Agilent .ch File Structure (FID)

This file format contains FID channel data.

Note that this is not the same as .ch files that contain UV, CAD, or ELSD data.

Files in this format are named something like FID1A.ch.

These files are comprised of a file header and data body.

File header

The file header contains metadata and information about the data. The header is 0x1800 bytes long.

The metadata is stored as plaintext strings that begin at fixed offsets. These strings adhere to a common pattern: a single byte specifying the length of the string, followed by the characters in the string separated by single null bytes. For example, the string “hello” would be stored as such:

 len   "h"       "e"       "l"       "l"       "o"
+----+----+----+----+----+----+----+----+----+----+----+
| 05 | 68 | 00 | 65 | 00 | 6C | 00 | 6C | 00 | 6F | 00 |
+----+----+----+----+----+----+----+----+----+----+----+

The strings are separated by large blocks of null bytes, which are likely used to reserve space for longer strings.

File Offset	Purpose	Example String
0x146	File type (number)	179
0x15B	File type (name)	GC DATA FILE
0x35A	Notebook name	cedrol_mix_01
0x758	Parent directory	mcminns
0x957	Date	17 Dec 19 10:04 am
0x9BC	UNKNOWN	7890
0x9E5	UNKNOWN	GC
0xA0E	Method	Rt-bDEX-SE_mcminn.M
0xC11	Instrument	Mustang ChemStation
0x104C	Units	pA
0x1075	Signal	Front Signal

There are also several groups of unreadable bytes at fixed offsets in the header. The purpose of most of these bytes are currently unknown. But there are a few known values that provide useful information about the data.

File Offset	Purpose	Data Type	Endianness
0x116	Number of data values	Unsigned int	Big
0x11A	First retention time (ms)	Float	Big
0x11E	Last retention time (ms)	Float	Big
0x127C	Scaling factor	Double	Big

Data body

The rest of the file contains the data values. The structure of the data body is relatively straightforward: little-endian doubles that each represent a single data value. These doubles are in ascending order with respect to time, so the first double corresponds to the first retention time.

These values must be scaled with the factor from the file header to acquire the actual data values.

The entire file can be visualized by the following diagram, where each double represents a data value. The segments are not drawn to scale.

+-----------------------------------+
|            file header            |
+--------+--------+--------+--------+
| double | double | double | double |
+--------+--------+--------+--------+
| double | double | double | double |
+--------+--------+--------+--------+
| double | double | double | double |
+--------+--------+--------+--------+
|     repeats for all data values   |
+-----------------------------------+