Download Mg28 Studies of the Molybdivanadate Method for Magnesium

AMERICAN JOURNAL OF CLINICAL PATHOLOGY
Vol. 31, No. 4, April, 1959, pp. 314-315
Printed in U.S.A.
MG 2 8 STUDIES OF T H E MOLYBDIVANADATE METHOD FOR MAGNESIUM
JERRY K. AIKAWA, M.D., AND ELOISE L. RHOADES, B.S.
Department of Medicine, University of Colorado School of Medicine, Denver, Colorado
In the course of our studies of the metabolism of magnesium with a radioactive
isotope of magnesium (Mg28), we needed a
rapid but reliable method for the determination of magnesium in biologic fluids and
materials. A survey of the various methods
currently in use suggested that the method
of Simonsen and co-workers1 might be most
suitable for our purpose. In this method,
calcium is separated from magnesium by
precipitation as the oxalate and the magnesium in the supernatant fluid is precipitated as magnesium ammonium phosphate.
The concentration of phosphate is determined colorimetrically with molybdivanadate.
The purpose of this report is to summarize
the results of our studies in which Mg28
was used as a tracer to modify and shorten
the method of Simonsen and co-workers.1
MATERIAL AND METHODS
The reagents used for the precipitation
of calcium and magnesium were those recommended by Simonsen and her associates.1
Mg28 was received as an acid solution of
MgCl2. It was neutralized with 1 N NaOH,
and the precipitate of Mg(OH) 2 was dissolved in 1 N H2SO4, in order to make a
dilute solution of MgS0 4 in distilled water.
Mg28 was assayed with a well scintillation counter and a scaling circuit. All countReceived, November 15, 1958; accepted for
publication December 19.
Dr. Aikawa is Established Investigator of the
American Heart Association, and Miss Rhoades
is a Research Associate.
This work was supported in part by a grant-inaid from the American Heart Association and the
Colorado Heart Association and in part by Contract No. AT(ll-l)-282 between the University
of Colorado and the U. S. Atomic Energy Commission.
Mg28 was supplied by the Brookhaven National Laboratory, Upton, Long Island, New
York, on allocation from the U. S. Atomic Energy
Commission.
ing rates were corrected for physical decay
of the isotope.
RESULTS
Preliminary studies in this laboratory revealed that determinations of magnesium
on duplicate and triplicate samples of pooled
serums performed on different days according to the original method of Simonsen and
her associates1 agreed within 2.0 =fc 3.7 per
cent of the over-all mean value in 11 separate runs. Triplicate determinations of the
phosphate standard on 12 occasions were
reproducible within 1.2 ± 1.5 per cent.
When known amounts of magnesium were
added to pooled serums, recovery of the
added magnesium was complete.
Mg-S Studies with Serum
Tracer quantities of Mg28 were added to
pooled serums in order to follow stepwise
the completeness of separation of magnesium
from calcium. Whether the calcium oxalate
precipitate was allowed to form for 5, 10,
15, 30, 60, 90, or 120 min., or 24 hr., a mean
of 2.98 per cent of the radioactivity was recovered with the precipitate after the supernatant fluid was carefully decanted off. The
supernatant fluid contained a mean of 96.80
per cent of the radioactivity originally
present in the mixture. Two washings of the
oxalate precipitate with 2 per cent NH 4 OH
removed all but 0.16 per cent of the radioactivity.
The minimal time interval necessary for
the complete precipitation of calcium from
the serum as calcium oxalate was next determined. Precipitation for 5, 10, 15, 20,
25, 30, 60, 90, or 120 min. resulted in the
same amount of calcium in the washed precipitate, as determined by titration of
permanganate.
The minimal time interval necessary for
the complete precipitation from serum of
magnesium as magnesium ammonium phosphate was determined as follows: The super-
314
METHOD FOR MAGNESIUM
April 1959
imtant fluid above the MgNH 4 P0 4 precipitate, allowed to form for 5, 10, and 15
min., contained up to 6.70 per cent of the
radioactivity. When precipitation for 30
min. or longer was allowed, the supernatant
fluid contained a negligible amount of radioactivity. After precipitation for 30 min., 2
washings of the MgNH 4 P04 precipitate
with alcoholic ammonium hydroxide resulted in a mean decrease of 1.90 per cent
in the radioactivity content of the precipitate. Thus, a mean of 94.90 per cent of the
original Mg28 was recovered in the final
precipitate.
The phosphate content of the MgNH 4 P04
precipitate at 5, 10, 15, 20, 25, and 30 min.
manifested a progressive increase with the
time allowed for precipitation. After the
precipitate had been allowed to form for
30, 60, 90, and 120 min., the concentrations
of phosphate of the precipitate were identical.
Mg28 Studies with Urine
A tracer amount of Mg28 was added to a
pooled specimen of human urine and the
fate of the radioactivity followed after the
precipitation of calcium for 10 min. and the
magnesium as phosphate for 30 min. A mean
of 2.17 per cent of the Mg28 was recovered
with the calcium oxalate precipitate and a
mean of 99.00 per cent was found in the
washed MgNH 4 P0 4 precipitate.
COMMENTS
Precipitation of calcium as calcium oxalate was complete within 5 min. Precipitation of magnesium as magnesium ammonium
phosphate was complete by 30 min. It was
essential that the mixture be stirred vigorously for at least 1 min. after the addition
of (NH 4 ) 3 P0 4 . With these reductions in
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time intervals for precipitation, the method
becomes rapid and suitable for clinical use.
Concentrations of Magnesium in Serum
from Human Beings
Two hundred and twenty determinations
of concentrations of magnesium in serum of
hospitalized persons by this modified method
revealed a mean value of 1.59 ± 0.32 mEq.
per 1. Values lower than 1.10 mEq. per 1.
were obtained in serums from patients with
congestive heart failure, cirrhosis, and renal
failure after dialysis. All values higher than
2.10 mEq. per 1. were found in patients with
renal failure prior to therapy.
SUMMARY
Mg28 was used as a tracer to study the
characteristics of the method of Simonsen
and her associates for the determination of
magnesium in serum and urine. Precipitation of calcium was complete within 5 min.
and the adsorption of magnesium to this
precipitate was minimal. Precipitation of
magnesium as magnesium ammonium phosphate was complete by 30 min.
SUMMARIO IN INTERLINGUA
28
Mg esseva usate como traciator in le
studio del characteristicas del methodo de
Simonsen e associatos pro determinar magnesium in sero e urina. Le precipitation de
calcium esseva complete intra 5 minutas.
Le adsorption de magnesium a iste precipitate esseva minimal. Le precipitation de
magnesium in le forma de phosphate ammoniaco-magnesian esseva complete in 30
minutas.
REFERENCE
1. SIMONSEN, D.
G.,
WESTOVBR, L. M.,
AND
WERTMAN, M.: The determination of serum
magnesium bv the molvbdivanadate method
for phosphate. J. Biol. Chem., 169: 39-47,
1947.