Contamination with cadmium from micropipette tips

Tulonru. Vol. 26. pp. 175-176 0 Pergamon Press Ltd1979. Printed in Great Britain

0039-9l40/79/0201-0175$02.00/0

ANNOTATIONS

CONTAMINATION WITH CADMIUM FROM MICROPIPETTE TIPS S. SALMELA* and E. VUORI Department of Public Health Science, University of Helsinki, Helsinki, Finland (Received 8 May 1978. Accepted 11 September 1978)

Summary-Contamination by cadmium from micropipette tips from two different manufacturers was tested. Tips were washed with two different acids and the washing solutions were analysed by atomic absorption (graphite furnace). The washed tips were controlled by analysing pure acids and were found decontaminated. Clear contamination was found in yellow Eppendorf pipettes, which varied according to the consignment. The decontamination of disposable micropipette tips is emphasized when trace metal contamination should be avoided.

A literature survey indicated that there are not many references concerning contamination with metals from disposable micropipette tips. However, the rapidly expanding use of these tips and the sensitivity of modern analytical methods warrant investigation of the possibility of such contamination. For example, zinc, iron and calcium have been warned about and rinsing of the tips with 1M hydrochloric acid and water has been recommended. When determining micro-amounts of cadmium in dry-ashed human milk samples by graphite-furnace atomic-absorption spectrometry, we found it impossible to obtain reproducible signals, which varied significantly from one to another. The volume pipetted was 20 ~1 and a manual pipetting procedure was used. The pipette tips were, as usual, rinsed several times with the sample liquid before use. After some experiments we suspected contamination caused by the pipette tips. Twelve microlitre pipette tips from the two manufacturers were left in 50ml of 2M hydrochloric acid overnight. The inside of the tips was also in contact with the acid. Next day the tips were rinsed several times with demineralized water and dried at room temperature before use. Three successive portions of 6M hydrochloric acid were introduced into the furnace with the same unwashed tip. The procedure was repeated with 2M hydrochloric acid. The acid used for soaking the tips was analysed three times, decontaminated tips being used to introduce it into the furnace.

spectrometer equipped with a deuterium backgroundcorrector, HGA-74 graphite cell, HGA-2100 controller unit, Servogor Model RE 511 recorder and cadmium hollow-cathode lamp were used. The purge gas used was argon. The operating parameters were according to the manufacturers’ manual. An Eppendorf 20+1 micropipette was used with yellow micropipette tips (for 5-100~1 pipettes, Eppendorf Geratebau Netheler + Hinz GmbH) and Finnpipette tips (Labsystems Oy, Finland). Reagents

Hydrochloric acid (Merck “Suprapur”) solutions (2M and 6M) were made by diluting with dis-

EXPERIMENTAL Apparatus

A

Perkin-Elmer

Model

300

atomic-absorption

* Present address: S.S. Department of Public Health 3, Science, University of Helsinki, Haartmaninkatu SF-00290 Helsinki 29, Finland. 175

Fig. 1. Three 20-~1 aliquots of 6M HCl introduced into the graphite furnace with the same unwashed pipette tip.

ANNOTATIONS

176

was 5 mV absorbance

and the chart-speed 120mm/min. values are seen in the figures.

The

RESULTS AND DISCUSSION

Fig. 2. Three 204 aliquots of 2M HCl introduced into the graphite furnace with the same unwashed pipette tip.

tilled and demineralized water (Santasalo Oy, Finland and Millipore Co., Super-Q System). Standard cadmium solutions were prepared from 1000 pg/ml stock solution (Titrisol, Merck) on the day of use. Procedure

Washing solutions and clean acids (2Oql portions) were pipetted manually into the graphite furnace with unwashed and with decontaminated pipette tips, then dried, ashed and atomized as usual. The argon purge gas was used in the continuous flow mode and ordinary graphite tubes were used. The recorder senstivity

3 ng/ml

washing

solution

Fig. 3. Three aliquots of the acid washing solution (50ml of 2M HCI) used for twelve micropipette tips, compared with a standard containing 3 ng/ml and pipetted with a decontaminated tip.

When 204 samples of 6M hydrochloric acid were introduced with an unwashed Eppendorf pipette tip three times in succession, diminishing signals for cadmium were obtained. The first sample was strongly contaminated with cadmium and the effect disappeared in the next two. In the case of 2M hydrochloric acid the effect was similar, but decontamination of the tip took longer. The height of the peak depended not only on the concentration of acid used, but also to some extent on the time that the acid stayed in the tip. The solution (50ml of 2M hydrochloric acid) used to wash the twelve micropipette tips was found to have a cadmium concentration of about 3ng/ml. When the washed tips were used to pipette pure 2M and 6M hydrochloric acid they were found to have been decontaminated. The whole procedure was repeated with nitric acid (“Suprapur”, Merck) and similar results were obtained. Another consignment of Eppendorf tips was tested as above and again found to be contaminated with cadmium, but not so strongly. No contamination was found in Finnpipette tips. When micro-amounts of cadmium are to be determined and disposable pipette tips are used, the tips must be decontaminated by washing with an acid solution, e.g., overnight with 2M hydrochloric acid. The cadmium contamination of the Eppendorf tips may be due to the yellow pigment of their casting. Other possible contaminants were not investigated.

REFERENCE

1. H. Hein and W. Schrader, Anulysentechnischr PerkinElmer, Uberhngen/Bodensee, 1975.

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