Standard, calibration

It has been demonstrated that, when many samples are to be analyzed in automatically injected sequences, the APCI mode is more stable than the ESI mode for quantitation [32]. However, the ESI mode is more sensitive for the detection ofthe pyrrolizidine- N-oxides. 

The apparent sensitivity and resolution of the HPLC-ESI-ion trap MS ion chromatograms could be enhanced by software manipulation. Thus, a reconstructed ion chromatogram displaying only the ion of interest, usually the molecular ion adduct (MH ), was capable of extracting quantifiable peaks from low abundance chromatograms or from superimposed peaks (figure 13.18). 

Because of the diversity of structures, including stereochemical orientations, in the pyrrolizidine alkaloids extracted from a plant, it is not usually possible to have authenticated standards for every alkaloid extracted. An alternative approach is to generate calibration curves using standard pyrrolizidine alkaloids that best approximate those extracted from the plant [35,41]. Thus, considerations such as N-oxidation, mono- or diesterification, open chain or cyclic diesterification, and the character of the esterifying acids are important in deciding which standards to use. The analytes are then described in terms of equivalents of the standard alkaloid. At a later stage, if it becomes important, specific authenticated alkaloids can be used 

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Fig. 2 Conventional flat-bottom hole calibration standard for corrosion detection.

Fig.3 Corrosion calibration standard for eddy curent instrument using the gradient method.

The operation is quite simple One sets the frequency to the lowest value, adjusts the gain and phase to the desired sensitivity using a special calibration standard discussed below and performs a zero-compensation on a defect free zone of the standard. Now one is ready to test. As one slides the probe across the surface of an aluminum structure, a signal response will be indicative of the presence of corrosion or of the presence of a subsurface edge. 

A linear regression analysis should not be accepted without evaluating the validity of the model on which the calculations were based. Perhaps the simplest way to evaluate a regression analysis is to calculate and plot the residual error for each value of x. The residual error for a single calibration standard, r , is given as... 

To ensure that S eas is determined accurately, we calibrate the equipment or instrument used to obtain the signal. Balances are calibrated using standard weights. When necessary, we can also correct for the buoyancy of air. Volumetric glassware can be calibrated by measuring the mass of water contained or delivered and using the density of water to calculate the true volume. Most instruments have calibration standards suggested by the manufacturer. 

The following absorbances were obtained for a set of Cu calibration standards... 

Linear regression of the calibration standards gives the relationship between absorbance and concentration as... 

In the DPD colorimetric method for the free chlorine residual, which is reported as parts per million of CI2, the oxidizing power of free chlorine converts the colorless amine N,N-diethyl-p-phenylenediamine to a colored dye that absorbs strongly over the wavelength range of 440-580 nm. Analysis of a set of calibration standards gave the following results... 

The amount of sulfur in aromatic monomers can be determined by differential pulse polarography. Standard solutions are prepared for analysis by dissolving 1.000 mb of the purified monomer in 25.00 mb of an electrolytic solvent, adding a known amount of S, deaerating, and measuring the peak current. The following results were obtained for a set of calibration standards... 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

Prepare a single multicomponent working standard from the stock standards by making appropriate dilutions with methanol. Concentrations in the working standards should be at such a level that a 20- xL sample added to 100 mL of water gives a calibration standard whose response for each trihalomethane is within 25% of that for the samples to be analyzed. 

Samples and calibration standards are prepared for analysis using a 10-mL syringe. Add 10.00 mL of each sample and standard to separate 14-mL screw-cap vials containing 2.00 mL of pentane. Shake vigorously for 1 min to effect the separation. Wait 60 s for the phases to separate. Inject 3.0-pL aliquots of the pentane layer into a GC equipped with a 2-mm internal diameter, 2-m long glass column packed with a stationary phase of 10% squalane on a packing material of 80/100 mesh Chromosorb WAW. Operate the column at 67 °C and a flow rate of 25 mL/min. 

Zhou and colleagues determined the %w/w H2O in methanol by GG, using a capillary column coated with a nonpolar stationary phase and a thermal conductivity detector. A series of calibration standards gave the following results. 

Deming and Pardue studied the kinetics for the hydrolysis of p-nitrophenyl phosphate by the enzyme alkaline phosphatase. The progress of the reaction was monitored by measuring the absorbance due to p-nitrophenol, which is one of the products of the reaction. A plot of the rate of the reaction (with units of pmol mL s ) versus the volume, V, (in milliliters) of a serum calibration standard containing the enzyme yielded a straight line with the following equation... 

A 10.00-mL sample of serum is analyzed, yielding a rate of 6.84 X 10 pmol mL s h How much more dilute is the enzyme in the serum sample than in the serum calibration standard ... 

The concentration of chloride in sea water can be determined by a flow injection analysis. The analysis of a set of calibration standards gives the results in the following table. 

Analysis of Standards The analysis of a standard containing a known concentration of analyte also can be used to monitor a system s state of statistical control. Ideally, a standard reference material (SRM) should be used, provided that the matrix of the SRM is similar to that of the samples being analyzed. A variety of appropriate SRMs are available from the National Institute of Standards and Technology (NIST). If a suitable SRM is not available, then an independently prepared synthetic sample can be used if it is prepared from reagents of known purity. At a minimum, a standardization of the method is verified by periodically analyzing one of the calibration standards. In all cases, the analyte s experimentally determined concentration in the standard must fall within predetermined limits if the system is to be considered under statistical control. 

Construct a precision control chart using the following 20 ranges, each determined from a duplicate analysis of a 10-ppm calibration standard... 

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