Blank standard deviation

ICP-SFMS (Thermo Finnigan, Flement) with cold vapour generation was developed with a guard electrode and a gold amalgamation device using an Au-sorbent for sample pre-concentration to improve the sensitivity. Instrumental parameters of ICP-SFMS such as take-up time, heating temperature of Au-sorbent, additional gas flow, and sample gas flow were optimized. Detection limit calculated as 3 times the standard deviation of 10 blanks was 0,05 ng/1, RSD = 7-9 %. 

The detection limit is another value which is often quoted, and this may be defined in a variety of ways. The most widely accepted definition is that the detection limit is the smallest concentration of a solution of an element that can be detected with 95 per cent certainty. This is the quantity of the element that gives a reading equal to twice the standard deviation of a series of at least ten determinations taken with solutions of concentrations which are close to the level of the blank. 

In 1975, the lUPAC defined the LOD in terms of concentration (cl) and the signal (xl) generated by a solution of concentration clI They defined the value of xl in terms of the mean blank signal (xb) and the standard deviation (5b) of these blank measurements as... 

As discussed earlier, a concentration that would produce a signal of approximately 10-12 times the standard deviation of the blank (or in this case xeloq) is considered to be the limit of quantification. Therefore, if the LOQ was set at 10 times 5eloq, for 7 replicates (6 degrees of freedom) of the fortified control samples... 

Sensitivity is a measure of the smallest concentration that can be either measured [limit of detection (LOD)] or accurately quantitated [limit of quantitation (LOQ)]. In the USA, the method for measuring LOD or LOQ is left up to the method developer. European requirements for determining LOD and LOQ are very specific the LOD is based on the mean plus three standard deviations for 20 control blank samples, and the LOQ is defined as the lowest concentration giving an acceptable CV. 

Net recoveries of cyfluthrin from matrices fortified at 0.01-5.05 mg kg ranged from 77 to 119%. The limit of detection (LOD) is defined as the lowest concentration that can be determined to be statistically different from a blank or control. Calculate the value by taking the standard deviation of the residue values from the analysis of the recovery samples at the limit of quantification (LOQ) and using the equation... 

Estimated standard deviation (ESD) of the estimated intercept (blank) a... 

Assuming that the standard deviation of the critical value is approximately equal to the standard deviation of the blank, scv Srl> it results for a = fi in... 

The DL and QL for chromatographic analytical methods can be defined in terms of the signal-to-noise ratio, with values of 2 1-3 1 defining the DL and a value of 10 1 defining the QL. Alternatively, in terms of the ratio of the standard deviation of the blank response, the residual standard deviation of the calibration line, or the standard deviation of intercept (s) and slope (5) can be used [40, 42], where ... 

Metal Seawater concentration (nmol/kg) Blank (nmol) Relative standard deviation at test level (average of 10 analyses %) Recovery (%)... 

The limit of detection (LoD) has already been mentioned in Section 4.3.1. This is the minimum concentration of analyte that can be detected with statistical confidence, based on the concept of an adequately low risk of failure to detect a determinand. Only one value is indicated in Figure 4.9 but there are many ways of estimating the value of the LoD and the choice depends on how well the level needs to be defined. It is determined by repeat analysis of a blank test portion or a test portion containing a very small amount of analyte. A measured signal of three times the standard deviation of the blank signal (3sbi) is unlikely to happen by chance and is commonly taken as an approximate estimation of the LoD. This approach is usually adequate if all of the analytical results are well above this value. The value of Sbi used should be the standard deviation of the results obtained from a large number of batches of blank or low-level spike solutions. In addition, the approximation only applies to results that are normally distributed and are quoted with a level of confidence of 95%. 

The limit of quantitation (LoQ) is the lowest concentration of analyte that can be determined with an acceptable level of uncertainty. This should be established by using an appropriate reference material or sample. It should not be determined by extrapolation. Various conventions take the approximate limit to be 5, 6 or 10 times the standard deviation of a number of measurements made on a blank or a low-level spiked solution. 

The determination of iron at trace levels can be accomplished by the spectrophotometric measurement of its complex with the reagent o-phenanthroline. The sensitivity of a particular method is 53 ppm per unit absorbance and the standard deviation of the blank estimated from 25 measurements is 0.002 of an absorbance unit. The practical detection limit is therefore 1.64aB or 0.0033 of an absorbance unit which corresponds to 0.17 ppm, and the theoretical detection limit is 3.28aB or 0.0066 of an absorbance unit which corresponds to 0.35 ppm. (The value of pB in this case is assumed to be zero.) Hence if a result is less than 0.17 ppm, the conclusion is that iron is not detected. If the value lies between 0.17 ppm and 0.35 ppm, the iron content should be reported as less than 0.35 ppm. ... 

The dwell time was 200 msec for the analytes and 100 msec for the IS. At least 500 extracted samples were injected onto each column without any column regeneration. No solvent evaporation and reconstitution steps were involved. Ethyl acetate was preferred over methyl t-butyl ether (MTBE) because MTBE caused pulp-up of the mat. Six blank plasma lots were tested for matrix interference and none was detected in the analyte or IS region. When 100 ng/mL of the analytes were spiked into the blank plasma samples, the relative standard deviations were 1.0 and 1.5% for omeprazole and its metabolite, respectively. Precision and accuracy figures are given in Table 1.9. 

Residue was incorporated to a volumetric flask. Determinations of REE were carried out with an ICP mass spectrometer VG Elemental model PQ3. Detection limits were calculated as the concentration equivalent to three times the standard deviation of five replicates of the blank... 

Quantification of the limits of detection (LOD), or minimum detectable levels (MDL statistically defined in Section 13.4), is an important part of any analysis. They are used to describe the smallest concentration of each element which can be determined, and will vary from element to element, from matrix to matrix, and from day to day. Any element in a sample which has a value below, or similar to, the limits of detection should be excluded from subsequent interpretation. A generally accepted definition of detection limit is the concentration equal to a signal of twice (95% confidence level) or three times (99% confidence) the standard deviation of the signal produced by the background noise at the position of the peak. In practice, detection limits in ICP-MS are usually based on ten runs of a matrix matched blank and a standard. In this case ... 

Here the concentration range of the analyte in the ran is relatively small, so a common value of standard deviation can be assumed. Insert a control material at least once per ran. Plot either the individual values obtained, or the mean value, on an appropriate control chart. Analyse in duplicate at least half of the test materials, selected at random. Insert at least one blank determination. 

Longer (e g. n > 20) frequent runs of similar materials Again a common level of standard deviation is assumed. Insert the control material at an approximate frequency of one per ten test materials. If the run size is likely to vary from run to run it is easier to standardise on a fixed number of insertions per run and plot the mean value on a control chart of means. Otherwise plot individual values. Analyse in duplicate a minimum of five test materials selected at random. Insert one blank determination per ten test materials. 

Here we cannot assume that a single value of standard deviation is applicable. Insert control materials in total numbers approximately as recommended above. However, there should be at least two levels of analyte represented, one close to the median level of typical test materials, and the other approximately at the upper or lower decile as appropriate. Enter values for the two control materials on separate control charts. Duplicate a minimum of five test materials, and insert one procedural blank per ten test materials. 

As shown in Reference ( ), it is sufficient to have an estimate of the blank (B) and its standard deviation (03) plus the variation of Oy with the signal magnitude (y) to specify a decision criterion or level (Lq) given a, and a detection limit (L3) given Lq and 3. (See especially Figure 2 in Reference ( )). If Oy is independent of signal magnitude (at and below the detection limit), and if y is normally distributed, one concludes that... 

Depending on method type or intent, the LOD or LOQ may need to be determined. ICH guidelines describe several approaches and allow alternative approaches, if scientifically justifiable. Suggested approaches include calculation based on signal-to-noise ratio (typically set at 3 1 for LOD and 10 1 for LOQ) standard deviation of the response and slope standard deviation of the response of a blank, or a calibration curve. 

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