Portable Analytical Instruments - Analytical Chemistry - ACS Publications


Portable Analytical Instruments - Analytical Chemistry - ACS Publicationspubs.acs.org/doi/abs/10.1021/ac00011a742Jun 1,...

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Portable Analytical Instruments

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ike the personal computer, succeeding generations of analytical I i n s t r u m e n t s h a v e s h r u n k in size and weight even as they have become more sophisticated and versatile. With these changes, instruments once restricted to fixed sites are being redesigned for portability. Furthermore, unlike previous oneof-a-kind devices flown on airplanes or spacecraft, some of these new portable instruments are being introduced into commercial markets. One of the key driving forces for portability is the need to monitor environmentally sensitive sites in situ. Mobile i n s t r u m e n t s for such work may have to operate in rugged areas, potentially without sources of electrical power, consumables (e.g., gases and solvents), or clean spaces for handling samples.

data reproducibility, and low power demands. The alternative, a quadrupole analyzer, might offer a more compact design for a portable mass s p e c t r o m e t e r b u t it d e m a n d s too much power. This particular mass analyzer incorporates a cycloidal (crossed-field) design t h a t focuses an ion beam in an unusually small volume. To accomplish this, ions are produced by electron ionization and accelerated by the potential difference between the source slit and the ionization chamber. The ionization chamber is located within the magnetic field so t h a t ions initially traveling parallel to the electric field and orthogonal to the

Backpacker's mass spectrometer

magnetic field trace out a cycloidal p a t h . "It is i n h e r e n t l y a doublefocusing instrument," says Hemond. The cycloidal design also offers the advantage of low operating voltages for scanning masses. However, to fit the ion source and plates between the magnet poles, a magnet t h a t produces a high volume of magnetic field is required. To meet this demand, Hemond installed a N d - F e magnet unit weighing about 9 kg. For detection, a commercial cycloid tube that operates in the mass range of 2-150 with a resolution of 1 u was chosen. The actual resolution has not yet reached this level (it drops off at

The ultimate in portability may be the backpack mass spectrometer being developed by MIT civil engineer Harry Hemond. With a total weight of about 32 kg, this unit can be transported to remote sites to collect envir o n m e n t a l d a t a . For e x a m p l e , it could provide rapid analysis of water along a shoreline fouled by an oil spill. To obtain this portability, Hemond balanced several factors to create a unit that is light, rugged, and energy efficient. A magnetic mass analyzer was chosen as the basic spectrometer configuration because of its stability,

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m a s s e s > 40); however, t h e m a s s spectrometer is still in the early stages of development. Hemond has just begun to look for the source of the resolution problem. A F a r a d a y cup collector is employed as a second detector, responding to masses between 2 and 11. This collector is situated so t h a t it can function when the electric field is shut off, converting the unit into a 180° magnetic sector mass spectrometer. The instrument could easily be calibrated in the field by bringing along appropriate standards or, as Hemond points out, by bleeding in air and looking at the H 2 0 , N 2 , and C 0 2 peaks. A 1 0 - t u r n potentiometer on t h e front panel of the mass spectrometer allows t h e o p e r a t o r to m a n u a l l y sweep masses. An ion current meter on the unit indicates response. In addition, the output of the mass scanning circuit can be fed to an external x-y recorder via a connector on the face of the instrument. A simple 1-mm 2 membrane interface serves as the sample inlet system. The small size guarantees that anything that crosses the membrane can be introduced into the mass spectrometer. Because liquid or gas samples are injected directly into the mass spectrometer without any preseparation, data must be collected and computationally deconvoluted. Hemond recently added an RS-232 port to the portable mass spectrome-

ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1, 1991 · 641 A

FOCUS ter for transferring data to a laptop computer. The computer, in turn, can be used to control the mass scanning circuit. If t h e high vacuum system h a s been vented, the mass spectrometer is first evacuated with an external rough vacuum pump. An 8-L/s diode sputter-ion pump within the system operates continuously to m a i n t a i n the high vacuum. If for any reason the high vacuum is lost, the portable battery-operated pump can be used to rough p u m p the spectrometer. Heaters for the filament and plates of the cycloid tube aid in degassing the system. Gas pressure at both the s p u t t e r - i o n p u m p a n d inside t h e sample inlet system are displayed on two external meters. Power for the mass spectrometer is supplied internally by using a pack of N i - C d batteries or externally by tap­ ping into a vehicle cigarette lighter ( 1 2 V d c ) . W h e n t h e cycloid t u b e heaters are switched off, the typical current load for the mass spectrome­ ter is 2 A. Separate power switches allow the operator to turn off idle subsystems, conserving battery power for several hours of field measure­ ments. The entire unit fits into a rectan­ gular welded aluminum framework, measuring 20 χ 36 χ 60 cm, t h a t can be mounted onto a backpack frame. Controls and meters are mounted on a front panel. In demonstration tests, the porta­ ble mass spectrometer yielded appro­ p r i a t e fragmentation p a t t e r n s for methane (after briefly bubbling the gas through water) and trichloroethylene at levels of 200 ppm in tap wa­ ter. Sensitivity, measured with a calibrated Ar gas leak into the in­ strument, is reported as ~ 50 pg/s. Hemond says that with modifica­ tions, such as simplifying the stain­ less steel vacuum envelope, using a high-energy product magnet for the sputter-ion pump, or reducing the thickness of the aluminum frame, t h e i n s t r u m e n t can become even lighter. "I would not be surprised if we could reduce the weight to about 23 kg," he says. Other modifications, such as mul­ tiple-membrane probes and vapordrying techniques, would increase the versatility of the device. How­ ever, even at this early stage, the backpack mass spectrometer is capa­ ble of collecting important environ­ mental data.

cence (XRF) analyzer that weighs just 8.5 kg and fits snugly into an overthe-shoulder bag. Various plug-in probes provide quantitative determi­ nation of elements ranging from Si to U in solids, liquids, and powders. The unit can be used for on-site analyses of commercial products, such as coat­ ings and catalyst beds, as well as for environmental field studies of contam­ inated soils and water. Radioactive sources are employed to g e n e r a t e t h e p r i m a r y X - r a y s which, in turn, generate the fluores­ cent X-rays t h a t characterize each element. Fluorescent X-rays are de­ tected by either an Ar or Ne gasfilled proportional counter or a solidstate Si(Li) device, which is chosen specifically for the desired elements. The s o l i d - s t a t e detector r e q u i r e s cooling and is equipped with a liquid nitrogen Dewar flask t h a t offers about 8 h of operation. For heavy elements (Ti-U), probes use 2 4 4 Cm, 109 Cd, or 2 4 1 Am as the X-ray source. Lighter elements are detected with a 5 5 Fe source. Tradi­ tionally, a He purge was necessary to measure fluorescent X-rays emitted from light elements because these w a v e l e n g t h s are absorbed by air. Outokumpu avoids this problem by designing a probe that can be posi­ tioned close to the sample. Radioac­ tivity levels outside the probe are so low t h a t film badges and special li­ censing are not required. Detection limits vary with each element. However, of the 24 elements the Environmental Protection Agen­ cy (EPA) regulates, the Outokumpu X-ray analyzer detects all but Be at levels of 100-200 ppm. In a typical environmental applica­ tion, the XRF unit directs research­ ers to where samples should be col­ l e c t e d for l a t e r a n a l y s i s in t h e laboratory, t h u s avoiding random sampling. The unit also plays a role in checking for compliance—ensur­ ing, for instance, that all contami­ nated soil has been removed. A 16-bit microprocessor and inter­ nal software included with the unit offer simultaneous determination of up to six elements, multivariate re­ gression analysis, a user-determined reference library of up to 400 stan­ dards, and several operating modes. The RS-232 port allows researchers to interface the instrument with a computer. A rechargeable b a t t e r y unit can provide approximately 10 h of portable operation.

X-ray fluorescence anywhere

FT-IR in the field

Outokumpu Electronics of Langhorne, PA, has introduced an X-ray fluores­

Portable radiometers for measuring IR emissions have been used previ­

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ously, especially for a t m o s p h e r i c transmission measurements. How­ ever, a m o n g t h e a p p l i c a t i o n s of Bomem's new MB series of instru­ ments are field pollution monitoring and in-plant analyses. The Bomem MB units weigh about 100 lbs. The i n s t r u m e n t s are de­ signed for ac but can be configured for dc operation. Depending on which detector and beamsplitter have been chosen, the FT-IR instruments record spectral ranges of 0.75 - 3 μηι, 0.75 22 μιη, 2 - 2 8 μτη, or 2 - 50 μιη. Resolu­ tion can be varied from 1 to 128 cm"1. In addition, the radiometers can be at­ tached to a telescope to measure a narrow field of view. All d e t e c t o r s e m p l o y e d by Bomem—such as HgCdTe (also la­ beled MCT), InGaAs, or Si—are solid state. Liquid nitrogen or thermoelectronic cooling may be required for some of t h e s e . B e a m s p l i t t e r s are made from KBr, KC1, or the glass known as BK7. Because components are pre-aligned at the factory, they are not interchangeable. However, once built, these rugged and stable machines have been transported all over the world and into space aboard the Shuttle. The Bomem FT-IR systems are be­ ing used in municipal waste-to-ener­ gy incinerators, where they contin o u s l y m o n i t o r e m i s s i o n s . By measuring the chemical composition of hot, wet chimney gas, the operator can ensure compliance with regula­ tory limits for environmentally dan­ gerous compounds. For measurements in a factory en­ vironment, the system can be mount­ ed on a cart and wheeled to an appro­ priate location. For instance, it can monitor vinyl chloride production by measuring the amount of HC1 in the monomer. The trickiest part of working with t h e s e i n s t r u m e n t s is c a l i b r a t i o n . "These are state-of-the-art instru­ ments," explains Bomem's Fred Baudais. "Absolute calibration is very difficult; it is much easier to do rela­ tive calibration." For field studies, the radiometer works best as a semi­ quantitative or qualitative detector. "From the peak areas we can deter­ mine threshold concentrations," adds Baudais. A number of researchers, including scientists from EPA, are now work­ ing on obtaining quantitative infor­ mation with these radiometers. Among the techniques being explored is t h e vapor r e l e a s e of s t a n d a r d amounts of chemicals. The unit has also been modified for absorption work by shining IR light

FOCUS through a telescope at a mirror sev­ eral hundred meters away and then collecting the reflected light. This ap­ proach offers a means of detecting "cold" gases. Laboratory on wheels When several i n s t r u m e n t s and/or wet chemical t e c h n i q u e s a r e r e ­ quired, it may be easier to bring the entire analytical laboratory to the sample. That is what E - N - G Mobile Systems of Concord, CA, has begun m a r k e t i n g . E - N - G will c u s t o m design and outfit a van, trailer, or truck with many of the standard fea­ tures of the analytical laboratory. To date, E - N - G has constructed more than 20 of these mobile laboratories. One advantage of this approach, e x p l a i n s E - N - G P r e s i d e n t Dick Glass, is t h a t it avoids specially de­ signed instruments. Equipped with standard laboratory instruments, the mobile laboratory can be used for du­ ties such as r o u n d - t h e - c l o c k air quality monitoring and soil analysis at hazardous waste sites. In the past, mobile laboratories have been designed by scientists fa­ miliar with the needs of a laboratory. E-N-G, to some degree, reverses the

process. As a major manufacturer of television n e w s t r u c k s , t h e com­ pany's expertise lies in engineering the vehicle for the equipment it con­ t a i n s . "Weight is a key consider­ ation," says Glass. An overloaded ve­ hicle is dangerous and can damage its suspension. In addition, a host of safety, security, and convenience fea­ tures are built into the vehicle. For instance, a typical mobile unit might include a water tank, sink, sol­ vent storage cabinet, waste water storage tank, fume hood, gas cylinder storage area, and air and/or hydro­ gen gas generator. To prevent con­ tamination, the air pumped into the lab to maintain a positive room pres­ sure is first passed through dust and charcoal filters. Air conditioning and, where necessary, heat are provided. Smoke and fire alarms, as well as a burglar alarm that triggers a beeper, can be included. Analytical instruments are secure­ ly fastened to bench tops. E-N-G's customers equip their mobile labora­ tories with commercially available instruments. One instrument suppli­ er, Hewlett-Packard, says that the only precaution needed while t r a n s ­ porting its instruments is that the

mass spectrometer source be shut off and the heads for reading computer hard disks be "parked." Electrical power is provided by on­ board generator(s). A standard de­ sign is two 6.0-kW generators to sup­ port two instruments such as a gas c h r o m a t o g r a p h and a m a s s spec­ trometer plus the necessary support­ ing appliances and facilities. Whether and how local and state municipalities will regulate the move­ m e n t of these mobile laboratories with respect to onboard gas cylinders still needs to be resolved. For in­ stance, travel through traffic tunnels could be prohibited. Glass admits that his company has received con­ flicting advice on this issue. Clearly, the scientific community a n d t h e general public will need to get used to the idea of mobile laboratories. Alan R. Newman Suggested reading Glass, D. American Environmental Labora­ tory, Feb. 1991, 26. Hemond, H. F. "A Backpack-Portable Mass Spectrometer for Measurement of Volatile Compounds in the Environ­ ment"; MIT Sea Grant College Program Technical Report MITSG 91-4; Massa­ chusetts Institute of Technology: Cam­ bridge, MA, 1991.

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