SOLID-PHASE EXTRACTION Principles, Techniques, and Applications

27 Pages·2006·1.35 MB·English

4
HANDLING LARGE VOLUME SAMPLES:
APPLICATIONS OF SPE TO
ENVIRONMENTAL MATRICES
Martha J. M. Wells, Tennessee Technological University, Cookeville, Tennessee
I. Introduction
11. Large Volume Samples
A. Liquid-Liquid Extraction Versus SPE
B. Breakthrough Volume
C. Practical Implications
111. Natural Organic Matter
A. Particulates
B. Dissolved Organic Matter
C. Significance of Matrix Variation
IV. HydrophilicIHydrophobic Extremes
A. Sorbent Effects
B. Sample Matrix Additives
C. Elution Considerations
D. Recommendations to Improve Recovery
v.
Conclusions
References
I. INTRODUCTION
Environmental applications of solid-phase extraction on bonded silicas were
first developed in the 1980s and have grown rapidly in the 1990s, primarily
as an alternative to liquid-liquid extraction. Protocol development mainly
focuses on analyses of water although applications in the areas of air pollu
Copyright © 2000 by Taylor & Francis Group LLC
tion and industrial hygiene, waste treatment and disposal, inorganic and or-
ganic analytical chemistry, agrochemical bio-regulators and fertilizers,
soils, and plant nutrition are increasing.
Environmental analyses may present the analyst with some or all of the
following challenges:
large volume samples
non-homogeneous samples containing particulates andlor dissolved
organic matter, and non-aqueous phase liquids
a wide range of analytes covering both hydrophilic to hydrophobic
extremes
Knowing these issues are potential problems is the first step toward
overcoming them. This chapter will examine analytical approaches to these
challenges during SPE. While it is possible to be unfortunate enough to
confront all these challenges in the same sample, for the purpose of this dis-
cussion, each point will be addressed individually.
11. LARGE VOLUME SAMPLES
To a clinical chemist, any volume greater than a few tenths of a milliliter
seems enormous, but environmental chemists are accustomed to dealing
with liters of samples. The reason for this volume difference is not so much
that a reservoir of water yields large volumes of samples more willingly
than does a human patient. Rather, the sample volume is determined by the
typical concentrations of potential analytes and, therefore, the degree to
which they will need to be concentrated from the sample prior analysis.
Pharmacological levels of drugs in bodily fluids are commonly found in the
part-per-million (ppm) or part-per-billion (ppb) levels while trace pollutants
are often tracked at the part-per-billion or part-per-trillion (ppt) levels. As
the analyte is recovered by extraction, the co-extracted interferences are
also recovered, presenting a challenge for the analyst.
Certainly, large volume samples may be obtained from sources of water
(i.e. drinking water, surface water, groundwater, wastewater, and seawater),
but large samples can also be derived from other matrices. An effective
strategy for extracting solid environmental matrices such as soil and plant
tissues (dealt with in detail in Chapter 3) is to turn them into a "water" sam-
ple before processing. For example, when soil samples are extracted with
water-miscible organicibuffer mixtures, several milliliters of sample may
result. This volume, in turn, is hrther diluted with water to reduce the
eluotropic strength of the sample.
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Non-linear Modes of Chromatog- Corresponding Solid-Phase Extraction2
raphyl modes
Column Preparation
Frontal Separation Sample Loading
Stepwise Desorption Column Wash
Displacement Development Sample Desorption
Figure 1. Solid-phase extraction as a combination of non-linear modes of
chromatography. 1. A.L. Lee, A.W. Liao and C. Horvath, (1988) J.Chromatogr., 443:
31-43 2. M.J.M. Wells and J.L. Michael, (1987) J.Chromatogr.Sci., 25: 345-350.
A. LIQUID-LIQUID EXTRACTION VERSUS SPE
There are many good experimental reasons for using SPE in preference to
liquid-liquid extraction (LLE) for extracting large volume environmental
samples. For example, exposure to and consumption of large volumes of
organic solvents is avoided; operator dedication to manually shaken sepa-
ratory funnels, which are expensive to purchase, tedious to clean, and sub-
ject to breakage, is eliminated; increased production through multiple si-
multaneous extractions is realized; and the formation of emulsions is re-
duced. For anyone who has ever experienced the frustration of attempting
to "break" an emulsion formed while extracting "real-world" samples by
LLE, this advantage alone might make SPE attractive. However, in the
author's experience, the primary advantage that SPE has over other separa-
tion and preconcentration procedures for large volume samples is the fun-
damental, theoretical difference inherent in the SPE approach SPE is a
-
non-equilibrium procedure.
In contrast, liquid-liquid extraction, batch adsorption, etc., are equilib-
rium processes. The problem with using an equilibrium process is that you
may never know when equilibrium has been reached, and the equilibrium
distribution may necessitate multiple extractions. Also, different analytes
may exhibit vastly different distribution coefficients between extracting
solvents and various matrices because of other contaminants.
LLE procedures depend on the principle of repeated extractions. If a
frog jumps 50% of the distance out of a well each time it jumps, it will,
theoretically, never succeed in achieving 100% of the distance. Similar to
this hapless frog, the extraction process in LLE is driven by the equilibrium
distribution or partition coefficient, equivalent to the fractional distance our
frog jumps out of the well: Repeated LLEs will yield a recovery closer and
closer to 100% without ever achieving complete extraction. Recovery from
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Frontal analysis
Elution analysis
Displacement development
Figure 2. Illustration of the three principal modes of elution chromatography
which are relevant to discussion of solid-phase extraction. A and B are the two
components to be separated and C is the carrier or displacement agent. Repro-
duced with permission from Ettre, L.S. and Horvath, C. Foundations of Modern
Liquid Chromatography, AnaLChem. g:422A. Copyright [I9751 American
Chemical Society.
samples extracted by separating funnel LLE may be matrix-dependent. The
composition of the sample itself, or the presence of contaminating soluble
or particulate material in actual samples, may alter the expected distribution
coefficient. Problems may result from the inability to adequately predict
when virtually complete extraction has been reached. That point will vary
from sample to sample as the nature of the matrix varies (Wells et al.,
1995). The differences in recovery are not always accurately predicted by
synthetically spiked samples. Therefore, equilibrium LLE by a generalized
procedure (for example, using a set extraction time or a specified number of
extractions) is not always appropriate for environmentally contaminated
samples.
SPE is a combination of non-linear modes of chromatography (Figure
1). The sample loading or retention step involves frontal chromatography
(frontal analysis or frontal separation), and the sample desorption, or elution
step, involves stepwise, or gradient, desorption or displacement develop-
ment (Lee et al., 1988; Wells et al., 1990). Non-linear chromatography is
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superior to linear chromatography in column load capacity, reduced mobile
phase consumption and simultaneous fi-actionation and concentration, and it
can produce greater concentrations than in the original samples (Lee et al.,
1988). Because SPE is a non-equilibrium, non-linear chromatographic pro-
cedure; it is able to produce simultaneous purification, fractionation, and
concentration.
Large volumes can be handled by SPE sorbents in either the column or
disc format. Most often the choice of using a disc or column is a personal
preference, although, for very large samples, a column format can be more
cost effective. Environmental laboratories already have filtration apparatus
including disc holders and side-arm vacuum flasks as standard equipment,
so discs or columns may be equally convenient.
Matrix solid-phase dispersion (MSPD) and solid-phase microextraction
(SPME) procedures are closely related to SPE. Each is a valuable analyti-
cal technique in its own right. However, the analyst should clearly under-
stand that both MSPD (batch adsorption) and SPME (partition-dependent)
are equilibrium procedures.
B. BREAKTHROUGH VOLUME
Most quantitative, analytical, chemistry texts for undergraduate college
level focus primarily on linear (or infinite dilution) chromatography, as
practiced in the pseudo-equilibrium techniques of high-performance liquid
chromatography (HPLC) and gas chromatography (GC). Such texts may
discuss nonlinear (finite concentration) modes of chromatography such as
frontal chromatography or displacement development briefly or not at all
(Figure 2). Linear chromatography creates a dilution of the solute because
the sample is applied in a small plug at the head of the column followed by
the continuous addition of mobile phase (Lee et al., 1988; Denney, 1976).
This is represented in Figure 3a. Conversely, in frontal chromatography the
sample is continuously added to the column. Only the least sorbed compo-
nent is obtained in a pure state (Ettre and Horvath, 1975; Denney, 1976).
This is represented in Figure 3b.
When dealing with large volume samples, the breakthrough volume of
certain analytes may be exceeded. Sample breakthrough is a function of the
strength of the interaction between the analyte and sorbent, the sample vol-
ume and the mass of sorbent. Sample breakthrough occurs regardless of the
type of sorbent. The solute-of-interest has some finite capacity factor in the
sample solvent. When the breakthrough volume is exceeded, the solute be-
gins to elute from the end of the column at the same time as sample is still
being added to the head of the column (Figure 3b).
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-
-
Increased sample loading
Strongly retamed
Elution development component
(sample behaves as a weak
Weakly retamed
eluent - not true SPE) component
M~xtureo f
- components
Frontal development
(both analytes retain but compete for
limited sorbent capacity - true SPE)
Figure 3. Cornpanson of elution development (A) as practiced in HPLC and
frontal development (B), as they would appear if applied to a SPE cartridge. Note
that as the sample size, and hence analyte loading, increases either the analytes
both migrate down the column (under conditions of weak retention) or one ana-
lyte competes effectively for the sorbent and reduces capacity for the other ana-
lyte.
Larrivee and Poole (1 994) defined breakthrough volume as "the volume
of sample, assumed to have a constant concentration, that can be passed
through the SPE device before the concentration of the analyte at the outlet
of the device reaches a certain fraction of the concentration of the analyte at
the inlet."
Bidlingmeyer (1 984) first described theoretical breakthrough volume
profiles in SPE. Hypothetical data is graphed (Figure 4) for two com-
pounds having assumed capacity factors of 10 and 30, respectively. With
an assumed column volume of 1.0 mL, recovery is theoretically 100% up to
and including a loading volume of 1 1 mL and 3 1 mL, respectively. That is,
a breakthrough volume of 1I column volumes is derived from a capacity
factor of 10 plus the volume of solvent in the column when the sample was
introduced (1.0 mL), and likewise for a breakthrough volume of 31 mL.
Beyond the point where the breakthrough volume is exceeded, recovery ef-
ficiency drops nonlinearly (Bidlingmeyer, 1984; Wells, 1985). Actual
breakthrough data are well modeled in the literature by theoretical profiles
for both column and disc formats for a variety of analytes (Larrivee and
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Poole, 1994; Wells, 1982;
Wells, 1985, Wells and Mi-
chael, 1987a; Wells et al.,
1990). The theoretical models
used are described in Chapter
6. The theoretical considera-
tions that are particularly ap-
plicable to large volume sam-
ples are discussed below.
Poole's research group has
published a substantial body of
research predicting SPE
breakthrough volumes by ex-
amining the strength of the in-
teraction between the analyte
and sorbent. Breakthrough
volumes were determined and
modeled using solvation or
VOLUME PASSED THROUGH
solvatochromic parameters to CARTRIDGE (m~)
characterize analyte retention A BIDLINGMEYER. LC MAGAZINE. 2 (198-
(Larivee and Poole, 1994;
Mayer et al., 1995). Solute Figure 4. The relationship between re-
size is identified as a primary covery, sample loading volume, and capac-
driving force for sorbent reten- ity factor by solid-phase extraction. Re-
tion under SPE conditions with printed with permission from Wells, M.J.M.
(1985) General Procedures for the Develop-
polar interactions favoring re-
ment of Adsorption Trapping Methods Used
tention in the aqueous mobile
in Herbicide Residue Analysis. The Second
phase and a decrease in the
Annual International Symposium, Sample
breakthrough volume (Miller
Preparation and Isolation Using Bonded
and Poole, 1994). The solva- Silicas, Analytichem International, Inc., 63-
tion parameter model also al- 68.
lows the prediction of break-
through volumes with different sample co-solvents. The selective sorption
of the organic solvent by the sorbent changes the stationary-phase volume
and system selectivity. This results in changes in up to an order of magni-
tude in the breakthrough volume when either methanol, propan- 1-01, tetra-
hydrofuran, or acetonitrile are added to the solvent as an organic modifier at
the 1% (vlv) level (Poole and Poole, 1995).
Hughes and Gunton (1995) applied a model based LLE theory to inter-
pret the elution profile for multi-component SPE data. Rather than a
graphical representation of recovery versus volume (as illustrated in Figure
4), they plotted extraction profiles as functions of [-ln(1-R,)] where RT is
the total extraction recovery, versus volume, V. The authors illustrated that
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analytes having extraction profiles with the same slopes exhibited no selec-
tivity i.e., no selective desorption. In these extraction profiles an intercept
not significantly different than zero indicated that none of the analyte was
irreversibly bound. The slope, intercept and relative shape of extraction
profiles are proposed by the authors to be more useful than the more com-
mon recoverylelution plots, allowing improved differentiation between
various SPE schemes. At the point where the profiles for different analytes
converge and became more linear, the effects of sample loading mass are
revealed.
Liska et al. (1990) measured breakthrough curves for polar compounds
from water. Using breakthrough volumes and widths of the elution curves,
theoretical preconcentration factors were calculated for all analyte-aqueous
sample-sorbent systems tested. Likewise, Nakamura et al. (1996) con-
cluded that attention to the sample volume is important to successful SPE
and reported that, for reversed-phase SPE, the appropriate sample volume
could be approximated in relation to the log Po,, values (logarithm of the n-
octanol/water partition coefficient) of the analytes. They determined that
for chemicals with a log Po,, value above approximately 3.5, there was no
breakthrough from alkyl-bonded, silica sorbents up to a sample volume of
one liter. However, the sorbent mass used in these experiments was un-
specified.
Bidlingmeyer (1984) reported that recovery is dependent on flow rate
through the SPE device because breakthrough volume is decreased due to
band-broadening at higher flow rates. Mayer and Poole (1994) found that
the recovery of analytes by SPE shows significant flow-rate dependence
when the sample volume exceeds the breakthrough volume of the analyte.
C. PRACTICAL IMPLICATIONS
The critical feature of an extraction is that it must be able to meet an antici-
pated detection limit for quantitation of the analyte. In order to meet this
requirement, the sample must be concentrated. The degree of concentration
is determined by the amount of sample that can be extracted without loss of
analyte in the breakthrough volume. For a given reversed-phase sorbent,
the breakthrough volume is a function of the hydrophobicity of the solute
and the mass of sorbent used. In such cases the dependence of analyte re-
covery on sample volume by SPE has been demonstrated for the strongly
hydrophobic C18 and polymer sorbents -both strongly hydrophobic. For
other sorbents, however, the analyte-sorbent interaction may depend on an
ion-exchange mechanism, weak van der Waals forces, or combinations of
these and other interactions. When the interaction is not a simple one, the
threefold relationship between sorbent mass, sample volume, and ana-
lytelsorbent interaction is less well explored.
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Loading Step Elution Step
4 Breakthrough check cartridge
Waste stream
Separate cartridges at elution
test eluents from both cartridges
Figure 5. Illustration of how a second cartridge, used in series with the primary
extraction cartridge, provides a simple indicator of breakthrough. If any of the
analyte appears in the eluent of the second cartridge after sample loading and sepa-
rate elution of each cartridge, then the sorbent capacity of the first cartridge has
clearly been exceeded.
When the analyte is not tightly adsorbed on the sorbent, the mass of
sorbent must be increased to maintain the desired sample volume. As the
strength of the interaction between the sorbent and the analyte increases, the
required sorbent mass decreases, and the sample volume that can be passed
over the sorbent before reaching breakthrough increases.
Our goal is to maximize the ratio of sample volume to sorbent mass,
since concentration achieved by an extraction depends not just on the vol-
ume of sample loaded but on the volume of the elution solvent required to
desorb the analyte. While breakthrough volume is roughly directly propor-
tional to bed mass, elution volume is roughly inversely proportional. The
loading capacity, or the total amount of material which the sorbent is physi-
cally capable of adsorbing, is not generally an issue in trace analyses. How-
ever, for analyses of weakly retained species or where poor analyte detec-
tivity requires that high microgram or low milligram quantities must be ex-
tracted, then the use of an additional column in series with the primary SPE
column is recommended as a backup to monitor for breakthrough (Figure
5).
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Pfaab and Jork (1994) determined that the ratio of sorbent and sample
volume necessary to avoid breakthrough of phenylurea herbicides during
SPE from drinking water should be 1 g of reversed-phase octadecyl bonded
sorbent per liter of water for the range of concentrations typically encoun-
tered. In our work (Wells and Michael, 1987a; Wells et al., 1994a) a 1 g
mass of sorbent has become a standard starting point for SPE method de-
velopment using reversed-phase sorbents, as well as for cation and anion
exchange sorbents. In addition to a sorbent mass of 1 g, a sample volume
of 100-200 mL and solute concentration of 100 ppb are reasonable starting
values when attempting to establish a new method. Many compounds will
be retained from a liter of water by a sorbent mass of 1 g.
Foreman et al. (1993) isolated multiple classes of pesticides from 10L
water samples using octadecyl bonded sorbent cartridges containing 10 g of
sorbent. For a wastewater containing dyes, surfactants, and dye carrier
components, a 1 g-to-100 mL ratio was established to prevent breakthrough
of the most hydrophilic, colored components of the effluent (Wells, et al.,
1994b). These parameters were successfully applied to the recovery of or-
ganics from wastewater by passing 9 L of sample through 90 g of reversed-
phase sorbent purchased as the bulk phase and packed into a chromatogra-
phy column.
111. NATURAL ORGANIC MATTER
Natural organic matter (NOM), comprising both particulate matter (PM)
and dissolved matter (often referred to as dissolved organic matter, DOM,
or dissolved organic carbon, DOC) is implicated in the environmental fate
and transport of chemicals. The presence of NOM is relevant to this discus-
sion, because it can complicate environmental analyses for several reasons.
In environmental matrices, analytes can exist in free form, or com-
plexed with particulate or dissolved organic matter. For example, NOM is
known to bind both metals and hydrophobic organic pollutants. The influ-
ence of this "associated" state on the transport of contaminants, their ulti-
mate degradation, or their bioavailability to the food chain, is unclear.
However, it clearly changes the retention properties of the bound analyte
compared to the free analyte. In the presence of sorbents used for SPE, or-
ganic matter and analytes complexed with organic matter, can also become
adsorbed on the sorbent, complicating analyses by changing the properties
of the extracting sorbent as the NOM binds to its surface.
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