Sample preparation methods Liquid samples

All reagents and solvents that are used to prepare the sample for analysis should be ultrapure to prevent contamination of the sample with impurities. Plastic ware should be avoided since these materials may contain ultratrace elements that can be leached into the analyte solutions. Chemically cleaned glassware is recommended for all sample preparation procedures. Liquid samples can be analyzed directly or after dilution when the concentrations are too high. Remember, all analytical errors are multiplied by dilution factors therefore, using atomic spectroscopy to determine high concentrations of elements may be less accurate than classical gravimetric methods. 

The preponderance of analytical sample preparation methods employs some type of extraction. Traditionally, these methods were liquid-liquid, liquid-solid, and hot... 

SPE has been applied to phthalate esters (plasticisers in PVC), polar pesticides (agricultural usage) and for other continuous pollution monitoring problems and environmental analyses [272]. For these applications SPE has largely displaced LLE as the preferred technique for the preparation of liquid samples, e.g. EPA method 506 is concerned with the determination of phthalates and adipate esters in drinking water. 

Direct injection of plasma or supernatant after protein precipitation on a short column with a high liquid flow rate is a common method for reducing analysis time in the pharmaceutical industry. The direct injection of a sample matrix is also known as the dilute-and-shoot (DAS) approach.62 DAS can be applied to all types of matrices and approaches and is the simplest sample preparation method with matrix dependency. Direct injection can also be approached through the extraction of eluent from PPT, SPE, and LLE onto a normal phase analytical column. The procedure is called hydrophilic interaction liquid chromatography (HILIC)70110111 and it avoids the evaporation and reconstitution steps that may cause loss of samples from heat degradation and absorption. 

For sample cleanup the typical methods like solid-phase extraction (SEE) and liquid—liquid extraction can be used in the same manner as they are used for HPLC. Please refer to the corresponding handbooks for a detailed background of these sample preparation methods. ... 

Semisolid samples. As with liquid samples, methods (B) and (C) are the best choices for this type of sample. The specific choice will depend on fhe rheological properties (viscosity, density, air retention) of the particular preparation. These samples are best measured in the transflectance mode. Liquid and semisolid samples may contain a mixture of solvents of disparate volatility which may evaporate separately during the measurement process. Differences in solvent volatility can alter the sample matrix and lead to errors in the determination which are best avoided by using a set of calibration samples spanning an expanded range of solvent proportions. ... 

As well as typical sample preparation methods such as filtration and liquid-liquid extraction, newer developments are now extensively used. The first of these is solid-phase extraction (SPE). This is a rapid, economical, and sensitive technique that uses several different types of cartridges and disks, with a variety of sorbents. Sample preparation and concentration can be achieved in a single step. Interfering sugars can be eluted with aqueous methanol on reversed-phase columns prior to elution of flavonoids with methanol. 

A systematical approach of sample preparation methods and optimisation of the quality aspects of sample preparation may enhance the efficiency of total analytical methods. This approach may also enhance the quality and knowledge of the methods developed, which actually enhances the quality of individual sample analyses. Unfortunately, in bioanalysis, systematical optimisation of sample preparation procedures is not common practice. Attention to systematical optimisation of assay methods has always been mainly on instrumental analyses problems, such as minimising detection limits and maximising resolution in HPLC. Optimisation of sample extraction has often been performed intuitively by trial and error. Only a few publications deal with systematical optimisation of liquid-liquid extraction of drugs from biological fluids [3,4,5]. 

Figure 4.1-9 Schematic of the sample preparation methods used to study liquid EXAFS (a) thin liquid film sandwich between low atomic weight plates, and (b) the liquid (circles) dispersed in a low atomic number matrix (polyhedrons). The figure has been redrawn from reference 40 with permission.

The compound to be analysed, the analyte, is generally contained in a liquid or solid matrix it is rarely found in a form that allows direct measurement. Interfering species that may lead to unwanted interactions, particularly during trace analysis in the presence of abundant matrix components, have to be eliminated. As a result, analysts have long acknowledged the need for efficient and reproducible sample preparation methods. The pre-treatment process has to take into account the analyte, matrix and measurement technique chosen. This situation has led to a number of specific sample pre-treatment protocols that describe sample treatment from sampling all the way to recording of the results (Fig. 20.1). 

As the levels of tocopherols and tocotrienols in meat samples are usually lower than in oil and fat samples, a larger sample size is needed in the sample preparation. The meat sample is homogenized to weaken the sample matrix. As in Basic Protocol 2, saponification, heating, and liquid/liquid extraction are used to increase the recovery and remove interference compounds. Satisfactory results can be achieved using a reversed-phase HPLC method. 

Treatment of the cream with concentrated ammonia followed by acidification with 1% acetic acid is suitable as a sample preparation method for the high performance liquid chromatographic determination (9). The clear supernatant is used for the analysis. 

The physical state of the chemical to be analyzed determines the sample preparation method. The majority of the chemicals listed in the Schedules of the CWC are liquids in room temperature. There are also gases or solids in each schedule. Some chemicals are borderline cases, which may be solids in room temperature but melt in the infrared beam, or liquids that are too volatile in the infrared beam. Table 2 summarizes some typical chemicals of each type in the Schedules. 

The character and hydrocarbon-type composition of several syncrudes have been investigated by adaptation of methods developed for heavier fractions of petroleum crude oils. The methods are reviewed briefly, and results are summarized for five coal liquids and a hydrotreated shale oil Refining requirements for removal of heteroatoms, especially nitrogen, and conversion of polynuclear aromatics are discussed in relation to the composition of the syncrudes and the character of refined products to be expected. A preliminary report is given on the preparation of liquid samples from coals of widely different rank to permit more systematic correlation of hydrocarbon character with coal source in relation to refining. 

A number of different sample preparation methods have been described in the literature [37,38], A collection of these protocols is accessible on the Internet [39,40], The original method that is always the most widely used has been called dried-droplet. This method consists of mixing some saturated matrix solution (5-10 pi) with a smaller volume (1-2 pi) of an analyte solution. Then, a droplet (0.5-2 pi) of the resulting mixture is placed on the MALDI probe, which usually consists of a metal plate with a regular array of sites for sample application. The droplet is dried at room temperature and when the liquid has completely evaporated to form crystals, the sample may be loaded into the mass spectrometer. 

Golge, O., Kabak, B. Evaluation of QuEChERS sample preparation and liquid chromatography-triple-quadrupole mass spectrometry method for the determination of 109 pesticide residues in tomatoes. Food Chem. 176, 319-332 (2015)... 

Solid Phase Microextraction Out of the many sample preparation methods, solid phase microextraction (SPME) is one of the most frequently used. SPME is used for the determination of VOCs in liquid, gas, and solid samples. The great advantage of the method is that it combines, in one stage, the isolation and enrichment of compounds, and completely eliminates the need for organic solvents. 

Liquid and gas samples do not need much preparation, but special cells to contain the samples are often necessary. The simplest method to prepare a liquid sample is to make a capillary thin film of the liquid. The capillary thin film is made by placing a drop of liquid on a KBr plate and sandwiching it with another KBr plate. This method, however, is not suitable for volatile liquids. Liquid cells can be used for volatile liquid and toxic liquid samples, particularly for quantitative analysis. The spacing between the bottom and the top of liquid cell is typically from 1 to 100 /u.m. The cell is made of an infrared-transparent material. Typically, KBr is used however, KBr should not be selected as the material for holding samples containing water because water dissolves KBr. Instead, ZeSe or AgCl should be used because they are infrared-transparent but not water soluble. Cells for gas samples are structurally similar to cells for liquid but the dimension is much larger. 

The fast-gradient method, in contrast, retains analytes on-column until well after the solvent front has eluted. Overall sample throughput is increased with fast-gradient methods due to reduced analytical run time, decreased method development time, and fewer repeat analyses. Onorato et al. [90] used a multiprobe autosampler for parallel sample injection, short, small-bore columns, high flow rates, and elevated HPLC column temperatures to perform LC separations of idoxifene and its metabolite at 10 s/sample. Sample preparation employed liquid liquid extraction in the 96-well format. An average run time of 23 s/sample was achieved for human clinical plasma samples. 

In this review we have summarized the results obtained by different chromatographic techniques and a variety of sample preparation methods for the analysis of antioxidants in polymers and in solutions. Efficient techniques including liquid and gas chromatography, mass spectrometry, traditional low pressure extraction techniques and newer high pressure techniques have been developed. These have made possible the accurate quantification and identification of antioxidants. The newer techniques offer versatile tools for further developments in this area of polymer analysis. 

With the sample preparation method described earlier, the optimal amount of total carbon for AMS analysis is around l.Omg. When the total carbon level is far below that level, a precise amount of suitable carrier carbon is added. Petroleum-based tributyrin is often used as the carrier because it is a nonvolatile liquid, contains depleted levels of 14C and 3H, and its measurement is highly reproducible [18]. 

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