Liquid samples sample-handling techniques

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

One of the most important factors in the selection of the sample handling technique is to attempt to analyze the sample, as it exists, without any form of chemical or physical modification. For gases and certain liquids, simple transmission cells, often with a flow-through configuration meet these requirements. 

Infrared spectrophotometry is a familiar established analytical technique which provides identification of compounds by fingerprint spectra, of which a vast library is available. Both liquid and gaseous samples may be easily analysed and therefore modifications of established sample handling techniques have enabled both GC and HPLC instruments to be readily interfaced. Ideally, scan times of less than 1 s are required to be able to record each peak and peak shoulders. Instrument sensitivity is sufficient so that on the fly recording of spectra can be obtained from GC and HPLC eluants which contain nanograms of sample per ml mobile phase, for example, 10 ng sample in 100 pi GC-IR sample cell. Fourier transform infrared (FTIR) instruments are able to meet these criteria but until recently the instrumentation and computer system have been too expensive for routine use. The new generation of... 

Sampling, sample handling, and storage and sample preparation methods are extensively covered, and modern methods such as accelerated solvent extraction, solid-phase microextraction (SPME), QuEChERS, and microwave techniques are included. Instrumentation, the analysis of liquids and solids, and applications of NMR are discussed in detail. A section on hyphenated NMR techniques is included, along with an expanded section on MRI and advanced imaging. The IR instrumentation section is focused on FTIR instrumentation. Absorption, emission, and reflectance spectroscopy are discussed, as is ETIR microscopy. ATR has been expanded. Near-IR instrumentation and applications are presented, and the topic of chemometrics is introduced. Coverage of Raman spectroscopy includes resonance Raman, surface-enhanced Raman, and Raman microscopy. 

THE INFRARED (IR) spectrum results from the interaction of radiation with molecular vibrations and, in gases, with molecular rotations. The spectrum itself is a plot of sample transmission of IR radiation as a function of wavelength or related units. Infrared spectroscopy is the physics that deals with the theory and interpretation of this spectrum and is one of the most popular techniques for identifying molecules. The IR spectrum can be used as a type of fingerprint unique to a molecule. In addition, the presence or absence of many chemical functional groups such as phenyls and carbonyls usually can be established from the spectrum. Quantitative analyses of mixtures can be obtained. Infrared spectra can be run for liquids, solids, or gases without special difficulties. Different types of spectrometers can be used, and a wide variety of sample handling techniques are available, many of which are described in this article. 

The popularity of this extraction method ebbs and flows as the years go by. SFE is typically used to extract nonpolar to moderately polar analytes from solid samples, especially in the environmental, food safety, and polymer sciences. The sample is placed in a special vessel and a supercritical gas such as CO2 is passed through the sample. The extracted analyte is then collected in solvent or on a sorbent. The advantages of this technique include better diffusivity and low viscosity of supercritical fluids, which allow more selective extractions. One recent application of SFE is the extraction of pesticide residues from honey [27]. In this research, liquid-liquid extraction with hexane/acetone was termed the conventional method. Honey was lyophilized and then mixed with acetone and acetonitrile in the SFE cell. Parameters such as temperature, pressure, and extraction time were optimized. The researchers found that SFE resulted in better precision (less than 6% RSD), less solvent consumption, less sample handling, and a faster extraction than the liquid-liquid method [27]. 

The micropipette tip containing solid phases is a relatively new sample preparation technique that permits handling of microliter to submicroliter amounts of liquid samples, using the techniques of SPE, dialysis, and enzyme digestion. Various phases (reversed-phase, affinity, size-exclusion, etc.) are packed, embedded, or coated on the walls of pipette, permitting liquid samples to be transferred without undue pressure drop or plugging (Fig. 2.5). 

Sample preparation refers to a family of solid/liquid handling techniques to extract or to enrich analytes from sample matrices into the final analyte solution. While SP techniques are well documented, few references address the specific requirements for drug product preparations, which tend to use the simple dilute and shoot approach. More elaborate SP is often needed for complex sample matrices (e.g., lotions and creams). Many newer SP technologies such as solid-phase extraction... 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

For these techniques, a dissolved sample is usually employed in the analysis to form a liquid spray which is delivered to an atomiser e.g. a flame or electrically generated plasma). Concerning optical spectrometry, techniques based on photon absorption, photon emission and fluorescence will be described (Section 1.2), while for mass spectrometry (MS) particular attention will be paid to the use of an inductively coupled plasma (TCP) as the atomisation/ionisation source (Section 1.3). The use of on-line coupled systems to the above liquid analysis techniques such as flow injection manifolds and chromatographic systems will be dealt with in Section 1.4 because they have become commonplace in most laboratories, opening up new opportunities for sample handling and pretreatment and also to obtain element-specific molecular information. 

Finally, proper handling technique is very important, especially when it comes to wiping the outside of the needle and the droplet at the tip of the needle prior to injection. Any residual liquid on the outside of the needle will be caught in the septum puncture and will slowly enter the column. This produces broad tailing, especially of the solvent, making separations difficult as well as introducing an unknown amount of sample. On the other hand, liquid in the needle cannot be removed via capillary action of the wiping towel. 

Supercritical fluid extraction (SFE) has been demonstrated as a technique that has eliminated some of the tedious steps of current liquid-liquid and solid-liquid extraction procedures. SFE also offers cleaner extracts, less sample handling and equivalent or better recoveries to conventional technologies. As a technique, it is cost effective, time efficient and low in solvent waste generation. 

The main virtues of chromatographic techniques are versatility, accuracy, speed of analysis and the ability to handle complex mixtures and separate the components accurately. Only very small samples are required, and the technique can detect and measure very small amounts e.g. 10 10 mol or less. Analysis times are of the order of a few seconds for liquid samples, and even shorter for gases. However, a lower limit around 10 3 s makes the technique unsuitable for species of shorter lifetime than this. 

A large part of optimizing analytical setups involves removing the main bottleneck that is slowing down the entire process. Often, several iterations are required in which the bottleneck is shifted back and forth between the reactor and analytical technique before an optimum is found. This optimization can also be performed on the synthetic side of the problem, with the utilization of automated liquid dispensing and sample handling systems. Unfortunately, the end point of this endeavor is often determined by purely monetary considerations. 

Bioproducts are usually secreted from animal cells in culture, and can be purified after cell removal by solid-liquid separation techniques (see Chapter 11). However, the product can sometimes be found within the cell and this requires its extraction from the cellular mass, which contains numerous molecular species that can have high viscosity and proteolytic activity, which increases the difficulty of sample handling. 

The appropriately prepared sample can then be extracted by a number of techniques. The main points to consider are to allow adequate time of exposure of the solvent system in the sample matrix and to limit sample handling steps, that is, avoid filtration steps by using soxhlet (sample in a glass thimble), extraction columns (sample matrix eluted after soaking in soxhlet), or semiauto mated systems (pressurized liquid extractors). 

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