mAtHemAtiCAL equAtion CorreCtion to SPeCtrAL AnD trAnSPort interFerenCeS in HigH-reSoLution Continuum SourCe FLAme AtomiC ABSorPtion SPeCtrometrY: DeterminAtion oF LeAD in PHoSPHoriC ACiD

In this work, a new mathematical equation correction approach for overcoming spectral and transport interferences was proposed. The proposal was applied to eliminate spectral interference caused by PO molecules at the 217.0005 nm Pb line, and the transport interference caused by variations in phosphoric acid concentrations. Correction may be necessary at 217.0005 nm to account for the contribution of PO, since Atotal 217.0005 nm = A Pb 217.0005 nm + A PO 217.0005 nm. This may be easily done by measuring other PO wavelengths (e.g. 217.0458 nm) and calculating the relative contribution of PO absorbance (APO) to the total absorbance (Atotal) at 217.0005 nm: A Pb 217.0005 nm = Atotal 217.0005 nm A PO 217.0005 nm = Atotal 217.0005 nm k (A PO 217.0458 nm). The correction factor k is calculated from slopes of calibration curves built up for phosphorous (P) standard solutions measured at 217.0005 and 217.0458 nm, i.e. k = (slope217.0005 nm/slope217.0458 nm). For wavelength integrated absorbance of 3 pixels, sample aspiration rate of 5.0 ml min-1, analytical curves in the 0.1 – 1.0 mg L-1 Pb range with linearity better than 0.9990 were consistently obtained. Calibration curves for P at 217.0005 and 217.0458 nm with linearity better than 0.998 were obtained. Relative standard deviations (RSD) of measurements (n = 12) in the range of 1.4 – 4.3% and 2.0 – 6.0% without and with mathematical equation correction approach were obtained respectively. The limit of detection calculated to analytical line at 217.0005 nm was 10 μg L-1 Pb. Recoveries for Pb spikes were in the 97.5 – 100% and 105 – 230% intervals with and without mathematical equation correction approach, respectively.

. However both main and secondary lines for Pb are interfered if variations in phosphoric acid concentrations occur, changing the aspiration rate of the nebulizer.
Correction of spectroscopy interferences by mathematical equations in inductively coupled plasma mass spectrometry (ICP-MS) technique has already been established [5,6].Nevertheless the adaptation of such mathematical equations to atomic absorption spectrometry for spectral interference correction was not feasible due to the lack of a truly simultaneous multi-element system.Recently and upon the introduction of a highresolution continuum source atomic absorption spectrometry (HR-CS AAS) [7], this problem has been overcome as the whole spectral environment around the analytical line can be acquired [8,9].Using this technique, the interference caused by PO molecules at the main line for Pb at 217.0005 nm may be eliminated using the least-squares background correction (LSBC) [1].The alternate line at 283.3060 nm is unaffected by PO, but both main and secondary lines are interfered by transport effects.A method using the LSBC plus internal standardization was recently proposed to minimize these interferences in the determination of Pb in phosphoric acid by HR-CS FAAS [1].
In this paper, a new approach to overcome spectral and transport interferences associated with the determination of Pb in phosphoric acid by HR-CS FAAS has been proposed.In principle, spectral overlap can be corrected providing that another wavelength of the interfering element is itself free from interference.The feasibility of using mathematical equation correction previously employed in ICP-MS, to eliminate spectral interference caused by PO molecules at the 217.0005 nm Pb line and transport interference caused by variations in phosphoric acid concentrations in the determination of Pb in phosphoric acid has been investigated.

materials and methods instrumentation
All measurements were carried out using an Analytik Jena ContrAA 300 high-resolution continuum source flame atomic absorption spectrometer equipped with a xenon short-arc lamp XBO 301 [10] with a nominal power of 300 W operating in a hot-spot mode as a continuum radiation source.This new equipment presents a compact high-resolution double-Echelle grating monochromator correspondent to a spectral band width < 2 pm per pixel in the far ultraviolet range and a charge-coupled device (CCD) array detector.
High-purity (99.7%) acetylene (Air Liquid, Brazil) was used as fuel gas.Air-acetylene oxidizing flame was used for analyte atomization.Solutions were directly aspirated by the burner / nebulizer system of the spectrometer using an injection module (SFS 6) enabling the computercontrolled aspiration of blanks, analytical solutions and samples.

Reagents, analytical solutions and samples
High purity de-ionized water (resistivity 18.2 MΩ cm) obtained using a Millipore Rios 5 ® reverse osmosis and a Millipore Milli-Q Academic ® deionizer system (Bedford, MA, USA), and Merck Suprapur ® nitric acid (Darmstadt, Germany) were used throughout to prepare all solutions.All chemical reagents used were of analytical grade.
Additionally, it should be stressed that the line for PO at 217.0458 nm is within the spectral window for Pb at 217.0005 nm (Figure 1).The ratio between slopes of curves for PO (slope 217.0005 nm /slope 217.0458 nm ) was calculated daily taking into consideration that changes in the operating parameters alter the measured absorbance.However, ratios obtained in different days were usually within 1.23 -1.29, but were not significantly different at the 95% confidence level (paired t-test).

Analysis of test solutions and samples
The feasibility of the mathematical correction for spectral and transport interferences was verified in the determination of Pb in test solutions and two phosphoric acid samples (Table 1).Results for samples were in agreement at a 95% confidence level (paired t-test) with those obtained by HR-CS FAAS using LSBC plus internal standardization.Precision and accuracy studies were also carried out using test solutions, or else, recovery experiments for diluted [1% -7.5% (m/v)] phosphoric acid samples spiked with 100 and 400 μg L -1 Pb.

Figure 1 .
Figure 1.Spectrum recording for spiked Pb in phosphoric acid.(A) and (B) correspond to lines at 217.0005 nm and 217.0458 nm, respectively.