Compensating for interferences

Examine a procedure from Standard Methods for the Analysis of Waters and Wastewaters (or another manual of standard analytical methods), and identify the steps taken to compensate for interferences, to calibrate equipment and instruments, to standardize the method, and to acquire a representative sample. 

The entire measurement system is very compact (the volume of the measurement cell is 3 cm3) with a very small internal volume in the tubing and pump. Besides making the instrument very compact, it allows accurate measurements to be performed on even very small sample volumes. This, together with the outstanding stability of the PAS detector and the ability to compensate for interference, is the biggest advantage of the system. 

Differential measurement has been used as a method for compensating for interferences and fouling. This is particularly true where direct measurement is difficult or impossible, such as with miniaturized biosensors. This method consists of subtracting the signal of a control electrode from the signal of the active working electrode. The control... 

The instrument response was determined, after which additional elements were added successively and analyzed, and adjustments made to compensate for interferences caused by peak overlap and nonlinear count versus concentration relationships. 

Metal salts may be used in the treatment of wool. Flame methods for the determination of aluminium [185], barium, chromium, copper, mercury, strontium, tin, zinc [186] and zirconium [187] in wool have been published. Standard additions to wool cleaned by soaking and washing it with disodium EDTA (800 ml of 0.5 M for 30g wool with soaking for 3 days and double washing) was used as the calibration technique. This compensated for interferences from hydrochloric acid and amino-acids. The samples were equilibrated to a constant humidity for 24 h and then 0.3 g sealed with 5 ml of constant boiling point hydrochloric acid in a glass tube. The tubes were placed in an oven at 110UC for 20 h. The nitrous oxide/acetylene flame was used for the determination of aluminium and zirconium. Sulphate, phosphate, citrate and silicate have been found to interfere in the determination of titanium and zirconium in fire-proofed wool [188], These flame... 

The control systems in each atomic absorption instrument will have individual settings, and in all cases the manufacturer s instructions should be followed. Whichever instrument is used, a facility for background correction is mandatory in order to compensate for interference from the complex biological medium. However, background correction does have limitations when wavelengths below 300 nm are used. Another approach to trace... 

Signal processed to improve signal to noise ratio, compensate for interferences, correlate multiple sensors and convert to concentration units... 

The effect of iron, nickel, and vanadium on the signals obtained from antimony solutions in 50% sulfuric acid was investigated. Antimony signals from solutions containing 15 ng Sb/ml and 28.2 /xg/ml each of iron, nickel, and vanadium were suppressed by 14% compared with those obtained when no metals were added. Consequently, the method of standard additions was adopted to compensate for interferences of this type. 

Another almost trivial, but important, deviation from adherence to Beer s law is caused by mismatched ceils. If the cells holding the analyte and blank solutions are not of equal path length and equivalent in optical characteristics, an intercept k will occur in the calibration curve and A F.hc r k will be the actual equation instead of Equation 1.3-1. This error ean be avoided by using either carefully matched cells or a linear regres-.sion procedure to calculate both the slope and intercept of the calibration curve. In most cases linear regression is the best strategy because an intercept can also occur if the blank solution does not totally compensate for interferences. Another way to avoid the... 

The final step in a chemical analysis is calculation of results. This step may consist of a few simple calculations, or it may involve a complicated data processing operation that calculates analyte levels and compensates for interferences in the method. In addition to providing a number for the quantity or percentage of analyte in a sample, the calculation of results usually involves an evaluation of the reliability of the data (precision and accuracy) of the analytical values. In modem analytical laboratories, results are calculated and stored by computers, frequently as part of the process by which analyte levels are measured with an appropriate instrument. 

It compensates for interferences due to signal enhancement or suppression, but does not compensate for spectral interferences. For this reason, an external blank solution must always be run. 

Compensating for an interference in the sample s matrix is more difficult. If the identity and concentration of the interferent are known, then it can be added to the reagent blank. In most analyses, however, the identity or concentration of matrix interferents is not known, and their contribution to S stead, the signal from the interferent is included as an additional term... 

When the identity of the matrix interference is unknown, or when it is impossible to adjust the flame to eliminate the interference, then other means must be used to compensate for the background interference. Several methods have been developed to compensate for matrix interferences, and most atomic absorption spectrophotometers include one or more of these methods. 

Background correction is used to compensate for background absorption and scattering due to interferents in the sample. Such interferences are most severe for analytes, such as Zn, that absorb at wavelengths of less than 300 nm. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Despite the variety of methods that had been developed, by 1960 kinetic methods were no longer in common use. The principal limitation to a broader acceptance of chemical kinetic methods was their greater susceptibility to errors from uncontrolled or poorly controlled variables, such as temperature and pH, and the presence of interferents that activate or inhibit catalytic reactions. Many of these limitations, however, were overcome during the 1960s, 1970s, and 1980s with the development of improved instrumentation and data analysis methods compensating for these errors. ... 

Miscellaneous Methods At the beginning of this section we noted that kinetic methods are susceptible to significant errors when experimental variables affecting the reaction s rate are difficult to control. Many variables, such as temperature, can be controlled with proper instrumentation. Other variables, such as interferents in the sample matrix, are more difficult to control and may lead to significant errors. Although not discussed in this text, direct-computation and curve-fitting methods have been developed that compensate for these sources of error. ... 

Don t use a flexible coupling to compensate for misalignment between the pump and motor shafts. The purpose of the flexible coupling is to compensate for temperature changes and to permit some axial movement of the shafts without interference, while they transfer energy from the motor to the pump. 

All instruments are limited in specificity of response to the contaminant they are supposed to measure. Interferences may cause false readings and, as pointed out earlier, care must be taken to identify (and, if possible, compensate for) these interferences. 

A more difficult criterion to meet with flow markers is that the polymer samples not contain interferents that coelute with or very near the flow marker and either affect its retention time or the ability of the analyst to reproducibly identify the retention time of the peak. Water is a ubiquitous problem in nonaqueous GPC and, when using a refractive index detector, it can cause a variable magnitude, negative area peak that may coelute with certain choices of totally permeated flow markers. This variable area negative peak may alter the apparent position of the flow marker when the flow rate has actually been invariant, thereby causing the user to falsely adjust data to compensate for the flow error. Similar problems can occur with the elution of positive peaks that are not exactly identical in elution to the totally permeated flow marker. Species that often contribute to these problems are residual monomer, reactants, surfactants, by-products, or buffers from the synthesis of the polymer. 

Obviously, the lowering in intensity of the laser beam incident through the beam expander must be compensated for by increasing the source output power. However, it has been found that the intensity loss when using the beam expander is less than that encountered with interference filters. 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

Basel et al. [23] have described methods of compensating for chloride, ammonia, and bicarbonate interferences in determining bromide in sabne waters with an automated segmented flow analyser utihsing the phenol red method. 

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