Interface soft ionisation

The principle of MS/MS for direct analysis of a multicomponent system is shown in Figure 6.18, in which the first mass spectrometer (MS I) operates with soft ionisation (FI, FD, Cl, LD), and thus produces an ensemble of molecular ions (M + H+, M — H+, or adducts). For identification of molecule ABC only ABC+ is allowed to enter an interface or fragmentation zone for excitation by collisional activation, laser radiation or surface-induced dissociation. Within the time of one vibration (10-13s), ABC+ dissociates into fragments characterising the original molecule. They are separated and detected by MS II [226]. Soft ionisation with FI/FD produces low ion yields, which may be insufficient for MS/MS LVEI (typically at 20 V) can be an alternative. Complete analysis of a multicomponent system is carried out in some 20 min. 

The FIA-MS screening approach using soft ionisation interfaces prior to any CID procedure provides an overview of the MS separation procedure, which is based on the different m/z ratios of the molecular or cluster ions generated. With the help of this very fast screening method—positive or negative FIA-MS by-passing the analytical column—the surfactant chemist is able to characterise complex blends and formulations without difficulty (Fig. 2.5.1) while the experienced analyst is able to make initial statements about the presence of frequently used and therefore most important surfactants in environmental samples (Fig. 2.5.2) despite the presence of complex matrices. The information provided by ESI or APCI—FIA—MS overview spectra for a first characterisation [8,17-19], which were also available with non-API soft ionising interfaces such as FAB [20] or TSI [9] in industrial blends as well as environmental samples, were obtained from ... 

In the qualitative analyses of surfactants, the FIA-MS screening method applying both soft ionising API interface types, APCI and ESI, provides the overview spectra that contain the molecular ions or adduct... 

MS techniques have met this need in the analysis of involatile, polar surfactants after coupling techniques of liquid chromatographic methods with MS became available. Different types of interfaces for off-line and on-line coupling of liquid chromatography (LC) and MS in the analyses of surfactants had been in use [7,16] while the methods applied at present were performed predominately with soft-ionising atmospheric pressure ionisation (API) interfaces [16-19],... 

Atmospheric pressure chemical ionisation (APCI) is a technique that also creates gas phase ions from the liquid sample. It too takes place at atmospheric pressure and uses a similar interface to that in ESI. As in ESI, the sample solution is mixed with a nebulising gas and the sample arrives in the spray chamber as a fine mist of droplets or spray. In APCI, an extra component - a corona discharge - is used to further ionise the analyte droplets in a manner similar to straightforward Cl (Figure 2.34). While a small amount of fragmentation may occur, the technique is still considered to be a soft ionisation one. The gas-phase ionisation in APCI is more effective than ESI for analysing less polar species. ESI and APCI are complementary methods. 

The two chapters that were selected for this topic one on GC-ion trap mass spectrometry, by SabUer and Fujii and the other by Schroder on LC-MS in environmental analysis give an excellent contribution to the application of GC-MS and LC-MS to environmental analysis. Both chapters include many practical aspects and examples in the environmental field and also cover the historical perspective of the techniques and show the perspective on ionisation and scanning modes. Advances achieved in GC-ion trap by the use of external ion sources and GC/MS/MS possi-bihties are discussed. The LC-MS chapter provides an overview of the first applications of LC/MS interfacing systems, such as moving belt, direct Uquid introduction (DLI) and particle beam (PB), and then on the more recent soft ionisation techniques, like thermospray and atmospheric pressure ionisation interfacing systems. 

One of the most serious drawbacks that has been observed in the ionisation process with TSP, APCI, ESI interfaces, and also with FAB, is the soft ionisation of the analytes which mostly leads to molecular ions or molecular adduct ions. Though molecular mass information is provided, there is little or no structural information at all observable with PBI or electron impact (El) MS. This soft ionisation is clearly disadvantageous for any identification of environmental contaminants, since it generates either considerably less or no fragments at all, and hence is unable to confirm the presence of such compounds of environmental concern. With the commercial availability of tandem devices, tandem mass spectrometry (MS/MS) helped to overcome these identification obstacles via coUision-induced dissociation (CID) in MS/MS mode or via ion trap in MS mode. Today, even bench-top machines provide the possibility of MS . However, when TSP began to become the method of choice in environmental analysis and became commercially available, MS/MS technology was still quite expensive. Users of TSP ionisation with spectrometers not amenable for MS/MS had the possibility to record... 

Games, D. E., 1981, Combined liquid chromatography-mass spectrometry with a moving-belt interface, in Soft Ionisation Biological Mass Spectrometry (H. R. Morris, ed.), pp. 54-68, Heyden and Son, London. 

Requirements for MWD determination by means of MS techniques are (i) absence of discrimination (ii) soft ionisation (DCI, FAB, FD, ESI, LD, MALDI) (Hi) equal ionisation efficiency for high- and low-MW molecules (iv) transmission efficiency (even very heavy molecules should reach the detector) (v) detection efficiency and (vi) calibration. Soft ionisation techniques have all been applied to assorted polymers to obtain average MWDs. Of these, LDI is recommended because it is easily interfaced to both ToF-MS (capable of very high m/z detection) and FTICR (offering high mass resolution and accuracy). Sample preparation with matrix selection (out of 20-30 matrices) and the need for a great excess of matrix (10" 1) are critical for success of MALDI-MS. Failures for correct MWD with MALDI are due to polydispersity (PD) discrimination (especially for PD > 1.1), crystallisation and ionisation processes. Mass discrimination effects observed for polymers have been attributed to sample... 

In the case of the API interface, both the electrospray and APCI modes are soft ionisation techniques which leave the molecular ion (or pseudomolecular ion) intact without much fragmentation. The interface therefore offers more sensitivity and is ideal for quantitative work at low levels (e.g., contamination of drug and food matrices by polymer additives/ breakdown products). 

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

---