SPECIAL REPORT 96-16 Development of an Analytical

Method for White Phosphorus (P4)

in Water and Sediment Using

Solid-Phase Microextraction

Marianne E. Walsh, Susan Taylor and Philip G. Thorne August 1996
Abstract: Headspace solid-phase microextraction

(SPME) methods were developed for white phosphorus in water and sediment/soil to minimize waste generated by methods based on solvent extraction.

Headspace SPME provided a rapid, non-exhaustive

extraction, based on equilibrium, of white phosphorus.

Comparison of results obtained by headspace SPME

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and solvent extraction shows that headspace SPME may

be used quantitatively for some water matrices and qualitatively for more complex matrices such as sediment/

soil. Because detection limits appear to be similar to

those obtained by solvent extraction, headspace SPME

can be used to rapidly screen samples for contamination, eliminating the need to solvent-extract most samples.
Special Report 96-16

Development of an Analytical

Method for White Phosphorus (P4)

in Water and Sediment Using

Solid-Phase Microextraction

Marianne E. Walsh, Susan Taylor and Philip G. Thorne August 1996

Prepared for

U.S. ARMY ENGINEER WATERWAYS EXPERIMENT STATION

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by Marianne E. Walsh, Chemical Engineer, Applied Research

Division; Susan Taylor, Research Physical Scientist, and Philip G. Thorne, Research Physical

Scientist, Geological Sciences Division; Research and Engineering Directorate, U.S. Army

Cold Regions Research and Engineering Laboratory (CRREL), Hanover, New Hampshire.

Funding for this work was provided by the U.S. Army Engineer Waterways Experiment

Station (WES), Vicksburg, Mississippi, through Environmental Quality Technology Project

AF25, Ann Strong, Project Monitor.

The authors gratefully acknowledge Ann Strong, WES, and Thomas F. Jenkins and

Alan Hewitt, CRREL, for their technical review of the manuscript.

This publication reflects the personal views of the authors and does not suggest or

reflect the policy, practices, programs, or doctrine of the U.S. Army or Government of the

United States. The contents of this report are not to be used for advertising or promotional purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.

ii
iii

CONTENTS

Preface ............................................................................................................................................. ii

Introduction .................................................................................................................................... 1

Methods .......................................................................................................................................... 2

Standards and spiking solutions ............................................................................................ 2

Matrices ...................................................................................................................................... 2

SPME........................................................................................................................................... 3

Solvent extraction ..................................................................................................................... 3

Gas chromatograph .................................................................................................................. 4

Results and discussion .................................................................................................................. 4

Detection capability .................................................................................................................. 4

Depletion of total analyte present .......................................................................................... 4

Calibration ................................................................................................................................. 5

Field samples ............................................................................................................................. 9

Advantages of headspace SPME ............................................................................................ 11

Further studies .......................................................................................................................... 11

Summary and conclusions ........................................................................................................... 11

Literature cited ............................................................................................................................... 11

Abstract ........................................................................................................................................... 13

ILLUSTRATIONS

Figure

1. SPME device ......................................................................................................................... 1

2. Headspace SPME followed by thermal desorption in injection port of gas

chromatograph .............................................................................................................. 3

3. Mass of white phosphorus detected by headspace SPME vs. aqueous concentration 6

4. Concentration of white phosphorus estimated by headspace SPME vs. solvent

extraction for field-contaminated water samples .................................................... 9

5. Mass of white phosphorus detected by headspace SPME and concentration found

in same sediment sample following solvent extraction .......................................... 10

6. Flow chart of analytical methods for white phosphorus with headspace SPME

used to screen samples for white phosphorus ......................................................... 10

TABLES

Table

1. Mean peak heights obtained following headspace SPME/GC for water and soil

samples ........................................................................................................................... 4

2. Concentration of calibration standards prepared for headspace SPME of water

samples ........................................................................................................................... 5
Table

3. Water quality measurements for matrices used for aqueous white phosphorus

standards ........................................................................................................................ 5

4. Regression coefficients for linear calibration models of white phosphorus................ 6

5. White phosphorus mass detected by headspace SPME for replicate 0.144 μg/L

aqueous standards ........................................................................................................ 6

6. Predicted mass of white phosphorus sorbed to the SPME phase during headspace

SPME............................................................................................................................... 7

7. Mass of white phosphorus found by headspace SPME for samples extracted with

sonicated and static aqueous phases.......................................................................... 8

8. Blocked ANOVA comparing means of sonicated and static samples .......................... 8

9. Estimates of white phosphorus concentrations by headspace SPME and solvent

extraction in field-contaminated water samples ...................................................... 9

iv
INTRODUCTION

Analytical methods have been developed for

white phosphorus (P4) residues in sediment and

water (Walsh and Taylor 1993, Walsh 1995, USEPA

1995). Both methods rely on solvent extraction

prior to gas chromatographic analysis. The extracting solvents are isooctane and diethyl ether for

soils/sediments and water, respectively. The

method for soils/sediments has been performed

successfully in field laboratories (Racine et al.

1993), but the safety hazards associated with diethyl ether restrict the analysis of water samples

to laboratories with fume hoods. In this study, we

describe methods that minimize or eliminate the

use of organic solvents.

Solid-phase microextraction (SPME) is

a new alternative to traditional techniques for extracting volatile or semivolatile organics (Belardi and Pawliszyn 1989, Boyd-Boland et al. 1994,

Zhang et al. 1994). First developed to

analyze for volatile chlorinated

organics, PCBs (polychlorinated

biphenyls) and BTEX (benzene, toluene, ethylbenzene, xylene) in water

(Arthur and Pawliszyn 1990; Arthur

et al. 1992a, b, d; Potter and Pawliszyn 1992, 1994; Buchholz and Pawliszyn 1993), the method has been

successfully used for a wide variety

of analytes in environmental, food

and pharmaceutical matrices (Hawthorne et al. 1992, Otu and Pawliszyn

1993, Buchholz and Pawliszyn 1994,

Horng and Huang 1994, Yang and

Peppard 1994, Zhang et al. 1994). The technique

has several advantages (fast, simple, precise, sensitive), and requires no solvent (Arthur et al.

1992e). For this technique, a thin fused silica fiber

coated with a stationary phase is exposed to a sample, either by immersion in a water or air sample

or to headspace above an aqueous or solid sample (Fig. 1). Analytes sorb to the stationary phase,

then the fiber is transferred directly to a heated

injection port of a gas chromatograph for thermal

desorption and analysis. The method can be automated and an SPME autosampler is available commercially (Arthur et al. 1992c, Berg 1993).

Development of an Analytical Method for White Phosphorus (P4)

in Water and Sediment Using Solid-Phase Microextraction

MARIANNE E. WALSH, SUSAN TAYLOR AND PHILIP G. THORNE

 



































Plugger

Barrel

Adjustable Needle


Guide/Depth Gauge

Septum-piercing Needle

Fiber Attachment Tubing

Coated Fused Silica Fiber

Figure 1. SPME device.
Headspace sampling works well for analytes

with a Henry’s Law constant, the equilibrium partition coefficient for a chemical between water and

the headspace above it, above 90 atm-cm3/mole

(Zhang and Pawliszyn 1993a, b). Since Henry’s

Law constant for white phosphorus is over 2000

atm-cm3/mole (Spanggord et al. 1985), headspace

SPME is feasible. We were particularly interested

in headspace sampling because environmental

matrices containing white phosphorus are potentially complex (surface waters, wastewater, sediments), and the SPME fiber would be damaged or

fouled by immersion into these matrices. Headspace sampling relies on the equilibrium partitioning of the analyte among the matrix, headspace and

SPME phase (Zhang and Pawliszyn 1993a, b). The

extraction is not exhaustive, and for semi-volatiles

only a small portion of the total analyte present

becomes associated with the fiber. If depletion of

the analyte is negligible, the same aliquot of a sample can be analyzed first by headspace, then by

solvent extraction. Analysis of the same sample

aliquot by headspace SPME followed by solvent

extraction allows direct comparison of the two

methods without the confounding effect of heterogeneity, and makes SPME a valuable tool for

screening samples for contamination. Because the

majority of samples sent to analytical labs for the

analysis of volatiles or semi-volatiles tend to be

blank (devoid of the analytes of interest at analytical detection limits), considerable time and solvent

could be saved by screening samples for contamination prior to solvent extraction and analysis.

The objectives of this study were to

1. Determine if headspace SPME provides a detection capability similar to solvent extraction

methods for white phosphorus in water and

sediment matrices.

2. Test the feasibility of calibrating headspace

SPME for the quantitative analysis of water

samples.

3. Evaluate the performance of headspace SPME

with field-contaminated water and sediment

samples.

4. Compare concentration estimates obtained

via headspace SPME with estimates obtained

by solvent extraction.

5. Evaluate the suitability of headspace SPME

for use in a field laboratory.

METHODS

Standards and spiking solutions

An analytical standard for white phosphorus

was obtained from Aldrich Chemical Co. (Milwaukee, Wisconsin). The white phosphorus was supplied as a 5-g stick. It came immersed in water and

had a white coating on its surface. Pieces (100–300

mg) of white phosphorus were cut from the stick

under water. Care was taken to ensure that the surfaces of each piece of white phosphorus were freshly

cut, lustrous in appearance, and showed no evidence of white coating. These translucent, colorless

pieces were transferred under water to a test tube,

and the test tube was placed in a beaker filled with

water. The water with white phosphorus pieces was

heated to 54°C to melt the white phosphorus, which

remains as a separate phase. A Gilson Microman

Positive Displacement Pipet was used to obtain 25-

μL (45 mg) droplets of white phosphorus, which

were placed in separate test tubes under water. The

water was cooled to solidify the droplets.

A stock solution for calibration standards was

prepared under nitrogen by dissolving 90 mg of

white phosphorus in 250 mL of toluene (Aldrich

Chemical Co.). Standards over the range 7.2 to 7200

μg/L were prepared by diluting the stock solution

with methanol. Aqueous standards over the range

of 0.0072 to 0.72 μg/L were prepared by adding 25

μL of a methanol standard to individual 25-mL aliquots of water in 40-mL glass vials sealed with a

septum.

Aqueous solutions of white phosphorus were

prepared by placing pieces of white phosphorus

into an amber jug containing 4 L of reagent-grade

water (Type I) (MilliQ, Millipore) with no headspace, and agitating the jug for more than 60 days.

Matrices

Blank matrices used to prepare spiked samples

were reagent-grade (Type I) water (MilliQ, Millipore); groundwater from a domestic well in

Weathersfield, Vermont; surface water from a pond

in Hanover, New Hampshire; surface water from a

salt marsh in Anchorage, Alaska; Ottawa sand purchased from U.S. Silica (Ottawa, Illinois); a loamy

soil from the U.S. Army Environmental Center (Aberdeen Proving Ground, Maryland); and a sandy

silt from Lebanon, New Hampshire. Soil samples

were wetted to 100% moisture (dry weight basis,

i.e., equal parts water and soil) prior to spiking.

Field-contaminated samples were obtained from

Eagle River Flats, Fort Richardson, Alaska. Water

samples were collected in 1-L amber glass bottles

and sediment samples were collected in 120-mL

wide-mouth jars filled so that there was no headspace. Samples were maintained at 4°C until extracted.

2
SPME

SPME fiber assemblies were obtained from Supelco (Bellefonte, Pennsylvania) (Fig. 1). These

assemblies are composed of a fused silica fiber

coated with a stationary phase (we purchased the

100-μm polydimethylsiloxane-coated fibers). The

fiber is attached to a holder with a septum-piercing sheath that protects the fiber. The holder also

allows precise positioning of the fiber in samples

and the injection port of the gas chromatograph.

In general, the fiber is exposed to a sample for a

short period, during which time analytes sorb to

the stationary phase. Then the fiber is placed into

the injection port of a gas chromatograph to thermally desorb the analytes. We used the SPME fibers as follows: for each water sample, a 25-mL

aliquot was placed in a 40-mL vial. The vial was

either allowed to stand statically at room temperature or was placed in a sonic bath for five or ten

minutes, during which time the SPME phase was

exposed to the headspace (Fig. 2a). The SPME

phase was immediately transferred to a heated

(200°C) injection port of the gas chromatograph

(Fig. 2b) described below.

For each sediment/soil sample, a 40-g subsample was placed in a 120-mL jar containing 10.0 mL

of reagent-grade water. The jar was sealed with a

cap equipped with a septum. Each sample was

shaken manually for 15 seconds, and then the

SPME phase was exposed statically to the headspace for five minutes such that there was no physical contact between the fiber and the sample. The

SPME phase was thermally desorbed as described

for the water samples.

Solvent extraction

For water samples with white phosphorus concentrations less than 0.1 μg/L, preconcentration

of the solvent extracts is required. A non-evaporative approach is used since white phosphorus is

volatile and air-sensitive (Walsh 1995). A 500-mL

aliquot of water was mixed with 50 mL of diethyl

ether by shaking in a 500-mL separatory funnel

for five minutes. After phase separation, all of the

ether layer was collected. The volume of the ether

layer recovered varied depending on the temperature and the ionic strength of the samples; it generally ranged from 3 to 10 mL. The volume of the

ether layer was further reduced to 0.5–1.0 mL by

adding the ether extract to approximately 50 mL

of reagent-grade water in a 125-mL separatory

funnel and shaking for one minute. After phase

separation, the ether layer was collected in a 5.0-

mL graduated cylinder and the exact volume measured. This procedure resulted in a preconcentration factor of 500 to 1000. White phosphorus concentration in the extract was then determined by

gas chromatography. Extracts were analyzed immediately to minimize loss due to solvent evaporation.

For samples with white phosphorus concentrations greater than 0.1 μg/L (Walsh 1995), simple

solvent extraction provides sufficient concentration. A 25-mL aliquot of water was shaken for five

3

Figure 2. (left) Headspace SPME followed by (right) thermal desorption in injection port of gas chromatograph.
minutes with 2.00 mL of isooctane. Following

phase separation, the isooctane extract was analyzed.

Wet sediment samples were extracted by placing a 40-g subsample into a 120-mL jar containing

10.0 mL of reagent-grade water. Then, 10.0 mL of

isooctane was added. Each jar was tightly sealed

with a Teflon-lined cap and vortex-mixed for one

minute, and then placed horizontally on a platform shaker for 18 hours. The sample then was

allowed to stand undisturbed for 15 minutes, and,

if necessary, centrifuged for five minutes, to permit phase separation. Extracts were analyzed within a few hours.

Gas chromatograph

For solvent extracts or calibration standards,

white phosphorus was determined by injecting a

1.0-μL aliquot on-column into an SRI Model 8610

gas chromatograph equipped with a nitrogen–

phosphorus detector. SPME fibers were thermally

desorbed at 200°C in the injection port of the same

GC, and, for convenience, the SPME fiber was left

in the injection port for the entire run time (five

minutes).

The methylsilicone fused silica column (J and

W DB–1, 0.53-mm-i.d., 15-m, 3.0-μm film thickness)

was maintained at 80°C. The carrier gas was nitrogen set at 30 mL/minute. Under these conditions, white phosphorus eluted at 2.7 minutes.

RESULTS AND DISCUSSION

Detection capability

We evaluated the detection capability of SPME

while gathering validation data for analytical

methods for white phosphorus by solvent extraction and gas chromatography (Walsh et al. 1995).

For each matrix (Table 1), ten replicate spiked samples (1 L for water and 40 g for wet soil) were prepared by adding an aqueous solution of white

phosphorus to yield concentrations near the method detection limits for the solvent extraction methods (0.01 μg/L for water and 1 μg/kg for soil).

Headspace SPME was performed and peak height

data were obtained.

For those water samples that had white phosphorus concentrations near 0.01 μg/L, the peak

heights obtained by headspace SPME were small,

but white phosphorus was consistently detectable

in all spiked samples (signal-to-noise ratios were

greater than 7). Therefore the detection capability

of the SPME appeared to be comparable to solvent

extraction. In addition, the SPME response factors

(means of the peak heights normalized to spiked

concentration for each water matrix) were similar. The similarity in response with different water matrices was further studied in terms of calibration, as described below.

 In contrast, SPME peak heights varied with the

different soil matrices. Peak heights were lowest

in the sample with the highest organic content and

grain size distribution. However, in all cases, the

peak heights were much larger than those obtained

by solvent extraction, indicating that detection

capability of the headspace SPME might be better

than that for solvent extraction. Further studies

were performed with field-contaminated samples

as described below.

Depletion of total analyte present

White phosphorus may be present in sediment

samples as heterogeneously distributed particles

of different masses. Due to potential loss of white

phosphorus by sublimation and oxidation, traditional homogenization methods involving drying,

grinding, sieving, mixing, and subsampling are

not applicable for white phosphorus-contaminat4

Table 1. Mean peak heights obtained following headspace SPME/GC for

water and soil samples spiked at white phosphorus concentrations near

the detection limit for solvent extraction methods.

Peak height

Spiked Standard RSD Response

Matrix conc. n Mean deviation (%) factor*

Reagent-grade water 0.012 μg/L 10 1,465 252 17 122,000

Well water 0.0097 μg/L 10 1,119 106 9.5 115,000

Pond water 0.010 μg/L 10 1,013 235 23 101,000

Sand 1.9 μg/kg 9 270,481 20,356 7.5 142,000

Lebanon (Sandy silt) 0.97 μg/kg 10 97,063 10,486 11 100,000

USAEC (Loam) 0.84 μg/kg 9 74,381 6,157 8.3 88,500

* (Peak height)/(Spiked concentration [μg/L]).
ed sediments. As a result, wide variations are routinely observed in concentration estimates of subsamples from individual field-contaminated sediment samples. This heterogeneity complicates the

comparison of different analytical methods.

One potential attribute of headspace SPME is

the ability to detect the presence of an analyte without altering a sample. Provided that headspace

SPME does not remove a significant amount of

analyte, the same subsample of sediment can be

analyzed by headspace SPME followed by solvent

extraction and GC determination. Using the same

subsample for both determinations, the results of

the two methods can be compared without the

confounding effects of heterogeneity. Since heterogeneity between subsamples of bulk water samples has not been observed, we are primarily concerned with soil/sediment samples.

To see if concentration estimates based on the

amount of white phosphorus in solvent extracts

would be biased if headspace SPME was performed, we used the peak height data in Table 1

and peak height data from direct injections of calibration standards to calculate the mass of white

phosphorus detected by SPME. Less than 0.4% of

the initial white phosphorus present was removed

by headspace SPME from each soil matrix, and less

than 1% from each water matrix. Thus the amount

of white phosphorus removed by one headspace

SPME extraction is negligible.

Calibration

Calibration, which is necessary for quantitation,

has been achieved for other analytes by spiking a

known amount of analyte, generally dissolved in

a water-miscible solvent, into a clean water matrix, then performing SPME as is done for regular

samples (Zhang et al. 1994). Calibration for a complex matrix such as soil is more complicated since

matrix–analyte interactions are much more pronounced. While methods such as standard additions or internal standards have been suggested

and work well for some analytes (Zhang et al.

1994), the difference in extractability between

spiked and native analyte can have a profound

influence, unless the extraction is exhaustive. Recently, exhaustive headspace SPME extraction of

some volatiles was achieved by simultaneous heating of the sample and cooling of the fiber coating

(Zhang and Pawliszyn 1995). Prior to that study,

quantitative results by SPME were reported primarily for water samples, and results for soil samples have been predominantly qualitative.

Linearity

To determine the feasibility of calibrating headspace SPME for water contaminated with white

phosphorus, a series of aqueous calibration standards over the range 0.0072 to 7.2 μg/L (Table 2)

was prepared in duplicate. Linearity of response

(peak heights) with respect to white phosphorus

concentration was tested for water from four sources (a MilliQ purification system, a well water, a

pond water and a salt marsh) (Table 3). These aqueous standards were prepared by adding 25 μL of

standards prepared in methanol to 25 mL of water,

which resulted in a methanol concentration of 0.1%.

Previous studies with BTEX showed that a methanol of less than 1% did not effect peak areas obtained by SPME (Arthur et al. 1992c). Peak heights

obtained by headspace SPME-GC were converted

to mass (pg) of white phosphorus from response

factors based on syringe injections of standards in

methanol (Zhang and Pawliszyn 1993a).

For data for each water matrix, a linear regression model with intercept (Fig. 3) and a linear regression model through the origin were tested for

lack of fit. In all cases, linear models adequately

described the data over the concentration range

0.0072 to 0.144 μg/L (the F ratio for lack of fit was

less than the critical value for 95% confidence)

5

Table 2. Concentration of calibration

standards prepared for headspace SPME

of water samples.

Mass of white

Conc. of Conc. of phosphorus in each

methanol std. aqueous std. aqueous std.

(μg/L) (μg/L)* (ng)

7.2 0.0072 0.18

14.4 0.0144 0.36

28.8 0.0288 0.72

72 0.072 1.8

144 0.144 3.6

1440 1.44 36

7200 7.2 180

* Prepared by adding 25 μL of methanol

standard to 25 mL of water.

Table 3. Water quality measurements for

matrices used for aqueous white phosphorus

standards.

Conductivity Redox

Matrix (mmho/cm) pH (mV)

Reagent-grade water 0

Well water 0.277 7.6 163

Pond water 0.201 7.4 14

Salt marsh water 23 7.9 133
(Table 4). Higher concentrations were beyond the

linear range of the detector. The slopes for the regression models for the four different waters were

not compared statistically because the data were

not generated on a single day; however, they appeared to be similar.

Matrix effects

To confirm the similarity in response (mass of

white phosphorus detected by headspace SPME)

among the four different matrices, five replicate

0.144-μg/L aqueous standards were prepared for

each matrix and analyzed on a single day. The

means of the responses obtained (Table 5) were

compared by ANOVA and were not significantly

different (p = 0.65) despite the fairly wide range

6

10

20

30

40

50

0 0.05 0.10 0.15

Reagent

Well 

Pond 

Salt Marsh 

White Phosphorus Mass (pg)

White Phosphorus Concentration (μg/L)

Figure 3. Mass of white phosphorus (pg) detected by

headspace SPME vs. aqueous concentration over range

of 0.0072 to 0.144 μg/L. Lines represent least squares

regression models.

Table 4. Regression coefficients for linear calibration models of

white phosphorus (pg) detected by headspace SPME vs. aqueous

concentration over range of 0.0072 to 0.144 μg/L.

Zero-intercept

Model* with intercept Model through origin hypothesis

Water Lack of fit Lack of fit Calculated

matrix Slope Intercept F ratio† Slope F ratio** F††

Reagent grade 226 –0.0067 0.15 226 0.11 0.00014

Well 272 –0.42 0.16 268 0.16 0.26

Pond 280 –0.27 0.88 278 0.71 0.21

Salt marsh 230 0.41 0.24 234 0.25 0.41

* y = mx + b, where y = mass of WP (pg) and x = aqueous WP concentration

(μg/L)

† Critical F0.95 (3,5) = 5.41

** Critical F0.95 (4,5) = 5.19

†† Critical F0.95 (1,8) = 5.32

Table 5. White phosphorus mass (pg) detected by headspace SPME for replicate

0.144 μg/L aqueous standards prepared in

four water matrices and analyzed on the

same day.

White phosphorus mass (pg)

Rep Reagent Well Pond Salt marsh

1 31.8 36.5 41.3 39.6

2 35.8 39.1 31.5 32.1

3 37.6 32.9 36.7 33.1

4 32.9 36.2 36.6 34.3

5 32.3 34.1 35.6 36.8

Mean 34.1 35.8 36.3 35.2

Std dev 2.5 2.4 3.5 3.0

% RSD 7.4% 6.8% 9.6% 8.6%

Mass (pg) 237 249 252 244

Conc. (μg/L)
of conductivities (0 to 23 mmho/cm) between the

matrices. These results, however, are consistent

with those observed for BTEX where salt concentrations below 1% did not significantly affect results (Arthur et al. 1992c). However, salt saturation has been used successfully to enhance response for BTEX by headspace SPME (MacGillivray et al. 1994).

The mean mass of white phosphorus detected

by headspace SPME for the combined data set

(Table 5) is 35.4 pg. To prepare each 25-mL aqueous standard at 0.144 μg/L, 3600 pg of white phosphorus was added. Therefore, the mass removed

by headspace SPME was 0.98% of the initial mass.

These data provide further confirmation that headspace SPME may be performed on the same sample prior to solvent extraction without biasing the

concentration estimates obtained by solvent

extraction. For water samples, sequential SPME–

solvent extraction of the same aliquot would be

appropriate for samples with white phosphorus

concentrations greater than 0.1 μg/L. For water

samples with lower concentrations, a larger aliquot (500 mL) of water is needed to provide adequate preconcentration using ether extraction.

Results using these various extraction options will

be discussed later in relation to field samples.

Model predictions

Zhang and Pawliszyn (1993a) derived the following equation for the mass of analyte sorbed

by the SPME phase at equilibrium during headspace SPME of water samples:

 

n C VV

V VV = K KKK K

1

1

2 012

21 23 2 + +

where K1 = SPME phase/gas partition coefficient

K2 = gas/water partition coefficient

C0 = initial concentration of the analyte in the aqueous solution

V1, V2, and V3 = volumes of the SPME phase,

aqueous solution and the headspace.

The gas/water partition coefficient (K2) is derived

from the Henry’s Law constant (KH)

 

K K

2

H = RT .

The SPME phase/gas partition coefficient (K1) can

be estimated from the relationship

K = K1K2

7

where K is close to the octanol water partition coefficient (Kow) for many analytes (Zhang and Pawliszyn 1993a, Louch et al. 1992, Dean et al. 1996).

For BTEX compounds, the ratios of Kow/K ranged

from 0.9 to 2.7 (Zhang and Pawliszyn 1993a). Kow

and KH have been measured for white phosphorus (Spanggord et al. 1985). The estimates are Kow

= 1200 and KH = 2100 atm-cm3/mole. Using these

estimates for Kow and KH, the predicted mass of

white phosphorus sorbed to the SPME phase was

computed for equilibrium with a 0.144-μg/L aqueous solution and the two size vials used in these

studies (Table 6).

Table 6. Predicted mass (pg) of white

phosphorus sorbed to the SPME

phase during headspace SPME based

on model by Zhang and Pawliszyn

(1993) and estimates of Kow and KH

by Spanggord et al. 1985.

Size of Volume (mL) Theoretical

vial Aqueous mass sorbed

(mL) Headspace solution (pg)

40 1 39 104

5 35 102

10 30 100

15 25 97

20 20 93

25 15 87

30 10 77

35 5 57

120 1 119 105

10 110 104

20 100 103

30 90 102

40 80 100

50 70 98

60 60 95

70 50 92

80 40 87

90 30 80

100 20 69

110 10 49

Predictions are shown for two sizes of extraction vials used for these studies and various

volumes of headspace. Numbers in bold correspond to volumes used in this study.

= mass of analyte

sorbed by the SPME phase at equilibrium

during headspace SPME.

V1= volume of SPME phase = 6.12 × 10–4

mL*

V2= volume of aqueous solution

V3= volume of headspace

C0= 0.144 μg/L

K1= SPME phase/gas partition coefficient

K2= gas/water partition coefficient = KH/

RT = 0.09

K1K2 ≈ KOW = 1200.

* Personal communication, Supelco, 1996.

 

n C VV

V VV = K KKK K

1

1

2 012

21 23 2 + +
For 25 mL of 0.144 μg/L aqueous solution in a

40-mL vial, the predicted mass of white phosphorus sorbed by the SPME phase is 97 pg. We found

approximately 35 pg, or about one-third of the predicted mass. Assuming that K2 is accurately predicted from KH, the amount of mass sorbed corresponds to a K of 410. The ratio between Kow and K

equals 2.9, which is slightly outside the range reported for BTEX. Therefore the model predictions

and experimental results are in fair agreement.

The model also predicts the effect of variable

volumes of headspace. Predicted mass sorbed increases as headspace volume shrinks, but the effect is minimal for the size vials we used when the

vials are at least half full. This characteristic is very

useful in a field laboratory since sample volumes

need to be measured with a precision of approximately ±1 mL. Sample aliquots could be obtained

by gentle pouring into graduated vials, eliminating the need for volumetric pipets and associated

glassware washing and rinsing.

Agitation

During headspace SPME, agitation of the sample hastens equilibrium (Arthur et al. 1992c). We

used a sonic bath (Motlagh and Pawliszyn 1993),

but found that temperature had to be carefully controlled. Fluctuating temperatures decrease precision. Higher temperatures increase the analyte vapor phase concentration but decrease analyte sorption into the fiber (MacGillivray et al. 1994, Zhang

and Pawliszyn 1995) since sorption is an exothermic process (Arthur et al. 1992c). Stirring was not

tested, but has been used by previous investigators (MacGillivray et al. 1994, Zhang and Pawliszyn

1995). The most convenient and least labor-intensive setup is static extraction for five minutes

(length of the GC run) at room temperature.

Zhang and Pawliszyn (1993a) predicted equilibration time for headspace SPME using a model

based on one-dimensional diffusion described by

Fick’s second law. Equilibration time was controlled by the gas/water and SPME phase/water

partition coefficients (K2 and K1 described above).

Extraction time profiles of analytes by headspace

SPME are characterized by a three-part curve: first,

by a rapid increase in mass sorbed within the first

minute of exposure of the SPME phase to the headspace, followed by a much slower increase, then

no change at equilibrium. The initial increase corresponds to the sorption of analyte initially present

in the headspace, and the slower increase is due to

the mass transfer of analytes from the aqueous

phase (Pawliszyn 1995). The transition between the

initial rapid rise and equilibrium is reduced by

agitation. Based on this model and the physical

properties of white phosphorus (Kow = 1200, KH =

2100 atm-cm3/mole), equilibration time for white

phosphorus should be only a few minutes for wellagitated samples. Even though equilibrium may

not be reached within a few minutes when the

sample is not agitated, the mass absorbed should

approach the mass at equilibrium because of the

shallow slope of the extraction profile.

To see if the elimination of the sonication significantly decreased the mass of white phosphorus sorbed to the SPME phase, four replicate 0.144-

μg/L aqueous standards were prepared in reagentgrade water and extracted and analyzed in random order by headspace SPME by three methods:

five minutes of sonication, ten minutes of sonication, and five minutes static. One sample was extracted statically for 20 minutes, then 80 minutes.

The results (Table 7) were compared by blocked

ANOVA (Table 8), and a significant difference was

found indicating that equilibrium was not reached

during five minutes of static headspace SPME.

However, the difference was small for practical

purposes, 30.6 pg vs. 34.0 pg (90% recovery). For

the one sample extracted statically for 20 minutes

and 80 minutes, the masses detected were 31.5 and

35.2 pg, respectively. Since sonication or lengthy

static extractions resulted in only modest increas8

Table 7. Mass of white phosphorus (pg)

found by headspace SPME for samples

extracted with sonicated and static aqueous phases.

White phosphorus mass (pg)

Rep 5 min sonic 10 min sonic 5 min static

1 30.3 32.7 29.1

2 36.8 34.5 31.5

3 36.8 35.1 30.8

4 31.2 33.6 31.0

Mean 33.8 34.0 30.6

Std dev 3.52 1.06 1.02

% RSD 10.4% 3.1% 3.3%

Table 8. Blocked ANOVA comparing means

of sonicated and static samples.

ss df MS F

Total 12971.74 12

Correction factor 12899.92 1

Between block 28.02 3 9.34 3.58

Between treatment 28.14 2 14.07 5.39

Error 15.66 6 2.61

F0.95(2,6) = 5.14
9

es in response, simple five-minute static extraction

should provide satisfactory precision and sensitivity if timing and extraction conditions are consistent. When analyzing samples in a field lab, eliminating the need for extra equipment and steps in

the method will save time and money.

Field samples

Water

Water samples were collected from a salt marsh

that was contaminated by white phosphorus munitions (Racine et al. 1993). Each of the five samples plus a blank were analyzed first by headspace

SPME and concentrations estimates obtained using aqueous calibration standards. Triplicate measurements were made on some samples to estimate

precision with real samples. White phosphorus was

detectable by headspace SPME in three of the five

water samples (Table 9).

Solvent extractions were then performed, either

with isooctane or diethyl ether, depending on the

concentration. One sample had an estimated concentration of 0.1 μg/L, which is barely detectable

by isooctane extraction, so both types of extractions

were performed, with the isooctane extraction performed on the same subsample as the headspace

SPME. One sample was overrange using the headspace SPME method. The sample was diluted by a

factor of 1:100 with reagent-grade water, and headspace SPME was repeated, followed by isooctane

extraction of the same aliquot of sample.

Agreement between the methods was good, as

was the precision of the headspace SPME method

(Table 9). Regression of the mean of the headspace

SPME determinations with those by solvent extraction yielded a linear model with slope of 1.07 and

R2 of 0.997 (Fig. 4). Although the data set is small

and more samples need to be analyzed for confirmation, these initial results showed good agreement between the different methods.

Sediment

A much larger number of sediment samples

were available for analysis. A total of 92 sediment

samples was analyzed by headspace SPME followed by solvent extraction. We used the same

subsample for both types of extractions. Of these

samples, 30 were blank and 62 were positive by

both methods; therefore, there were no false positives or false negatives for the headspace SPME

method. The sediment samples varied widely in

salt and organic matter content, and this matrix

heterogeneity resulted in a poor correlation between the amount of white phosphorus detected

by headspace SPME and the concentration found

by solvent extraction (Fig. 5a, b). Nonetheless,

headspace SPME proved to be an excellent screenTable 9. Estimates of white phosphorus concentrations by headspace SPME and solvent extraction in field-contaminated water samples.

Estimated concentration (μg/L)

Headspace Solvent extraction

Sample SPME isooctane* Ether

1 < d 0.003

< d

< d

2 < d < d

< d < d

<d

3 0.0348 0.0149

0.0354 0.0187

0.0355

4 0.109 0.0834 0.0759

0.106

0.0972

5 >17.2 (overrange) 39.7

38.7

5 (1/100 dilution) 0.399 0.369

Blank < d < d

* Isooctane extraction of the same subsample as headspace

 SPME.

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

White Phosphorus Concentration (μg/L) by SPME

White Phosphorus Concentration (μg/L) by Solvent Extraction

Figure 4. Concentration (μg/L) of white phosphorus

estimated by headspace SPME vs. solvent extraction

for field-contaminated water samples.
10

20

40

60

80

100

0 0.005 0.010 0.015 0.020

WP Mass (pg) by SPME

WP Concentration (μg/g) by Solvent Extraction

2

4

6

8

0 0.0003 0.0006 0.0009 0.0012 0.0015

White Phosphorus Mass (pg) by SPME

White Phosphorus Concentration (μg/g) by Solvent Extraction

a. Data within linear range of detector.

Figure 5. Mass of white phosphorus detected by headspace SPME and concentration found in same sediment sample

following solvent extraction.

b. Same data as a, showing results at lowest concentrations detected.

Figure 6. Flow chart of analytical methods for white

phosphorus with headspace SPME used to screen

samples for white phosphorus.

Sediment/ Soil

(40 g plus water to form slurry)

Headspace Solid Phase

Micro-Extraction (SPME) (5 min)

and

Thermal Desorption in Gas Chromatograph

Nitrogen Phosphorus Detector (NPD)

White Phosphorus Detected by SPME?

No

Yes

Extract Sediment/Soil with 10 mL Isooctane

Analyze Extracts by Gas Chromatography

Nitrogen Phosphorus Detector

Calculate White Phosphorus Concentration

Based on Calibration Standards in Isooctane

Stop

Ma. Wa

MaW 0

Water

(25 mL)

Headspace Solid Phase

Micro-Extraction (SPME) (5 min)

and

Thermal Desorption in Gas Chromatograph

Nitrogen Phosphorus Detector (NPD)

White Phosphorus Detected by SPME?

No

Stop

Yes

Yes

No

Avoid Solvent

Extraction?

Estimate White Phosphorus

Concentration Based on Matrix

Calibration Standards

Extract 500 mL Water with 50 mL Diethyl Ether

Collect Ether Extract

Reduce Volume of Ether Extract to 1.0 mL

by Mixing with Reagent Water

Analyze Extracts by Gas Chromatography

Nitrogen Phosphorus Detector

Calculate White Phosphorus Concentration

Based on Calibration Standards in Ether

b. Water method.

a. Sediment/soil method.
ing method. Based on the results from these fieldcontaminated sediment samples, the detection limit

of the headspace SPME appeared to be similar to

that of the solvent extraction (Fig. 5b).

The samples used for this experiment were collected from an area that was known to be contaminated, thus the high percentage of positive samples. Typically, most samples submitted for analysis are blank. We suggest that solvent extraction be

used only for those sediment and soil samples

where quantitative results are desired (Fig. 6).

Advantages of headspace SPME

When samples are analyzed for white phosphorus, headspace SPME provides the following

advantages over the methods based on solvent

extraction:

1. Time required for extraction/preconcentration is reduced to five minutes for water samples.

2. Water samples may be analyzed easily in a

field laboratory.

3. Sediment/soil samples can be screened for

the presence of white phosphorus.

4. Solvent extraction of blank samples can be

eliminated, reducing the production of hazardous laboratory waste and reducing costs.

Further studies

Beyond these initial studies, the following

experiments will be performed.

1. Gather more data to validate quantitative

results for field-contaminated water samples.

2. Determine if salt saturation, which generally

increases response of headspace SPME for

BTEX, is appropriate for white phosphorus.

3. Explore ways to calibrate headspace SPME for

sediments/soils.

SUMMARY AND CONCLUSIONS

Using the spiked and field-contaminated water

and sediment samples, we tested headspace SPME

for the analysis of white phosphorus. The SPME

fiber was simply exposed to the headspace above

a sample and then thermally desorbed in the injection port of the gas chromatograph. We calibrated

the method to obtain quantitative results for water

samples. Matrix dissimilarities confounded quantitation for sediment samples; however, headspace

SPME successfully detected the presence of white

phosphorus in sediment. Using headspace SPME

of water and sediment matrices, detection limits

appear to be similar to those obtained by solvent

extraction.

11

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August 1996

Development of an Analytical Method for White Phosphorus (P4) in

Water and Sediment Using Solid-Phase Microextraction

Marianne E. Walsh, Susan Taylor and Philip G. Thorne

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17

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

Headspace solid-phase microextraction (SPME) methods were developed for white phosphorus in water and

sediment/soil to minimize waste generated by methods based on solvent extraction. Headspace SPME provided a rapid, non-exhaustive extraction, based on equilibrium, of white phosphorus. Comparison of results

obtained by headspace SPME and solvent extraction shows that headspace SPME may be used quantitatively

for some water matrices and qualitatively for more complex matrices such as sediment/soil. Because detection limits appear to be similar to those obtained by solvent extraction, headspace SPME can be used to rapidly screen samples for contamination, eliminating the need to solvent-extract most samples.

Analytical method

P4

Solid-phase microextraction

SPME

White phosphorus