A miniature mass spectrometer capable of detecting analytes eluting from a high-performance liquid chromatography (HPLC) system is described and demonstrated for the first time. The entire instrument, including all pumps and the computer, is contained within a single enclosure that may be conveniently accommodated at the base of the HPLC stack. The microspray ion source, vacuum interface, ion guide, and quadrupole ion filter are all microengineered. These components are fabricated in batches using microelectromechanical systems (MEMS) techniques and considered to be consumables. When coupled to a standard HPLC system using an integrated passive split, the limit of detection for reserpine while scanning the full mass range is 5 ng on-column (1 pg of which is passed to the microspray). The mass range is m/z 100-800, and each spectrum is typically acquired at a rate of 1 scan per second.
Miniaturized mass spectrometers are becoming increasingly capable, enabling the development of many novel field and laboratory applications. However, to date, triple quadrupole tandem mass spectrometers, the workhorses of quantitative analysis, have not been significantly reduced in size. Here, the basis of a field-deployable triple quadrupole is described. The key development is a highly miniaturized ion optical assembly in which a train of six microengineered components are employed to generate ions at atmospheric pressure, provide a vacuum interface, effect ion guiding, and perform pre-filtering, fragmentation and mass analysis. Despite its small dimensions, the collision cell efficiently fragments precursor ions and yields product ion spectra that are very similar to those recorded using conventional instruments. The instrument has been used to detect Thiabendazole, a common pesticide, in apple pulp at a level of 10 ppb.3
Microfabricated quadrupole mass spectrometers with Brubaker pre-filters are demonstrated for the first time. Complete filters are assembled from two dies, each carrying two pairs of rods providing the pre-filter and main filter sections. The rods are held in precision silicon mounts that are fabricated using wafer scale deep reactive ion etching and anodic bonding to glass substrates. Improvements to ion transmission are obtained by tuning the bias potential applied to the pre-filter. The effect is explained in terms of a simple analytic theory for ion motion in the pre-filter. Mass filtering with a range of m/z = 0-1200 and a resolution of m/Δm ≈ 150 at 10% of peak height is demonstrated using 2-4 mm long pre-filter electrodes, 30 mm long main electrodes (both of 650 µm diameter) and a RF drive at ≈ 6.5 MHz.
The limitations of conventional machining and assembly techniques require that designs for quadrupole mass analyzers with rod diameters less than a millimeter are not merely scale versions of larger instruments. We show how silicon planar processing techniques and microelectromechanical systems (MEMS) design concepts can be used to incorporate complex features into the construction of a miniature quadrupole mass filter chip that could not easily be achieved using other microengineering approaches. Three designs for the entrance and exit to the filter consistent with the chosen materials and techniques have been evaluated. The differences between these seemingly similar structures have a significant effect on the performance. Although one of the designs results in severe attenuation of transmission with increasing mass, the other two can be scanned to m/z ϭ 400 without any corruption of the mass spectrum. At m/z ϭ 219, the variation in the transmission of the three designs was found to be approximately four orders of magnitude. A maximum resolution of M/⌬M ϭ 87 at 10% peak height has been achieved at m/z ϭ 219 with a filter operated at 6 MHz and constructed using rods measuring ( nterest in the miniaturization of mass analyzers has been growing rapidly in recent years, largely driven by the need for compact, lightweight systems for use in environmental, security, and space applications. While there have been numerous attempts to make small analyzers, not all can reasonably be called miniature [1]. However, quadrupole mass filters [2-6] and various types of ion traps [7][8][9] with characteristic dimensions of the order of a few millimeters or less have been demonstrated. The size and weight contribution to a complete mass spectrometer system is not the only benefit of a miniaturized quadrupole mass filter. The rf amplitude required to achieve a particular mass range increases with the square of the rod radius. Hence, the rf supplies for miniature filters can be smaller and require less power than those needed for larger filters. More importantly, smaller pumps may be used, as the short path length of ions in the filter means that a higher pressure can be tolerated. Although this paper is focused on miniaturization of the filter, reports on the development of other miniaturized components that might be incorporated into a system, such as ion sources, gauges, and pumps can be found elsewhere [10 -12].The factors that determine the size of a miniature quadrupole mass filter are the required signal level, the accuracy of the construction technique, and the number of rf cycles needed to achieve the desired resolution. Clearly, as the size of the entrance aperture decreases, there will be an inevitable loss of signal, although some of this can be recovered through the use of an array [2,4]. There is a well-known correlation [13] between the fractional geometrical error and ultimate resolution. Hence, one fundamental limitation is set by the accuracy of the fabrication technique and the minimum acceptable resolution. Si...
An electrospray ionisation mass spectrometer (ESI-MS) whose main components are all fabricated using silicon microelectromechanical systems (MEMS) techniques is demonstrated for the first time. The ion source consists of a microengineered alignment bench containing a V-groove mounting for a nanospray capillary, an ion extraction electrode, and a pneumatic nebuliser. The vacuum interface consists of two plates, each carrying a 50 µm diameter capillary, that are selectively etched and bonded together to provide a differentially pumped internal cavity. The quadrupole filter consists of a microfabricated frame that provides mountings for stainless steel rods measuring 650 µm in diameter and 30 mm in length. Two different quadrupoles are compared: a firstgeneration bonded silicon device, and a second-generation silicon-on-glass device with a Brubaker prefilter. Differential pumping of a MEMS component is demonstrated for the first time, atmospheric pressure ionisation and ion transfer into vacuum are characterised, ESI-MS operation is demonstrated and spectra are presented for a variety of compounds. KEYWORDS:Mass spectrometry, Electrospray, Quadrupole filter, MEMS IntroductionOver 25 years have passed since an electrospray ionisation (ESI) source was first coupled to a quadrupole mass spectrometer (MS) by Yamashita and Fenn [1]. Since then, ESI-MS has become a workhorse of analytical chemistry, allowing analytes to be ionised at atmospheric pressure with little fragmentation and passed into a vacuum system for analysis.Conventional ESI sources employ spray tips with an internal diameter (ID) of ≈100 µm[2]. In order to establish a Taylor cone and maintain stable emission of charged droplets, minimum flow rates of 0.5 -5 µL min -1 and voltages of 2.5 -4 kV are used. However, it has been convincingly demonstrated that high sensitivity and stable emission can be achieved at much lower flow rates and voltages using spray tips with IDs of ≈ 5 µm, and silica nanospray capillaries with a range of coatings have become widely available [3][4][5][6][7][8].Unfortunately, while these are inexpensive, operator expertise and costly positioning apparatus are often required for stable and reproducible performance.Electrosprayed ions are passed into a low-pressure chamber via a vacuum interface. The simplest interface is a single orifice or capillary that allows gas and entrained ions to pass directly into the vacuum chamber. Clearly, the orifice must be small enough or the pumps large enough to ensure that the pressure is consistent with mass analysis. However, the susceptibility of very small orifices to clogging and the inconvenience of large pumps led to this approach being abandoned. Modern instruments employ one or more stages of 4 differential pumping. Larger orifices can be tolerated since only a fraction of the flow is transmitted to the vacuum chamber, while the majority of the gas load can be pumped away at higher pressure using more modest pumps.The effective transfer of ions and molecules from atmospheric pressu...
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