An ion source for a mass spectrometer comprising: an ultrasonic transducer arranged and adapted to focus ultrasonic energy onto a surface of a sample fluid without said ultrasonic transducer directly contacting said sample fluid, wherein said ultrasonic transducer is arranged and adapted to eject one or more droplets from said sample fluid in a substantially controlled manner, wherein said one or more droplets comprise a majority of un-ionised droplets; and characterised in that: said ion source further comprises: an Electrospray ion source arranged and adapted to ionise said one or more droplets ejected from said sample fluid by said ultrasonic transducer; and a control system arranged and adapted to determine the density of said sample fluid by determining the intensity or energy of a reflected sonar pulse and to control the focusing of said ultrasonic energy onto said surface of said sample fluid based upon said determined density of said sample fluid and the time of flight of a reflected sonar pulse; wherein said sonar pulse has an energy in the range < 100 mW, 100-200 mW, 200-300 mW, 300-400 mW or 400-500 mW and/or has a relative low energy so as not to cause ejection of droplets from said sample fluid. an ultrasonic transducer arranged and adapted to focus ultrasonic energy onto a surface of a sample fluid without said ultrasonic transducer directly contacting said sample fluid, wherein said ultrasonic transducer is arranged and adapted to eject one or more droplets from said sample fluid in a substantially controlled manner, wherein said one or more droplets comprise a majority of un-ionised droplets; and characterised in that: said ion source further comprises: an Electrospray ion source arranged and adapted to ionise said one or more droplets ejected from said sample fluid by said ultrasonic transducer; and a control system arranged and adapted to determine the density of said sample fluid by determining the intensity or energy of a reflected sonar pulse and to control the focusing of said ultrasonic energy onto said surface of said sample fluid based upon said determined density of said sample fluid and the time of flight of a reflected sonar pulse; wherein said sonar pulse has an energy in the range < 100 mW, 100-200 mW, 200-300 mW, 300-400 mW or 400-500 mW and/or has a relative low energy so as not to cause ejection of droplets from said sample fluid. said ion source further comprises: an Electrospray ion source arranged and adapted to ionise said one or more droplets ejected from said sample fluid by said ultrasonic transducer; and a control system arranged and adapted to determine the density of said sample fluid by determining the intensity or energy of a reflected sonar pulse and to control the focusing of said ultrasonic energy onto said surface of said sample fluid based upon said determined density of said sample fluid and the time of flight of a reflected sonar pulse; wherein said sonar pulse has an energy in the range < 100 mW, 100-200 mW, 200-300 mW, 300-400 mW or 400-500 mW and/or has a relative low energy so as not to cause ejection of droplets from said sample fluid. an Electrospray ion source arranged and adapted to ionise said one or more droplets ejected from said sample fluid by said ultrasonic transducer; and a control system arranged and adapted to determine the density of said sample fluid by determining the intensity or energy of a reflected sonar pulse and to control the focusing of said ultrasonic energy onto said surface of said sample fluid based upon said determined density of said sample fluid and the time of flight of a reflected sonar pulse; wherein said sonar pulse has an energy in the range < 100 mW, 100-200 mW, 200-300 mW, 300-400 mW or 400-500 mW and/or has a relative low energy so as not to cause ejection of droplets from said sample fluid.