The carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions of bone and tooth collagen provide a powerful tool for studying past biological, environmental, and cultural phenomena. Collagen has a well-understood chemical composition that has enabled the development of invaluable quality control (QC) criteria for isotopic data – something that is extremely rare among biogeochemical research on ancient biomolecules as a whole. The most important of these collagen QC criteria is atomic C:N ratio (ratio of carbon to nitrogen atoms), which provides an indicator of the extent to which the amount of carbon and nitrogen present in a sample matches the known composition of pure collagen, thereby indicating whether contamination or diagenesis may be influencing a sample's isotopic compositions. We present a model describing the relationship between the carbon and nitrogen isotopic and elemental compositions that accounts for the isotopic composition of the endogenous collagen and exogenous contaminants as well as taxon-specific information about the collagen's amino acid composition. In some cases the traditional C:N QC parameters are applicable, while in others they can result in the inclusion of unreliable (altered) isotopic data primarily due to contamination from humic substances. Using new and previously published data on taxa commonly encountered in ancient studies, we further illustrate how using traditional C:N QC parameters may lead to the inclusion of altered isotopic compositions in real archaeological and paleontological scenarios. We argue that the traditional ‘one size fits all’ approach to the C:N QC criterion should be avoided and we outline new collagen QC criteria specific to certain taxa and environments on the basis of the results of our model. These revised criteria will help to improve the interpretation of isotopic data by more accurately identifying samples with isotopic compositions altered by contamination.

碳 ( δ ¹³ C) 和氮 ( δ ¹⁵N) 骨和牙齿胶原的同位素组成为研究过去的生物、环境和文化现象提供了有力的工具。胶原蛋白具有易于理解的化学成分，这使得为同位素数据制定了宝贵的质量控制 (QC) 标准——这在整个古代生物分子的生物地球化学研究中极为罕见。这些胶原质 QC 标准中最重要的是原子 C:N 比（碳与氮原子的比率），它提供了样品中碳和氮含量与已知纯胶原蛋白组成相匹配程度的指标，从而表明污染或成岩作用是否可能影响样品的同位素组成。我们提出了一个模型，描述了碳和氮同位素与元素组成之间的关系，该模型解释了内源性胶原蛋白和外源性污染物的同位素组成以及有关胶原蛋白氨基酸组成的分类群特定信息。在某些情况下，传统的 C:N QC 参数是适用的，而在其他情况下，它们可能导致包含不可靠（改变）的同位素数据，主要是由于腐殖质的污染。使用古代研究中常见的分类群的新的和以前发表的数据，我们进一步说明了使用传统的 C:N QC 参数如何导致在真实的考古和古生物学场景中包含改变的同位素组成。我们认为传统的“一刀切”的 C 方法：应避免使用 N QC 标准，我们根据我们的模型结果概述了特定于某些分类群和环境的新胶原质 QC 标准。这些修订后的标准将有助于通过更准确地识别同位素组成因污染而改变的样品来改进对同位素数据的解释。