5.N-SING 单数名词含量;成分 You can use content to refer to the amount or proportion of something that a substance contains.

【搭配模式】：n N

Sunflower margarine has the same fat content as butter...

向日葵所制人造黄油的脂肪含量与黄油脂肪含量相同。

He was astonished at the high gold content in the 340 million-year-old rock.

他对这块有3.4亿年历史的岩石的高含金量大为惊异。

从collins字典，可知content是单数可数名词，应该不是属于零冠词的情况，所以我个人观点是 at different  moisture content 应该改为 at different  moisture contents. 参考来源是 google scholar中的 nature杂志，共有9条记录。既然nature杂志都有类似记录，我个人认为level of是可以省略的。

我之前也被审稿人要求修改语言（http://ask.yygrammar.com/q-33562.html），这是我一开始写的，

The relative contents of  five elements (C, N, O, Si, and Al) at surface depth of 3-5 nm were detected by using a Kα X-ray photoelectron spectrometer (Thermo Fisher Scientific, UK) with a monochromatic Al Kα X-ray source (excitation energy=1468.6 eV).

下面是找机构润色后的句子：

The relative contents of the five elements (C, N, O, Si, and Al) at surface depths of 3-5 nm were detected by using a Kα X-ray photoelectron spectrometer (Thermo Fisher Scientific, UK) with a monochromatic Al Kα X-ray source (excitation energy=1468.6 eV).

由上述例子表明，content在表示含量的含义时，是可以用复数的。另外，如果有条件的话，可以找母语者帮你润色一下，尤其是冠词的使用方面，一直是科技论文的难点和要点。