ASB's big jump in net interest margin in the December 2010 half was mainly the result of a drop in interest expense on term deposits and issued paper, rather than a rise in income as more mortgages moved to floating rates.   At the time of announcement of its December half result, ASB said its net interest margin rose by 28 basis points to 1.92 per cent, from 1.64 per cent in June. ASB's parent, Commonwealth Bank, announced yesterday that Barbara Chapman, the bank's group executive human resources and group services, had been appointed ASB chief executive. Chapman takes up her new role next month. A break-up of ASB's interest expense shows that while term deposits rose to NZ$19.09 billion in December, compared with $18.12 billion in June, interest expense on such deposits fell to $576 million in the latest half-year period compared with $1.41 billion for the full June financial year. The amount of issued paper was little changed during the period, but interest expense fell to $89 million during the latest half, against $220 million during the 12-month period to June. Interest income from advances to customers didn't see much variation at $1.81 billion for the half-year period, compared with $3.71 billion during the previous full-year period. Interestingly, while ASB has managed to increase its deposit book recently, the proportion of deposits due after 12 months has been falling. In December last year, 5.7 per cent of deposits were due after 12 months. This fell to 3.4 per cent in June and dropped further, to 2.5 per cent, in December 2010. Asset quality has been deteriorating, mainly because of a hit on corporate loans. The bank's 90-day past due assets rose to $387 million, from $369 million, in June. This was thanks to a near doubling in corporate past due loans to $66 million from $36 million. Total impaired assets rose to $499 million from $484 million, with a very big chunk of this coming from corporate impairments, at $317 million.