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Frequency-to-M/z Conversion

Frequency-to-M/z Conversion

32. Frequency-to-M/z Conversion​

FTMS analyzers (Orbitrap, LTQ-FT) store profile data in the frequency domain. The ScanEvent contains conversion coefficients to translate frequencies to M/z values.

32.1 LTQ-FT Conversion (nparam == 4)​

Coefficients: [unknown, A, B, C]

M/z = A + B/f + C/f²

Where f is the frequency value.

32.2 Orbitrap Conversion (nparam == 5 or 7)​

Coefficients: [unknown, (I,) A, B, C (, D, E)]

M/z = A + B/f² + C/f⁴

32.3 Inverse Conversion (M/z to Frequency)​

Used when looking up a specific M/z in frequency-domain profile data:

LTQ-FT: Solve quadratic C + Bf + (A - Mz)f² = 0:

f = (-B - sqrt(B² - 4C(A - Mz))) / (2(A - Mz))

Orbitrap: Solve (A - Mz) + B/f² + C/f⁴ = 0:

Let x = 1/f²:

Cx² + Bx + (A - Mz) = 0
x = (-B - sqrt(B² - 4C(A - Mz))) / (2C)
f = 1/sqrt(x)

32.4 Direct M/z Data​

When profile.step > 0 (positive step), the data is directly in M/z domain and no frequency conversion is needed. The M/z of bin i is:

mz = profile.first_value + i * profile.step