Problems pipets

Micro-pipetting instruments such as the "Eppendorf or "Oxford pipettors with disposable plastic cone tips are customarily employed to dispense the liquid samples into electrothermal atomizers. Sampling problems which are associated with the use of these pipettors are among the troublesome aspects of electrothermal atomic absorption spectrometry (67,75). The plastic cone-tips are frequently contaminated with metals, and they should invariably be cleaned before use by soaking in dilute "ultra pure nitric acid, followed by multiple rinses with demineralized water which has been distilled in a quartz still. 

Aqueous standard solutions are a source of certain difficulties In electrothermal atomic absorption spectrometry of trace metals In biological fluids The viscosities and surface tensions of aqueous standard solutions are substantially less than the viscosities and surface tensions of serum, blood and other proteln-contalnlng fluids These factors Introduce volumetric disparities In pipetting of standard solutions and body fluids, and also cause differences In penetration of these liquids Into porous graphite tubes or rods Preliminary treatment of porous graphite with xylene may help to minimize the differences of liquid penetration (53,67) A more satisfactory solution of this problem Is preparation of standards In aqueous solutions of metal-free dextran (50-60 g/llter), as first proposed by Pekarek et al ( ) for the standardization of serum chromium analyses This practice has been used successfully by the present author for standardization of analyses of serum nickel The standard solutions which are prepared In aqueous dextran resemble serum In regard to viscosity and surface tension Introduction of dextran-contalnlng standard solutions Is an Important contribution to electrothermal atomic absorption analysis of trace metals In body fluids. 

Pipets pose a special problem. Brushes cannot be used because of the shape of some pipets and the narrowness of the openings. If soap is to be used, one must resort to soaking with a warm soapy water solution for a period of time proportional to the severity of the particular cleaning problem. Commercial soaking and washing units are available for this latter technique. Soap tablets are manufactured for such units and are easy to use. 

For one of the methods in problem 43, you needed a pipet. Is the pipet you would choose calibrated for blowout How can you tell by looking at the markings on the pipet What is the name of the pipet you would choose Give specific details as to how the volume is delivered using this pipet. 

Implementation of microanalytical devices presents some issues mostly related to the scale of the volumes. In fact, successive reduction in the sample volume may compromise analysis either because the measurement limit of the analytical method is exceeded or because the sample is no longer representative of the bulk specimen. Another drawback for microchip devices is microvolume evaporation of both sample and reagent from the microchip, compromising quantitative determination or inducing unwanted hydrodynamic flows. This problem has been addressed by designing pipetting systems that automatically replace fluid lost by evaporation or by enclosing the chip in a controlled... 

An aliquot of 250 pi of plasma to be analysed and 250 pi of the standard solution were pipetted in an 11.5 ml glass tube. Then, 250 pi of acetate buffer solution was added and mixed for ten seconds on a vortex mixer. 9 ml of extraction liquid was added and the solution in the tubes was extracted on a Heidolph tumble mixer. A potential problem is present if the solvents used are mixed in different compositions a composition can possibly be selected that has a density equal to the density of the aqueous layer. This may give rise to problems with the phase separation. 

The steep thermal gradient along the tube means that any variation in the sample position (e.g. because of pipetting, or spreading due to surface tension and viscosity effects) will alter the atomization peak shape. Peak area integration will help to minimize this problem, as will a rapid heating ramp and isothermal operation (see Sections 3.6.2 and 3.6.3). 

If the electrode response is sluggish, first check/replace the membrane since most 02 electrode problems are due to dirty or leaky membranes. Remove the cell plug and rinse the cell thoroughly with distilled water using a plastic disposable pipet to avoid the risk of damage to the membrane. 

Proper reagent care can reduce problems stemming from contamination, heat or excessive light exposure. Reagent contamination can be avoided by the use of clean pipet tips. Prompt return of reagents to proper storage conditions will prolong their shelf life. 

It is usual with electrothermal atomisers to pipette between 5 and 100 pi samples into the device using a micropipette. With petroleum samples dissolved in organic solvents this may be a problem. Due to the low surface tension of many of these solvents they do not pipette easily and often dry irregularly in the atomiser, both factors giving rise to poor reproducibility. The problem of poor drying characteristics may be overcome with many solvents by pipetting into a pre-heated atomiser at approximately 80°C. The solvent is removed immediately, leaving the analyte on a reproducible spot each time. This technique, however, requires some care so as not to melt or contaminate the pipette tip. 

Several devices of liquid handling are commercially available. Pipetting a volume of several microliter usually presents few problems. Below about 1 /d, due to obstacles... 

The best way to overcome this problem is to use an electronically controlled multiway pipet such that, for example, 230 pL of solution can be drawn up and then 4 x 55 pL dispensed, leaving 10 pL in the tips. The plates are then mixed on a vortex-type plate mixer. An alternative method using a conventional multiway pipet is to draw up 55 pL of solution A or B, dispense it in the appropriate wells, mix the contents of tips and wells by pipeting up and down a few times, and then use new tips for the next row of wells. 

Monolayer Properties. For solubility reasons it was necessary to spread poly (L-leucine) from chloroform containing 20% trifluoracetic acid. This solvent causes drops of liquid to creep up the outside of the pipet, and the observed area per residue of 16 may be slightly low. Use of a glass pipet treated with a silicone minimized the problem. Both specimens of poly(L-methionine) behaved in a similar manner. Poly(L-nor-leucine) has been investigated previously (12,16). The results presented here are in general agreement, but the work has been repeated under the same experimental conditions as for the other polymers to enable a precise comparison (Figure 3). 

The assay sample buffer we have used is 0.14 M sodium chloride with 10 mM phosphate, pH 7.0, phenol red at 10 mg/liter, and an inert protein, usually 0.1% gelatin. The protein is included in all samples to minimize adsorptive losses. Gelatin has proved to be most useful because it is free of most potentially cross-reactive proteins that occasionally contaminate some preparations (e.g., luteinizing hormone in crystalline bovine serum albumin) it is free of most small, nonproteinaceous molecules that occasionally contaminate other preparations (e.g., steroids in ovalbumin) it effectively reduces nonspecific adsorption it is inexpensive and it does not cause foaming or create problems with valves on some automatic pipetting equipment. The concentration of phosphate is low and could be increased or supplemented. In effect we accomplish this by including 50 mM EDTA in the buffer used for the first antibody. The phenol red is included to serve as an indicator of dangerous pH shifts upon addition of sample. 

After the operator has selected the desired method menu of the relevant samples and has started the instrument, all subsequent steps are fully automated. Since 1987 it is also possible to effect a direct identification of the sample so that there are no longer any problems in respect of a dialogue with a central EDP system. The samples are taken from the sample vessel by means of disposable single use pipette tips that are used for one sample only and exchanged via a computer-monitored pipetting unit. This method excludes the possibility of a carry-over between samples. In accordance with the preset conditions, the required slides are automatically moved to the sample dosage unit (see Fig. 23). Samples of 11 pi serum or plasma will be sufficient for kinetic measurements (enzymes), 10 pi of sample for all other tests. As soon as application of the sample has been completed, the slide is moved to the appropriate incubation chamber by means of the slide rotor (see Fig. 23). The chemical reactions take place in these chambers. This is followed by measurement either by reflectometer (end point or kinetic) or a potentiometric measurement unit. 

The most convenient way to overcome this problem is to use an electronically controlled multiway pipettor such that, for example, 230 pL can be drawn up and then 4 x 55 pL dispensed, leaving 10 pL in the tips. A manual pipettor should be used in the reverse pipetting mode. 

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