Quantitative analysis internal standards

Recoveries also depend on the wine matrix white and red wines differ for volatiles and polyphenols, and contents of alcohol and sugars. For quantitative analysis, internal standards have to be spiked to the sample in suitable concentration, such as dimethyl sulfide-d6 (d6-DMS) 25 pg/L, dipropyl disulfide (DPDS) 25 pg/L, 4-methylthiazole (MT) lOpg/L, and 3-(methylthio)-l-hexanol (MTH) 50pg/L. A white wine matrix can be used [e.g., 10% v/v ethanol, sugar content <4g/L, and a polyphenolic content of 115mg/L expressed as (+)-catechin] for preparation of standard solutions to calculate the calibration curve for analytes, containing total S02 corrected to 100 g/L and previously treated twice with charcoal 3 g/L to remove sulfur and less polar volatile compounds (higher alcohols are not removed). 

Internal rather than external standards are recommended for quantitative analysis. If standard additions are used for calibration all the parameters listed in this table must be held constant. 

Internal standard calibration is used when the changes in the analytical system are known to be frequent and substantial. To compensate for these changes, internal standards at known concentrations are added to all calibration standards, field samples, and laboratory QC samples prior to analysis. Internal standards are synthetic analogs of specific target compounds or compounds that are similar in nature to the target analytes and that are not found in environmental samples. Internal standard calibration is a requirement of GC/MS methods. Laboratories sometimes use it for GC methods as it significantly improves the accuracy of compound quantitation. 

ABA is detected and quantified by enzyme immunoassay with a monoclonal antibody.578 The detection limit of immunoassay with a monoclonal antibody is about 0.02 pmol however, enzyme immunoassay cannot exclude cross-reactivity with unknown substances, so GC- or LC-SIM analysis using the stable isotope dilution method is recommended for correct quantitation. As internal standards labeled with the stable isotope, deuterated derivatives of ABA such as [3, 5, 5, 7, 7, 7 -2H6]ABA (ABA-//6)579 and [7, 7, 7 -2H3] derivatives of phaseic and dihydrophaseic acids580 are used. 

For a given spectrometer, a set t>f relative values of. V can be developed for the elements of interest. Note that the ratio US is directly proportional to the concentration n on the surface. The quantity / is usually taken as the peak area, although peak heights are also used. Often, for quantitative work, internal standards are used. Relative precisions of about 5% are typical. For the analysis of solids and liquids, it is necessary to assume that ihc surface composition of the sample is the same as its bulk composition, l- or many applications this assumption can lead to signiticani errors. Detection of an element hy XPS requires that it be pres-cnl at a level of at least 0.1 %. Quantitative analysis can usually he performed if 5 m of the element is present,... 

FAB or LSIMS using a probe inlet does not readily lend itself to quantitative work. Firstly, it is not possible to know how much of the sample has been consumed in the analysis. Secondly, discrimination effects (see section 12.3.3) prevent the comparison of intensities between species of differing surface activity. Semiquantitative results may be readily obtained if discrimination effects are assumed to be constant for the species of interest, for example the determination of homologue distributions in a mixture. For accurate quantitation an internal standard of an isotopically enriched analogue of the analyte should be used. For example, in the determination of cationic surfactants in environmental samples [10], quantitation was achieved by using an internal standard of a trideuterated form of the analyte. In this way the standard will be subject to the same level of discrimination as the analyte. Discrimination effects between different cationic species may also be reduced by adding to the sample an excess of a highly surface-active anionic surfactant. The anionic species will dominate the matrix surface and attract cations into the surface monolayer [10]. 

Standardization—External standards, standard additions, and internal standards are a common feature of many quantitative analyses. Suggested experiments using these standardization methods are found in later chapters. A good project experiment for introducing external standardization, standard additions, and the importance of the sample s matrix is to explore the effect of pH on the quantitative analysis of an acid-base indicator. Using bromothymol blue as an example, external standards can be prepared in a pH 9 buffer and used to analyze samples buffered to different pHs in the range of 6-10. Results can be compared with those obtained using a standard addition. 

Troost and Olavesen investigated the application of an internal standardization to the quantitative analysis of polynuclear aromatic hydrocarbons. The following results were obtained for the analysis of the analyte phenanthrene using isotopically labeled phenanthrene as an internal standard... 

Samples of analyte are dissolved in a suitable solvent and placed on the IR card. After the solvent evaporates, the sample s spectrum is obtained. Because the thickness of the PE or PTEE film is not uniform, the primary use for IR cards has been for qualitative analysis. Zhao and Malinowski showed how a quantitative analysis for polystyrene could be performed by adding an internal standard of KSCN to the sample. Polystyrene was monitored at 1494 cm- and KSCN at 2064 cm-. Standard solutions were prepared by placing weighed portions of polystyrene in a 10-mL volumetric flask and diluting to volume with a solution of 10 g/L KSCN in... 

Precision The precision of a gas chromatographic analysis includes contributions from sampling, sample preparation, and the instrument. The relative standard deviation due to the gas chromatographic portion of the analysis is typically 1-5%, although it can be significantly higher. The principal limitations to precision are detector noise and the reproducibility of injection volumes. In quantitative work, the use of an internal standard compensates for any variability in injection volumes. 

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 ... 

This experiment describes a quantitative analysis for caffeine, theobromine, and theophylline in tea, pain killers, and cocoa. Separations are accomplished by MEKC using a pH 8.25 borate-phosphate buffer with added SDS. A UV detector set to 214 nm is used to record the electropherogram. An internal standard of phenobarbital is included for quantitative work. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

An hplc assay was developed suitable for the analysis of enantiomers of ketoprofen (KT), a 2-arylpropionic acid nonsteroidal antiinflammatory dmg (NSAID), in plasma and urine (59). Following the addition of racemic fenprofen as internal standard (IS), plasma containing the KT enantiomers and IS was extracted by Hquid-Hquid extraction at an acidic pH. After evaporation of the organic layer, the dmg and IS were reconstituted in the mobile phase and injected onto the hplc column. The enantiomers were separated at ambient temperature on a commercially available 250 x 4.6 mm amylose carbamate-packed chiral column (chiral AD) with hexane—isopropyl alcohol—trifluoroacetic acid (80 19.9 0.1) as the mobile phase pumped at 1.0 mL/min. The enantiomers of KT were quantified by uv detection with the wavelength set at 254 nm. The assay allows direct quantitation of KT enantiomers in clinical studies in human plasma and urine after adrninistration of therapeutic doses. 

Liquid chromatography was performed on symmetry 5 p.m (100 X 4.6 mm i.d) column at 40°C. The mobile phase consisted of acetronitrile 0.043 M H PO (36 63, v/v) adjusted to pH 6.7 with 5 M NaOH and pumped at a flow rate of 1.2 ml/min. Detection of clarithromycin and azithromycin as an internal standard (I.S) was monitored on an electrochemical detector operated at a potential of 0.85 Volt. Each analysis required no longer than 14 min. Quantitation over the range of 0.05 - 5.0 p.g/ml was made by correlating peak area ratio of the dmg to that of the I.S versus concentration. A linear relationship was verified as indicated by a correlation coefficient, r, better than 0.999. 

Because of the complex nature of the discharge conditions, GD-OES is a comparative analytical method and standard reference materials must be used to establish a unique relationship between the measured line intensities and the elemental concentration. In quantitative bulk analysis, which has been developed to very high standards, calibration is performed with a set of calibration samples of composition similar to the unknown samples. Normally, a major element is used as reference and the internal standard method is applied. This approach is not generally applicable in depth-profile analysis, because the different layers encountered in a depth profile of ten comprise widely different types of material which means that a common reference element is not available. 

High performance liquid chromatography is used for the separation and quantitative analysis of a wide variety of mixtures, especially those in which the components are insufficiently volatile and/or thermally stable to be separated by gas chromatography. This is illustrated by the following method which may be used for the quantitative determination of aspirin and caffeine in the common analgesic tablets, using phenacetin as internal standard where APC tablets are available the phenacetin can also be determined by this procedure. 

Quantitative analysis using the internal standard method. The height and area of chromatographic peaks are affected not only by the amount of sample but also by fluctuations of the carrier gas flow rate, the column and detector temperatures, etc., i.e. by variations of those factors which influence the sensitivity and response of the detector. The effect of such variations can be eliminated by use of the internal standard method in which a known amount of a reference substance is added to the sample to be analysed before injection into the column. The requirements for an effective internal standard (Section 4.5) may be summarised as follows ... 

It is possible to carry out a chromatographic separation, collect all, or selected, fractions and then, after removal of the majority of the volatile solvent, transfer the analyte to the mass spectrometer by using the conventional inlet (probe) for solid analytes. The direct coupling of the two techniques is advantageous in many respects, including the speed of analysis, the convenience, particularly for the analysis of multi-component mixtures, the reduced possibility of sample loss, the ability to carry out accurate quantitation using isotopically labelled internal standards, and the ability to carry out certain tasks, such as the evaluation of peak purity, which would not otherwise be possible. 

HPA catalyzed liquid phase nitration was eairied out in a Teflon-lined stainless autoclave of 200 mL equipped with a magnetic stirrer. Reactants and HPA were quantitatively added to the autoclave, which was sealed and heated in an oil-bath. Products were analyzed by GC with OV-101 30 m capillary column and FID detector by using calibrated area normalization and internal standard method. All products were confirmed by GC-MASS analysis. 

This method requires about 40 g of tobacco which are extracted with ethyl acetate in the presence of ascorbic acid. A trace amount of C-NDELA is added as an internal standard for quantitative analytical work. The filtered extract is concentrated and NDELA is enriched by column chromatography of the concentrate on silica gel. The residues of fractions with p-activity are pooled and redissolved in acetonitrile. Initially, we attempted to separate NDELA on a 3% OV-225 Chromosorb W HP column at 210 C using a GC-TEA system with direct interface similar to the technique developed by Edwards a. for the analysis of NDELA in urine (18). We found this method satisfactory for reference compounds however, it was not useful for an optimal separation of NDELA from the crude concentrate of the tobacco extract (Figure 4). Therefore, we silylated the crude concentrate with BSTFA and an aliquot was analyzed by GC-TEA with direct interface. The chromatographic conditions were 6 ft glass column filled with 3% OV-... 

An internal standard is desirable in any quantitative trace environmental analysis. The ideal internal standard should behave in a manner identical to that of the analyte in all the procedures followed for isolation, purification, and determination without producing interference. This is a difficult requirement to meet for nitrosamines, especially for NDMA. 

---