Not always. The reason being,  increasing atomic number doesn't always equate to increasing mass is because many atoms don't have a number of neutrons equal to the number of protons. In other words, several isotopes of an element may exist. If a sizeable portion of an element of lower atomic number exists in the form of heavy isotopes, then the mass of that element may (overall) be heavier than that of the next element. If there were no isotopes and all elements had a number of neutrons equal to the number of protons, then atomic mass would be approximately twice the atomic number (approximately because protons and neutrons don't have exactly the same mass... the mass of electrons is so small that it is negligible). Different periodic tables give differing atomic masses because the percentages of isotopes of an element may be considered changed from one publication to another.

The element symbol for oxygen is O and its atomic number is 8. The mass numbers for oxygen must be 8 + 8 = 16; 8 + 9 = 17; 8 + 10 = 18. respectively

In confirmatory tests, a procedure specific to the class or family of drug for which the sample screened positive is employed to "wash" or separate the drug itself from the matrix, i.e. blood or urine, in which it resides. The procedure of separating the drug from the sample is called an extraction. The Toxicology Section has different procedures for each class of drug. The general scheme of an extraction is to adjust the pH of the sample, expose the sample to an organic solvent to wash out the drug, collect and concentrate the organic solvent, and derivatize the drug in the sample. Some extractions have a but a few steps in the scheme, while others have many steps. The extraction process is one of the most time-consuming aspects of drug testing.