and thermally stable molecules. Polar involatile compounds, separated on silica gel, generally strongly adsorb on to the matrix, and decompose when heat is applied for volatilisation [817]. Use of less-adsorbent phases, such as polyamide, is particularly useful for TLC-EIMS work, because the analytes are not as strongly adsorbed to this phase and do not require high probe temperatures [818,819]. For compounds that are not suitable candidates for TLC-EIMS, FAB can be employed. Chemical ionisation, although suitable for TLC-MS, appears to have been little used. 

Cold plasma with reduced temperature is another way to cope with the most annoying problems from interferences, even in the case of low-resolution instruments [394], The effect consists of weaker ionisation conditions coming close to chemical ionisation [395]. In particular, argides are reduced by orders of magnitude in comparison to conventional ICP operation. However, at lower plasma temperatures, evaporation of analyte material is considerably reduced. Reducing the plasma temperature also has a dramatic effect on the ionisation (and therefore sensitivity) of many elements. Table 8.65 shows the ion population as a function of plasma temperature and ionisation potential. As a result, the cold plasma technique is only advantageous for a rather small number of elements and applications. 

Desorption chemical ionisation (DCI) mass spectrometry has been used for detecting additives extracted from polymers [51,52] by a solvent as volatile as possible. To use the DCI probe, 1 -2 iL of the sample, in solution, are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. The sample is then subject to fast temperature ramping. DCI does not seem to be the most suitable mass-spectrometric method for analysis of dissolved polymer/additive matrices, because ... 

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