化学文献中经常出现ca.这个单词,不知道是什么缩写

化学文献中经常出现ca.这个单词,不知道是什么缩写
比如说The photocatalytic oxidation of MO(甲基橙)in aerobic aqueous solution containing 5 mg of titania nanodisks was also observed by monitoring its absorption spectral change as reported earlier,which exhibited ca.90% destruction of the absorption maximum band at 465 nm with a new species detected at ca.410 nm upon 45 min of irradiation.
不是钙……可以读一读我写上的这个文献,依据内容来讲不是钙,且这个后面还有一个点,字母还是小写的!