Preparation of standard solutions

The ethyl acetate extraction solvent is prepared by adding triethylamine (0.01% v/v). Use this modified solvent (referred to hereafter as solvent) to prepare all standard and sample solutions. 

To prepare primary standard solutions, weigh out separately 100 mg of each alkaloid (nicotine, 4-ethenylpyridine, etc.) into individual 100-ml volumetric flanks and dilute to volume with solvent. To prepare secondary standard solutions, pipet 1ml of nicotine primary standard into 100-ml volumetric flask. Also, pipet 0.50ml of 4-ethenylpyridine into the same volumetric flask (along with appropriate volume of any additional alkaloid primary solutions) and dilute with solvent. 

Prepare quinoline (internal standard) solution by adding 10 pi quinoline to 100 ml solvent. [Note quinoline can be detected in highly concentrated ETS at ca. 1% of the nicotine concentration (M.W. Ogden 1989, unpubl. data). This will not interfere with the quantitation of the remaining ETS alkaloids in the normal application of this method. However, if large amounts of ETS are concentrated, the standard procedure should be modified by either increasing the amount of quinoline internal standard added or by employing a different internal standard.] 

Prepare at least two sets of five calibration solutions that cover concentration range of interest (typically 0.1-2 pg/ml nicotine) by adding, for example, 10, 20, 50, 100, and 200 pL of alkaloid secondary stock to each of five 2-ml glass vials. Add 50 pi quinoline secondary standard and 1ml solvent to each vial, cap, and shake to mix. [Note no volume corrections are necessary for standards prepared in this fashion because the weight ratios of analytes to quinoline are constant, regardless of additional solvent volumes added.] 

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Full quantitation is accomplished in the same manner as for most analytical instrumentation. This involves the preparation of standard solutions and matching of the matrix as much as possible. Since matrix interferences are usually minimized in ICPMS (relative to other techniques), the process is usually easier. 

Hydrochloric acid and sulphuric acid are widely employed in the preparation of standard solutions of acids. Both of these are commercially available as concentrated solutions concentrated hydrochloric acid is about 10.5- 12M, and concentrated sulphuric acid is about 18M. By suitable dilution, solutions of any desired approximate concentration may be readily prepared. Hydrochloric acid is generally preferred, since most chlorides are soluble in water. Sulphuric acid forms insoluble salts with calcium and barium hydroxides for titration of hot liquids or for determinations which require boiling for some time with excess of acid, standard sulphuric acid is, however, preferable. Nitric acid is rarely employed, because it almost invariably contains a little nitrous acid, which has a destructive action upon many indicators. 

Preparation of standard solutions for calibration curves. The following concentrations are suitable ... 

Lam, R. B., and Isenhour, T. L., Minimizing Relative Error in the Preparation of Standard Solutions by Judicious Choice of Volumetric Glassware, Anal. Chem. 52, 1980, 1158-1161. 

One obvious procedural refinement would be to precoat the catalyst on the Teflon tape. This would allow low loadings to be delivered by length as opposed to mass measurements, or the tedious preparation of standard solutions. Accordingly, strips of tape were added to a solution of 16-Rf6 in CFaCeFii. The solvent was removed under an inert gas stream to give a yel-... 

Smith, B.W. and Parsons, M.L., Preparation of standard solutions critically selected compounds, J. Chem. Educ., 50, 679, 1973. 

As seen in the previous section, measured values are not absolute, but are obtained with a certain degree of uncertainty. The uncertainty is caused by the combined effect of several error sources. Four major sources for data uncertainty were described in the previous section reproducibility, accuracy, resolution, and limit of detection. To these may be added other factors instability of instrumentation, contamination, accuracy in preparation of standard solutions, etc. The sum of all uncertainties is called the analytical error. The analytical error is a cumulative outcome of all errors... 

Preparation of Standard Solutions Prepare a standard solution of each of the organic compounds to be quantitated in Hydrochloric Acid (known to be free of interfering impurities) at approximate concentrations of 5 mg/kg, or within 50% of the concentrations in the samples to be analyzed. 

Reference materials, that is, materials of known composition of components (usually determined on the basis of interlaboratory tests), can be of great help in proper preparation of standard solutions. They usually serve for verification of trueness of a given analytical procedure and accuracy of the obtained analytical result. If the composition of a reference material matches that of the analyzed sample, and the substance to be analyzed is one of its components, then the material can be utilized for calibration in such a way that a series of standard solutions is prepared via addition of analyte in known quantities. 

Irrespectively of historical aspects, the word standard should be reserved, at present, for cases dealing with the highest metrological quality. For instance, very pure metals and high purity compounds with exactly established stoichiometry can be regarded as primary measurement standards and serve for the preparation of standard solutions. 

The reported values for PCA-1, and in particular PCA-70, were higher than their respective true values. It is not clear why results for the PCA-70 mixture, whose GC profile and composition are similar to those of the PCA-60 standard, were less accurate then the results for the PCA-1 sample, whose GC profile and composition were quite different to the external standard. One possible explanation could be the amount of additives/stabilizers used by the manufactures, which are not measurable using ECNI or ECD detection. This makes the preparation of standard solutions from commercial products problematic for quantitation of PCAs and suggests that only pure PCA commercial formulations or synthetic mixtures prepared by free-radical chlorination of pure n-alkanes should be used for the preparation of external standards. 

Most of the elements are obtainable in a state of high purity. Those available with a purity of at least 99.9% or 99.99% are shown in Figure 5-2. Many, but certainly not all, of these elements can be used as primary standards to prepare standard solutions determinately. Smith and Parsons listed suitable choices for preparation of standard solutions for 72 of the elements. 

Determination of iron The most important applications of dichromate involve either directly or indirectly the titration of Fe(II). An excess of standard Fe(II) can be added to determine oxidants, or an excess of Fe(III) to determine reductants. These determinations usually can be carried out equally well with Ce(IV). For routine applications, however, the low cost and ease of preparation of standard solutions and the great stability of dichromate offer some advantages. Permanganate is at a disadvantage, expecially if hydrochloric acid solutions are to be used. 

Volumetric flasks (Figure 2-20) are manufactured with capacities ranging from 5 mL to 5 L and are usually calibrated to contain a specified volume when filled to a line etched on the neck. They are used for the preparation of standard solutions and for the dilution of samples to a fixed volume prior to taking aliquots with a pipet. Some are also calibrated on a to-deliver basis these are readily distinguished by two reference lines on the neck. If delivery of the stated volume is desired, the flask is filled to the upper line. 

For most purposes, reagent-grade potassium dichromate is sufficiently pure to permit the direct preparation of standard solutions the solid is simply dried at 150°C to 200°C before being weighed. 

Compounds Recommended for the Preparation of Standard Solutions of Some Common Elements ... 

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