Sample Introduction Methods

The efficient transfer of an analyte from its original condition to the ionization region of an ion mobility spectrometer (IMS) is the topic of this chapter. Snccessfnl detection and identification of an analyte by IMS depend on many steps but none more important than those by which a sample is introduced into an instrument. IMS instruments are used for the detection and identification of analytes found in air, water, biological fiuids and tissues, industrial solvents and on surfaces. Because ion mobility spectrometry is such a universal analytical instrument, sample introduction methods are diverse and depend on the type of sample analyzed. Atmospheric pressure operation makes IMS suitable for interfacing with several sample introduction systems as a detector as well as a selective filter for mass spectrometric techniques. 

Because IMS is a method that separates gas phase ions through collisions with a buffer gas, all analytes must be transported from the sample matrix and converted to a gas phase ion before ion mobility separation and detection can be performed. Thus, the type of introduction method largely depends on the physical characteristics of the analyte. The remainder of this chapter is divided into four sections based on the characteristics of the sample vapor, semivolatile, aqueous, and solid. While these categories are somewhat arbitrary with significant sample overlap, it is useful to think of volatile samples as those compounds that exist or partially exist as vapors under ambient temperature and pressure semivolatile samples as those compounds that can be volatilized but have vapor pressures too low to detect by IMS under ambient temperature and pressure aqueous samples as those compounds that are not volatile but can be dissolved in water and solid samples as compounds not in a solution. Table 3.1 lists a number of example analytes according to the categories discussed in this chapter. 

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Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid particles 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. 

An exeuaple of a nodular apparatus for capillary electrophoretic separation methods, is shown in Figure 4.43 [637-639,681-684]. It Offers a choice of automated sample introduction methods with on-column detection and has a... 

There are several sample introduction methods that are used in conjunction with ICP, including nebulization, electrothermal evaporation, gas chromatography, hydride generation, and laser ablation [30]. Laser ablation combined with ICP (LA-ICP) is useful for analysis of solids. In such a source the sample is positioned in a chamber prior to the ICP source, the ablation cell. Argon gas at atmosperic pressure flows through the cell towards the ICP source. The sample is irradiated by a laser beam and... 

Although is has been in use for over 50 years and has become one of the most widely used routine analysis techniques, GC research remains vibrant and challenging. There are two key areas in which dramatic advancements are being made sampling and sample introduction methods and multi-dimensional separations. A summary of sampling techniques in use with GC is shown in Table 14.9, including the basic principle of the technique and some key applications. These techniques have become critical in extending the use of GC into the diverse fields described in the Applications section. 

An excellent review of modern sorptive sampling techniques that could be considered for the enrichment of volatiles from mammalian secretions appeared recently [10]. To be on the safe side, more than one sample preparation and sample enrichment method should be used to analyze mammalian secretions. If GC and GC-MS analyses are employed, the results obtained with split/split-less, on-column, SPME and solventless (solid) sample introduction methods [11,12] should be compared. 

Because of its capability for rapid multielement analysis, ICP-MS is particularly suited to sample introduction methods which give rise to transient signals. For example, electrothermal vaporization, flow injection and chromatographic methods can be interfaced and many elements monitored in a single run (see Chapter 7). 

Other mass spectral techniques that use LC and capillary electrophoresis (CE) as the sample introduction method make it possible to analyze chemicals that should otherwise be derivatized for GC analysis, and also those nonvolatile and nonderiva-tizable chemicals that cannot be analyzed at all with GC. Many of these chemicals could be analyzed with FUR without GC separation, but in the environment, they may be in, for example, water or soil samples (which possibly have to be extracted with water). Water samples are difficult to analyze with FTIR since water is quite a poor solvent for FTIR due to very high molar absorptivity. 

In summary, peak area is the preferred measurement especially if there are any changes in chromatographic conditions, such as partition ratio, temperature, or sample introduction method, that can cause changes in peak height or width (but not area). However, peak height measurements are less affected by overlapping peaks, noise, and sloping baselines. It must also be remembered that the commonly used concentration detectors are flow sensitive and prone to errors if areas are used for quantitative analysis. 

In this chapter, the description of applications highlights the selection of the instrumentation used as well as the alkaline solubilization sample preparation (often the most critical part of a complete analytical method). The main characteristics of the suspension sample introduction method are also emphasized. A brief discussion on subgroups of these methods, identibed by the atomic spectrometric instrumental approach, is bnally presented. 

Z. Mester, J. Earn, R. Sturgeon, J. Pawliszyn, Determination of methylmercury by solid-phase microextraction inductively coupled plasma mass spectrometry a new sample introduction method for volatile metal species, J. Anal. Atom. Spectrom., 15 (2000), 837 D 842. 

Common Sample Introduction Methods using Electrospray Ionisation... 

Recent improvements of well-known sample preparation technic ues, together with new methodologies are presented. Their possibilities for different kinds of samples are reviewed. Problems associated with the use of standard on4ine technic ues ate discussed in relation to vaporizing and on-column sample introduction methods. 

Sample introduction is probably one of the most important stages for reproducible measurements and is related to the efficiency of sample uptake to the plasma source. Normally samples are introduced in solution form and in latter years sample introduction as solids and gases directly or from GC columns is now commonly employed on a routine basis where applicable. Selection for the best sample introduction method needs careful consideration, keeping in mind that the properties of the atomiser will dictate its design and operation. For adequate thermal dissociation, volatilisation, excitation and atomisation of aerosol particles, the efficiency of nebulisation will determine the sensitivity and reproducibility of analyte response. The following requirements must be considered when analysing samples using atomic emission methods ... 

Figure 2.9 Overview of sample introduction methods and hyphenated techniques used in ICP-AES. (A) Pneumatic concentric (sometimes called the Meinhard nebuliser) (B) Babington (C) fritted disc (D) Hildebrand nebuliser (E) cross flow (G) standard ultrasonic nebuliser for aqueous and non-aqueous solvents (H) electro-thermal graphite ( ) electro-thermal carbon cup (K) graphite tip filament (L) laser ablation (M) hydride generation (P) flow injection...

Figure 28-5 Continuous sample introduction methods. Samples are frequently introduced into plasmas or flames by means of a nebulizer, which produces a mist or spray. Samples can be introduced directly to the nebulizer or by means of flow injection (FIA) or high-performance liquid chromatography (HPLC). In some cases, samples are separately converted to a vapor by a vapor generator, such as a hydride generator or an electrothermal vaporizer.

J.F. Tyson, J.M.H. Appleton, A.B. Idris, Flow injection sample introduction methods for atomic absorption spectrometry, Analyst 108 (1983) 153. 

The use of vapor-generation methods prior to IGP-MS or AAS detection improves the LOD of the method. This is because the sample introduction methods facilitated by liquid aspiration are very inefficient (1-5%), compared to methods for gaseous or vapor sample introduction (>90%). 

A number of important sample preparation techniques rely upon gas extraction or the analysis of samples in the gas phase. These samples usually contain low concentrations of volatile analytes and higher concentrations of water vapor in a low molecular weight gas or air. Direct sample introduction by syringe or valves is only suitable for small volumes of relatively concentrated samples (section 3.4.2). More common sample introduction methods involve analyte accumulation by sorbent or cryogenic trapping followed by vaporization in the presence of a flow of gas to transport the... 

For the first live atomization sources listed in Table 8-1, samples arc usually iiilroduccd in the form of aqueous solulions (occa.sionally, nonaqueous soUilion.s are used) or less often as slurries (a slurry is a suspension of a ftnely divided powder in a liquid). For samples lhai are difficull lo dissolve, however, several methods have been used lo introduce samples into the atomizer in ihe form of solids or finely dispersed powders. Gen-crallv, solid sample-introduction techniques arc less reproducible and more subject lo various errors and as a result are not nearly as widely applied as aqueous solution techniques. Table 8-2 lists the common sample-introduction methods for atomic spcciro.scopy and the ispe of samples lo which each method is applicable. 

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