The Internal Standard

Several authors indicate that modern spectrochemical analysis should be dated with the introduction of the concept of the internal standard by Gerlach in 1925. A variety of difficult to control operating parameters influence spectral line intensity. Among them are arc wandering, arc length, arc current, failure to time the exposure exactly, small errors in sample weight, and small losses of sample in transfer to the electrode. 

In 1925 Gerlach introduced the concept of the internal standard and listed the criteria that should be used for the selection of an internal standard with his homologous line pair concept. An internal standard can be a weak line of a major constituent of the sample, but more likely will be a substance, not already a constituent of the sample, that is added to all samples and standards at a constant concentration level. In practice, an internal standard, whose concentration is constant, produces a spectral emission line whose 

To function effectively as an internal standard, the element selected should meet certain criteria. These include the following  

The internal standard to be added should not be a constituent of the sample. 

The internal standard and the unknown element should have similar rates of volatilization. 

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There are two standard procedures for determining the octane numbers Research or FI and the Motor or F2 methods. The corresponding numbers are designated as RON (Research Octane Number) and MON (Motor Octane Number) which have become the international standard. 

The ISO 8681 standard, which treats all the petroleum products, groups lubricants, industrial oils and related products in the L Class. The international standard ISO 6743/0, accepted as the French standard NF T 60-162, subdivides the L Class into 18 families or categories. 

Furthermore, each sub-category given in Table 6.2 can be divided according to product viscosities, which are classified in the international standard ISO 3448 (French standard NF ISO 3448, index T 60-141). 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

Since the analyte and internal standard in any sample or standard receive the same treatment, the ratio of their signals will be unaffected by any lack of reproducibility in the procedure. If a solution contains an analyte of concentration Ca, and an internal standard of concentration, Cis, then the signals due to the analyte, Sa, and the internal standard. Sis, are... 

Because equation 5.10 is defined in terms of a ratio, K, of the analyte s sensitivity and the internal standard s sensitivity, it is not necessary to independently determine values for either or kjs. 

A single-point internal standardization has the same limitations as a singlepoint normal calibration. To construct an internal standard calibration curve, it is necessary to prepare several standards containing different concentrations of analyte. These standards are usually prepared such that the internal standard s concentration is constant. Under these conditions a calibration curve of (SA/Sis)stand versus Ca is linear with a slope of K/Cis-... 

When the internal standard s concentration cannot be held constant the data must be plotted as (SA/Sis)stand versus Ca/Cis, giving a linear calibration curve with a slope of K. 

A standard sample was prepared containing 10.0 ppm of an analyte and 15.0 ppm of an internal standard. Analysis of the sample gave signals for the analyte and internal standard of 0.155 and 0.233 (arbitrary units), respectively. Sufficient internal standard was added to a sample to make it 15.0 ppm in the internal standard. Analysis of the sample yielded signals for the analyte and internal standard of 0.274 and 0.198, respectively. Report the concentration of analyte in the sample. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Assume that p-xylene is the analyte and that methylisobutylketone is the internal standard. Determine the 95% confidence interval for a single-point standardization, with and without using the internal standard. 

For a single-point external standard (omitting the internal standard) the relationship between peak area, A2, and the concentration, C2, ofp-xylene is... 

For an internal standardization, the relationship between the peak areas for the analyte, A2, and the internal standard, Ai, and their respective concentrations, Ci and C2, is... 

One advantage of an HPLC analysis is that a loop injector often eliminates the need for an internal standard. Why is an internal standard used in this analysis What assumption(s) must we make about the internal standard ... 

A quantitative analysis for vitamin Bi was carried out using this procedure. When a solution of 100.0 ppm Bi and 100.0 ppm o-ethoxybenzamide was analyzed, the peak area for vitamin Bi was 71 % of that for the internal standard. The analysis of a 0.125-g vitamin B complex tablet gave a peak area for vitamin Bi that was 1.82 times as great as that for the internal standard. How many milligrams of vitamin Bi are in the tablet ... 

After heating to 50 °C in a water bath, the sample was cooled to below room temperature and filtered. The residue was washed with two 5-mL portions of CCI4, and the combined filtrates were collected in a 25-mL volumetric flask. After adding 2.00 mL of the internal standard solution, the contents of the flask were diluted to volume with CCI4. Analysis of an approximately 2- tL sample gave LfD signals of f3.5 for the terpene hydrate and 24.9 for the camphor. Report the %w/w camphor in the analgesic ointment. 

After adding a 10.00-mL portion of the internal standard, the solution was filtered. Analysis of the sample gave a peak height ratio of 23.2 for ASA and 17.9 for CAE. 

A form of internal standardization in which a radioactive form of the analyte serves as the internal standard. 

Internal standards at a known concentration are added to the sample after its preparation but prior to analysis to check for GC retention-time accuracy and response stability. If the internal standard responses are in error by more than a factor of two, the analysis must be stopped and the initial calibration repeated. Only if all the criteria have been met can sample analysis begin. 

Chromatographic peak areas are calculated automatically by the data system by reference to the response obtained from certain specified, compound-dependent ions. From the peak areas of the target compounds, quantification is achieved by comparison with the internal standards, which are present in known concentration. The laboratory responsible for the analysis must report the target compounds and all tentatively identified (nontarget) compounds. Standard EPA forms must be completed and submitted. A laboratory is said to be in compliance when it has satisfied all aspects of its CLP contract. 

To produce a quantitative result, chromatographic peak areas of identified target compounds are compared with peak areas of the internal standards, which are of known concentration. 

ASTM D4181 calls out standard specifications for acetal mol ding and extmsion materials. Homopolymer and copolymer are treated separately. Within each class of resin, materials are graded according to melt flow rate. The International Standards Organization (ISO) is expected to issue a specification for acetal resins before 1992. 

The fine chemicals standards discussed herein are primarily those originating in the United States. Much discussion has occurred regarding harmonization of the wodd s standards. It is not yet clear, however, what impact the International Standards Organization QuaHty Management Standards (ISO 9000) may have on the manufacture and specifications of fine chemicals. 

The International Standards Organization 9000 (ISO 9000) standard, developed by the European Economic Community (EEC), also impacts manufacturers implementation of LIMS systems. ISO 9000 is a set of standards which are required for manufacturers selling products to the EEC. The 9000 standards are credited with playing an important part in the impetus to greater computerization of laboratory information management (9). 

The atoms and molecules at the interface between a Hquid (or soHd) and a vacuum are attracted more strongly toward the interior than toward the vacuum. The material parameter used to characterize this imbalance is the interfacial energy density y, usually called surface tension. It is highest for metals (<1 J/m ) (1 J/m = N/m), moderate for metal oxides (<0.1 J/m ), and lowest for hydrocarbons and fluorocarbons (0.02 J /m minimum) (4). The International Standards Organization describes weU-estabHshed methods for determining surface tension, eg, ISO 304 for Hquids containing surfactants and ISO 6889 for two-Hquid systems containing surfactants. 

A significant revision to the SMR scheme was introduced in 1991 in response to consumer desire for greater consistency in natural mbber (13). Other producing countries have similar specification schemes (14), as does the International Standards Organization (ISO) (15). An example of the specifications for TSR is given in Table 1 for the present Standard Malaysian scheme. Except for SMR 5, rheograph and cure test data (delta torque, optimum cure time, and scorch) are provided. 

Finished zinc and zinc aHoys are usuaHy analyzed for metals other than zinc by emission spectroscopy and the zinc determined by difference. ASTM method E 27 describes a technique using a dissolved sample and photographic detection. The internal standard is the zinc line at 267.0 nm. However, procedures using soHd samples are generaHy preferred and photoelectric detection often replaces optical detection. Samples are cast and machined on the surface where the arc is stmck. Up to 15 elements can be determined in a few minutes by modem automatic spectrometers. ASTM gives wet chemical methods for metals other than zinc (79). 

TABLE 2-236 Thermodynamic Properties of the International Standard Atmosphere ... 

Calibration Cahbration entails the adjustment of a measurement device so that the value from the measurement device agrees with the value from a standard. The International Standards Organization (ISO) has developed a number of standards specifically directed to cahbration of measurement devices. Furthermore, compliance with the ISO 9000 standards requires that the working standard used to cahbrate a measurement device must be traceable to an internationally recognized standard such as those maintained by the National Institute of Standards and Technology (NIST). 

Approved techniques for manual and mechanical sampling are often documented for various commodities handled in commerce by industiy groups. Examples are the International Standards Organization (ISO), British Standards Association (BSA), Japan Institute of Standards (JIS), American Society for Testing Materi s (ASTM), and the Fertihzer Institute. Sampling standards developed for use in specified industry applications frequently include instructions for labora-toiy work in sample preparation and analysis—steps (2) and (3) above. 

At X-ray fluorescence analysis (XRF) of samples of the limited weight is perspective to prepare for specimens as polymeric films on a basis of methylcellulose [1]. By the example of definition of heavy metals in film specimens have studied dependence of intensity of X-ray radiation from their chemical compound, surface density (P ) and the size (D) particles of the powder introduced to polymer. Have theoretically established, that the basic source of an error of results XRF is dependence of intensity (F) analytical lines of determined elements from a specimen. Thus the best account of variations P provides a method of the internal standard at change P from 2 up to 6 mg/sm the coefficient of variation describing an error of definition Mo, Zn, Cu, Co, Fe and Mn in a method of the direct external standard, reaches 40 %, and at use of a method of the internal standard (an element of comparison Ga) value does not exceed 2,2 %. Experiment within the limits of a casual error (V changes from 2,9 up to 7,4 %) has confirmed theoretical conclusions. 

For a quick estimate of total dissolved solids (TDS) in water one can run a conductivity measurement. The unit for the measurement is mhos/cm. An mho is the reciprocal of an ohm. The mho has been renamed the Sieman (S) by the International Standard Organization. Both mhos/cm and S/cm are accepted as correct terms. In water supplies (surface, well, etc.) conductivity will run about 10 S/cm or 1 iS/cm. 

The international standard ISO 1940 is based on a linear equation similar to Equation 9.3 [3]. The specification calls for balance quality level by Grade Numbers with the lower the number the lower the permissible... 

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