Product
Review
DSC
BALANCES DSC continues to assimilate advanced components for better heat measurement and control
OUT
Differential scanning calorimetry (DSC), like the closely related technique of differential thermal analysis (DTA; see the Product Review in the Oct. 15,1994, issue [Anal. Chem. 1994, 66,1035 A-1038 A]), is an established method with roots that stretch back to the days of Lavoisier and his ice calorimeter. Modern calorimetry finds applications in the pharmaceutical, food and beverage, and polymer industries as well as in research areas such as metabolism and catalyst characterization. DTA measures the difference in temtemperature across afixedthermal path perature between a sample and a therbetween the sample and a reference. All mally inert reference material when both three methods can provide quantitative are heated according to the same pro- values for enthalpy and heat capacity. gram. In power compensation DSC, the Aside from energy profiles, specific types temperature of the sample is constantly ad- of information that can be extracted from justed to match that of the thermally inthe data include phase transition temperert reference material as the temperature atures, percent crystallinity in polymers, range is scanned. In heat flux DSC, heat and food or pharmaceutical shelf life and flow is determined from the differential stability.
Analytical Chemistry asked Lee Hansen of Brigham Young University (Provo, UT) to comment on current trends in DSC and to offer advice to potential buyers. Table 1, although not intended to be comprehensive, contains a representative selection of commercially available differential scanning calorimeters. Modes of measurement
"There are three ways of measuring heat. Power compensation or heat flux are measured for DSC, whereas the rise in temperature is measured for DTA" Hansen explains. "DSC methods aren't new— they go back to at least the turn of the century. Tian and Calvet developed heat flux theory in the 1920s. Power compensation came in about 30 years ago when we started to get quality temperature controllers." Where DSC instruments have changed is in basic construction. New
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 323 A
Product
Review
Table 1. Summary of representative products Product Company
DSC 550 Instrument Specialists 2402 Spring Ridge Drive Suite Β Spring Grove, IL 60081 815-675-1550 $16,500; replacement cell $2200 48x15x36 Heat flux
Micro DSC Astra Scientific International 6900 Koll Center Pkwy Suite 417 Pleasanton, CA 94566 510-426-6900 $40,000 28 χ 61 χ 46 Heat flux
CSC 5100 Nano DSC Calorimetry Sciences 515 East 1860 South Provo, UT 84606 801-375-8181
Scan modes
Proportional integrated derivative (PID) controller; thermocouple detector Heating, cooling
Computerized controller; Pt detector; thermoelectric heating/cooling array Heating, cooling
Sample—Chromel-alumel thermo couple; Platinel II dual PID control; chromel or constantan heat flux plate Heating, cooling scans; timed isother mal modes; up to 10 program steps
Temperature range
-45°C-120°C
0°C-120°C
-150°C-550°C
Temperature accuracy and precision Heating rates Heat sensitivity/differential heating rate accuracy Sample chamber Crucibles/ampules
Accuracy ± 0.001 °C; precision + 0.001 °C 0.01 °C/min-2.0 °C/min ±0.1 μνν accuracy
Accuracy ± 0.1 °C; precision ±0.01 °C 0-2 °C/min 2 μθ8ΐ/°0 sensitivity; ± 2 μοβΙ/'Ό baseline noise at 1 °C/min
± 0.5% accuracy; quadratic corrections with multiple standards 0.1-100 °C/min ± 0.5% accuracy; quadratic correc tions with multiple standards
Hastelloy C
24-K Au capillary
Crimped AI, Cu, Au, Pt, and graphite; hermetic AI and Au
Volume
1 mi-
0.87 mL
50 mm3 crimped; 10 mm3 hermetic
Sample delivery
Removable cell (batch delivery); flow-through cell All
Fixed cell; flow-through loading and cleaning Pressurized to 3 atm
Removable crucible
Data system
486-based PC with MS-DOS-based data acquisition and instrument control software
Built-in computer with Windowsbased software for control, data acquisition, and analysis
486-based PC with software that runs in the background for multitasking; DSC, TGA, and TMA applications; baseline slope correction, heat of fusion, heat capacity, partial area, peak purity, 4 glass transition meth ods; 10 curve overlays with 2 y-scales; ASCII export; automatic or manual calculation of onset, maximum, or ending temperatures
Scope and applications
Pharmaceutical stability, enzymatic reactions, protein decomposition, gelation studies
Research, quality control
Special features
High sensitivity and large sample size for biotechnology and food applications
Conformation, solvation, stability, and structure for biopolymers in solution; lipid membrane structure; ligand interactions; helixto-coil transitions for polynucleotides Determines absolute heat capacity; baseline repeatability 10 μοΆ\Ι°0 at 1 °C/min
Options
ΙΝΑ
Version with multiple removable cells for temperatures up to 200 °C
Water or LN 2 cooling accessory; automatic gas switching
Reader Service No.
401
402
403
Price Dimensions (h χ w χ d; cm) Type Furnace/chiller Control and temperature detection
Atmospheres
ΙΝΑ = Information not available at press time
324 A
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
$46,000-$75,000 74 χ 53 χ 23 Power compensation
Noncorrosive inert, reducing, or oxidizing gases
ΙΝΑ
TA 8000 DSC 820 Mettler Toledo 69 Princeton-Hightstown Rd. P.O. Box 71 Hightstown, NJ 08520 800-638-8537 $46,000 and up 65 χ 45 χ 28 cm Heat flux
DSC 7 Perkin Elmer 761 Main Ave. Norwalk, CT 06859 800-762-4000
DSC-50 Shimadzu Scientific Instruments 7102 Riverwood Dr. Columbia, MD 21046 410-381-1227
DSC 2920 TA Instruments 109 Lukens Ave. New Castle, DE 19720 302-427-4000
$23,000-$36,000 49 χ 90 χ 58 Power compensation
$16,000 40x17.3x54 Heat flux
$45,000-$60,000 50 χ 58 χ 46 Heat flux
Pt100 sensor; 14-pile Au/Au-Pd thermopile
Dual independent Pt-lr furnaces; Pt resistance thermometers/heaters
Chromel-alumel thermocouple for sample and furnace temperatures
Heating, cooling scans; isothermal mode; options for alternating DSC
Heating, cooling scans; isothermal mode; options for dynamic DSC
Ambient-700 °C standard; -160 °C-700°C optional ΔΤ sensitivity ± 0.001 °C; reproducibility ± 0.1 °C 0.01-250 °C/min < 0.7 nW sensitivity; noise 1 μνν
-170°C-725°C
Heating, cooling scans; timed isothermal mode; up to 100 program steps Ambient-725 °C standard; -150 "C-400 °C with optional cooling unit Accuracy ± 0.1 °C; precision ±0.1 °C 0.1-99.9 °C/min; 0.1-99.9 °C/h 10 μ\Λ/ sensitivity; 2 μ\Λ/ peak-topeak noise
Platinel II control thermocouple; chromel-alumel temperature detector Heating, cooling scans; options for modulated DSC, pressure DSC, differential photocalorimetry -180°C-725°C
Standard AI, Au, Cu, Pt; mediumand high-pressure stainless steel or gold-plated crucibles
Standard AI, Au, Pt, Cu, graphite, Al 2 0 3 ; volatile AI and Au; largevolume stainless steel; high-pressure stainless steel; gold-plated stainless steel and Ti; vapor pressure AI 10-60 μ ι
Standard AI crimp cell, Pt cell with lid, alumina, quartz, Ni, Cu, largevolume Pt and AI, hermetic AI, high-pressure AI, high-pressure hermetic stainless steel 40 μι. standard
Removable crucible
Removable crucible
Static or dynamic N2, Ar, He, C0 2 , air, 0 2 , or other inert or active gases over full temperature range 66-MHz 486-based PC with UNIXbased software for up to 6 analyzers with fictive temperature, 2- and 3curve Cp methods; advanced software for purity, specific heat, dynamic kinetics, and isothermal kinetics calculations
Inert gases, N2, He for subambient operation; 20-50 mL/min flow rate; reduced pressure Windows-based thermal analysis workstation TA-60WSI for control of up to 4 instruments; data analy sis for DSC, DTA, TGA, or TMA
Quality assurance and control, research-level experiments
Research or routine work; charac terization of ceramics, biopolymers and foods, high-MW polymers, pharmaceuticals, and cosmetics
Research or routine work; indus trial or academic investigation of polymers, pharmaceuticals, foods, and inorganic materials
Simultaneous operation with 5 other modules for DSC, TGA, TMA, DMA, and DTA plus two additional task regions Dynamic DSC, high-pressure cell, DPA 7 photocalorimeter, robotic autosampler, GSA 7 gas selector, CCA 7 controlled cooling accessory, mechanical cooling devices, TA gas station 405
Stand-alone modular analyzers with simultaneous multitasking operation
Simultaneous operation with 7 other modules for TGA, TGADTA, TMA, DMA, and DEA or with a rheometer Modulated DSC, pressure DSC, high-temperature DTA, differen tial photocalorimetry; LN 2 cool ing accessory, refrigerated cool ing system; dual-sample DSC; autosampler for 62 samples 407
40-150 μ ι Removable crucible; automatic or manual insertion Air, N 2 , 0 2 , Ar, He; other inert or active gases 60-MHz Pentium PC with UNIX operating system and X-Window Motif interface, INGRES relational database software for method and data storage, true multitasking and multimodule control; data analysis options for DSC, TGA, or TMA; options for purity, model-free kinetics, ASTM kinetics, and heat capacity calculations R&D, quality control
Alternating DSC; ISO-9001 approval; built-in connections for two purge gases Sample changer, gas-switching unit; water, cryostat, or LN 2 cooling; software modules
404
Accuracy ± 0.1 °C; precision
±0.1 °C 0.1-500 °C/min ± 1 % accuracy
Low-temperature cooling unit
406
Accuracy ± 0.1 °C; precision ± 0.05 °C 0.01-200 °C/min 0.2 μνν sensitivity
Crimped AI, Cu, Au, Pt, and graphite; hermetic AI, alodined AI, and Au; AI solid fat index pan 50 mm3 crimped and solid fat index; 10 mm3 hermetic Removable crucible Noncorrosive inert, reducing, or oxidizing gases 486-based or Pentium IBM PS/2 PC with GPIB board, RMX or OS/2 Warp platform with stan dard data analysis software and options for heat capacity.dynamic purity, Borchardt and Daniels kinetics, isothermal kinetics, ASTM E-698 thermal stability kinetics, and oxidative stability
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 325 A
Product
Review
electronics, detectors, and thermocouples; precise-tolerance machining; and advanced computer control and temperature programming have combined to enhance instrument performance. Generally, Hansen says, DTA and DSC can be used in similar ways. "The output data looks essentially the same for all three methods. Many people who own a unit don't know whether it's DTA or DSC." However, the differences in instrument construction for each method lead to some trade-offs in performance and applications. DTA instruments, which are the simplest of the three, use thermocouples attached to the sample and reference cells to measure the temperature difference at each point in the program. They are generally optimized to allow operation at higher temperatures than either of the DSC types and are often used for high-temperature (~ 600-1700 °C) studies of ceramics and inorganic materials. Power compensation-type calorimeters operate through electronic control and direct measurement of the power that is actively delivered to the heater for each cell to maintain matched temperatures. Heat-flux calorimeters, which are based on the Tian-Calvet model, provide passive temperature adjustment through a fixed thermal conduction path that allows sample and reference temperatures to equilibrate. As both cells are heated or cooled, the measured cell temperatures reflect the flow of heat between them. Heat-flux instruments are the most sensitive of the three types, according to Hansen; some provide up to 1000-fold lower detection limits than power compensation or DTA instruments do. "DTA instruments often are not constructed to take advantage of their better detection limits," Hansen says. He notes that the added complexity of electronic controls for active temperature matching tends to limit the resolution of power compensation DSC instruments. All three types of instruments are generally designed with furnaces or chillers to operate over the range from liquid nitrogen temperatures up to ~ 700 °C. Sample size
What differentiates DSC instruments most, says Hansen, is not so much the mode of heat measurement as the sample size needed for an application. Because heat is a universal physical property rather than a chemically specific one, sample size is a much more important consideration for DSC than it would be for spec326 A
troscopy. Sample size, he says, determines the appropriate scan rate, the sensitivity, the precision, and often the mode that will be used for measurement. DSC instruments fall into two classes with regard to sample size: small volume (a few microliters) and large volume (up to a few milliliters). One reason for this division is the need to make all heat transfers as efficient as possible; a sample chamber should heat or cool the sample uniformly. Most small-volume instruments are either power compensation or DTA, but most large-volume instruments are heatflux calorimeters, says Hansen. Small-volume applications, which are usually performed with dry samples, include measuring heat capacities, characterizing glass phase transitions in polymers, and defining hydrates and solvates for pharmaceuticals. Large-volume applications include characterizing phase transitions in solutions of lipids and pro-
Sample size affects the choice of heating rate and the method sensitivity teins and testing product stability and shelf life. "Large-volume DSC is being used more and more for measuring metabolic rates," Hansen says. "The tissue is simply placed under an oxygen atmosphere in an isothermal or scanning system to measure the heat it generates." Scanning and sensitivity
DSC provides unique energy profile information for many applications, but it is not fast. A single sample can take from an hour to more than a day to scan—and autosampling, though convenient, doesn't speed things up, although some calorimeters have more than one sample chamber. For shelf life and stability testing, however, DSC is comparatively efficient, measuring in a day what might otherwise take months to discover. Small-volume instruments typically scan at 10-20 °C/min, whereas large-volume instruments typically scan at 10-20 °C/h.
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
Sample size affects both the choice of heating rate and the sensitivity in a given application. Faster heating rates can be used for a small sample without causing temperature inhomogeneities in the sample. On the other hand, small samples provide less sensitivity and baseline resolution for heat measurement than large ones do. A small-volume instrument may have baseline heat resolution of 100 pW. Largevolume models may have heat resolution and sensitivity in the low-pW range. DSC modules often have different temperature range capabilities, and most systems will scan down as well as up. There are four general temperature ranges: cryogenic (down to liquid N2 temperatures), ~ 40 °C-100 °C for biological applications, up to a few hundred degrees for routine work (polymers, consumer products, and pharmaceuticals) and melting point determinations, and up to ~ 1700 °C for applications involving ceramics, zeolites, catalysts, and inorganic materials. The most significant development in DSC in recent years, Hansen says, is modulated or oscillatory DSC, a temperature programming method in which a sine wave is superimposed on the temperature scan so that the temperature oscillates around a regular programmed gradient. However, Hansen says, "This method is still being argued about. There's a lot of debate over how to calculate thermodynamic quantities from the data. The main claim for oscillating DSC is that it can distinguish reversible from irreversible processes and separate overlapping peaks. However, these questions can usually be resolved by other, although timeconsuming, methods." Sample holders
Sample holders also differ between smalland large-volume instruments, says Hansen. Disposable ampules that hold 5-10 pL are used in small-volume calorimeters. Most of these ampules are made of aluminum and have a crimp lid, but gold and inert metal alloys are also commonly used. Some laboratories use glass ampules for lower temperature applications that require high accuracy in determining specific heats. The sample containers used in largevolume calorimeters may be either removable or fixed ampules. Removable ampules can be used for solids or liquids; they are generally made of alloys such as Hastelloy (a nickel alloy) or stainless steel and are machined to very close tolerances of 0.0002-0.0005 in.
In thefixedampules, samples are in jected with a syringe andflushedout after a run. The precise fit of afixedampule in the instrument prevents baseline shifts due to changes in heat transfer coeffi cient. However, because the ampule can't be removed from the instrument, only liquid samples can be used. Reference materials
Appropriate reference materials for heat capacity determinations by DSC should be thermally inert over the temperature range of the experiment and should have a specific heat close to that of the sample. Reference materials for small-volume ap plications are fairly well established, says Hansen. These include melting point stan dards for temperature and heat calibra tion and standards for heat capacity. For these, little has changed in the past 5-10 years. However, Hansen notes, "No good sys tems have been worked out to match the conditions for large-volume samples, most of which are solutions." Despite the need for reliable standards and reference mate rials in this area of DSC, he says there is little hope that any will be developed in the near future. 'The real problem is that there's no public funding to support the work. The National Institutes of Health, the National Science Foundation, and the National Institute of Standards and Tech nology have all been cutting back in those areas." Stretching the limits
DSC has reached the point at which much of the emphasis in design goes to adapt ing it for new applications. The sample chambers for most DSC instruments ac commodate a variety of atmospheres. Small-volume instruments generally al low afixedpressure, but according to Han sen, all the manufacturers of largevolume instruments have introduced highpressure ampules with limits of about 2000 psi. Applications include kinetics studies of gas-sample reactions, pressure effects on metabolic rates, and phase transitions in lipids. In addition, at least one commercial heat-flux calorimeter has been designed to match the high tempera ture range of DTA instruments; another has an extremely high sample capacity of about 50 mL. Several manufacturers offer interchangeable modules for performing DSC, DTA TGA and other methods in the same instrument, and DSC also has been coupled with conductometry, GC, MS, and FT-IR spectrometry. Deborah Noble
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