Quantitative analysis spiking method

Precision and accuracy Quantitative analysis by NMR is very precise with relative standard deviations for independent measurements usually much lower than 5%. The largest errors in NMR measurements are likely due to sample preparation, not the NMR method itself. If a good set of standards is available and all NMR measurements for the test and standard samples are performed under the same acquisition conditions, the quantitative results can be readily reproduced on different instruments operated by different analysts at different times. Therefore, good intermediate precision can also be achieved. An accurate quantitative NMR assay will require accurately prepared standards. The accuracy of an NMR assay can be assessed, for example, by measuring an independently prepared standard or an accurate reference sample with the assay. In many cases, a spike recovery experiment can also be used to demonstrate the accuracy of an NMR assay. 

Analysis under the Convention is focused primarily on qualitative analysis (unambiguous identification) rather than on quantitative analysis. For the presence or absence of Schedule 1 chemicals, no quantitative limits are set in the Convention. Sample preparation methods for GC/MS proceed with a certain efficiency, and for accurate quantitative analysis, a determination of the recovery efficiency will be necessary. This may be performed by spiking a blank background sample and carrying out... 

An important issue in the method development for quantitative analysis using LC-MS is the possible occurrence of matrix effects. A matrix effect is an (unexpected) suppression or enhancement of the analyte response due to coeluting matrix constituents. It can be easily detected by comparing responses between a standard solution and a spiked pre-treated sample (post-extraction spike). Detailed studies on matrix effects revealed that the ion suppression or enhancement is frequently accompanied by significant deterioration of the precision of the analytical method [115-116]. Therefore, it can be useful to discriminate the two type of matrix effects. The absolute matrix effect indicates the difference in response between the solvent standard and the post-extraction spike, while the relative matrix effect indicates the difference in response between various lots of post-extraction spiked samples [116]. Unless counteraction is taken, an absolute matrix effect will primarily affect the accuracy... 

A more important method of quantitative analysis is the use of internal standards. Here, the sample and standards are spiked with an equal amount of a solute whose retention time is near that of the analyte. The ratio of the area of the standard or analyte to that of the internal standard is used to prepare the calibration curve and determine the unknown concentration. This method compensates for variations in physical parameters, especially inaccuracies in pipetting and injecting microliter volumes of samples. Also, the relative retention should remain constant, even if the flow rate should vary somewhat. 

Ma et al. have described a method for the simultaneous determination of 22 sulfonamides in cosmetics. Various cosmetic samples, including, cream, lotion, powder, shampoo, and lipstick, were extracted under ultrasonica-tion. Qualitative and quantitative analysis was carried out by using UPLC-MS/MS. The limits of detection for the set of sulfonamides were 3.5-14.1 (xg/kg. The mean recoveries at the three spiked concentrations were 80.3-103.6% with intra-day precision less than 12% and inter-day precision less than 15%. 

Some EPA methods refer to spiking with a standard referred to as a surrogate. The requirements of the surrogate and the reasons for using it are very similar to those of an internal standard. However, a surrogate is not used for quantitative analysis so the two terms are not the same and... 

Overspotting can be performed (also with the Linomat), in which more than one sample can be applied to a single initial zone position. These samples can include multiple standard reference compounds from different vials to prepare an in situ mixture, sample plus spiking solution, for validation of quantitative analysis by the standard addition method, or sample plus reagent, for in situ prechroma-tographic derivatization. 

One of the most useful techniques for the determination of a single analyte element in a known or unknown matrix is to use an internal standard. The technique is one of the oldest methods of quantitative analysis and is based on the addition of a known concentration of an element that gives a wavelength close to that of the analyte. The assumption is made that the effect of the matrix on the internal standard is essentially the same as the effect of the matrix on the analyte element. Internal standards are best suited to the measurement of analyte concentrations below about 10%. Care must also be taken to ensure that the particle sizes of specimen and internal standard are about the same, and that the two components are adequately mixed. Where an appropriate internal standard cannot be found, it may be possible to use the analyte itself as an internal standard. This method is a special case of standard addition, and it is generally referred to as spiking. 

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. 

Method validation is needed to demonstrate the acceptability of the analytical method. A recovery test on a chemical being determined should be performed in order to verify the reliability of the series of analyses. Recovery studies are usually conducted by spiking untreated sediment with the target chemical at the deteetion limit, quantitation limit and in the range of 10-50 times the detection limit. The method is considered acceptable when the recoveries typically are greater than 70%. When the recovery is less than 70%, an improvement in the analytical methods is needed. Where this is not possible for technical reasons, then lower recovery levels may be acceptable provided that method validation has demonstrated that reproducible recoveries are obtained at a lower level of recovery. Analysis is usually done in duplicate or more, and the coefficient of variation (CV) should be less than 10% to ensure that recoveries will be consistently within the range 70-110%. 

The main advantages of plasma-source mass spectrometry (PS-MS) over other analytical techniques, such as PS-AES and ETAAS, are the possibilities of quantitative isotope determination and isotope dilution analysis the rapid spectral scanning capability of the mass spectrometer and semiquantitative determinations to within a factor of two or three. Several labelling methods are used for the quantification of analytes present in complex mixtures. In these methods, the sample is spiked... 

Stable labelled isotopes are spiked into samples before extraction and the ratio of unlabelled compound and stable labelled isotope was used to quantitate the unlabelled compound. Analysis is by high-resolution gas chromatography-mass spectrometry. Fifteen standard water samples and ten standard soil samples containing 2,4-D at known concentrations were analysed. Compound concentrations ranged from 100 to lOOOOug per kg for soil samples. Average recoveries were over 84% and method precision, given as relative standard deviation, was better than 19%. 

This 2D-method was validated for the concentration range between 0.005 and 0.5 pmol for D-amino acids and 0.05-5 pmol for L-amino acids. Within-day and interday precisions were always better than 8% relative standard deviation (RSD) and the accuracies for spiked rat plasma samples were between 95.5% and 100.2%. Limit of detections (LCDs) and limit of quantitations (LOQs) were reported to be as low as 3 fmol (S/N = 3-5 corresponding to 0.15 nmolg wet tissue) and 5 fmol (corresponding to 0.25 nmolg wet tissue). It was concluded that this assay is supposed to be one of the most sensitive analysis method for amino acid enantiomers in mammalian samples. 

If a certified material is not available, as is usually the case in drug residue analysis, an approximation can be obtained by spiking the blank sample matrix to a nominal concentration. The accuracy of the method is then determined by assessing the agreement between the measured and nominal concentrations of the analytes in the spiked drug-free matrix sample. The spiking levels should cover the range of concern and should include one concentration close to the quantitation limit. 

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