John V. Hinshaw
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Chromatographers have high expectations of their instrumentation. They want to see an instrument system deliver high performance
levels for their individual samples, and if it does not, they want to know why. Manufacturers write instrument specifications
for specific test and evaluation samples under specific analytical conditions. Real-world samples and instrument conditions
seldom closely resemble the test setup, so the manufacturers' examples often set analysts' expectation levels too high. Instrument
specifications are better thought of as a basis for comparing two or more instrument systems before purchase, much as car
buyers might compare the mileage rating or horsepower of several competitive models.
Even if chromatographers were to verify overall instrument system performance by duplicating the manufacturer's test protocols,
the instrument might not deliver that level of performance for their specific samples. They should consider any chromatographic
sample in terms of its relationship to the instrument subcomponents — inlet, column, detector, and data handling — when setting
performance expectations. A careful analysis of a sample as it passes through each instrument subcomponent before committing
to a particular set of instrument options and accessories will help tremendously to reduce or eliminate surprises and disappointments
later.
Even when a system is already in use, and its performance is less than desired, this type of analysis will point out problem
areas that chromatographers can adjust and optimize for the specific analytical problem at hand.
Optimization Goals
Successful optimization requires realistic expectations and a clear understanding of the goals to be met. Optimization, whether
of the hardware or the gas chromatography (GC) method, takes two forms. The first and most common form involves adjustment
of operating parameters to achieve improved performance. The second form buys better performance through equipment or column
upgrades. Chromatographers should expect their instrumentation to deliver the performance of which it is capable, but they
should also realize that going beyond that level might require investment in upgraded or new equipment. Strong interdependence
of the various instrument components on each other often makes major upgrades expensive or impractical, especially in GC.
For example, converting a packed-column instrument to capillary-column capabilities or going from conventional to high-speed
capillary column performance may require the replacement of most of the instrument components, from the pneumatics through
to the detector. In such situations, a new instrument might be the best solution. On the other hand, adding a selective detector
or an on-column inlet to an existing system capable of accepting such new options is a reasonable upgrade that adds capabilities
without requiring extensive modifications.
The best opportunities for hardware optimization exist when instrumentation fails to deliver desired performance goals that
lie within its capabilities. For example, an improperly adjusted or configured split–splitless inlet may compromise results'
accuracy and repeatability. Adjusting an inlet's operating conditions and configuration might establish acceptable performance
but only if the inlet can deliver that performance in the context of the specific sample, its components, and their concentrations.
In addition, the column must accept the sample without compromising resolution or quantitation as it comes from the inlet.
Analytes must be eluted from the column with mass levels and peak profiles that do not exceed the detector's capabilities,
and the data-handling system must process the raw chromatographic data accurately and consistently from run to run. Unrealistic
expectations cause a perceived problem that requires either an adjustment of the expectations or the installation of hardware
upgrades. The majority of operational problems arise when the analytical components fail to deliver the performance of which
they are capable; improper setup or operation causes most of these difficulties.
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