For a wide array of analytical techniques, accurate analysis of air or other gas contaminants requires the generation of standard atmospheres to calibrate and verify instrument response. Direct injection of a standard atmosphere into an analytical system can also be used as a diagnostic tool to evaluate the performance of individual system components and the overall system as a whole. This real-time diagnostic information can then be used to compensate for changes in instrument response due to changes in the operating environment or the deterioration of system components.
Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. These microanalytical systems should provide a high chemical selectivity, to discriminate against potential interferents, and the ability to perform the chemical analysis on a short time scale. These microanalytical systems also require accurate, reliable, and field-portable standards for calibration and evaluation of instrument response.
A common analytical technique, particularly for microanalytical systems, is gas chromatography. A conventional gas chromatograph comprises a means for injection of a sample to be analyzed, a supply of carrier gas, a column for separating the components of the sample, and a detector for detecting the separated components that are eluted from the column. For quantitative analysis, it is necessary to accurately determine how much of a component is in the unknown sample. Generally, quantitative analysis requires the comparison of the size of the chromatographic peak of a component in an unknown sample to a chromatographic peak of the same component in a known calibration standard.
In addition to calibration, standards can be used to evaluate quantitative errors in the chromatographic process. Quantitative errors can be associated with peak size measurement, standardization, sampling techniques and sample introduction, and chromatographic system errors. In particular, standards can be used to identify column-induced changes in the sample character and detector errors due to overload or other factors. For example, a standard can be used to determine and correct for retention time shifts due to adsorption or other problems during gas chromatography measurements.
Both static and dynamic methods have been used to produce gas or vapor standards. Dynamic methods, based on continuous flow of a standard-containing gas mixture, have the advantages of reducing adsorption problems and being able to vary the concentration of the standard by simple change in the diluent flow. In general, the standard should be as close to the unknown sample as possible, stable, and of high purity. Especially with trace analysis, sample size of the standard and the unknown sample should be kept the same within measurement error.
One dynamic method to generate vapor standards is to use the diffusion of vapor through a capillary to add small amounts of the vapor to a flowing gas stream. In a conventional diffusion source, the liquid whose vapor is to provide the standard of interest is contained in a reservoir at a known temperature. The liquid is allowed to evaporate from the reservoir and the vapor diffuses through the capillary into a flowing diluent gas stream. The vapor concentration in the resulting gas mixture can then be determined from rate of diffusion of the vapor through the capillary and the flow-rate of the diluent gas.
Whereas such diffusion tubes are well developed for conventional systems, a need exists for accurate, reliable, and field-portable standards for the calibration and evaluation of microanalytical systems. The present invention comprises a microfabricated diffusion source for use with such microanalytical systems. The invention can provide a very small sized calibration source that can be integrated with the fabrication of the microanalytical system by microelectromechanical systems (MEMS) technologies. This integrated fabrication eliminates the need for an external calibration source and also takes advantage of the economies of scale and low cost inherent with MEMS manufacturing. In addition, due to the very low calibrant flux requirements, which are typically in the nanograms or picograms per second, the lifetime of the microfabricated diffusion source can be months to years. These lifetimes can minimize, or in some cases eliminate, the need to periodically replenish the calibration source material.