Cross-contamination sampling techniques

Solutions. Correction for cXjj is most often made using a standard-sample bracketing technique (e g., Galy et al. 2001). In this protocol, standard and sample isotope ratios obtained by multiple measurement cycles are compared and the sample result expressed as a deviation from the standard. Cross contamination between the sample and the standard is avoided by washing the analytical instrumentation with dilute (usually about O.IN) HNO3 for several minutes between analyses. Introduction of Mg in dilute HNO3 (e.g., 0. IN) into the MC-ICPMS... 

Human error is always a significant factor in elemental analysis. Inadequately trained, inexperienced or simply inattentive analysts may not be able to recognize a developing problem with a potential for blank and sample contamination and to handle it correctly. Unskilled analysts may cause laboratory contamination or cross-contamination between samples due to poor laboratory technique. There is no substitute to proper training and experience for the recognition and prevention of laboratory contamination in the trace level elemental analysis. 

Each preparation is separated into four phases (I) The tissue treatment phase includes how to handle tissue so there is no cross-contamination of samples and how to grind, powder, or macerate the tissue to enhance isolation. (II) The lysis phase includes how to differentially lyse the cellular membrane and the mitochondrial membrane and how to isolate mitochondria away from the rest of the cell. (Ill) The purification phase includes techniques for extracting purified and enzyme-sensitive DNA from the lysis phase. (IV) The assay phase shows how to assay the final product of the preparation by agarose gel electrophoresis and offers some suggestions as to how to assess the purity of the isolated DNA. 

If plates are chosen as storage containers, it is necessary to isolate each sample from cross contamination and interaction with ambient air. This can be done by attaching lids or sealing the plates. A lid can be as simple as a plastic cover or a proprietary device that allows automated placement and removal. Sealing techniques range from cap mats, to adhesive seals, to heat seals. Traditionally, a sealed plate requires a manual step for seal removal. Recently a new technology provides automated seal removal. 

Rock samples, especially those containing sulphides, should be crushed in a percussion mortar and then ground to about 20 mm in an agate mortar. High-energy crushing and grinding techniques are to be avoided and particular care should be taken to minimise contamination of sulphides by atmospheric Hg and cross-contamination of unmineralised samples by sulphides. 

Solid samples, varying from rocks, through tablets to biological tissues, can present a major problem for automation. The physical form of the sample can limit both the technique and the instrument that can be applied. This is an area that needs radical thought to solve the problems posed by the sample. For example, the automated microwave digestion of minerals still requires the crushing of the rock to be a manual offline process. To automate this and eliminate cross-contamination may not be easy or cost-effective. 

The Camag Automatic TLC Sampler 4 (ATS 4) is an advanced, fuUy automated, PC-controlled device for sequential application of up to 66 samples from a rack of 2 ml septum-covered vials or 96 samples from well plates (15 05 45 mm height) as spots by contact transfer (0.1-5 xl) or as bands by the spray-on technique (0.5->50 xl) a motor driven dosing syringe sucks up the sample volume and feeds a steel capillary connected to a capillary atomizer. The speed, volume, and X- and Y-position pattern of application are controllable, and a programmable rinse cycle between the applications can eliminate cross-contamination. 

Sloppy analytical technique can also lead to widespread laboratory contamination, so it is imperative that any analyst working at trace levels has a demonstrated expertise when it comes to general sample handling and analytical techniques, including the operation and maintenance of apparatus used in the laboratory. This is especially true for methods that use automated liquid handling devices that could be prone to dripping or other phenomena that cause cross-contamination. Extraction schemes and formats in which extracts are arranged in close proximity to one another, e.g. 96-well or other similar formats, can be especially problematic in this respect (an example is described in Section 9.7.1b). 

On-line multicolumn preffactionation is an efficient but complex method of isolating fractions for HPLC analysis [I]. Switching devices can implement the transfer of fractions to other extraction columns or to an analytical column. Each solvent used for extraction must be compatible with the subsequent step. Such online procedures minimize sample loss from manual manipulations. On-line multi-column techniques are most effective for high-volume repetitive procedures where the time involved in method development is inconsequential due to throughput. If cross-contamination must be avoided, columns or cartridges must be discarded after each use In other cases, regeneration is possible. 

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