several hours, and remained steady at this value. The phosphor containing the pentene, however, rose to a transient maximum counting rate; t h e count rate then started to fall and was still decreasing 30 hours after injection of the p h x p h o r into the flask, After burning r;maller quantities of methyl iodide thct count rates of both phosphors rose to their maximum levels in a much shorter time as shown in Figure 3. Other \;eights, up to 25 mg., of methyl iodide (not shown in Figure 3 ) gave traces with a similar overall shape, the count rate of the standard phosphor always rising to a steady maximum within 12 hours of injection. The effect of irradiating t h e phosphors with ultraviolet light after burning iodinated compounds had little effect on the standard phosphor but i t radically altered the plot of the phosphor containing pentene. Immediately after irradiation the count rate of the phosphor containing pen-
tene was a maximum; the counting efficiency then decreased, rapidly a t first, to a few per cent over a period of 4 to 8 hours. CONCLUSIONS
The oxygen flask method described earlier ( 1 ) can be successfully applied to the assay of carbon-14 in halogenic compounds. S o modification to the standard procedure is required after the combustion of chlorinated compounds which results in a slight lowering of the counting efficiency. T o obtain reliable and reproducible results it is necessary to modify the method after burning substances containing bromine and iodine. The incorporation of 10% v./v. 2-pentene in the phosphor prevents the considerable reduction in counting efficiency encountered after the combustion of brominated samples. Counting the sample should be delayed for 2 hours after injection of the phosphor into t h e flask t o allow the count rate to
reach a constant level. This time lapse can be reduced to 1 hour if the aliquot is irradiated with ultraviolet light before counting. The incorporation of Zpentene in the phosphor used for iodinated compounds has distinct disadvantages and is not therefore recommended. The use of standard phosphor is advocated. It may be necessary to allow the aliquot to stand for 12 hours before counting to enable i t to reach a steady counting rate. ACKNOWLEDGMENT
I thank D. G. Humphreys for his practical assistance with the combustions. LITERATURE CITED
(1) Dobbs, H. E., ANAL.CHEM.35, 783
(1963).
HORACE E. DOBBS U. K. Atomic Energy Research Establishment Wantage Berkshire, England
Determination of Moisture in Foods by Gas Chromatography SIR: There are a great variety of method. for determining the moisture content of foods. Many of the methods are highly empirical; consequently, they are product-specific. The technique of ga. chromatography is well suited t o the determination of water in a variety of food product., especially highmoisture food product. which are difficult to analme by other procedures. A suitable ga- liquid chromatographic (GLC) method can sa1 i-fy most criteria of a good quality coiitrol procedurenamely. ipeed. accuracy, and simplicity. Recently Stein and Ambrov (2) reported on the determination of water in aluminum aipirin using GLC. They used a column packed with Fluoropak 80 coated with 6% LAC2R446 and 0.4% phosphoric acicl. Earlier, Carlstrom. Spencer, and ,John-on ( 1 ) had w e d a column packed n i t h 30% polyethylene glycol on firehick to determine trace water in butane. The procedure tlevribed herein utilize> a column prellared b y coating Fluoropak 80 x i t h 10% of Carbowau 400. Thic column gives \vater a low retention time which allo\\s rapid ana1y.i.. Secondary butanol is used as a n internal standard and methanol is used to evtract water from the product to be analyzed. EXPERIMENTAL
Apparatus. Two chromatographs have been used for this s-ork, a Perkin Elmer 154C equippzd with 10,000-
ohm thermistors a n d a Research Specialties D u a l Column, Model 60-3, equipped with a hot wire (Katharometer) detector. GLC Column. Aluminum tubing, '/(-inch o.d. X &foot, packed with Fluoropak SO coated with 10% Carbowax 400. T h e Carbowax 400 was dissolved in acetone, slurried with t h e Fluoropak 80, a n d dried, in vacuo, with constant swirling. Operating Conditions. Column temperature, 120' C.; injection port temperature, 150' C.; dftector temperature, 200' C. (Research Specialties) a n d 120" C. (Perkin Elmer); helium flow rate, 65 ml. per minute; sample size, 2 pl. Preparation of Sample for Analysis. A R a r i n g nlendor fitted with a vented, gasketed, screw cap Blendor jar is used t o comminute t h e sample. I n a typical a n a l p i s 15 grams of a food product are placed in t h e Blendor jar containing 100 ml. of absolute methanol a n d several milliliters of sec-butanol. T h e mixture is comminuted for a period of minutes depending on t h e type of food product. T h e quantity of sec-butanol added is dependent of t h e water content of t h e food product a n d Table I includes illustrations of t h e quantities used. Table I also lists d a t a on time for sample preparations. After blending, the mixture is allowed to settle for 15 seconds, then a sample of the clear supernatent is drawn into a microliter syringe and 2 pl. are injected into the chromatograph. Kith certain products it is necessary to filter the blended mixture through coarse filter paper. The 2 4 . sample is drawn
Table I.
Sample Preparation Data Approx. amount of
Product
see-butyl hIoisture alcohol I3lending range, added, time, ml mm.
Cereal pe!lets 20-30 7 0 i.O Raisins 8-12 3 0 3 0 Bread 28-38 9 0 3 0 Rice 8-12 3 0 7 0 Dog food 6 0 3.0 20-25 Glues 5045 i.O 3.0 Flour 10-15 3.0 3.0 Yon-food product of interest to food processors. A 7.0-gram sample was used. Table II. Relationship of Peak Height Ratio to Weight Ratio Wt. ratio of Peak height ratio water and see-butyl of water to see-butyl alcohol ( R w ) alcohol ( R H ) 0.45 0 92 0.90 1.i.S 1.35 2.60 Reproducibility of GLC Analysis RH Values Mixture equivalent 1 . 4, 1 .IS, 1.14, t o 25% moisture 1 5, 1.15, 1,14, I . 5, 1.15, 1.16, Table 111.
Mixture equivalent t o 30y0 moisture
1. 1
5
i , 1.39, 1.38,
1 8, 1 .39, 1.38, 1 8
VOL. 36, NO. 3, MARCH 1964
b
689
r
I
Table IV.
Food product Cereal pellets Dried raisins (KO. 1 ) Dried raisins (No. 2) Flour (No. 1 ) Flour (No. 2)
m
m 0
* 0 W
w E
e
Reproducibility of Entire Procedure
Moisture (yo) Proposed GLC procedure Other 21.5, 21.7, 21.3, and 21.5 18.7, infrared balance 9 . 2 , 9 . 2 , and 9 . 3 7 . 1 , Vat. oven ( 6 hr.) 11.3, Vac. oven ( 6 hr.) 13.6, 13.8, and 13.8 12.6, 12.5, 12.6, and 12.6 12.5, Air oven 130' C. 13.5, Air oven 130' C. 13.5, 13.7, and 13.6
0
W
a
Figure 1 , Typical gas chromatogram of the methanol extract of a food product 1. 2. 3.
Methanol sec-Butyl alcohol
Water
from the clear filtrate. The GLC analysis takes about 5 minutes. Calculations. T h e ratio of peak height of water t o peak height of .set-butanol ( R H ) is calculated from t h e d a t a on t h e chromatogram a n d converted t o the weight ratio of water t o sec-butanol (Rw)by using a cali-
bration curve prepared from data such as t h a t included in Table 11. R w is multiplied by milliliters of sec-butanol to get milliliters of water and this in turn can be converted to per cent moisture in the original sample. DISCUSSION
The reproducibility of the GLC portion of this procedure was checked by preparing two mixtures of water, sec-butanol, and methanol equivalent to a mixture which would be obtained from 15-gram samples of food containing 25% and 30% moisture, respectively. The Rx values so obtained are tabulated in Table 111. A typical chromatogram is illustrated in Figure 1.
The reproducibility of the entire method is illustrated in Table IV. This method has been for routine quality control work for Over a year. The column is very stable if not heated above 130" C. We have used one column continuously for a year with little change in resolution. LITERATURE CITED
(1) Carlstrom, A. A., S encer, C. F., Johnson, J. F., ANAL.&EM. 32, 1056 (1960). (2) Stein, H. H., Ambrose, J. M., Ibid., 35, 550-2 (1963).
WARRENM. SCHWECKE JOHN H. KELSON Quality Control Department General Mills, Inc. Minneapolis, Minn.
Separation of Certain Cations from Mixtures of Various Cations on Ion-Exchange Papers-Arsenic, Barium, Cadmium, Tin, and Zinc SIR: Previous reports in this series have demonstrated the separation of silver or thallium ( 1 ) and arsenic or antimony (2) from virtually all other ions by development with complexing reagents on ion-exchange resin loaded filter papers. Exactly the same techniques and procedures have now been employed to test various other selective reagents (all in aqueous solution) for such separations. Exploratory tests were performed by developing the following 26 ions individually: Ag, T1, Pb, Cd, Cu, Co, Xi, Hg(I), Hg(II), As(III), Fe(III), Sb(III), V(V), Bi, Sn(IV), Au(III), Pt(IV), Al, Ce(III), Ce(IV), Mg, Zn, Ba, Mn(II), Cr(III), and U(V1). Test solutions (0.050M) were prepared by dissolving the reagent-grade nitrate, chloride, sulfate, or acetate in water. Mercury(1) nitrate was stabilized with 48 ml. of HNO, per liter of solution. Platinum(1V) and gold(II1) were prepared as the usual chloro complexes. T o dissolve uranium(V1) acetate, the minimum necessary amount of glacial acetic acid was added. Vanadium(V) was a saturated solution of VSOS in 0.10M sulfuric acid. Antimony(III), tin(IV), and arsenic(II1) chloride solutions were stabilized with 6M HCl. 690
0
ANALYTICAL CHEMISTRY
Bismuth chloride was dissolved in 1:5 HCl. T o dissolve cerium(1V) sulfate, the minimum necessary amount of sulfuric acid was added. The promising reagents were then tested with a mixture containing 15 compatible, representative ions, including the ion t o be separated. One such mixture contained Ag, -41, Ba, Cd, Ce (IV), Cr, Co, Cu, Fe, Pb, Mg, Mn, Ni, T1, and Zn. Four spray reagents, previously described ( 1 , 2, 4)served for the detection of all the ions listed above. The reagents tested were the same ones which were used as background electrolytes in earlier electrochromatographic studies of the ions (3, 4). We therefore know the sign of the charge of each ion in each developing solution. The results given below are based upon at least four migrations of each ion in each system, with stated RFvalues having a standard deviation of +0.04 or less, Migration distances were generally 30 t o 35 cm., which took about 2 hours with the cation-exchange paper and 75 minutes with the anion-exchange paper. Arsenic and barium were separated from the 24 other ions and from each other by development with a solution of 0.0125M E D T A and 0.05M ammonia,
p H 9.2, on filter paper loaded with strongly basic anion-exchange resin in the (ethylenedinitri1o)tetraacetate form (Reeve Angel Grade SB-2, control -4-10297). Both arsenic and barium were complexed to form anionic species in this system. Arsenic had RF 0.78 and was separated by 2 cm. from the closest zone (Bi, RP0.58). Barium had RF = 0.89 and was separated by 3 cm. from the arsenic zone. The vanadium zone could not be detected in this system, so its position is indeterminate. Arsenic and cadmium were separated from the 24 other ions and from each other by development with a solution of 0.5031 sodium chloride, p H 6.5, on filter paper loaded with strongly acidic cationexchange resin in the sodium form (Reeve Angel Grade SA-2, control A-7802-1,2). Both arsenic and cadmium were cationic in this system. Cadmium had RF 0.46 and was separated by 3 cm. from the closest zone, vanadium, which trailed back to the origin. -4rsenic had RF 0.91 and was separated by about 10 cm. from the cadmium zone. Arsenic was separated from the 25 other ions, and tin was separated from all except antimonous, vanadium, mercuric, and cadmium ions by development
