LITERATURE CITED

(1) J. M. Miller, J: Ross, J. Rustenberg, and G. L. Wilson, Anal. Chern., 45,

(2) J . H. Beynon and R . G. Cooks, Res./Develop., 22 (11). 26(1971). (3) K. H. Maurer, C. Brunee, G. Kappus, K. Habfast, U. Schroder, and P.

Schuize, 19th Conference on Mass Spectrometry, Atlanta. Ga., 1971, Paper K-9.

(4) J. M. Miller and G. L. Wilson, h t . J. Mass Spectrom. ion phys., 12, 225 (1973).

(5) E. Watanabe. Y. Itagaki, T. Aoyarna, and E. Yarnauchi, Anal. Chem., 40,

(6) T . W. Shannon, T. E. Mead, C. G. Warner, and F. W. McLafferty, Anal.

(7) N. R. Daly, A. McCormick, R. E. Powell, and R. Hayes, lnt. J. Mass Spec-

RECEIVED for review July 19, 1974. Accepted September 16, 1974. The authors thank the National Research Council of Canada for financial support.

1000 (1968).

Chem., 39, 1748 (1967).

rrom. /on fhys.. 11, 255 (1973). 627 (1973).

Gravimetric Calibration of Volumetric Microsyringes

Gene Kritchevsky and James L. Bolen

Division of Neurosciences, City of Hope National Medical Center, 1500 E. Duarte Rd., Duarte, Calif. 9 1010

Ralph 0. Leonard

Perkin-Elmer Corp., Norwalk, Conn. 06856

We are routinely using a simple gravimetric technique for calibration of microsyringes. I t involves drying a known volume of a potassium biphthalate solution of known con- centration, followed by determination of the nonvolatile so- lute weight on a rapid-response microbalance. The tech- nique has reduced the relative standard deviation due to variability in operators and syringes to 0.5% or less.

Our studies have shown that cumulative accuracy and precision errors in routine dispensing of small volumes of liquids from microsyringes can easily reach 5% or higher. These errors are the result of poor calibration of the sy- ringe, non-linear scales, poor operator techniques, and vari- ability between operators. The relatively simple approach of having each operator calibrate his own syringes a t the volume(s) to be dispensed reduces errors in accuracy and precision to a minimum. A variety of calibration techniques have been used to achieve this end: dispensing and weigh- ing mercury; dispensing solvents onto a recording balance and extrapolating the evaporation curve back to time zero; dispensing solutions of radioactive materials and counting; and dispensing an exact volume of solution containing a known concentration of a nonvolatile solute, evaporation to dryness, and weighing the residue on a high speed automat- ic microbalance. We found the last technique to be very satisfactory. as detailed below.

EXPERIMENTAL Apparatus. Pans. Commercial aluminum foil is cut into circles

of about lh-in. diameter on a cork boring machine. About six layers can be cut at a time if they are sandwiched between sheets of card- hoard to prevent tearing. The circles are indented with a flat tipped dowel with smooth edges, a small glass stopper, or a pestle, and cleaned with chloroform/methanol (2:1), then chloroform to remove oils and other interfering materials.

Heating Block. Any flat top hot plate which can maintain a rea- sonably constant temperature ( f2 .5 “C) between 110 and 120 “C will be adequate.

Desiccator. A standard laboratory glass desiccator with a Petri dish containing potassium hydroxide pellets is used. It is conve- nient to cover the top of a tall 150-ml beaker with aluminum foil and place it on the porcelain plate of the desiccator as this pro- vides an elevated surface for the sample pan. It makes the pan transfer quite convenient, and it helps to speed cooling.

Microbalance. Electronic microbalances (Autobalance Model AD-2, Perkin-Elmer Corporation, Norwalk, Conn.) with servo con- trolled beams and automatic digital readouts, which speed the pro- cedure and reduce the possibility of operator error, have proved especially satisfactory for routine accurate weighing of other non- volatile materials by the same procedure.

Reagents. Potassium biphthalate (Baker Analyzed reagent, pri-

mary standard, catalog No. 2958) is used because of its time-hon- ored purity (100.02%), wide use as a primary standard, and its very low volatility under the experimental conditions. The concentra- tion of the solution should provide an adequate solute weight (at least 100 micrograms) in the volume dispensed. This volume is, with rare exception, the total volume of the syringe. The reagent should be dissolved in redistilled water. Accuracy of the concentra- tion is maximized by using large quantities, by weighing the re- agent (previously dried and cooled) on an analytical balance, and by dissolving the reagent to give an accurately known volume of solution. For convenience of initial and subsequent use, the well mixed solution can be stored in a number of tightly capped, Teflon stoppered, graduated tubes (e.g., Glenco, 3121 White Oak Drive, Houston, Texas 77007; catalog No. 20901, 1-, 2-, or 3-ml tubes). After each use, residual volume of solution is noted, as well as pos- sible evaporation loss, which should be minimal. I t is generally not necessary to run a solvent blank.

Procedure: 1) a) Heat sample pans on the hot plate at 110-120 “C for 3 min. b) Place sample pan on beaker in desiccator to cool for 2 min. c) Place sample pan on balance tray for 1 min and note reading. d ) Repeat cycle a-c to ensure reproducible tare weight. 2) Dispense solution into weighed pan. 3) Repeat step 1 (a-d). Usual- ly 10 minutes of heating ( la ) is desirable to ensure complete drying.

DISCUSSION The timing of these cycles permits the weighing of more

than one sample pan a t a time. The weight of the residue in micrograms divided by the known concentration in micro- grams per microliter gives the true volume of the solution dispensed onto the pan. This volume, of course, is equal to the volume held by the syringe, if the transfer of solution is performed carefully. RECEIVED for review March 20, 1974. Accepted September 16, 1974.

CORRECTION

Simultaneous Determination of Alpha-Emitting Nuclides of Radium through Californium in Soil

In the paper by C. W. Sill, K. W. Puphal, and F. D. Hindman, Anal. Chem., 46, 1725 (1974), some words were inadvertently transposed, making the meaning unintelligi- ble. On page 1736, the second sentence under “Calcula- tions” should read as follows. “Let X, Y, and 2 be the total counts obtained in the energy intervals for the 239Pu being determined, the e36Pu tracer recovered through the proce- dure, and the 236Pu obtained in the standardization for the same quantity of tracer, respectively.”

192 ANALYTICAL CHEMISTRY, VOL. 47, NO. I , JANUARY 1975