Mass standard addition procedure

Adequate precision and accuracy are only likely to be achieved if some standardization procedure is employed and the nature of this, internal or external standards or the method of standard additions, needs to be chosen carefully. If internal standardization procedures are adopted then appropriate compound(s) must be chosen and their effect on the chromatographic and mass spectrometry methods assessed. The ideal internal standard is an isotopically labelled analogue of the analyte but, although there are a number of commercial companies who produce a range of such molecules, these are not always readily available. An analytical laboratory is then faced with the choice of carrying out the synthesis of the internal standard themselves or choosing a less appropriate alternative with implications on the accuracy and precision of the method to be developed. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

The direct determination of trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr, and Zn) by ICP-MS in powdered milk was reported [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Mass intensity losses were not observed. The quantitative ICP-MS procedure was applied the standard additions method with a Y internal reference. This direct technique is as fast as the slurry approach without particle size effects or sensitivity losses. 

A different perspective on mass spectral fragmentation can be obtained considering the energy of the molecular ion formed during the El process. The ions with low energy states can be related to thermal processes [21.22], and this may provide information on the thermal decompositions. In addition to standard ionization procedures, special techniques that provide milder ionization conditions such as field ionization (see Section 5.4) offer an even closer similarity to the pyrolytic process. 

The ionization process that takes place in the ion source of the mass spectrometer can be carried out by standard ionization procedures such as electron impact (El), by chemical ionization (Cl), or by other special techniques (desorption ionization, etc.). In Py-MS, in addition to the decomposition of the sample by heat in the pyrolyzer, the pyrolysate may suffer fragmentations in the MS during the process of ion formation. 

Matrix effects can be important during the analysis of petroleum products. They generally disturb intensities emitted by a detected isotope [10]. Organic matrix may modify element ionization in the plasma and consequently cause a variation of sensitivity. For these reasons, a standard addition method was performed for the calibration procedure to control the matrix effects. Concentrations of metals in analyzed organic samples must be compatible with ICP-MS potential in order to obtain reliable results. Experience shows that concentrations detected by the mass spectrometer ideally should be in the range of 1-100 ng/g. Thus, for optimal detection conditions, samples were diluted in xylene according to their pre-estimated element concentration range. 

Table 19.3 Results of niacin determinations for milk samples. Niacin determinations by liquid chromatography-isotope dilution mass spectrometry (LC-IDMS) are compared to expected values for four milk samples. Expected niacin levels for milk are roughly 1 ppm, according to the USDA Nutrient Database for Standard Reference (US Department of Agriculture 2010) and results obtained for two commercial milk samples (Brands F and G) are a little under 1 ppm. The result for sample NFY0409F6 is about 30% lower, but is consistent with results obtained for other milk samples from the same source. In addition, the niaein level for NFY0409F6 was estimated by a standard additions experiment, the result from which is in agreement with the estimate from the normal LC-IDMS procedure. The level obtained for the reference material (RM) RM 8435 whole milk powder, reported on a dry mass basis, is in agreement with the reference value. Data are from Goldschmidt and Wolf (2007), with permission from the publisher.

Two procedures are most commonly used for evaluation of the samples. The routine method is based on a calibration curve, which can be prepared either from aqueous standards with the same HNO3 concentration as in the procedure, or by spiking pre-extracted water samples (if the extraction efficiency is close to 100 %). The slopes of both curves, however, should agree with the slope of the standard addition method (see Fig. 12-6) only thus can it be guaranteed that all interferences have been eliminated. It is also good practice to check the sensitivity of the instrument from time to time (Le., to identify the mass of element per 0.004 absorbance) when analysing standard solutions, and to compare the determined characteristic mass with the manufacturer s nominal values. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

It is clear that neither NMEA nor NDPA is appropriate for an internal standard in NDMA determination if criteria are interpreted strictly, but both compounds have been used for this purpose. Addition of a nitrosamine, not normally present in the sample, is helpful in detecting any gross errors in the procedure, but the addition should not be considered to be internal standardization. Utilization of NMEA or NDPA to indicate recovery of NDMA can lead to significant errors. In most reports of the application of these "internal standards", recovery of all nitrosamines was close to 100%. Under these conditions, any added compound would appear to be a good internal standard, but none is necessary. NDMA is a particularly difficult compound for use of internal standardization because of its anomalous distribution behavior. I mass j ectrometry is employed for quantitative determination, H- or N-labeled NDMA could be added as internal standard. Because the labeled material would coelute from GC columns with the unlabeled NDMA, this approach is unworkable when GC-TEA is employed or when high resolution MS selected ion monitoring is used with the equipment described above. 

To validate the analytical procedure recovery experiments are performed. To this end, the CRM is spiked with a known mass of the analytes at a variety of concentration levels (at least three different levels) and the concentrations measured are compared to the expected concentrations in at least three separate experiments. The extraction step has been shown to be a critical step in the analytical procedure and it may be responsible for poor recoveries. The efficiency of this step can be assessed either by repetitive extraction of the sample or by the addition of internal standards prior to the extraction step with the assumption that the latter actually represent the behavior of the analytes of interest. 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

The first strategy to compensate for mass spectral drift is to tune the instrument. This is typically achieved with the volatile standard, perfluorokerosene, and tuning so that mlz 181 is one-tenth of m/z 69. Unfortunately, this procedure is insufficient to compensate for all the instrumental drift and additional methods are required. 

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