ochratoxin a , rifampicin , apigenin , 
 tamoxifen , paclitaxel , cyclosporin a , phloretin , deferoxamine mesylate , 
 and pepstatin a were purchased from sigma chemical co. ( st . louis , 
 mo , usa ) .
hematoxylin and cycloheximide were purchased from fisher 
 scientific ( hampton , nh , usa ) and a.g .
samples were prepared in meoh , etoh , or ch3cn depending on solubility at concentrations from 0.20 to 
 100 ng/l .
hplc flow from a shimadzu lc-20at 
 pump ( shimadzu corp . , kyoto , japan ) was directed to a shimadzu sil-20acht 
 autosampler that was directly connected to a shimadzu elsd - ltii evaporative 
 light scattering detector with a low - flow nebulizer .
elsd parameters 
 were set as follows : n2 pressure = 350 kpa , temperature 
 = 50 c , gain = 8 ( maximum gain = 12 ) , flow rate = 1 ml / min . 

samples were subjected to isocratic mobile phase of methanol and water 
 ( 90% ) at a flow of 1 ml / min for an acquisition time of 0.17 min .
elsd 
 response area was obtained through autointegration of peaks by shimadzu 
 lcsolution version 1.25 software .
serial dilutions yielded 
 samples in 0.200 , 0.390 , 0.780 , 1.56 , 3.13 , 6.25 , 12.5 , 25.0 , 50.0 , 
 and 100 ng/l solutions .
a volume of 10 l was directly 
 injected into the hplc flow , and the hplc outflow was directed into 
 elsd - ltii without splitting .
each sample was injected in triplicate , 
 and response peak areas were transformed and averaged .
plots of log(elsd 
 response area ) versus log(amount ) were constructed to yield calibration 
 curves .
data points that had residuals greater than 3 ( 3 
 times the standard deviation ) from the true mean value , or clearly 
 deviated from the linear curve , were determined as outliers and were 
 omitted from the weighted least - squares analysis by assigning no weight . 

samples of cylcoheximide 
 were prepared in concentrations of 50.0 , 60.0 , 70.0 , 80.0 , and 90.0 
 ng/l .
using residuals , an injection of 800 ng was determined to be nonlinear ; 
 however , 500700 ng was linear .
the lol of cycloheximide was 
 determined to be 700 ng . the transformed average elsd areas for all standards were averaged 
 at each sample concentration .
a weighted least - squares analysis was performed to generate 
 a line for the  universal  calibration curve .
positive and negative error were calculated for the 
 average elsd response area for each injection quantity .
the 
  universal  calibration curve yielded a quantity for 
 each average elsd response area .
quantities 
 of 250 ng of rifampicin , cycloheximide , ochratoxin a , and apigenin 
 were injected .
error values were calculated to be 4.8 , 0.4 , 19.2 , 
 and 53.2% , respectively .
injection quantities of 100 , 200 , and 400 
 ng of deferoxamine mesylate yielded errors of 9.2 , 24.3 , and 39.7% , 
 respectively , following quantification using the  universal  
 calibration curve
. samples of 
 rifampicin , cycloheximide , apigenin , and ochratoxin a were injected 
 in quantities of 250 ng .
 a lack of good methods for absolute quantification of 
 natural products has limited the accuracy of high - throughput screening . 
 many currently used methods for quantification are either too slow 
 or not amenable to the structural diversity of natural products . 
 recent 
 developments in low - temperature evaporative light scattering detectors 
 ( elsd - lt ) have overcome several historical limitations of elsds , including 
 analyte decomposition and low sensitivity . 
 primarily , elsds have been 
 used for relative quantification and detection of compounds that lack 
 a uv chromophore . in this study 
 , we employ an elsd - lt for absolute 
 quantification of natural products . 
 calibration curves were constructed 
 using a weighted least - squares analysis for a diverse set of natural 
 products and other compounds . 
 an average calibration curve was evaluated 
 for the  universal  quantification of natural products . 
 
 optimization of elsd - lt hardware and parameters improved sensitivity 
 and throughput and established the utility of elsd - lt for quantification 
 of large natural product libraries . 
Experimental Section