Method and system for determining the concentration of an analyte in a fluid sample

Abstract

A method and system are provided for detecting the concentration of an analyte in a fluid sample. The method and system involve analysis of a volatilized, ionized fluid sample using a mass spectrometer or other ionic analyte detection device that provides a signal proportional in intensity to the quantity of ionized analyte detected. The improvement involves replacement of a necessary non-analyte component in the fluid sample with a substitute component that serves the same purpose as the original component but is either more volatile than the original component and/or the analyte or undergoes a reaction to provide lower molecular weight reaction products, and results in an increased intensity in signal and signal-to-noise ratio. Acoustic fluid ejection is a preferred method of generating nanoliter-sized droplets of fluid sample that are then volatilized, ionized, and analyzed. Also provided are zwitterionic compounds suitable as the substitute components that when ionized and heated decompose to provide carbonic dioxide, a nitrogenous species such as ammonia, an amine, or nitrogen gas, and a volatile aromatic compound.

1 . An improved method for determining the concentration of an analyte in a fluid sample that additionally comprises a necessary non-analyte original component, the method comprising volatilizing and ionizing the sample, and introducing the ionized, volatilized sample into an ionic analyte detection device that provides a signal proportional in intensity to the quantity of ionized analyte detected, wherein the improvement comprises:
 acoustically generating nanoliter-sized droplets of the fluid sample prior to volatilization and ionization, such that the fluid sample is introduced into the ionic analyte detection device in the form of nanoliter-sized droplets; and   substituting for the necessary non-analyte original component a substitute component that: (a) functions as the original component in the fluid sample; (b) (i) is more volatile than the necessary original component or (ii) upon volatilizing the fluid sample, undergoes a reaction to yield at least one reaction product that is more volatile than the necessary original component; and (c) results in an increase in the intensity of the signal and/or a greater signal-to-noise ratio than either the signal intensity or signal to noise ratio obtained using the original component.   
     
     
         2 . (canceled) 
     
     
         3 . The method of  claim 1 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 5 nl. 
     
     
         4 . The method of  claim 3 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 2.5 nl. 
     
     
         5 . The method of  claim 4 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 50 pl. 
     
     
         6 . The method of  claim 5 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 1 pl. 
     
     
         7 . The method of  claim 1 , wherein the improvement further includes using focused acoustic ejection to generate the nanoliter-sized droplets of the fluid sample. 
     
     
         8 . The method of  claim 7 , wherein the acoustic ejection is carried out using an acoustic ejector that directs focused acoustic energy into a reservoir containing the fluid sample in a manner that results in the rapid ejection of consistently sized fluid droplets from the surface of the fluid sample. 
     
     
         9 . The method of  claim 1 , wherein the ionic analyte detection device comprises a mass spectrometer. 
     
     
         10 . The method of  claim 9 , wherein the ionizing comprises chemical ionization, field desorption ionization, electrospray ionization, atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, or inductively coupled plasma ionization. 
     
     
         11 . The method of  claim 1 , wherein the analyte comprises a drug, a metabolite, an inhibitor, a ligand, a receptor, a catalyst, a synthetic polymer, or an allosteric effector. 
     
     
         12 . The method of  claim 1 , wherein the analyte is a biomolecule. 
     
     
         13 . The method of  claim 12 , wherein the biomolecule comprises a nucleotide analyte, a peptidic analyte, or a saccharidic analyte. 
     
     
         14 . The method of  claim 1 , wherein the necessary non-analyte original component comprises an original salt and the substitute component comprises a substitute salt. 
     
     
         15 . The method of  claim 14 , wherein the original salt and the substitute salt function as buffer salts for the fluid sample, such that volatilization of the fluid sample results in gas phase extraction of the substitute buffer salt. 
     
     
         16 . The method of  claim 15 , wherein the substitute salt comprises singly charged ions formed from weak acids or weak bases. 
     
     
         17 . The method of  claim 16 , wherein the substitute salt comprises ammonium bicarbonate, ammonium formate, ammonium acetate, pyridinium acetate, pyridinium formate, ethylmorpholinium acetate, trimethylamino acetate, or trimethylamino formate. 
     
     
         18 . The method of  claim 17 , wherein the substitute salt comprises ammonium bicarbonate, ammonium formate, or ammonium acetate. 
     
     
         19 - 40 . (canceled) 
     
     
         41 . The method of  claim 1 , where the increase in analyte signal intensity and/or signal-to-noise ratio is at least 10%. 
     
     
         42 . The improved method of  claim 41 , where the increase in analyte signal intensity and/or signal-to-noise ratio is at least 25%. 
     
     
         43 . The method of  claim 1 , wherein the droplets introduced into the ionic analyte detection device comprise the analyte and the substitute component. 
     
     
         44 . An improved method for determining the concentration of an analyte in each of a plurality of fluid samples that additionally comprises a necessary non-analyte original component, the method comprising volatilizing and ionizing the samples and introducing each ionized, volatilized sample into an ionic analyte detection device that provides a signal proportional in intensity to the quantity of ionized analyte detected,
 wherein the improvement comprises substituting for the necessary non-analyte original component a substitute component that functions as the original component in the fluid sample, is more volatile than the necessary original component, and results in an increase in the intensity of the signal and/or a greater signal-to-noise ratio than either the signal intensity or signal to noise ratio obtained using the original component, and additionally comprises   (a) providing the fluid samples in each of a plurality of fluid reservoirs;   (b) acoustically coupling an acoustic droplet ejector to a first of the fluid reservoirs;   (c) activating the ejector to generate focused acoustic radiation toward the first reservoir and into the fluid sample therein, in a manner effective to eject nanoliter-sized droplets of the fluid sample into the ionic analyte detection device;   (d) positioning another of the fluid reservoirs and the acoustic droplet ejector in acoustic coupling relationship;   (e) repeating step (c); and   (f) repeating steps (d) and (e) with additional fluid reservoirs in the plurality of fluid reservoirs at a rate of greater than 5 reservoirs per second.   
     
     
         45 . The method of  claim 44 , wherein the droplets comprise both the analyte and the substitute component. 
     
     
         46 . The method of  claim 44 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 5 nl. 
     
     
         47 . The method of  claim 46 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 2.5 nl. 
     
     
         48 . The method of  claim 47 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 50 pl. 
     
     
         49 . The method of  claim 48 , wherein the nanoliter-sized droplets of fluid sample have a mean droplet size of less than about approximately 1 pl. 
     
     
         50 . The method of  claim 44 , wherein the rate is greater than 10 reservoirs per second. 
     
     
         51 . The method of  claim 50 , wherein the rate is greater than 25 reservoirs per second. 
     
     
         52 . The method of  claim 44 , wherein the ionic analyte detection device comprises a mass spectrometer. 
     
     
         53 . The method of  claim 52 , wherein the ionizing comprises chemical ionization, field desorption ionization, electrospray ionization, atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, or inductively coupled plasma ionization. 
     
     
         54 . The method of  claim 44 , wherein the analyte comprises a drug, a metabolite, an inhibitor, a ligand, a receptor, a catalyst, a synthetic polymer, or an allosteric effector. 
     
     
         55 . The method of  claim 44 , wherein the analyte is a biomolecule. 
     
     
         56 . The method of  claim 44 , wherein the biomolecule comprises a nucleotide analyte, a peptidic analyte, or a saccharidic analyte. 
     
     
         57 . The method of  claim 44 , wherein the necessary non-analyte original component comprises an original salt and the substitute component comprises a substitute salt. 
     
     
         58 . The method of  claim 57 , wherein the original salt and the substitute salt function as buffer salts for the fluid sample, such that volatilization of the fluid sample results in gas phase extraction of the substitute buffer salt. 
     
     
         59 . The method of  claim 58 , wherein the substitute salt comprises singly charged ions formed from weak acids or weak bases. 
     
     
         60 . The method of  claim 59 , wherein the substitute salt comprises ammonium bicarbonate, ammonium formate, ammonium acetate, pyridinium acetate, pyridinium formate, ethylmorpholinium acetate, trimethylamino acetate, or trimethylamino formate. 
     
     
         61 . The method of  claim 60 , wherein the substitute salt comprises ammonium bicarbonate, ammonium formate, or ammonium acetate. 
     
     
         62 . The method of  claim 44 , wherein the fluid reservoirs are arranged in an array. 
     
     
         63 . The method of  claim 62 , wherein the fluid reservoirs are contained within a substrate comprising an integrated multiple reservoir unit. 
     
     
         64 . The method of  claim 63 , wherein the integrated multiple reservoir unit is a microwell plate and the fluid reservoirs are wells therein. 
     
     
         65 . The method of  claim 62 , wherein the fluid reservoirs are tubes in a tube rack. 
     
     
         66 . An improved method for determining the concentration of an analyte in each of a plurality of fluid samples that additionally comprises a necessary non-analyte original component, the method comprising volatilizing and ionizing the samples and introducing each ionized, volatilized sample into an ionic analyte detection device that provides a signal proportional in intensity to the quantity of ionized analyte detected,
 wherein the improvement comprises substituting for the necessary non-analyte original component a substitute component that functions as the original component in the fluid sample, is more volatile than the necessary original component, and results in an increase in the intensity of the signal and/or a greater signal-to-noise ratio than either the signal intensity or signal to noise ratio obtained using the original component, and additionally comprises   (a) providing the fluid samples in each of a plurality of fluid reservoirs;   (b) acoustically coupling an acoustic droplet ejector to a first of the fluid reservoirs;   (c) activating the ejector to generate focused acoustic radiation toward the first reservoir and into the fluid sample therein, in a manner effective to eject nanoliter-sized droplets of the fluid sample into the ionic analyte detection device, wherein the droplets have a mean droplet size of less than approximately 50 pl;   (d) positioning another of the fluid reservoirs and the acoustic droplet ejector in acoustic coupling relationship;   (e) repeating step (c); and   (f) repeating steps (d) and (e) with additional fluid reservoirs in the plurality of fluid reservoirs at a rate of greater than 10 reservoirs per second.