Bedford Institute of Oceanography (4~ I'lnstitut oceanographique de Bedford

Dartmouth/Nova Scotia/Canada

A Modified Niskin Bottle for Trace-Element Sample Collection

J. M. Bewers, W. W. Hall, and I. D. ay

Report Series/BI-R-74-2IFebruary 1974

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BEDFORD INSTITUTE OF OCEANOGRAPHY

Dartmouth, Nova Scotia Canada

A MODIFIED NISKIN BOTTLE

FOR TRACE-ELEMENT SAMPLE COLLECTION

by

J.M. Bewers, W.W. Hall and I.D. Macaulay

February 1974

Atlantic Oceanographic Laboratory Marine Sciences Directorate

Department of the Environment

REPORT SERIES BI-R-74-2

ABSTRACT .I

The design and use of a modified Niskin bottle for trace-element

sample-collection in marine waters is discussed. The modification involves

the replacement of the internal rubber spring by a PFTE-sheathed shock cord.

RESUME

On discute Ie dessin et l'emploi d'un echantillonneur Niskin qui a

ete modifie pour la collection des echantillons de l'eau de mer pour

l l'analyses d'elements en concentrations tres petites. La modification se I

compose du remplacement du ressort interieur de caoutchouc avec un corde de

choc couvert d'un gaine de PFTE.

DISCUSSION

Niskin(TM) sampling bottles (Fig. 1), manufactured of polyvinyl

chloride (PVC), are widely used for the collection of sea water samples for

trace element analysis. These bottles contain an internal spring which,

unless otherwise specified, consists of surgical rubber tubing. Spencer

and Brewer (1970) have discussed these samplers and sanctioned their use

for trace element sample collection provided that the sample does not come

in contact with the rubber spring. Unless this condition is observed the

sample can become contaminated with significant amounts of zinc, copper and

antimony. These authors have recommended that, for trace element work, the

Niskin bottle be modified by replacement of the rubber spring with one made

of teflon-coated stainless steel. This modification subsequently became an

option available from the manufacturer.

Our experience with teflon-coated steel springs has indicated that

they have two main disadvantages. First, unless replaced frequently, they

are subject to chafing with subsequent peeling of the teflon coating. This

leads to the exposure of bare metal which may corrode and contaminate the

sample. Second, the springs are susceptible to fatigue and in the case of

breakage during oceanographic operations one end cap is usually lost.

In view of the disadvantages of rubber and teflon-coated steel

springs, we have replaced them with polytetrafluoroethylene (P.T.F.E.)

sheathed shock cords. Lengths of rubber shock cord are placed within

R~clad(TM) 3/B-inch diameter shrinkable P.T.F.E. tubing. The sheath is

heat-shrunk onto the shock cord and each end of the assembly is then sealed

into threaded PVC plugs which can be screwed directly into modified bottle

end caps (Fig. 2). Figure 3 is an end view of the open bottle after final

assembly. The use of the threaded end plugs allows springs to be replaced

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easily at sea when required. Assembly instructions and working drawings are

included in the appendix to this report. To date these modifications have

only been applied to l2-litre Niskin bottles to which external assemblies,

consisting of tripping mechanism and handle, are cemented with PVC dowels.

The 30-litre Niskin bottle differs in that some external parts are attached

~

with metal bolts passing through the bottle wall. Epoxy plugs are used to

seal off the recessed bolt heads internally. There are indications

(J .M. Edmonds, private communication, 1973) that due to poor binding of the'

plugs, channels containing iron oxide particles have formed in the PVC-epoxy

interface. It is not known whether these particles can actually contaminate

samples.

It is possible to acid-leach the Niskin bottles after the sheathed

springs have been installed, although normally they are only washed with

deionized water before each cruise. During use it is to be expected that

some water will penetrate the spring sheath due to the large pressure dif-

ferentials involved. Since this might lead to sequential cross contamina-

tion of samples, it was thought necessary to determine the amount of water

which can be exchanged through the spring sheath. This was done by rinsing

a modified Niskin bottle, immediately after use at sea, with deionized water

and allowing it to stand for a few days. Changes in the sodium concentration

during storage indicate that the maximum amount of sea water released from

the sheath assembly is 20 ~£. This volume represents only 2xlO-4% of the

sample volume and, therefore, contamination of samples by this mechanism is

very small.

In conjunction with this work a weighted PVC messenger was developed

(Fig. 4 and 5) to minimize the quantity of metal particles which slough off

the hydrowire during its descent.

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REFERENCES

SPENCER, D.W., and P.G. BREWER. 1970. Analytical methods in Oceanography,

I. Inorganic Methods. Critical Reviews in Solid State Sciences,

Chemical Rubber Company: 409-478.

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FIGURE 3: Detail of modified l2-litre Niskin bottle showing sheathed spring and modified end cap

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FIGURE 4: PVC-sheathed, weighted messenger

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FIGURE 6: Modifications to 'Niskin' Sampling Bottle

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APPENDIX

MODIFIED NISKIN BOTTLE ASSEMBLY INSTRUCTIONS

1. Unravel the braiding at one end of a 4 ft length of 3/8 inch diameter

shock cord.

2. Tie a length of string to the rubber core exposed by unravelling.

3. Crimp the opposite end of the shock cord in a vice.

4. Stretch the shock cord to reduce its diameter.

5. Thread string and shock cord through a 3~ ft length of 3/8 inch diameter

shrinkable tubing.

6. Relax the shock cord and cut a 3 ft length of both cord and sheath

with a sharp knife, leaving neat square ends.

7. Thread into the assembly the two end adapters ensuring that they are

the correct way round.

8. Thread onto the assembly the nylon ferrules positioning them about

1/8 inch from each end.

9. Thread two 7~ inch lengths of 3/4 inch diameter shrinkable tubing over

the assembly so that each piece overlaps the nylon ferrules by about ~ inch.

10. Shrink the 3/4 inch diameter tubing, with a heat gun, to fit the assembly

and trim end.

11. After cooling, thread the adapters over the larger shrunken tubing so

that they are hard against the ferrules.

12. Push the spreaders into each end of the assembly and if necessary trim

back the end of the sleeving to allow for expansion of the core.

13. Place the assembly inside the Niskin bottle and screw the adapters into

the modified end caps.