Postnatal development of urea synthesis in VLBW-infants as estimated by 15N-ammonium chloride urine test: Differences between AGA- and SGA-Infants

A prototype off-axis integrated cavity output spectrometer (OA-ICOS) utilizing two identical cavities together with a near-infrared (1.63 μm) external cavity tunable diode laser is described. The two-cavity design—one for a reference gas and one for a sample gas—takes advantage of classical double-beam infrared spectrometer characteristics in reducing uncertainties due to laser scan or power instabilities and major temperature variations by a factor of three or better compared with a single-cavity scheme. This is the first OA-ICOS instrument designed to determine ¹³C/¹²C and ¹⁸O/¹⁶O ratios from CO₂ rotation/vibration fine structure in three different combination bands. Preliminary results indicate that at 0.8 Hz a precision of 3.3 and 2.8 \permil\ is obtained for δ¹³C and δ¹⁸O, respectively, over a period of 10 h and a pure CO₂ gas sample at 26 hPa. By averaging 100 spectra over a subset of the data, we achieved a precision of 1.6 and 0.8 \permil\ for δ¹³C and δ¹⁸O, respectively.