Chemical ionisation source

PTR-MS combines a soft, sensitive and efficient mode of chemical ionisation, adapted to the analysis of trace VOCs. Briefly, headspace gas is continuously introduced into the chemical ionisation cell, which contains besides buffer-gas a controlled ion density of H3O. VOCs that have proton affinities larger than water (proton affinity of H2O is 166.5 kcal/mol) are ionised by proton transfer from H3O+, and the protonated VOCs are mass-analysed. The chemical ionisation source was specifically designed to achieve versatility, high sensitivity and little fragmentation, and to allow for absolute quantification of VOCs. To... 

Figure 2.34 Schematic diagram of an atmospheric pressure chemical ionisation source (Figure used by kind permission of Dr Paul Gates, School of Chemistry, University of Bristol, UK),...

On-line mass spectrometry has been implemented in pharmaceutical processes for monitoring raw materials andproducts ". In this particular application, dilution of the samples is carried out by a membrane interface coupled directly to the atmospheric pressure chemical ionisation source of a quadrupole mass spectrometer for real-time analysis. Continuous online MS has also been used for monitoring fermentation processes in the brewing industry. ... 

Figure 7.4 Quadrupole mass analyser with a chemical ionisation source.

Hewlett Packard, for example, supply the HP 5988A and HP 5987A mass-selective detectors for use with LC (see Appendix 1). This particle beam LC-MS uses the same switchable electron impact chemical ionisation source and the same software and data systems that are used for a GC-MS system. Adding a GC creates a versatile particle-beam LC-GC-MS system that can be switched from LC-MS to GC-MS in an instant. 

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]. 

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. 

Principles and Characteristics In the early mass-spectrometric ionisation techniques, such as El and Cl, the sample needs to be present in the ionisation source in its gaseous phase. Volatilisation by applying heat renders more difficult the analysis of thermally labile and involatile compounds, including highly polar samples and those of very high molecular mass. Although chemical derivatisation may be used to improve volatility and thermal stability, many compounds have eluded mass-spectrometric analysis until the emergence of fast atom bombardment (FAB) [72]. 

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. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

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. 

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 ... 

With APCI, the LC column eluent is nebulised in a heated vaporiser. Once vaporised, the plasma of eluent and sample components enters the APCI source where it encounters electrons emitted from the tip of a corona discharge needle. The eluent molecules are ionised and act as the reagent gas in a chemical ionisation process, transferring charge to the analyte. 

APCI. The column effluent is nebulised into an atmospheric-pressure ion source. Through a corona discharge, electrons initiate the reactant gas-mediated ionisation of the analytes. Proton transfers are typical reactions generating [M + H]+ or [M — H] ions, although radical ion formation is possible as in high vacuum chemical ionisation (Cl). The ions formed are injected into the high vacuum of the mass spectrometer. APCI typically accepts flow rates of up to 2 mL min-1. 

Undoubtedly, mass spectrometric detection has a substantial role to play in condensed-phase chromatographic analyses of toxic impurities. As in GC/MS, it can be highly sensitive, although this is probably more analyte-specific than in GC/MS. Selectivity can be gained by SIM on single quadrupoles or, if necessary, SRM on MS/MS instruments. What must be considered is the appropriate ionisation mode to be used in LC/MS. Most modern instruments use atmospheric pressure ionisation sources, including electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI) and more recently atmospheric pressure photoionisation (APPI). 

Chemical ionisation results from the gas-phase collision between the analyte and species formed from the reagent gas introduced concomitantly in the ion source and bombarded by electrons. Methane, ammonium or isobutane are often used as reagent gases (Fig. 16.17). The reagent gas is introduced into the ion source at a pressure of a few hundred pascals, which reduces the mean free path and favours collision. Chemical ionisation produces positively and negatively charged species. 

The chemical ionisation (Cl) mass spectrum Fig. 3, was recorded on a Finnigan 4000 Mass Spectrometer with ion source pressure 0.3 Torr, ion source temperature 150°C, emission current 300 yA, electron energy 100 eV using methane as a reagent gas. The electron impact (El) mass spectrum Fig. 4, was recorded on Varian MAT 311 Spectrometer, with an ion source pressure 10 6 Torr, ion source temperature 180, emission current 300 yA and electron energy of 70 eV. 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

Modem mass spectrometers within the pharmaceutical industry are more usually fitted with atmospheric pressure ionisation sources that are ideally suited to be connected to HPLC equipment. They are very robust which enables them to be used unattended for many weeks without the need for source cleaning or routine maintenance. There are two types of atmospheric ionisation sources, namely Electrospray Ionisation (ESP) or Atmospheric Pressure Chemical Ionisation (APCl) [15]. Both ionisation modes provide soft ionisation which favours quasi-molecular ion production with little or no fragmentation. Most typically MH ions are observed but MNa, MNHj and MK may also be produced. 

---