Direct liquid injection inlet to a laser photoionization apparatus
View Patent ↗A method and apparatus are provided for analyzing an analyte at low concentration in a liquid sample by photoionization and mass spectrometry. An inlet system is provided for direct injection of the liquid sample using a capillary tube. The method and apparatus allow for 20 to 2000-fold improvement of the lower detection limit of an analyte in a liquid sample compared to a conventional liquid chromatography/mass spectrometer apparatus.
1. A method for analyzing an analyte at low concentration in a liquid sample by mass spectrometry comprising the steps of:
(a) introducing a liquid sample containing said analyte into a capillary tube having a proximal end for receiving a liquid sample and a distal end for exit of said sample;
(b) forming the liquid sample exiting said distal end into a directed stream of droplets along a path toward a zone of photoionization under a gradient of successively lower pressure such that substantial condensation of said analyte along said path is avoided;
(c) directing said stream into said zone of photoionization to ionize said analyte to form analyte ions; and
(d) passing said analyte ions into a mass analyzer of a mass spectrometer for mass analysis of said ions.
2. A method according to claim 1 wherein in said step (a) said sample exits into a region under atmospheric pressure.
3. A method according to claim 1 wherein in said step (a) said sample exits into a region under under subatmospheric pressure.
4. A method according to claim 1 wherein said analyte is quantitatively analyzed.
5. A method according to claim 1 wherein in said step (c) said stream is directed into said zone of photoionization at a pressure in the range of 10 −4 to 10 −5 torr.
6. A method according to claim 1 wherein said stream is passed along said path through a collimator directly into said zone of photoionization.
7. A method according to claim 6 wherein the distance from said collimator to said zone of photoionization is in the range of about 12 to about 0.5 centimeters.
8. A method according to claim 7 wherein said distance and the pressure at which said stream is introduced into said zone of photoionization are selected so that the analyte is detectable at a minimal concentration in a sample of about 0.0001 μg/ml.
9. A method according to claim 1 wherein said zone of photoionization is provided by a laser.
10. An apparatus for irradiation of a liquid sample containing an analyte to be ionized, comprising:
(a) a capillary tube for introducing said liquid sample as droplets into said apparatus;
(b) a zone of photoionization for irradiating evaporated droplets of said liquid sample at subatmospheric pressure to form ionizable species;
(c) a region of subatmospheric pressure characterized by a gradient of successively lower pressure along a path toward said zone of photoionization;
(d) a collimator for directing a collimated stream of evaporated droplets of said sample along said path directly into said zone of photoionization; and
(e) a mass spectrometer for determining the m/e ratio of ions found by irradiating said sample.
11. An apparatus according to claim 10 wherein said capillary tube is located to introduce said droplets into a region under atmospheric pressure in said apparatus.
12. An apparatus according to claim 10 wherein said capillary tube is located to introduce said droplets into a region under subatmospheric pressure in said apparatus.
13. An apparatus according to claim 10 wherein the distance from said collimator to said zone of photoionization is in a range of about 12 centimeters to about 0.5 centimeters.
14. An apparatus according to claim 10 wherein said zone of photoionization is provided by a laser.