John V. Hinshaw

Requirements for the Best Detector Operation

Obtaining consistent high detector performance over extended periods requires the analyst to adhere to specific requirements for setup and operation. Some, such as hydrogen and air flow, are determined largely by the manufacturer and design and cannot be changed significantly without loss of detector performance; straying too far can cause a failure to operate at all. Others, such as amplification range, are dictated by sample size, analyte-specific response, and their relationship to minimum detection requirements. By encoding these items in a well-written method and adhering to them, chromatographers can achieve the detector noise, sensitivity, drift, and reliability that their analytical methodology requires.

Each detector has unique gas supply requirements as well as other external requirements that chromatographers must establish and maintain. Instrument manufacturers specify basic requirements for the electrical supply and environmental conditions for their instruments, and they will communicate these conditions upon request. These prerequisites affect all aspects of an instrument and should be adhered to as closely as possible.

Electrical: As electrical transducers that operate in very sensitive ranges, GC detectors are particularly sensitive to electrical supply quality and to external interferences. In general, each gas chromatograph requires a dedicated grounded alternating current (AC) electrical supply with its own circuit breaker, usually 120 V at 20 A or 240 V at 10 A. The appropriate voltage depends upon the original country or area for which the instrument was manufactured. GC users should not attempt to change the voltage supply level for which the instrument was manufactured. The internal electronics usually are capable of running with multiple line supply voltages, but in most units the inlet, detector, auxiliary, and oven heaters are voltage-dependent, so it is not possible to switch from 120 to 240 V or vice versa without replacing these heaters with those designed for the target line voltage. The line voltage also must stay within a certain window above and below the nominal voltage level: one manufacturer specifies ±10 %, for example. The majority of modern GC systems are not affected by the AC supply frequency — they use internal crystal clocks for a time base — so either 50- or 60-Hz supplies are appropriate.

Most importantly for detectors, the electrical supply must be free of significant electrical spikes, high-frequency signals, and rapid voltage fluctuations. Some manufacturers specify line quality and, in cases in which such interferences are suspected, a line voltage monitor can reveal the nature of any unusual fluctuations. In addition to the quality of directly connected supply lines, the vicinity must be free from sources of significant radio-frequency (RF) interference. Various country and international standards specify both the degree to which instruments can be influenced by incoming RF signals, as well as the frequency distribution and signal strength of any potentially interfering RF signals that originate from the instrument. Instruments will meet these specifications only if all covers and attached grounding wires are in place. Even so, some cell phones and two-way radios can produce significant detector interference if operated very close to an instrument. It is good practice to require that cell phones be turned off or simply not present in the proximity of sensitive laboratory instruments.