Error in Reproducibility

I just have a problem regarding the quantitative analysis of fuels using the Agilent Cary 630 FTIR. We were able to establish a good calibration curve based on peak height from the Microlab Quant, and we are using that curve to make a component type method in the Microlab PC. After establishing the method in the Microlab PC, we started sample analysis, then it was observed from the repeated scans of the same sample that the concentration values differ from each run (e.g. trial 1 gave 1.55% concentration, while trial 2 gave 2.7% using the same sample). We cannot establish an acceptable reproducibility. Also, it was observed that the amount of sample placed in the sample interface also affects the concentration, which is not quite acceptable/ideal. Do you have any suggestions or recommendations?

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