Sample Introduction Devices

Programmed-temperature vaporizers are flexible sample-introduction devices offering a variety of modes of operation such as spHt/sphtless, cool-sample introduction, and solvent elimination. Usually the sample is introduced onto a cool injection port liner so that no sample discrimination occurs as in hot injections. After injection, the temperature is increased to vaporize the sample. 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Ionisation in an API source can take place in a variety of ways depending on the type of applications, namely by gas-phase ionisation, liquid- and plasma-based ionisation. At present, there are three major application areas of API-MS air or gas analysis (industrial emissions), on-line LC-MS (largest commercial application), and ICP-MS. A wide variety of sample introduction devices are available for gas analysis by API-MS. For use in ICP-MS, ions are sampled directly from the inductively... 

An ion mobility spectrometer consists of a sample-introduction device a drift tube where ionisation and separation of ions takes place and a detector. Ionisation sources of choice include radioactive sources (e.g. a 63Ni foil), photoionisation methods, corona-spray ionisation, flame ionisation and corona discharge. The most common detection method used to measure the... 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

Wang, S.R. and Jiang, S.J. (1991) An ultrasonic nebuliser as the sample introduction device for high-performance liquid-chromatography combined with inductively coupled plasma atomic emission spectrometry./. Chinese Chem. Soc., 38, 327-332. 

Fig. 4.8 Schematic diagram of glass sample introduction device...

There are several drawbacks to ultrasonic nebulizer/desolvation systems. Precision is typically somewhat poorer (1% to 3% relative standard deviation) than for pneumatic nebulizers (0.5% to 1.0% relative standard deviation) and washout times are often longer (60 to 90 sec compared to 20 to 30 sec for a pneumatic nebulizer/spray chamber without desolvation). Furthermore, chemical matrix effects are dependent on the amount of concomitant species that enter the ICP per second. Therefore, use of any sample introduction device that increases the amount of sample entering the plasma per second also naturally leads to more severe matrix effects when the sample contains high concentrations of concomitant species. 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

Gas chromatography (GC) is the most common and successful method of soil-gas analysis. The detection limits are about 1-10 ppb by volume. The basic components of a gas chromatographic system are a carrier gas and a flow control system, a column packed with a gas-separating material, an oven for temperature control of the column, a sample introduction device, a detector and a recording system (Fig. 8-8). 

The direct analysis of homogenized solid material can be used if the sample of interest is hard to dissolve, or if the sample pre-treatment increases the risk of contamination or losses of the analyte. Solid samples (e.g. grape stones, skins) can be processed using lasers or heated cells to vaporize the sample. The vaporized sample can be directly introduced into the ICP-MS and measured. This technique is generally characterized by lower sample consumption and higher sensitivity. Solid sampling can be carried out for ICP-MS using electrothermal vaporization (ETV) or laser ablation (LA) sample introduction devices. 

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