• at least 10 replications per level
• at least three distinct levels, including one for process blank response (mass = 0)

Dimensional units for mass levels and responses, while not identified, must be consistent. The web-tool will report the limit of detection value in the same dimensional unit as the mass inputs.

Before calculating the limit of detection, the web-tool checks data quality and will abort for any of the conditions below:

• Less than 10 replications in any level
• Less than three distinct levels
• Absence of a process blank level (mass = 0)
• Number mismatch between mass level and response variables
• Responses in the highest mass level are not significantly different from those in the process blank level

If data quality passes, the web-tool will perform the calculation and return the estimate for the limit of detection and the 90% upper confidence LOD90 limit (a measure of uncertainty). Also, other results may be returned if those options are preselected. Results may also be returned with a message that data quality was marginal, and offer a suggestion for improving the quality.

The LOD90 value determined is the best estimate of the minimum mass of a particular analyte required to elicit a real and reliable response in the detector tested. This minimum detector response is the Critical Value (CV, an optional printed output), which should relate to the peak detection threshold value that can usually be set manually in the trace detector. The peak detection threshold value is usually set higher than the CV unless the LOD90 is determined using realistic chemical background.

Caveat: The LOD90 determination assumes normality for the non-zero measurements but will calculate an LOD90 value with data that may contain statistical outliers. Large biases in LOD90 estimates are possible depending on the value of the outlier and the total number of replicates in that mass level. The user is responsible for checking the quality of the data. To help with this assessment, the following analyses and plots may be generated:

• a Grubbs test for a single outlier is generated for each level of the mass for the non-zero measurements.
• a scatter plot of the measurements against their associated mass levels
• a multiplot showing a normal probability plot of the non-zero measurements for each distinct level of the mass.

The graphs are generated in Scalable Vector Graphics (SVG) format. SVG is now natively supported in most major browsers. The one exception is Internet Explorer (IE). Native SVG support is available for version 9 of IE. However, an SVG plugin is required if you use an earlier version. You can request that the graphs be generated in JPEG format if you do not have an SVG capable browser. You can also request that the graphs not be displayed (regardless of which browser you are using).