Instrument blanks

15 Determination of dissolved organic carbon and nitrogen by high temperature combustion 

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Blanks may be differentiated into instrumental blank (background and baseline, respectively) and chemical blank (analyte blank). 

A common source of error in ICP-MS is that negative masses are produced for unknown solutions. This occurs when the calibration line crosses the vertical axis at a value significantly above zero (Fig. 9.6(b)), which is often the result of high counts from the instrument blank, usually caused by an uncorrected interference. Any unknown samples which have a CPS below this value will then produce negative masses. One solution to this is to computationally force the line through the origin, or to remove outliers from the calibration line, but it is much better to identify the true cause of the problem (i.e., find the interference, if this is the source), and to rerun the samples. 

During a single run, which may take all day if a large number of samples are to be analyzed, the instrument may drift from its optimum settings. To detect this drift in solution-based techniques, and also to compensate for some matrix effects, a known amount of an element may be added to each sample before analysis. This internal standard (also called a spike) is added to all the samples and blanks, with the exception of the instrument blank (which is defined as zero concentration for all elements see below). It is important that the element (or isotope) chosen as the spike is not an element which is to be determined in the samples, and preferably which does not occur naturally in the samples. It must not be an element which will cause, or suffer from, interference with the other elements to be determined. In solution ICP-MS,... 

Two types of blanks need to be prepared with each batch of samples. To subtract background levels of contamination occurring during instrumental analysis, an instrument blank, which consists of a matrix-matched solution with no internal standard, is made. Generally, an instrument blank is run at the start of the analysis and will be included in the calibration with assigned concentrations of zero for all elements to be measured, so, in effect, this is subtracted from all the subsequent samples. 

Organize Analytical Batches—These include analytical QC checks (instrument blanks and other QC checks as required by the method), CCV standards, prepared field samples, and laboratory QC samples. 

Because memory effects (carryover) from high concentration samples may take place in the nebulizer and in the plasma torch, the system must be flushed between samples. To recognize and prevent memory effects, instrument blanks are analyzed after high concentration samples additional frequent instrument blanks intersperse analytical sequences as a QC measure. 

An analytical batch is a group of samples, extracts or digestates, which are analyzed sequentially using the same calibration curve and which have common analytical QC checks. These are the samples that are bracketed by the same CCVs, have the same instrument blanks, and other QC checks that may be required by the method (for example, the DDT and Endrin breakdown product check in organochlorine pesticide analysis by EPA Methods 8081). If the CCV or any of the analytical QC checks are outside the method acceptance criteria, the whole analytical batch or only the affected samples are reanalyzed. 

The instrument blank is the reagent used in the final preparation of the samples, which is injected into the analytical system after high level standards and samples have been analyzed. Analysis of instrument blanks removes memory effects from the analytical system. A calibration blank used for flushing the system in trace element analysis is an example of an instrument blank. 

Instrument blank Analyzed as needed to eliminate memory effects must be free of contamination before samples are analyzed. 

Do instrument blanks indicate the presence of memory effects ... 

Table 5.4 summarizes the acceptance criteria for instrument, calibration, and method blanks. No contaminants of concern should be present in method blanks above the laboratory PQL. Equally important is that instrument blanks show no memory effects. If these conditions are not met, a possibility for false positive sample results becomes real. For decision on sample data with contaminated method blanks the chemist may rely on the following rules of the Functional Guidelines ... 

System or Instrument Blank. It is a measure of system contamination and is the instrumental response in the absence of any sample. When the background signal is constant and measurable, the usual practice is to consider that level to be the zero setting. It is generally used for analytical instruments but is also applicable for instruments for sample preparation. 

Instrument blank —a blank analysed with field samples to assess the presence or absence of instrument contamination. 

Intercomparisons between the persulfate and HTCO techniques showed considerable differences early in the investigations [36,80], with later work [81-83] displaying much closer agreement between the methods. It is probable that some of the rapprochement results from a better understanding of the factors influencing the HTCO method, including choice and conditioning of the catalyst and the proper measurement of the instrument blank [84,85]. 

For this top-down approach, it is assumed that the quality control (QC) precision and recovery data have been collected over a sufficiently large number of runs and period of time to allow for natural variation of all factors that can affect the results. These factors include different analysts, analytical instruments, blank tissue lots, lot numbers of reagents, and preparations of standard solutions. Note that other factors that can affect analytical results, such as method bias, variations in the sample matrix, sampling, sample storage and treatment, subsampling, homogeneity, standard purity, and the preparation of standard solutions, are not included in this discussion. 

Method and instrument blanks and, where possible, field blanks should be analyzed with each batch of samples. A method blank uses water, usually high-purity (double-distilled and deionized) water, which is processed through all laboratory steps in the same way as samples. An instrument blank uses the same water directly introduced into the detector. A field blank is high-purity water that has been bottled in the laboratory, shipped with sample bottles to the sampling site, processed, and preserved as a routine sample and returned with the routine samples to the laboratory for analysis. The analysis of a blank should not yield a value higher than that allowed by the acceptance criteria. Blanks are used to determine the limit of detection of a method and to monitor all aspects of the analytical process. 

Fig. 15-1. TOC 5000 instrument blank with (a) new Pt-Al20j catalyst, (b) with the same catalyst after 5 d of washing with Milli-Q water.

Adjust instrument sensitivity, baseline stability and perform instrument blanking procedures following manufacturer s guidelines. 

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