Finding and Verifying CAS Registry Numbers® on STN®
— March 1993 —
It is very important that you verify your answer by carefully examining the fields in the displayed record. Getting an answer does not mean it is the right one for your needs! You are responsible for determining if the answer truly corresponds to your substance. Here are some ideas for verifying your answer.
1. Look through the chemical names
The names are each labeled CN in the record display.
2. Compare the molecular formulas
If you have a molecular formula, compare it with the one in the record display. Molecular formulas are in the MF field. Please remember that the formula is not enough information by itself to uniquely identify a substance. However, it is a useful check.
Sometimes the molecular formula is UNSPECIFIED in a display because the substance has an unknown composition and there is no specific molecular formula available.
3. Follow Hill System Order
Your formula may not look the same as the formula in the display. Chemical Abstracts Service uses the Hill System Order for specifying molecular formulas. Not everyone uses Hill Order when writing a formula. For example, the common way of noting the formula for sulfuric acid is H2S04. But the Hill Order is H204S.
A. Hill Order for substances containing carbon (organic compounds)
- Carbon atoms are cited first.
- Hydrogen atoms (if any) are cited next.
- Any remaining elements are arranged in alphabetical order by element symbol.
B. Hill Order for substances without carbon (inorganic compounds)
All element symbols are arranged in alphabetical order.
For more information on the Molecular Formula field, /MF, please see the Registry File: Dictionary Searching manual. If you think your answer is correct but looks different from the molecular formula in the record, call the STN Help Desk. For details, see Appendix 5 — Additional Support.
4. Compare structure diagrams
If you have a structure diagram for the substance, compare it with the one that is displayed at the end of the record. You may need help from a chemist for a complicated structure. Even if the structures don't immediately look the same, check to see if yours is rotated, upside down, or a mirror image of the online structure.
5. Check for input errors
If the answer records do not seem to match your substance, this could be due to typographical or information errors. To help you figure out what could be wrong, look over the information in STEP 1 — Prepare in the appropriate section: SEARCHING WITH A NAME or SEARCHING WITH A REGISTRY NUMBER. For name searching, also see STEP 4 — EXPAND.
6. Help for verifying your answer
If you do not have a chemistry background or experience with online searching, it may be difficult to verify your answer. Be sure to ask for help from trained personnel if you are not confident that the answer record is the right one for your substance. It is important that you use the correct Registry Number.
If you have trouble verifying your answer, there are sources you can go to for help. Please refer to the section entitled Appendix 5 — Additional Support.
7. The Registry Number Locator field
The Registry Number Locator field (LC) specifies the names of other STN files and lists that contain CAS Registry Numbers and other substance information. Some of these files and lists are for regulated substances. Your substance is contained in the files and lists that display in the Locator field. Here are some Locator field entries for regulated substances:
- EINECS — European Inventory of Existing Commercial Chemical Substances
- TSCA — Environmental Protection Agency Toxic Substances Control Act Inventory (U.S.A.)
- CHEMLIST — An STN file containing information on TSCA regulated substances
- WHO — World Health Organization publications