Measurement and Results - Lead in a Manganese Matrix

I am analyzing a manganese rich material for trace metals content. One of the analytes of interest is lead (Pb). The solution resulting from sample preparation contains ~3,000ppm Mn going into the ICP-OES (axial). When using the Pb line at 220.353nm, we consistently see a peak that equates to ~75ppm Pb in the original sample. The Pb line at 217.000nm shows no signal for lead in the sample preparation. The calibration standards are all prepared in dilute HNO3 and the r2 values are >0.99. Lead spikes into the sample preparations show good recovery on both the 220.353 and 217.000 lines. We use an internal standard to normalize matrix effects. We ran the sample preparations on ICP-MS as a 'referee' and found no signal for lead suggesting that there is something happening on the 220.353nm line. We prepared a solution containing 3,000ppm Mn from a 10,000ppm stock solution and saw the same trend on the ICP-OES with the two emission lines. I cannot find a reference to an interference at 220.353nm for manganese. Can anyone shed some light here?