Sample clean Subject

Another aspect to take into account is that surfactants are often ingredients in the cleaning products used in chemical laboratories. The sampling (as discussed above) and sample handling to which the samples are subjected always carries with it the risk of contamination. It is therefore necessary to process sufficient numbers of blank samples together with the real samples. Numerous samples of oceanic seawater and fossil seawater (taken from wells) have shown traces of LAS (around 1 ppb). Therefore, environmental concentrations found at similar levels should be regarded with caution. 

The wet assay technique to measure dust in cotton was a modification of the method described by Thibodeaux (11). A 400-mg tuft of cotton, randomly selected from a bulk sample, was subjected to multiple ultrasonic washings in methanol. Clean methanol (200 ml) was used for each of three 5-min washings. The combined methanol washings were filtered through a 17 ym sizing screen (the screen was identical for both wet and dry assay procedures) and collected on a 0.5 ym filter. Increase in filter weight provided the measure of dust content (%) in cotton by wet assay. 

Sometimes, matrix effects can be corrected by normalization of the NSB/ B0. This must be tested by showing that the interference is consistent in multiple samples taken at different times from the untreated individual. In other instances, predose samples from each patient (or test animal) are used to construct the calibrator standards for quantitation. In such cases, it may be better to do sample clean-up to eliminate matrix interferences, because it is not always possible to obtain adequate predose sample volumes from each subject. 

Fig. 12. Trace element/Ca ratios of foraminifera subject to different cleaning procedures, (a) Mn/Ca, (b) Mg/Ca and (c) Sr/Ca. Samples cleaned using procedure A were subject to ultrasonication in methanol, oxidation and a weak acid leach. Those cleaned using procedure B were subject to additional reductive cleaning, and those cleaned using procedure C were subject to additional reductive cleaning as well as cleaning in DTPA (diethylene triamine pentaacetic acid). DTPA removes refractory phases rich in Ba and the rare earth elements (e.g. Haley Klinkhammer 2002). Note that the introduction of the reductive cleaning step significantly lowers Mn/Ca and also lowers Mg/Ca. Reductive cleaning has no resolvable effect on Sr/Ca. DTPA has little effect on Mn/Ca, Mg/Ca or Sr/Ca. Error bars (where visible) represent the standard deviation of the mean of two (0. universa) or three (G. conglomerata) separate analyses. Data are from Hathorne (2004).

In the end, poly water samples were subjected to much closer scrutiny and were all shown to contain some contamination with impurities of substances suspended in ordinary water. When the original experiments were repeated with extraordinary care given to cleaning the apparatus, pol5iwater could no longer be produced By 1972, most of the world s scientists considered the case closed, and by 1973, even Deryagin conceded that polywater did not exist. 

Contrastingly, in MALDI-IMS, in which the tissue sample is directly measured, researchers should pay special attention to the fact that samples are extremely complex mixtures of biomolecules. Because tissues and cells are subjected to MALDI-IMS, the sample clean-up procedure is limited. Figure 3.4 shows IMS results of spotted peptide solution (0.5 pL of 100 nM adrenocorticotropic hormone [ACTH]) on both indium tin oxide (ITO) glass slide surface and brain section, and it clearly demonstrates the severe ion suppression effect on the tissue surface the spotted ACTH peptide was detected only from the ITO surface, but not from tissue surface. As this example indicates, an important point to consider when executing an IMS experiment is the optimization of the sample s condition, so that analyte objects can be efficiently ionized from crude mixtures. 

The vapor pressure of materials as well as the desorption rate of adsorbates increase with rising temperature. Therefore, once closed, UHV chambers are subjected to a bake-out procedure, that is, the whole vacuum system is heated to 150-250 °C for many hours while pumping. Likewise, filaments and the samples themselves have to be preheated to temperatures higher than in the following experiments in order to outgas unwanted contaminants (specific sample cleaning, see Section 3.1.4.1). Obviously, bake-out and outgassing temperatures put constraint on the materials that can be used for and in a UHV system. 

A powerful tool now employed is that of diode array detection (DAD). This function allows peaks detected by UV to be scanned, and provides a spectral profile for each suspected microcystin. Microcystins have characteristic absorption profiles in the wavelength range 200-300 nm, and these can be used as an indication of identity without the concomitant use of purified microcystin standards for all variants. A HPLC-DAD analytical method has also been devised for measurement of intracellular and extracellular microcystins in water samples containing cyanobacteria. This method involves filtration of the cyanobacteria from the water sample. The cyanobacterial cells present on the filter are extracted with methanol and analysed by HPLC. The filtered water is subjected to solid-phase clean-up using C g cartridges, before elution with methanol and then HPLC analysis. 

The sensory technique used for assessing human perception of odors is called olfactometry. The basic technique is to present odorants at different concentrations to a panel of subjects and assess their response. The process favored by the U.S. National Academy of Sciences is dynamic olfactometry (16). This technique involves a sample dilution method in which a flow of clean, nonodorous air is mixed with the odorant under dynamic or constant... 

In view of the foregoing remarks, it is clear that all glassware used in the preliminary treatment of samples to be subjected to stripping voltammetry, as well as the apparatus to be used in the actual determination, must be scrupulously cleaned. It is usually recommended that glassware be soaked for some hours in pure nitric acid (6 M), or in a 10 per cent solution of pure 70 per cent perchloric acid, followed by washing with de-ionised water. 

In trace organic analysis there is usually an extraction or clean-up process, rather than a sample dissolution. Here not only must the matrix effect be considered, but also the recovery yield of the extraction. Frequently an external spike standard is added, but there is often no way of knowing if the recovery of the spike standard matches the analyte in question. There is considerable evidence that the U S E P A method for VOA analysis (Minnich 1993) is subject to such error, as reported by Schumacher and Ward (7997). The analyst must always consider the possibility of such an error, especially when using CRMs to control methods that are applied in routine mode. 

Plant materials are homogenized with methanol. Hexythiazox residue is extracted with hexane and then transferred to acetonitirile by liquid-liquid partitioning. The acetonitirile is removed by rotary evaporation and the sample is cleaned up using Florisil PR column chromatography. The concentrated eluate is subjected to high-performance liquid chromatography (HPLC) analysis. 

Before adopting this method at the ordnance plant, sections of pipelines were chosen for test samples, to determine if the swab and pig method would satisfactorily clean these contaminated pipes. One half the sections were cleaned by this method and the other half was thoroughly flushed with water. They were allowed to dry and then were subjected to initiation by fires. The sections that had been flushed with water ignited and burned vigorously. The sections that had been subjected to cleaning with the swab and pig had no product remaining that would support combustion. 

A gas chromatographic procedure using electron capture detection has been described for the determination of Dursban (0,0-diethyl-0 (3, 5, 6-trichloro-2-pyridyl phosphorothioate) in water and silt [95]. In this method, water samples are extracted with dichloromethane, the extract is evaporated, and a solution of the residue is cleaned up on a column of silicic acid, Dursban being eluted with hexane. The eluate is evaporated to dryness under reduced pressure, and a solution of the residue in hexane is subjected... 

Thibaut et al. [14] published a procedure for determination of NP and NP transformation products in snails, duckweed and trout liver and viscera. Samples were homogenised in MeOH, and either directly chromatographed on HPLC (liver, duckweed), or subjected to a further clean-up using L/L partitioning with methanol, chloroform and acetonitrile/isooctane, successively. 

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