System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water

doi:10.4236/jasmi.2012.24031

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Journal of Analytical Sciences, Methods and Instrumentation, 2012, 2, 203-207

http://dx.doi.org/10.4236/jasmi.2012.24031 Published Online December 2012 (http://www.SciRP.org/journal/jasmi) 203

System for High Throughput Water Extraction from Soil

Material for Stable Isotope Analysis of Water

Timothy S. Goebel, Robert J. Lascano*

Wind, Erosion, and Water Conservation Research Unit, Cropping Systems Research Laboratory USDA-ARS#, Lubbock, USA.

Email: *[email protected]

Received September 25th, 2012; revised October 26th, 2012; accepted November 2nd, 2012

ABSTRACT

A major limitation in the use of stable isotop e of water in ecological studies is the time that is required to extract water

from soil and plant samples. Using vacuum distillation the extraction time can be less than one hour per sample. There-

fore, assembling a distillation system that can process multiple samples simultaneously is advantageous and necessary

for ecological or hydrological investigations. Presented here is a vacuum distillation apparatus, having six ports, that

can process up to 30 samples per day. The distillation system coupled with the Los Gatos Research DLT-100 Liquid

Water Isotope Analyzer is capable of analyzing all of th e samples that are generated by vacuum distillatio n. These two

systems allow larger sampling rates making investigations into water movement through an eco logical system possible

at higher temporal and spatial resolution.

Keywords: Isotope; Vacuum Distillation; Soil

1. Introduction

Stable Isotopes of water have been studied in the fields’

of ecology and geology since the 1950’s with increased

attention over the last 30 years [1-6]. Historically the sam-

ples were analyzed using isotope ratio mass spectrometry

(IRMS) that requires the evolution of the sample into a

gas. This process is very sensitive; however, it is also

very time consuming [3]. With the advent of instrument-

tation using off-axis integrated cavity output spectros-

copy (OA-ICOS), such as the Liquid Water Stable Iso-

ope Analyzer (LIWA) DLT-100 from Los Gatos Inc. or

the L2130 from Picarr o Inc., the time required to analyze

samples is no longer the bottleneck in sample pro cessing

for stable isotope analysis [7]. Currently, the major bot-

tleneck for studying stable isotopes has shifted to the

time required for complete extraction of water from soil

samples collected from the field [8]. The larger the num-

ber of samples taken in the field the greater the temporal

and spatial resolution the samples will represent. There

are several different extraction methods currently used to

extract water from plant and soil samples. Examples of

distillation meth ods include azio tropic and cryog enic dis-

tillation techniques [9-12]. The objective was to develop

a multi-port water extraction system capable of produc-

ing unfractionated water samples from multiple soil sam-

ples in a day. Presented here are the details of the design

of a cryogenic vacuum distillation system. This system

follows the port design developed by West et al. [8]

having six ports that allow extraction of several sets of

soil samples in a day. A manifold that allows the iso-

lation of individual ports for troubleshooting and main-

tenance connects these ports.

2. Experimental

2.1. Build Details

2.1.1. Assembly of Extraction Port

The most basic component of the Vacuum Distillation

System is the port as illustrated in Figure 1. The port

connects the ignition tube and the collection test tube to

the vacuum. The ignition tube (Pyrex, Ignition Tube, 25 

200 mm, Rimless, Heavy Wall, # 9860-25) contains the

soil or plant material. It is connected to the glass “T” by a

custom made Swagelok adapter (# 8 in Figure 1). The

collection tube (Pyrex, Tube, 12  75 mm, Rimless, #

9820-12) contains the water extracted from the soil sam-

ple. It is connected to the glass “Tee” by another cus-

tom-ma de Sw agelok ad aptor (# 9 in Figure 1). The glass

“Tee” is a custom-made glass piece made by “Q” Glass

*Corresponding a uthor.

#The US Department of Agriculture (USDA) prohibits discrimination

in all its programs and activities on the basis of race, color, national

origin, age, disability, and where applicable, sex, marital status, famil-

ial status, parental status, religion, sexual orientation, genetic informa-

tion, political beliefs, reprisal, or because all or part of an individual’s

income is derived from any publi c assistance program.

System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water

204

II, LLC. It is a 3/8-inch (~10 mm) glass tube connecting

both the sample tube and the collection tube to the vacu-

um manifold. The glass “Tee” is then connected to an-

other Swagelok adaptor (# 3 in Figure 1). The next piece

is a Swagelok Union Tee (# 2 in Figure 1) allowing for

the addition of a thermocoup le gauge tube (# 5 in Figure

1) to the port through the use of another Swagelok adap-

tor (# 6 in Figure 1). The port is then connected to a

Swagelok Plug Valve (# 1 in Figure 1) that can isolate

the port from the manifold.

2.1.2. Assembl y of Mani f ol d

The manifold (Figure 2) was made using seven Swagelok

Plug Valves, one for each port plus one to release vacu-

um. The plug valves were connected using eight 3/8-inch

(~10 mm) copper “Tee”s connected by 4 cm long copper

tubing with the outside “Tee”s fitted with a cap. One of

the copper “Tee”s faces the opposite direction and is

connected to the vacuum (Edwards # 5). All of the cop-

per fittings were sized and assembled prior to soldering.

The completed manifold was then fastened to the frame

to stabilize the system. The assembled port was con-

nected to the manifold using the included swagelok com-

pression ferrule and 3/8-inch (~10 mm) copper tubing

that can be bent to fit multiple ports to the manifold (Fig-

ure 3a).

2.1.3. Construction of Support Base

The base of the system was manufactured from plywood

with the length of the base measuring 3.0 m and the

width measuring 0.6 m (Figure 3b). The platform that

the ports and manifold are fastened to was 0.3 m wide

and 3.0 m long with two braces in th e middle f or sup por t.

Given that the length of the sample tube and the collec-

tion tube are different, wooden 4  4 blocks were cut as

stands for the water beaker and the liquid nitrogen Dewar

(Figure 3b). Adjacent to each port, a heating coil was

fastened to the platform at an appropriate height to main-

tain boiling water in the beaker containing the sample

tube. While cost can vary based on location and time the

total cost for construction of the distillation setup was

less than $10,000 USD, making this a reasonably priced

solution for water extraction from soil samples for iso-

topic analysis.

Figure 1. Diagram of the port assembly with parts list.

System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water 205

2.2. Soil Sample Collection and Preparation

Soil samples were collected from a field adjacent to the

USDA-ARS research facility in Lubbock Texas, USA

(33˚3540 N, 101˚54'00 W). The soil texture was a sandy

clay loam and was ground and passed through a 2-mm

sieve before being oven dried at 225˚C for 24 hours.

From the bulk sample, 10 g of soil was weighed and

added to a pre-weighed ignition tube. The ignition tube

was then weighed again. One mL of water was then

added to the soil in the ignition tube and the weight was

taken again. Glass wool was then added to the ignition

tube to prevent soil loss during the ex traction process an d

the ignition tube was weighed again. The ignition tube

was then sealed with a rubber stopper and placed in a

freezer until it is time for extraction.

2.3. Extraction Procedure

The process of extraction begins by removing the sam-

ple-tube from the freezer and placing in LN2 (liquid Ni-

trogen). Next, the collection tube is placed on the port

with a “dummy” sample-tube. The vacuum is applied to

the system and allowed to draw down for 20 minutes to

remove any possible water contamination. During this

process the plug valve on the port was turned off to iso-

late the port from the vacuum to check for leaks. The

port should hold vacuum at <0.013 kPa (100 milliTorr)

or adaptors should be tightened unt i l 0.013 kPa is achi eved

and maintained.

The vacuum is then released and the “dummy” tube is

removed and replaced with sample-tube frozen in LN2.

The LN2 Dewar is placed under the sample-tube to

maintain the temperature at –210˚C preventing any loss

of water from the sample as vacuum is applied to the port.

The vacuum is then drawn down <0.013 kPa and the

system is again checked for leaks. If a leak is present it

must be from the adap tor connected to th e sample tu be as

Figure 2. Diagram of manifold.

the other junctions were tested previously. Once the port

is isolated from the vacuum pump and is holding vacuum

the LN2 Dewar is removed from the sample side of the

port and placed under the collection tube. A 1000 mL

beaker was placed under the sample tube and boiling

water was poured into the beaker up to the top of the

heating mantle and a timer was started.

The amount of time the sample was allowed to distill

was varied between 4 and 90 minutes for each soil sam-

ple. When the distillation ti me was complete the vacuum

was released from the port and the collection tube was

removed from the port, sealed with parafilm®, and placed

in a water bath to thaw. Once the collectio n tube is in the

bath, the sample-tube is removed from the port and al-

lowed to cool before weighing. All water samples that

were collected were then analyzed using a LGR-DLT

100 Liquid Water Isotope Analyzer following the IAEA

procedure for analysis of stable isotopes [13].

3. Results and Discussion

When using vacuum distillation it is imperative that the

resulting water extract be unfractionated. However, the

distillation process follows a Rayleigh distillation curve

where the heaver water (i.e., ) will condense first

and the lighter water (i.e., ) being condensed last.

This process creates a fractionated sample if the distilla-

tion process is stopped before all of the water has been

evaporated from the sample and condens ed in the collec-

tion tube. Calibration of the extraction timing is an es-

sential first step to develop a protocol that can be used in

the extraction system for stable isotope measurements.

As an example of this protocol, results from the calibra-

tion of the different ports are shown in Figure 4. These

results clearly show that the minimum extraction time

required to develop an unfractiona ted water sample from

soil samples is 30 minutes, based on both the isotope

signature as well as the percent water. There was no dif-

ference in the extraction times between the different ports

regardless of the distance from the vacuum source. While

the minimum extraction time was found to be 30 minutes,

in our labor atory we use one h our as our stand ard extrac-

tion time. This allows enough time to start all six ports

and the time necessary to make adjustments to a port

before the hour is up on first po rt.

4. Conclusion

The distillation apparatus described allows for a high

throughput water extraction from soil samples. The sys-

tem has six ports that allow us to extract 30 or more

samples in a 8-hour working day using one hour as the

standard extraction time. This system in conju nction with

the LGR DLT-100 allows for the extraction and analysis

of 30 samples per day making it possible to study water

System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water

206

(a)

(b)

Figure 3. (a) Diagram of the copper tubing used to connect the manifold to the six ports; (b) Photograph of the assembled

distillation apparatus.

Figure 4. Extraction timing curves for each port. The δ18O is represented by and the water content is represented by .

he dashed line represents the minimum extraction time to obtain an unfractionated water sample. T

System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water 207

movement in the soil at a higher spatial and temporal

resolution than has been previously used. Extraction times

may vary based on the soil type, e.g., clay vs. sand, and

as such calibration should be performed for differing soil

types. The system described here can also be used to ex-

tract water form plant material such as leaves and stems;

however, some concern has been noted about the inter-

ference of organics that can co-distill with the water

causing errors in the spectroscopic analysis of these wa-

ter samples [14,15]. More recently, a study has shown

that it is possible to calibrate the instrument for these

types of contaminants and remove the interference from

the measurement [16]. The possibility of using this ex-

traction technique for analysis of samples from both soil

and plant material makes this a simple cost effective sys-

tem for in depth analysis of water movement through an

ecosystem.

5. Acknowledgements

This research was supported in part by the Ogallala Aq-

uifer Program, a consortium between USDA-Agricultural

Research Service, Kansas State University, Texas Agri-

Life Research, Texas AgriLife Extension Service, Texas

Tech University, and West Texas A&M University.

We would like to thank Blake Bradley for his help in

developing the drawings of the apparatus. We would also

like to thank Ja mey Deusterhaus and Jill Booker for their

help and insight.

Mention of trade names or commercial products in this

publication is solely for the purpose of providing specific

information and does not imply recommendation or en-

dorsement by the US Department of Agriculture.

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