TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Component: The optical radiation is generated from a single laser diode and the control current comprises a bias component which is alternated between two values respectively defining lambda 1 and lambda 2. The apparatus includes means operative to calculate quantities Q 1 and Q 2, separate estimates of the amount of target gas in the monitored space based upon the normalized magnitude of frequency components f 1 and f 2, compare quantities Q 1 and Q 2 to determine the quality of their agreement with each other and previous results for measurements made through the monitored space, apply rules dependent upon this quality, and use Q 1 and Q 2 in combination with previous results to calculate the quantity of target gas present in the monitored space. The apparatus comprises two or more laser diodes (102), each laser diode being driven by a bias current which causes it to operate at a mean wavelength close to a different optical absorption line of the same target gas and being scanned across this line by a sinusoidal current component at a frequency which is non-harmonically related to any other scanning frequency. The apparatus includes an optical splitter operative to split the radiation into two fractions of which one fraction is transmitted across the monitored space to the first optical receiver and the other is passed through a retained sample of the target gas to a second optical receiver. The control current is produced by a digital synthesizer including a digital-to-analogue converter (DAC) to output a sequence of voltages and a voltage-to-current (V-I) converter to convert the voltages into a current. Preferred Method: Each target gas absorption line is scanned at two, nonharmonically related electrical frequencies and measurements of any absorption by such lines are made by determining the magnitude of the two, similar order harmonics of the nonharmonically related scanning frequencies, this process being carried out for each absorption line being scanned and where this process is performed simultaneously, all electrical scanning frequencies being chosen to be non-harmonically related. The amount of methane gas present in the monitored space is determined and then the amount of hydrogen sulfide present is estimated using a coefficient relating the amount of methane to the amount of hydrogen sulfide for the solution gas of a particular field or facility.