Sample transport interferences

The acid concentration in the final solution being presented to the instrument should ideally be 2-3% maximum because of the sample transport interferences associated with high concentrations of mineral acids. 

During method development, special attention must be given to correct for matrix and spectral interferences. Matrix suppression and sample transport interferences are compensated very well by the selection of suitable internal standards, which are matched to the ionization properties of the analyte elements. This is a routine and... 

During method development, special attention must be given to correct for matrix and spectral interferences. Matrix suppression and sample transport interferences are... 

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. 

Physical (transport) interferences. This source of interference is particularly important in all nebulisation-based methods because the liquid sample must be aspirated and transported reproducibly. Changes in the solvent, viscosity, density and surface tension of the aspirated solutions will affect the final efficiency of the nebulisation and transport processes and will modify the final density of analyte atoms in the atomiser. 

The choice of the appropriate mineral acid and oxidant depends on the final analytical technique to be used. For instance, HC1 is not recommended for furnace analysis as it can cause Cl interferences, while H2S04 is not desirable with ICP-AES or ICP-MS because of transport interferences derived from its viscosity. With ICP-MS HNO3 and H202 are preferred since the effect of polyatomic interferences is minimal as compared with HC1, HC104 and H2S04, which introduce polyatomic ions such as C10+ and SO+ [28-32]. In any case, in order to minimize corrosion of the metal sampler and skimmer cones with ICP-MS, final sample solutions should not contain high acid concentrations (e.g., above 10 percent for HNO3). 

Many examples are known where the FIA technique is used for sample transport only and an example of this is where sample contains a concentration of interfering matrices. These samples can be injected in very small volumes (10 to 100 pi) into a carrier stream to minimise these interferences due to excessive dilution. Standard addition and internal standard methods can equally be applied to FIA techniques to reduce matrix, spectral and other potential interfering effects. Ion exchange columns connected in the sample feed... 

Inductively coupled plasma atomic emission spectrometry (ICP-AES) involves a plasma, usually argon, at temperatures between 6000 and 8000 K as excitation source. The analyte enters the plasma as an aerosol. The droplets are dried, desol-vated, and the matrix is decomposed in the plasma. In the high-temperature region of the plasma, molecular, atomic, and ionic species in various energy states are formed. The emission lines can then be exploited for analytical purposes. Typical detection limits achievable for arsenic with this technique are 30 J,g As/L (23). Due to the rather high detection limit, ICP-AES is not frequently used for the determination of arsenic in biological samples. The use of special nebulizers, such as ultrasonic nebulization, increases the sample transport efficiency from 1-2% (conventional pneumatic nebulizer) to 10-20% and, therefore, improves the detection limits for most elements 10-fold. In addition to the fact that the ultrasonic nebulizer is rather expensive, it was reported to be matrix sensitive (24). Inductively coupled plasma atomic emission spectrometry is known to suffer from interferences due to the rather complex emission spectrum consisting of atomic as... 

Among the nonspectral interferences transport interferences in the nebulizer are relatively common in the analysis of body fluids. This is certainly no problem when 10- or 20-fold diluted serum is used for the determination of the electrolytes. If, however, undiluted or only slightly diluted body fluids are aspirated directly, the viscosity of these liquids can impair aspiration rate and nebulization efficiency relative to the reference solutions used. If the sample solution cannot be diluted sufficiently to avoid this interference, a frequently used alternative is matrix-matched standards, i.e., reference solutions with a viscosity close to that of the samples. Another alternative is to use the method of additions, which can perfectly correct for this interference. This calibration technique, however, is labor-intensive and time consuming, and is restricted to the linear part of the calibration curve. Viscosity of the sample solutions is much less of a problem when FI techniques are used for sample introduction. This is because samples are not aspirated but pumped to the nebulizer, because much smaller sample volumes are used, and because the sample is always in a carrier solution which supports nebulization and removes all potential residues in the nebulizer-bumer system. 

Another group of elements which is frequently determined by FAAS is copper, iron, and zinc in tissues and body fluids, and total iron-binding capacity. Concentrations of these elements are significantly lower compared to the electrolytes, so that samples cannot be diluted very much. This means that the viscosity of the measurement solutions must be carefully controlled in order to avoid transport interferences. A recent review article by Taylor [7] gives a concise overview over clinical applications of FAAS. 

One of the advantages of ETAAS is that it is fiee from transport interferences because usually no nebulizers are used for sample introduction. A measured volume of sample is deposited on the platform in the atomizer by an autosampler, so that sample viscosity and other physical parameters do not play a significant role. The major sources of errors in ETAAS are loss of the analyte element by volatilization in the pyrolysis stage prior to atomization and the formation of stable compounds with concomitants in the gas phase. 

Electrothennal evaporation (EXE) in addition to its advantages for microanalysis and improving sample transport, has the additional advantage in ICP-MS of introducing a dry analyte vapor into the plasma. Accordingly, it has been found useful for elements of which the detection limits are high as a result of spectral interference with cluster ions. In the case of Fe, which is interfered by °ArO. Park et al. [311] showed that the detection limit could be considerably improved by ETE. For similar reasons, the direct insertion of samples in ICP-MS maximizes the absolute power of detection [312], [313]. 

Let us now look at the other class of interference in ICP-MS—suppression of the signal by the matrix itself. There are basically three types of matrix-induced interferences. The first and simplest to overcome is often called a sample transport effect and is a physical suppression of the analyte signal, brought on by the level of dissolved solids or acid concentration in the sample. It is caused by the sample s impact on droplet formation in the nebulizer or droplet size selection in the spray chamber. 

The previously described space charge effect phenomenon offers an explanation for many of the observed sample matrix interference effects in ICP-MS.The masses of both the analyte element and the matrix are important. The transport of analyte species is suppressed more by heavy matrix ions than light ones, and heavy analyte ions are suppressed less severely than light analyte ions. 

Other types of physical interference effects include those affecting the nebulization/sample introduction process. These sample transport effects, which result from differences in viscosity, surface tension, and volatility, can be minimized by dilution.The use of a deHvery pump to transport the sample solution to the nebulizer will normalize these effects to a limited extent. Whenever mineral acids are used in sample decomposition or preservation, equivalent quantities added to the calibration standards compensate for differences in solution properties and hence minimize sample transport effects. 

---