petroleum-based materials. The standard deviation for precision based on the pooled data is 0.02 % hydrogen. DISCUSSIONThe apparatus has been in continuous routine service for over a year without combustion mishaps. Because of the availability of other methods for analyzing volatile samples, we have used this method only for samples boiling above 200 °C, and weighed the samples in open porcelain boats. The use of ampoules (7) to contain the samples would undoubtedly extend the method to lower-boiling materials. Although the apparatus as described is suitable only for hydrogen determinations, simple modifications would permit carbon determinations also. This would require, for example, attaching to the water absorbers two additional aluminum absorbers, the first containing manganese dioxide to absorb nitrogen oxides (7, 3) and the second containing Ascarite to(3) R. Belcher and G. Ingram, Anal. Chim. Acta, 4, 401 (1950). collect the carbon dioxide. A coating of Teflon (Du Pont) or similar material sprayed on the inside surface of the absorber tube would be necessary to prevent reaction between aluminum and the alkaline Ascarite. The approximate cost of parts needed to construct the apparatus is $1000. The estimated cost of materials and reagents is $1.00 per analysis, assuming replacement of the combustion tube and packing after 60 analyses and no recovery of reagents.By dovetailing combustion and purging operations, one operator can perform seven analyses in an 8-hour day. The construction of a dual apparatus with automatic cycle timers to control the rate of heating and the changes in gas flow should permit 14 analyses per day per operator.

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An Isotope dilution mass spectrometrlc (ID/MS) method for serum creatinine Is described which uses creatlnlne-13C2 as the labeled Internal standard. Creatinine Is separated from creatine and converted to the ethyl ester of A/-(4,6-dlmethyl-2-pyrlmldlnyl)-IV-methylglyclne. Combined capillary column gas chromatography and electron Impact mass spectrometry are used to obtain the abundance ratio of the unlabeled and labeled [M -COOC2Hs]+ Ions from the derivative. Quantitation Is achieved by measurement of each sample between measurements of two standards whose unlabeled/labeled ratios bracket that of the sample. Four freeze-dried human serum pools Including National Bureau of Standards Standard Reference Material 909 were analyzed with this method. The coefficients of variation for a single measurement ranged from 0.15 to 0.27% for the four pools. The measurements were found to be free of Interference. The high precision and absence of significant bias qualify this method as a candidate definitive method.Serum creatinine concentrations are commonly measured in clinical laboratories in evaluating renal function. Clinical methods generally involve a version of the Jaffe method or an enzymatic procedure (1). Because of significant discrepancies in results among the methods in use (2), a need exists for a method of demonstrated accuracy and precision, i.e., a definitive method, to provide an accuracy base against which the other methods can be judged. Universal acceptance of

Two isotope dilution mass spectrometric methods have been developed for the determination of D-glucose in human serum. Each uses a uniformly labeled (13C)glucose as the internal standard. The first method involves conversion of glucose into 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose and an extensive clean-up, followed by quantitation using packed column gas chromatography mass spectrometry. In the second method, glucose is converted into alpha-D-glucofuranose cyclic 1,2:3,5-bis(butylboronate)-6-acetate. The wet chemistry work-up is simpler, but analysis by capillary gas chromatography mass spectrometry is required. Both methods exhibit excellent precision (coefficients of variation less than 0.3%) and provided mean values that agree within 1% for all serum pools tested.

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