B American Society for Mass Spectrometry, 2017 J. Am. Soc. Mass Spectrom. (2017) 28:1987Y1990
Gain Switching for a Detection System to Accommodate
a Newly Developed MALDI-Based Quantification Method
Sung Hee Ahn,
Myung Soo Kim,
Jeong Hee Moon
Department of Chemistry, Seoul National University, Seoul, 151-747, Korea
Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 151-742, Korea
Disease Target Structure Research Center, KRIBB, Daejeon, 305-806, Korea
= low gain, V
= high gain
Abstract. In matrix-assisted laser desorption ionization time-of-flight mass spectrom-
etry (MALDI-TOF), matrix-derived ions are routinely deflected away to avoid prob-
lems with ion detection. This, however, limits the use of a quantification method that
utilizes the analyte-to-matrix ion abundance ratio. In this work, we will show that it is
possible to measure this ratio by a minor instrumental modification of a simple form of
MALDI-TOF. This involves detector gain switching.
Keywords: Detector gain switching, MALDI-TOF instrument, MALDI quantification
Received: 31 January 2017/Revised: 21 April 2017/Accepted: 5 May 2017/Published Online: 10 July 2017
ecently, we reported a method to quantify an analyte (A)
by utilizing matrix-assisted laser desorption ionization
(MALDI) time-of-flight (TOF) mass spectrometry [1, 2]. The
method is based on the observation that the analyte-to-matrix
ion abundance ratio, I([A + H]
)/I([M + H]
), to be called the
ion ratio in this work, is proportional to the amount or concen-
tration (c([A])) of the analyte in an analyte-matrix mixture.
Four main features and/or requirements for a reliable quantifi-
cation based on the ion ratio are (1) sample homogeneity, (2)
control of the effective temperature in the early matrix plume,
(3) construction of the calibration curve by plotting I([A + H]
I([M + H]
) versus c([A]), and (4) to quantify only those
samples with the degree of matrix suppression (S) smaller than
a critical value. S was defined as 1 − I([M + H]
([M + H])
with I and I
representing the matrix ion abundances in the
presence and absence of analytes, respectively. We have found
that the above quantification method was applicable to mix-
tures and heavily contaminated samples also . S has been
found to provide a good guideline to check whether a heavily
contaminated sample can be quantified by the present method.
One difference between the ion ratio method and other
MALDI-based quantification methods is that the former meth-
od requires the abundance data not only for [A + H]
for [M + H]
For a sample with a low analyte concentration, its
MALDI spectrum is dominated by matrix-derived ions.
When one attempts to measure the abundances of both
analyte- and matrix-derived ions in such a case, one may
encounter a problem related to the linear dynamic range of
the detection system. This can occur when using commer-
cial MALDI-TOF equipped with a detection system incor-
porating a single 8-bit analog-to-digital (A/D) converter. In
most studies involving MALDI, however, one wants to
improve the spectral data only for analyte-derived ions.
Therefore, an operator routinely deflects away the matrix-
derived ions and adopts a high detection gain for analyte-
derived ions .
In the instruments built in this laboratory, the above
problem has been handled in two ways. The first method,
used with our instruments at present, involves the use of an
A/D with a large dynamic range, a dual 10-bit A/D to be
specific. This is costly. An alternative is to provide two
different tracks and detection systems, one for the matrix-
derived ions and the other for the analyte-derived ions .
This dual-track instrument is even more costly. Another
problem related to the linear dynamic range of a detection
system is the saturation of the detector used, which is
usually a microchannel plate (MCP) .
Electronic supplementary material The online version of this article (doi:10.
1007/s13361-017-1711-2) contains supplementary material, which is available
to authorized users.
Correspondence to: Myung Kim; e-mail: firstname.lastname@example.org, Jeong Moon;