The FDA has introduced a new method for detecting some chemicals, such as NDMA:
10/16/2018: UPDATE - FDA releases additional NDMA/NDEA detection method
Source: https://www.fda.gov/drugs/drug-safety-and-availability/fda-updates-and-press-announcements-angiotensin-ii-receptor-blocker-arb-recalls-valsartan-losartan
See https://www.fda.gov/media/117807/download for details of the new process.
This has detected NDMA where previously it wasn't detected.
Wikipedia states:
In October 2019, the U.S. Food and Drug Administration (FDA) observed that a third-party laboratory was using higher temperatures in its tests to look for nitrosamine impurities. The NDMA was generated by the added heat but the higher temperatures were recommended for using a gas chromatography–mass spectrometry (GC/MS) method to test for NDMA in valsartan and angiotensin II receptor blockers (ARBs).[64] The FDA stated that it recommends using a liquid chromatography-high resolution mass spectrometry (LC-HRMS) testing protocol to test samples of ranitidine.[65] Its LC-HRMS testing method does not use elevated temperatures and has shown the presence of much lower levels of NDMA in ranitidine medicines than reported by the third-party laboratory. International regulators using similar LC-MS testing methods have also shown the presence of low levels of NDMA in ranitidine samples.[13] The FDA provided additional guidance about using another liquid chromatography-mass spectrometry (LC-MS) method based on a triple-quadrupole MS platform.[13][66]