Effect of sequence length, sequence frequency, and data acquisition rate on the performance of a Hadamard transform time-of-flight mass spectrometer

Various factors influencing the performance of a Hadamard transform time-of -flight mass spectrometer (HT-TOFMS) have been investigated. Using a nitrog en corona discharge to produce an ion stream of N-2(+), N-3(+), and N-4(+), it is found for spectra containing only N-4(+) that the signal-to-noise ra tio (SNR) closely approaches the value calculated from the ion background b y assuming that the ion background follows a Poisson distribution. In contr ast, for a more intense beam containing N-2(+), N-3(+), and N-4(+), the SNR is less than its theoretical value because of the appearance of discrete s pikes in the mass spectrum caused by deviations in the actual modulation se quence from the ideal one. These spikes can be reduced, however, by decreas ing the modulation voltage. Under these optimized conditions, the pseudo-ra ndom sequence length is varied to understand how it alters SNR, mass resolu tion, and scan speed. When the length of the pseudo-random sequence is doub led, the SNR increases by root2 while the time necessary to record a mass s pectrum also doubles. Mass resolution can be varied between 500 and 1200 at m/z = 609 as the sequence length, modulation speed (10 MHz, 25 MHz), and a cquisition rate (up to 50 MHz) are changed. Scan speeds of 6000 passes per s can be obtained using a sequence containing 4095 elements modulated at 25 MHz. The capability to tailor the HT-TOFMS to increase the scan speed and resolution with a constant 50% duty cycle makes the technique extremely app ealing as a mass analyzer for measuring rapid changes in the composition of an ion stream. (C) 2001 American Society for Mass Spectrometry.