Samples pre-treatment

Seawater samples should normally be analysed immediately upon sampling to prevent loss of analyte such as due to changes in the redox speciation or adsorption on the container (see also Chapter 1). Adsorption on the sample bottles can be mostly prevented in stored samples by sample acidification using nitric or hydrochloric acid. Addition of nitric acid can lead to interference with the voltanunetric determination of chromium in seawater, so in that case hydrochloric acid should be used. The amount of acid should be minimized as it will have to be neutralized prior to the voltanunetric determination of most elements. Reagent use should generally be minimized to avoid sample contamination. 

Dissolved organic matter in seawater interferes with the voltanunetric antdysis as it reduces the sensitivity due to physical effects as a result of competitive adsorption on the 

Voltammetric equipment is available from several sources BAS (USA), Eco Chemie (Netherlands), EG G (USA), Metrohm (Switzerland), Radiometer (Denmark/France), Sycopel Scientific (UK) and other companies such as in the Czech Republic and China. The equipment consists of a hanging mercury drop electrode (HMDE) and a potentiostat. Nowadays the electrodes have a valve to dispense a new mercury drop. These electrodes and potentiostats are computer controlled which is convenient as it automates the timing. 

The first requirement for a sample for liquid chromatography is that it must be a clear and homogeneous solution. Any particles present in the sample must be 

Substance Reagent Pre/post Minimum detectable Refs. 

Amino acids/peptides phenylisothiocyanate pre-column 10 pmol 38 

Carboxylic acids S-chloromethylanthracene 4-bromomethyl-7- pre-column 5 fmol 54 

Whenever possible, samples should be dissolved in the mobile phase to provide longest column life and maximum precision in quantitative analysis. The sample solvent must not have stronger eluting properties than the mobile phase, since this will result in wider peaks, and possibly in peak distortion. 

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The proposed PAS-FTIR procedure allows the determination of 30 samples h does not require any sample pre-treatment and avoids waste generation. So, it can conclude that the PAS-FTIR methodology is a reliable alternative to long and tedious classical method of analysis of Mancozeb and improves the sample frequency achieved by the FTIR procedure based on the measurement of KBr disks using internal standard. 

Sample pre-treatment. Novel procedures of electrochemical sample treatment have been proposed to decrease the signal interference with native cholinesterase inhibitors present in fruits and vegetables. Polyphenolic compounds were removed by electrolysis with soluble A1 anode followed by the oxidation of thionic pesticides with electrogenerated chlorine. The procedure proposed makes it possible to decrease the background current and the matrix effect by 80-90%. Thus, the detection limits of about 5 ppb of Pai athion-Methyl and Chloropyrifos-Methyl were obtained in spiked grape juice without any additional sepai ation or pre-concentration stages. 

A. Earjam, J. J. Vreuls, W. J. G. M. Cuppen, U. A. Th Brinkman and G. J. de Jong, Duect introduction of large-volume urine samples into an on-line immunoaffinity sample pre-treatment-capillary gas chr omatogr aphy system . Awn/. Chem. 63 2481-2487 (1991). 

Bermejo-Barrera P, I Ernanuez-Nocelo S, Moreda-PiSeiro A and Bekmejo-Barrera A (1999) Useftilness of enzymatic hydrolysis procedures based on the use of pronase E as sample pre-treatment for multi-element determination in biological materials. J Anal At Spcctrom 14 1893-1900. 

Garn, M. B., Gisin, M., Gross, H., King, P., Schmidt, W. and Thommen, C., Extensive flow-injection dilution for in-line sample pre-treatment, Anal. Chim. Acta, 207, 225, 1988. 

Although absorptiometric detection via preceding colour reactions is still mainly used, CFA has shown increasing versatility not only in detection choice but also in sample pre-treatment, such as (partial) dialysis, distillation, evaporation to dryness, digestion, long-term incubation and coated-tube separation a review was published by Snyder83. 

As a consequence of the previous considerations Kieber et al. [75] have developed an enzymic method to quantify formic acid in non-saline water samples at sub-micromolar concentrations. The method is based on the oxidation of formate by formate dehydrogenase with corresponding reduction of /3-nicotinamide adenine dinucleotide (j6-NAD+) to reduced -NAD+(/3-NADH) jS-NADH is quantified by reversed-phase high performance liquid chromatography with fluorimetric detection. An important feature of this method is that the enzymic reaction occurs directly in aqueous media, even seawater, and does not require sample pre-treatment other than simple filtration. The reaction proceeds at room temperature at a slightly alkaline pH (7.5-8.5), and is specific for formate with a detection limit of 0.5 im (SIN = 4) for a 200 xl injection. The precision of the method was 4.6% relative standard deviation (n = 6) for a 0.6 xM standard addition of formate to Sargasso seawater. Average re-... 

Anderson, R., Sample Pre-treatment and Separation, Wiley, Chichester, 1987... 

Monoliths Low backpressure, suited for conventional HPLC Higher separation efficiency by column coupling Rugged against delay volume and extra-column band broadening fast column re-equilibration Reduced maintenance on pumps and injector seals Reduced need for sample pre-treatment... 

An internal standard should always be used for every analysis carried out. This is an amino acid that is known to be absent from the sample under investigation. For instance in blood plasma analysis either of the non-physio-logical amino acids, nor-leucine or a-amino-/3-guanidinobutyric acid, may be used. This should be added in a known amount to the sample prior to any sample pre-treatment (for example, removal of protein). 

Bayliss and co-workers [10] combined ultra-high flow rates, parallel LC columns, a multiplex electrospray source, and mass spectrometric detection for the rapid determination of pharmaceuticals in plasma using four narrow bore (50 mm x 1 mm, 30 pm Oasis HLB) or capillary (50 mm x 0.18 mm, 25 pm Oasis HLB) HPLC columns with large particle sizes (to avoid high system back-pressure) in parallel with a multiple probe injector and a MUX MS interface. Small sample aliquots were injected directly into the system without sample pre-treatment procedure, obtaining very low limits of quantification (from 1 to 5 ng/mL). 

Choosing the sample pre-treatment method is difficult. The most important consideration is the final condition of the target compounds. What kind of solution is obtained The type of solvent and the concentration of the target compounds are very important in the selection of the separation conditions. 

Partial least-sqnares (PLS) is the most widely used chemometric technique employed to solve data-analysis problems [17-19], This method has the advantage of using full spectral information, and allow for a rapid determination of mixture components, often with no need or prior separation or sample pre-treatment [20-22],... 

In some cases, the signal is still influenced by the sample matrix despite peak purity, e.g., because of sample pre-treatment operations. If matrix effects are considered to be very unlikely, the following check can be skipped. They would still be discovered testing for accuracy. However, matrix effects lately discovered imply a lot of extra work, since the method must then be changed and validated once again. 

In addition to the normally suggested, sample pre-treatment procedures used prior to l.c. injection to avoid column contamination, Cig bonded silica-gel cartridges are particularly useful for removal of lignin-derived degradation-products that occur in biomass-conversion samples. 

Instrumental methods of analysis generally offer greater sensitivity and selectivity than the TLC approach outlined above. Different techniques are required for inorganic or organic analytes, as well as for compounds having limited volatility or thermal stability. In general, the greater sensitivity offered by instrumental methods is accompanied by a need for some form of sample pre-treatment. 

Figure 1 Sample pre-treatment scheme for instrumental analysis.

Recently, turbulent flow chromatography (TFC) has shown a great potential for online sample pre-treatment in the analysis of PFCs. Up to now, the use of this technique in food and environmental analysis is scarce, but some successful applications have been developed. Among them, the analysis of PFCs has been carried out in cord blood and also in less invasive human samples, hair and urine. In these works, the main advantages presented were the simplified sample preparation, robustness and sensitivity. In addition in the case of cord blood, a low volume of sample was required. 

The majority of commercial developments which relate to the automation of GC and HPLC pay little attention to sample preparation. There are few examples where pretreatment is not required. A fully automated system was developed by Stockwell and Sawyer [23] for the determination of the ethanol content of tinctures and essences to estimate the tax payable on them. An instrument was designed and patented which coupled the sample pre-treatment modules, based on conventional AutoAnalyzer modules, to a GC incorporating data-processing facihties. A unique sample-injection interface is used to transfer samples from the manifold onto the GC column. The pretreated samples are directed to the interface vessel hy a simple hi directional valve. An ahquot (of the order of 1 ml) can then he injected on to the GC column through the capillary tube using a time-over pressure system. 

Food/Topic Sample Pre-Treatment Column Mobile Phase Detector Reference... 

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