LC-MS

Useful interface which Is applicable to a wide range of molecules. The volatile solvent molecules are stripped from the sample and lost in a process similar to that used in the early jet separators used in GC-MS. The heavier sample molecules enter the MS and can be ionised by the standard methods of El, PICI or NICI. Gives spectra with El fragmentation which can be referred for identification to El spectral libraries built up over many years. No solvent background thus sensitive to the 1(h g level. Solvent flow rate up to 1 ml/min, mass range up to 1000 amu 

The eluent from the column is vapourised and a portion of the vapour (ca 1 %) is transferred to the mass spectrometer and the rest of the vapour is pumped to waste. The spectra produced are like Cl spectra since the presence of solvent vapour with the sample reduces the energy of the ionisation process and adducts can be formed with the solvent. Sensitive to the 10 g level mass range up to 2000 amu 

The mobile phase enters the instrument directly so that the flow rates can only be ca 10 pl/min. The mobile contains 1% of an involatile matrix, e.g. glycerol. The sample flows out onto the centre of a porous disc and the solvent, apart from the involatile matrix, evaporates. The sample in the matrix is struck by fast atoms (Xe or Cs) from a FAB gun and the high energy of the atoms generates ions from the sample. Soft ionisation technique produces limited fragmentation. Sensitive to 10 g level for lipophilic compounds, mass range up to 2000 amu or more 

The most common LC-MS interface. Flow rates up to 1 ml/min but best at 200 pl/min or below. A charged aerosol is generated at atmospheric pressure and the solvent is largely stripped away with a flow of N gas. The charged molecules are drawn into the MS by eiectrostaticaiiy charged plates. 

Can determine both small molecules and molecules up to 200 000 amu. Spectra can be simple, containing molecular ion only, or fragmentation can be induced by varying the cone voltage. More suitable for polar molecules 

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For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

Specialized detectors and inlet systems for GC/MS and LC/MS are described in Chapters 36 and 37, respectively. 

In an LC/MS combination, passage of the separated components (A, B, C, D) successively into the mass spectrometer yields their individual spectra. 

By connecting a liquid chromatograph to a suitable mass spectrometer through an interface and including a data system, the combined method of LC/MS (sometimes written HPLC/MS) can be used routinely to separate complex mixtures into their individual components, identify the components, and estimate their amounts. The technique is widely used. 

Another development arising from FAB has been its transformation from a static to a dynamic technique, with a continuous flow of a solution traveling from a reservoir through a capillary to the probe tip. Samples are injected either directly or through a liquid chromatography (LC) column. The technique is known as dynamic or continuous flow FAB/LSIMS and provides a convenient direct LC/MS coupling for the on-line analysis of mixtures (Figure 40.2). 

A detector is needed to sense when the separated substances are emerging from the end of the column. A mass spectrometer (MS) makes a very good, sensitive detector and can be coupled to either GC or LC to give the combined techniques of GC/MS or LC/MS, respectively. 

The coupled methods, GC/MS and LC/MS, form very powerful combinations for simultaneous separation and identification of components of mixtures. Hence, these techniques have been used in such widely disparate enterprises as looking for evidence of life forms on Mars and for testing racehorses or athletes for the presence of banned drugs. 

Liquid chromatography/mass spectrometry (LC/MS) is an analytical technique combining the advantages of an LC instrument with those of a mass spectrometer. 

LC operates in the liquid phase, while MS is a gas-phase method, so it is not a simple matter to connect the two. An interface is needed to pass separated components of a mixture from the LC to the MS. With an effective interface, LC/MS becomes a very powerful analytical technique. 

The combined LC/MS system provides more information than is obvious from the simple sum of the two separate instruments. 

A good LC/MS instrument routinely provides a means for obtaining the identities and amounts of mixture components rapidly and efficiently. It is not unusual to examine micrograms or less of materia). LC/MS is used in a wide range of applications, including environmental, archaeological, medical, forensic, and space sciences, chemistry, biochemistry, and control boards for athletics and horse racing. 

By combining an LC instrument to an MS through an interface, the powerful combination of LC/MS can be used to analyze, both qualitatively and quantitatively, complex mixtures arising from a wide variety of sources. 

Dynamic/continuous-flow FAB allows a continuous stream of liquid into the FAB source hence it constitutes an LC/MS interface for analyses of peptide mixtures. 

Samples containing mixtures of peptides can be analyzed directly by electrospray. Alternatively, the peptides can be separated and analyzed by LC/MS coupling techniques such as electrospray or atmospheric pressure chemical ionization (APCI). 

Liquid chromatograph/mass spectrometer (LC/MS) interface. An interface between a liquid chromatograph and a mass spectrometer that provides continuous introduction of the effluent from a liquid chromatograph to a mass spectrometer ion source. 

LC/MS. liquid chromatography and mass spectrometry used as a combined technique... 

Fig. 6. Particle beam lc/ms analysis of a complex ha2ardous waste sample (a) TIC showing peak at 23.23 min (b) mass spectmm of 23.23 min peak of...

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

This paper deseribes a rapid and versatile on-line-SPE LC-MS/MS method developed for the determination of various pestieides and tlieir metabolites in water. 28 pestieides, ineluding various triazines, phenylureas, organophosphorous eompounds and other speeies, were seleeted for systematie investigations. 

II) On-line eoupling of SPE to LC-MS/MS witli luixing of the organie SPE effluent witli aqueous eluent for optiiuuiu band foeussing on tlie EC eolumn... 

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