Volume correction standard addition method

Ag, At, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Tl, V, and Zn ETAAS Metals are measured by ETAAS using the manufacturer s recommended conditions and an injection volume of 20 pL Applicable to surface, ground, tap, and waste water. A high CE concentration interferes. To minimize the matrix effect, the chemical modification, standard addition method, and background correction systems may be used. 105... 

In applying tihe standard additions method, it is most convenient to add small volumes of concentrated standard to the sample solution in order to miniinize volume change and thereby make volume corrections unnecessary. For example, 100 /rL of a 1000-ppm standard might be added to 10 mL of sample to increase the concentration by 10 ppm. The volume change is only 1% and can probably be ignored. The concentration increments should be close to the unknown concentration. 

Normally in applying the standard addition method, a calibration curve is constructed, similar to Figure 14.6, where the Y axis would be the atomic emission or absorbance signal. If the volumes of added standards are appreciable, the signals are corrected for dilution by multiplying by (V/ -I- v)V-, or VJV-, where V is the initial volume and v is the added volume, and V, is the total volume. 

Total arsenic in urine was determined by the graphite furnace with Zeeman effect background correction. Using matrix modification with 5% nickel nitrate, sampie volumes of 20 /tL, charring up to 1500 °C, atomisation at 2800 °C. and using the standard addition method was reported to achieve a detection limit of 10 hqIL if the urine dilution factor (2) was considered (Edgar and Lum, 1983). 

Assume that several aliquots of the unknown solution with a concentration r, are transferred to volumetric llasks having a volume V. To each of these flasks is added a variable volume V,t)f a standard solution of the analyte having a known concentration Suitable reagents are then added, and each solution is diluted to volume. Instrumental measurements are then made on eacii of these solutions and corrected for any blank response to yield a nei instrument response S. If the blank-corrected instrument rcspon.se is proportional to concentration, as is assumed in the standard-addition method, wc ma) write... 

Quantitation by the standard addition technique Matrix interferences result from the bulk physical properties of the sample, e.g viscosity, surface tension, and density. As these factors commonly affect nebulisation efficiency, they will lead to a different response of standards and the sample, particularly with flame atomisation. The most common way to overcome such matrix interferences is to employ the method of standard additions. This method in fact creates a calibration curve in the matrix by adding incremental sample amounts of a concentrated standard solution to the sample. As only small volumes of standard solutions are to be added, the additions do not alter the bulk properties of the sample significantly, and the matrix remains essentially the same. Since the technique is based on linear extrapolation, particular care has to be taken to ensure that one operates in the linear range of the calibration curve, otherwise significant errors may result. Also, proper background correction is essential. It should be emphasised that the standard addition method is only able to compensate for proportional systematic errors. Constant systematic errors can neither be uncovered nor corrected with this technique. 

The method of standard additions is a useful procedure for checking the accuracy of a determination and overcoming interferences when the composition of the sample is unknown. It should be noted that the method cannot be used to correct for spectral interferences and background changes. At least three aliquots of the sample are taken. One is left untreated to the others known additions of the analyte are made. The additions should preferably be about 0.5x, x and 2x, where x is the concentration of the unknown. It should also be noted that the volume of the addition should be negligible in comparison with the sample solution. This is to prevent dilution effects... 

In AAS, similar to other spectroscopic techniques, the correlation between analyte concentration or mass and the measured value (in A or Ai t) is established by the use of calibration samples, usually calibration solutions. In the standard calibration method a calibration function (calibration curve) is established using calibration solutions ranging from A = 0 (Ajnt = 0) to the highest (integrated) absorbance expected for the measurement solutions. The standard calibration method can only be applied if interferences by concomitants are absent. The method of additions, where calibration solutions are made by adding increasing masses of the analyte to equal volumes of the sample solution, permits correction for some interferences (see Sec. 1.6). 

The criteria for acceptable linearity of least squares fit and zero intercept when plotting ratios of analyte to internal standard areas vs. concentration are similar to the case for external standard calibrations described earlier. More than one IS can be used, both for calculating RRTs to compensate for retention time variations as well as the RRFs for improving quantitation. The variations that a quantitation IS can compensate for depend upon the point at which it is introduced in the analysis. If it is put into the final extract prior to injection on the chromatograph, it can correct for concentration variations due to evaporative volume changes, variations in injection volume, and variations in detector response. This is called an injection internal standard. If the internal standard is put into the initial sample, and into calibration standards prepared in an equivalent matrix, it can additionally correct for variations in recovery during the sample preparation process. This is called a method internal standard. Combined use of separate compounds for each purpose can aid in determining the causes of peak area variability. 

Among the nonspectral interferences transport interferences in the nebulizer are relatively common in the analysis of body fluids. This is certainly no problem when 10- or 20-fold diluted serum is used for the determination of the electrolytes. If, however, undiluted or only slightly diluted body fluids are aspirated directly, the viscosity of these liquids can impair aspiration rate and nebulization efficiency relative to the reference solutions used. If the sample solution cannot be diluted sufficiently to avoid this interference, a frequently used alternative is matrix-matched standards, i.e., reference solutions with a viscosity close to that of the samples. Another alternative is to use the method of additions, which can perfectly correct for this interference. This calibration technique, however, is labor-intensive and time consuming, and is restricted to the linear part of the calibration curve. Viscosity of the sample solutions is much less of a problem when FI techniques are used for sample introduction. This is because samples are not aspirated but pumped to the nebulizer, because much smaller sample volumes are used, and because the sample is always in a carrier solution which supports nebulization and removes all potential residues in the nebulizer-bumer system. 

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