in quantity 和quantity 有什么区别呢?

quantity本身是名词,一般用形容词来修饰,可者前面直接跟动词.
有a small quantity/ a large quantiey /quantities/等用法.没有你讲的直接跟介词一起用的.
如:
Mathematics is the science of pure quantity.
数学是研究纯量之科学.
There is a small quantity of water left in the bottle.
瓶子里还剩下少量的水.
She has quantities of good clothes.
她有许多好衣服
It was a bad year for films,in terms of both quantity and quality.
今年的电影无论从数量上还是质量上都说不上好.
This company is more concerned with quality than with quantity.
这家公司对质量比对产量更重视.
A large quantity of air-conditioners has been sold since the temperature is high.
因为气温很高,有大量空调售出.
The factory turns out a large quantity of paper products.
这家工厂生产大量的纸制品.
The new sales director is still a bit of an unknown quantity.
新来的销售部主任大家还不大了解.