Signal depression

Matrix influences may affect the analyte (317) signal directly by interferences or indirectly by signal depression or amplification. 

SeS2 and Se02 Recoveries for NBS-SRM 339 were <10 percent. It was quickly realized that the high iron matrix in the sample was playing a major role in signal depression, as the sample otherwise appeared completely solubilized. Although addition of iron to the standards improved the recovery to >90 percent, the sensitivity was still poor, 5-10 percent of that obtained in an iron-free matrix. 

Another calibration technique - standard addition - minimizes matrix effects because analytes with well defined increasing concentrations are added to a set of sample solutions to be analyzed. The measured calibration curve in the standard addition mode plots the measured ion intensities of analytes versus the concentration added to the sample solution. The concentration of analytes in the undoped sample is then determined by extrapolation of the calibration curve with the x-axis. Matrix matching is subsequently performed and the matrix effects (signal depression or interference problems) are considered. An example of the standard addition technique is described in Section 6.2.6 using solution based calibration in LA-ICP-MS. 

The study showed that at concentrations of 100 mg l-1 fluoride and 1.2 percent KI (as the prereducing agent), the formation of stibine from Sb(V) was suppressed, while TMeSbO showed no signal depression. On the other hand, the use of 100 mg 1 1 fluoride in the absence of KI led to complete signal suppression for Sb(V) with apparent no influence on the signal intensity of Sb(III) and TMeSbO. While the concentration of TMeSbO was measured directly, Sb(III) and Sb(V) concentrations were calculated on the basis of the three analyzing steps, giving LoDs of 1.1, 1.2, and 1.4 pg l-1, respectively... 

Anode coatings used in chlorine/caustic soda cells may consist of baked layers containing ruthenium and iridium on a titanium substrate. These metals have been determined by fusing the coating with potassium hydroxide/potassium nitrate [73]. The melt was dissolved in hydrochloric acid and titanium added as a buffer for ruthenium (measurement at 349.9 nm) and potassium for iridium (measurement at 285.0 nm) to overcome signal depressions in the air/acetylene flame. 

FLIR devices can be divided in two classes by their detecting waveband. Mid-IR detectors, like the StarSAFIRE series mentioned above, are commonly used in humid environments since they show less signal depression by water vapour. However the detection of oil spills shows variable results and is not favourable as a stand-alone sensor for this purpose. Long-IR detectors (8-14 pm, sometimes realised by QWIP detectors with smaller... 

Among the most common chemical interferences in flame spectrometric methods, is the signal depression of alkaline earth metals in the presence of phosphate. This phenomenon occurs both in absorption and emission measurements. The interference effect is due to the formation of solid alkaline earth metal pyrophosphates which are difficult to vaporize. If calcium and phosphate solutions are simultaneously aspirated into the flame with two different nebulizers, signal depression is not observed. This is an indication that calcium phosphate does not form in the flame, but at an earlier stage of the process. The effect of phosphate can be removed by... 

Gas-phase Interferences. Virtually, all hydride forming elements interfere mutually. In some cases even very small concentrations of the interfering element cause severe signal depressions. The extent of the interference depends only on the concentration of the interfering element, not on the interferent-analyte ratio. After sufficient dilution of the sample solution these interferences do not appear. 

One of the most important factors involved in control of cation-anion interference effects is flame temperature. If compound formation occurs in the flame, as happens with calcium and the phosphate ion, a severe signal depression is observed. Use of high-temperature flames can minimize this effect. Flames that have been used successfully for this purpose are the nitrous oxide-acetylene flame and the premixed oxygen-acetylene flame. 

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