Simple Inexpensive Freeze-Drying Procedure

James W. Young

The Department of the Treasury, Bureau of Alcohol, Tobacco and Firearms, Cincinnati, Ohio 45202

Gary D. Christian

Department of Chemistry, University of Washington, Seattle, Wash. 98795

A rapid, inexpensive technique has been developed that enables lyophilization of small quantities of serum. The serum is dried quickly and efficiently without losses, and can subsequently be used for neutron activation analysis or other analytical procedures. Readily accessible labora- tory equipment is utilized throughout.

A dry ice-acetone bath is prepared using a flat-bot- tomed rectangular Pyrex baking dish (10 X 12 inches) with a glass cover. Acetone is added to a depth of 112 inch and small pieces of dry ice are added until the bath ap- proaches equilibrium temperature. The cover aids in rap- idly reaching the desired temperature.

The serum (1 ml) is pipetted into a 10-ml beaker. The beaker is carefully immersed in the dry ice-acetone bath for approximately 5 to 10 min. The beaker is swirled gen- tly using tongs while immersed so that the serum is frozen on the glass in as thin a film as possible. This enables the sample to be dried in a minimum of time.

Suitable modifications of the freezing apparatus are, of

course, possible. For freezing multiple samples, a rack to hold the beakers could be designed that would allow the contents of several beakers to be swirled simultaneously, rather than intermittently.

Following the freezing step, the beaker is placed in a small vacuum desiccator and a vacuum applied until the sample is dry. Total sample drying time is approximately 30 min for 1 ml of serum. Several samples can be dried simultaneously. The samples can then be transferred under dry conditions to quartz tubes for neutron activa- tion analysis. This procedure has been used for prepara- tion of samples for selenium determination. No losses of selenium were incurred during the process.

A Welch (Series 1405) pump was used to obtain the vacuum on a Pyrex brand vacuum type desiccator (Thom- as, No. 3750-M10, 160-mm diameter).

Received for review November 30, 1972. Accepted Febru- ary 5, 1973.

CORRECTION

A Penicillin Selective Enzyme Electrode

In the paper by G. J. Papariello, A. K. Mukherji, and C. M. Shearer [Anal. Chem., 45, 790(1973)], the sentence starting on line 16, page 790 which now reads, “Guilbault and Montalvo (6, 7) first introduced the concept of the en- zyme electrode by describing an electrode responsive to urea prepared by immobilizing . . .” should be changed to read “Clark and Lyons [Annals of the Neu York Academy of Sciences, 102, 29(1962)] introduced the concept of the en- zyme electrode. Guilbault and Montalvo (6, 7 ) developed a working enzyme electrode which is responsive to urea. This electrode is prepared by immobilizing. . .”

1296 ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973