Standard Substances

Other techniques for mass measurement are available, but they are not as popular as those outlined above. These other methods include mass measurements on a standard substance to calibrate the instrument. The standard is then withdrawn, and the unknown is let into the instrument to obtain a new spectrum that is compared with that of the standard. It is assumed that there are no instrumental variations during this changeover. Generally, this technique is less reliable than when the standard and unknown are in the instrument together. Fourier-transform techniques are used with ion cyclotron mass spectrometers and give excellent mass accuracy at lower mass but not at higher. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

Because variations in accurate isotope ratio measurements typically concern only a few parts per 1000 by mass and there are no universal absolute ratios, it is necessary to define some standards. For this purpose, samples of standard substances are produced and made available at two major centers IAEA (International Atomic Energy Authority, U.K.) and NIST (National Institute for Standards and Technology, U.S.). Standards from other sources are also available. These primary standards can be used as such, or alternative standards can be employed if the primary ones are not available. However, any alternative standards need to be related accurately to the primary ones (see formulae below). For example, the material PDB (PeeDee belemnite), used particularly as a standard for the ratio of isotopes, is no longer readily available, and a new standard, VPDB,... 

The measured ratio of abundances for two isotopes (A,B) is defined and illustrated for the two standard substances PDB and VPDB. 

Differences in measured isotope ratios (R) can be compared for two substances by using a new ratio a. The two substances may constitute part of a series of measurements on a sample, which are to be internally compared, or they may constitute results from a series that is compared with an external value from a standard substance. 

For a sample substance (S) for which an isotope ratio (R ) has been measured and then compared with the same isotope ratio (R ) for a standard substance (A),... 

The 5 values for a sample measured against a standard substance can be changed into S values against a second standard substance if the S value for the two standards is known. 

For example, from figure 4 for the two standard substances VPDB, PDB,... 

The difference in 5 values for two substances (P,Q) measured against a standard substance is approximately equal to 1000 times the natural logarithm of their fractionation factor (app). 

A 6-value is used to compare a measured isotope ratio in a sample with that for a standard substance containing the same isotopes but in known abundance ratio. 

Standard substances are available from such agencies as the International Atomic Energy Authority (U.K.) and the National Institute for Standards and Technology (U.S.). 

The physical properties of argon, krypton, and xenon are frequendy selected as standard substances to which the properties of other substances are compared. Examples are the dipole moments, nonspherical shapes, quantum mechanical effects, etc. The principle of corresponding states asserts that the reduced properties of all substances are similar. The reduced properties are dimensionless ratios such as the ratio of a material s temperature to its critical... 

To aid in solute identification, a standard substance is usually added to a mixture and the separation ratio of the solutes of interest to the standard is used for identification purposes. In practice, separation ratios are calculated as the ratio of the distances in centimeters between the dead point and the maximum of each peak. If the flow rate is sufficiently constant and data processing is employed, then the corresponding retention times can be used. 

A chromatogram is produced by developing a TLC/HPTLC plate, but it may be necessary to employ one of the reagents descnbed to make the positions, structures and sizes of the chromatogram zones apparent so that they can be recorded If the Rf values are the same a companson of the sizes of the zones of the sample and standard substances gives an indication for estimating the amounts If, as a result of matnx effects, the Rf values of sample and standard are not the same then their... 

In physics, the word specific implies a ratio. Weight is the measure of the earth s attraction for a body, which is called gravity. Thus, the ratio of the weight of a unit volume of some substance to the weight of an equal volume of a standard substance, measured under standard pressure and temperature, is called specific gravity. The terms specific weight and specific density are also sometimes used to express this ratio. 

To obtain reliable, accurate, and reproducible methods for quantitative estimation of deoxy sugars, certain conditions must be fulfilled. Thus, it is necessary that the chromogen be formed quantitatively from the sugar. The chromogen must then react quantitatively with the compound used for color formation, and lastly, the dye, once formed, should be stable and have a well defined molar extinction coefficient. In methods in which all of these conditions are not or cannot be fulfilled, recourse must be had to simultaneous determinations with suitable standard substances, a requirement not always easy to fulfil. 

The absolute value of a proportional error depends upon the amount of the constituent. Thus a proportional error may arise from an impurity in a standard substance, which leads to an incorrect value for the molarity of a standard solution. Other proportional errors may not vary linearly with the amount of the constituent, but will at least exhibit an increase with the amount of constituent present. One example is the ignition of aluminium oxide at 1200°C the aluminium oxide is anhydrous and virtually non-hygroscopic ignition of various weights at an appreciably lower temperature will show a proportional type of error. 

Running a control determination. This consists in carrying out a determination under as nearly as possible identical experimental conditions upon a quantity of a standard substance which contains the same weight of the constituent as is contained in the unknown sample. The weight of the constituent in the unknown can then be calculated from the relation ... 

OTHER STANDARD SUBSTANCES FOR ACIDIMETRY AND ALKALIMETRY 102S... 

In addition to the standard substances already detailed for use in standardising acids and alkalis, numerous others have been proposed. A number of these will be briefly described. 

Precipitation methods 418 Precipitation reactions 340 theory of, 340, 342, 579 Precision 13, 129 Preparation for analysis 109 solution of sample, 110 Preventive solution 368 Primary amines see Amines Primary standard substances requirements of, 261... 

Standard curves in spectrophotometry, 674 Standard deviation I 34 Standard potentials 62, 63, 66 Standard series method 652, 654 Standard solutions 107, 257, 259 for pH, 569, 831 prepn. of, 107, 260, 285, 802 storage of, 108 Standard substances for acidimetry and alkalimetry ... 

Standard substances - continued for redox titrations - continued iodine, 384 iron, 262... 

Fig. 1 Chromatogram of ipecacuahna tincture and of the standard substances 1 = emetine, 2 = tincture, 3 = cephaeline.

Fig. 2 Fluorescence scan of the chromatogram tracks of the standard substances cephaeline (A) and emetine (B) and of the ipecacuanha extract (C). Amounts applied cephaeline 0.5 pg, emetine 0.7 pg per 10 mm track length.

Standard substances of defined purity must be available for major components and the main impurities (criteria mole fraction, toxicity, legal... 

Until 2004, no betalain standards were commercially available and current products lack the required purity. Preparative isolation from plant material is laborious and costly and the resulting standard substances vary in relative humidity, crystal water, and salts, resulting in over- or under-estimation of pigment contents... 

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