Ionisation techniques

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

Principles and Characteristics Ionisation processes are the basis for mass-spectrometric detection. Each of the ionisation techniques occupies its own position in mass spectrometry. The optimum performance of any ionisation method (and therefore the result) will depend critically on the characteristics and reliability of the mass spectrometer. Ionisation may occur in the gas, liquid or condensed phase, and may be either hard or soft , i.e. with or without extensive... 

Table 6.10 reports the main areas of application of the various ionisation methods and the principal ions detected. A breakdown of MS techniques applied to various types of analytes is as follows thermally stable, low-MW Cl, El thermally instable, low-MW APCI (FLA, LC-MS), ESI and high-MW DCI, FD, FAB, LD, ESI (FLA, LC-MS, CZE-MS). Soft ionisation techniques such as FL, FAB and LD are useful for the detection of non-volatile, sometimes oligomeric, polymer additives. Recent developments in ionisation techniques have allowed the analysis of polar, ionic, and high-MW compounds, previously not amenable to mass-spectrometric analysis. Figure 6.4 shows the applicability of various atmospheric pressure ionisation techniques in terms of molar mass and polarity. 

Figure 6.4 Applications of various atmospheric pressure ionisation techniques...

The DCI probe is particularly attractive for samples that are susceptible to thermal decomposition, although it can equally well be used as a general means of introducing samples into the ionisation source, i.e. as an alternative to the direct insertion probe. The types of sample which benefit most from DCI probing are higher-molecular-weight, less-volatile compounds, organometallics, and any thermally sensitive compounds [40,67]. DCI is considered to be a soft ionisation technique. 

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]. 

Generally FAB produces protonated, MH+, or depro-tonated, (M — H) , quasi-molecular ions with a little excess energy which will sometimes produce fragment ions of low intensity. FAB is therefore a mild to soft ionisation technique which produces primarily molecular weight information and some structural information. Positive and negative ionisation mass spectra are produced with equal facility. FAB was originally used with magnetic sector mass spectrometers, but lately mainly with quadrupole mass spectrometers (Table 6.10). 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

Although introduction of FAB was a milestone in the development of mass-spectrometric ionisation techniques solving many biochemical-related problems, and was and still is a popular technique to use, it appears to be declining since the advent of electrospray. 

Applications Early MS work on the analysis of polymer additives has focused on the use of El, Cl, and GC-MS. The major drawback to these methods is that they are limited to thermally stable and relatively volatile compounds and therefore are not suitable for many high-MW polymer additives. This problem has largely been overcome by the development of soft ionisation techniques, such as FAB, FD, LD, etc. and secondary-ion mass spectrometry. These techniques all have shown their potential in the analysis of additives from solvent extract and/or from bulk polymeric material. Although FAB has a reputation of being the most often used soft ionisation method, Johlman el al. [83] have shown that LD is superior to FAB in the analysis of polymer additives, mainly because polymer additives fragment extensively under FAB conditions. 

In the deformulation of PE/additive systems by mass spectrometry, much less fragmentation was observed with DCI-MS/MS using ammonia as a reagent gas, than with FAB-MS [69]. FAB did not detect all the additives in the extracts. The softness and the lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Applications of hyphenated FAB-MS techniques are described elsewhere low-flow LC-MS (Section 7.3.3.2) and CE-MS (Section 7.3.6.1) for polar nonvolatile organics, and TLC-MS (Section 7.3.5.4). 

Desorption/ionisation techniques such as LSIMS are quite practical, as they give abundant molecular ion signals and fragmentation for structural information. In the conditions of Jackson et al. [96], all the molecular ion and/or protonated molecule ion species were observed in the LSIMS spectrum when only 1 pmol of each additive was placed on the probe tip. However, as mentioned above, in LSIMS/MS experiments the choice of the matrix (e.g. NBA, m-nitrobenzylalcohol) is very important. Matrix effects can lead to suppression of the generation of molecular ions for some additives. LSIMS is not ideal for the quantitative detection of polymer additives, as matrix effects are very important [96]. 

LSIMS is a more suitable ionisation technique than FD for analysis of mixtures by means of tandem mass spectrometry, because of the higher ion currents generated from polymer additives using LSIMS. LSIMS/MS experiments may be used in conjunction with FD-MS as a screen to determine class, molecular weight and structure of mixtures of organic polymer additives. 

Principles and Characteristics The pioneering technique of field ionisation (FI) was the first soft ionisation technique, introduced in 1954 [105]. For FI analysis of a reasonably volatile sample, the compound under investigation is volatilised by heat close to the emitter, so that its vapour can condense on to an emitter needle. Hence,... 

Table 6.18 lists the main characteristics of FD-MS. FD is a superior ionisation technique for quantitative analysis, as there are no matrix effects as in LSIMS or MALDI which might suppress the generation of ions from certain additives. However, the technique has some serious drawbacks. The primary difficulty is that FD produces only short-lived, highly variable currents of analyte ions. These analyte ion currents are also very... 

From the characteristics of the methods, it would appear that FD-MS can profitably be applied to poly-mer/additive dissolutions (without precipitation of the polymer or separation of the additive components). The FD approach was considered to be too difficult and fraught with inherent complications to be of routine use in the characterisation of anionic surfactants. The technique does, however, have a niche application in the area of nonpolar compound classes such as hydrocarbons and lubricants, compounds which are difficult to study using other mass-spectrometry ionisation techniques. 

Thermospray (TSP) is another soft ionisation technique which produces predominantly MH+ or (M — H) ions, together with some fragmentation. TSP is best suited to the analysis of organic compounds of low molecular mass (<1000 Da) that exhibit some polarity. Polymer additive molecules fall in this wide category. 

API-MS quickly and reliably analyses any size of molecule in almost any matrix, without contamination or degradation. As it is usually not obvious which technique to use, it is therefore often worthwhile analysing the samples of interest with both ionisation techniques and making the final decision empirically. 

The process of ion formation is extremely soft usually no fragmentation occurs. Mass spectra in ESI+ mode are dominated by pseudo-molecular ions (M+, [M + H] + or [M + Na]+) and cluster ions formed by the addition of one or more solvent molecules. For samples with molecular masses up to ca. 1000 Da (as in case of polymer additives), the ions produced from ESI are similar to those formed by other soft ionisation techniques, namely a protonated molecular ion (MH+) in the positive ionisation mode for basic compounds such... 

Coupled on-line techniques (GC-MS, LC-MS, MS/ MS, etc.) provide for indirect mixture analysis, while many of the newer desorption/ionisation methods are well suited for direct analysis of mixtures. DI techniques, applied either directly or with prior liquid chromatographic separations, provide molecular weight information up to 5000 Da, but little or no additional structural information. Higher molecular weight (or more labile) additives can be detected more readily in the isolated extract, since desorption/ionisation techniques (e.g. FD and FAB) can be used with the extract but not with the compounded polymer. Major increases in sensitivity will be needed to support imaging experiments with DI in which the spatial distribution of ions in the x — y plane are followed with resolutions of a few tens of microns, and the total ion current obtained is a few hundreds of ions. 

MS/MS has especially found analytical applications in combination with soft ionisation techniques, where, without MS/MS, only molecular weight information on... 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

An effective means to facilitate the mass-spectral analysis of rubber acetone extracts is to use desorp-tion/ionisation techniques, such as FD [92,113] and FAB [92]. FAB mass spectra for rubber extracts are generally more complex (due to fragment ions) than FD spectra of the same materials. Nevertheless, the FAB spectra are often complementary to FD, since ... 

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