Chemical and Ionization Interference

Chanical interferences are rare in plasma anission spectroscopy because of the efficiency of atomization in the high-tanperatnre plasma. Eor example, in EAAS, there is a severe suppression of the calcinm signal in samples containing high amounts of Al, as seen in Eigure 7.35. 

The same figure shows the lack of chemical interference in the ICP emission signal for Ca in solutions containing Al np to very high Al/Ca ratios. In the few cases where chanical interference is fonnd to occnr, increasing the RF power to the plasma and optimizing the Ar flow rates usually 

The alkali metals Li, Na, and K are easily ionized in flames and plasmas. Concentrations of an EIE greater than 1000 ppm in solution can result in suppression or enhancanent of the signals from other analytes. The EIE effect is wavelength dependent ion lines are affected differently than atom lines. The EIE effect can be minimized by optimizing the RF power and plasma conditions, by matrix matching, by choosing a different wavelength that is not subject to the EIE effect, or by application of mathematical correction factors. 

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Conventional AA use a liquid sample. The determination of several elements per sample is slow and requires larger volumes of solution due to the sequential nature of the method. Chemical and ionization interferences must be corrected by modification of the sample solution. 

ICP offers good detection limits and a wide linear range for most elements. With a direct reading instrument multi-element analysis is extremely fast. Chemical and ionization interferences frequently found in atomic absorption spectroscopy are suppressed in ICP analysis. Since all samples are converted to simple aqueous or organic matrices prior to analysis, the need for standards matched to the matrix of the original sample is eliminated. 

Whereas the incidence and extent of physical, chemical, and ionization interferences is similar in FES and flame AAS and AFS, spectral interferences are significantly different in the three techniques, and must be considered technique by technique. 

Generally physical, chemical, and ionization interferences are similar in terms of incidence and extent in all three flame analytical atomic spectrometric techniques, but they are not a severe problem provided the analyst is aware of their existence, and takes the necessary precautions. Spectral interferences are not regarded as a serious problem in flame AAS or flame AFS, but are potentially much more serious in FES. Unless the analyst is certain that a particular FES determination is spectral interference-free for the samples in question, scanning and careful scrutiny of emission spectra from samples and standards is advisable, together with reliability checks using certified reference materials and/or determination at more than one wavelength. 

The plasma sources can achieve temperatures in the range of 5000 to 10 000 K which are advantageous over flame emission that can only achieve temperature ranges of 1500 to 2500K. The flame in AAS lends itself to self absorption, spectral, chemical and ionization interferences which gives rise to noisy background. These interferences including ionisation are not very severe in plasmas because the extra electrons released by EIEs have little effect on the ionisation equilibrium of other elements and the extra electrons form a small portion of the total electron concentration in the plasmas. 

In atomic spectroscopy, absorption, emission, or fluorescence from gaseous atoms is measured. Liquids may be atomized by a plasma, a furnace, or a flame. Flame temperatures are usually in the range 2 300-3 400 K. The choice of fuel and oxidant determines the temperature of the flame and affects the extent of spectral, chemical, or ionization interference that will be encountered. Temperature instability affects atomization in atomic absorption and has an even larger effect on atomic emission, because the excited-state popula-... 

Chemical, physical, and ionization interferences are examples of analyte interferences. These influence the magnitude of the analyte signal itself. 

FAAS is subject to certain interferences associated with the nature of biological specimens. Mechanisms of the more important ionization, chemical and matrix interferences (Table 3) are discussed elsewhere. 

Minimizing Chemical Interferences The quantitative analysis of some elements is complicated by chemical interferences occurring during atomization. The two most common chemical interferences are the formation of nonvolatile compounds containing the analyte and ionization of the analyte. One example of a chemical interference due to the formation of nonvolatile compounds is observed when P04 or AP+ is added to solutions of Ca +. In one study, for example, adding 100 ppm AP+ to a solution of 5 ppm Ca + decreased the calcium ion s absorbance from 0.50 to 0.14, whereas adding 500 ppm POp to a similar solution of Ca + decreased the absorbance from 0.50 to 0.38. These interferences were attributed to the formation of refractory particles of Ca3(P04)2 and an Al-Ca-O oxide. 

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. 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

Explain what is meant by spectral, chemical, ionization, and isobaric interference. 

In atomic absorption spectroscopy (AAS) both ionization and chemical interferences may occur. These interferences are caused by other ions in the sample and result in a reduction of the number of neutral atoms in the flame. Ionization interference is avoided by adding a relatively high amount of an easily ionized element to the samples and calibration solutions. For the determination of sodium and potassium, cesium is added. To eliminate chemical interferences from, for example, aluminum and phosphate, lanthanum can be added to the samples and calibration solutions. 

Obviously, chemical induction is of great interest, because, on one hand, it allows the induction and acceleration of non-spontaneous reaction and, intrinsically, remains the unique method by which to affect such reactions (except for reactions proceeding under the influence of photochemical and ionizing radiation). On the other hand, chemical induction plays a significant role in biochemical processes. The literal translation from Latin term interference is mutual (inter) collision (ferio), which shows the total situation. 

Strontium. Strontium like Rb and Pd was included in anticipation of a standard. Strontium may be subject to more ionization and chemical interferences than other alkaline earths. Thorough studies of interfernces from mineral acids, HC1, HNO, and l SO are reported (3) as well as from Al, Si, and other ionization enhancement elements. Sr is strongly ionized, 84%, in nitrous oxide/acetylene the addition of 1000 ppm Cs is very important in suppressing Al, Si, and other interferences that cause enhancement. The 460.7 nm line is significant for AAS analysis of Sr. 

The two common sources of analytical error are interference during the analytical measurement and contamination during sample preparation. During the analytical measurement, potential interferences include, matrix effects chemical interferences, ionization interferences, and spectral interferences. 

Only chemical interferences were observed sodium and potassium ionized in the air-acetylene flame, and aluminum ionized in the nitrous oxide-acetylene flame magnesium and calcium exhibited evidence of interference by both phosphorus and aluminum. All the other elements were found to be interference-free. The addition of 1000 ppm of cesium as an ionization suppressor effectively removed the ionization interference in the sodium and potassium solutions. Similarly, 1000 ppm of lanthanum removed the interference due to phosphorus and aluminum in the magnesium and calcium solutions and suppressed the ionization of aluminum. 

Analyte Atomization and Ionization By the time the analyte atoms and ions reach the observation point in the plasma, they have spent about 2 ms in the plasma at temperatures ranging from 6000 to 8000 K. These times and temperatures are two to three times greater than those attainable in the hottest combustion flames (acetylene/nitrous oxide). As a consequence, desolvation and vaporization are essentially complete, and the atomization efficiency is quite high. Therefore, there are fewer chemical interferences in ICPs than in combustion flames. Surprisingly, ionization interference effects are small or nonexistent because the large concentration of electrons from the ionization of argon maintains a more-or-less constant electron concentration in the plasma. 

The temperatures are high, which favors the formation of atoms and ions. Sample residence times are long so that desolvation and vaporization are essentially complete. The atoms and ions are formed in a nearly chemically inert environment. The high and relatively constant electron concentration leads to fewer ionization interferences. 

The occurrence of molecular absorbance and scatter in AAS can be overcome by the use of background correction methods. Various types of correction procedures are common, e.g. continuum source, Smith-Hieftje and the Zeeman effect. In addition, other problems can occur and include those based on chemical, ionization, physical and spectral interferences. 

Several modifications of MALDI have been developed to couple additional sampling and reaction capabilities to this technique. Surface-enhanced laser desorption ionization (SELDI) is one type of modified MALDI and describes an ionization process that involves reacting a sample with an enhanced surface. With SELDI, the sample interacts with a surface modified with some chemical functionality prior to laser desorption ionization and mass analysis. For example, an analyte could bind with receptors or affinity media on the surface, and be selectively captured and sampled by laser desorption. A SELDI surface can be modified for chemical (hydrophobic, ionic, immunoaffinity) or biochemical (antibody, DNA, enzyme, receptor) interactions with the sample. This technique can act as another dimension of separation or sample cleanup for analytes in complex matrices. As discussed before, one disadvantage of MALDI is that the matrix (usually a substituted cinnamic acid) that is mixed with the sample can directly interfere with the analysis of small molecules. There have been several areas of research to overcome this issue.Direct ionization on silicon (DIOS) is an example of a modification of MADLI that eliminates the matrix. In this case, analytes are captured on a silicon surface prior to laser desorption and ionization. Other examples of matrix-free laser desorption techniques include the use of siloxane or carbon-based polymers. 

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