Separation of Soil Organic Phosphorus Compounds
Using Reverse-Phase Ion-Pair Chromatography
ABSTRACT
Methods were developed for the extraction and separation of soil organic
phosphorus compounds using reverse-phase ion-pair chromatography
(RP-ICP). Nucleotides (ATP, ADP, and AMP) were separated using a
mobile phase of 15 mM TBAHS, 15 mM KH2PO4, and 7% acetonitrile.
Inositol hexakisphosphate was separated using a mobile phase
composition of 0.05 M formic acid:methanol (49:51 v/v) and
1.5 mL/100 mL of TBAOH. Extraction procedures were developed for
the nucleotides which would be compatible with the RP-ICP system
developed for their separation.
INTRODUCTION
The transport of phosphorus (P) from agricultural land to watercourses
can contribute to the growth of algae and general water quality deterioration
associated with eutrophication." 1 Areas with intensive livestock production
can contribute P to runoff through land application of manure which can
increase the organic P fraction of soils and therefore potentially lead to
enhanced losses of P. Understanding how organic P compounds behave in
soils requires the ability to accurately measure organic P compounds in soils,
manures and other organic amendments.
Traditionally, soil organic P has been crudely determined by the
difference between total P and inorganic P. E2-41 This method is commonly
used because of the difficulties associated with direct measurement of organic
P compounds. While the difference method may be useful for estimating the
total soil organic P fraction, there may be substantial and unknown error
associated with its measurement as well as the inability to identify specific
organic P compounds. The development and implementation of improved
detection methods are necessary to ensure accurate measurement of the
concentration of organic P compounds in soil in order to study their
behavior. [53
The use of high performance liquid chromatography (HPLC) has been
used to identify and measure the organic P fraction in plants, foods, and
aqueous samples. E6-1°1 More recently the identification of organic P
compounds in soils and soil extracts has been accomplished using
phosphorus-3 1 nuclear magnetic resonance (NMR) spectroscopy." 1 -133
Although these methods have been developed and used successfully,
there are still some drawbacks to the utilization of these methods for
studying the reactivity and transport of organic P compounds in soils.
Many HPLC procedures utilize ion exchange columns for the separation of
organic P. These columns tend to be expensive, not very durable, and
impractical for analysis of large numbers of samples. Sample preparation
can also be very time consuming and manipulation of samples prior to
analysis can involve many steps that potentially alter or degrade organic P
compounds. The use of NMR is limited by the requirement for sample
pre-concentration and long run times, which can be both expensive and
cause sample degradation.
Practical methods to identify organic P in soil extracts and aqueous
samples are necessary when performing research requiring high throughput of
samples. In addition, reliable extraction methods must also allow recovery of
these compounds from the soil without modification and be compatible with
the analytical procedure.
Separation of Select Organic Phosphorus Compounds
Reverse phase ion pairing chromatography (RP-ICP) was used in this
study for the separation and detection of Inositol Hexakisphosphate (lHP),
Adenosine 5' Triphosphate (ATP), Adenosine 5' Diphosphate (ADP), and
Adenosine 5' Monophosphate (AMP). The RP-1PC system uses elements of
both reverse phase and ion exchange chromatography. The reverse phase
chromatography system is composed of a non-polar stationary phase, usually
carbon chains of varying length, with a polar mobile phase. Unlike regular
reverse phase chromatography, the RP-ICP system has a mobile phase
containing an ion-pairing reagent, typically a charged molecule with a
hydrophobic portion that will 'pair up' with ions in solution. The ion pairs are
retained by the hydrophobic stationary phase. Ion pairs do not exist in the
aqueous solution, but in the nonpolar stationary phase. Because the stationary
phase retains ion pairs, the reverse-phase column effectively functions as an
ion exchange column. This methodology offers several advantages. First,
ionic compounds can be separated without having to use an ion exchange
column, which eliminates complicated sample preparation. Second, reverse
phase chromatography is relatively easy to use and the columns are durable
and inexpensive. This allows for the analysis of large sample sets with
minimal preparation time and without costly sample preparation techniques.
Extraction of Organic Phosphorus Compounds
Sequential acid and alkali extraction has been commonly utilized to
determine soil organic P.[14-17] Initial soil extraction with strong acid
followed by an alkali solution removes more organic P than when either
extractant is used alone. In some procedures, elevated temperatures are also
used, which can cause hydrolysis of labile organic P forms.u4,15,171 With this
extraction procedure, a major concern is recovering all the organic P from the
soil without hydrolyzing any to the inorganic form.
A method proposed by Bowman and Moir [181 to extract soil organic P in
one step involves the use of Na2EDTA with NaOH. The NaOH is used to
solubilize organic P associated with soil organic matter, therefore removing
the majority of the organic P fraction in the soil. The resistance of soil organic
P to alkaline extraction is overcome by adding EDTA, which extracts organic
P adsorbed via a metal cationic bridge. EDTA is a hexadentate ligand that
binds strongly to Group 2 cations such as Mg2± and Ca2± and with other
common soil metals such as Fe 3± and A13±. These metal-EDTA complexes
have 1:1 formation constants ranging from 5.8 X 107 to 5.0 X 1010.[19]
With such high formation constants, EDTA is able to complex metal cations
that are binding organic P molecules to soil particles and soil organic matter,
thereby increasing organic P recovery from the soil. This method is a one-step
procedure, which is an improvement upon the acid–base extraction.
MATERIALS AND METHODS
Reagents
Tetrabutylammonium hydrogen sulfate (BAHS) and KH 2PO4 were
obtained from Fisher Scientific. Tetrabutylammonium hydroxide (TBAOH;
40% in water) was obtained from Fluka. Standards of IHP, ATP, ADP, and
AMP, were obtained from Sigma Chemical Company.
Mobile Phase
Preliminary experiments were conducted using different mixtures of the
solvents and ion-pairing reagents described by Patthy et al. F1 to determine the
optimal mixture for nucleotide separation. It was determined that the optimal
mobile phase composition for separation of all three nucleotides consisted of
15 mM TBAHS, 15 mM KH2PO4, and 7% acetonitrile. The mobile phase was
adjusted to pH 5.5 with concentrated H2504, filtered (0.45 imn) and degassed.
The mobile phase used for separation of IHP consisted of 0.05 M formic
acid:methanol (49:51 v/v) and 1.5 mL TBAOH/100 mL. E61 The pH was
adjusted to 4.3 with H2504, filtered (0.45 imn) and degassed.
HPLC Procedure
A reverse phase C-18 column (Waters Nova-Pak C18 3.9 x 75 mm) 4 μm
particle size was equilibrated with the mobile phase overnight. Analyses were
conducted with a Waters HPLC system, and either a refractive index detector
(Waters 410) for the 1HP or a photo diode array (PDA) detector (Waters 991)
for the nucleotides. Separation of ADP and AMP was optimal with a
1 mL min-1 flow rate, 10 1.11 injection volume, and detection at 260 nm
(Fig. 1). Separation of ATP was optimal at a flow rate of 2.5 mL min -1 , 10 ILL
injection volume and detection at 260 nm (Fig. 2). Separation of IHP was
optimal at a flow rate of 3 mL min -1 with a 30 μL injection volume (Fig. 3).
Retention times and peak areas were calculated with the Millennium
Extraction Techniques
The surface horizon of three soils were used to determine the
extractability and recovery of added organic P compounds (Table 1).
Several combinations of the extracting solution composition, extraction
time, amount of soil, and amount of added organic P were tested (Table 2).
All extractions were conducted at 25°C, to avoid potential compound
degradation from elevated temperatures. The initial extraction procedures
were as follows: 4 or 8 mg of ATP was added to 4 or 8 g of soil (screened
to <2mm), 25 mL of extracting solution (NaOH-EDTA or H 2O) was added
five minutes after addition of the ATP to the soil and shaken for 1 or 1.5 hr.
Some samples were also extracted with water: these were shaken for an
additional 0.5 hr and then combined with the original extracted solution
(Table 2). All extracts were filtered through Whatman 42 filter paper,
adjusted to pH 6 with H2504 and filtered (0.45 ilm) prior to HPLC analysis.
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