Agilent MassHunter DAD File Structure

This file format contains diode-array detector data acquired under MassHunter rather than Chemstation. It holds the same two views of a run that Chemstation splits into .ch and .uv files, one chromatogram per signal and the spectra, in an unrelated binary layout.

The files sit in the AcqData subdirectory and are named after the detector that wrote them. They come in descriptor/data pairs.

File	Type tag	Information
DAD1.cd	0x0200	Describes the signals stored in DAD1.cg
DAD1.cg	0x0201	One chromatogram per signal
DAD1.sd	0x0202	Describes the spectra stored in DAD1.sp
DAD1.sp	0x0203	The spectra

A type tag names the kind of file, not the detector that wrote it. Pumps, autosamplers and column compartments write .cd/.cg pairs under the same two tags, so rainbow finds the absorbance data by device name rather than by tag. Only the .sd/.sp spectra are unique to a detector.

Each descriptor gives the byte offset of every record in its data file, along with the record’s length in bytes (.sd) or in points (.cd). Records are therefore located rather than assumed to be evenly spaced. Assume little-endianness. Absorbances are plain doubles: unlike the Chemstation formats there is no delta encoding and no scaling factor.

All four files open with the same 68-byte header, whose only populated field is a 16-bit type tag at offset 0x00 identifying which file it is.

DAD1.cd stores the number of signals at 0x4C and its first signal record at 0x50. A record interleaves length-prefixed strings with fixed-width fields, so records vary in width and are read in sequence. The strings are UTF-8, since a detector reports temperatures in degrees Celsius.

Field (in order)	Example
Signal letter (length-prefixed)	A
Description (length-prefixed)	Sig=254.0,4.0  Ref=360.0,100.0
Kind: 1 for absorbance, 2 for telemetry	1
Byte offset of this signal’s data in DAD1.cg	68
Always zero, perhaps the high half of that offset	0
Number of points	3525
Unit (length-prefixed), after further fields not decoded here	mAU

A description names the band the signal integrates and the band subtracted from it: 254 nm over a 4 nm bandwidth, referenced to 360 nm over 100 nm. A chromatogram is therefore not a slice of the spectra; it is a band average, less a reference. A detector also records telemetry here (lamp voltage, board and optical unit temperature), sampled four times as often as its absorbance signals and carrying its own units. rainbow separates the two by unit and description rather than by the kind field, which the other devices do not use as consistently.

DAD1.sd stores the byte offset of its first spectrum record at 0x4C, the number of spectra at 0x50, and the wavelength range at 0x54 and 0x5C. Its records are a fixed 80 bytes.

Record Offset	Purpose	Example
0x04	Retention time in minutes (double)	0.0029167
0x20	Byte offset of this spectrum in DAD1.sp	68
0x28	Its length in bytes	1464
0x2C	Number of wavelengths	181
0x30	First wavelength (double)	190.0
0x38	Last wavelength (double)	550.0

The remaining fields hold the same values in every run available here, so their meaning could not be determined and rainbow does not read them.

DAD1.cg and DAD1.sp store each record as an axis prefix followed by its values, at the offset the descriptor gives.

.cg, per signal:
+------------------+------------------+--------------------------+
| first time (dbl) | time step  (dbl) | num_points x double      |
+------------------+------------------+--------------------------+

.sp, per spectrum:
+------------------+------------------+--------------------------+
| first wavelength | wavelength step  | num_wavelengths x double |
+------------------+------------------+--------------------------+

Note

Because the wavelength axis is repeated per spectrum, an acquisition may in principle vary it during a run. Such spectra do not share a grid and cannot form a dense two-dimensional array, so rainbow checks every spectrum’s axis and declines them with a warning rather than reshaping mismatched rows. The axis is also stated as a range in the .sd, and the two statements are checked against each other.

DAD parsing needs no flag, as the Chemstation UV formats need none. The detector’s telemetry traces are returned as analog data only under telemetry=True, or when named in requested_files.