Standard Addition or Spiking

Ideally, the standards should be made up in a solution containing the same normally expected levels of matrix elements as occur in the sample solution. It should be borne in mind that even if they exert no chemical interference, they could possibly exert a viscosity effect on a nebulized solution (especially with high concentrations of phosphoric or sulphuric acids). If it is not possible to determine the matrix components or prepare standards in a matrix solution, and unless experiments have shown matrix interference to be insignificant, then the method of standard additions, or spiking, should be carried out. This is where known amounts of the analyte are added to the sample or sample solution before determination by, e.g. AAS or colorimetry. 

ADAS (1987) The Use of Sewage Sludge on Agricultural Land. Booklet 2409. MAFF Publications, Alnwick. 

Adriano, D.C., Paulsen, G.M. and Murphy, L.S. (1971) Phosphorus-iron and phosphorus-zinc relationships in corn (Zea mays L.) seedlings as affected by mineral nutrition. Agronomy Journal 63(1), 36. 

McKnight, D.M., Wershaw, R.L. and MacCarthy, P. (1985) An introduction to humic substances in soil, sediment and waters. In Aiken, G.R., MacCarthy, P., Malcolm, R.L. and Swift, P.S. (eds) Humic Substances in Soil, Sediment and Water. John Wiley Sons, New York, pp. 1-9. 

Albrecht, W.A. (1942) Buying fertilizers wisely. Missouri Agricultural Experiment Station Circular 227, April 1942. 

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Method of standard additions or spiking method consists of adding known amounts of pure component a to a mixture containing X and Xb of a and b phases. It requires the preparation of several samples and measurement of several diffraction patterns containing different yet known additions (7o) of phase a. Other phases in the mixture are not analyzed but at least one of them b) should have a reference Bragg peak hkl) which does not overlap any reflection from phase a. The intensity ratio for this method is given as ... 

If the volume of the standard addition or spike, v, is very much smaller than the original solution volume V(l% or less), then any volume change can be ignored and Equation 3.7 simplifies somewhat to... 

Spike Recoveries One of the most important quality assessment tools is the recovery of a known addition, or spike, of analyte to a method blank, field blank, or sample. To determine a spike recovery, the blank or sample is split into two portions, and a known amount of a standard solution of the analyte is added to one portion. The concentration of the analyte is determined for both the spiked, F, and unspiked portions, I, and the percent recovery, %R, is calculated as... 

Whereas carbonyl chloride itself is very moisture sensitive and requires the corresponding precautions such as efficiently dried glassware and solvents, the Box derivative is very stable and can be analysed by high performance liquid chromatography (HPLC) with ultra violet (UV) detection at 270 nm. Quantification is achieved by the standard addition procedure spiking carbonyl chloride into the test polymer solution. However, since Box is a commercially available chemical, it is advisable to work also with Box standards, especially when the method is used for the first time and when problems are experienced in the HPLC determination or the derivation procedure. The standards of the carbonyl chloride derivative are particularly useful to establish the analytical system and to check linearity of detector response as well as for the recovery check. 

Calibration is carried either by means of internal standard addition or against a calibration curve from lead-spiked blood. For routine purposes the much simpler and faster method of calibration with lead-spiked blood can be applied. To minimise matrix effects the use of Zeeman GF-AAS and pyrolytically coated graphite tubes with a L vov platform is recommended, but this is instrument-dependent. 

Whilst these standards are mostly prepared simply within the solvent matrix used, variations on this theme may result in inclusion of these standards actually within the sample matrix. These procedures are often referred to as internal standardization, standard addition, or standard recovery, depending on the actual implementation of how the known analyte addition is used within the spectrophoto-metric method. These procedures either provide spectral calibration or method quality assurance data. For example, spectroscopic methods will often require pretreatment to remove the interfering sample matrix. Addition of a known amount of the test analyte spike (at the expected level) to a sample is... 

As a result of the difficult nature of the samples typically subjected to analysis via ETV-ICP-MS, external cahbration versus an aqueous standard solution is only applicable in a minority of apphcations. Most often, single standard addition or isotope dilution is used for quantification purposes. Of course, when the ETV unit is equipped with an autosampler, the addition of ahquots of modifier, standard or isotopic spike solution into the graphite furnace is facUitated. 

Illustration showing an alternative form of the method of standard additions. In this case a sample containing the analyte is spiked with a known volume of a standard solution of analyte without further diluting either the sample or the spiked sample. 

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

Also, a chromatographic profile or fingerprint of trace unknowns can be established and monitored, so that if product performance unexpectedly changes, there will be a starting point for troubleshooting. The effects of experimental variables on sample recoveries should be measured directly by controlled variation of an experimental factor, using the reference standard, or suitable external standards, or spiked addition of an external standard to the reference standard. A detailed example of the use of internal and external standards is presented in Chapter 4. 

Quantitation Poor reproducibility Comparison to spiked blank matrix or standard addition... 

In isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) the spike, the unspiked and a spiked sample are measured by ICP-MS in order to determine the isotope ratio. Using this technique, more precise and accurate results can be obtained than by using a calibration graph or by standard addition. This is due to elimination of various systematic errors. Isotopes behave identically in most chemical and physical processes. Signal suppression and enhancement due to the matrix in ICP-MS affects both isotopes equally. The same holds for most long-term instrumental fluctuations and drift. Accuracy and precision obtained with ID-ICP-QMS are better than with other ICP-QMS calibration... 

The accuracy of an analytical method is given by the extent by which the value obtained deviates from the true value. One estimation of the accuracy of a method entails analyzing a sample with known concentration and then comparing the results between the measured and the true value. The second approach is to compare test results obtained from the new method to the results obtained from an existing method known to be accurate. Other approaches are based on determinations of the per cent recovery of known analyte spiked into blank matrices or products (i.e., the standard addition method). For samples spiked into blank matrices, it is recommended to prepare the sample at five different concentration levels, ranging over 80-120%, or 75-125%, of the target concentration. These preparations used for accuracy studies usually called synthetic mixtures or laboratory-made preparations . 

Salgado Ordonez et al. [28] used di-2-pyridylketone 2-furoyl-hydrazone as a reagent for the fluorometric determination of down to 0.2 pg aluminium in seawater. A buffer solution at pH 6.3, and 1 ml of the reagent solution were added to the samples containing between 0.25 to 2.50 pg aluminium. Fluorescence was measured at 465 nm, and the aluminium in the sample determined either from a calibration graph prepared under the same conditions or a standard addition procedure. Aluminium could be determined in the 10-100 pg/1 range. The method was satisfactorily applied to spiked and natural seawater samples. 

Standard addition to wet sediment should be made in a water-miscible solvent (e. g., acetone or methanol). Any convenient solvent can be used to spike dry sediment. Standard addition to tissue samples can be made by first spiking a small amount of silica and allowing the solvent to evaporate. The silica is then ground with the tissue prior to extraction. 

Accuracy is defined as the bias of the method, it is a measure of how close the observed result is to the specified quantity. It is usually tested using a method of spiked placebos or of standard addition [3,4]. The expected performance of a method with respect to its accuracy varies enormously from sample to sample. A simple drug formulation, such as a tablet or injection, assayed by HPLC can expect an accuracy of around +/- 1%. 

Multi-element analysis by ETV-ICP-MS using ultrasonic slurry sampling of silicon based minerals such as talc or quartz is described by Rodriguez et al.33 The calibration was performed in the standard addition mode, where slurries were spiked with different weights (volumes) of a multi-element standard solution of the analytes of interest.33... 

The advantage of IDMS in comparison to other quantification strategies (external calibration or standard addition) is that analyte recovery does not have to be quantitative, providing that isotopic equilibrium has been achieved between all of the analyte and the added spike material. High accuracy analysis by IDMS is now well established.53 In the following paragraphs selected examples of IDMS use will be discussed briefly. 

Standard Addition. The sample is analyzed with and widioul the addition of a known umuunl or a compound that is also in the sample (spiking). The concentration is calculated from the observed increase in area. 

In this method, portions (2 g) of the plant material were digested in 30 ml of digestion acid until fumes of nitric acid are produced then the solution is cooled, and diluted to 50 ml in 15% hydrochloric acid-10% sulfuric acid vlv. This solution was used for analysis. Recovery studies were made by spiking the digestion acid or sample just prior to the digestion procedure with varying amounts of standard solution. Quantification of the six elements was made from linear calibration curves verified by the method of standard additions. 

In the standard addition method, the sample is analyzed first. A small, known amount of a known compound, thought to be in the sample, is then added to the sample, and the mixture is reanalyzed. If a quantitative increase is obtained in the peak of the compound corresponding to the standard, the compound representing the peak is tentatively identified. The technique of spiking is applicable provided that the sample is known to contain certain components appropriate standards can then be added to enable each peak to be identified. If the identities of the components in the sample are totally unknown, the spiking technique is inappropriate, and additional analytical or comparative techniques will be required. 

Direct analysis or spiking with stock solutions to apply standard addition technique... 

Wang et al. (1990) used acetonitrile-water to extract four isoflavones and coumestrol from soybean and processed soy products. Reversed-phase C18 HPLC followed by UV or fluorescence detection were used to separate and quantitate isoflavones in 20 min. Peaks were identified by retention time and standard addition. The method was sensitive to 2 ppm with UV detection at 254 nm for isoflavones and was sensitive to 0.5 ppm for coumestrol detection (using the fluorescent detector with excitation at 365 nm and emission at 418 nm). Recoveries of isoflavones and coumestrol ranged from 75% to 110% in spiked samples. Coefficients of variation for three soy products were below 9%. They compared three extraction solvents in... 

An SSF spectrum of a standard mixture of several of the PAHs found in a typical hydrocracker oil is shown in Figure 6, while that of an oil sample is shown in Figure 7. Quantitation is accomplished by generating a calibration curve from known concentrations of dicoronylene or by spiking samples through standard additions. 

We use the method of standard additions when it is difficult or impossible to duplicate the sample matrix. In general, the sample is spiked with a known amount or amounts of a standard solution of the analyte. In the single-point standard addition method, two portions of the sample are taken. One portion is measured as usual, but a known amount of standard analyte solution is added to the second portion. The responses for the two portions are then used to calculate the unknown concentration, assuming a linear relationship between response and analyte concentration (see Example 8-8). In the multiple additions method, additions of known amounts of standard analyte solution are made to several portions of the sample, and a multiple additions cahbration eurve is obtained. The multiple additions method gives some... 

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