Abstract: The invention provides a method and apparatus for the mass spectrometric determination of the isotopic composition of an element comprised in a sample by gas chromatography-isotopic ratio mass spectrometry. As a sample elutes from the column of the gas chromatograph it enters a sample conversion means which converts the element to an analysis gas (a gas comprising the element in elemental form or in the form of a simple compound, e.g. nitrogen or carbon dioxide) which is admitted into the mass spectrometer only during a second time interval corresponding to the elution of the peak containing the sample from the column. One or more calibration samples of a reference gas are admitted into the spectrometer during first time intervals which do not overlap the second time interval, thereby minimizing the contribution of the analysis gas to the reference gas mass analysis, and vice versa. Various devices are provided to minimize disturbance to the flow of gas into the spectrometer during practice of the method.

Abstract: The invention comprises methods and apparatus for the mass spectrometric determination of the isotopic composition of an element comprised in an analysis gas (that is, a gas comprising the element in elemental form or in the form of a simple compound, e.g., nitrogen or carbon dioxide). First and second flows of a carrier gas (e.g., helium) are simultaneously passed into the mass spectrometer, and samples of analysis gas and a reference gas are respectively introduced into the first and second flows during selected time intervals. The time intervals are selected so that mass spectrometric measurements are made on the analysis gas samples in the absence of any signal from the reference gas samples, and v.v. A preferred embodiment comprises a gas chroatograph through which a sample containing the element is passed, and conversion means to convert the eluting sample into the analysis gas.

Abstract: A valve arrangement for controlling the flow of a sample of blood gases along a capillaric streamway from a probe towards an analyzing apparatus includes a body 12 having an inlet 36 for control fluid and a reciprocable valve head 20 movable from a first position in which inlet 36 is isolated from a second inlet 30 for carrier gas and blood gases towards a second position in which inlets 36, 30 are in communication. The arrangement is such, that in the first position of the valve head 20 the carrier gas and blood gases can flow from inlet 30 towards an outlet 32. However, in the second position the pressure of the control fluid prevents the flow of the carrier gas and blood gases through the inlet 30.