31 The type of sample matrix can influence the detection capabilities, which can in turn affect the degree of ionisation that will occur in the plasma or allow the formation of species that may interfere with the analyte determination (Jarvis et al., 1992). The speed, accuracy and multi elements that can be determined in a single analysis make the ICP-MS a very cost-effective technique. The advantage using plasma compared to other ionisation methods, such as flame ionisation, are that complete ionisation is guaranteed and as such prevents oxide formation since it occurs in a chemically inert environment.
Additionally, the benefits associated with the use of ICP-MS include the ability to perform speciation studies when coupled with separation techniques such as HPLC.
The technique enables measurement of isotopic composition in nutritional studies and to identify sources of environmental exposure (Hill, 2007; Petridis et al., 2014).
Consequently, the ICP-MS limitations are based on the amount of total dissolved solids in the samples of interest. High total dissolved solids cause instrumental drifts, decreased sensitivity and detection capability due to the blockages that may occur in the apertures of the cones. To account for this, proper dilutions should be carried out prior sample analysis to minimise the high levels of total dissolved solids (McCurdy and Proper, 2014).
2.7. ANALYTICAL PERFORMANCE CHARACTERISTICS FOR DETERMINATION
32 Subsequently, an internal standard is used for calibration by plotting the ratio of the analyte signal to the internal standard signal as a function of the analyte standard concentration to correct analyte losses during sample preparation (Zenkevich and Makarov, 2007). The calculated ratio of the analyte signal and the internal standard signal for the samples is then used to determine the concentration of the analyte from a calibration curve.
Furthermore, an internal standard should have a similar trend to the response of an analyte (Skoog et al., 1998). An internal standard with mass number close to that of the analyte in study, improves the precision. The choice of an internal standard should be based on the analyte in study.
The use of internal standard methods is based on condition that one internal standard will not be adequate for a wide range of analytes varying in atomic masses (Vanhaecke et al., 1992; Bechlin et al., 2014). Such use accounts for the correction of the instrument fluctuations and signal drifts such as signal enhancement and reduction by sample matrix mismatch. For ICP-AES, the choice of an internal standard is based on the existence of similar excitation energies for the internal standard and the analyte of interest and the absence of spectral interferences between an analyte and an internal standard (Chiweshe et al., 2016).
Atomic weight, ionisation energy closely to the analyte and the absence of isotopes of internal standard in the sample are some of the selection parameters on the choice of an internal standard in ICP-MS (Sartoros and Salin, 1999).
2.7.1.2 External calibration
In an external calibration method, a known data from a calibration standard and unknown data from the sample are combined to generate a quantitative report. The external calibration method is advantageous since it is simple to perform and is applicable to different analysis. From the responses, a plot of concentration versus detector signal is made for each respective analyte. Linearity of the plot allows easier statistical calculations and extrapolation to estimate the concentration levels of analytes in samples. This linearity is verified by performing a linear regression R2=1 on the data set obtained. The closer the R2 value to 1, the more accurate the data is (Harris, 2007).
33 2.7.2 Limit of detection (LOD)
The limit of detection (LOD) is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limited generally to 1%. The LOD of an analytical method tells how low a concentration can be said to be measurable (Proctor, 2008). The LOD is estimated from the mean of the blank, and the standard deviation of the blank (MacDougall and Crummett, 1980).
For methods that employ a calibration curve, the LOD is defined as the analyte concentration yielding a response of concentration factor k higher than the standard deviation of the blank (Sb). The LOD can be calculated as three times the value of the standard deviation of the blank concentration values (Ingle and Crouch, 1988). The LOD formula was reported as:
LOD= 3Sb1 Equation…..2
Where 3Sb1 is three times the standard deviation of the blank concentration.
2.7.3 Limit of quantification (LOQ)
Limit of quantification (LOQ) is the lowest concentration at which the analyte in a sample is detectable at good precision and accuracy.
The LOQ is expressed as the analyte concentration which corresponds to the blank sample plus 10 times the standard deviation of the results for a series of replicates which are used to determine the LOD. It describes the lowest detectable concentration of the analyte measured by the employed analytical sample preparation procedure.
The distinguished equation used for the calculation of LOQ was reported by Shrivastava and Gupta (2011):
LOQ= 10Sb1 Equation…..3
Where 10Sb1 is ten times the standard deviation of the blank concentration.
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