Instruments
Relative Response
Whereas some instruments may detect more than one chemical, equal concentrations may not give equal response. The relationship between the instrument's response and the actual concentration of the chemical is termed the "relative response."
For example, if an instrument reading for a 100 ppm concentration of acetone is 63, then the relative response for that instrument and acetone is 0.63 or 63%. Relative responses vary with chemical and instrument. The same chemical may have a relative response of 63% for one instrument and 120% response for another. Calibration also affects relative response.
Detection Range
The operating range is the lower and upper use limits of the instrument. It is defined by the lower detection limit at one end and the saturation concentration at the other end. The lower detection limit is the lowest concentration to which an instrument will respond. It is important to use an instrument with an operating range that will accurately measure the concentration in the range of concern.
Some instruments may respond to the chemical for a range of concentrations but not give a consistent response throughout the range. The linear range is the range of concentrations over which the instrument gives response proportional to the chemical concentration.
Response Time
Response time is the time between initial sample contact and readout of the full chemical concentration. In direct-reading instruments, a rapid response time is desired. Response time for direct-reading instruments can be from seconds to minutes.
For methods that require air sample collection and analysis, the response time is referred to as the turnaround time. In other words, how long was the time between collection of the sample and receipt of results from the laboratory?
Knowledge Check Choose the best answer for the question.
4-6. In direct-reading instruments, what kind of response time is desired?
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