Flow-injection analysis sampling error

For determination of creatinine, the nonspecific Jaffe method, although subject to perturbation by many interfering substances of endogenous and exogenous origin, is the most widely used. However, a batchwise kinetic procedure and flow injection analysis have shown the possibility to determine creatinine in human urine samples by this reaction, free from any systematic error. Enzymatic assays have higher specificities, but still suffer... 

Some authors claim that detection limits (DLs) of 5 mgL are easily obtainable [167] using the batch PCV detection method. However, it has been reported that daily DLs range between 5 and 10 mg L and that personnel who are highly acquainted with the method were needed to achieve DLs down to <10 mg L [168]. This poses a problem when the non-labile or monomeric organic fraction of aluminium to be measured is very small. Errors for Alni can become high, and, because the Driscoll method is a subtraction method, the uncertainty of the calculated Ali fraction (Alim—Alni) can become unacceptable. To increase the precision, sample throughput and practicality of the Driscoll method, several flow injection analysis (FIA) systems have been designed both for the PCV method [169,170] and for detection with a fluorimetric determination of the 8-hydroxyquinoline-5-sulfonic acid (8-HQS) aluminium complex in a micellar medium (cetyltri-methylammonium bromide, CTAB) [168,171]. 

The common spectrophotometric method for creatinine detection is based on the Jaffe reaction between creatinine and picric acid in alkaline solution to form a red-yellow complex. However, substances of endogenous and exogenous origin usually cause interference. In spite of these problems, the colorimetric method of Jaffe is still used today for the determination of creatinine in biological samples.A batchwise kinetic procedure and flow injection analysis have shown the possibility to determine creatinine in human urine samples by this reaction, free from any systematic error.Enzymatic methods have been reported to increase specificity/selectivity but still suffer from interferences. To avoid these problems, new analytical methods were developed. Several electtoanalytical techniques, based on potentiometric or amperometric detection, are available. Potentiometric methods using several sensors and biosensors were known for the... 

The ICP-OES-FIA technique allows a rapid and routine method of analysis for both major and trace levels of metals in aqueous and non-aqueous solutions in most samples provided that the sample is in solution form. The flow injection method can be used to correct for baseline drift that may originate from uncontrollable thermal and electronic noise during analysis. However, these errors can be corrected if the peak obtained is measured over at least three points, i.e. immediately before the peak, at the peak and immediately after the peak and the height or area is integrated over these points. The elaborate time consuming correction procedures required for batch operations are not required for FIA methods and the baseline is defined by the emission obtained from the carrier liquid and is reproduced between each sample injection. A typical FIA analysis of signals for standards and samples is shown in Figure 7.11 for triplicate injections of variable concentrations of boron. 

Another development in HPLC analysis is the interfacing of on-line sample clean-up technologies at the front end of the HPLC analytical column. Using a switching valve, complex samples can be injected directly onto a sample preparation column, the stationary phase of which is designed to bind the analyte(s) of interest to the exclusion of the rest of the matrix, which runs to waste. Then the valve is switched and the mobile phase flows through the sample preparation column where it picks up the analyte(s) and carries them onto the analytical column and through to the detector in the normal way. This automation of sample preparation saves time and effort and reduces errors. There are many applications of HPLC assays in conjunction with on-line sample clean-up methods in the literature ... 

S.3.3.2 Mass Accuracy Before measuring the samples, accurate mass calibration was performed using flow injection of a 1,3,6-polytyrosine solution (m/z 181, 507, and 997) in methanol/water (50/50 v/v) with 0.1% formic acid at a flow rate of 10 pL/min. No recalibration was performed during the experiments. While for identification of unknowns the mass accmacy is of greatest importance (Hogenboom et al., 2009), this is not a strict prerequisite for target analysis, such as in this study, as the mass error could be accounted for from the injection of calibration solutions. Still, the mass accuracy should be stable over a whole analytical sequence to ensure accurate peak detection and integration. To study the mass accuracy, the drift in mass... 

Chapters on sample introduction and hyphenated sample treatment and ICP systems have also been further updated since the last edition. No doubt that chromatographic, electrophoresis, flow injection and field flow fraction separations have extended ICP-MS (and AES) measurements as the mainstay of elanental specia-tion measurements in biological and environmental fields. Without the combination of these separation techniques and ICP measurements, elemental speciation applications would be severely hampered... if not impossible (Chapter 18). The ability to measure P and S with high sensitivity has opened up new opportunities in proteomics, for example. Species-specific and unspecific isotopic dilution (ID-MS) has been critical in quantifying speciation analysis and revealing recovery errors (Chapter 13). Species-specific techniques have been applied to identify species transformations, resulting in the development of multi-species methods whereas, hyphenated species-unspecific ICP-ID-MS determinations of heteroatoms such as sulfur have become a common quantification technique in proteomics. 

Analytical procedures are sometimes susceptible to systematic errors that are difficult to detect. To a lesser extent, this also holds for flow analysis. A typical situation is the partial overlap of sample and reagent zones in sequential injection systems. The samples and reagents are sequentially inserted into the same analytical channel, and the analytical... 

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

It is equally important to choose an appropriate sample concentration, injection or loading volume, flow rate, column temperature, pore size, particle size, and detection method in order to obtain MWDs with high fidelity and adequate resolution. The effects of these parameters are relatively straightforward, except for pore size and detection method, which are discussed later in Error Analysis . The choices of those parameters are based on the same considerations as for commonly known polymers, which are discussed in several excellent monographs, one of which is listed in the Bibliography (Yau, Kirkland, and Bly). 

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