A novel strategy for MALDI-TOF MS analysis of small molecules
Shu Zhang
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Jian'an Liu
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Yi Chen
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Shaoxiang Xiong
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Guanghui Wang
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Jun Chen
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Guoqiang Yang
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Address reprint requests to Dr. Y. Chen and Dr. S. Xiong,
Institute of Chemistry, Chinese Academy of Sciences
,
Beijing 100190, China
1
Published online October 6, 2009 Received April 27, 2009 Revised September 22, 2009 Accepted September 26, 2009
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Beijing National Laboratory for Molecular Science
,
Beijing 100190, China
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Institute of Chemistry, Chinese Academy of Sciences
,
Beijing 100190, China
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) does not work efficiently on small molecules (usually with molecular weight below 500 Da) because of the interference of matrix-related peaks in low m/z region. The previous methods developed for this problem focused on reducing the peaks caused by the traditional matrices. Here, we report a novel strategy to analyze small molecules in a high and interference-free mass range by using metal-phthalocyanines (MPcs) as matrices which should be capable of forming matrix-analyte adducts. The mass of the target analyte was calculated by subtracting the mass of MPc from the mass of the MPc-analyte adduct. MPcs were also detectable and could serve as internal standards. Various MPcs with aromatic or aliphatic groups and different metal centers were then synthesized and explored. Aluminum-phthalocyanines (AlPcs), gallium-phthalocyanines (GaPcs), and indium-phthalocyanines (InPcs) were efficient matrices to form MPc-analyte adducts in either the positive or negative ion mode. The detection limits varied from 17 to 75 fmol, depending on analyte types. The mechanism of adducts formation was also proposed. Collectively, our strategy provides a novel and efficient way to analyze small molecules by MALDI-TOF MS. (J Am Soc Mass Spectrom 2010, 21, 154 -160) 2010 American Society for Mass Spectrometry
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Some researchers explored new matrices, such as
carbon nanotubes [19], graphite [20, 21], ionic liquid [22,
23], and also some high-mass molecules. Ayorinde et al.
[24] have reported the use of meso-tetrakis
(pentafluorophenyl) porphyrin (F20TPP) as a matrix in MALDI-TOF
MS analysis of some commercial nonylphenol ethoxylates
and the fatty acids of saponified vegetable oil.
The strategy involved in these discussed methods lies
in reducing the interference in the low mass region, which
is difficult to completely realize. These methods also have
other shortcomings, such as irreproducibility of silicon
surface, significant reduction addition of CTAB, and
contamination of ion source from carbon nanotubes.
Herein, we developed a novel strategy, which is
completely different from the approaches discussed
above, to analyze small molecules by MALDI-TOF
MS. The traditional stainless-steel plate was used,
and no additives were used to suppress the matrix
effect in this method. Small analytes were detected in
the form of matrix-analyte adduct in a higher mass
region by using metal-phthalocyanines (MPcs) as
matrices, which resulted in the elimination of the
interference peaks in low mass region derived from
traditional matrices. Unlike porphyrin matrix [24],
the MPcs matrices not only absorb laser energy but
also can form MPcanalyte adducts.
Several key factors affecting the formation of MPc
analyte adducts, such as matrix/analyte molar ratio,
central metal ions, and substituent groups of MPcs, as
Mof-flight mass spectrometry (MALDI-TOF MS)
atrix-assisted laser desorption/ionization
time[1, 2] is limited for the detection of small
molecules (usually with molar mass below 500 Da)
because of the interference peaks caused by matrix. To
solve this problem, several methods have been
reported, of which the most attractive is the matrix-free
laser desorption/ionization [313]. Siuzdak et al. [3]
designed a porous silicon surface; the explorations were
extended to not only the new types of silicon materials
(e.g., silicon nanocavity [6] and silicon nanowires [7]),
but also non-silicon substances such as porous
aluminum [8] and zinc oxide nanoparticles [9].
Surfaceenhanced laser desorption/ionization (SELDI) introduced
by Hutchens and Yip [10] is also a matrix-free method for
biological samples analysis [11, 12]. This technique is
combined with porous silicon surface through modifying
the surface to make it capture analytes [13, 14].
Use of matrix additivies [1518] is capable of reducing
background peaks in low m/z region as well. Guo et al.
[15] added a surfactant of cetrimonium bromide to a
conventional matrix of -cyano-4-hydroxycinnamic acid
(CHCA) to suppress substantially the matrix-induced
background.
well as samples pKa, are discussed in detail. This new
method has been successfully applied to the analyses of
various types of small molecules, including peptides,
fatty acids, and phenol.
Experimental
Gibberellic acid (GA3) was purchased from Acros (Fair
Lawn, New Jersey, USA). 2,5-Dihydroxy benzoic acid,
quercetin, and l (...truncated)