Quantitative precipitation assay

In addition to the MRM scan, full-scan data can be obtained using the EMS function on the QTRAP to provide valuable information for evaluation of the bioanalysis method. For example, dosing vehicle polyethylene glycol (PEG) has been found to cause ion suppression in electrospray ionization, and chromatographic resolution of PEG from the analytes of interest is crucial for reliable performance of a quantitative bioanalytical assay. The EMS scan of the incurred samples could provide valuable information on the presence and the retention time of PEG, which cannot be obtained by MRM (King et al., 2003). Phospholipids have been shown to cause ion suppression, especially when a generic extraction method such as protein precipitation is used. The EMS function on QTRAP can be used to elucidate the phospholipid profile and to monitor the separation of phospholipids from the analyte and the internal standard (King et al., 2003). 

It must be emphasised that this method is only suitable for the assay of benzylpenicillin in samples consisting substantially of benzylpenicillin. The assay should be repeated using the Standard Preparation of benzylpenicillin (dried crystalline sodium salt obtainable from the National Institute for Medical Research, assayed in terms of International Standard penicillin). The result should indicate that 95 8 per cent of the Standard is sodium benzylpenicillin if a lower value (but not less than 93 0 per cent) is obtained the assay may be considered valid and a proportionate correction should be applied to the result obtained on the sample under test. The purity of reagents is essential, particularly for the AT-ethylpiperidine. The temperature is important and should be ice cold whenever possible. If penicillin X (/)-hydroxy benzylpenicillin) is present the method may give an erroneous result, since a non-quantitative precipitation of the salt of penicillin X takes place. 

Turbidimetric Agglutination Immunoassays. Agglutination—precipitation immunoassays were among the first practical appHcations of the antigen—antibody reaction in diagnostic tests. These assays are not as widely used in the 1990s as EIA and FIA because they are either not quantitative enough or lack the sensitivity limits of RIA, EIA, and EIA. 

Assay of beryUium metal and beryUium compounds is usuaUy accompHshed by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryUium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryUium content of the sample is calculated from the titration volume. Standards containing known beryUium concentrations must be analyzed along with the samples, as complexation of beryUium by fluoride is not quantitative. Titration rate and hold times ate critical therefore use of an automatic titrator is recommended. Other fluotide-complexing elements such as aluminum, sUicon, zirconium, hafnium, uranium, thorium, and rate earth elements must be absent, or must be corrected for if present in smaU amounts. Copper-beryUium and nickel—beryUium aUoys can be analyzed by titration if the beryUium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

Theory Benzylpenicillin (sodium or potassium salt) may be assayed gravimetrically by quantitative conversion to the 1-ethylpiperidinium benzylpenicillin derivative. The ultimate precipitation is caused by 1-ethyl piperidine after the respective sodium or potassium salt of benzylpencillin has been duly converted with phosphoric acid to the corresponding penicillanic acid (i.e. parent acid) and the latter finally extracted with amyl alcohol. The reactions may be expressed as follows ... 

Many qualitative and quantitative methods for soluble antigens are based on their precipitation by antibodies. It is important that the conditions for each assay are carefully optimized in order to achieve the desired end. 

In addition to increasing throughput, researchers are finding ways to utilize the increased sensitivity of the new HPLC-MS/MS systems. For example, Xu et al. [113] recently described the development of a low sample volume assay for preclinical studies. In this assay, only a lO-pL plasma sample volume is required for the analysis. The small volume is prepared by protein precipitation (1 6 = plasmaiacetonitrile) using a special low volume 96-well plate. Only 5 pL of the precipitated sample is injected onto the HPLC-MS/MS system. In spite of these low volumes, the example assay is reported to have a limit of quantitation (LOQ) of 0.1 ng mL h It can be predicted that there will be more reports of improved LOQs and reduced sample volumes as new LC-MS/MS instrumentation is introduced to more laboratories. 

Figure 6. Partial purification of Inhibitors I and II mRNA. Fractions containing Inhibitors I and II mRNA determined by in vitro translation analyses were recovered from an initial 15-30% linear sucrose gradient, precipitated by cold ethanol, and applied to a 10-25% linear sucrose gradient. The sample was centrifuged for 36 h at 25,000 rpm. Fractions of the gradient were collected and subjected to in vitro translation analyses. The upper graph represents total methionine incorporation assayed with 1 jiL of the translation mixture as described (ll). The bottom figure quantitates the radiolabel incorporated specifically into Inhibitor I (solid bars) and Inhibitor II (open bars).

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