In this report we explore the use of MALDI-FTICR mass spectrometry for the 
quantitative analysis of five HIV-1 protease inhibitors in cell lysates. 
2,5-Dihydroxybenzoic acid (DHB) was used as the matrix. From a quantitative 
perspective, DHB is usually a poor matrix due to its poor shot-to-shot and poor 
spot-to-spot reproducibilities. We found that the quantitative precisions 
improved significantly when DMSO (dimethylsulfoxide) was added to the matrix 
solution. For lopinavir and ritonavir, currently the most frequently prescribed 
HIV-1 protease inhibitors, the signal-to-noise ratios improved significantly 
when potassium iodide was added to the matrix solution. The mean quantitative 
precisions, expressed as % relative standard deviation, were 6.4% for 
saquinavir, 7.3% for lopinavir, 8.5% for ritonavir, 11.1% for indinavir, and 
7.2% for nelfinavir. The mean quantitative accuracies, expressed as % deviation, 
were 4.5% for saquinavir, 6.0% for lopinavir, 5.9% for ritonavir, 6.6% for 
indinavir, and 8.0% for nelfinavir. The concentrations measured for the 
individual quality control samples were all within 85-117% of the theoretical 
concentrations. The lower limits of quantification in cell lysates were 4 
fmol/microL for saquinavir, 16 fmol/microL for lopinavir, 31 fmol/microL for 
ritonavir, and 100 fmol/microL for indinavir and nelfinavir. The mean mass 
accuracies for the protease inhibitors were 0.28 ppm using external calibration. 
Our results show that MALDI-FTICR mass spectrometry can be successfully used for 
precise, accurate, and selective quantitative analyses of HIV-1 protease 
inhibitors in cell lysates. In addition, the lower limits of quantification 
obtained allow clinical applications of the technique.