Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue7-8.2000:
Organic Geochemistry 31 (2000) 627±634
www.elsevier.nl/locate/orggeochem
The eect of laser wavelength and power density on the
laser desorption mass spectrum of fulvic acid
Teresa L. Brown 1, James A. Rice *
South Dakota State University, Department of Chemistry and Biochemistry, Box 2202, Brookings, SD 57007-0896, USA
Abstract
Fulvic acid (FA) is a water-soluble component of natural organic matter whose environmental and organic geochemistry is routinely correlated to its molecular weight. In an attempt to examine the usefulness of laser desorption
(LD) mass spectrometry to unambiguously determine this characteristic, laser wavelengths of 10.6 mm, 1.06 mm and
355 nm were evaluated using the four International Humic Substances Society FAs. Under the conditions employed,
FA desorption and ionization were optimal when an infrared wavelength is used and thermal desorption conditions
dominate. Mass distributions observed in the 10.6 and 1.06 mm FA positive-ion LD mass spectra were centered at
500±600 m/z, and ranged from 200 m/z to beyond 800 m/z. Lower m/z distributions were generally observed in the
corresponding negative-ion spectra. Increasing power density, or use of the UV wavelength, resulted in extensive
fragmentation of the FA component molecules. The positive-ion spectrum at 355 nm, or at a higher power density,
consisted predominantly of low-mass fragments (7), and fulvic acid (FA) is soluble at all pH
values. Each of these fractions is itself a complex mixture that has de®ed all attempts to separate it into discrete compounds (MacCarthy and Rice, 1991). Because
of its lower molecular weight, and ostensibly simpler
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00050-4
628
T.L. Brown, J.A. Rice / Organic Geochemistry 31 (2000) 627±634
nature (Stevenson, 1982; Hayes et al., 1989), fulvic acid
was chosen for the study described here.
In order to understand how humic materials ful®ll
their functions in the natural environment, it is imperative that we be able to describe their molecular structure. A basic component of a structural characterization
is molecular weight. As pointed out by Wershaw and
Aiken (1985), a method for the accurate molecular
weight determination of humic materials will allow for the
establishment of approximate molecular formulas when
used in conjunction with other characterization data,
determination of stoichiometric relationships between
humic materials and other chemical species, and the
accurate comparison of diagenetic dierences between
humic materials from dierent environments. In fact,
molecular weight is the one characteristic which is routinely correlated with many of the environmental behaviors and reactions of humic materials. Yet it is a
characteristic of FA whose absolute nature has remained
an enigma.
A principal method for obtaining a substance's molecular weight (or more appropriately here the molecular
weight distribution) is mass spectrometry. Its use in the
characterization of FA, and humic materials in general,
has been limited by the requirement that a sample must
be ionized and introduced into the gas phase for mass
spectrometric analysis.
The applicability of desorption ionization methods to
the study of high-mass biomolecules (Karas and Hillenkamp, 1988; Karas et al., 1989) suggests that they may
also be applicable to the mass spectrometric characterization of fulvic acid and its molecular weight. It has
been shown that laser desorption Fourier-transform ion
cyclotron resonances mass spectrometry (LD FT-ICR
MS) can be employed in the characterization of FA. Mass
distributions (100±1100 m/z) with number average
molecular weights ranging from 400 to 600 m/z for the
four International Humic Substances Society (IHSS)
reference fulvic acids have been reported (Novotny et al.,
1995). However, since LD processes have been found to
be dependent on a number of experimental parameters (i.e.
laser wavelength, laser power, and analyte nature (van
Vaeck et al., 1993)), it is likely that the molecular weight
distributions observed by LD FTMS would be dependent
on the desorption conditions employed. The purpose of
this study was to compare the desorption mass spectra of
FA obtained with laser wavelengths of 10.6 mm, 1.06 mm,
and 355 nm and two dierent laser power-densities.
Malcolm, 1985) and fulvic acid extraction procedure
(Swift, 1996) for each of these samples are described
elsewhere. Chemical characteristics of each FA sample
are given by Novotny (1993). The fulvic acid samples
were prepared by individually dissolving each sample in
deionized, distilled water to give a concentration of 5
mg/ml. Several drops of sample solution were placed on
a stainless-steel probe-tip and air dried. An alkali kraft
lignin and tannic acid were purchased from Aldrich and
Sigma, respectively, and used as received.
Positive±and negative-ion LD FT-ICR MS experiments with a laser wavelength of 10.6 mm were performed
on an 3T Extrel FTMS 2000 mass spectrometer coupled
to a pulsed CO2 laser according to the conditions speci®ed by Novotny et al. (1995). Laser-desorption FT-ICR
MS experiments at 1.06 mm and 355 nm were performed
on a 3T Extrel FTMS 2000 coupled to a pulsed
Nd:YAG laser with positive- and negative-ion detection
at the National High-Field FT-ICR Mass Spectrometry
Facility at the National High Magnetic Field Laboratory (Florida State University, Tallahassee, FL) and
Oak Ridge National Laboratory (Analytical Sciences
Division), respectively. Generation of the 355 nm laser
pulses was accomplished by frequency tripling. At each
wavelength, the laser power and delay time before excitation were optimized to give maximum ion intensity and
mass distribution for comparison purposes. The parameters were considered optimized at the point at which
the observed m/z range was maximized. Additional
experimental details are given by Brown (1998).
At 10.6 mm, the power density was 107 W/cm2.
Experiments performed at 1.06 mm utilized two dierent
power densities, 108 and 109 W/cm2 to speci®cally
demonstrate the eect that this variable has on the
observed spectrum. Spectra produced with the higher
power density will be designated as 1.06 mm-H. At 355
nm the power density was 107±108 W/cm2. To increase
the signal-to-noise ratio, ten laser events at dierent
spots on the probe tip were averaged during the acquisition of the 10.6 mm spectra. Low-mass ion ejection via
a chirp excitation was necessary in the 10.6 mm and 355
nm desorption experiments to remove high-intensity low
mass ions (
www.elsevier.nl/locate/orggeochem
The eect of laser wavelength and power density on the
laser desorption mass spectrum of fulvic acid
Teresa L. Brown 1, James A. Rice *
South Dakota State University, Department of Chemistry and Biochemistry, Box 2202, Brookings, SD 57007-0896, USA
Abstract
Fulvic acid (FA) is a water-soluble component of natural organic matter whose environmental and organic geochemistry is routinely correlated to its molecular weight. In an attempt to examine the usefulness of laser desorption
(LD) mass spectrometry to unambiguously determine this characteristic, laser wavelengths of 10.6 mm, 1.06 mm and
355 nm were evaluated using the four International Humic Substances Society FAs. Under the conditions employed,
FA desorption and ionization were optimal when an infrared wavelength is used and thermal desorption conditions
dominate. Mass distributions observed in the 10.6 and 1.06 mm FA positive-ion LD mass spectra were centered at
500±600 m/z, and ranged from 200 m/z to beyond 800 m/z. Lower m/z distributions were generally observed in the
corresponding negative-ion spectra. Increasing power density, or use of the UV wavelength, resulted in extensive
fragmentation of the FA component molecules. The positive-ion spectrum at 355 nm, or at a higher power density,
consisted predominantly of low-mass fragments (7), and fulvic acid (FA) is soluble at all pH
values. Each of these fractions is itself a complex mixture that has de®ed all attempts to separate it into discrete compounds (MacCarthy and Rice, 1991). Because
of its lower molecular weight, and ostensibly simpler
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00050-4
628
T.L. Brown, J.A. Rice / Organic Geochemistry 31 (2000) 627±634
nature (Stevenson, 1982; Hayes et al., 1989), fulvic acid
was chosen for the study described here.
In order to understand how humic materials ful®ll
their functions in the natural environment, it is imperative that we be able to describe their molecular structure. A basic component of a structural characterization
is molecular weight. As pointed out by Wershaw and
Aiken (1985), a method for the accurate molecular
weight determination of humic materials will allow for the
establishment of approximate molecular formulas when
used in conjunction with other characterization data,
determination of stoichiometric relationships between
humic materials and other chemical species, and the
accurate comparison of diagenetic dierences between
humic materials from dierent environments. In fact,
molecular weight is the one characteristic which is routinely correlated with many of the environmental behaviors and reactions of humic materials. Yet it is a
characteristic of FA whose absolute nature has remained
an enigma.
A principal method for obtaining a substance's molecular weight (or more appropriately here the molecular
weight distribution) is mass spectrometry. Its use in the
characterization of FA, and humic materials in general,
has been limited by the requirement that a sample must
be ionized and introduced into the gas phase for mass
spectrometric analysis.
The applicability of desorption ionization methods to
the study of high-mass biomolecules (Karas and Hillenkamp, 1988; Karas et al., 1989) suggests that they may
also be applicable to the mass spectrometric characterization of fulvic acid and its molecular weight. It has
been shown that laser desorption Fourier-transform ion
cyclotron resonances mass spectrometry (LD FT-ICR
MS) can be employed in the characterization of FA. Mass
distributions (100±1100 m/z) with number average
molecular weights ranging from 400 to 600 m/z for the
four International Humic Substances Society (IHSS)
reference fulvic acids have been reported (Novotny et al.,
1995). However, since LD processes have been found to
be dependent on a number of experimental parameters (i.e.
laser wavelength, laser power, and analyte nature (van
Vaeck et al., 1993)), it is likely that the molecular weight
distributions observed by LD FTMS would be dependent
on the desorption conditions employed. The purpose of
this study was to compare the desorption mass spectra of
FA obtained with laser wavelengths of 10.6 mm, 1.06 mm,
and 355 nm and two dierent laser power-densities.
Malcolm, 1985) and fulvic acid extraction procedure
(Swift, 1996) for each of these samples are described
elsewhere. Chemical characteristics of each FA sample
are given by Novotny (1993). The fulvic acid samples
were prepared by individually dissolving each sample in
deionized, distilled water to give a concentration of 5
mg/ml. Several drops of sample solution were placed on
a stainless-steel probe-tip and air dried. An alkali kraft
lignin and tannic acid were purchased from Aldrich and
Sigma, respectively, and used as received.
Positive±and negative-ion LD FT-ICR MS experiments with a laser wavelength of 10.6 mm were performed
on an 3T Extrel FTMS 2000 mass spectrometer coupled
to a pulsed CO2 laser according to the conditions speci®ed by Novotny et al. (1995). Laser-desorption FT-ICR
MS experiments at 1.06 mm and 355 nm were performed
on a 3T Extrel FTMS 2000 coupled to a pulsed
Nd:YAG laser with positive- and negative-ion detection
at the National High-Field FT-ICR Mass Spectrometry
Facility at the National High Magnetic Field Laboratory (Florida State University, Tallahassee, FL) and
Oak Ridge National Laboratory (Analytical Sciences
Division), respectively. Generation of the 355 nm laser
pulses was accomplished by frequency tripling. At each
wavelength, the laser power and delay time before excitation were optimized to give maximum ion intensity and
mass distribution for comparison purposes. The parameters were considered optimized at the point at which
the observed m/z range was maximized. Additional
experimental details are given by Brown (1998).
At 10.6 mm, the power density was 107 W/cm2.
Experiments performed at 1.06 mm utilized two dierent
power densities, 108 and 109 W/cm2 to speci®cally
demonstrate the eect that this variable has on the
observed spectrum. Spectra produced with the higher
power density will be designated as 1.06 mm-H. At 355
nm the power density was 107±108 W/cm2. To increase
the signal-to-noise ratio, ten laser events at dierent
spots on the probe tip were averaged during the acquisition of the 10.6 mm spectra. Low-mass ion ejection via
a chirp excitation was necessary in the 10.6 mm and 355
nm desorption experiments to remove high-intensity low
mass ions (