Preparation of Liquid Samples

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. 

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. 

For many years, liquid-liquid extraction (LLE) was the classical technique for the preparation of liquid samples. In spite of several drawbacks, it was widely used in all fields of analysis. It usually involves mixing of an aqueous sample solution with an equal volume of immiscible organic solvent for a period of time, and then... 

Membrane filters are used to remove particulates from samples and solvents prior to HPLC analysis and also for the preparation of liquid samples, where no solvent is used. Typical materials of construction for membrane filters are usually synthetic polymeric materials, although natural substances, such as cellulose, and inorganic materials, such as glass fibers, are also used acryhc... 

The preparation of solid sample by the film technique is the same as the preparation of liquid samples. In the two cases, there are many special requirements for the solvent used. The most important of them are as follows solvent must be chemically inert to the sample analyzed solvent must be chemically and physically inert to the material of the cuvette... 

Figure 6 The microslide - a thin, flat rectangular micro-capillary of glass, useful for preparation of liquid samples vulnerable to...

Preparation of liquid samples (enviromnental or biological) is usually simpler and may involve filtering to remove suspended solids and pH adjustment to prevent precipitation of uranium. If separation is required, the same strategies outlined earlier can be deployed. 

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). 

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. 

It is important to describe the preparation of these samples, which were annealed at high temperature in dry nitrogen gas and then rapidly quenched in liquid nitrogen. Because they were thereby rendered amorphous, there was no scattering due to the PTFE-like crystallinity that would complicate the data interpretation. Membranes were boiled in NiCL solutions and then soaked in H2O, D2O, and H2O/ D2O = 50 50 solutions. Due to the elimination of crystallinity from the melt-quench pretreatment, these samples were considerably hydrated at 41 vol %. 

The earliest applications for quantitative analysis of liquid samples and solid preparations entailed sample dissolution in an appropriate solvent. A number of moisture determinations in APIs and pharmaceutical preparations based on both reflectance and transmission measurements have been reported. Their results are comparable to those of the KF method. The high sensitivity provided by the NIR technique has fostered its use in the determination of moisture in freeze-dried pharmaceuticals. ° The noninvasive nature of NIR has been exploited in determination of moisture in sealed glass vials. " " ... 

Sample preparation is probably the most important step in any analytical procedure. Poor preparation of lipid samples will only yield inferior or questionable results. Some commonly performed sample-preparation procedures for gas-liquid chromatographic (GC) analysis of fatty acids in food samples are introduced in this unit. Since the introduction of gas chromatography in the 1950s, significant progress has been made in fatty acid analysis of lipids however, fatty acid methyl esters (FAMEs) are still the most commonly used fatty acid derivative for routine analysis of food fatty acid composition. 

As indicated above, the preparation of the sample depends on the type of material and the homogeneity of that material. With dry powdered or liquid materials, the whole or parts of the samples collected can simply be thoroughly mixed prior to analysis. 

However, the use of IR spectra is limited for the analysis of food because sample preparation is more complicated in comparison with NIR. Dehydration, homogenization, dissolution or dispersion of samples is necessary. In addition, to make an IR transmission spectra of liquid samples, it is necessary to use a cuvette with very narrow path length of 0.001 - 0.1 mm because the absoiptivities are very high, which causes a sample loading... 

In static headspace extraction, sample preparation for liquid samples is usually quite simple—most often, the sample can just be transferred to the headspace sample vial and sealed immediately following collection of sample to minimize storage and handling losses [13],... 

Table 3 Preparation of mammalian samples by liquid-liquid extraction (LLE) prior to LC-MS analysis... 

Bruker EM 640S Portable GC/MS with AMDIS Data Analysis Software (Blinded Mode) for Qualitative Analysis of Liquid Samples Prepared by On-site Methods... 

Standard Operating Procedure (SOP) SOP-LAB-EQP-005, Bruker EM640S Portable GC/MS with AMDIS Data Analysis Software (Blinded Mode) for Qualitative Analyses of Liquid Samples Prepared by On-site Methods, Version 3, The Technical Secretariat of the Organization for the Prohibition of Chemical Weapons, The Hague, 1999. 

While these methods both share the distinct advantage of looking directly at the active ingredient of the formulation they also share a number of disadvantages. Because of the small quantities released, sample preparation techniques, can frequently be elaborate and therefore very time consuming. Since each step in the preparation of a sample is a potential source of error, this increased complexity can also decrease the accuracy of the method. Considerations of this type led this laboratory to the use of labeled pheromones to decrease sample handling and to increase the quantitative accuracy, however, liquid scintillation counting does not provide qualitative information about the labeled species. 

As a rule, a separation method should be used for both purification and concentration of the sample. The classic method for peptides and proteins is a reverse-phase liquid chromatography preparation of the sample, followed by a concentration step (often lyophiliza-tion) of the fraction of interest. During those steps performed on very small quantities of sample, loss on the sample can occur if care is not taken to avoid it. Lyophilization, for instance, can lead to the loss of the sample absorbed on the walls of the vial. The use of separation methods on-line with the mass spectrometer often are preferred. Micro- or nano-HPLC [32,33] and capillary electrophoresis [34], both coupled mainly to electrospray ionization/mass spectrometry (ESI-MS), are used more and more. 

Digestion of liquid samples may not be necessary. Analyses of a distilled liquor, beer and wines have been reported [25, 36, 169, 208]. Determination of wines by direct aspiration after filtration if required, comparing absorbances to those of standards prepared in a synthetic solution simulating the wine matrix has been described by Ecrement [36] and is outlined in note 5 to Section IV.B.l. Iron in orange juice has been determined following hydrolysis with HN03 [174]. 

The surface energies of several materials have been determined by measuring the change of the lattice constant (Table 2). One problem of the technique lies in the preparation of the sample. Only a limited number of substances can be prepared as small spherical particles with a defined radius on a carbon support. Often the particles are not spherical, which limits the applicability of the above equation. The surface stress can only be determined for the solid/vacuum interface, not in gas or liquids. In addition, the interpretation of diffraction effects from small particles becomes increasingly difficult with diminishing particle size (43,44],... 

In most cases, the use of chromatography in trace analysis needs preliminary preparation of analytical samples. For such purpose, beside classical hquid-liquid extraction, special systems were developed for extraction from or into the gas phase, or for extraction with the participation of solid phases. These systems enabled very low and very selective determination in the final step, even for microsamples. 

Currently, the most commonly used method for determining drug impurities is HPLC-MS. Such analysis requires proper preparation of the sample, adequate adjustment of separation parameters, and use of an ionization method. The use of soft ionization produces the molecular ion of the impurity and enables its molecular weight to be established. Use of tandem MS/MS spectrometry allows the chemical structure of the impurity to be established by marking its fragmentation schemes. Liquid chromatography coupled with MS detection was used in purity studies of zaleplon [63], etoricoxib [64], ethanediol, diacetate [65], dicloxacillin [66], quinapril [67], and others described in several review articles [68-70]. 

Membrane extraction with sorbent interface (MESI) is an interesting example of an extraction device, which is the most useful system for interfacing with GC. In this approach, the donor phase is a gas or a liquid sample, and the acceptor phase is a gas. The volatiles are continuously trapped on sorbent and then desorbed into GC [112]. Another solution is a combination of off-line GC-MESI through a cryogenic trap, which allows preparation of environmental samples in the field and performance of GC analysis after transportation to the laboratory [113,114]. MESI allows the extraction of volatile and relatively nonpolar analytes. 

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