Systematic error, blank

Spike recoveries on method blanks and field blanks are used to evaluate the general performance of an analytical procedure. The concentration of analyte added to the blank should be between 5 and 50 times the method s detection limit. Systematic errors occurring during sampling and transport will result in an unacceptable recovery for the field blank, but not for the method blank. Systematic errors occurring in the laboratory, however, will affect the recoveries for both the field and method blanks. 

The first sample to be analyzed is the field blank. If its spike recovery is unacceptable, indicating that a systematic error is present, then a laboratory method blank. Dp, is prepared and analyzed. If the spike recovery for the method blank is also unsatisfactory, then the systematic error originated in the laboratory. An acceptable spike recovery for the method blank, however, indicates that the systematic error occurred in the field or during transport to the laboratory. Systematic errors in the laboratory can be corrected, and the analysis continued. Any systematic errors occurring in the field, however, cast uncertainty on the quality of the samples, making it necessary to collect new samples. 

Activation analysis is based on a principle different from that of other analytical techniques, and is subject to other types of systematic error. Although other analytical techniques can compete with NAA in terms of sensitivity, selectivity, and multi-element capability, its potential for blank-free, matrix-independent multielement determination makes it an excellent reference technique. NAA has been used for validation of XRF and TXRF. 

In contrast, a systematic error remains constant or varies in a predictable way over a series of measurements. This type of error differs from random error in that it cannot be reduced by making multiple measurements. Systematic error can be corrected for if it is detected, but the correction would not be exact since there would inevitably be some uncertainty about the exact value of the systematic error. As an example, in analytical chemistry we very often run a blank determination to assess the contribution of the reagents to the measured response, in the known absence of the analyte. The value of this blank measurement is subtracted from the values of the sample and standard measurements before the final result is calculated. If we did not subtract the blank reading (assuming it to be non-zero) from our measurements, then this would introduce a systematic error into our final result. 

Sudden shifts to lower or higher values in the T plot are generally operator related. Different operators may use slightly different procedures that lead to bias. New reagent lots may introduce systematic error through blank contamination or different potency. Sudden undocumented environmental events may... 

Proportional systematic errors are detected with a Recovery Rate Chart, but not constant systematic errors (e.g. too high blank values). Additionally the spiked analyte might be bound to the matrix differently. This possibly results in a higher recovery rate for the spike than for the originally bound analyte. 

The G-BASE project collects samples in random number order (Plant, 1973), as this helps identify any correctable systematic errors introduced during sample preparation and analysis, processes in which the samples are handled in numeric order. For every block of one hundred numbers, five numbers are reserved for control samples so when they are submitted within a batch of samples they are blind to the analyst. The control samples inserted are one duplicate sample, two replicate samples, two blanks, and two secondary reference materials (SRM) used to monitor accuracy and precision as well as to level data between different field campaigns (see Johnson et al, 2008). Along with the original sample ofthe duplicate pair, this means 8% of samples submitted are control samples, a point not to be overlooked in setting the budget for analyses. 

Titration error — A numerical difference in volume, charge, or mass between the - equivalence point and the - endpoint. It is commonly related to a systematic -> error that can be corrected by carrying out a -> blank determination. See also - error (of measurement) [i]. 

Activation analysis is a blank-free technique. In general, blanks not oiJy determine the limits of detection, but at low concentrations they cause the main problems with respect to accuracy, because the small amounts to be determined have to be conveyed through all the steps of the chemical procedures, from sampHng to detection, without introducing systematic errors. These problems are not encountered in activation analysis, because contamination by other radionucHdes can, in general, be excluded and losses of the radionuclides to be determined can easily be detected by activity measurements. 

Blank determination The process of performing all steps of an analysis in the absence of sample used to detect and compensate for systematic errors in an analysis. 

The standard deviation of the null signal in this expression is given in terms of counting statistics if Poisson statistics are not likely to account for most of the random counting error, then it would be prudent to deduce Op from a moderate number of replicates -- le, replace the second term in the numerator of the second factor by 2t Sg Jri, where t is Student s-t and Sp is the estimated standard deviation for the blank (counts). Bounds for systematic error should be based on sound experience or analysis of the measurement process default values that reflect much low-level radionuclide measurement experience are set at 1% [baseline], 5% [blank], and 10% [calibration], respectively. Poisson deviations from normality are adequately accounted for by this expression down to B - 5 background... 

Two points merit emphasis in the above exercise a), The statistical confidence Interval for the outcome s based on S and its SE (using a 2-sided Student s-t) SE but not S is used also for the estimation of Lp. b) The confidence Interval, and Lj, and Lp (and its upper limit) are correct for normally distributed random errors. Faired T, B comparisons and a moderate number of replicates tend to make these assumptions reasonably good this is an important precaution, given the widely varying blank distributions of such difficult measurements. Perhaps the most important consequence of the paired comparison InjJuced, symmetry, is that the expected value for the null signal [B - B ] will be zero -- ie, unbiased. Systematic error bounds, some deeper implications of paired... 

Operation of a recovery control chart, when systematic errors from matrix interferences are expected Measurement of two blank solutions at the beginning and at the end of a batch in order to identify contamination of reagents, of the measurement system and instrumental faults and documentation of the blank values on a blank control chart... 

Detection limits down to the pg per kg level have been attained for the most favorable instrumental analyses (e.g., carbon and nitrogen in molybdenum and tungsten) and for radiochemical analyses (e.g., cadmium and thallium in zinc) at least if no nuclear interferences occur. This is below the concentration levels at which these impurities influence the material characteristics and below the detection limit attainable by more common methods of analysis. A precision (reproducibility) of a few percent is possible at the mg per kg concentration level in the most favorable cases. However, at higher concentration levels the precision will not improve significantly. Many systematic errors can be checked experimentally (e.g., interferences, yield of a radiochemical separation) others can be avoided experimentally (e.g., surface contamination). Systematic errors due to reagent blanks do not arise. 

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