Labile analytes

There should be high sample transfer to the mass spectrometer or, if this takes place in the interface, ionization efficiency. This is of particular imporfance when frace-level componenfs are of inferesf or when polar and/or labile analytes are involved. 

Both El and Cl require the analyte of interest to be in the vapour phase before ionization can take place and this precludes the study of a significant number of polar, involatile and thermally labile analytes. 

The use of spray deposition increases the range of solvents which can be used in moving-belt LC-MS and the range of solutes that can be studied by this technique. Since less heat is required to remove the solvent, it is less likely that the solute will be inadvertently removed from the belt or undergo thermal degradation. It is not, however, unknown for particularly volatile and labile analytes to be lost when using spray deposition. 

Since the sample is ionized directly from solution it is protected from heat and many thermally labile analytes may be studied with httle or uo degradation. 

Decomposition of some thermally labile analytes is observed. 

The advent of the electrospray interface has allowed the full potential of LC-MS to be achieved. It is now probably the most widely used LC-MS interface as it is applicable to a wide range of polar and thermally labile analytes of both low and high molecular weight and is compatible with a wide range of HPLC conditions. 

SFC-FID is widely used for the analysis of (nonvolatile) textile finish components. An application of SFC in fuel product analysis is the determination of lubricating oil additives, which consist of complex mixtures of compounds such as zinc dialkylthiophosphates, organic sulfur compounds (e.g. nonylphenyl sulfides), hindered phenols (e.g. 2,6-di-f-butyl-4-methylphenol), hindered amines (e.g. dioctyldiphenylamines) and surfactants (sulfonic acid salts). Classical TLC, SEC and LC analysis are not satisfactory here because of the complexity of such mixtures of compounds, while their lability precludes GC determination. Both cSFC and pSFC enable analysis of most of these chemical classes [305]. Rather few examples have been reported of thermally unstable compounds analysed by SFC an example of thermally labile polymer additives are fire retardants [360]. pSFC has been used for the separation of a mixture of methylvinylsilicones and peroxides (thermally labile analytes) [361]. 

Wide application range (recommended for reactive and thermally labile analytes)... 

Flow limitations restrict application of the DFI interface for pSFC-MS coupling. pSFC-DFI-MS with electron-capture negative ionisation (ECNI) has been reported [421], The flow-rate of eluent associated with pSFC (either analytical scale - 4.6 mm i.d. - or microbore scale 1-2 mm, i.d.) renders this technique more compatible with other LC-MS interfaces, notably TSP and PB. There are few reports on workable pSFC-TSP-MS couplings that have solved real analytical problems. Two interfaces have been used for pSFC-EI-MS the moving-belt (MB) [422] and particle-beam (PB) interfaces [408]. pSFC-MB-MS suffers from mechanical complexity of the interface decomposition of thermally labile analytes problems with quantitative transfer of nonvolatile analytes and poor sensitivity (low ng range). The PB interface is mechanically simpler but requires complex optimisation and poor mass transfer to the ion source results in a limited sensitivity. Table 7.39 lists the main characteristics of pSFC-PB-MS. Jedrzejewski... 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

High performance liquid chromatography (HPLC) ESI, APCI, APPI Separation of polar, ionic, nonvolatile, high molecular weight and thermally labile analytes... 

Split Simple Starting point for method development Use with isothermal and temperature programmed GC Fast very sharp peaks Choice of glass sleeve not trivial Limits detection concentration to ppm Most sample wasted through split vent Loss of low-volatility, labile analytes... 

MS operation is based on magnetic and electric fields that exert forces on charged ions in a vacuum. Therefore, a compound must be charged or ionized in the source to be introduced in the gas phase into the vacuum system of the MS. This is easily attainable for gaseous or heat-volatile samples. However, many thermally labile analytes may decompose upon heating. Such samples require either desorption or desolvation methods if they are to be analyzed by MS. Although ionization and desorption/desolvation are usually separate processes, the term ionization method is commonly used to refer to both ionization and desorption or desolvation methods. 

Typically splitless injection is used for trace analysis by capillary GC. Splitless injections can exhibit problems with carryover, poor repeatability, and labile analytes. Penton (1991) reports improved results with the temperature-programmable injector. With a temperature-programmable injector, samples are injected into a glass insert at an injector temperature below the boiling point of the analysis solvent the injector temperature is then rapidly programmed to a higher value. Penton reported this technique offered greater ease of optimization and improved precision. 

In the case of GC, it also makes less volatile or thermally labile analytes amenable to GC separation. 

As with polar bears, wolverines and Arctic foxes also appear to have sizable capacity for biotransforming POPs, as evidenced by enantiomer analysis. Wolverines captured from Iceland and the Canadian Arctic had enrichments of (—)-PCBs 136 and 149 in livers, with mean EFs of 0.41 and 0.46, respectively [268]. While these enrichments were similar to those of a-HCH (mean EF of 0.42) and heptachlor epoxide (mean EF of 0.55), they were not as stereoselective as those of /ra 5-chlordane (mean EF of 0.65) and oxychlordane (mean EF of 0.71) [268]. Populations of Arctic foxes in Iceland feeding mostly on marine mammals had much higher POP concentrations in liver tissue than those with a terrestrial diet, but had similar enrichments of (-l-)-chlordane [267]. As with wolverines, Arctic fox livers [268] had modest enrichments of (—)-a-HCH, (+)-PCBs 136 and 149, and El -PCB 95 on Chirasil-Dex, with mean EFs of 0.41, 0.48, 0.54, and 0.55 respectively. These enrichments were again minor compared to that of the OC compounds, which had mean EFs of 0.61 for cw-chlordane, 0.89 for /ra 5-chlordane, 0.68 for oxychlordane, and 0.73 for heptachlor epoxide. Depletions of labile analytes compared to recalcitrant compounds... 

Derivatization is another form of sample preparation. It is utilized for the analysis of labile analytes or to enhance retention or detection with a preferred type of detector. Derivatization can be performed to enhance detection by UV/Vis, fluorescence, or electrochemical detection. Consideration must be given to the stability of the derivatize to solvolysis and thermal degradation. In our labs alendronate, a bisphosphonate with a primary amine functionality, was derivatized with FMOC to enhance detection by UV/Vis as well as to increase retention in RPLC mode [19]. An acylchloride was derivatized with... 

Applicability While applicable to virtually all classes of analytes, IAs work particularly well for those classes of polar and/or labile analytes which are not readily amenable to GC and HPLC analysis. 

The coupling of liquid chromatography (LC) with mass spectrometry (MS) has undergone much evolution since its initial inception [1,2], Atmospheric pressure ionization techniques such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) opened the door for the ionization and analysis of nonvolatile or thermally labile analytes. This technique revolutionized drug discovery and development allowing for dramatic improvements in sensitivity, selectivity, and speed. This area continues to grow, and significant advances have been and continue to be achieved in all three areas [3-5],... 

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