Nebulization Interferences

Interference effects in plasma atomic emission spectrometry comprise (1) nebulization interferences, (2) chemical interferences, (3) ionization interferences, and (4) spectral interferences. The degree of interference varies from one different plasma source and spectrometer used to another. However, the most significant impediment to the effective use of any plasma-AES equipment is spectral interference. 

Nebulization interferences are observed if the quantity of the sample nebulized varies considerably as a function of time. Uneven nebulization may be caused by matrix salts or organic compounds and solvents by alterations in the viscosity, in the surface tension, or in the solution density. This may also occur to some extent for solutions with high mineral acid concentrations. 

Apart from this, memory effects may exist if long tubes and large vessel surfaces are used in the nebulization and desolvation system. This type of interference is not the fault of the plasma source, and can be easily avoided and controlled. 

Interferences caused by desolvation are related to differences in volatility between the matrix and analyte, and are noticed as a change of the analyte concentration in the aerosol mist during desolvation. 

The interference by copper on the determination of iodide is explained by the following reaction taking place in the nebulization chamber  

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The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

The concept of selectivity and specificity has been applied to characterize interferences appearing in two different ICP-MS techniques (Horn [2000]). Classical ICP-MS with pneumatic nebulization and ETV-ICP-MS are compared for the determination of traces of zinc in sea-water. Whereas spectral interferences decrease using the ETV device, nonspectral interferences increase significantly (Bjorn et al. [1998]). A quantitative comparison of the both analytical procedures, here called PN (pneumatic nebulization) and ETV (electrothermal vaporization, Sturgeon and Lam [1999]) is possible by means the specificity as a function of the Zn concentration (Horn [2000]). The spectral interferences on the four zinc isotopes are listed in Table 7.4. 

Bjorn E, Freeh W, Hoffmann E, Liidke C (1998) Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction. Spectrochim Acta 53B 1766... 

Chemical interferences are the result of problems with the sample matrix. For example, viscosity and surface tension affect the aspiration rate and the nebulized droplet size, which, in turn, affect the measured absorbance. The most useful solution to the problem is matrix matching, matching the matrix... 

Greenfield ef. ai.l l) observed a reduction of signal intensity that correlates with sample intake effects from the modified solution viscosity and/or surface tension of mineral acids. This, coupled with peristaltic pumping of solutions into the nebulizer, considerably reduces physical interferences. Increased salt concentration also has an effect on solution physical properties. In the experience of these authors, the high levels of salt in the matrix also increases the noise from the nebulizer system. This degradation of nebulizer performance, which is not necessarily accompanied by a proportional reduction in sensitivity, is the cause of the observed deterioration of detection limits in real samples as opposed to ideal solutions. 

ICP-AES was validated for the simultaneous determination of Al, B, Ba, Be, Cd, Co, Cr, Cu, Fe, Li, Mn, Ni, Pb, Se, Sr and Zn in human serum in a clinical laboratory. The samples underwent digestion and yttrium was used as an internal standard. The LOD were as follows 0.002-0.003 (xM for Ba, Cd, Mn and Sr 0.014-0.07 (xM for Be, Co, Cr, Cu, Fe, Li, Ni, Pb and Zn and 0.2-0.9 (xM for Al, B and Se. The concentrations of Al, Be and Co in human serum were found to be above the LOD, while those of Cd, Cr, Ni and Pb were below the LOQ however, in case of acute intoxication with the latter elements the method is valid . Matrix effects were evaluated for ICP-AES analysis using solution nebulization and laser ablation (LA) techniques. The main matrix-related interferences stem from elements with a low second ionization potential however, these are drastically reduced when pure He is used as carrier gas. This points to Ar (the usual carrier) participation in the interference mechanism, probably by interacting with doubly charged species. ... 

Ideally, the standards should be made up in a solution containing the same normally expected levels of matrix elements as occur in the sample solution. It should be borne in mind that even if they exert no chemical interference, they could possibly exert a viscosity effect on a nebulized solution (especially with high concentrations of phosphoric or sulphuric acids). If it is not possible to determine the matrix components or prepare standards in a matrix solution, and unless experiments have shown matrix interference to be insignificant, then the method of standard additions, or spiking, should be carried out. This is where known amounts of the analyte are added to the sample or sample solution before determination by, e.g. AAS or colorimetry. 

Interference is defined as an effect causing a systematic deviation in the measurement of the signal when a sample is nebulized, as compared with the measure that would be obtained for a solution of equal analyte concentration in the same solvent, but in the absence of concomitants. The interference may be due to a particular concomitant or to the combined effect of several concomitants. A concomitant causing an interference is called an interferent. Interference only causes an error if not adequately corrected for during an analysis. Uncorrected interferences may lead to either enhancements or depressions. Additionally, errors may arise in analytical methods in other ways, e g. in sample pretreatment via the... 

Since AAS is a ratio method, many instrumental errors (e.g. long-term source drift, small monochromator drifts) should cancel out, as 7 is ratioed to I . However, a stable uptake rate, or aspiration rate, is required. This falls as the viscosity of the solution sprayed is increased. Nebulizer uptake interferences can be minimized if the dissolved salts content of samples and standards is approximately matched. For example, when determining pg cm sodium levels in 2 M phosphoric acid, ensure that the standards are also dissolved in 2 M phosphoric acid, using a blank to check for contamination. 

In addition to conventional aspiration, using a nebulizer and spray chamber, samples may be introduced in to atomic spectrometers in a number of different ways. This may be because a knowledge of speciation (i.e. the organometallic form or oxidation state of an element) is required, to introduce the sample while minimizing interferences, to increase sample transport efficiency to the atom cell or when there is a limited amount of sample available. 

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