MS - American Chemical Society

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product review

Old reliable benchtop GC/MS The market for GC/MS remains strong for food and environmental applications. Rajendrani Mukhopadhyay

T

he old saying, “If it ain’t broke, don’t fix it,” could be applied to benchtop GC/MS systems. While experts agree that the general performance and functional capabilities have increased since Analytical Chemistry last reviewed the instruments (1999, 71, 401 A–406 A), they say the changes have not been revolutionary. “There have been improvements over the last five years, but I would say the improvements have generally been incremental. In the benchtop GC/MS area, there hasn’t been a shining new instrument that has been developed,” says Nicholas Snow of Seton Hall University. “I think that’s an indication of the fact that the technology has been well developed,” says James Yano of Agilent Technologies. Experts also think GC/MS hasn’t seen any major changes recently because instrument companies have focused their attention on techniques such as LC/MS for biomedical and pharmaceutical applications. Nonetheless, the market for GC/MS systems remains stable. Tables 1–4 list selected commercially available quadrupole, ion trap, TOF, and magnetic sector benchtop GC/MS instruments. The tables are not meant to be comprehensive; some vendors also offer products not listed here. Experts agree that two changes have occurred in GC/MS instrumentation over the past five years. For one, the general cost per analysis has decreased. Matthew Booth of the University of Florida states, “I think you can get a lot more for your dollars now than [five years ago]. GC/MS instruments with differential pumping and chemical ionization sources used to be an option only for research laboratories. These options are now available for benchtop systems for under $100,000, making them very affordable for almost every laboratory.” Second, the instrument has become more user-friendly. “That’s where the vendors have made the most improvement in the last five years— ease of use,” Snow says. The ease of use has mainly come about because of better software. Some vendors argue that software improvements are just as critical as instrumentation changes. “Technology allows you to generate a certain type of data, but the other important part is what you allow the user to do with that data,” explains Steve Smith of Waters. Even though GC/MS has seen few innovations, the market for the instrument remains substantial. Environmental applications, forensics, and the food industry continue to be the major © 2004 AMERICAN CHEMICAL SOCIETY

users of GC/MS, and the pharmaceutical and petrochemical industries are becoming more visible markets. Within the established core of GC/MS users, Yano says that some lines have blurred between environmental and food applications. He points to the example of import and export of foodstuffs, where an increased interchange of food among countries has caused a rise in pesticide analysis.

The subtle differences In terms of GC/MS instrumentation, the gas chromatograph has changed the least. Although more types of columns are commercially available, the basic GC technology has not changed. Several vendors now offer the capability to do large volume injections, where up to 50 µL of sample can be introduced into the GC column, instead of the conventional 1 µL. Large volume injection improves the sensitivity of MS detection; however, some experts are wary of it for two reasons. First, the large volume inlet is an additional expense. Second, only highly pure samples can be introduced into the instrument in large volumes. As Booth and other experts point out, injections of impure samples in large volumes will only result in contaminating the GC column. The mass spectrometers of benchtop GC/MS instruments have evolved in the past five years to provide more user choices. Instruments are now available with more than one ionization mode, where the ionization source can be switched from elecJ U N E 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y

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product review

Table 1. Quadrupole benchtop GC/MS instruments.1 Product

5973 Inert MSD

Clarus 500

QP-2010

Quattro Micro GC

1200L

Finnigan Trace DSQ

Company

Agilent Technologies 395 Page Mill Rd. P.O. Box 10395 Palo Alto, CA 94303 800-227-9770

PerkinElmer 549 Albany St. Boston, MA 02118 800-762-4000

Shimadzu America/ Shimadzu Scientific Instruments 7102 Riverwood Dr. Columbia, MD 21046 800-477-1227

Waters 34 Maple St. Milford, MA 01757 800-252-4752

Varian Instruments 2700 Mitchell Dr. Walnut Creek, CA 94598 800-926-3000

Thermo Electron Corp. 81 Wyman St. Waltham, MA 02454 781-622-1000

URL

www.chem.agilent.com www.perkinelmer.com www.ssi.shimadzu.com www.waters.com

www.varianinc.com www.thermo.com

Cost (U.S.D.)

Not available

$58,500

$74,450 EI only

$200,000

$100,000–150,000

Not available

Mass range (m/z)

1.6–800

1–1200

1.5–1024

4–1500

10–1500

1–1050

Unit mass

Unit mass

>1500 (fwhm)

1 amu (unit mass resolution across entire mass range)

Better than unit mass resolution (10% valley) over the entire mass range

EI, CI

EI, CI

EI, CI

EI, CI

EI, CI

LC, GC, DIP, DEP

Liquid and headspace autosamplers, DEP, DIP solids probe

Mass resolving power Unit mass

Ion source

Inert EI, CI

Sample introduction

Split/splitless, PTV, on- Capillary up to 5 mL/ column, headspace, min purge and trap

Split/splitless injection, GC, DIP, DCI on-column injection, PTV, DIP, pyrolyzer

Modes of operation

EI, NCI, PCI, EI with CI EI, NCI, PCI ion source

EI, NCI, PCI

Selected ion monitor- EI, PCI, NCI, ing, MS/MS, positive Townsend Discharge, ESI, APCI and negative ion

Options

Performance turbo, PCI/NCI, deconvolution software

Liquid autosamplers, headspace thermal desorption, cryogenic oven cooling, GC
GC, standard highspeed GC/MS

CI, solids introduction probe, GC autosampler, TargetLynx and QuanLynx software, Wiley and NIST libraries

Water cooling

PCI, NCI, PPINICI

Triple quadrupole, Vacuum interlock proprietary Chromatoprobe for solid sample introduction

1 Most companies offer multiple instruments. Contact the vendors for full product lines. APCI, atmospheric pressure chemical ionization; CI, chemical ionization; DCI, direct chemical ionization; DEP, direct exposure probe; DIP, direct insertion probe; EI, electron ionization; ESI, electrospray ionization; FD, field desorption; NCI, negative chemical ionization; PCI, positive chemical ionization; PPINICI, pulsed positive ion negative ion chemical ionization; PTV, programmed temperature vaporizer.

tron impact (EI) mode to chemical ionization (CI) mode. Experts say that scientists should keep in mind some general caveats when they consider sources capable of both EI and CI modes. Aviv Amirav of Tel Aviv University points out, “When one designs an ion source that will be able to switch from EI to CI by a click of a mouse, then usually there are some trade-offs in sensitivity. The ion source is not optimized to either ionization mode.” Instruments offering dedicated ion sources that can be interchanged present two problems. For some instruments, the vacuum needs to be broken in order to change the ion source. Instruments that do not need venting tend to be more expensive. The ultimate choice in instrument will boil down to the user’s comfort level. Snow compares purchasing a benchtop GC/MS instrument to buying a car. He says, “When you look at all these vendors’ systems and analyses, they’ll do the job, just like driving a car will get you to work, but a lot of us pick our car on how comfortable we are in it and if we can drive it easily. I tell all new users to make sure they test drive. To the uninitiated 214 A

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eye, [the instruments] all do look similar to each other. A lot of differences come out in how you test drive them.”

Quadrupoles and ion traps The quadrupole is still the most common type of mass filter in benchtop GC/MS instruments; ion trap, TOF, and magnetic sector instruments are less prevalent. In most systems, the ion detection system is based on conversion dynode electron multipliers. Agilent Technologies recently introduced the 5973 Inert MSD, which is a quadrupole instrument with a proprietary inert ion source that facilitates the analysis of highly reactive compounds. The 5973 Inert MSD can be purchased with both EI and CI capabilities, and the switch from EI to CI can be achieved with a mouse click. Shimadzu’s QP-2010 has a quadrupole mass filter and offers both EI and CI modes without a change in ion source. PerkinElmer’s current quadrupole GC/MS is the Clarus 500, which evolved from the TurboMass instrument. The Clarus 500 has large volume injection capability and offers both EI and CI.

product review

Table 2. Ion trap benchtop GC/MS instruments. Product

4000

Finnigan PolarisQ

Company

Varian Instruments 2700 Mitchell Dr. Walnut Creek, CA 94598 800-926-3000

Thermo Electron Corp. 81 Wyman St. Waltham, MA 02454 781-622-1000

URL

www.varianinc.com

www.thermo.com

Cost (U.S.D.)

$90,000

Not available

Mass range (m/z)

10–1000

10–1000

Mass resolving power 1 amu (unit mass resolution across entire mass range)

Unit mass resolution (resolution based on 10% valley definition) over entire mass range

Ion source

EI, CI

EI, CI

Sample introduction

GC only

Liquid and headspace autosamplers, DEP, DIP solids probe

Modes of operation

EI, internal PCI, external PCI/NCI, PCI, NCI, PPINICI hybrid-mode PCI/NCI, EI/MS/MS, CI/MS/MS, and MSn

Options

Proprietary Chromatoprobe for Variable damping gas control solid sample introduction, liquid module, fast switching power CI, GC detectors supply for PPINICI

The detection system is based on photomultipliers rather than electron multipliers; according to the manufacturer, photomultipliers tend to last the lifetime of the instrument. Waters offers the Quattro Micro GC with a quadrupole mass filter. It comes with interchangeable EI and CI sources that do not require venting for source exchange. Like PerkinElmer’s, Waters’ instrument uses a photomultiplier detector. Thermo Electron and Varian offer both quadrupoles and ion trap GC/MS instruments. Varian has four products on the market. The Saturn series and the 4000 are ion trap instruments. The ion source in the two Saturns is EI, with optional positive CI; a change in hardware is not necessary to switch from one ionization mode to another. The 4000 has three separate ionization configurations—internal, external, and hybrid chemical ionizations. Varian’s fourth instrument, the 1200L, can be purchased either as a single- or triple-quadrupole mass spectrometer. The instrument can switch between GC/MS and LC/MS in the field. Thermo Electron offers three types of instruments. The Finnigan Trace DSQ uses a single quadrupole, and the Finnigan PolarisQ has an ion trap. The GC column, ion source, and detectors are the same for both instruments. The ion source is EI, with an optional hardware upgrade to a CI source or a combination EI/CI source. Thermo Electron’s third instrument, the Finnigan Focus DSQ, is similar to the Trace DSQ but has a smaller footprint.

Magnetic sector

Magnetic sector GC/MS instruments are also commercially available. JEOL offers GCmateII and its variation, the TEEMmate (Tunable-Energy Electron Monochromator). The difference between the two instrument systems lies in the ion source. The GCmateII comes with both EI and CI capabilities. In the TEEMmate, the electron monochromator forms electrons at the exact resonance energy needed to produce the analyte ions, eliminating the need for high-pressure ioniza-

Table 3. TOF benchtop GC/MS instruments. Product

Pegasus III

GCT

Company

LECO Corp. 3000 Lakeview Ave. St. Joseph, MI 49085 800-292-6141

Waters 34 Maple St. Milford, MA 01757 800-252-4752

URL

www.leco.com

www.waters.com

Cost (U.S.D.)

~$129,000

$200,000

Mass range (m/z)

5–1000

20–2000

Mass resolving power Unit mass

>7000 (fwhm)

Ion source

EI

EI, CI, FI, FD

Sample introduction

GC, DIP, DEP

GC, DIP, DCI

Modes of operation

Full mass range acquisition up to 500 spectra/s

Exact mass full scan, positive and negative ion

Options

GC
GC

GC autosampler, deconvolution software (ChromaLynx)

TOFs Benchtop GC/MS instruments with TOF mass filters are recent introductions into the market. These instruments are aimed mainly toward the pharmaceutical and petrochemical

industries, which need to analyze highly complex mixtures in a short amount of time. Both Waters and LECO offer TOF GC/MS instruments, but each vendor uses the TOF mass filter to different advantages. LECO’s TOF mass spectrometer is used to generate spectra very quickly, at an acquisition rate of up to 5000 spectra/s. LECO offers two instruments, the Pegasus III and the Pegasus 4D. The ionization source in the instruments is EI only, and the detector is a microchannel plate. The Pegasus III consists of an Agilent 6890 GC column attached to a TOF mass spectrometer. The recently introduced Pegasus 4D can do GC
GC, which allows complicated samples with multiple components to be sequentially separated on two columns of different phases before TOF analysis. Waters’ GCT instrument comes with EI, CI, and field ionization (FI) capabilities. FI provides a strong molecular ion signal with very little fragmentation. The TOF mass spectrometer in the GCT can be used to determine the specific elemental composition of the compounds eluting from the GC column. The detector, like LECO’s, is a microchannel plate.

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Table 4. Magnetic sector GC/MS instruments. Product

AMD QuAS3AR

GCmateII

Company

AMD Intectra GmbH Koenigsberger Str. 1 27243 Harpstedt, Germany 49-4244-1062

JEOL USA 11 Dearborn Rd. Peabody, MA 01960 978-535-5900

URL

www.AMD-Intectra.de

www.jeol.com

Cost (U.S.D.)

$250,000–300,000

Not available

Mass range (m/z)

1–2000 at 6-kV accelerating voltage, 1–1000 at 2.5 kV; up to m/z 3000 at 2– 4000 at 3-kV accelerating voltage reduced accelerating potential

Mass resolving power 300–5000 (10% valley)

500–5000 (20% valley)

Ion source

EI/CI and EI/FI in dual ionization configuration

EI, CI

Sample introduction

GC, DIP, DEP/DCI, batch inlet

GC, DIP

Modes of operation

Magnet scan, rapid acceleration voltage scan at constant magnetic strength, linked scans

Scan or selected ion monitoring, positive and negative ions

Options

APCI, GC/LC/MS configuration

Fast atom bombardment, linked-scan MS/MS

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tion. AMD Intectra, a company based in Germany, offers the AMD QuAS3AR system. The instrument is available with EI/CI and EI/FI sources and can be configured to run in both GC/MS and LC/MS modes.

Is there room for improvement? Experts point to two major avenues that still need to be explored: increasing the range of compounds amenable to GC/MS analysis, and decreasing the time of a GC/MS run. “One needs to extend the range of compounds one can analyze by GC/MS. That’s one obvious need that is ignored,” says Amirav. The run times for most GC/MS instruments, except the TOFs, continue to stretch between 30 min and 1.5 h. Booth argues that the coupling of short GC columns to TOF mass filters could very well lead to analysis times on the order of a few minutes, reducing the risk of analyte decomposition. Rajendrani Mukhopadhyay is an associate editor of Analytical Chemistry.