Volatile samples

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Preparing a Volatile Sample Gas chromatography can be used to separate analytes in complex matrices. Not every sample that can potentially be analyzed by GG, however, can be injected directly into the instrument. To move through the column, the sample s constituents must be volatile. Solutes of low volatility may be retained by the column and continue to elute during the analysis of subsequent samples. Nonvolatile solutes condense on the column, degrading the column s performance. 

In some inlet devices, the volatile sample materials are first separated from entrained hydrogen gas or air by condensing them in a coolant bath. Subsequently, when all of the volatile sample components have been condensed and the hydrogen or air has been swept away, the sample is reheated and sent to the plasma flame. 

Experimental Problems Water Solubility, Volatility, Sampling,... 

Unfortunately, the preset split flow ratio is only an approximate indication of the sample split ratio. The latter depends in a complex way on many parameters, including the range of sample volatilities, sample solvent, volume of sample injected. 

Thermal desorption of solid traps by microwave energy is unsuitable for thermally labile compounds. In microwave thermal analysis [431] the (solid) sample is heated directly via interactions of the microwaves with the sample, providing more even heating and reduction of temperature gradients in comparison to heating with electrical furnaces. By passing air over a microwave-heated volatile sample evolved gases may be collected [432]. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

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 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,... 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

Volatile samples require quick action. Cyclohexene, for example, has been known to evaporate from the prisms of unthermostatted refracto-meters more quickly than you can obtain the index. It may take several tries as you readjust the light, turn the sample and scale image adjust, and so on. 

Our focus in this chapter is on the analysis of organic analytes in sample matrices that are organic and/or inorganic in nature. These organic analytes can be subclassified into volatile, semivolatile, or nonvolatile. The matrix can be gas (or volatile samples), solid, or liquid. Both the anticipated concentration of the analyte and the type of sample dictate the instrumentation that can be used, as well as the sample preparation technique required. 

Volatile samples are usually analyzed by gas chromatography. There are many different sample pretreatment methods for gases. Let us review these techniques ... 

The solid or liquid sample is placed into a glass vial, which is heated, with enough empty space in the vial above the sample to allow the analyte to reach equilibrium between the gas and solid (or liquid) phase. The gas phase is sampled and analyzed, usually by GC. This type of sampling is generally used for analyzing trace amounts of volatile samples. 

Infrared analysis is usually used as a qualitative method to identify substances. Liquids are usually analyzed as pure substances in cells with very small optical path lengths of 0.1-1.0 mm. Usable spectra can be obtained by placing a drop of relatively non-volatile sample between two sodium chloride plates, allowing them to be held together by capillary action. 

Gas chromatography (GC or, less commonly, GLC) is the most widely used separation technique for volatile samples. The resolution is sufficient to routinely separate components, such as homologous series, saturated from unsaturated fatty acids, terpenoids, triacylglycerols, etc. The use of a mass spectrometer to identify the separated components (GC-MS) is discussed in Section 8.4. 

These methods require that the sample is either a gas or, at least, a volatile substance which can be easily converted into a gas (this explains the utility of mass spectrometry in the field of organic chemistry). In inorganic chemistry it is often more difficult to obtain a gaseous sample, and so other ionization sources have been developed. If the sample is thermally stable, it may be volatilized by depositing it on a filament and heating the filament (thermal ionization mass spectrometry - see below). In restricted cases (e.g., organometallic chemistry), chemical treatment of the sample may give a more volatile sample. 

Mass spectrometry is traditionally a gas phase technique for the analysis of relatively volatile samples. Effluents from gas chromatographs are already in a suitable form and other readily vaporized samples could be fairly easily accommodated. However the coupling of mass spectrometry to liquid streams, e.g. HPLC and capillary electrophoresis, posed a new problem and several different methods are now in use. These include the spray methods mentioned below and bombarding with atoms (fast atom bombardment, FAB) or ions (secondary-ion mass spectrometry, SIMS). The part of the instrument in which ionization of the neutral molecules occurs is called the ion source. The commonest method of... 

To prevent more volatile samples from sudden evaporation a circulation water cooling is often incorporated in the DIP, and refrigerated probes for more volatile samples have also been developed. [43] Sometimes, glass wool is placed into the sample vial to increase the surface for adsorption, and thus to slow down evaporation of the sample. Sudden evaporation causes distorted spectra and may even result in a temporary breakdown of the high vacuum. 

The enormous temperatures attained on resistively heated sample holders can also be used to intentionally enforce the decomposition of non-volatile samples, thereby yielding characteristic pyrolysis products. Pyrolysis mass spectrometry (Py-MS) can be applied to synthetic polymers, [54] fossil biomaterial, [55] food [56] and soil [57] analysis and even to characterize whole bacteria. [58]... 

Highly volatile samples cannot be introduced into the ion source by means of a direct insertion probe even when cooling is applied. A reference inlet system or reservoir inlet system is better suited for that purpose. [59] The name of this type of... 

Reference inlets serve equally well for the analysis of gases, solvents, and similar volatile samples. They are especially convenient when a continuous signal is desired for instrument tuning or long-lasting MS/MS experiments in ion chemistry. In addition, the components of a mixture are admitted to the ion source without fractionation, i.e., without affecting their partial pressures. This property of reservior inlets has extensively been used in the petroleum industry. 

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