MONITORING METHODOLOGY
6.4 Chemical Analyses of Samples using ICP-OES
The ICP-OES was used for the analysis of six metals (Mo, Mn, Fe, Na, Zn, P and Cr) present in solution in Company A's phosphating line. The ICP-OES was used for this analysis based on its ability to rapidly analyse several metals simultaneously. It also offers the advantage of low chemical interference, unlike techniques such as atomic absorption spectroscopy.181 In Section 6.4.1 how the instrument works is described, while in Sections 6.4.2 and 6.4.3 how the standards and samples were prepared for the analysis is described.
6.4.1 Principles of Inductively Coupled Plasma Optical Emission Spectrophotometry (ICP-OES)
The apparatus consists of two main components: an ICP source and a spectrometer. There are two types of spectrometers that can be used: a polychromator or a monochromator.
For the purpose of elemental analysis in this project, the sequential type of monochromator was used. The ICP source consists of a radio frequency generator that produces an operating power of at least 1.10 kW. Other components include a torch, coil, nebulizer, spray chamber and a drain, whose operating conditions are specified in Table 6.4. The torch can be mounted in two ways, either in an axial position or in a radial position. The axial position generates a higher light intensity while the radial position requires less maintenance and consumes less argon. The radial position is used for normal analysis and complex materials, while the axial position is used for the analysis elements with lower detection limits.182 The technique is based upon the ionization of a flowing stream of argon gas by an applied oscillating radio frequency field, which is inductively coupled to the ionized gas by a water-cooled coil. The coil is wound around a quartz torch that confines the plasma. The liquid sample is pumped to the nebulizer and generates aerosols in the spray chamber. This aerosol is then injected into the ICP, at temperatures of between 6000 and 8000 K. These high temperatures perform a dual function: they ionize the atoms, producing an emission spectrum, and they reduce chemical interferences by completely dissociating the molecules of compounds formed.
The light emitted is then focused onto the monochromator for detection.
Spectral interferences can occur and may alter the net signal intensity. These interferences are caused by ion-atom recombination, spectral line overlaps, molecular band emission, or stray light. Spectral interferences can be overcome by choosing appropriate analytical wavelengths and by making background corrections. This instrument was the most appropriate to use for this project as it was used for multi- elemental analysis and has the appropriate detection limits required. In this project the concentrations of seven elements were determined and the detection limit of 0.001 mg/L for Fe, Mn and Zn was low enough to measure these concentrations. The operating conditions of the ICP-OES used in this analysis for this project, are summarized in Table 6.4.
Table 6.4 Specifications and operating conditions for ICP-OES Instrument
Torch Mounting Nebuliser
Operating Power Nebuliser Pressure Photomultiplier Voltage Plasma Argon Flow Auxiliary Argon Flow Pump Speed
Varian Liberty 150AX Turbo Axial
Pneumatic (Concentric) LlOkW
240 kPa 800 v 15.0L/min 1.50L/min 15.0 rpm
6.4.2 Preparation of Standards
For the ICP-OES analysis, seven calibration standards were prepared from a mixed stock standard solution containing Mo, Mn, Fe, Zn, Na, and P. These standards covered a wide range of concentrations of these six elements. These are represented in Tables 6.5 to 6.6.
A second set of standards was prepared, for Cr analysis only and is shown in Table 6.7.
The mixed stock solution was prepared from 1000 mg/L Mo, Mn, Fe, Zn and P solutions purchased from Fluka. The Na solution was prepared using NaCl, supplied by
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Riedel-de Haen, which was dried overnight in an oven set at 110 °C and left to cool in a dessicator. The mass of NaCl, required to obtain a 1000 mg/L (which equals to mg/L) solution of Na, was calculated.
C Na (required) = 1000 mgL"1 VNa = 250 mL = 0.25 L
The concentration of 1000 mgL"1 is required. The volume the of the flask used to prepare the standard solution was 250 mL. Then using Equation 6.1, the mass of Na required, was calculated.
MN a =CN aX VN a Equation 6.
MNa (required) = lOOOmgL"1 x 0.25L M Na (required) = 1 OOOmgL"1 x 0.25L
= 250mg
Using the molar mass ratios of Na and NaCl, the mass of NaCl required could be calculated. This is shown by equation 6.2.
M = M x NaCI Mr
l v lN a C l mN a * Mr M
M
NaCl
NaCl
250 mgx
Na
58.44 22.99 635.5 mg = 0.635 g
Equation 6.2
Table 6.5 Preparation of mixed stock solution from proprietary 1000 mg/L solutions Metal
P Na Zn Mo Mn Fe
Purchased Solution Cone 1000 1000 1000 1000 1000 1000
Mixed Stock Solution Cone 400 200 80 80 80 80
From this mixed stock, seven working calibration standards were prepared. The concentrations of the six elements in each standard are shown in Table 6.6.
Table 6.6 Contents and concentration (mg/L) of the seven working calibration standards
Metal P Na Zn Mo Mn Fe
Concentration Stdl
0.8 0.4 0.16 0.16 0.16 0.16
Std2 4 2 0.8 0.8 0.8 0.8
Std3 8 4 1.6 1.6 1.6 1.6
Std4 20 10 4 4 4 4
Std5 40 20 8 8 8 8
Std6 80 40 16 16 16 16
Std7 200 100 40 40 40 40
Table 6.7 Preparation of Cr stock solution (standard 8) and three working standards Mixed Stock Solution (Standard 8)
Cone (mg/L) 5
Standard 9 1
Standard 10 0.2
Standard 11 0.04 Two calibrations were performed for each analysis: one before the analysis and one after the analysis. These calibration curves are presented in Appendix A.
6.4.3 Preparation of Samples
All samples obtained were filtered for ICP-OES analysis using 20 ml plastic syringes and 0.45 u.m nylon filters manufactured by Anatech Instruments (Pty) Ltd. Many samples had to be diluted by an appropriate dilution factor to ensure that the concentrations of the metal ions were within the calibration range. These dilution factors ranged from 4 to 2000. The samples were stored in a refrigerator at a temperature of approximately 8 °C.
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